WO1994018603A2 - Holography, particularly, edge illuminated holography - Google Patents

Abstract

A metal halide sensitised medium recording a phase modulated image wherein voids in the medium modulate the image and the medium is substantially free of the metal of the metal halide and metal halide itself and a method of recording a phase modulated image.

Description

TECHNICAL FIELD

HOLOGRAPHY, PARTICULARLY, EDGE ILLUMINATED HOLOGRAPHY

This invention relates to metal halide phase modulated recording media; a process for developing such recording media; and regenerative and hardening bleaches therefore. Thus, it relates to the recording of holograms.

This invention relates to edge illuminated holograms. More particularly it relates to such holograms utilizing a single pass, grazing incidence, reference beam in the edge illuminated substrate without the use of special index matching fluids or the like between the substrate and the holographic recording medium. The invention also relates to photopolymers containing photopolymerizable monomers having a higher index for use as a recording medium.

This invention relates to holograms and light panels, more particularly it relates to edge-lit and steep reference angle holograms and displays; holograms which are light panels used to illuminate other holograms, displays, and electronically switched pixelated screens, such as those used in computer and television displays (e.g. liquid crystal displays); and to methods of making such holograms and holographic light panels. BACKGROUND ART

Although silver bromide films can produce finer grain images than that of dichromated gelatin,, it has not been possible to produce holograms in silver halide sensitive gelatin of the phase modulated variety that are as good as those that may be made in a dichromated gelatin medium. Unfortunately, the dichromated gelatin does not provide the fine grain images of a silver halide gelatin system recording amplitude modulated images.

Grazing incidence single pass edge illuminated holograms have not been achieved in the prior art without the use of special index matching fluids or layers between the holographic medium and the substrate and the use of such does not lead to great precision in the holograms provided thereby.

Although invented in the late 1940's by Dennis Gabor (who later received a Nobel Prize for his invention), holography has been slow to gain widespread public acceptance. The invention of the laser in the early 1960's touched off an explosion of holographic research throughout the decade. Display holography was made possible by the invention of the laser-viewable, off-axis transmission hologram by Leith and Upatnieks, and the development simultaneously of the white light viewable reflection hologram by Denisyuk in the Soviet Union. Many companies spent millions of dollars researching holography in the 1960's, but significant improvements did not come fast enough to enable widespread public marketing.

Conductron/McDonnell Douglas set up the first mass production facility. Holograms were included with the World Book Encyclopedia, holograms were given as premiums in cereal boxes, but still the technology was lacking. White light viewable reflection holograms were dark and difficult to see, or one needed a laser to view transmission holograms, which obviously was not found in the' common household. This prompted most of the large companies to scale back or cut off their research efforts.

Since then progress in Display Holography has been reasonably steady, with various incremental improvements occurring over the years in optical components, special photosensitive emulsions and their processing chemistries, vibration stability and a growing mastery of techniques by practitioners. With these developments came the breakup of holography into many subcategories. In 1968 Benton developed a technique to restrict the 3D information in the vertical direction, known as vertical parallax, creating a transmission hologram which could be replayed using white light. This was the birth of the "rainbow" hologram. These techniques were then quickly applied to reflection holograms as well.

Since the development of embossing techniques in the early 1970's to mass produce holograms into various plastic substrates, in combination with Benton 's rainbow techniques, we have seen a proliferation of low cost, high volume display holograms as security devices on credit cards, as novelty stickers, in books and magazines, and so forth. The widespread attention to this field in the mid 1980's was due in no small part to a cover story on holography in National Geographic written by ImEdge's John Caulfield and read by 25 million people.

Other advances since then such as live portraiture, natural color holograms, holographic stereograms made from movies and computer graphics, computer generated holograms and many other techniques have finally begun to permeate our everyday lives and make the public much more .aware of this technology. There are other media in which holograms are made. The most common is silver halide emulsions made by Kodak, Agfa and Ilford. Kodak was very active in developing new emulsions until, the early 1970's when it scaled back research and just kept a few of it's emulsions on the market. Agfa has since dominated the field. Ilford very recently made a decision not to make holographic emulsions anymore. Another common medium, most familiarly seen in holographic jewelry, pendants, and the like is dichromated gelatin (DCG). DCG holograms are very bright, and noise-free, but very humidity sensitive. Some processing techniques for silver halide and DCG can be toxic, and require special safety equipment and precautions. DuPont has a new photopolymer which holds great promise for the field. Polaroid also has a proprietary photopolymer, known as DMP-128, and has recently set up a mass production facility to address the display market with DMP-128 holograms.

Problems with Public Acceptance

Regardless of the material, serious impediments to mass market acceptance have still remained. There are many reasons for this. These include:

1. Color - Display holograms have mostly been monochromatic, and frequently in unattractive or unrealistic colors. More recently pseudocolor techniques have vastly improved, yielding multicolor images. But few full, real color

holograms exist in the marketplace.

2. Appearance - Many hologram images are grainy in appearance.

3. Brightness - Often hologram images are too dark, or too dim to see well.

4. Contrast - Holographic media scatter and diffract unwanted light causing background noise which diminishes image

contrast.

5. Cut-off - Many holographic images, particularly deep ones can only be seen within a small angular range, and beyond that disappears, or "cuts-off".

6. Lack of Vertical Parallax/Rainbow Effects - Many people find rainbow holograms , the result of elimination of vertical parallax information disturbing or unattractive and still not color-realistic enough over a wide enough angular viewing range.

7. Obstruction - If one wants to view a reflection hologram carefully up close, the viewer's head gets in the way of the "reconstruction beam.

8. Inconvenience of Display - A special light must be hung and aimed from the ceiling or across the room. Room lights can cause annoying secondary images.

These problems are all being addressed by numerous researchers, and great strides are being made to eliminate these problems. Some are severely constrained by the laws of physics, and require very clever schemes, or technical band-aids to overcome. It will turn out that solving these problems creates new, unanticipated uses for holography. This is a major item of technical leverage we will explore below. For the moment, however, let us concentrate on improving display holography.

The Primary Problem : Illumination is the Key

Although the above problems are significant, the primary problem leading to lack of mass market acceptance of display holograms has been the difficulty in illuminating them to achieve a suitable attractive image. Holograms require an external light source, and not just any light source, but a point source light, of adequate intensity, in order to properly view the image. These are typically strong incoherent sources with small filaments, lasers, or the sun. In addition, the light source must be located at just the right distance and angle. Holographic images are not viewable at all or look very poor in standard diffuse room light. But, we live in a world with mostly diffuse artificial light sources. This has been a very severe restriction. On the other hand, point-like room lights produce weak but annoying secondary images.

Thus the point-of-purchase and sign markets, for example, have been reticent to embrace holography, since the set-up of appropriate lighting involves special space, viewing and light source location requirements which necessitate more cost and effort than these industries are interested in expending. The same holds true for the home wall picture market, and many others.

Definition of Transmission and Reflection Holograms

As noted above, traditionally Display Holograms have been divided into two major categories: Transmission and Reflection. Display Holograms are made by splitting the beam from a laser, (usually an Argon, Krypton, Helium-Neon or Helium-Cadmium laser) into two beams, using one, the "object beam" to illuminate an object and the other, the "reference beam" travelling directly to the photosensitive medium. Light reflected from the object reaches the photosensitive medium and interferes with the reference beam, to form (depending on the medium) an amplitude and/or phase modulated region within the medium. After appropriate photochemical processing, the image of the object in the finished hologram can be viewed by illuminating it with a reasonable duplicate of the reference beam (called the reconstruction beam).

Without getting into details of the many subcategories of holograms that exist now, in simple terms, a transmission hologram is one where the object beam and the reference beam strike the photosensitive medium from the same side of the medium (Figure 1). When viewing the image, the reconstruction beam is on the opposite side of the hologram from the viewer, thus transmitting the light through the hologram to the viewer. (Figure 2). A reflection hologram is made with the object beam and the reference beam on opposite sides of the photosensitive medium. (Figure 3). The image is viewed with the reconstruction beam on the same side of the hologram as the viewer, thus the light reflects off the hologram to the viewer. (Figure 4). Over the years, the physics of reflection and transmission holograms have been studied and described in great detail in numerous publications. Many beautiful and exciting transmission and reflection holograms have been made and displayed. Viewers ooh and aah and marvel at the magic of viewing depth from a planar hologram. But people haven't bought them. The market has been disappointingly slow to get off the ground.

Edge Illumination

In the late 1960's and early 1970' s researchers realized that another possibility existed for creating and reconstructing holograms - rather than having the reference beam enter the hologram from one side or the other, the reference beam can enter from the edge. Nassenstein^1,2, Bryngdahl³, Stetson⁴, Lin⁵ and others started exploring this concept,-But the display holographers either didn't know about this work, didn't understand it or didn't appreciate it's significance. Standard transmission and reflection holograms are easier to make, and the market was in the early stages of being explored.

As the fiber optic and integrated circuit industries grew in the 1970's, and researchers gained better knowledge of guided waves, edge-lit, or waveguide holograms were proposed for integrated optics systems for holographic memories and other optical data processing and communications purposes (e.g. see Suhara⁶ and Miler ⁷).

Several years ago Upatnieks^8,9,18 proposed the use of an edge- illuminated hologram to create a compact holographic sight. He coupled a thick cover plate to a photographic plate with index matching fluid. The reference beam was introduced through a polished edge of the cover plate. However, this method was intended for narrow bandwidth or laser illumination, since the wavelength selectivity of this type of hologram is not great enough to permit white light viewing.

Moss^10,11, working on the problem of holographic head-up displays for automobiles realized that the edge illumination concept provided a compact efficient solution. He sends the reference beam directly into the holographic layer. Reconstruction, as with Upatnieks' system is done with light of the same wavelength as the reference beam. As the wavelength bandwidth of the reconstruction beam increases, the resolution of the image decreases.

The evolution of the developments described above, coupled with the unsuccessful marketing experience for many years of standard reflection and transmission holograms, has led to a great resurgence of interest in edge-illuminated holograms for display purposes. As will be proposed here, we

propose not only to investigate making better displays, but also the new concept of holographic illuminators, which will have a major impact on a wide range of industries as a new and better type of light source. Current Edge Illumination Work, Waveguide Holograms:

CAULFIELD, PHILLIPS, BENTON

In this section we will present current work being done on edge- illuminated display holograms, and the problems associated with various schemes.

Recently, Prof. H. John Caulfield, Prof. Nicholas Phillips, Prof. Stephen Benton, and others have started exploring various previously described concepts, and have developed new techniques to create holograms that can be illuminated from the edge. It is also known that researchers in the Soviet Union, Japan and other countries are working on edge-lit holographic techniques. Much of the significant ground-laying work in the field has been done by members of the ImEdge Technology, Inc. team. This work along with associated problems and solutions will be described in detail below.

Benton 's Work

Prof. Benton^12,13 of MIT Media Labs, has applied an edge-lit technique, similar to that of Upatnieks, with a thick cover plate, to his well known rainbow holographic stereograms. Later experiments used a thinner, plastic cover plate, and a specially made index matching exposure tank, allowing white light to successfully be used for reconstruction.

The Waveguide Hologram

Prof. H. John Caulfieid, and his graduate student, Qiang Huang¹⁴, have applied waveguide techniques to the creation of edge-lit display holograms and holographic illuminators. A waveguide hologram (WGH) consists of three important parts: the input coupler, the waveguide and the holographic emulsion, as shown in Figure 5. Light is conducted from the source into the waveguide by the input coupler, which can be a prism, a grating, or other edge-lighting mechanism. The waveguide used is typically a sheet of transparent material, such as glass or plastic, with two surfaces that are locally parallel and optically polished. In order to achieve waveguiding, or total internal reflection, the index of refraction of the waveguide must be higher than the index of the environment it is immersed in. Light propagates in a zigzag path through the waveguide, confined by total internal reflection from the parallel waveguide surfaces. The holographic photosensitive material placed parallel to the waveguide, in optical contact via index matching fluid. A guided wave similar to the one used for a reference beam is used to reconstruct the holographic image.

Waveguide holograms have many unique properties compared to- conventional holography. Several of these include increased image- to-background contrast, multiple and thus more efficient use of the illumination beam, the twin image effect, and the multimode image blurring effect. It has also been shown that employing the WGH method, diffraction efficiency of a hologram can be increased dramatically. With respect to image contrast, consider the image reconstruction process of a conventional hologram as shown in Figure 6a. When the illuminating beam enters from one side of the emulsion, only a small amount of light energy is diffracted to create an image if the diffraction efficiency of the hologram is not high. The major portion of the incident light may not be diffracted at all. Consequently, the undiffracted light may increase the brightness of the background. As viewers usually judge the brightness of the image of a hologram by contrasting it with the background, many holograms are not impressive because they lack image-to-background brightness contrast. As shown in Figure 6b, the WGH employs an illumination beam which is confined inside the waveguide by total internal reflection. Thus the undiffracted light makes no contribution to the background brightness. Therefore, a bright image with an inefficient hologram can be obtained by simply increasing the power of the illumination beam.

During the image reconstruction process of a conventional hologram or an edge illuminated hologram which does not utilize controlled multiple bounces, the illumination wave is utilized only once. Multiple utilization of the illumination wave can be achieved by the WGH technique. Consider the WGH illumination process shown in Figure 7. Assume the guided illumination beam is collimated. When it reaches the area where a hologram is placed, the beam encounters region 1 of the hologram first. Part of the light is diffracted as the reconstruction of the image, and the rest of the light is reflected. After the total internal reflection at the other waveguide surface, the residual light illuminates region 2 on the hologram and undergoes the second reconstruction. This process repeats until the illumination beam passes the hologram area. Because of this multiple utilization of the illumination beam, WGHs can reconstruct a holographic image more efficiently.

WGH's have a unique property, which we call the twin image effect, which can be a blessing or a curse depending on the particular product one is designing. In a WGH, two images can be reconstructed simultaneously, one on each side of the holographic recording medium. This effect is caused by the total internal reflection occurring at the hologram surface. Consider a WGH recorded as shown in Figure 8a. The object beam is simply a plane wave which is vertically incident on the holographic emulsion. The reference beam is a guided wave incident obliquely at angle θ. The recording of the interference of the two beams yields a WGH grating. Thereafter, as illustrated in Figure 8b, this WGH is illuminated with a guided wave which is identical to the previous reference beam. Because of diffraction, a.portion of the light is coupled out of the hologram in the direction of the original object beam and becomes the first reconstructed image beam. The rest of the illuminating wave travels in the original direction until it reaches the emulsion boundary. Because the emulsion has a larger refractive index than the environment, the undiffracted illumination wave suffers total internal reflection and creates a reflected beam. This situation is equivalent to having another illumination beam incident on the hologram with an angle of π - θ. Accordingly, a portion of this beam is diffracted and propagates in the opposite direction from the original object wave. This wave is the second reconstructed image beam. If the original object beam is not vertically incident but makes an angle Φ with respect to the normal of the hologram, the reconstructed twin beams will propagate along the angular directions of Φ and π - Φ. Usually, the output beam which propagates in the same direction as the original object beam (the first image) is stronger than the other one. The ratio of the intensity of these beams is related to the local diffraction efficiency of the recording material.

During the image reconstruction process of a WGH, there is an effect called multimode blurring, which causes many images to be reconstructed simultaneously, and overlapped to one another. The multimode blurring effect is caused by the angular divergence of the guide illumination beam. As shown in Figure 9, a diverging illumination beam with a circular cross section is coupled into the waveguide. It propagates and illuminates an elliptical area on one of the waveguide surfaces when it encounters the first total internal reflection. The reflected beam then travels toward the outer surface of the waveguide and undergoes its second total internal reflection. After that, the beam illuminates the previous surface again, but with a larger elliptical area. This process continues and distributes a series of illuminated elliptical areas with growing sizes along the propagating direction of the light beam. In the portion of the waveguide surface shown as the shaded area of Figure 9, these illuminated ellipses overlap one another. If a hologram is placed at these overlapping illuminated areas, multiple images are observed because they are reconstructed simultaneously by two or more illumination waves which have slightly different incident angles. These images are spatially overlapped, so the resultant image is degraded. Illumination by a group of light waves with different incident angles is called multimode illumination. Therefore we refer to this process as multimode blurring. It has been shown that multimode blurring can be eliminated by the combination of a proper input coupler and a slit to limit the divergence in one direction and control the extent of the reference beam in the other.

Experimental results have so far proven that the WGH technique is practical for generating 2-D and 3-D images for display, but work is still necessary to improve and optimize the quality of the images. Wavelength selectivity using the above WGH technique has so far not been satisfactory enough to allow for white light illumination of 3-D images, but 2-D images are on the hologram plane, and do not require that the illumination light have high spatial coherence. Therefore good results were obtained for 2-D images (whose uses are noted elsewhere in this document) with white light illumination, such as directly from a light bulb or with a fiber-optic ribbon conducting light from a remote source. WGH-Practical, But Needs Work

Additional experiments¹⁵ have demonstrated that by utilizing a thick holographic recording layer and a multimode waveguide, or simply the substrate that the recording layer is on, a single, white light reconstructible image can be obtained from a waveguide hologram. However, if the substrate is a multimode waveguide, the spatial coherence is limited, requiring that images be in or close to the hologram plane.

The WGH system, then, has the following advantages:

1. It is compact. The usual alignment necessary for conventional holographic recording and playback systems is not necessary.

2. The optical fiber, laser or incoherent source can be remotely located.

3. The reconstruction beam cannot be blocked, leaving the image obstruction-free.

4. No undiffracted light can enter the viewer's eye to cause either danger (from laser reconstruction) or contrast loss. Use of laser light for reconstruction of products for which monochromatic images are acceptable would allow the public to benefit from crisper, deeper images than are typically obtained with multicolor holograms or even monochromatic holograms made to be reconstructed with white light. The safety factor has prevented laser illuminated holograms from public display in the past.

5. The image can be very bright because of high image-to- background contrast and multiple utilization of the illumination beam.

6. The image can only be reconstructed by the light inside the waveguide. Other light sources cannot affect the quality of the WGH image.

7. The WGH system need not be planar. For example, it can be cylindrically shaped.

Mr. Huang and Prof. Caulfield¹⁶ have also investigated another recording scheme to produce white light illuminated, edge-lit rainbow reflection holograms. Using a two step process, a standard transmission master (H1) hologram is generated using a collimated reference beam. A second hologram (H2) using the edge-lit concept is then recorded, where the object beam is the projected pseudoscopic real image from H1, which is illuminated with the optical phase conjugate beam of the original reference beam. Following Benton's rainbow hologram concepts, a slit aperture is placed in front of the H1 hologram to eliminate the information contained in the vertical parallax. A three-dimensional white light edge-lit reconstructed image was produced with this method.

Prof. Phillips - Another Approach

Prof. Nicholas. Phillips¹⁷ has investigated another way of feeding light into the edge-lit or waveguide hologram instead of the input coupling prism described by Prof. Caulfield. Prof. Phillips fed a laser beam through a single mode fiber, used as a spatial filter, then split the beam into object and reference beams. The reference beam was fed into cylindrical expansion and collimating lenses. The cylindrically collimated laser beam was then introduced into the polished edge of a substrate onto which photosensitive material was coated. (Figure 10). Care was taken to avoid non-uniformity and unwanted divergence of the reference beam so that the reference beam created a collimated sheet of light passing through the substrate. The object beam was sent in normal to the recording medium. Results were evaluated using Dupont photopolymer and Ilford silver halide material. An interesting and unexpected result was achieved with the DuPont material. By careful alignment, the system could be set up so that the fluorescence in the recording layer became extremely uniform. In this case, it is proposed, that the reference beam was not bounced around inside the recording layer substrate structure, but rather continuously fed the recording layer from a travelling wave "diffractive feed" system. This concept may form the basis for eliminating the problem of lack of illumination uniformity (known as illumination depletion) present in the WGH scheme where the illumination beam is progressively more depleted as it bounces through the waveguide, resulting in a gradient in the brightness of the reconstructed image.

One important problem common to silver halide holographic recording materials is that they shrink after processing. Tanning developers can prevent that, but add an absorptive stain which reduces efficiency, particularly with transmission holograms. The shrinkage changes the slant of the fringes inside the hologram which then changes the acceptable input angle for the reconstruction beam. This angle can put the hologram out of the edge-lit regime. This phenomenon must be well understood and controlled in order to successfully use silver halide materials for edge-lit products.

Since edge-lit holograms rely on a fringe slant which is different from conventional reflection and transmission holograms, the dispersion of the image on reconstruction is not subject to familiar rules. Optimization of edge-lit images may require some bandwidth reduction of the reconstructing light or a concept such as the use of single parallax master images.

This work on the various edge-lit recording methods means that holograms can now be displayed in a compact, self-contained format, where the illuminating light source can be hidden within a frame or otherwise mounted to the hologram. Combining the new edge-lit concept with new advances being made, both in the West and the Soviet Union, in holographic materials and processing techniques, as well as improved methods of achieving realistic, full color images, provides a major new impetus toward the mass marketing of display holograms. 1. Commercial Images and Displays

The world is poised to experience an explosion in the use of holograms for both industrial and consumer products. As holograms become more common, as they have on the credit cards that millions of Americans carry in their pockets, as well as magazine and comic book covers, baseball cards and cereal boxes, the public expects to see technological marvels such as the holographic projection of Princess Lea's message we all loved in "Star Wars" become available. This dream may yet be a little too far from current reality, but as holographic images become more widespread much residual resistance to the current state of the art will diminish. This resistance, to a large degree, can be traced to burdensome illumination requirements. The necessity of a hard (point) source of light kept at a specific angle and distance from, the image is a formidable obstacle to holography achieving the pervasive or even dominant role it should achieve in display images. In spite of the proliferation of embossed holograms which can be viewed in "room" light, the real "quality" images with true depth and sharp detail still cannot be viewed without special lighting. The edge lit principle of being able to illuminate one of these holograms with an incorporated light source overcomes this major obstacle.

The first half of this discussion of edge-illumination, then, centers on the economic benefits of holography as a display medium. The thesis is that holography will become a pervasive display medium when reconstruction illumination is simplified.

Magical Medium

The special nature of a hologram that differentiates it from other imaging media is that the image is "reconstructed" rather than simply illuminated. The very nature of edge illumination truly adds another dimension to the benefits of holography. By reconstructing an image of an externally illuminated hologram with any light source, we also must necessarily illuminate whatever is in the

vicinity as well. Likewise, holograms whose images are meant to be reconstructed by using external light sources have an image which will be adversely affected by "stray" light from other sources which happens to fall on this hologram. In contrast, an edge illuminated hologram can have the unique and wonderful characteristic that it will not be affected by any light source other than the edge illumination. Its image is . virtually nonexistent unless and until the reconstruction illuminator at its edge is turned on. The nearest analogy may be the nature of backlit display transparencies, which, when properly set up, can compete favorably with extraneous light. They can be also made to appear black when their illumination is turned off. Although this can be an effective display technique, it pales in comparison to the fact that when an edge lit hologram's reconstruction illumination is turned off, you can see through it as if it were clear glass. This "magical" quality can do more than make it the most impressive display medium. It can make these holograms useful in totally new and unusual ways.

This new technology we have invented will allow us to bring the world of holography into its next stage. The expected growth of the U.S. technology and knowledge base should be exponential. Entine industries will need to adopt this technology in ways that we can only begin to predict or imagine. Much as photography has become the dominant medium in the print segment of our visual society which needs to capture the visual realities of our world, so holography is the logical next step. It is the only viable three-dimensional method which can be viewed, anywhere, without the aid of ssecial glasses or other encumbering paraphernalia or computer hookups.

Actually entering the various commercial markets required that we perfect techniques and inventions with respect to:

1. Determining the optimum edge-lit scheme to produce marketable 2D and 3D holograms. This involves understanding the theory, and achieving experimental success with a given scheme.

2. Size of the holograms. Size is an essential ingredient of most displays. If we can make 2 foot by 2 foot high quality holograms, this will open up the point of sales market (probably the largest market area we have identified). Billboards and other outdoor signs are usually larger, but we can use arrays of these "small" holograms for them. For prior holograms, size offers no fundamental problems. For our holograms, large size lead to nonuniformity through illumination depletion.

compensate for this nonuniformity.

3. Uniformity of the image, (see 2)

4. Efficiency of the image.

5. Image quality.

6. Reconstruction beam type and quality, (see 1)

7. Scatter from the hologram. Scatter is a source of unwanted noise. It is worst in the blue/ultraviolet region of the spectrum. We

develop

alow scatter holography.

8. Illumination means and mechanisms (which may vary with the application). Illumination methods impact many aspects of these waveguide/edge-lit holograms. Some of the key issues are:

a. Compactness

b. Ruggedness

c. Image or Beam Quality

d. Color or White Light imaging or illumination

e. Coupling efficiency

f. Cost

g. Safety for laser illumination.

9. Determine the optimum recording material.

HOLOGRAPHIC LIGHT PANELS

Holographic light panels are a new development originally conceived by Professor Caulfield. These panels constitute a new kind of light source, where light enters through or near the edge of the light panel and is then re-emitted in a controlled pattern from the face. These low cost, thin, flat light panels can produce uniform, directed beams of light, which can be white light or laser light, depending on whether the light entering the panel edge is white or laser generated. Furthermore, the panels can be designed to produce a beam or multiple beams which can be narrow, highly directed or wide angle or even fully diffused. Such light sources have numerous applications including: converting standard holograms to edge-lit ones, image projection, flat panel television displays, security and biotechnology applications. They allow significant reduction in the physical volume necessary for illumination of LCD's, transparencies, holograms, and various other objects.

DISPLAY HOLOGRAPHY

In the past, the striking three dimensional images produced by traditional display holograms could only be seen when the holograms were illuminated either by a distant, precisely positioned, narrow width, bright spot light or by a laser. This constraint has severely limited the markets available for traditional holographic displays. Home and office decorators, point-of-purchase advertisers, and others involved in display .industries have consistently shown fascination, with the potential of three dimensional holographic displays. However, the inability to view the images under standard room lighting conditions, without setting up special permanent external spot lighting directed to the hologram has prevented the acceptance of holography for these applications. Steep Reference Angle Display Holograms

Steep reference angle display holograms have many

(though not all) of the applications of edge illumination without the engineering necessary to commercially achieve with the edge-lit approach. Steep reference angle display holograms may be used in movie theater lobbies.

Edge-Lit Holograms

Advances in edge illumination technology are being pursued at various universities and government sponsored facilities.

Nevertheless, even if this or some of the other industrial or academic technology is made available for commercial exploitation, we are confident that we have the advantage of a broader perspective coupled with superior independent technology. As one typical example, we do not know of any other group which has appreciated the potential power of edge-lit technology as the basis for the holographic light panel.

One of the major applications of an edge-lit holographic light panel is back-lighting of liquid crystal displays (LCDs), particularly for notebook computers.

The requirements are most stringent for back-lighting the LCDs used in notebook computers. These requirements affect the size, weight and battery life of the notebook computer and in turn its competitive position in the marketplace. The requirements for a competitive LCD backlighting system are:

1. Produce even illumination fully covering the user's entire comfortable field of view with no reduction in display clarity caused by the illuminator and its optics. 2. Have minimum bulk and weight to make the notebook computer as thin and light as possible.

3. Produce the required display brightness with minimum battery power in order to increase battery life. Short battery life is one of the greatest complaints from users of notebook computers.

There are several non-holographic competitors for LCD back-lighting that attempt to meet all these requirements.

Many manufacturers of LCDs and notebook computers which use LCDs have expended engineering efforts to meet the requirements listed above. Notable among these is COMPAQ Computer Corporation, with two issued back-lighting optics patents assigned to it. These optics use a series of horizontal steps to achieve the required illumination coverage. However, these horizontal steps interact with the line structure of the LCD producing an undesirable moire effect in the display.

3M has developed plastic sheeting that is designed as a general purpose optical element to produce a sheet of illumination similar to that of the COMPAQ Computer Corporation patented optics. The 3M product comes in two forms: a reflective form named RAF (right angle film) and a transmissive form named TRAF (transmissive right angle film). Both of these are made with very fine grooves which, like the step structure patented by COMPAQ, interact with the line structure of the LCD producing the same sort of undesirable moire effect.

In both of these products, the undesirable effect of moire pattern in the display is reduced by adding a diffusing element to the optics. This increases, the bulk and weight of the package and reduces the display brightness through the reduction in optical efficiency.

One of the many advantages of our holographic light panel is that, unlike the above products, it has no inherent structure to produce moire effects in the LCD display. Thus, it does not require the added weight and bulk of a diffuser.

A basic inherent advantage of the holographic technology is that the physics of holograms causes the light to be redirected in a highly efficient manner, in contrast with competitive systems which inefficiently direct light by means of scattering it and/or diffusing it.

OBJECTS OF THE INVENTION

Among the objects of the invention are:

to provide true one pass edge illuminated holograms ; to provide improved steep reference angle holograms ; to provide holographic illuminators; to provide such illuminators that direct the light eminating therefrom as desired; to provide such illuminators that are true one pass edge illuminated and steep angle illuminated; to provide such illuminators that provide pixelated illumination in mono chrome and in three primary colors; to provide a holographically illuminated liquid crystal display; and to provide apparatus and methods of making and achieving the foregoing.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the features of construction, elements, arrangements of parts; articles of manufacture comprising features and properties and relations of elements; and methods comprdsing the several steps and the relation of one or more of said stages with respect to each of the others - all of which will be exemplified in the construction, articles, and methods herein described.

The scope of the invention will be indicated in the claims. FURTHER APPLICATIONS OF THE HOLOGRAPHIC LIGHT PANEL

Beyond providingback-lighting for LCD's, otherapplicationsoftheholographic lightpanel includemedical, diagnostic and laboratory tools, all ofwhich would be improved by this unique ability to pump light into areas that are extremely close to the source ofthe light without having any interference in viewing or photographing the area. The holographic light panel also can easily be configured to place the heat generating source ofthe input light remotely, resulting in a completely "cold" illumination source.

Photo studio and dark room procedures could be revolutionized by the holographic light panel:

The simplest applications here are for displaying photographic transparencies. Every userofand worker in photography, from the photographer himselfto the advertising agency, to the printer, to the ultimate client use light boxes. These boxes are big and bulky, and command a significant price along with their significant claim to work-top real estate. A compact light box that is only one quarter inch thick should take-by-storm the photographic industry which prizes sleek high-tech appearance and light weight compactness. In fact, photographers and theirsales representatives have long sought a truly portable unit which can accommodate large format transparencies (8 X 10 in.) and still be stored in their sleek custom tailored portfolios. Similar reductions in size, weight and power can be obtained for all types of projectors: slide, movie and overhead.

Holographic lightpanels can also beused to providesimilaradvantages indarkroom lightsources particularly for photographic enlargers by reducing the enlarger bulk and the heat that is generated by current light sources. They will also simplify the enlarger by allowing a change from a condenser enlarger to a diffusion source with modular switching between holographic heads.

One of the bulkiest pieces of a "location" photographer's travel baggage contains the light "heads" and reflectors. The reflectors, by their nature must be large parabolas requiring bulky shipping cases. A holographic light panel can accomplish the same task with a small fraction of the depth necessary. The holographic light panel can generate light in almostany desired shape. With the holographic light panel, it is possible to view or even shoot through the light source. It is important to remember that the holographic light panel will be transparent from the "back". When looking through or photographing through it, the holographic-light panel acts as if the light is coming right from the viewer's eye.

Another photographic advantage: one ofthe most flattering ways to light a model's face is to surround it with even lighting from all directions. This eliminates shadows that accentuate wrinkles or folds in the skin and "washes out" pores, blemishes and other imperfections. At present, the methods used are not only very awkward and time consuming to set up but they either create some shadow from the photographer or make it difficult for him/her to move about. Theperfectsolution is a large holographic light panel which eliminates all shadows, yet give the photographer freedom ofmovement. It would also reduce the heat generated in the photo studio, which creates a major problem today in keeping models from profusely perspiring or melting. In the photographing offood displays or any other product that is heat sensitive, the advantages of reduced heat from the illuminator are selfevident.

We have found ready interest from manufacturers oflight boxes and lighting equipment to license this technology for next generation illuminators.

Photography is not the only non-computer application that can use efficient illuminators to project light along the line ofsight without obstructing it. Other possibilities include: * Microscope Illuminators - allowing for precise lighting to be applied from close proximity * Law Enforcement - Finger illumination for optical fingerprint reading. Equipment can easily be carried in a patrol car. (We are already in negotiation to develop this.)

* Doctor/Dental/Surgery lights- apreciselighting instrumentwith line-of-sightprojection forsmall orifices and cavities

* Display case lighting without the source being visible (forjewelry, stores, museums, etc.) * Illuminating protective glass covering for works of art in a museum or gallery.

* Compact biosensing and immunochemistry instruments. In this application, the light is reflected

(orscattered, fluoresced, etc.) from theobjectbeing examined, passes through the hologram with no distortionor loss ofinformation to a detectorviewing theobject through the panel illuminator.

DISCLOSURE OF THE INVENTION

Method and Apparatus for Recording Holograms and Light Panels Using Effervescent Wave

Caused Index Matching

I have discovered that by utilizing a photopolymer comprising a photopolymerizable monomer- having a higher index of refraction than the polymer component as the recording medium on a substrate having an index of refraction slightly greater than that of the recording medium, such grazing incidence, one pass, reference beam edge illuminated holograms may be produced since the evanescent wave formed in the recording medium during the recording process will polymerize the photopolymerizable monomer in the evanescent layer and then cause the migration of additional monomer to the evanescent layer which causes that layer to increase in index of refraction by as much as .01 to .07.

During the recording process, you can see that the evanescent layer glow due to the fluorescence of the material and after a certain length of exposure time, the hologram will bloom as the index at the interface in the photopolymer rises to match that of the substrate.

I use exposure times of a few tens of seconds, usually about half a minute or so. The photopolymer obtained from DuPont's Optical Element Venture Group has a bulk index of approximately 1.5 and I have used substrates of silica, acrylic, BK7 and BK10 glasses successfully. The index change achievable in the photopolymer can be made to range from about .01 to about .07. Thus the difference in index of refraction of the recording medium and the substrate can vary by this amount. BEST MODE FOR CARRYING OUT THE INVENTION

Method and Apparatus for Recording Holograms

and Light Panels Using Effervescent Wave

Caused Index Matching

Holograms in the edge-illuminated geometry - new materials developments

ABSTRACT

Some rigorous statements are made about the recording ofholograms in the true edge-illuminated geometry. An essential asymmetry is outlined between the role of the substrate refractive index and that of the recording medium. Laboratory observations indicate that unique possibilities exist for recording in this regime using the photopolymers of Du Pont. Unlike any other regime of holography, numerous optical criteria have to be met simultaneously. We believe iliat self-induced index matching between the recording polymerlayerand thesubstrateon which it is mounted is thekey toa workable recording system.

1.INTRODUCTION

In recording holograms in the edge-illuminated geometry, the assumption dial the usual constraints apply is misleading. In fact thegeometry isvery restrictiveuswe shall see below. Anedge illuminatedhologram is perforceone in which compactness isthe prime targetofthe exercise but although this is very laudable it is the causeofmajorrestrictionson theproperties ofthe recordingmaterialsused.Arecentpaperby Upatnicks¹ givesaconcise andeducated viewofihe problem.

We shall enlargeon the remarks ofthat paper in what follows and examine the materials requirements that are demanded by the regime. The relationship between the refractive indices ofthe recording material and its substrate is the most important aspectof the work. This is outlined in the discussion. In practice, the matching requirements of these indices is normally very light arid either impractical or impossible toachieve in a simple non-fussy manner.

In this paper, we report laboratory observations dial indicate novel regimes ofrecording that are enabled by the unusual properties ofthenewrange ofDu Pontimaging polymers.

We fust examine the geometry and consider the case in which the indices of the recording material and its substrate are in the relationships n_m/n_s>I and n_m/n_s<l where n_m is the index ofthe recording medium and n_s is the index ofihe substrate.These two casesare importantly and subtlydifferent; neither looks favourable lotherecording process at firstsight. We begin the paper by a detailedconsideration ofgeometry and then reportsome interestingobservations in the laboratory.

2. CONSIDERATIONS OFGEOMETRY

Sincetheoverriding importance in therecordingapparatusrelatesto thesubstrate geometry, we show a view ofthe substrateand itsinputreference in Figure 1.

Figure 1-Showing thesubstrategeometryandtheminimalangle ofattackofa reference beam launchedfrom

theedgeofthe hologram into ihesubstrate,given by tan Φ_s = L/1

Theapparatusthatwe haveused isshowninFigure2.Beamshapesandcleanlinessareadheredtowithmaximumrigour.

Figure2-The recordingset-upforihe edge-lithologram. The objectbeam is usedto illuminateH₁, with the halfwaveplate usedto match thesignalpolarization with thefinalreference beam. The reference beam isexpandedverticallythrough a cylindricallensandiscoupledinto thephotopolymeratatypicalangle of88°

It isobvious thatconsiderablerestriction is imposed on theangleofattack ofthe incoming reference on the interface between the substrate and therecordingmaterial. Wesee thattheangle Φ_s isoftheorderofthatgiven by the relationship

(2.1) whereL is thelength ofthesubstrateand tits thickness.

Theamplitude oflight transmitted atthe interface between the substrateandovercoated recording layeris given by therelationship

(2.2)

whereΦ_sandΦ_m aretheangleswith respect tothenormal totheinterfaceinthesubstrateandmedium respectively.To thisequation, weadd Snell'sLaw.

n_mSinΦ_m = n_ssinΦ_s (2.3) where n_m and n_s aredefined above.

First, weconsiderthecasen_m>n_sso thattheraydiagram looks like thatin Figure3.

Returning to equation (2.2), itcan beexpressed in the form

(2.4)

Where we havewritten n_m =n_s + Δn and Δn is regarded as small enough. This expression reveals that unless the criterion

holds then as Φ_s approachesπ/2 then the transmittance drops lo zero. Since we have restricted the values oftan Φ_s by therelationship tan Φ_s = L/t, wecan write the abovecriterion in the form

This sets a stringent demand on index matching in the recording process. Figure 4 shows a set ofplots ofintensity transmittance againstn fora setofvaluesofΦ_s.

Figure4-Transmittedintensityversusrefractive indexdifferenceforanglesapproaching grazing incidence UsingFresnel's relationsand Snell's law wecan derivethe intensity transmittance in the form

Figure4showsdatawith n_s starling at 1.49andapproaching 1.495 with n_m= 1.495 forΦ_sapproaching grazing.Figure4 illustrates a solution to theproblem ofcouplinga reference beam into thehologram at asteepangle. By choosing theindex ofthe substrateto bejust below that ofthe recording material, we are able lo couple light reasonably efficiently into the film atangles approaching grazingincidence

When the relationship between the indices is such that n_rn<n_s then the situation is much more subtle. Once theangle ofattack Φ_s exceeds thecritical angle

given by the relation

(2.5

then total internal reflection occurs and the light fails toenterconventionally thesecond medium. Moreover, an evanescent layer is created, Figure 5,just inside ihe interface in the recording medium. This effect is well known and is the result ofa detailed study in the excellentanicle by Bryngdahl². Thepenetration depth oftheevanescent field depends on the angle ofattack Φ_s and inessencedecreasesasΦ_s increases.

Figure5-Thecreation ofanevanescentlayerbyreflectionoflightwhichisincidentfromthedensermedium(n_s> n_m). The reflectedlightissubjecttoaspatialshift—the GOOSHänchen shift(seeBryngdahl²)

Detailed calculation shows that ~

(2.6)

where the evansecent waveamplitudefalls offas

λ_pis thus apenetration parameter,λ_a is iheairwavelength and z is a coordinate normal to the interface.

Given the considerationsabove,wecan writeexpression (2.6) in theapproximate form

(2.7) whereΔn=n_s - n_m

Equation (2.7) is finallywritten in ihef (Δn<<n)

(2.8)

(2.9)

Now suppose that weletΔn→ 0from abovezero. Wecan seethatpenetration isperfectfor

(2.10)

Thesediscussionsabovearefundamental lo theprocess oftransmission at the interface in thecasen_s≥n_m.

3. CONSIDERATIONOFTRANSMISSION OFLIGHTTHROUGH A DIELECTRIC LAYER Here we use theconventional diagram as inFigure6.

Figure6-Basicgeometryforlightreflectingoffandtransmittingthroughalayerofdielectricmaterialofthicknessd Wededuce thefamiliarresults thatthetotal transmittedamplitude through alayerisgiven by

(3.1)

Thusyielding the transmitted intensity in theform

(3.2)

The properties ofthis equation are well known but are subject to a new importance when the angle ofattack θ→ π/2. As the lightapproaches grazingincidencethen theangularsensitivity of(3.2)diminishes.Thisisbecause

(3.3) (3.4)

/

where n is the index of the layer. Evidently, coarse angling of the layer can 'tune' the transmission with relative ease and this effectisobserved in thelaboratory. Here S COSΦ = d and

whereλ_m is the wavelength oflight in the medium and as usual λ_m = λ_a/n whereλ_a is the air wavelength. The transmittances t,t' and reflectancesr, r'are given by the usual Fresnel relationships.

Paradoxically,theintroductionofthepartial wavesystem encouragedbyreflectionoffthesecondfaceofthelayerin factenablesthetransmittanceofthelayerandavoidstheshortcomingsofreflectanceshown in thesingleinterfacecalculation.Thiseffectisofcourseendemicinany etalonsituationwheretwohighlyreflectivemetalfilmscancombinetotransmitstrongly undertherightconditionsofseparation.However,in ouredgelithologramcase,failuretosuppressthepartialreflectionswillleadtointolerable'orangepeel'cosmeticsintherecording.

4. POLARIZATION RESTRICTIONS INTHE RECORDING PROCESS

A major drawback of recording of edge lit holograms lies in the geometrical restrictions imposed on the polarization of the recording lightTheproblem is illustrated in Figure 7.

Figure 7-Polarization isrestrictedto thesmoderegimedenotedby⊙: thiscan leadtoseriousorange-peelfringesinH₂ and more importantly,bi-refringentbasematerialforpolymerrecording willleadto variationsofpolarization ofthe angularspectrumofwavesfromH₁ representedbyA₁ andA₂. This complexissueisdifficult toresolve

The polarizations are directed by the symbol⊙ implying an s polarized regime of recording. This is unavoidable since thep modeleads toa truenegation offringecontrast.

Another irritating problem arises ifbi-refringentbase material isused loback therecording layereg a halidefilm on MylarorDu Pont polymer on Mylar. Such bi-refringence then varies across the angular spectrum from the master thus leading to some strange fringecontrastperformancein the H₂ copy hologram.

5. RECORDING MATERIAL DEVELOPMENTS - DU PONT'S PHOTOPOLYMER

This regime ofrecording is extremely intolerantofmaterial performance. Shrinkage in the recording layer is not permitted without therisk ofspurious replay via the largesurfaceofthe layer.

Therefractiveindexofsilverhalide layers is notadjustable and inanycase, thatmedium isatriskofshrinkageand generallypoorperformance.Newmethodscurrentlyunderdevelopmentby theauthorsmayhelptoeasethissituation however.(Phillipsetal³1993).

Theprimecandidateforrecordingin thisregimeisundoubtedly theDuPontfamilyofphotopolymers.Theseareideal materialsduetotheir lowscatteranduseful modulation indices.Theydohoweverhaveaseriousdrawback.Theirdissociationfrom theirbi-refringentbaseMylar isdifficultandspecialtechniquesarenecessarytoavoidproblemsfromthisareaasoutlined in section4.Theydoshrinkbutthemagnitudeis unlikely to shiftthereplayreference geometry from edge to largesurface. However, the majorexpansion oflayers permittedby Du Pont's novel monomer transferprocess is ofcourse questionable since that

the slanting fringes of the regime out of their tolerable direction.Theirmainandextremelysubtlecontributionisself-inducedindexmatching,whichisdiscussedin thenextsection.

6. SELFINDUCED INDEX MATCHING—A NOVEL PHENOMENON ASSOCIATED WITH

THE DU PONT POLYMERS

Wefirstmake thefollowingobservation:

The bulk index ofthe Du Pont materials (measured at the interface) used as an overcoaied layer at an interface lit from the substrate side is only likely to increase. This is due lo shrinkageaftersolvent evolution orby the action oflight. This remark needs to be well qualified and can be understood with reference to Figure 8.

Figure8-Showing theselfindexmatchingphenomenon inDuPont'sselfimagingphotopolymers. As thereference lightgeneratestheevanescentregion, monomerisin effectdrawn towardsit thusenhancing thepenetration ofthe reference beam.Allsuccessfulsimplisticrecordingsmade byourselvesworkaccording to thisprinciple. The effectisobservedbynoting the increasing fluorescence in lightfrom thesensitizing dye in thephotopolymer

The evanescent layer can encourage monomer to diffuse towards it thus increasing the index in the polymer adjacent to the interface. This phenomenon has been observed by us in frequent cases. It is a novel and fascinating effect signified by the progressive increaseoffluorescentlight from the sensitising dye in thepolymerlayer. This effectpermits a novel and effectively uniqueapproach to local optimisation oflight transmittanceat the interface.

The observation ofthis phenomenon is, in our work, an important diagnostic feature and extremely helpful in developing aclear understanding oftheoverall method.

7. CONCLUSIONS

This restrictive geometry is still in the early stages ofstudy. Generally, thin edge-lit holograms are needed and therestrictionsare palpably severe. The discussion of optical geometry reveals that the coupling of the reference into the layer is a subtle process which can be helped by self-generated attributes ofthephotopolymer layers.

Surprisingly, theregimethatlooksostensibly unpromising, ie that in which total reflection and evanescence is induced, looks the most interesting and in fact, promising.

8. ACKNOWLEDGEMENTS

The authors especially wish to thank Michael Metz and Carl Flatow ofIm EdgeTechnology Inc for their personal and financial supportin partofthis work and Mrs Kathy Phillipsand colleague PeterMarsh for theirefforts in preparation ofthemanuscript.

9. REFERENCES

1 UPATNIEKS,J.,Appl. Optics Vol.31 No.8, 10 Mar 1992

2 BRYNGDAHL, O., Progress in Optics Vol.XI. Ed.E. Wolf, North Holland, 1973

3 PHILLIPS,NJ.; RALLISON,R.D.; BARNETT,C.A.; SCHICKER, S.R.,Dichromatedgelatin-some hereticalcomments, Proc.S.P.I.E..SanJosé,Feb 1993 in thepress It has been observedthat in the formation ofedge illuminated holograms, the action of the signal wave canincrease the refraction index ofthe recording layerthus increasing the coupling ofthe refereace wave into the hologram when it is incident at an angle closeto graringincidence.

NB The recording material is chosento have a refractive indexjust belowthat ofthe substrate.

It is noted that where the signal beam arrives, the hologram is seen to be switched

regions ofhigh signal strength thus indicating that the refractive index has increased in thatlocalitythus enablingthepenetration ofthereference wavebyindex:matching.

We note that enhancement ofthe refractive index at the interface can be achieved by either reference or signal wave activity. Such enhancement could be achieved by for example by exposing the recording layer to a diffuse page ofsignal wave on its own prior to exposureto theholographicpattern

.

The bulk index ofthe recorderlayeris thus increased.

DISCLOSURE OF THE INVENTION

alide Phase Recording Medium, and

Regenerative Bleach Therefore

I have found that the large values of index modulation observed in dichromated gelatin are probably caused by gelatin hydrolysis in the nodal parts of the image. When collapse of the recording layer is inhibited, void modulation occurs ranging between the refractive indices of gelatin and air. I found that the use of a persulphate is very effective in preventing the collapse of the voids in the gelatin, particularly, Potassium Persulphate and this is one aspect of my invention as this may be applied to both dichromated gelatin and silver halide-sensitized gelatin.

My novel systems for recording phase modulated images in silver halide-sensitized gelatin, involve exposing the film to the desire image; developing the film conventionally; removing the silver filaments formed at the image anti- nodes by bleaching and hardening the area around the voids by releasing Chromium 3 ions where the metal grains were; then in the first process uniformly exposing the film to sensitized all of the silver halide grains remaining in the film; developing them into silver filaments; and then again removing the silver filaments by a solvent bleach which forms a small void where the silver was and hardening the gelatin adjacent to the voids. The voids formed in the antinode regions of the image, in this second bleaching step, appear to be smaller than those formed in the nodal areas, due to the previous hardening of the gelatin during the first bleach and hardening step. I then amplify all voids using hot or graded propanol drying. In a second preferred process, after the first bleaching and hardening step, I merely fix the film using conventional hypo (sodium thiosulphate) or rapid fixer (amonium thiocyanate) and then amplify the voids using hot or graded proponal drying.

I used Potassium Dichromate to supply the chromium ion through reaction with the silver, Potassium Persulphate for regeneration of the reduce silver oxidate. Potassium or Sodium Hydrogen Sulphate, and Distilled Water in my bleaching hardening solutions.

The Ferric Nitrate (or Sulphate) mentioned below in formula SSI cannot be used as it tends to soften rather than harden the gelatin around the voids in my process for recording phase modulated images in silver halide films. The formula SS2 is preferred in the Repeat (2 development) Method; SS3 in the one step preferred method. Other metals beside silver could be used, such as, platinum, gold, or the like in the metal halide.

Those skilled in the art will note, that, ideally, in my repeat method for the formation of phase modulated gelatin which has been activated by silver bromide, the antinodes of the image should underexpose the film so that some silver bromide grains will be left at the exposed antinodes to form voids in the second bleaching step. Furthermore, those skilled in the art will realize that the illumination necessary in the second exposure is uniform in the sense that it activates all remaining silver bromide grains in the gelatin. Any form of illumination which accomplishes this result may be utilized. In the preferred one step process, the voids predominate in the antinodal regions and are few, smaller, or absent in the nodal regions. Holograms produced by this process are startlingly good.

In the one step process, I reverse the proportion of Dichromate and acid in the bleaching solution so that the gelatin will harden throughout. This apparently reduces or prevents the formation of voids at the nodal regions when the silver bromide is removed by the fixer.

DETAILED DESCRIPTION

Halide Phase Recording Medium, and

Regenerative Bleach Therefore

Dichromated gelatin -some heretical comments

ABSTRACT

Thispaperlooksatsomeideasoldand new thatrelatetothe formationofimages in dichromated gelatin. The traditional viewthat the dichromated system hardens gelatin thus preventing solubilizadon ofthe material must be balanced against other observations such as that of the reduction of the bulk index of the gelatin layer and the appearance of gelatin in the processing solutions. We revisit the problem and take a look at some new chemical ideas that relate to the behaviourofgelatin during solvent bleaching of silverhalides. A new method for the processing ofsilverhalide-sensitized gelatin (S.H.G.) is proposed which we call the Repeat Method.

1. INTRODUCTION

The subject of dichromated gelatin as a recording medium for holograms has received enormous attention over the years due to the importance of the medium to the manufacture of headsup displays for military and latterly civilian cockpit applications in aeroplanesand motorvehicles.

Dichromated gelatin (D.C.G.) is a complex structure involving care and consistency with the recording and processing technique in order to exploit its full potential. It is not possible to give a full bibliography of the subject since much important Soviet work has remained buried due to their needs of classification and just general lack of communication. In the West, the important contributionsofShankoff and Chang² amongst numerousother workers cover key details ofthe process. Much interest has oflatecentred on hybrid techniques in which silver halide layersare converted to pure phase modulated gelatin revisited later on in Figure 4, using subtle process techniques. We should perhaps single out the work of the Spanish group under Fimia³ in this area ofactivity which builds on early developments by Chang and co-workers⁴, Graver et al⁵, Angell⁶ and others. Silver halide gelatin (S.H.G.) essentially combines the high signal to noise possibilities of D.C.G. with the high innate light sensitivity of the silver halide materials. There are however serious penalties to pay when one extracts salts ofsilver from a gelatin layer, the most important being thecollapseofthe

Bragg structure and the need to find some finishing method of 'propping up' the voided material. Scattermaynot indeed beas lowusdesirabledue to the fact thatsilverhalide when removed leaves voids in thegelatin which themselvesact as scattering centres.

Thepurposeofthis paperis toprovide some food forthought in thebackground to theprocess for D.C.G. and S.H.G. We shall look at a number ofkey laboratory observations based upon the study ofthe silver halides that pose important and as yet only partially answered questions. We do not offeran entirely new route to themanufacture ofD.C.G. at this stage bul the way is left clear for a range ofdetailed studies that may ultimately enable us toreplace D.C.G.as such with new chemical formalismsand processes.

What we propose in this paper is that the large values of index modulation observed in D.C.G. are plausibly caused by gelatin hydrolysis in the nodal parts of the image structure. If the collapse of the recording layer can then be reversed or inhibited, void modulation occurs, ranging between the refractive indicesofgelatin pureand air.

Westructure this paperby first spelling out what are seen asa set ofkey observations in the laboratory. These are then collated to provide guidance fora furtherunderstanding ofthe D.C.G.and S.H.G. processes. Whilst there is no disputing the hardening ofgelatin by dichromate, we must add to diis the observation that dichromate bleaches used in theirrehalogenating orsolvent form on the silver imagescan cause layersoftening and disruption ofthe images.

2.KEY LABORATORY OBSERVATIONSTHAT RELATETOTHEIMAGE FORMING MECHANISM OFD.C.G.

(a) Majorsolvent bleach agents forsilver are capable ofdisruption ofgelatin layers.

Here, we observe that the removal ofdeveloped silverby asolvent bleach in order toprovide modulation ofa hologram can cause thedisruption ofthe gelatin layer.

We specify such solvent bleaches as belonging to the class ofconstituents shown below; their ionic Redox potentials accompany them.

Sodium Dichromate

Potassium Dichromate 1.33 Volts-lowpHconditions

Ammonium Dichromate

Pyridinium Dichromate

Potassium Permanganate 1.65 Volts- low pH conditions

The dichromate family and permanganate all exhibitlarge redox potentials which enable theircorrosive ability when attacking silver inagelatin layer. Bycomparison,wehaveanodd manoutoflowpotential:

Cupric Sulphate 0.16 Volts

To thisfamily, we have recentlyadded a newcombination:

Ferric Sulphate(orNitrate) 0.77 Volts

+ Potassium Persulphate 2.00 Volts

This strange combination relies on the intrinsic oxidising power of the Ferric compounds together with the regeneration of the reduced form ofthese compounds (as a resultofattacking silvermetal) back to theiroriginal Iron III form. Potassium Persulphate itselfcannotattack silverstrongly despite its large redox potential becauseofacharge barrierproblem thatprevents close proximity of the Persulphate ion to the silver site. However, Persulphate can be used to regenerate (or oxidise) other compounds in common solution. Bjelkhagen, Phillips, Wang⁷.

We have compounded a novel silversolvent bleach as follows:

FerricNitrate(orSulphate) 30gm

Potassium Persulphate 20gm

Potassium Hydrogen Sulphate 50gm

Distilled Water 1 Litre

This solution will remove silver from a developed layer. The pH is low due to the acidity ofthePotassium (orSodium) Hydrogen Sulphate. Rates ofsilver solution can be controlled much more sensibly than with the usually over-rapid dichromates. To obtaina similarrate ofsilver solution with a typical dichromate and the same acid buffer, we might usejust one quarter ofa gram of the dichromate This is not a happy concentration for a properly buffered solution. In fact, the Ferric compounds can be replaced by dichromates in the above list ifitbedesired to do so. The weights will havetobejudged in terms ofbleach rate ofcourse. Wehavealso examined theroleofthe Kodaketch bleach EB2compoundedas follows:

PanACopperSulphate 120gm

Potassium Bromide 7.5 gm

Citric Acid 150gm

DistilledWater 500 ml

PartB3% HydrogenPeroxide solution

Distilled Water 500 ml

N.B. Theaddition ofPotassium Bromideeffectively enables the Cupric Ion with its lowRedox potential of0.16 volts toattack the silver. WithouttheBromide, the action will notproceed.

Equal parts are mixedjustpriorto use. Theroleofdiis bleach is traditionally toprovideetched bars ofgelatin in large scale (e.g. > a few μm in scale) lithographic images. Typically, such a bleach will destroy layers ofhigh spatial frequency gratings and cause the developedzone topeel offintotheprocessing solution.

It isnotable that the Ferric Nitrate-Persulphate bleach mentioned previously tends to exhibit a similar performance to the EB2 in terms ofgelatin disruption when Peroxide is added, though it can still be disruptive in the absence ofPeroxide.

(b) Acidified dichromatesolution tends to increase the granularscatter ofbleached silver halide layers.

An often used technique to reduce printout in bleached silver-halide layers is to immerse the bleached layer in acidified dichromate solution. Thereduction ofprintout sensitivity is dubious buta notable effectis an observed increase ofgrain in the bleached layer. Such ripening implies a detachment of silver halide from its protective colloid (the gelatin) which then permits a form of Ostwald growth.

(c) If a Lippmann layer recorded in silver halide using a solvent bleach, is dried conventionally, it shrinks - moving its reflectance from the red ofthe HeNe for example, to a green colour. Ifit is dried in hot or graded Propanol, it emerges a red colourafter drying.

This effect is most striking and illustrates the powerofShankoffs method (Smith⁸) by which thebulk ofthe dried layer is recovered in the drying process. Obviously, such voiding as is generated by the removal ofthe silver is manifestly demonstrated when rapid drying in hotpropanol is invoked. A case history isoutlined in Figure 1.

Figure 1 -A possible two cell history ofthe solvent bleaching ofa silver halide layer

S

Figure 1.4-Conditionsafter hot orgradedpropanoldrying ofthesolvent bleachedlayer.

Modulation is then determinedbyair voidsandsilverhalidegrains. Scattercan be high because

the voidsareshapedfrom the originaldevelopedsilverfilaments

(d)When D.C.G. layersare processed,thena gradual build-upofdeposited gelatin isobserved in the processing dishesorvessels.

This observation has been confirmed by one ofus (RR) over many years ofwork in the field ofD.C.G. processing. One has to ask why does this situation arise- is itrelevant to the underlying mechanism in the formation ofthe D.C.G. image?

(e) The bulk index ofprocessed D.C.G. is in general less than in its preprocessed state.

This observation again confirmed by one of us (RR) but is the subject of much argument between various working groups. It seems extremely unlikely that the mean index ofa D.C.G. layer can be in excess of its original unprocessed values. Perhaps we could be more precise and say that the integrated optical path measured orthogonally to a gelatin layer is unlikely to increase as a result ofD.C.G. recording and processing. Our firm beliefis that itsalways decreases.

(f)Ifsolvent bleaching ofsilver halide layers isconducted with ultrasonicagitation then the disruption ofthe gelatin layer is considerably greater than when the solvent action takes placewithoutsuch agitation.

Ultrasonic agitation has been found by us to be an effect way ofclearing away the debris of gelatin hydrolysis which occurs when the bleach EB2 or our synthetic bleach Ferric Nitrate (Sulphate) + Potassium Persulphate is used. In traditional crude etch bleaching, the gelatin surface is wiped over to remove the debris with, however, the risk of damage to the layer. Evidently, the loosening ofgelatin presentwhen etch bleaches are applied can be effected by either thedeliberate addition ofHydrogen Peroxide to thebleach or the exploitation ofsome otheraspectofthechemical formulation toachieve the same end result.

3. A COLLATION OF IDEAS BASED ONTHE OBSERVATIONS OFSECTION 2

In Section 2, we have observed a set of key phenomena which we believe give some clues as to the true fundamentals of the D.C.G.process. Let us consideraplausible train ofevents as depicted in Figure2.

Figure2.3 Conditionsafter 'development'-thehardeningphotograpliicfixer usedincreasesbulkhardnessofthe unexposedzoneandatthesame timeremovesloosenedgelatin. Modulation isamplifiedbyvoidenlargement.

Hardeningfixers contain ChromeAlum hardenerandagelatin solvent-AceticAcid.

Figure2.4-Conditions afterhotorgradediso-propanoldrying in voidsare nowamplified,

emulsion swellsandlayerthicknessreturns

(i) Thedichromatedlayerisexposed to the holographic lightpattern.

(ii) In the antinodal regions, a photo-induced hardening reaction takes place. We shall clearly not argue against this long accepted and well proven idea. We might, however, take issue with the assumption that photo-reduced dichromate is only responsible for the hardening process forreasonswhich will become apparent later in this paper.

(iii) In the nodal regions some softening takesplacetogetherwith a degreeofgelatin hydrolysis which is controlled by 'development' in a hardening fixer. Here 'development' describes a process involving some hardening of the layer and we should remember thatsome hydrolysis maybe encouraged bycertain constituentsofthe fixer.

(iv) Shankoff's rapid or graded drying process allows the regions ofpartially hydrolysed gelatin to expand so as to contain amplifiedairvoids.

In sub-section (iii) we see an essential and simplistic ingredientin which the unusually large levels ofindex modulation are easily explained.

Clearly, thephotohardeningreaction is very importantinprovidingresistance tohydrolysis. The hydrolysis itselfand its origins will be discussed in detail in the nextsection.

4. REASONS FOR THE HYDROLYSIS OF GELATIN

Ifweconsiderthe action ofthe bleachEB2 then wecan see thathydrogen peroxide is added toprovoke hydrolysisofthe gelatin. It does this by acting on the loose ends of gelatin created when the silver removal breaks the protective colloidal structure of gelatin thus liberating free endsofthe protein structures. These looseended structures arc made hydrophylic by theattachment of functional groups which originate from the peroxide in solution. Many othergelatin solventsexist, the most pertinent being Acetic Acidcommonly used in fixerbuffers.

When gelatin is photo-hardened then neighbouring zones will have loose ends available as can be seen from a binary two zone model shown inFigure 3.

Figure 3 A-comparison ofthe twochosen modelsshowing the basic difference between D.C.G.

and thesolventbleached silver halide

Figure3.2-Thefinalresultforasolventbleachsilverhalide layerdriedin hotorgradedIso-propanol.

Thehighestbulkindexcoincideswith thepattern node

On the right hand side ofthe interface then loose ends from strands ofgelatin in region2 cannot be lied up in the hardening reaction present in region 1. Under such conditions, the ends at the boundary ofregion 2 can be caused to have an affinity to water (hydrolysis by the action of H₂O₂). The problem is a little like the Van der Waals forces at the surface of a gas or liquid - an essentialanisotropy is introduced.

We note that the observed granulation ofsilver halide layers mentioned in Section 2(b) is explainable by a degree of hydrolysis caused by an interaction between the gelatin and the bleach solution. It seems likely that any solution containing agents of high enoughredoxpotential iscapableofasimilareffect. Wemightexpectthe hydrolysisofgelatin (astheattachedprotectivecolloidof the silver halide layer) to be triggered offby the presence ofany suitable agent such as H₂O₂ which can create hydrophylic conditionsforloosegelatinstrands.Inthecaseofthesilverhalides,thenecessaryloosening iscreated by thesolvationofsilvergrains.

Wenow interjectacompletelynewchemicalprocess forthephase modulation ofgelatin.

This method was invented in the beliefthatprevious studiesweresomewhatrestrictiveand toan extent flawed since they failedto provide adequate spatial frequency response ofthe recording. The inherent advance ofthis new method lies with avoidanceofthe use of fixer or excessively rapid solvent bleaches which attack the oxidation products of either developer or bleach. Such questionablesolventbleaches includePermanganate and Ammonium dichromate. Theessence ofthe method is tobe extremely gentle with theeffectsoftanning ofthelayer thuspreserving the structureofvoids inthegelatin.

DISCUSSION

Dichromatedgelatin offers a uniquely interesting setofattributes both in its capabilities forhigh index modulation and in the simplicityofprocessing. Possibly, the substitution ofthe dichromate family by otherchemical compounds is inhibited by the need for those other compounds to have a similarly potent photo-hardening reaction. For example, the ferric compounds (Nitrate or Sulphate) being brown certainly have a reasonableability to absorb actinic lightoreven mid-spectrum green. The general wisdom however is that their photo-hardening ability, though significant, is not the sameas thatofthedichromate family.

The reader is referred to the book by Kosar⁹ (page 39) for a discussion ofthe role ofFerric and Ferrous compounds on the cross- linking ofgelatin. Evidently, the Ferric ion can assist gelation ofa gelatin solution though the ion is converted lo Ferrous by the action ofactinic light. Ferrous ion causesgelatin to soften making itprone to hydrolysis.

This situation allows some fascinating processes such as the True-to-Scale Process in which Ferricyanide from a blue print transfers by contact into a moist gel containing Ferrous ion. The oxidation of the Ferrous ion to Ferric by the ferricyanice then insolubilises the gelatin in that region ofcontact. Interestingly, the Ferric bleach mentioned by us in this papercontains a regeneration mechanism for the Ferric ion. As the Ferric ion is reduced by oxidation of local silver then the Potassium Persulphate can regenerate the original Ferric population from the Ferrous population. It is ourbelief that further studies oftherelative proportions ofFerric (Nitrate/Sulphate) and Potassium Persulphate will reveal new mediods of manipulation for the silver halide layers discussed earlier. A word of caution is needed however since the Persulphate may itself promote the hydrolysis of gelatin via its reaction with water.

Thesoftening ofgelatinbyFerrous ion reveals some interesting properties ofthesolvent bleach process discussed in thispaper. As thesilveris removed dius leaving voids in the layer then thegelatin is simultaneously softened by thegeneration ofFerrous ions.

The overall integrity ofthe gelatin thus suffers and indeed, the recorded region ofsilverhalide mayjust peel offthe substrateafter bleaching especially ifultrasonic agitation is invoked.

Now we can reconsider an alternative solvent bleach namely acidified dichromate solution. Does such a silver solvent bleach tan the gelatin layer or does it soften it? Here the evidence is sparse, but the action ofreduced dichromate ion present after oxidation ofthe silver must be questioned. It seems likely diat softening lakes place as evidenced by the removal of the recorded region by acidified dichromate under ultrasonic agitation (i.e. in the case ofa small patch ofrecorded grating say, then this patch will detach from the substrate).

The work related in this paper is ofcourse derived in the main from studies of silver halide layers in which development is followed by removal of silver. To simulate the photo-hardening reaction in the antinodal regions offered by the dichromate process we have used developers that tan the layer in the locality of the developed silver (notably those containing catechol as in CWC2 for example). However, such powerful tanning developers can promote layer distortion and persistent inhomogeneous stain images.

We found thatnon-tanning developers such as Ascorbic Acid leftthe layersprone toattack by hydrolysisand hence riskeddetachment of the gelatin layer during solvent bleaching. Since the tanning developers such as Catcchol create oxidation products that tan, it isassumed that thenodal regions are tanned by diffusion ofthese products from the antinodal region.

The interesting difference ofintegrity oflayers which were ultrasonically agitated during bleaching and diose that were free from agitation provided strong insights into thepresence ofgelatin hydrolysis in the weakening ofthe recorded layer.

Some ofthe reporting ofbulk index changes in D.C.G. layers have a degree ofunreliability. Obviously, the bulk or average index ofa gelatin layercan only decrease bearing in mind the effect ofhydrolysis and could only increase ifserious physical compression was invoked.

The observation 2(c) ofrecovery of layer thickness during hot isopropanol drying is most interesting because it implies a sort of 'memory' built in to the gelatin layer thatprc-disposes itto return, when forced, back to its original thickness before recording. Of course such an observation is not precise and is probably capable ofserious modification ifrequired but its simplistic description provides a working startpoint.

6. CONCLUSIONS

This paper makes a comment on the technology ofD.C.G. and S.H.G. The complex and often tortuous discussions of where the modulation levelscomefrom areeasily focussed ifgelatin hydrolysis is invoked. The set ofobservationspresented here providea talking point for new investigations and offer a degree ofthe 'heresy' suggested by the tide namely that dichromate can both tan (orcross-link) gelatin andat the same time effect its hydrolysis.

An entirely newprocess for the creation ofS.H.G. has been presented. Ata firstlook, it seems tobe freeofthe uncertainty ofprevious mediods though much is still lo bedone to optimiseand even explain theprocedure. It isclear from ourstudies thatbleaches that contain rapid acting dichromate or those that contain Ammoniacal compounds with their concommitent silver halide solution action do not properly relate to this process. Ferric Nitrate and Persulphate form a uniquely controlled and regenerative bleach system apparendy ideal for this new process. Our finding is that fixercan result in uncertain behaviour of such solvent processes and is bestleftalone. Wedid in fact obtain a competent resultby fixing the layerand then solventbleaching out the developed silver. In truth however, the results were notat allcompetitive with the Repeat Method. Itis probablytrueto say that the S.H.G. process relieson the integrityofcomplex oxidation productdeposits in thegelatin-liberated at both the develop and bleach stages. TheFerric-Persulphatebleach went through two clear phases- (i) removal ofsilver leaving a dark brown stain residue and (ii) a gradual clearance of that stain until the layer was notionally colourless. Ammonium dichromate as ableach oxidising agentattacked the stain with extremerapidity and the overall process failed to workefficiently.

From extensive tests of what we might call complementary methods using fixation, we concluded that fixer can damage the integrity ofoxidation productdeposits.

7. ACKNOWLEDGMENTS

Theauthors wish to thank Mrs Kathy Phillips and colleague Peter Marsh fortheirStirling efforts in thepreparation ofthe structure ofthispaper.

8. REFERENCES

1. SHANKOFF,T.A. Appl. Optics 7,2101, 1968

2 CHANG, B.J. Proc S.P.I.E.177, 1979.71-81

3 FIMIA, A. et al Proc. S.P.I.E.1136, 1989

4 CHANG. B.J. & WINICK. K. S.P.I.E.Proc215, 1980

5 GRAVER. W.R.etal. Appl. Optics Vol.19No.9.1980

6 ANGELL, D.K. Proc S.P.I.E.883.106.1988

7 BJELKHAGEN,H.I.; PHILLIPS.N.J.;WANG, C.Proc S.P.I.E. SanJose, 1991

8 SMITH, H.M. Holographic recording materials. SpringerVerlag,Topics on Applied PhysicsVol.20p 75-99, 1977 KOSAR,J.,Lightsensitive systems. Wiley,p3940, 1965

10 FIMIA, A.; BELÉNDEZ. A.; PASCUAL,I.Journal ofModem Optics Vol.38 No.102043-2051, 1991

CHEMICAL FORMULARY

Silversolvent bleaches

SS1. Ferric Nitrate (Sulphate) 30gms SS3. Potassium Dichromate 10 gms

Potassium Persulphate 20gms Potassium Persulphate 20 gms

Potassium or Sodium Hydrogen Sulphate 2 gms

Potassium orSodium Hydrogen Sulphate 50gms Distilled water to 1 Litre

Distilled waterto 1 Litre

SS2. Potassium Dichromate 0.5 gms

Potassium Persulphate 20gms

Potassium orSodium Hydrogen Sulphate 50gms

Distilled waterto 1 Litre

N.B. In these bleachesnovelregeneration ofthe reducedsilveroxidatant is effectedby the Persulphate which itselfcannot effectivelyattacksilverbecause ofadouble layerproblem. Thishelpstheconfinementofthereducedoxidant tositesclose to the silver site that isunderattack. Wespecificallydo notrecommendthe useofAmmoniumdichromate.

3. Eastman Kodak'setchbleach EB2

Thisbleachproducesmajorgelatin hydrolysis.Itisoftenprecededby hardeningfixationorachromealumbathtoavoidmajordisruption.

PartA Cupric Sulphate 120gms

Potassium Bromide 7.5 gms

CitricAcid 150gms

Distilled waterto 500 ml

PartB Hydrogen Peroxide3% 500 ml

Mixequalpartsjust before use.

N.B.BleachSS1 willetchinjustthesamewayasEB2 bytheadditionofHydrogenPeroxide.

Developer for phase modulated gelatin (S.H.G.)

The Eastman-Kodak concentrated developer HC 110 is an ideal simplistic developerfor the Repeat Method. Dilution can be 1 + 30to 1 + 60.Basedon concentratedQuinol, we believe thisdevelopertoendowjust the right leveloftanningaction.

Reactivatorsolution RA

Sodium Sulphite(Anhydrous) 50gms

Sodium Hexametophosphate 1 gm

Distilled waterto 1 Litre

The Repeat Method for the formation ofphase modulated gelatin

Beforeproceeding,read Fimia etal.¹⁰forinstructions on presoftening ofAgfa gelatin.

1. Develop in HC 110 developerdiluted to 1 + 30 to 1 + 60 at20°C.

Typically to density of2.5 forAgfa's 8E56 HD or8E75 HD.

N.B. Eastman Kodak's maximum resolution plate type 1 performs exceptionally wellprobably due to its softer gelatin compared to theAgfaproduct. 2. Wash 5 minutes.

3. Solvent bleach in SS2:

Allow time forall developerstain toclear.

4. Wash 5 minutes.

5. Reactivate in solution RA - 2.minutes. Remember that this contains a silver halide solvent (Sodium Sulphite) so do not allow too much protractionofthis step.

6. Wash 2 minutes.

7. Expose toan intense white light.

8. Develop tocompletion inHC 110.

9. Wash 5 minutes.

10. Solventbleach in SS1. Wait until all slain iscleared. Do notagitate.

11. Wash scrupulously and use hot orgraded Propanol dehydration.

The one step Method for the formation of

phase modulated gelatin.

Before proceeding, read Fimia et al.¹⁰ for instructions

on presoftening of Agfa gelatin.

1. Develop in HC 110 developer diluted to 1 + 30 to 1 + 60 at 20ºC.

Typically to density of 2.5 for Agfa's 8E56 HD or 8E75 HD.

N.B. Eastman Kodak's maximum resolution plate type 1 performs

exceptionally well probably due to its softer gelation compared to the

Agfa product.

2. Wash 5 minutes.

3. Solvent bleach in SS3. Allow time for all developer stain to clear.

4. Wash 5 minutes.

5. Thoroughly fix using sodium thiosuphate or amonium thiocyanate.

Noveldeveloping agentforHolography andLithography This invention describes a novel developingagentbased on the developing activity of

Ascorbic Acid in combination with

p and other specified compounds. The

oxidationproducts ofAscorbic Acid have a strong solvencyeffect on silverhalides and thus provide partial fixing, action of a silver halide layer which is undergoing development. Such effects have a deterrent effect on the dynamic range and achieved density ofholograms developed in AscorbicAcid.

A novel combination of agents provides for unusual developer energy, enhanced dynamic range and elimination ofthe silverhalide solution problem.

Whereas

developing agents applied to for example an Agfa-

ahaert 8E95HD layer will create an optical density of at most 6. We have observed densities approaching 9. Thus illustrating a greatly increased dynamic range. In a subsequent bleached hologram, agreat improvement ofsignal to noise ratio is observed. The second embodiment ofthe process ofindex matchingby light induced effects may set throughout the layers as distinct from localised index matching induced by the evanescent field ofthe reference wave near the interface between recording medium and substrate.

In either method, the effects are to be employed just prior to the recording of the holographic pattern.

DISCLOSURE OF THE INVENTION AND BEST MODE

FOR CARRYING OUT THE INVENTION

COLOR DISPLAYS

The holographic light panel is a two dimensional application of a hologram. The HLP can replace many existing

illuminators and light sources which require bulky and expensive optics with a much thinner, lighter, less

expensive "sheet illumination". The concept would be to combine the HLP and the LCD.

Person Viewing The Display

The HLP would be edge-lit. This would allow the light source for the hologram to be at the base of the hologram or at a location remote from the display. The light would then be routed to the display via fiber optics and distributed by teh hologram for uniform illumination across the LCD.

One advantage of the HLP is that the light can be shaped so that light from the display can be sent out in small solid angles or large solid angles. Color Displays from Mono-LCDs.

A colored (red, blue, green) illuminator would be used. No color filters would be employed. The LCD would be a monocromatic device. The illuminator would provide the color to the LCD display. A brightness advantage over current color LCDs of 10x or more is expected by taking advantage of the efficiency of light transfer via the HLP and by shaping the light to match the specific requirements of each display. Additional brightness is expected because this process will generate color pictures without filtering. This process would allow the use of large remote light sources. There would be no Red, Blue, Grene (RGB) point failures because the color would be coming from the HLP and not from the LCD. Point failures within the LCD would be possible/ but the probability would be reduced by using a monochromatic LCD.

99.99% of all holograms are copies of original holograms. The copy process is simple, exact, and inexpensive. Once a "perfect" HLP has been produced for the mono or color application, large numbers of low-cost copies can be

produced that will have the same properties as the master.

Depixelator:

An additional HLP would be used in front of the LCD display. The depixelator would expand the light from each pixel so that no black mask would be required between the pixels.

This would result in a 10% to 30% increase in brightness of the display as well as incresed image fidility.

C

w A



SUMMARY

This is a review of some of the overall concepts regarding holographic light panels. So far we are making two kinds of"edge-lit" holograms: True Edge Lit (ELH) and Steep Reference Angle (SRA). The third, Waveguide Hologram (WGH) has yet to be explored.

Our light panels will be characterized by whether they are ELH, SRA or WGH, and what their output is. There are several possible outputs that we foresee right now:

Narrow beam (NB)

This is where we want there to be a beam out with a narrow field of view, i.e. a collimated beam (over a distance of several mm to several feet, depending on the application) comes out of the face of the hologram. This is good for sensors (like for fingerprints).

Lambertian beam (LB)

The object was a diffuser, and the light out of the face of the light panel travels in all directions. This is good for transparency illumination.

Pixellated beam (PB)

This is good for LCD displays: flat panel television, computer monitors, etc. Computer monitors, for example would require a narrow beam output;

For visual applications, a pure white orpure monochromatic beam is required. Uniformity will be required in most cases. For detection applications, uniformity and/or chromatism may not be an issue. For example, with a Fingerprint sensor, high transmissivity is important since we want the light to come in the edge, go out the face, illuminate the finger, pass through the hologram to a detector. Uniformity may not be an issue, since that can be compensated for in software. 1. EXPLANATORY POINTS

(a) True Edge Lit (ELH) will be defined as single pass. Steep Reference Angle (SRA) by its very nature is external reconstruction, and therefore single pass. Waveguide Holograms (WGH) are covered by Caulfield, and are multi-pass.

(b) What will come out of the hologram is not Lambertian, and maybe not semi- Lambertian. we will need a separate microdiffuser if color blending, consistency or whiteness is important. We can make a hologram create a diffusepatch in air, but it willpick up chromatic aberration as the light travels. The diffuser will smear the colors to create white or whitish. We may want to call it "widely diffused light" or some other such thing.

We can control the NA of the exiting beam from being narrow to wide.

(c) The sensor (finger or bio or whatever) detector can be considered to be "in the rear" (the opposite side of the light panel from the finger).

(e) Thediagrams formaking all ofthecombinations we suggest are.

(f) The "holy grail" for a pixellated light panel is separate RGB dots. This can be done with a carefully masked multiple exposure process, or three separate carefully aligned, holograms. VERY IMPORTANT NOTE: The true edge lit (ELH) creates POLARIZED output, which would eliminate the need for separate polarizers with the LCD panel (the cases of using them or not should both be covered in the claims). Since the colors, ifthey were in a single hologram, will not travel well over a long range (they'll aberrate, or change with viewing perspective) we will pick, up the light as it begins to disperse and send

it through a microdiffuser before it hits the window in the LCD. We can also make crude white dots, instead of RGB, but they will also abenate and would probably need to be augmented with a diffuser.

2. One key point is that the monochromatic true edgelit hologram

isa TRANSMISSION NOTCH FILTER. Holographic notch filters, like the one in patent, are REFLECTION holograms. We are not aware of anyone who has ever maue a TRANSMISSION HOLOGRAPHIC NOTCH FILTER. As with the reflection notch filters, though, the thicker the hologram, the more wavelength selective it will get; the thinner the hologram, the more broadband the output.

3. With the true edge lit, the reference beam appears to be generated WITHIN THE SUBSTRATE, therefore the effects ofSnell's Law are quite different than with the SRA, where the reference beam is extenal from air, as with standard holograms.

4. With the ELH, we have used BK10 glass to make it work during creation of the hologram. The BK10, or any equivalent substrate whose index is tightly matched to that of the monomer, or slightly higher (when using DuPont photopolymer) is essential to making high quality ELH's. Even with the SRA, using BK10 helps to minimize microfringing, which causes unpleasant cosmetic problems. After the hologram is formed, it is delaminated from the BK10 substrate, and then relaminated onto Acrylic or some otherless expensive substrate, which works acceptably for replay of the hologram.

5. Just a note for future reference: another method that may be considered for the multicolor or 3 dot process is to use rainbow holograms...simultaneous rainbow masters whose images are overlapped properly.

Wethink, however, that this would severely limit the usable viewing angle in the vertical direction.

6. One advantage of using the light panel is that it can direct the light beam from the hologram into the clear apertures of the LCD without the light going to the TFT area, thus utilizing the.light more efficiently. You make the master H1 which replays an array of spots, use the aerial image of the dots a little remote from the H2 (not exactly in the plane of the H2) so that the light can converge into the LCD's windows. (H1 and H2 refer to first and second generation holograms in the recording process.)

7. Let us define an SRA to have a reconstruction beam greater than 80 degrees to the normal to the plane of the hologram.

8. Note that the ctøόmatic aberration is minimal the closer the image from H1 is to the planeofH2, and increases as the image from H1 is displaced from the plane ofH2.

9. The use of the large lens, as depicted in the diagrams requires very careful attention to the conjugate relationship of the feeding beam to that lens. You need a very circular patch of light coming from a carefully collimated system (or whatever

conjugate the lens was designed for). Otherwise the hologram will suffer from spherical and chromatic aberrations. Note that Benton's edge lit patent makes use'of such a large lens.

10. To change the uniformity of the output light across the light panel, we can manipulate the recording process with an amplitude masking filter

or (for a pixellated beam) make the apertures change in size from one end ofthe hologram to the other. This latter solution may not be practical for LCD applications, but may be ok for other types of applications.

11. The light out of the ELH is strongly polarized. This is an important claim.

12. If the angle between the final hologram and the hologram used to illuminate it increases, the output light becomes more and more monochromatic yielding a transmission notch filter. When the angle gets steep enough, there exists cross coupling between the holograms to allow one hologram to be thereconstruction beam for an ELH. (The reference beam is generated by H2 which is reconstructed by an external beam (we show a point source).

13. The use of the acrylic as shown which dumps the back reflection ofthe hologram is also novel. The hologram is replayed by a beam diverging in the acrylic. The advantage is that it increases the solid angle of the light coming from the source, and removes the back reflected light.

14. For an SRA ideally we want to make it with a convergent reference beam, so we can replay it with a diverging reconstruction beam. Collimated reference and reconstruction beam will also work, but practically, particularly for larger sizes, a point source reconstruction is better. (Otherwise the collimating optics get large and expensive and voluminous.) SOME FURTHER NOTES:

1. We have invented not only LCD illumination, but other light panel uses, outlined above (fingers, biosensors, transparency illumination, etc.)

2. Note one of the key features of the invention (for the transmission notch filter) is the use of one hologram to get the light into a second hologram which is true edge-lit.

3. Note that another key feature of the invention is that we are making full color LCD displays by using a monochrome LCD panel - eliminating the need for the dyes in the LCD part. We are making multicolor spots from an incoming white beam. These can be done by lamination or ultimately putting all the spots in one hologram. Registration is important.

4. We can also make a white output (with or without spots) or a monochrome output (with or without spots). It is a matter of using the mask or not.

5. The dots can be any size we want. We can produce 5-8 micron dots now. We will soon have 2 micron capability. LCD features are typically in the 80 to 100 micron size.

6. We believe that it is only necessary to depixellate the image if one is magnifying (such as in a virtual reality helmet) or projecting. SOME INDUSTRIAL APPLICATIONS

C.1 EDGE-LIT HOLOGRAMS

In order to understand how edge-lit holograms can be used to improve Head Mounted Displays, one must first understand what an edge-lit hologram is, and what its advantages are.

(C.1.1) Background

Although invented in the late 1940's by Dennis Gabor (who later received a Nobel Prize for his invention), holography has been slow to gain widespread public acceptance. The invention of the laser in the early 1960's touched off an explosion of holographic research throughout the decade. Display holography was made possible by the invention of the laser-viewable, off-axis transmission hologram by Leith and Upatnieks, and thedevelopment simultaneously of the white light viewable reflection hologram by Denisyuk in the Soviet Union. Many companies spent millions of dollars researching holography in the 1960's, but significant improvements did not come fast enough to enable widespread public marketing.

A hologram is made by interfering coherent light from an object, the object beam with light derived from the same light source, the reference beam within a recording medium such as a photographic emulsion. Two basic types of hologram existed, the reflection hologram, where the viewer is on the same side of the hologram as the light source, and the transmission hologram where the light source is on the opposite side of the hologram as the viewer. ImEdge Technology, Inc. is producing a third kind of hologram, which is a hybrid between a reflection and a transmission hologram, the edge-lit hologram.

Reflection holograms are white light viewable but are often dark and difficult to see. Transmission holograms typically need a laser to view them, or have reduced vertical parallax, which creates the rainbow which most people are familiar with from holograms such as are found on their credit cards.

Progress in holography over the years has been reasonably steady, with various incremental improvements occurring in optical components, special photosensitive emulsions and their processing chemistries, vibration stability and a growing mastery of techniques by practitioners. More widespread attention to this field has been given more recently, due in no small part to a cover story on holography in National Geographic written by ImEdge's John Caulfield and read by 25 million people.

ITI has researched many of the problems typically associated with holography, and has developed techniques to improve the brightness, eliminate the grainy appearance, provide better contrast, and control the angle of view. Edge-lit holograms offer significant advantages over standard holograms. Several of these are:

• The incoming or outgoing beam is not obstructed by a viewer, or optics as is the case with some standard holograms (e.g. reflection holograms).

• Improved contrast is achieved since edge-lit holograms are not affected by room light or other outside sources.

• Edge-lit holograms allow displays to be made in a much more compact package than standard holograms.

• Laser light can be used for public or commercial displays safely. It cannot with standard holograms.

C.1.2 Holographic Light Panels: Brief Overview

Holographic optical elements (HOEs) are gaining more widespread use. One type ofHOE takes light in, operates on that light, and sends light out. Thus we can conceive of a hologram which takes light in (the reconstruction beam, which is the conjugate of the reference beam) and, rather than diffracting light out in a pattern that we perceive as an image, diffracting light out in a prearranged pattern ofjust light, which is used to perform an illumination function. If we make the hologram an edge-lit hologram, then we have a new, compact, surface emitting light 'source'. This light source can be highly efficient, polarized, monochromatic, multicolored, or white. It can be uniform or patterned, for example, in a way to generate pixels. Therefore it is an excellent way to make a backlight for an LCD panel.

C.2 FLAT DISPLAYS. LCDs

This proposal describes a fundamentally new concept for providing backlighting for fiat displays of both the passive and active matrix LCD type. By using an edge-lit hologram as an integral part of the backlighting system, we will show that the light efficiency for LCD panels can be increased dramatically.

For our purposes, we would like to define the flat panel display in terms of the following modules:

(1) a light source,

(2) a coupler of the light to the display,

(3) an active pattern-forming element, and

(4) output beam conditioners.

In some systems, many ofthose items are combined or eliminated. An electroluminescent panel, for instance, combines items 1,2, and 3 and omits item 4. The LCD panel enjoys the distinction of being the best pattern-forming element by combining reliability, and maturity ofdevelopment with the most satisfying appearance and acceptable cost. Many companies (some of them American!) have wonderful LCD technology. Accordingly, it is our intent to assume that item 3 will be an LCD and to optimize the supporting elements (items 1,2 and 4) for DOD and commercial purposes using our proprietary technologies. Note, however, that should a better pattern forming element be devised which requires backlighting, our technology is adaptable.

The ideal combination of items 1 and 2 would be an ordinary, commercially-available, white light source feeding a passive, flat panel which would emit all of the light (no color filters which absorb significant amounts of light) in RGB pixels matched to the LCD.

The ideal version of item 4 for head mounted and projection displays would be a depixellating diffuser to produce a smooth image of controlled angular extent. We will show below that we can accomplish these goals.

C.2.1 LCD Backlighting

All high density gray scale LCD's and Active Matrix Color LCD's require a backlight.⁸ Interest in improving backlight performance is increasing rapidly driven by the large world-wide investment in high performance LCD display screens. At the simplest level, all backlights consist of one or more light sources and an optical system.

Todays technology offers four solutions to light sources for backlights: electroluminescent panels, light boxes, edge lights and flat lamps.

Electroluminescent panels are not very power efficient, and suffer from short operation life.

Light boxes use tubular fluorescent lamps, a holder, reflectors and diffusers. There may be several singular lamps or serpentine lamps. Power efficiency is good but it takes a relatively thick box to achieve uniformity. This is totally unsuitable for head mounted displays.

Edge Lights take the light from a fluorescent tube and couple it into the edge of a specially designed plastic (or glass) plate. The plate is optically designed so that light is directed out the front surface. It is possible to have a thin backlight profile but the intensity is limited since the light is generated in one fluorescent lamp and spread out over a relatively large area through these special plates which are not the most efficient means. These are not easily pixellated into ultrasmall spots.

Flat lamps are being developed by a number of companies.

The associated optics for LCD backlighting inherently incur large light losses. These typically include a pair of polarizers and a compensation filter to convert the natural blue of the LCD to a more eye-pleasing black. Combined, these can reduce light throughput by as much as 60%. In the case of color LCDs, the color filter layer absorbs typically 70 - 80% of the light entering it. In addition, active matrix LCDs (AMLCDs) have a series of windows, surrounded by interstices where the control transistors reside. The ratio ofwindow area to the entire panel area varies with different designs and manufacturers. Typical throughput loss from light that strikes the interstices rather than the windows can be around 50% or more. Overall, of light that is available to go through the LCD panel, only about 15% actually makes it through.

This severe light loss means that brighter lamps are required, which use more electricity, and therefore cost more money, and, in the case of battery operated systems, significantly reduces battery life. For consumer applications, the power consumption of the light source is also strongly restricted, with respect to weight, volume, cost of the power supply unit for the light source and audible noise from the fan for cooling both TFT-LCD's and the light source.³ For color active matrix LCD's the color filter array currently represents the largest single component expense. The backlight subsystem is an additional expense for all current LCD designs. Backlighting technology is in a constant state of improvement. New designs are focusing on using a single tube placed along the edge rather than multiple tubes located behind the panel. Tubes are only part of the backlight cost. Inverters, reflectors, light pipes and diffusers also contribute.¹⁰ Today the predominant light source used for backlighting is a fluorescent tube⁴.

The ideal backlight for LCD displays would be a thin film that efficiently converts low voltage direct current into white light with a uniform surface brightness up to 3000 cd/m². There is no such source. Electroluminescent panels are the closest physical approximation to this ideal, but they have relatively low brightness. Flat fluorescent lamps have promise, but have not yet achieved the desired combination of low cost, lightness, reliability and thinness necessary for them to see widespread use. Therefore most of today's high brightness backlighting applications use light sources that are not planar, but which produce sufficient light with reasonable efficiency, e.g. cold cathode tubular fluorescent lamp, and the slightly more efficient hot cathode tubular fluorescent lamp.⁵

C.3 ITI's Technology

Clearly a need exists for better, more efficient backlighting technology. ITI has spent the past two and a half years developing trade secret and proprietary techniques for producing high quality edge-lit holograms. A detailed background and explanation ofedge-lit holograms can be found in Sections III-C and III-F. Briefly, holograms in the past have typically been illuminated by an external source with special characteristics (e.g. collimated light or a distant point source, or a laser). ITI's technology allows light to enter a substrate from the edge. Light travels through the substrate is redirected by a hologram laminated to the substrate. This hologram forces the light to exit the substrate in a controlled direction, with structured characteristics which are determined by how the hologram was made. We call such a holographic system a 'holographic light panel' (HLP). Therefore, the most basic HLP consists of light travelling through a substrate, and then being emitted by the hologram to illuminate an object such as an LCD (active or passive) panel. ITI has developed a patent pending technique to use a white light illumination source and have the hologram emit light within a narrow wavelength band. This is a white-to-monochromatic system. It is also possible to create a white-to-white system, that is, white light illuminating source, and white light emitted by the HLP, by using a two hologram system. Here the light source illuminates a hologram on the face of the substrate which couples the light into the substrate. The light travels through the substrate and gets emitted by the output hologram. The output holograms can be made to couple the light out with reasonable uniformity (see Section III-D) and significant efficiency (theoretically approaching 100% at a given wavelength).

For AMLCDs, the next step to improving efficiency would be for the output hologram to emit light only where the AMLCD windows are and not in the location of the interstices. ITI has already demonstrated a prototype of such a device. In this prototype, light input to the edge of a plastic substrate strikes the output hologram, and is emitted in a checkerboard pattern. ITI has one prototype consisting ofapproximately 16001.4mm square monochromatic pixels, where light from each pixel focuses approximately 3 millimeters into space above the plane of the hologram. The machine used to make the mask for this hologram has the capability ofproducing pixels in any pattern (lines, triads, checkerboard, etc.) down to 0.2 micrometers per pixel. It should be emphasized that considerable efficiency is gained by only sending through the windows, and not at the interstices between the windows. In addition, by only sending light through the windows of a TFT-LCD, the temperature rise associated with such systems is diminished because the illumination energy normally absorbed by the interstices is eliminated.

Going one step further, ITI has already begun experiments, but requires funding assistance, toward making the pixel array noted above in RGB colors. Therefore, the most expensive and light inefficient component of and LCD display, the color filter layer can be eliminated! This reduces costs and enhances efficiency.

Another feature that has been discovered regarding ITI's edge-lit holograms, is that they maintain at least some degree of polarization. If the polarization ratio can be sufficiently controlled, this would eliminate one of the polarizers commonly used in LCDs, allowing a further light throughput gain.

Taking the ultimate step, ITI has also begun experiments on a technique to modulate the pixels in the hologram. If this concept is successful, it could lead to the elimination of the LCD itself!

ITI's holograms are madein photopolymer. Once the master mated and registered to a particular LCD model is perfected, replication of the holograms should ultimately prove to be very inexpensive, certainly compared with current AMLCD manufacturing costs.

These advantages over current backlighting technology offer the potential for significant efficiency improvement. C.4 DEPIXELLATION

One of the biggest problems associated with head mounted displays, orprojection systems is that the pixels in these systems are optically magnified, causing annoying 'pixellation' in the displayed images. Professor Phillips has invented and can produce a photopolymeric microlens structure which is registered with the pixels (or stripes, where needed) for a given LCD system, which effectively merges the red, blue and green subpixel images into white, yielding the most effective depixellating system that we have seen. We propose to marry our hologram technology with this depixellation technology in order to achieve the highly effective, efficient and clear display imagery necessary for successful head mounted displays.

Detailed Technical Approach

ITI has already developed successful techniques for making edge-lit holograms. Several types we have achieved include:

• true edge lit, waveguide hologram

• true edge-lit, no bounce hologram

• 2 hologram, face-lit, waveguide hologram

• 2 hologram, face-lit, no bounce hologram

• steep reference angle hologram.

Since the quality of image bearing holograms is quite subjective, no formal measurements have been made of the samples we have produced. However, Holographic Light Panel quality must be objective, and correspond to strict, measurable specifications. Our technical plan, then, involves utilizing and enhancing the techniques we have already developed to make light panel samples, carefully measuring their characteristics, as noted in Section III-A2, and then optimizing our processes to achieve results as close to the desired specifications as possible. While we have already made impressive proofofconcept samples, considerable work still needs to be done to achieve holograms optimized for LCD applications.

Some of the history and background behind our present technical status is described in Section III-F. We will describe here some of the characteristics necessary for a successful backlighting system, as noted in Section III-A2, and how we propose to approach achieving these goals.

UNIFORMITY

It is important for each area of the backlight HLP, or each pixel with a pixellated HLP to emit the same amount of light as any other area or pixel, within a given tolerance, e.g.10% or 20% differential, depending on the application. Fortunately, the eye is very forgiving of such luminance differences, which would tend to make looser tolerances more acceptable than with some other features.

In order to achieve uniformity in our hologram outputs, we propose the following concept. In particular, as light travels through a substrate, or a waveguide, or a hologram at an oblique angle, it gets absorbed along the way, according to Beers Law, so that the part nearest to the light source sees the most light, and the part furthest away sees a significantly depleted amount of light. This can be a factor in both the recording and the playback of the hologram. The problem is what we call illumination depletion. It is possible in principle to precompensate for this problem during the recording. The primary goal of this component of the technical plan is to perfect that precompensation method. Four distinct tasks can be identified as necessary to the accomplishment of this component of the plan.

Task1

Task 1 is the quantification of the output pattern of the hologram. We will record a standard hologram, illuminate it in a standard way, and plot the output intensity with specially designed equipment. The hologram we will record will have a plane wave as an object. This means that the field of view for the reading equipment will not be a problem. Readout will be via an optical fiber connected to a detector. The optical fiber will be translated under computer control in x and y to plot out the output intensity pattern.

Task 2

We must record a precompensation mask. Remembering the previous discussion, we understand that we must record a hologram with a uniform plane wave. The reconstructed pattern will then be used to expose a photographic mask. There are many ways we can vary the linearity and base density of that recorded pattern.

Task 3

Task 3 is to record the plane wave through various precompensating masks. Obtaining the proper exposure will be a significant concern.

Task 4

Task 4 will be to quantify the 2-dimensionaI output pattern derived in this manner. In principle, this pattern will be quite uniform. In practice it will inevitably require considerable experimentation to perfect this technology.

This cycle of precompensation, recording, and quantification will be repeated as needed until we have perfected the ability to record holograms ofplane waves in this manner. It remains then to go through the cycle several more times recording various objects, to convince ourselves that the precompensation mask made for plane waves is adequate for recording extended objects. We have no reason to doubt this, because the subtle errors it will introduce are likely to be totally lost to the human eye. That is, the eye is very tolerant of small gradual variations of illuminations.

DIFFRACTION EFFICIENCY / LUMINOSITY

At a given wavelength, it is well known that holograms have the capability of achieving nearly 100%. efficiency. That is, nearly 100% of the incoming light can get channeled into one of the first diffraction orders. Achieving high diffraction efficiencies in holography is dependent on many factors, such as the material used, the exposure density, the processing chemistry, the vibrational stability of the system, and the index of refraction modulation achieved in the material. We have the advantage of having Prof. Phillips on our team, who is one of the most skilled and knowledgeable holographers in the world. We have successfully achieved very bright edge-lit hologram prototypes, but their actual efficiency and luminosity have not yet been quantified. This quantification is an important part ofthe effort proposed here, and is commonly done by comparing the light emitted by the hologram, collected by a radiometer, with the light entering the hologram. One great advantage that we have is that even if our holograms only achieve modest efficiency, we believe that the comparative gain over current systems, whose efficiency is very poor (see Section III-C), will be significant. PIXEL RESOLUTION

In order to produce pixellated HLPs, we have developed special techniques for making masks which arc used as the object for our holograms. Larger masks are computer generated, smaller feature masks are made by standard high resolution printing techniques, or electron beam lithography. This is a straightforward process. Masks can easily be made to match any known LCD pixel array pattern, down to a feature size of around 0.2 microns, which is much smaller than any feature required for current high resolution LCD technology.

VIEWING ANGLE

Holograms inherently have a limited angle of view. The deeper the image, or the smaller the construction aperture, the smaller the region where the image is viewable. In the case of HLPs, the image is either the extent of the exiting light field, or the image of the pixel mask. Images in the plane of the hologram exhibit the widest viewing angle. Restricting the angle of view is quite straightforward, using known techniques for apcrturing the object beam used in the construction of the hologram. In the case ofHead (or Helmet) Mounted Displays, typically only a small viewing angle is required. The smaller the viewing angle, the more efficient the display, since more light is channeled to a smaller solid angle. One of the ways that we will be able to control the viewing angle, making it smaller or larger, according to desired specifications is by the special photopolymeric gradient index microlens diffusers that Prof. Phillips has developed and patented. By varying the size and thickness of the microlenses and the gradient index variation, we can achieve a wide range of viewing angles, which can be tailored, for example, to maximize in the horizontal plane and minimize in the vertical, or achieve peak transmission in any desired direction.

RGB PIXELS

Techniques for making full color holograms have been known for many years. Recently, both the Russians and the Japanese have made some excellent image-bearing full color holograms. However, since edge-lit holograms require light to travel along a much longer path length in the holographic medium than standard holograms, we need to develop new techniques or special materials in order to achieve full color. DuPont has a polychromatic photopolymer, which has the capability ofachieving good full color standard holograms, and the capability to "tune" the colors by allowing post-processing monomer migration. In early experiments with this material we have not met with great success for edge-lit holography. However we are working closely with DuPont, and they are developing special photopolymer formulations for us to our specifications, specifically for use with edge-lit holography. The materials issue is really the key to achieving success at making an RGB pixellated HLP. There are several techniques that we will explore, including working with the new DuPont materials, and creating special masks to allow us to expose red, a-blue and a green mask image in the same hologram. We will devise special techniques to allow proper registration of each mask. If we are unsuccessful with this technique, there are several others we will try, including differential swelling and shrinking of localized areas of the hologram, which will create variations in interference fringe spacings, which translates to different color outputs in different areas of the hologram. Another technique would be to laminate three holograms together in registration. COUPLING TECHNOLOGY

The least understood aspect of this work is the coupling of light into the waveguide light panel. Theoretical analysis by Prof. Caulfield shows that the only way to avoid lateral chromatic dispersion altogether is

• use a grating (hologram) input coupler, and

• arrange the illumination beam, the input hologram, and the output hologram so that light enters and leaves the hologram in the same direction (probably normal to the waveguide).

Taking this as our goal, the unanswered questions include these:

• In what ways do point and line sources differ in their net coupling efficiency?

• In what ways do the angular divergences in the two directions (in-plane and out- of-plane) impact efficiency?

• How are all of these parameters affected by waveguide geometry and mode structure?

• How does the hologram design impact efficiency? How close to 100% of the input light can we get out of the hologram?

• What kind of optics should we use? What beam conditioning elements?

This part of the total program will be performed by and at Alabama A&M University (AAMU). The first step will be a complete modelling of the illuminator system. This can and will involve computer ray tracing, but it must also explicitly include the mode structure of the waveguide.

A recent PhD project under Prof. Caulfield (the AAMU PI) provided for the first time, the analytical tools to handle thick waveguide holograms in a rigorous manner. We will apply this methodology (see appendix) to the illuminator hologram. Modelling the source, the beam conditioning, the source optics,a nd the coupler will be complicated by the fact that there are so many choices.

For instance, we have

• filament systems (line, coiled, etc.)

• arc lamps

• LED's (singles, arrays)

• laser (diode, gas, solid)

• etc.

Within limits, each can be shaped by

• lenses (refractive, reflective, diffractive, holographic, etc.)

• stops, slits, pinholes, etc. Thus besides building up the models, we must develop some preferences and rules of thumb to help us decide which systems should be modelled.

Once wecan model a total system, we can at least partially optimize it. Both simulated annealing and genetic algorithms will be so slow as to be totally impractical, but steepest ascent can be used to find the local optima of the systems initially modelled. Thereafter we can downselect to a few of the locally optimized systems.

Through continuing interaction with the other team members, we should have a good system ready to test soon after the optimization is complete.

The next AAMU task is to design and assemble tests to verify system performance. As all metrology is circular, these test must trace back to primary or secondary standards. That is, they must be quantitatively and absolutely calibrated.

Next, in collaboration with its partners, AAMU will assemble multiple couplers of each type on multiple holograms. These will then be tested, and the results will be compared with theory. Inevitably, this requires some adjustment of both test and theory to accomplish full reconciliation.

Finally AAMU will use all of its experience and analysis to that point to "optimize" the couplers for any particular applications ARPA and other team members suggest.

BIBLIOGRAPHY

1. K. Nassenstein, "Interference, Diffraction and Holography with Surface Waves ("Subwaves"). I., Optik 29 (1969), 597

2. H. Nassenstein, "Interference, Diffraction and Holography with Surface Wave ("Subwaves"). II., Optik 30 (1969), 44

3. O. Bryngdahl, "Evanescent Waves in Optical. Imaging", Progress in Optics, XI (1973), 167

4. K. Stetson, "An Analysis of the Properties of Total Internal Reflection Holograms", Optik 29 (1969), 520

5. L. Lin, "Edge Illuminated Hologram", J.Opt. Soc. Amer. 60 (1970), 714A

6. T. Suhara, H. Nishihara, J. Koyama, "Waveguide Holograms: A New Approach, tb Hologram Integration", Opt. Comm. 19 (3) (1976), 353

7. M. Miler, V.N. Morozov, A.N. Putilin, "Diffraction Components for Integrated Optics", Sov. J. Quant. Electron. 19 (3) (1989), 276

8. J. Upatnieks, "Compact Holographic Sight", SPIE Proceedings, Vol. 883 (1988), 171

9. J. Upatnieks, "Method and Apparatus for Recording and Displaying Edge-Illuminated Holograms", U. S. Patent 4,643,515

10. G. Moss, "Holographic Display Panel for a Vehicle Windshield", U.S. Patent 4,790,613

11. G. Moss, "Holographic Indicator for Determining Vehicle Perimeter", U.S. Patent 4,737,001

12. S. Benton, S. Birner, A. Shirakura, "Edge-Lit Rainbow Holograms", SPIE Proceedings, Vol. 1212 (1990), 149

13. W. Farmer, S. Benton, M. Klug, "The Application of the Edge- Lit Format to Holographic Stereograms", SPIE Proceedings, Vol. 1461 (1991)

14. Q. Huang, H. J. Caulfield, "Waveguide Holography and its Applications", SPIE Proceedings, Vol. 1461 (1991)

15. A.N. Putilin, V.N. Morozov, Q. Huang, H.J. Caulfield, "Waveguide Holograms with White Light Illumination", Opt. Eng., 30(10), 1615

16. Q. Huang, H. J. Caulfield, "Edge-Lit Reflection Holograms", SPIE Vol. 1600 (1991), 182

17. N. J. Phillips, C. Wang, T.E. Yeo, "Edge Illuminated Holograms, Evanescent Waves and Related Optical Phenomena", SPIE Vol. 1600 (1991), 18

18. J. Upatnieks, "Edge-illuminated holograms", Appl. Opt.

31(8), 1048

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above construction, articles, and methods without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

Claims

1. A metal halide sensitized medium recording a phase modulated image wherein voids in the medium modulate the image and the medium is substantially free of the metal of the metal halide and the metal halide itself.

2. The medium defined in claim 1 wherein said voids are greater in volume at the antinodes in the recorded image than at the nodes thereof.

3. The method of recording a phase modulated image in a metal halide sensitized recording medium comprising the steps of:

A. exposing said medium to an image;

B. developing said medium by converting exposed metal halide grains to metal grains; and,

C. bleaching and hardening said medium to remove said metal grains and form voids therein.

4. The method defined in claim 3; and the additional steps of:

D. uniformly exposing said film to activate substantially all of the remaining metal halide grains therein;

E. developing said film to convert substantially all the remaining metal halide grains therein to metal grains; and,

F. bleaching said medium to remove said metal grains and form voids therein.

5. The method defined in claim 4 wherein said medium is gelatin and said hardening is accompanied by release of chromium ion at said metal grains.

6. The method defined in claim 3 wherein said medium is gelatin and said hardening is accompanied by release of chromium ion at said metal grains.

7. The method defined in claim 6 wherein said bleaching step is accompanied by use of a Dichromate.

8. The method defined in claim 7 wherein said Dichromate is Potassium Dichromate.

9. The method defined in claim 8 wherein said bleaching step is accomplished using a Persulphate.

10. The mothod defined in claim 4 and the additional step of:

G. amplifying said voids by applying a Propanol to said medium.

11. The method defined in claim 3 and the additional step of:

D. fixing said medium.

12. The method defined in claim 11 wherein said medium is gelatin and said hardening is accompanied by release of chromium ion at said metal grains.

13. The method defined in claim 12 wherein said bleaching step is accompanied by use of Dichromate.

14. The method defined in claim 12 wherein said bleaching step is accomplished using a Persulphate.

15. The method defined in claim 11 and the additional step of:

G. amplifying said voids by applying a Propanol to said medium.

16. A bleaching and hardening solution for forming a phase modulated image in a metal halide recording medium comprising:

A. a dichromate; and

B. a persulphate.

17. The solution defined in claim 16 further comprising:

C. a hydrogen sulphate.

18. The solution defined in claim 17 wherein the proportion of said hydrogen sulphate in said solution is much less than said dichromate.

19. The solution defined in claim 16 wherein said dichromate is potassium dichromate.

20. The solution defined in claim 19 wherein said persulphate is potassium persulphate.

21. The solution defined in claim 16 wherein said persulphate is potassium persulphate.

22. A hardening bleach for a gelatin phase modulated image recording medium comprising a dichromate.

23. A regenerative bleach for a metal halide phase modulated image recording medium comprising a persulphate.

24. A bleaching and hardening solution for forming a phase modulated image in a metal halide recording medium comprising, in proportion:

X-Dichromate, about 10 grams;

X-Persulphate, about 20 grams;

X-Hydrogen sulphate, about 2 grams; and pure water to 1 Litre; where X is an alkali metal.