WO2000049431A1

WO2000049431A1 - Optical filter system

Info

Publication number: WO2000049431A1
Application number: PCT/US2000/003931
Authority: WO
Inventors: Milan M. Popovich
Original assignee: Digilens Inc.
Priority date: 1999-02-18
Filing date: 2000-02-16
Publication date: 2000-08-24
Also published as: AU2882100A

Abstract

An optical filter system (10) that includes a holographic device (12) having a plurality of holographic optical elements (30, 32, 34) switchable between an active state wherein light from a light source (14) is diffracted by the device and a passive state wherein the light is not diffracted by the device. Each of the holographic optical elements is operable to form a light beam having a wavelength band different from the light beams formed by the other optical elements. A color filter (18) is positioned to receive the light beams from the holographic device and operable to reduce a wavelength band profile of the light beams. The system further includes an optical device (26) operable to collimate the light beams emitted from the color filter.

Description

OPTICAL FILTER SYSTEM

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application Serial No. 60/120,119, filed February 18, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to an optical filter system, and more particularly, to an optical filter system employing electrically switchable holograms.

Image display systems typically include a display screen configured to display monochrome images. When a multi-color display is required, a sequence of images are displayed and illuminated sequentially with red, green, and blue lights. The switching from one image to the next is performed rapidly (e.g., at a rate faster than the response time of a human eye) so that a color image is created in the viewer's eye due to the integration of red, green, and blue monochrome images. This allows a viewer to see a full color image generated from a display system having a display screen operable to produce only monochrome images. The display system typically includes a white light source and a rotating color wheel having red, green, and blue filters. However, these rotating filters are often susceptible to mechanical failures and tend to be large and noisy.

The risk of mechanical failures, noise and size may be reduced by using solid state techniques such as liquid crystal polarization switches. However, these switches work only with polarized light, thus, half of the light produced by the light source is never projected from the display screen, resulting in low illumination. Furthermore, the liquid crystal polarization switches and mechanical rotating wheels often do not provide sufficiently fast switching speeds between the different colors. If the switching speed is too slow, the image projected from the display screen will not appear as a multicolor image.

There is, therefore, a need for a reliable, efficient, and fast switching optical filter system operable to generate red, green, and blue light from a white light source.

SUMMARY OF THE INVENTION

An optical filter system operable to receive polychromatic light and modify the light to form a plurality of output beams, each having a respective wavelength band profile is disclosed.

An optical filter system of the present invention generally comprises a holographic device comprising a plurality of holographic optical elements switchable between an active state wherein light from a light source is diffracted by the device and a passive state wherein the light is not diffracted by the device. Each of the holographic optical elements is operable to form a light beam having a wavelength band different from the light beams formed by the other optical elements. A color filter is positioned to receive the light beams from the holographic device and operable to reduce a wavelength band profile of the light beams. The system may further include an optical device operable to collimate the light beams emitted from the color filter.

The plurality of holographic optical elements preferably comprises three holographic optical elements, each element having a hologram optimized to diffract red, green, or blue light. The optical device may also include switchable holographic optical elements. The filter system may further include a lens positioned between the light source and the holographic diffraction device and operable to collimate light from the light source.

A display system of the present invention generally comprises a light source and a holographic device operable to receive light from the light source and produce a plurality of light beams each having a different wavelength band. A color filter is positioned to receive light beams from the holographic device and operable to reduce wavelength band profiles of the light beams. The system may further include an optical device operable to collimate the light beams emitted from the color filter and a display device positioned for illumination by light emitted from the optical device. The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is schematic of one embodiment of a filter system of the present invention.

Fig. 2 is a perspective view of a holographic optical element and light source for use with the filter system of Fig. 1.

Fig. 3 is a partial front view of the holographic optical element of Fig. 2 illustrating an electrode and electric circuit of the holographic optical element.

Fig. 4 is a schematic of a holographic device having three holographic optical elements each optimized to diffract red, green, or blue light.

Fig. 5 is a diagram illustrating spectral output characteristics of the filter system shown in Fig. 1.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable one of ordinary skill in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples and various modifications will be readily apparent to those skilled in the art. The general principles described herein may be applied to other embodiments and applications without departing from the scope of the invention. Thus, the present invention is not to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

Referring now to the drawings, and first to Fig. 1, an optical filter system, generally indicated at 10, is shown. The optical filter system 10 may be used in color sequential image generating systems to illuminate a monochrome display screen 28 sequentially with red, green, and blue, light, for example. By switching between the resultant red, green, and blue images produced by the display screen in a very rapid manner, a viewer will perceive what is effectively a full color image.

The system includes a holographic diffraction device, generally indicated at 12, configured to receive incoherent white light 15 from a light source 14. The white light includes red, green, and blue bandwidth light components. The diffraction device 12 produces three separate output beams 20, 22, 24 having wavelength bands in the red, green, and blue regions, respectively. The holographic device 12 is preferably configured to function as a cylindrical lens to focus the beams 20, 22, 24 at specific locations which are laterally from one another on a color filter 18, which is positioned to receive light transmitted through the diffraction device 12. The system 10 further includes a lens 16 positioned between the light source 14 and the diffraction device 12 and operable to collimate light emitted from the light source. An optical device 26 is positioned to receive light transmitted through the color filter 18 and project color light to illuminate the display device 28.

The holographic diffraction device 12 includes three holographic optical elements 30, 32, 34 each switchable between an active, diffracting state and a passive, non-diffracting state. It is to be understood that in the passive state (non-diffracting state), the incoming light may still be slightly diffracted, however, the light is not substantially altered. The holographic optical elements 30, 32, 34 each include a hologram interposed between two electrodes 40 (Figs. 2 and 3). The hologram may be a Bragg (thick or volume) hologram or Raman-Nath (thin) hologram. Raman-Nath holograms are thinner and require less voltage to switch light between various moods of the hologram, however, Raman-Nath holograms are not as efficient as Bragg holograms. The Bragg holograms provide high diffraction efficiencies for incident beams with wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition and within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition.

The hologram is used to control transmitted light beams based on the principles of diffraction. The hologram selectively directs an incoming light beam from light source 14 either towards or away from a viewer and selectively diffracts light at certain wavelengths into different modes in response to a voltage applied to the electrodes 40. Light passing through the hologram in the same direction as the light is received from the light source 14 is referred to as the zeroth (0th) order mode 44 (Fig. 2). When no voltage is applied to the electrodes 40, liquid crystal droplets within the holographic optical element 30, 32, 34 are oriented such that the hologram is present in the element and light is diffracted from the zeroth order mode to a first (1st) order mode 46 of the hologram. When a voltage is applied to the holographic optical element 30, 32, 34, the liquid crystal droplets become realigned effectively erasing the hologram, and the incoming light passes through the holographic optical element in the zeroth order mode 44.

It is to be understood that the holographic optical elements 30, 32, 34 may also be reflective rather than transmissive as shown in Figure 2 and described above. In the case of a reflective holographic optical element, the arrangement of the holographic device and color filter 18 would be modified to utilize reflective properties of the hologram rather than the transmissive properties described herein. The transmission holograms are sensitive to the polarization state of incident light and exhibit maximum diffraction efficiency for p-polarized light, with the response to s-polarized light being around 1% for that of p-polarized light. In order to make use of the full output of the light source 14, the filter system 10 may include optical filters which make use of both the p-polarized light and s-polarized light, such as disclosed in U.S. patent application serial number , filed January 5, 2000, (Attorney Reference No. 5454-

00700/RDP029) which is incorporated herein by reference in its entirety. If reflection holograms are used, these additional provisions are not required since the holograms are not polarization sensitive. Reflection holograms only start to become polarization sensitive at large angles of incidence, typically much greater than 45 degrees.

The light that passes through the hologram is diffracted by interference fringes recorded in the hologram to form an image. Depending on the recording, the hologram is able to perform various optical functions which are associated with traditional optical elements, such as lenses and prisms, as well as more sophisticated optical operations. The hologram may be configured to perform operations such as deflection, focusing, or color filtering of the light, for example.

The holograms are preferably recorded in a photopolymer/liquid crystal composite material (emulsion) such as a holographic photopolymeric film which has been combined with liquid crystal, for example. The presence of the liquid crystal allows the hologram to exhibit optical characteristics which are dependent on an applied electrical field. The photopolymeric film may be composed of a polymerizable monomer having dipentaerythritol hydroxypentacrylate, as described in PCT Publication, Application Serial No. PCT US97/12577, by Sutherland et al. The liquid crystal may be suffused into the pores of the photopolymeric film and may include a surfactant.

The diffractive properties of the holographic optical elements 30, 32, 34 depend primarily on the recorded holographic fringes in the photopolymeric film. The interference fringes may be created by applying beams of light to the photopolymeric film. Alternatively, the interference fringes may be artificially created by using highly accurate laser writing devices or other replication techniques, as is well known by those skilled in the art. The holographic fringes may be recorded in the photopolymeric film either prior to or after the photopolymeric film is combined with the liquid crystal. In the preferred embodiment, the photopolymeric material is combined with the liquid crystal prior to the recording. In this preferred embodiment, the liquid crystal and the polymer material are pre-mixed and the phase separation takes place during the recording of the hologram, such that the holographic fringes become populated with a high concentration of liquid crystal droplets. This process can be regarded as a "dry" process, which is advantageous in terms of mass production of the switchable holographic optical elements. The electrodes (electrode layers) 40 are positioned on opposite sides of the emulsion and are preferably transparent so that they do not interfere with light passing through the hologram. The electrodes 40 may be formed from a vapor deposition of Indium Tin Oxide (ITO) which typically has a transmission efficiency of greater than 80%, or any other suitable substantially transparent conducting material. The electrodes 40 are connected to an electric circuit 48 operable to apply a voltage to the electrodes, to generate an electric field (Fig. 3). Initially, with no voltage applied to the electrodes 40, the hologram is in the diffractive (active) state and the holographic optical element 30, 32, 34 diffracts propagating light in a predefined manner. When an electrical field is generated in the hologram by applying a voltage to the electrodes 40 of the holographic optical element 30, 32, 34, the operating state of the hologram switches from the diffractive state to the passive state and the holographic optical element does not optically alter the propagating light. It is to be understood that the electrodes may be different than described herein.

For example, a plurality of smaller electrodes may be used rather than two large electrodes which substantially cover surfaces of the holograms.

Each holographic optical element 30, 32, 34 is holographically configured such that only a particular monochromatic light is diffracted by the hologram. The red optical element 30 has a hologram which is optimized to diffract red light, the green optical element 32 has a hologram which is optimized to diffract green light, and the blue optical element 34 has a hologram which is optimized to diffract blue light. A holographic device controller 50 drives switching circuitry 54 associated with the electrodes 40 on each of the optical elements 30, 32, 34 to apply a voltage to the electrodes (Figs. 3 and 4). The electrodes 40 are individually coupled to the device controller 50 through a voltage controller 56 which selectively provides an excitation signal to the electrodes 40 of a selected holographic optical element 30, 32, 34, switching the hologram to the passive state. The voltage controller 56 also determines the specific voltage level to be applied to each electrode 40.

Preferably, only one pair of the electrodes 40 associated with one of the three holographic optical elements 30, 32, 34 is energized at one time. In order to display a color image, the voltage controller 56 operates to sequentially display three monochromatic images of the color input image. The electrodes 40 attached to each of the holograms 30, 32, 34 are sequentially enabled such that a selected amount of red, green, and blue light is directed towards the viewer. For example, when a red monochromatic image is projected, the voltage controller 56 switches the green and blue holograms 32, 34 to the passive state by applying voltages to their respective electrodes 40. The supplied voltages to the electrodes 40 of the green and blue holograms 32, 34 create a potential difference between the electrodes, thereby generating an electrical field within the green and blue holograms. The presence of the generated electrical field switches the optical characteristic of the holograms 32, 34 to the passive state. With the green and blue holograms 32, 34 in the passive state and the red hologram 30 in the diffractive state, only the red hologram optically diffracts the projected red image. Thus, only the portion of the visible light spectrum corresponding to the red light is diffracted to the viewer. The green hologram 32 is next changed to the diffractive state by deenergizing the corresponding electrodes 40 and the electrodes of the red hologram 30 are energized to change the red hologram to the passive state so that only green light is diffracted. The blue hologram 34 is then changed to the diffractive state by deenergizing its electrodes 40 and the electrodes of the green hologram 32 are energized to change the green hologram to the passive state so that only blue light is diffracted.

The holograms are sequentially enabled with a refresh rate (e.g., less than 150 microseconds) which is faster than the response time of a human eye so that a color image will be created in the viewer's eye due to the integration of the red, green, and blue monochrome images created from each of the red, green, and blue holograms. Consequently, the display device 28 will be illuminated sequentially by red, green, and blue lights so that the final viewable image will appear to be displayed as a composite color. The red, green, and blue holographic elements 30, 32, 34 may be cycled on and off in any order.

It is to be understood that the holographic diffraction device may be different than described herein without departing from the scope of the invention. For example, the diffraction device may include additional holographic optical elements that perform optical functions other than color filtering. Also, the diffraction device may include more than one holographic optical element configured to diffract each color wavelength band.

The color filter 18 is positioned at a location where the output beams

20, 22, 24 are focused by the holographic diffraction device 12 and forms color filtered output beams 70, 72, 74. The color filter 18 comprises red, green, and blue narrow bandpass filters 60, 62, 64, which are positioned to receive the red, green, and blue output beams 20, 22, 24, respectively. The filter components are preferably interference filters having multi-layer dielectric coatings such as available from Optical Coating Laboratory, Inc. (OCLI) of 2789 Northpoint Pkwy., Santa Rosa, California, for example.

Fig. 5 is a graph of transmission vs. wavelength and shows wavelength band profiles R, G, B for the output beams 20, 22, 24 emitted from the diffraction device 12 and wavelength band profiles R", G\ B" for the color filtered beams 70, 72, 74, respectively. The wavelength band profiles R, G, B of the output beams 20, 22, 24 are relatively broad and of Gaussian form so that the sides of the profile tends to fall off in a non-abrupt manner. This may result in sidebands where there is an area S of overlap (indicated by hatching) in wavelength between the red, green, and blue output beams. The color filter

18 reduces the wavelength band profile by effectively selecting only a central portion of each color band, thus substantially eliminating overlapping sidebands S.

The light beams 70, 72, 74 formed by the color filter 18 are directed to the optical device 26, which is configured to compensate for the spectral dispersion between the beams created by the holographic diffraction device

12. The optical device 26 preferably collimates the beams and corrects for color dispersion to provide a uniform distribution of light. The optical device 26 preferably comprises three holographic optical elements 80, 82, 84 which are designed to act upon red, green, and blue light, respectively, as described above for the optical elements 30, 32, 34. These elements 80, 82, 84 are preferably switchable between an active, diffracting state and a passive, non- diffracting state by a controller 88. The controller 88 is electronically coupled to the diffraction device controller 50 so that corresponding holographic optical elements (i.e., 30 and 80, 32 and 82, 34 and 84) are switched in synchronism. The optical device 26 may also be passive (i.e., non- switchable). As described above for a switchable reflection hologram, non- switchable transmission and non-switchable reflection holograms are not polarization sensitive.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations made to the embodiments without departing from the scope of the present invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. An optical filter system comprising:

a holographic device comprising a plurality of holographic optical elements switchable between an active state wherein light from a light source is diffracted by the device and a passive state wherein said light is not diffracted by the device, each of said holographic optical elements being operable to form a light beam having a wavelength band different from the light beam formed by the other optical elements; and

a color filter positioned to receive said light beams from the holographic device and operable to reduce a profile of the wavelength band of said light beams.

2. The optical filter system of claim 1 wherein the holographic optical elements each comprise a hologram interposed between two electrode layers operable to apply an electrical field to the hologram.

3. The optical filter system of claim 2 wherein the hologram is formed from a polymer and liquid crystal material.

4. The optical filter system of claim 1 wherein said plurality of holographic optical elements comprises three holographic optical elements, each element having a hologram recorded therein that is optimized to diffract red, green, or blue light.

5. The optical filter system of claim 4 wherein each hologram is interposed between two electrode layers operable to apply an electrical field to the hologram to diffract the red, green, or blue light.

6. The optical filter system of claim 5 further comprising a controller operable to sequentially supply voltage to and remove voltage from the electrode layers of each of the holographic optical elements.

7. The optical filter system of claim 1 further comprising an optical device operable to collimate said light beams emitted from the color filter.

8. The optical filter system of claim 7 wherein the optical device comprises three holographic optical elements, each element having a hologram recorded therein which is optimized to diffract red, green, or blue light.

9. The optical filter system of claim 8 wherein the holographic optical elements of the optical device are switchable between said active state and said passive state in synchronization with the holographic optical elements of the holographic device.

10. The optical filter system of claim 1 further comprising a lens positioned between the light source and the holographic device and operable to collimate light emitted from the light source.

11. The optical filter system of claim 1 wherein each of the holographic optical elements of the holographic device are operable to focus the light beam emitted therefrom at a different location on the color filter.

12. The optical filter system of claim 1 wherein the color filter comprises red, green, and blue bandpass filters.

13. A display system comprising:

a light source; a holographic device operable to receive light from the light source and produce a plurality of light beams;

a color filter positioned to receive said light beams from the holographic device and operable to reduce a profile of the wavelength band of said light beams; and

a display device positioned for illumination by light emitted from the optical device.

14. The display system of claim 12 wherein the holographic device comprises a plurality of switchable holographic optical elements.

15. The display system of claim 14 wherein said plurality of holographic optical elements comprises three holographic optical elements, each element having a hologram recorded therein that is optimized to diffract red, green, or blue light.

16. The display system of claim 15 wherein the hologram is formed from a polymer and liquid crystal material.

17. The display system of claim 15 wherein the hologram is interposed between two electrode layers operable to apply an electrical field to the hologram to diffract the red, green, or blue light.

18. The display system of claim 17 further comprising a controller operable to sequentially supply voltage to and remove voltage from the electrode layers of each of the holographic optical elements to create a sequence of monochrome images which are combined to form a color image.

19. The display system of claim 13 further comprising an optical device interposed between the color filter and the display device and operable to collimate said light beams transmitted from the color filter.

20. The display system of claim 19 wherein the optical device comprises three holographic optical elements, each element having a hologram recorded therein that is optimized to diffract red, green, or blue light.

21. The display system of claim 20 wherein the holographic optical elements of the optical device are switchable between said active state and said passive state in synchronization with the holographic optical elements of the holographic device.

22. The display system of claim 13 further comprising a lens positioned between the light source and the holographic device and operable to collimate light from the light source.

23. The display system of claim 13 wherein the light source is a white light.