WO2007107752A1

WO2007107752A1 - Diffuse reflector for light enhancement of lcd screens

Info

Publication number: WO2007107752A1
Application number: PCT/GB2007/001005
Authority: WO
Inventors: Hans I. Bjelkhagen
Original assignee: Optopreneurs Limited
Priority date: 2006-03-23
Filing date: 2007-03-21
Publication date: 2007-09-27
Also published as: GB0605814D0

Abstract

A method of making a substantially panchromatic holographic optic element (HOE) is disclosed, comprising depositing a layer of finely granulated silver halide in emulsion or suspension onto a polymeric, plastics or film layer to a depth of less than lOOμm or preferably lOOμm and then exposing to laser light of at least 3 different wavelengths in the visible light spectrum either consecutively or simultaneously and then curing the resulting holographic image recorded in said silver halide layer. The invention also covers a display screen capable of receiving an image signal and interpreting said signal to display said image represented thereby on an image projecting surface of said display screen, said display screen (2) comprising an electronically controlled light transmissive panel (4) , the electronic control of which results in differential illumination of portions of said panel, and at least one reflector (6) disposed adjacent and behind said panel such that light impinging on the surface of said panel can pass through the panel and be reflected to a certain extent by said reflector which is made by the aforementioned method. This renders said substrate and said panel together substantially panchromatically holographically reflective to visible light giving rise to exception brilliance under normal ambient and/or day light.

Description

Diffuse Reflector for Light Enhancement of LCD Screens

This invention relates to the provision of a diffuse reflector for enhancing the illumination of liquid crystal display (LCD) screens of the sort typically used in laptop computer visual display units, free standing or wall mounted monitors, and more recently in televisions. More particularly, the present invention is concerned with enhancing of the illumination of such screens (and possibly other types of light emitting display technologies such as thin film transistor (TFT), active LCD screens, displays involving twisted nematic technologies (TN), super twisted nematic (STN) and the like). Indeed this invention should be considered as having application to any display technology involving a screen on which an image is intended to be displayed by virtue of the differential illumination of pixels or other portions defined on that screen, said screen being typically flat and allowing a certain degree of light transmission therethrough such that ambient or specific light incident on the screen is subsequently incident on a substrate disposed behind said screen which then reflects at least some of the light back through said screen to enhance the illumination thereof. The screens used on modern mobile, hand-held computers, video games and digital cameras telephones fall within these parameters.

Accordingly, while the following description of the invention is provided with almost exclusive reference to the provision of a means of enhancing the illumination of LCD screens, it is to be understood that the invention is not necessarily limited to this particular application.

BACKGROUND

The image produced on LCD screens is generally (provided the screen has not been damaged) of uniform brightness, or of a brightness which can be adjusted over a relatively narrow range. In modulated lighting environments such as offices, homes or other commercial premises, this brightness is generally adequate, and the user of the screen experiences little difficulty in viewing the image presented on the screen, particularly when the user is situated, as they usually are, at the optimum position in front of the screen.

It has been known for some time that users have great difficulty in viewing the images on LCD screens where the ambient light, e.g. sunlight, is strong, or where unwanted light from a coherent source (such as a spotlight) impinges on the screen to give rise to glare and undesirable reflection images (glare).

In an effort to mitigate against these disadvantages, the use of diffuse reflection holography has been proposed. Specifically, in a presentation made at the International Symposium of the Society for Information Display in San Diego in 1996, Phillip J. RaIIi and Michael M. Wenyon disclosed the idea that the diffuse reflection of light provided by holograms might be used to better illuminate LCD screens. Their work was subsequently covered in US Patent No. 5659408 to Wenyon. In his paper and patent, Wenyon states that ambient-illuminated reflective LCDs, such as the super-twisted nematic (STN) display, are widely used in portable, battery-operated equipment where low cost and low power are important. He suggests a new type of holographic reflector which can be positioned immediately or proximately behind LCD panels having polarizers thereon to enhance the brightness of the screen in low-level ambient light, eliminate glare, and provide the LCD with a uniform backlit appearance.

He also suggests that the holographic reflector element might also be used in backlit LCDs (which can be viewed in the dark) to allow high transmission of the backlight at one colour, with holographic reflection at another colour for normal use in ambient light. The trade name used by Polaroid Corporation, of which entity Wenyon was an employee at the time of the presentation and filing date of abovementioned US Patent, is I MAG IX™.

In both cases, the function of the hologram disposed behind the LCD is to transform the angles of light arriving from external sources or ambiently so that the brightest reflection exits though the LCD in a controllable zone which is ideally centred normal to the plane of the LCD panel in the display. In a normal hologram, this diffuse reflection of light is that which is seen by the viewer who sees an image appear, either on the surface of the hologram substrate, or in front of or behind it in the case of 3D holograms. The angle of diffuse reflection is very different from the more conventional angle of so-called specular reflection which is the same as that in the case where the holographic substrate was simply a mirror. It is for this reason that holograms are typically illuminated from above in a manner which ensures that the conventionally reflected light (manifested as an image of the light source) is not seen when the viewer is in the standard viewing position.

In the construction of the device of Wenyon, the holographic layer is laminated to the rear polarizer of the LCD stack, in front of the normal metallic reflective or transflective layer. Importantly, it is to be understood that the creation of the holographic layer for this particular application is stated as being of the photopolymer type in which a layer of 10μm photopolymer layer, coated and chemically bonded to flexible polyester film, is exposed to two laser beams of light which interfere constructively in certain areas thus giving rise to a photopolymerization reaction. This causes microscopically interspersed layers of alternating polymer and voids filled with air. The localized differences in refractive index act as mirror-like interfaces, thus creating a volume holographic reflector which can both diffuse and redirect light.

While the arrangement described above works well, it is disadvantaged in that photopolymer holographic optical elements (HOEs) are only typically capable of giving rise to diffuse reflection of light at one particular wavelength. This means that any screen which is provided with such a hologram therebehind can be further illuminated or have its backlighting enhanced in only a single colour. Thus this arrangement does work well for monochromatic LCD displays, such as those commonly found in digital watches, but in general of far less use in modern applications, such as for modern LCD computers screens and LCD televisions, which are necessarily panchromatic.

Indeed, the use of a reflective holographic element of the monochrome type for LCD displays has already been described in U.S. Pat. No. 5,663,816 to Chen et al. entitled: Liquid crystal display device comprising reflective holographic.

In US5812229 in the name of Motorola (Inventor: Chen et al.), there is a described a liquid crystal display device with the following structure components: a front polarizer effective to transmit linearly polarized light having frequencies within a broad spectral band, a liquid crystal cell having at least one region having a transparent mode effective to transmit elliptically polarized light, a retardation film effective to transmit elliptically polarized light; a back polarizer optically coupled to the liquid crystal cell and the retardation film, and a reflective holographic optical element optically coupled to the back polarizer.

In this structure, the liquid crystal cell and the retardation film are optically coupled to emit elliptically polarized light within a selected spectral band, and the holographic element filters light transmits polarized light having an axis which corresponds to the predetermined polarization axis of the light passing through the first polarizer, but again the holographic element is a photopolymer which is generally only capable of achieving diffuse reflection of light at one particular wavelength or over a relatively narrow range of wavelengths, i.e. visible red, blue or green. Indeed, this patent refers to the IMAGIX material developed by Polaroid and Wenyon above.

Sato et al. (Tsukuba Research Laboratory, Technical Research Institute, Toppan Printing Co. Ltd., SPIE Vol. 3010 pp293-299) have already proposed a transmission hologram having low wavelength selectivity which allegedly produces a near-white colour reflector for reflective LCDs, but their proposal involves a volume-type transmission hologram and a metallic reflection layer which necessarily increases the cost of the overall arrangement, and additionally the volume-type transmission hologram is again fabricated from photpolymer .

One of the fundamental problems with photopolymer holographic films is that while they are light sensitive and useful for recording holograms, they cannot be made truly panchromatic. A generally unsatisfactory approach to this problem has been to record holograms on three separate photopolymer substrates using red, green and blue laser light respectively, and then to combine the three films with accurate registration so that white light impinging on the surface of the combined film can be reflected diffusely. The reason this approach is unsatisfactory is that it is very difficult to control the photopolymerization reaction caused by exposing the photopolymer films to laser light, and the creation of voids interspersed with regions of photopolymer has an unavoidable degree of randomness.

A further reason that attempts to produce truly panchromatic photopolymer holograms have proved unsatisfactory is the shrinkage which occurs as part of the photopolymer processing. Once a particular photopolymer film has been exposed to laser light to record a hologram therein, the photopolymer bust be baked to set the arrangement of voids and photopolymer which give rise to the hologram. This baking process causes unavoidable shrinkage in the film regardless of the substrate on which it may be mounted, and this destroys or at least substantially impairs the colour balance of the photopolymer film.

Thus while all the prior art arrangements describe the use of Holographic Optical Elements (HOE) and holograms in general, none mentions the possibility of generating a reflective multi-wavelength, panchromatic HOE which can generate white light based on the combination of three primary special wavelengths.

It is an object of the present invention to provide a product which mitigates against the above disadvantages.

According to a first aspect of the invention there is provided a display screen capable of receiving an image signal and interpreting said signal to display said image represented thereby on an image projecting surface of said display screen, said display screen comprising an electronically controlled light transmissive panel, the electronic control of which results in differential illumination of portions of said panel, and at least one reflector disposed adjacent and behind said panel such that light impinging on the surface of said panel can pass through the panel and be reflected to a certain extent by said reflector, characterised in that said reflector comprises a substrate on which a layer of silver halide has been deposited and subsequently exposed to at least three different wavelengths of laser light in the visible spectrum to render said layer and said substrate together substantially panchromatically holographically reflective to visible light.

According to a second aspect of the invention there is provide a method of making a substantially panchromatic holographic optic element comprising depositing a layer of finely granulated silver halide in emulsion or suspension onto a polymeric, plastics or film layer to a depth of less than 50μm and then exposing to laser light of at least 3 different wavelengths in the visible light spectrum either consecutively or simultaneously and then curing the resulting holographic image recorded in said silver halide layer.

Preferably, the silver halide layer is exposed to at least five different wavelengths of laser light in the visible spectrum

Preferably, the substrate onto which the silver halide emulsion or suspension is deposited is a plastic film.

Preferably, the laser light emanates from one of a continuous wave laser, a RGB laser, or a pulsed laser.

Preferably, the exposure of the silver halide layer to the different laser light wavelengths occurs consecutively. Alternatively, the exposure of the silver halide layer to the different laser light wavelengths occurs simultaneously.

Preferably the silver halide layer is between 5 and 20μm in thickness.

Preferably, the silver halide emulsion or suspension is one in which the particulate silver halide material has fine grains of less than 50μm in diameter.

It has been found that LCD screens provided with the revolutionary silver halide holographic optical element behind them (as determined to the incident ambient light which impinges on the front surface of the LCD screen) exhibit startling and indeed superb brightness and clarity.

By virtue of the fact that the silver halide coated film consists of three holographic recordings in a single layer, recorded at red, blue and green laser wavelengths, this allows for an extremely efficient reflector with the correct colour balance for the viewing of colour displays in natural illumination.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic representation of the device according to the invention.

Referring to Figure 1 , there is shown a device indicated generally at 2 comprising the essential elements of i. a light transmissive LCD-type image display screen 4, and a white-light (panchromatic) reflective holographic optical element 6. As can be seen from the figure, light emanating from a source 8 is specularly reflected at 10 by the front surface of the LCD panel 4-such reflections do not interfere with the viewing at notional eye position 12. Indeed, it is because of the diffuse light reflection characteristics of the HOE 6 towards the eye position 12 that any images, whether static or dynamic, appearing on LCD screen 4 are so brightly displayed. The HOE 6 involves the use of an extremely high-resolution recording material (i.e. extremely fine grain silver halide coated film), and this characteristic, combined with the inherent high light-sensitivity allows for the generation of large-format colour reflectors.

A new simple method of fabricating such a white light or panchromatic HOE reflector is also disclosed.

As has been proposed in the past for the large-scale production of holograms, it is proposed that a silver-halide master hologram of extremely high resolution be produced. Typically such masters are provided on glass, as opposed to plastics or polymer film substrates to increase the longevity and reusability of the said masters. In all but this regard, the HOEs of the present invention are identical.

As mentioned above, the HOE 6 is intended to be used as a reflective HOE, which means that cross-talk between different colours is easily eliminated. When illuminated with regular ambient white light, the recorded master colour HOE reflector generates strong reflected white light directed towards the observer (perpendicular to the HOE).

The master HOE can be used in a copying system to generate large quantities of colour HOEs on flexible holographic film. There are two possibilities for the copying: a full-beam exposure in a step-and-repeat process, or a three-laser beam scanning system to perform the replication. The first method is somewhat slow, but can produce high-quality copies. The scanning system is faster, and may be more suitable for high-volume production.

Claims

1. A method of making a substantially panchromatic holographic optic element (HOE) comprising depositing a layer of finely granulated silver halide in emulsion or suspension onto a polymeric, plastics or film layer to a depth of less than 100μm and then exposing to laser light of at least 3 different wavelengths in the visible light spectrum either consecutively or simultaneously and then curing the resulting holographic image recorded in said silver halide layer.

2. A method according to claim 1 wherein the silver halide layer is exposed to at least five different wavelengths of laser light in the visible spectrum

3. A method according to any preceding claim wherein the substrate onto which the silver halide emulsion or suspension is deposited is a plastic film.

4. A method according to any preceding claim wherein the laser light emanates from one of a continuous wave laser, a RGB laser, or a pulsed laser.

5. A method according to any preceding claim wherein the exposure of the silver halide layer to the different laser light wavelengths occurs consecutively.

6. A method according to any of claims 1-4 wherein the exposure of the silver halide layer to the different laser light wavelengths occurs simultaneously.

7. A method according to any preceding claim wherein the silver halide layer is between 5 and 20μm in thickness.

8. A method according to any preceding claim wherein the silver halide emulsion or suspension is one in which the particulate silver halide material has fine grains of less than 50μm in diameter.

9. A method according to any preceding claim wherein the layer of finely granulated silver halide in emulsion or suspension is deposited onto said polymeric, plastics or film layer to a depth of less than 50μm

10. A display screen capable of receiving an image signal and interpreting said signal to display said image represented thereby on an image projecting surface of said display screen, said display screen comprising an electronically controlled light transmissive panel, the electronic control of which results in differential illumination of portions of said panel, and at least one reflector disposed adjacent and behind said panel such that light impinging on the surface of said panel can pass through the panel and be reflected to a certain extent by said reflector, characterised in that said reflector comprises a substrate on which a layer of silver halide has been deposited by the method of any preceding claim to render said reflector and said panel together substantially panchromatically holographically reflective to visible light.