GB2235990A - Rear projection screens and projection systems incorporating such - Google Patents

Abstract

A rear projection screen (14) of a projection system onto which is projected an image generated by an image source (10) such as a CRT, LC (liquid crystal) device or transparency, comprises at least in part, an electro-optic diffusing element which has a selectively controllable light dispersion characteristic whereby light spreading at the screen, and hence angle of view, can be adjusted to suit requirements. An LC element providing an electrically controllable light scattering effect can be used, preferably of the kind having pockets or cells of LC material in a layer between opposing electrodes, for example a polymer dispersed LC film. The screen may be used with a lenticular array or a Fresnel lens. <IMAGE>

Description

DESCRIPTION REAR PROJECTION SCREENS AND PROJECTION SYSTEMS INCORPORATING SUCH This invention relates to a rear projection screen having diffusing means and to a rear projection display system which includes such a projection screen.

Generally, in a rear projection display system a projection lens projects a primary image produced by an image source onto one side, the so-called rear side, of the projection screen.

This projected image is then observed by a viewer situated on the opposite, front, side of the screen.

Rear projection display systems, comprising a single image source in the form of a cathode ray tube for monochrome reproduction or three image sources in the form of red, green and blue emitting cathode ray tubes for full colour reproduction, may be employed for video display purposes, for example, TV pictures, with a display size which is substantially larger than that attainable with a conventional cathode ray picture tube.In another form of rear projection display system the image source may consist of one or more flat display panels, such as liquid crystal panels, comprising a matrix of individual picture cells and one or more associated light sources, the picture cells being electrically controllable to act as light shutters by selectively absorbing and transmitting or reflecting light depending on whether the panel is operated in a transmissive or reflective mode whose outputs are projected onto a screen via a projection lens system. In yet another kind of rear projection display system, the image source may comprise a transparency in the form of a slide or a film and a light source for illuminating the transparency.

With such projection display systems the light from an image source located behind the screen is projected forward along a projection axis, which could be folded to reduce the overall size of the system, towards the screen to form an image at the plane of the screen which is distributed to viewers at the front side of the screen. The light energy tends to be concentrated along the projection axis. In applications in which a room-size audience is contemplated, this may cause problems. The required angle of view will normally vary little in the vertical direction but widely in the horizontal direction. Most viewers are located at generally the same vertical position but are spaced horizontally.

Proposals have been made to improve the performance of rear projection screens. For example US-A-4573764 describes a screen which provides a very wide viewing angle with a good uniformity of luminance at any horizontal viewing angle. This is achieved by providing the screen with a light focussing circular Fresnel lens and a light distributing lenticular lens array (of vertical rib structure) with each lenticular element having specific design parameters giving comparatively high horizontal light dispersion. Dispersion of light in the vertical direction is obtained by a diffusion component situated between the Fresnel lens and the lenticular array.This diffusion component, which can be due to surface texturising, a bulk property of the screen material or a purposely provided diffusion layer, tends also to reduce colour shift, in the case where different colour projection beams are converged on the screen, by achieving a more even horizontal dispersion of light rays. The fresnel lens acts first as a field lens to bend the ray bundles back towards the main axis. The diffusing elements of the screen then diffuse light into a desired half-cone angle and horizontally spreads the light as widely as possible.

Further examples of similar rear projection screens are described in EP-A-233662 and EP-A-0148529. As also described in the latter reference, light absorbing material may be introduced between the vertical lenticular elements to increase image contrast by reducing ambient light reflections. It is suggested in EP-A-0148529 that a surface of the screen may be roughened or that a light-dispersing material may be incorporated in the screen to provide the screen with light dispersing properties.

For similar purposes it has also been proposed that a so-called "bulk diffuser" be provided in the screen which consists of grains embedded in matrix material, the material of the grains having a refractive index different to that of the matrix.

It is seen then these known examples of screens comprise diffusing means of various types which scatter, or spread, light whose strength, and hence the degree of scattering, controls the angle of view of the projected display. The strength of the diffusing means also effects the display brightness and this controls the gain of the screen, the screen gain being defined as the ratio of the brightness of the display on the screen to that of a display projected on to a perfect or Lambertian diffuser.

With such screens there is, therefore, a requirement for compromise between screen gain and angle of view. Generally, the greater the scattering the lower the gain and the greater the angle of view. Accordingly, screens designed with one specific kind of display in mind have fixed performance characteristics which cannot be varied.

It is an object of the present invention to provide an improved rear projection screen whose light spreading properties can be controlled to some extent according to desired use.

According to one aspect of the present invention there is provided a rear projection screen having light diffusing means which is characterised in that the diffusing means comprises at least in part an electro-optic diffusing element which is selectively controllable to adjust light dispersion.

By varying appropriately the light dispersion characteristics of the electro-optic diffusing element light spreading at the screen, and accordingly the angle of view of the displayed image, can be adjusted as desired to suit differing requirements.

Preferably, the electro-optic diffusing element comprises a liquid crystal element providing an electrically controllable light scattering effect. It is envisaged, however, that other kinds of electro-optic element capable of exhibiting a light scattering property which is variable in accordance with an applied voltage so as to provide an adjustable light diffusion effect may be used.

In one embodiment, the liquid crystal element comprises pockets, or cells, of liquid crystal material in a layer disposed between opposing electrodes. The liquid crystal element may comprise a polymer dispersed liquid crystal film.

Polymer dispersed liquid crystal films, sometimes referred to also as an encapsulated liquid crystal films, can be regarded as an electrically controllable variable light scattering devices. Such films are now generally well-known. Typically, they consist of a multitude of droplets of liquid crystal embedded in a solid polymer layer on whose surfaces electrodes are provided, with an additional supporting layer or layers if required. Their proposed uses heretofore have been restricted to light control and display applications. In operation they act as a light shutter, or as a matrix of individually operable light shutters in the case of a display device, to control the transmission of light in accordance with an applied potential.

The mechanism by which this is achieved is light scattering. In the presence of an electric field established between the electrodes the liquid crystal droplets are aligned perpendicular to the film surfaces so that light entering the film is transmitted through the film and suffers little scattering. In the absence of an electric field the orientation of the liquid crystal droplet axes is random and maximum scattering of light occurs. The amount of scattering can be varied to an extent between maximum and minimum values in accordance with the strength of the applied electric field.

Thus, diffusing properties of the screen comprising such an electro-optic element, and accordingly the distribution or spread of the screen output, can be controlled by suitable selection of a voltage applied to the element. With no applied voltage, maximum diffusion by the element is obtained giving a large viewing angle and rendering a display on the screen viewable to a large audience, although inevitably with a reduced brightness.

With an applied voltage sufficient to fully align the liquid crystal, very little scattering of light takes place so that a display on the screen has a comparatively small viewing angle but with improved gain. Thus, a viewer can selectively control the light spreading characteristics of the screen in accordance with subsisting requirements simply by appropriate adjustment of the voltage applied to the element to produce a high brightness picture with restricted viewing angle or a more modest brightness picture with greater viewing angle for a larger audience.

Intermediate states may also be possible.

Other forms of liquid crystal; elements having pockets or cells of liquid crystal material may be used. Another example of electrically controllable liquid crystal element having cells of liquid crystal comprises a cellular structure disposed between opposing electrodes which defines a multiplicity of cells containing liquid crystal material. An example of such a construction is described in EP-A-0278721, details of which are incorporated herein by reference. This construction uses a structure of insulative material, e.g. polyimide, providing microcells which contain liquid crystal material. The microcells can be closed so that discrete, and mutually isolated pockets of liquid crystal are obtained.

In another embodiment the electro-optic diffusing element may comprise a dynamic scattering liquid crystal device. An example of known dynamic scattering LC device comprises a conductively doped nematic liquid crystal material of negative dielectric anisotropy and positive conductance anisotropy disposed between two transparent, for example glass, plates having transparent electrodes on their opposing surfaces which electrodes are surface treated to determine the alignment of the liquid crystal immediately adjacent to them. The plates are maintained a fixed distance apart, for example 20 micrometres, by means of spacer elements.Upon the application of an alternating voltage across the electrodes of sufficient magnitude to exceed a threshold level space charge forces created exceed stabilisation forces in the LC material producing turbulence and consequently local variations in refractive index. These variations produce light scattering. The effect of this device on light projected onto the screen is, therefore, similar to that achieved with the above-described polymer dispersed liquid crystal film, except that scattering occurs upon the application of a voltage rather than in the absence of an applied voltage. The extent of scattering is dependent on the magnitude of the applied voltage so that by adjustment of this voltage, the degree of dispersion can be varied correspondingly by a viewer to alter the display characteristics in a manner similar to that described previously.

Compared for example with polymer dispered liquid crystal films, dynamic scattering devices are considered to be less favourable since they require more power and their construction requires two accurately spaced, e.g. glass, plates whereas the former type is simpler to fabricate and can be in a flexible form. Also, it might be expected that the regulation of the amount of light scattering by a dynamic scattering device may be more difficult to control.

The electro-optic diffusing element may serve by itself as the screen with the element including perhaps one or more transparent supporting sheets.

Alternatively, the screen may further comprise additional elements known per se in the art. For example, the electro-optic diffusing element may be used in combination with a conventional diffusing layer, or in combination with one or more Fresnel lenses for collimating the input, directional diffusers of the lenticular lens array kind and light absorbant materials as previously described.

According to another aspect of the present invention there is provided a rear projection display system having at least one image source, a rear projection screen and a lens system for projecting an image generated by the image source onto the screen, the screen comprising a rear projection screen according to the one aspect of the present invention.

The image source may be one or more cathode ray tubes emitting light which is projected onto the screen.

Alternatively, the image source may comprise a non-emissive component such as a transparency or a matrix liquid crystal display device having a multiplicity of picture elements each acting as a variable light shutter in which case the projection system further includes a light source for illuminating the image source. The screen of the present invention offers the further advantage when used in the latter kind of projection display system in that when a small viewing angle, high gain mode of operation is selected for the screen a lower power light source can be used to conserve energy. This is especially important in, for example, a portable liquid crystal projection display system which operates on batteries since the light sources consumes the greatest amount of power in the system.A solitary viewer can therefore select the narrow viewing angle mode to conserve energy consumption while still obtaining an adequate display brightness.

Rear projection screens and projection display systems incorporating such screens in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates schematically and in simplified form a rear projection display system according to the invention comprising an image source, projecting lens system and a rear projection screen operating in one mode; Figure 2 illustrates the projection system of Figure 1 operating in an alternative mode; Figure 3 is a schematic cross-sectional view, not to scale, through a typical part of the rear projection screen of the system of Figure 1 and 2; and Figure 4 illustrates an embodiment of projection system according to the invention comprising a full-colour liquid crystal projection system.

Referring to Figure 1, the rear screen projection display system comprises an image source 10, producing an image to be displayed which image is projected along a projection axis 15 via a projection lens system 12 onto the rear side of a projection screen 14 to produce a display which is observed by a viewer 16 situated in front of the screen.

The image source 10 can comprise, for example, a single projection cathode ray tube for monochrome reproduction.

Alternatively, three projection cathode ray tubes, respectively providing a red, green and blue display component, may be employed for colour reproduction, each being associated with a respective projection lens arranged to converge the three primary colour display components on the screen.

The image source 10 may instead comprise one or more matrix liquid crystal panels of known kind, the panels each having a row and column array of individually operable picture elements which modulate light from an associated light source, the modulated light then being projected onto the screen, as will be described more fully with regard to Figure 4.

The above image sources may be used to provide video displays, e.g. T.V. pictures, or datagraphic displays.

As another possible example, the image source may comprise a transparency such as a slide or film together with a source of illumination, the transparency modulating light from the source which is then projected onto the screen.

It will be appreciated that the simplified arrangement shown in Figure I is by way of example only and that in practice the projection axis may in effect be folded in a known manner, using reflective mirrors for example, to reduce the overall dimensions of the system.

In accordance with the present invention, the rear projection screen 14 comprises an electro-optic diffusing means.

In this embodiment the electro-optic diffusing means is in the form of a liquid crystal film in which the liquid crystal material is contained in pockets, or cells, extending as a layer between opposing electrodes to which a voltage is applied. By varying the applied voltage the light scattering effect of the liquid crystal, and hence the light diffusing properties, can be controlled selectively. More particularly, the dispersion or distribution of light in horizontal and vertical directions at the screen's front surface, the output, can be adjusted to give a high gain, that is high brightness output, with a limited viewing angle by limiting the extent of dispersion, or a low gain output with larger viewing angle by increasing the extent of dispersion.The two modes are illustrated respectively in Figures 1 and 2 which show the dispersion characteristics for a typical ray of light along the projection axis 15. Thus, in Figure 1 a high brightness display is obtained but with limited viewing angle as only a pencil like beam of minimally scattered light is produced. This display is suited to viewing by a single or small number of viewers. In contrast, in Figure 2 a wider viewing angle display is obtained by virtue of a comparatively wide cone of forward scattered light being produced rendering the display suitable for a larger audience but, as a consequence, with diminished brightness.

The electro-optic diffusing means can be adjusted also to dispersion states intermediate these two extremes.

Referring to Figure 3, there is shown schematically in cross-section a portion of rear projection screen 14 which in this case comprises, at least in part, a polymer dispersed liquid crystal film, 20, serving as the electro-optic diffusing element. Polymer dispersed liquid crystal films, or encapsulated liquid crystal films, are now well known and widely reported in the literature. Such films are produced by Taliq Corporation, ICI and others. Examples of these films are described for instance in EP-A-238626, EP-A-0088126, and the article entitled "Polymer Encapsulated Nematic Liquid Crystals for Display and Light Control Applications" by J. L. Fergason published in SID 85 DIGEST at pages 68-69 to which reference is invited.Briefly, they consist of a multiplicity of pockets or cells of liquid crystal material in a layer arranged between two opposing electrodes to which control voltages are applied. These pockets are constituted by separate, generally spherical, droplets or capsules 22 of nematic liquid crystal material having positive dielectric anisotropy, only a few of which are shown in Figure 3 for simplicity, dispersed in a solid polymer matrix layer 23.

The droplets may typically be in the region of 1.0 to 20 micrometres in diameter. On either side of the matrix material 23 are formed transparent electrodes 24 and 25, for example of ITO, extending continuously over the area of the screen. The matrix material 23 together with the electrodes is carried on an insulative supporting layer 26 of transparent plastics material, which may be rigid or flexible, although another material such as glass could be used instead. A further supporting layer could be provided on the opposite side of the layer 23.

The thickness of the polymer matrix material 23 can be around 5 to 50 micrometres and accordingly may accommodate one or several "layers" of droplets 22. The droplets 22 are formed and fixed in random dispersement in the polymer matrix by known techniques such as chemical, thermal or light induced phase separation. The matrix material can be of any substantially transparent type known in the art, for example latex, polyvinyl alcohol, epoxy resin, polyurethane resin, polycarbonate resin, polyvinyl butyral resin, etc. but preferably of a type which exhibits minium absorption.The film may be one of the group in which the liquid crystal is encapsulated in polymer shells which are subsequently embedded in a polymer continuium constituting the matrix rather than one of the group in which the droplets are spontaneously dispersed in the matrix material during formation of the matrix layer.

The film 20 employed for the screen follows conventional forms of polymer dispersed LC films in these respects and, accordingly, reference may be made to the aforementioned publications describing typical examples for further information regarding its fabrication and the theory behind its behaviour in operation.

When light enters the film 20 it is scattered at the interfaces between the polymer matrix and the droplets by an amount which depends on the difference in refractive indices and, as the liquid crystal is birefringent, also on the orientation of the liquid crystal relative to the direction of the light and, if applicable, the direction of polarisation of the light. If the refractive index of the polymer chosen is close to the "ordinary" refractive index (that is, the refractive index perpendicular to the director) of the liquid crystal then very little light is scattered when the light is parallel to the crystal axis. With a sufficient electric field applied across the polymer matrix the droplets align perpendicular to the film surfaces so that in this condition light entering the film perpendicularly suffers very little scattering.On the other hand, with no applied field the orientation of the liquid crystal axes is random and maximum scattering occurs. The nature of optical scattering in such films is a complex subject as is apparent from the article by Zumer et al entitled "Light scattering from a small nematic droplet" published by The American Physical Society in Physical Review Vol. 34, No. 4, October 1986 at pages 3373 to 3386. The distribution of scattered light is dependent on the difference between the refractive indices of the polymer and the extraordinary direction of the liquid crystal. If this difference is small the scattering pattern will have a large forward direction bias. The number and size of the droplets is also an important consideration. For simplicity it is assumed that the film scatters uniformly in the unenergised state producing, since the film is planar, a Lambertian distribution in the forward direction with reference to the direction of illumination.

In operation of the screen, therefore, the degree of scattering is controllable and determined by the electric field established by applying a voltage difference across the electrodes 24 and 25. This regulating of the light scattering performance of the film is usable to provide differing display characteristics.

Considering for example a ray of light, A, projected onto the screen from the projection lens, then in the absence of an electric field across the layer 23 the droplets 22 are in the isotropic, non-aligned, phase, so that the light is dispersed at the output, i.e. the viewing side, of the film 20, as represented by the arrows B in Figure 3. Upon the application of an electric field across the layer 23, the layer 23 effectively becomes transparent, assuming a suitable choice of refractive indices for the materials, and allows light to be transmitted directly therethrough in the same direction with minimal dispersion, as represented by the arrow A'. By varying the magnitude of the applied field the degree of light spreading and hence viewing angle, can be varied correspondingly.

Thus the viewing angle and gain of the screen display produced can be selectively tailored according to the number and spread of viewers by varying at will the applied voltage. The amount of scattering should be restricted bearing in mind that display brightness decreases with increases in scattering and can be controlled by suitably limiting the maximum voltage applied between the electrodes.

The screen may include further components for enhancing display quality using known techniques depending on the requirements of the projection system. For example, a Fresnel lens may be disposed at the input side of the film 20 which is designed to image the aperture of the projection lens on to the audience to provide improved uniformity of illumination across the screen.

A component for assisting the spread or distribution of light horizontally in known manner, such as a lenticular lens array consisting of vertically-extending parallel ribs, may be incorporated. This component may be provided on the side of the film 20 remote from the support layer 26 with the ribs facing outwardly. Alternatively, the screen of Figure 3 may be reversed in operation such that light from the projection lens enters the film 20 through the electrode 25. Ribs forming the lenticular lens array can then be formed on the outer surface of the support 26 itself. In this case horizontal spreading of light is determined to a large extent by the additional component and the film 20 serves primarily to control the vertical distribution of light so as to enable an audience of viewers at different heights with respect to the screen to be accommodated.

Light absorbent material may be provided in known manner to reduce the effects of ambient light and enhance the contrast ratio of the display.

In a modified version of the screen 14, an alternative construction of controllable liquid crystal light diffusing film 20 similarly having a multiplicity of cells of liquid crystal material in a layer between two opposing control electrodes is employed. In this film construction, a cellular structure of insulative material is disposed between the opposing electrodes and defines a multitude of microcells, each containing liquid crystal material and being bounded by portions of the structure.

An example of this kind of film is described in EP-A-0278721 to which reference is invited. The film consists of two supporting layers of transparent and flexible material carrying opposing transparent electrodes. A net-like structure of polyimide material extends between the electrodes to provide the microcells, each typically 50 to 500 micrometres across, in which the liquid crystal material is retained. The microcells can be closed so that the liquid crystal contained therein is isolated with each cell holding a discrete portion which is separate from neighbouring microcells. The microcells extend as a single layer parallel to the electrodes and the supporting layers.

This film operates in generally similar manner to the polymer-dispersed liquid crystal film with the scattering effect on light entering the film being controllable in accordance with voltage applied to the electrodes. Thus, by altering the applied voltage, the amount of diffusion, and hence the angle of view and gain of the screen display, can be adjusted in like fashion.

Again, the screen may include further optical components such as a Fresnel lens, a lenticular lens array or light absorbent materials.

Referring now to Figure 4, there is shown an embodiment of a rear projection display system according to the invention, this embodiment comprising a full colour liquid crytal projection system.

In the system, white light from a lamp/reflector assembly 60 is directed through a condensing lens 61 to a crossed pair of dichroic mirrors 70 where it is broken into red, green and blue portions of the spectrum. These three colour components are then passed respectively either directly or by reflection from mirrors 71 to three matrix liquid crystal display devices 62 of conventional form having a row and column array of individually operable picture elements acting as light modulators. Each display device has input and output polarising sheets although these are not shown.

After modulation by their respective display devices, the three colour components are recombined by way of dichroic prism 74. The recombined light is then projected by the projection lens 12 to the rear projection screen 14.

The screen of the system may be physically separate from the other components. Alternatively, for a compact, portable system especially, the screen may be connected with the housing containing the other components and movable between storage and operative positions. For true portability, the system may be battery powered. In this case a switchable light source may be provided so that the viewer has the option of setting the screen to its high gain, minimum dispersion, mode and using a lower power light source in order to conserve battery energy while still obtaining an adequately bright display.

While in the above embodiments particular forms of light diffusing liquid crystal elements have been described, it is envisaged that other forms of electro-optic element which are capable of exhibiting a variable light scattering effect on incident light which is controllable in accordance with an applied voltage can be utilised to provide the desired adjustable light diffusing characteristic.

It will be appreciated also that various modifications to the above-described screens and projection systems are possible, as will be apparent to persons skilled in the art, within the scope of the invention as set out in the appendant claims.

Claims (10)

CLAIM(S)

1. A rear projection screen having light diffusing means, characterised in that the diffusing means comprises an electro-optic diffusing element which is selectively controllable to adjust light dispersion.

2. A rear projection screen according to Claim 1, characterised in that the electro-optic diffusing element comprises a liquid crystal element providing an electrically controllable light scattering effect.

3. A rear projection screen according to Claim 2, characterised in that the liquid crystal element comprises pockets or cells of liquid crystal material in a layer disposed between opposing electrodes.

4. A rear projection screen according to Claim 3, characterised in that the liquid crystal element comprises a polymer dispersed liquid crystal film.

5. A rear projection display system comprising at least one image source, a screen and a lens system for projecting an image generated by the image source onto the screen1 characterised in that the screen comprises a rear projection screen according to any one of the preceding claims.

6. A rear projection system according to Claim 5, characterised in that the or each image source comprises a cathode ray tube.

7. A rear projection system according to Claim 5, characterised in that the or each image source comprises a matrix liquid crystal display device and means for illuminating the liquid crystal display device.

8. A rear projection system according to Claim 5, characterised in that the image source comprises a transparency and means for illuminating the transparency.

9. A rear projection screen substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 3 of the accompanying drawings.

10. A rear projection system substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 4 of the accompanying drawings.