WO1990005429A1

WO1990005429A1 - Frame-sequential colour display system

Info

Publication number: WO1990005429A1
Application number: PCT/GB1989/001297
Authority: WO
Inventors: David Mansel Williams; Michael Stephen Baigrie; Philip Jones
Original assignee: Raychem Limited
Priority date: 1988-10-31
Filing date: 1989-10-31
Publication date: 1990-05-17
Also published as: GB8825429D0; EP0441861A1; CA2001868A1

Abstract

A frame-sequential colour display system using a striped colour filter with correspondingly striped liquid crystal 'shutter' provides frame-sequential colour display without the disadvantages of moving parts and/or polarising filters.

Description

_PP&MB-_SEQUENTIAL COLOUR DISPLAY SYSTEM

This invention relates to a frame-sequential colour display system.

There are various methods currently used in the market place to produce colour displays. The most familiar of these is the cathode ray tube ⁽CRT⁾ used in television sets. Here the colour is generated by the electron beams falling on separate, individual spots of red, green and blue light emitting phosphors. Each picture element (pixel) is, therefore, made up of three smaller elements. The eye, viewing from a distance, cannot resolve the individual spots and adds the pri^¬ maries together to give the illusion of a wide range of colours. _Cathode ray tubes of course suffer from limi^¬ tations in size and image resolution.

A similar technique has been used for liquid crystal (L.C.) displays, for example using an array of thin film transistors (TFT's) in a backlit mode with a L._C. cell overlying a series of fine line colour filters on the front panel. The filters are arranged in lines of red, green and blue, with a row of TFT's arranged to actuate the L.C. cell material at the desired pixel points along each line such that the 640 x 600 array of individual pixels becomes a 640 x 200 array of colour pixels. This is extremely wasteful. Not only are such fine lines of filter (few hundred micrometres across) difficult to deposit, but they — Δ -

waste two thirds of the image resolution (as does the CRT colour system) .

Frame-sequential colour avoids this sacrifice of resolution by illuminating all of an image with a sequence of colours in consecutive frames, usually a red frame followed by a green frame and a blue frame. The eye thus sees red, green and blue information in turn and, if presented at sufficient speed adds colours together. It is the earliest from of colour display, a version having been demonstrated by Baird in 1928. By redefining the light-transmitting areas of the image for each colour frame, a fully coloured image can be seen.

Some known frame-sequential colour display systems use mechanically-movable colour filters, such as rotating colour-filter wheels, to generate the necessary sequence of coloured lights, but such arrangements of moving parts tend to pose mechanical difficulties which can be commercially unacceptable, especially for large area displays. Other known systems use flashing differently coloured light sour¬ ces, but these can present problems owing to limited lamp life and/or the relatively slow colour-change speeds obtainable.

Another known system is a television camera view finder which uses a striped colour filter between crossed polarisers placed close to the viewer's eyepiece, with a correspondingly striped polarisation shifter to transmit the camera image to the eye through the differently coloured filter stripes in sequence.

This arrangement is also inconvenient, especially for large area displays, since the filter must be close to the viewer's eye, and crossed polarisers in any case have the disadvantage of dissipating an undesirably high proportion of the incident light. The present invention provides a system which is remarkably free from the above disadvantages and limi¬ tations, and can be used with advantage for large area displays.

_One aspect of the invention provides a frame- sequential colour display system comprising (1) image- defining means for defining an image, (2) a light source, and (3) stationary colour-filtering means bet¬ ween the light source and the image-defining means, the filtering means comprising at least one switchable material arranged to be repeatedly electrically switched between a colour-transmitting state and a second state so as to transmit to the image-defining means repeating sequences of differently coloured lights at a rate of at least 30 sequences per second, and the image-defining means being capable of rede¬ fining the image for each transmission of each dif¬ ferently coloured light.

The colour-filtering means might for example comprise separate layers of suitably coloured guest/host L.C. materials could be superposed to give a greater range of colours on switching between a coloured state and a substantially transparent state, but the choice of materials could be problematical and undesirably high light losses could occur in such a multi-layered construction.

Preferable, therefore, is a system wherein the colour-filtering means comprises at least two dif¬ ferently coloured areas of colour filter material arranged to impart different colours respectively to transversely separate portions of light transmitted from the source to the image-defining means, and the switchable material is switchable to cause light coloured by the respectively coloured filter areas selectively to be transmitted to the image-άefininσ means. Although in theory there is no limitation as to the number and arrangement of the differently coloured filter areas, it will be convenient in practice to use two or more areas of each of the respective colours interspersed with one another in a sense transverse to the transmission path of the light to the image- defining means, preferably in the form of strips of the respective colours arranged substantially parallel to one another, for example in the form of a striped colour filter member, or in the form arranged so as to separate the incoming light into transverse separate red, green and blue beams.

The positioning of the colour-filtering means according to this invention between the light source and the image-defining means has the advantage that light-spreading means can be, and preferably is, posi¬ tioned between the filtering means and the image- defining means to spread the respectively coloured lights over the image in operation of the system. Such spreading reduces or avoids undesirable patterning of the viewable image by the striped, or otherwise pat¬ terned, filter. A diffusion plate or a de-focussed Fresnel lens could for example be used as the light- spreading means.

For full-colour displays, it will be preferable to cause the system to transmit red, green, and blue lights in sequence to the image-defining means, and filter areas imparting those colours to the filtered light are accordingly preferred. Appropriate wave¬ lengths and strengths of the colours are known per se, as are suitable coloured filter materials, on suitable multilayer dichroic filters or reflectors.

When the switchable material is to serve as a "shutter" to select which of the respectively coloured lights will be transmitted to the image-defining means, the switchable material may be either between the light source and the colour filter material, thus causing the light to fall only on the selected filter areas, or between the filter material and the image-defining means, thus transmitting the coloured light only from the selected filter areas. The switchable material could also be between two filters or on both sides of a single filter. The switchable material could also be incorporated in the coloured filter materials, for example as an intermediate layer, or in the form of dispersed particles or droplets.

In most cases, it will be convenient for the switchable material to be highly translucent or relati¬ vely transparent, preferably substantially transparent, in its colour-transmitting state, and relatively light- scattering or opaque, preferably highly light- scattering or substantially opaque, in its second state.

The switchable material may be selected from known materials, but it is highly advantageous according to the present invention to use liquid crystalline material as the switchable material, and another aspect of the present invention accordingly provides a frame- sequential colour display system comprising (1) image- defining means, for defining an image, (2) a light source, (3) stationary colour-filtering means comprising at least two differently coloured areas of colour filter material arranged to impart different colours respectively to transversely separate portions of the light transmitted from the source to the image- defining means, and (4) liquid crystalline material arranged to be electrically switchable between a colour-transmitting state and a second state so as to transmit to the image-defining means repeating sequen- ces of lights coloured by the respectively coloured fil_ter areas at a rate of at least 30 sequences per second, the image-defining means being capable of re¬ defining the image for each transmission of each dif¬ ferently coloured light.

A black guest-host type liquid crystalline material may be especially useful as a "shutter" material, but other types could also be used provided that they are sufficiently opaque or light-scattering in one state and sufficiently translucent or transparent when electrically switched to another s_tate. The black guest/host type is preferred for backlit displays, while the scattering type is pre¬ ferred for projection displays. Suitable liquid crystalline materials for various versions of this invention may be selected as required. For scattering systems, liquid crystals whose ordinary index of refraction substantially matches the refractive index of the matrix polymer are preferred. For a polyvinyl alcohol matrix, liquid crystals such as E7 or E63 from British Drug House (BDH) or ZLI 1840 from Merck can be used. Pleochroic dyes can be mixed with these liquid crystals. Exemplary suitable dyes are D109, D85, and D106 from BDH. There are also liquid crystal materials commercially available which have dyes premixed in them, such as D109E63 from BDH or ZLI 3499 from Merck.

When the liquid-crystalline (L.C.) material is to act as a "shutter" as aforesaid, it is preferable to use minute droplets or particles of the L.C. material dispersed in a suitable polymeric, preferably organic polymeric, matrix. Such encapsulated droplets or par¬ ticles of L.C. material have the advantage that they can readily be made small enough to allow very fast switching from the molecularly aligned light- transmitting state to the molecularly-non-aligned highly light-scattering state. In order to achieve 30 sequences of three colours per second, it will clearly be necessary to switch the "shutter" material controlling transmission of each colour "on and off" thirty times per second, while allowing as much time as possible between switching operations.

It has been determined according to the present invention that practical switching response times of not more than 10 milliseconds, preferably not more than 7 milliseconds, more preferably 3 to 5 milliseconds, are desirable, and these can be achieved using the aforementioned encapsulated L.C. materials.

Suitable encapsulated liquid crystal materials for the present purposes are described and claimed in U.S. Patents Nos. 4435047, 4671618, 4673255, 4685771, 4688900, 4579423, 4605284, 4616903, and 4707080, the disclosures of all of which are incorporated herein by reference. Reference may also be made to PCT applica¬ tion publication No. WO85/04262, and Japanese 61047427A; the disclosure of both of which is incor¬ porated herein by reference.

Materials consisting of droplets or particles of liquid crystal materials in a continuous polymeric matrix are advantageous, since adequate switching speeds for high frame rate dynamic displays can be achieved with a majority, preferably at least 75%, more preferably at least 90%, and most preferably substan¬ tially all, of the droplets or particles being less than 20 micrometres, preferably less than 10 micro¬ metres, more preferably not more than 5 micrometres, in average thickness in the direction of the electrical field to be applied in use. Such small thicknesses are extremely difficult to achieve in cells where the liquid crystal material is merely confined between two plates, since accurate spacing cr the plates is dif¬ ficult to achieve, especially over large areas.

Suitable electronic circuitry and electrodes for controlling the switching of the switchable material, notably L.C. material, to transmit the required sequen¬ ces of coloured lights can readily be devised by per¬ sons experienced in the relevant technology. For example, an electronic circuit for controlling the switching of the shutter would consist of inputs from the image-defining-means-driving circuit which define the start of the frames in the sequence of pictures required to be, e.g., red, green, or blue. These inputs would sequentially enable a driver circuit to apply voltages to the shutter material covering the red, green and blue filters so that only the red shutter was open when the image intended to be red was defined on the image forming means and so on.

The image-defining means may be of any kind capable of re-defining the image at least once for each transmission of each coloured light. Cathode ray tubes (black and white) could be of some limited use, but larger and more adaptable displays can be prepared using multiplexed arrays of pixels, various formats of multiplexed display being known. Preferably, the image-defining means will define the image in terms of light-transmitting and light-scattering (or substan¬ tially opaque) areas.

In a back-lit display, the light-transmitting areas of the partial image defined for each colour frame will allow the respectively coloured light to pass through to the viewers eye, while the scattering or opaque areas will block the light. The resulting differently-coloured partial images are integrated by the viewer's eye to provide a complete image in full colour, owing to the rapid transmission of the repeating sequences of colour frames, preferably at a rate faster than the flicker response of the human eye. In a front-lit display, the scattering areas of each partial image will reflect the coloured light to the viewer's eye, while the light-transmitting areas will allow most of the incident light to pass through away from the viewer.

A preferred form of multiplexed image-defining means uses L.C. material, preferably in the aforemen¬ tione_d form of encapsulated droplets or particles, with suitable transparent pixel electrodes (usually indium/tin oxide) controlled by field-effect tran¬ sistors for convenient addressing of the individual pixels. These systems can achieve high image resolu¬ tion using small individual pixels (e.g. each 1 to 4 square millimetres in area), and can do so over relati¬ vely large display areas while achieving acceptable production yields by using the techniques described in our copending British Patent Application No. 8617866, the disclosure of which is incorporated herein by reference. Larger or smaller pixels can also be used according to the eye and viewing distance of the _display. _The encapsulated L.C. material is preferably of the same particle or droplet size, with the same advantages, as hereinbefore described for the L.C. "shutter" material. Black guest/host materials are preferred for optimum "on/off" contrast.

It will be understood that the novel colour- filtering means comprising the switchable L.C. "shutter" is itself an aspect of this invention. This aspect accordingly provides a selective colour filter device comprising (1) at least two differently-coloured areas of colour filter material and (2) liquid crystalline material selected areas of which corresponding to the said coloured areas are arranged to be electrically switchable between a light- transmitting state and a second state so as to select w_hich of the said coloured areas will impart colour to light which will pass through the device in use. The preferred kinds and arrangements of the L.C. material apply as indicated above.

It will be seen that colour television and other coloured moving-picture displays can advantageously be based on the frame sequential colour display system of the present invention.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, wherein

Figure 1 shows schematically a backlit display system according to the present invention, and

Figure 2 shows schematically in enlarged detail a portion of the colour-filtering means of the system shown in Figure 1.

Figure 3 shows schematically a projection display system according to this invention.

Referring to the drawings, Figure 1 shows a light source 10 schematically depicted as a fluorescent tube, although incandescent filament sources or other sources could be used instead. Light from the source 10, indi¬ cated by the chained lines, reaches colour-filtering means 20 shown in schematic cross-section, comprising a striped colour filter member 22 having red (R) , green (G) and blue (B) stripes and a liquid crystal "shutter" layer 24.

As will be described in more detail hereinafter, the "shutter" layer 24 is shown as energised for the green part of the colour transmission sequence, those parts of the liquid crystal material corresponding to _the green colour filter stripes being subjected to an orienting electrical field which aligns the L.C. mole¬ cules so as to allow light to pass through, while the other parts of the L.C. material remain in a highly light-scattering unoriented state. Thus, light from source 10 passes through only the green filter stripes, and the resulting green light passes to light-spreading means 30, which spreads the green light more or less uniformly over the image-defining means 40, so that the viewer will see green light in the transmissive parts of the image. The red image and the blue image for each three-colour sequence are similarly transmitted to the viewer by orienting the appropriate parts of the L.C. "shutter" to let the lic,ht pass through the red filter stripes and the blue filter stripes respec¬ tively, with the image re-defined by means 40 to render transmissive only those parts of the total image area which are to be coloured red or blue respectively. Of course, those parts of the total image area which require mixing of the basic red, green and blue colours will be rendered transmissive for the required two, or for all three, of the colour transmissions in a cycle, as known per se.

The light-spreading means 30 is shown schemati¬ cally as spreading the light (chained lines) in the manner of a concave lens, but could in fact be in any convenient form, for example a diffusion plate posi¬ tioned either as shown or in contact with the image- defining means 40 (but not in contact with the colour-filtering means), or could be a Fresnel lens spaced from the image-defining means by a distance of more than twice its focal length, to allow spreading of the light after passing through the focal plane F-F (e.g. as indicated by the short-dotted lines), or could be a series of lenslets with focal length shorter than the lenslet plane-image defining means distance. In a projection system the colour shutter can be advan¬ tageously placed in the condenser part of the optics where the light is approximately parallel to the axis.

The image-forming means is shown schematically as the aforementioned preferred multiplexed L.C. display, the preferred matrix 42 of encapsulated droplets or particles of black guest/host liquid crystal material being between a transparent common electrode 44 of indium/tin oxide and a multiplexed array of transparent pixel electrodes 46, also of indium/tin oxide. The pixel electrodes are individually controlled by field- effect transistors which can be addressed electrically to render the L.C. material light-transmitting in selected pixels and light-scattering in others, thus redefining the light-transmitting image areas for each transmission of each coloured light, and enabling full- colour moving images to be generated.

Suitable control circuitry can again readily be devised along the lines hereinbefore described for controlling the switchable L.C. "shutter", which may be progressively switched from top to bottom to follow progressively scanned images.

Figure 2 shows in exploded detail the operation of the colour-filtering means 20, a transparent common electrode 23 being shown between the striped colour filter 22 and the matrix 24 of encapsulated L.C. material. Transparent strip electrodes 25 are arranged in register with the coloured filter stripes, that aligned with the green stripe being shown as energised to orient the underlying L.C. material so that it transmits the incident light (arrows 26). The L.C. material underlying the other electrodes 25, which are not energised, is shown schematically in random, light- scattering molecular arrangement, thus preventing substantially all of the incident light from passing through the red and blue filter stripes.

The electrodes 23 and 25 may be adhered to the opposite surfaces of the L.C. matrix 24, or may be coated on suitable transparent carriers such as glass plates (not shown) and held in non-adherent contact with the matrix. The colour filter stripes may be incorporated in a separate filter sheet or plate 22, or could be coated on or adhered to the common electrode 23 (or its carrier) or could be coated on or adhered to the individual strip electrodes 25 (or their carrier). Suitable materials for the colour filter stripes, for example coloured polyester film or printed stripes of suitable dyes or inks, can readily be selected by per¬ sons familiar with this field of technology. Interference coatings or plasma-deposited coloured coatings could also be used.

Figure 3 shows a projection display according to this invention, employing dichroic filters, in this case designed to operate at a 45° angle. A light beam having red, green and blue components (labeled r, g and b, respectively) from light source 50, which may be for example a quartz halogen or xenon short arc lamp with a suitable reflector, is collimated by reflector 51 and/or subsidiary lens 52. First dichroic filter 53 reflects the red but passes the green and blue com¬ ponents. The green and blue components are further separated by dichroic filters 54 and 55, which reflect green and blue respectively. Since the light incident on dichroic filter 55 has only a blue component, it may be replaced if desired by a plain mirror. The three transversely separated beams are recombined by further dichroic filters 56,57, and 58, to illuminate image forming display cell 59. Dichroic filter 56 is red reflective; dichroic filter 57 is green reflective, but permits red light to pass through; and dichroic filter 58 is blue reflective, but permits red and green light to pass through. Because only red light is incident en dichroic filter 56, it may be replaced if desired by a plain mirror. The image on display cell 59 is pro¬ jected onto screen 60 by projection lens 61. Intercepting the transversely separated beams are switchable shutters 62, 63, and 64, preferably made of an L.C. material as hereinabove described and also pre¬ ferably placed approximately equidistant optically from image forming display cell 59. During operation, shut¬ ters 62, 63, and 64 are independantly switchable bet¬ ween light transmitting and light blocking states, so as to controllably allow red, green or blue light to illuminate image forming display cell 59. For progressively scanned images, shutters 62, 63, and 64 may contain striped electrodes so that the clear por¬ tion of the respective shutter progressively increases during the scan. This gives a colour boundary that sweeps downwards on image forming display cell 59 as the information corresponding to that colour is changed progressively to that for the next colour. In this case a light spreading means 65, such as diffuser, may be placed in the light path to obscure the detail of the striped electrode structure.

Claims

--._>CLAIMS ;

1. A frame-sequential colour display system comprising (1) image-defining means for defining an image, (2) a light source, and (3) stationary colour- filtering means between the light source and the image- defining means, the filtering means comprising (4) at least one switchable material arranged to be repeatedly electrically switched between a colour-transmitting state and a second state so as to transmit to the image-defining means repeating sequences of differently coloured lights at a rate of at least 30 sequences per second, and the image-defining means being capable of redefining the image for each transmission of each dif¬ ferently coloured light.

2. A system according to claim 1 wherein the colour- filtering means comprises at least two differently coloured areas of colour filter material arranged to impart different colours respectively to transversely separate portions of the light transmitted from the source to the image-defining means, and the switchable material is switchable to cause light coloured by the respectively coloured filter areas selectively to be transmitted to the image-defining means.

3. A system according to claim 2, comprising two or more areas of each of the respective colours interspersed with one another in a sense transverse to the transmission path of the light to the image- defining means.

4. A system according to claim 3, comprising strips of the respective colours arranged substantially parallel to one another.

5. A system according to any of the preceding claims, wherein light-spreading means is positioned between the light-filtering means and the image—defining means to spread the respectively coloured lights in operation.

6. A system according to any of the preceding claims wherein a light combining means is positioned between the light filtering means and the image-defining means to combine the coloured light in operation.

7. A system according to any of the preceding claims, arranged to be capable of transmitting a red frame, a green frame and a blue frame in each sequence.

8. A system according to any of the preceding claims, wherein the switchable material is substantially transparent or highly translucent in its colour- transmitting state, and substantially opaque or highly light-scattering in its second state.

9. A system according to any of the preceding claims, wherein the switchable material comprises liquid crystalline material.

10. A frame-sequential colour display system comprising (1) image-defining means, for defining an image, (2) a light source, (3) stationary colour- filtering means comprising at least two differently coloured areas of colour filter material arranged to impart different colours respectively to transversely separate portions of the light transmitted from the source to the image-defining means, and (4) liquid crystalline material arranged to be electrically switchable between a colour-transmitting state and a second state so as to transmit to the image-defining means repeating sequences of lights coloured by the respectively coloured filter areas at a rate of at least 30 sequences per second, the image-defining means being capable of re-defining the image for each transmission of each differently coloured light.

11. A system according to any of the preceding claims, wherein the image-defining means comprises liquid crystalline material in a multiplexed pixel array.

12. A system according to claim 9, 10, or 11 wherein the liquid crystalline material is in the form of minute droplets or particles dispersed in a polymeric matrix.

13. A system according to claim 12, wherein substan¬ tially all of the particles or droplets are of average thickness less than 20 micrometres, preferably less than 10 micrometres, in the direction of the activating electrical field to be applied thereto in use.

14. A system according to any of claims 9 to 13, wherein the liquid crystalline material is in the form of a black guest/host material.

15. A system according to any of the preceding claims in the form of a back-lit display.

16. A selective colour filter device comprising (1) at least two differently-coloured areas of colour filter material and (2) liquid crystalline material selected areas of which corresponding to the said coloured areas are arranged to be electrically switchable between a light-transmitting state and a second state so as to select which of the said coloured areas will impart colour to light which will pass through the device in use.

17. A system according to any of the preceding claims in the form of a front projection display.

18. A system according to any of claims 1 to 16 in the form of a back projection display.

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