WO1998052359A1

WO1998052359A1 - Display system

Info

Publication number: WO1998052359A1
Application number: PCT/GB1998/001396
Authority: WO
Inventors: William Alden Crossland; Anthony Bernard Davey; Vincent Glenn Geake; Ian David Springle; Timothy Martin Coker
Original assignee: The Secretary Of State For Defence
Priority date: 1997-05-15
Filing date: 1998-05-15
Publication date: 1998-11-19
Also published as: GB9709926D0; AU7440698A; EP0981911A1

Abstract

The PLLCD (photoluminescent LCD) principle is applied to a projection display. A source (11) of largely monochromatic, near-visible UV light directs excitation light through a liquid-crystal (or acousto-optic) modulator (15). The photo-luminescent materials (31) that emit visible light when struck by the excitation light are mounted on a screen (3) and the modulated excitation light is projected onto this screen by a projector (17) in order to form a visible image. The image is controlled by a feedback mechanism involving reflective spots (33) on the screen, a video camera (121) and a controller (23). Alternatively a scanning technique can be used. Since only monochromatic light is required for the projection the system can be considerably simpler than RGB projection displays.

Description

DISPLAY SYSTEM

The invention is concerned with displays, particularly with liquid-crystal displays (LCDs) of the photoluminescent LCD or PLLCD type, that is to say ones in which excitation light, usually in the near-visible ultra-violet, is modulated by a liquid-crystal layer to impinge upon an output layer or screen, which may be made of phosphor dots or pixels. Various architectures for PLLCDs have been proposed, in which the location of the output material which converts the UV to visible light varies. In some cases this material may actually be within the LC cells (see US-A-4830469 (US Philips)), in other cases, e.g. WO 95/27590 (Crossland et al) or EP-A1-0522910

(Thomson-CSF) , the material is on the front of the cell (the normal layout, possibly with a polariser in between) , and in others the material is mounted on a separate screen optically separated from the cell (e.g. in EP-A-185495 (Canter) or other TIR systems, where the separation is necessary to ensure optical decoupling of the excitation light) . As the phosphor material becomes more distant from the LC shutter, a greater degree of collimation is required of the light emerging from the LC cell in order to avoid crosstalk.

This material can be of various types, such as fluorescers which emit a visible wavelength solely whilst being excited, phosphors which emit visible wavelengths during and for a time after excitation, and photochromies which change their absorption characteristics during and (for a time) after excitation.

The invention concerns a display device including: a liquid-crystal modulator comprising liquid-crystal or other optical elements adapted to modulate activating light, a screen having display elements activatable by ' the activating light, and a projection means for directing the modulated light from the liquid-crystal elements across free space and on to the corresponding elements on the screen. As the display elements, which may be of phosphor material though photochromies are conceivable, are moved away from the LC module to form a free-standing screen the display becomes what is known as a projection display, where the screen is large in comparison to the LC layer. In this case the phosphors are on the viewing screen, and the UV light is projected onto it. The screen is viewed either from the side from which the UV is incident, or from the far side . An advantage of this scheme is that the image is projected by a single projection head working at a narrow range of wavelengths, rather than the three projection heads at relatively broad wavelength bands (R,G,B) or a single projection head with a full visible spectrum colour image. Furthermore, the optics, especially in the projection head, are not affected by chromatic abberation and can therefore be simpler than in prior art devices, such as those in which light modulated by a liquid crystal is projected onto a reflective screen, which is then viewed by the audience .

The screen for such a projection display preferably includes an additional layer or layers to improve the efficiency by reflecting forwards (towards the viewer) the visible light emitted backwards that would otherwise be lost. Simplistically this would increase the efficiency of the overall display system by up to 100%. Depending on the configuration of the display, that is, rear or front projection, different types of reflecting layer would be required.

For front projection the layer would simply be required to reflect the whole visible spectrum; an enhanced metallic reflector might be most suitable for this. On the other hand the requirement for rear projection is slightly more complex in that, whilst the visible light is still reflected, the exciting light, most probably UN, must be transmitted with as little loss as possible. For that reason an interference filter, made from a plurality of dielectric layers and designed to meet these requirements, would be employed. The liquid-crystal display preferably also includes a source of collimated monochromatic light. This may be a laser, or a UV tube and a suitable collimator .

The display may in one embodiment have a single modulating cell, or at least a subset of the total number of pixels of the screen, such as one line, in which case the projection head scans the modulated light across the screen. For a single cell raster- scanning would be used, and for a line of cells a simple vertical scan suffices. A particularly powerful source of monochromatic light, such as a laser, is required in this version of the invention. Moreover, for video capability a very rapidly modulating liquid crystal should be used, such as a ferroelectric, or else a different kind of modulator such as an acousto-optic modulator. The modulator should have a low absorption in its transmissive state, and so be less likely to overheat. A cholesteric- mirror liquid-crystal arrangement would be suitable here. It is also preferable in this version to use a true phosphor, which emits light for a short period after the excitation has finished.

Alternatively the display can have a modulator with a plurality of liquid-crystal cells each separately modulating the monochromatic light. The whole screen, or a substantial part of it, can thus be simultaneously illuminated without the need to scan the modulated light.

In order to achieve the scanning or projection accuracy required to illuminate the correct pixel some distance from the liquid-crystal cell, a feedback system is advantageously provided in order to align the system. The system can comprise a series of reflective points on the screen, the reflections from which are detected at the projector and the projecting head adjusted accordingly. The distance from the projection head to the screen also needs to be accurately controlled, or at least determined, and a system of dots may be provided to range the screen, which may be the same as the reflective dots used for alignment.

The projection may be from either in front of or behind the screen. When the projection head is located in front of the screen, i.e. on the same side as the audience, then the monochromatic activating light can be short-wavelength visible, so as to avoid any concerns, however unjustified, about safety; visible- light excitation of phosphor is described for instance in PCT/GB 97/878.

In an alternative arrangement the projection arrangement may be behind the screen. In this case the screen may block the monochromatic light, which allows the frequency of the light to be chosen freely; it may be for example UV light, especially UVA.

Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying figures, which show schematic representations of the embodiments.

In Fig. 1 the liquid-crystal display has a projector 1 and a screen 3. The audience sits on the other side of the screen 3 from the projector 1.

The projector 1 has a high-intensity source 11 of essentially monochromatic UV light at a suitable wavelength, such as 365 nm, which is widely available, or (preferably) 388 nm, which is passed through a collimator 13 to provide collimated light.

The collimated light then passes through a liquid- crystal cell 15 that modulates the light. In order not to absorb too much of the high-intensity light, a cholesteric liquid crystal is used with circularly polarised light, as described in WO 97/05520.

The light then passes through the projection head 17. The projection head 17 diverges the collimated light in a predetermined manner. Both the precise direction of projection and the amount of divergence are controllable by a controller 23 connected to a video camera 21, as explained below. The light is then incident on a screen 3. The screen is made of a material that is opaque to ultraviolet light at the wavelength used, but transparent to visible light. The screen 3 is approximately 2 m wide by 1.3 m high.

A large number of phosphor dots 31 are provided on the interior (projector-side) surface of the screen 3, arranged in groups of three phosphor dots, each group having one phosphor dot that emits red light when hit by the UV light, one phosphor dot that emits blue light and one phosphor dot that emits green light. The phosphor dots may be made of standard phosphors, for example those described in the aforementioned patent US-4830469. The screen 3, in this example, has 625 rows of 1000 groups of phosphor dots: each group covers an area of approximately 2 mm by 2 mm. The screen 3 also has four highly reflective points 33 arranged on the projector side of the screen 3 for alignment purposes, preferably on the periphery so as no to interfere with the image.

The light from the projection head must be very accurately aligned with the individual pixel on the screen. In a rigid system this could be achieved by manual adjustment, but in this embodiment it is done by means of adaptive optics. The camera 21 records an image of the rear of the screen, which is then passed to the controller 23. The controller 23 is a small computer that determines the position of the four reflective dots 33 in the image. This analysis of the image is not difficult, since these four dots dominate the image. The controller determines from the position and spacing of the dots in the image how far away the screen is and its precise direction, and adjusts the projection head for direction and divergence of the light accordingly.

Fig. 2 shows a schematic section through the screen 3 representing a slight variation on the screen of Fig. 1 in that the phosphors are on the far side from the projector, while it has in common that it is to be viewed from the far side of the screen. The screen is based on a sheet substrate 37 of plastics material, on which the phosphor layer 31, here shown as a continuous layer, is formed. Between the substrate and the phosphors is a selectively reflecting layer 35. This layer is in essence an interference filter formed of a stack of layers of refractive indices chosen to pass the activation light (UV) but to reflect all visible wavelengths.

The effect of the filter is almost to double the output of the display because the light from the phosphors 31 is all directed towards the viewer, whereas without the filter half of it would have been directed backwards and would have been lost in the system.

The above embodiment uses amplitude or intensity modulation and either full -image projection or raster scanning on to the screen. It would also be possible to use phase modulation to produce the image. Here the liquid crystal is addressed so as to produce a diffraction pattern which has the effect of diffracting the input light in some directions and not others, giving rise to an image. Again this method can be applied with either a full-frame or a raster-scan technique .

Fig. 3 shows a second embodiment of the invention, illustrating the scanning technique. For visible-light laser-scanned displays reference may be made, for instance, to US 5646766 (Conemac/ Advanced laser Technologies Inc.), US 5311321 (Crowley/ Corporation for Laser Optics Research) or US 5424771 (Yu/ Samsung) . In the present embodiment a UV or near-UV visible laser 41 emits a light beam which is modulated by a liquid- crystal modulator 45 or, as in US 5311321, an acoustic- optic modulator. The beam is then directed at a screen 3 of similar constitution to that in the previous embodiment .

This architecture is analogous to the traditional CRT but has significant advantages: • Light path folding techniques can be used to make it more compact ;

• A large glass envelope that contains a vacuum is not required; and

• The screen can be flat and of arbitrary size as it is not limited by the size and weight (not to mention bulk) of the glass envelope. The viewing characteristics remain those of a CRT otherwise, and in contrast to prior-art laser projectors the projection optics need not be corrected for chromatic aberrations because the projected light is largely monochromatic. Moreover the fact that only one laser, rather than three, is needed is an enormous simplification.

Ideally the excitation laser would be UV, but the principle would still work with a visible laser

(although the coating mentioned above would not be as effective) .

In the scanning type of embodiment the "divergence" is effected by the modulator unit 45 itself, which simultaneously controls the amplitude modulation of the laser beam and its direction, so no projective optics are required.

As a further modification, the laser and the scanning and modulating device could be replaced by a CRT laser screen such as is described in Nasibov et al , "Full colour TV projector based on A₂B₆ electron-beam pumped semiconductor lasers", Journal of Crystal Growth 117 (1992) pp. 1040-1045 or US 5339003 (Koslovsky et al) . Here the phosphor screen of a CRT is replaced by a layer of lasing material pumped locally by the electron beam so as to emit electromagnetic radiation. In this case projection optics might be required in order to project the modulated light correctly from the CRT laser screen onto the viewing screen. Even so these optics would be considerably simpler than current projection optics since they do not need to correct for chromatic aberration, the projected light being largely monochromatic .

Claims

1. A display device including: a modulator (15, 45) comprising at least one element adapted to modulate activating light, a screen (3) having display elements (31) activatable by the activating light, and a means (17, 45) for directing the modulated light across free space and on to the respective elements on the screen.

2. A display as claimed in claim 1, in which the screen (3) includes a reflecting layer (35) behind the display elements (31) as seen by the viewer, adapted to reflect any rearwardly directed light from the display elements back towards the viewing side.

3. A display as claimed in claim 1 or 2 , in which the excitation light is projected on to the front, i.e. viewing, side of the screen.

4. A display as claimed in claim 1 or 2 , in which the excitation light is projected on to the rear of the screen.

5. A display as claimed in any preceding claim, in which the display elements are made of phosphor material or of photochromic material .

6. A display as claimed in any preceding claim, in which the excitation light is modulated simultaneously by a plurality of elements.

7. A display as claimed in claim 6, in which the means for directing the modulated light is in the form of a diverging projection head.

8. A display as claimed in any of claims 1 to 6, in which the modulated excitation light is scanned across the screen.

9. A display as claimed in any preceding claim, in which the excitation light is in the ultra-violet or short-wavelength visible range.

10. A display as claimed in any preceding claim and including an adjustment means (23) for aligning the modulated light with the screen.

11. A display as claimed in claim 10 and further including a feedback system (21, 23) for controlling the alignment accuracy between the optics and the screen.

12. A display as claimed in claim 11, in which the feedback system includes a video camera (21) monitoring the screen.

13. A display as claimed in any preceding claim, in which the modulator (15) is a liquid-crystal device.

14. A display as claimed in claim 13, in which the liquid crystal is cholesteric, nematic or smectic.

15. A display as claimed in any preceding claim and further including a source (11, 41) of excitation light.

16. A display as claimed in claim 15, in which the source is a laser.