WO2006054056A1

WO2006054056A1 - Light selector for flat projection displays

Info

Publication number: WO2006054056A1
Application number: PCT/GB2005/004381
Authority: WO
Inventors: Adrian Robert Leigh Travis
Original assignee: Cambridge Flat Projection Displays Ltd
Current assignee: Cambridge Flat Projection Displays Ltd
Priority date: 2004-11-16
Filing date: 2005-11-15
Publication date: 2006-05-26
Anticipated expiration: 2007-05-16
Also published as: GB0425215D0; TW200617571A

Abstract

A transparent screen for use with flat projection displays comprises means such as an array of prisms (1) for deflecting rays incident at glancing angles on the screen through a given angle into the screen towards its normal. The rays are then condensed by suitable measures, such as curved facets (7), onto an array of points on the far side of the screen, where all rays except those originally incident on the rear at a glancing angle are absorbed. The screen can be used to select light projected into a tapered waveguide (4) so that only the light emitted at a glancing angle, after the first bounce not to be totally internally reflected, passes through the slits (2). The apparatus can be run in reverse as a camera.

Description

Light selector for flat projection displays

This invention is concerned principally with light- selecting or redirecting devices such as diffusers for flat projection displays or cameras. ^■

Rear-projection televisions are less expensive than plasma displays or liquid-crystal displays, but are bulky. WO 01/72037 by the present- inventor describes how to make a thin projection display by pointing a video projector into the thick end of a lightguide in the form of a wedge-shaped or tapered sheet. The injected light bounces off the faces of the lightguide at ever steeper angles, until it exceeds the initial angle and escapes. The greater the difference between the injection angle of a ray and the critical angle, the more times the ray must reflect internally off the faces of the guide before it emerges from the surface of the waveguide. The angle of injection therefore determines how far a ray is from the point of input when it emerges from the face of the guide, so that a magnified version of the projected image appears on the face of the guide.

Wedge-shaped lightguides are commonly used, as shown for instance in EP 663600 (Nitto Jushi), to spread the illumination from a fluorescent tube across the rear face of a liquid-crystal display, and rays emerge travelling towards the tip, but with varying degrees of divergence from the plane of the wedge. A film of extruded prisms, such as that shown in Figure 1 and described for instance in EP 762183, is commonly placed with the prisms against the lightguide so as to bend rays round approximately to the perpendicular angle, from which liquid-crystal displays are usually viewed. This technique can be applied to projector displays or, as described in the inventor's earlier WO 02/45413, cameras, but the requirements are stricter because scattered light should be suppressed. The diffusive screens commonly used in rear-projection television, on the other hand, are most effective when light is normally incident, so a Fresnel lens is often placed behind a diffusive screen as shown in Figure 2 so as to collimate light from the video projector. The diffusive screen often comprises on one side an array of lenslets and on the other side an array of slits, the lenslets being arranged to condense the light through the slits. This has the dual advantage of both diffusing the light with a controllable angle of divergence, and providing a front surface which is mostly black so that ambient illumination does not degrade contrast.

Conventional wedge-shaped or tapered lightguides emit light travelling towards the tip, nearly parallel to the surface, because the light has only just escaped the total-internal- reflection regime. As a result the emission is ill prepared for the kind of diffuser conventionally used in rear-projection television. This is not only because rays travel in the wrong direction to be collimated by the Fresnel lens. It is also because, as explained in the applicant's earlier WO 02/060187, conventional wedge lightguides have straight sides (i.e. flat faces), so the emerging rays have periodic angles of divergence, which means that they cannot be collimated so as to be normally incident on the diffusive screen.

There is the further problem in flat projection using the tapered waveguide technique that rays incident on a material/air surface at slightly less than the critical angle are not in fact transmitted without reflection, so a component is reflected back into the guide and goes on to cause a ghost image at the next bounce or bounces. The reflection can be reduced by an anti-reflection coating but not eliminated; some means of blocking these ghost rays is thus required.

A wedge lightguide can be made to work as a flat-panel camera, as is explained in WO 02/45413. Ghost images are formed in the flat-panel camera for the same reason as in flat projection: rays are partially reflected when incident on a dielectric interface.

WO 03/013151, also by the present applicant, explains how to design a tapered lightguide so that the emitted rays are all close to parallel. It also explains why the angle at which ghost rays emerge into air is substantially different from that of the wanted rays, and shows how a set of louvers aligned to pass the wanted rays will absorb the ' ghost rays. However, this arrangement does not take advantage of all the technology that has been developed to make diffusive screens for rear-projection television.

According to the invention there is provided a transparent screen comprising means for deflecting rays incident at glancing angles on the rear face of the screen through a given angle into the screen towards its normal, means for condensing the rays onto an array of points on the front face of the screen, and means for absorbing substantially- all rays except those originally incident on the rear face at a glancing angle. Here "front" and "rear" are intended to refer to the relation to a viewer.

The screen is preferably used with a tapered waveguide to make a camera or projector. Here "tapered" is to be taken to include any profile that makes internal bounces progressively steeper; this can also be done by GRIN techniques, for instance. Light emerging at the intended place from such a tapered panel will always be at this glancing angle, because it is the first "bounce" to be just steep enough relative to the surface of the panel to escape the total-internal-reflection regime and thus to emerge. The glancing angle is just less than the arcsin of the reciprocal of the refractive index of the material through which the ray is travelling.

The deflecting means can be an array of prisms, for instance, arranged in parallel fashion transverse to the direction of propagation through the tapered waveguide. Alternatively it can be a mirror-type array, preferably embedded in a transparent material. The condensing means can be the same as or separate from the deflecting means and would normally include curved reflecting or refracting surfaces focussing parallel rays in transverse strips onto slit-shaped apertures in a light-absorbent material.

In this way the "ghost bounces", that emerge at steeper angles further along the tapered waveguide than the intended point, are focussed by the condensing means onto the absorbing means and do not spoil the image seen by the viewer. Of course, the absorbing means can also act in the usual way to absorb ambient light.

For a better understanding of the invention, embodiments will now be described by way of example, referring to the accompanying drawings, in which:

Figure 1 shows the prismatic sheet conventionally used with a wedge lightguide in the backlight of a liquid-crystal display;

Figure 2 shows a particular kind of diffuser that is sometimes used in conjunction with a Fresnel lens in rear-projection televisions;

Figure 3 shows a first embodiment of this invention comprising a prismatic sheet and a lenslet array on which is printed a pattern of slits opposite the lenslets;

Figure 4 shows a second embodiment of this invention, in which the pattern of slits is printed on the prismatic sheet opposite the prisms, and one facet of the prisms is curved so as to condense light;

Figure 5 shows how the pattern of slits can be separated into two, the combined action of the two layers being such as to absorb skew rays; Figure 6 shows how the slit itself can be embossed with a lens or grating so that it diffuses light with the required angle of divergence;

Figure 7 shows how the diffuser can be bonded to the wedge lightguide by a glue of low refractive index; and

Figure 8 shows how the prisms can be curved so as to collimate rays whose wave vector has a component perpendicular to the wedge axis.

Figure 3 shows a first embodiment of the present invention, in which there is provided a transparent sheet flat on one face and embossed on the other with a prism array 1 shaped like an extruded sawtooth, and a second transparent sheet embossed on one side - the rear as seen by a viewer - with an array of lenslets 3 facing the flat side of the first sheet, and on the opposite side with an array of slits 2, one opposite each lenslet. The prisms and slits typically have a pitch of about lOOμm.

The prisms 3 of the prism sheet are applied to or placed against the exit face of a tapered waveguide 4 into the thick end of which a projector injects an image, formed by rays 5, whose angle at a given point is a function of row height in the projector image.

The angles of the prism facets are such that rays emerging from the tapered waveguide 4, which are parallel and incident at a glancing angle on the array and perpendicular to the direction of extrusion, are refracted by the first facet, as the light enters the prism, and reflected by the second so as to emerge approximately perpendicular to the opposite surface of the sheet. These rays then pass through the lenslets 3 and are condensed onto the slits 2 in such a way that that light incident perpendicular to the plane of the sheet is condensed by each lenslet through one slit. The thickness of the sheet thus corresponds to the focal length of the lenslets.

Preferably the dimensions of the slits are such as to pass substantially all unscattered light emerging from the tapered waveguide display 4 after a specified number of reflections, but to block all scattered light and all light 6 which has undergone any but the specified number of reflections, i.e. the number of reflections at which the light just exceeds the critical angle of incidence. Although the change in angle within the waveguide of a ray that has undergone one extra double bounce is only marginally greater (e.g. a fraction of a degree) than the previous bounce, the change of angle outside the waveguide is usually much greater (several degrees) , so a high selectivity can be achieved. For further detail see

WO 02/60187 mentioned above. Optionally there may also be provided a diffuser which rays pass through after leaving the lenslet and slit array in order to achieve complete diffusion.

The disadvantage of providing three separate sheets is that there are three times as many material/air interfaces and therefore three times as much reflection as there is with a single diffuser.

A second embodiment of the invention is therefore described, as shown in Figure 4, in which there is provided a transparent sheet on one side of which is embossed an extruded array of prisms, one facet 7 of each prism, namely the second one encountered by a ray, being curved so as to condense rays as they undergo refraction and reflection. On the opposite side of the sheet is printed an array of slits 2, and the radius of prism curvature and slit position should be such that parallel rays 5 incident at a glancing angle on the rear of the sheet are fully condensed as they reach the opposite side of the sheet and pass through a slit. The light from a video projector is strongly directional, so that the ratio of the slit width to the focal length of each prism or lenslet can in principle be small. Small slits are difficult to print, but if the focal length of the prisms or lenslets is increased then there is a chance that rays from one prism or lenslet might stray through the slit intended for another. This can be prevented by making the slits very deep so that each acts as a tunnel whose walls absorb any skew rays. Figure 5 shows how the same effect can be achieved by splitting the slit array into two layers, one layer 2a being embedded within the transparent sheet at an appropriate depth below the second layer 2b so as to block both undesired rays 6a and 6b.

If the focal length of the lenslets is increased so as to ease the precision required to print the slits, then the angle at which rays diverge thereafter is reduced. Preferably, therefore, the slit is embossed with a lenslet or grating 8 so as to diffuse the transient light with the angle of divergence required for diffusion, as shown in Figure 6.

In a typical RPTV system such as shown in Figure 2 the thickness of the lenslet sheet might be comparable to the spacing of the lenslets; the slits run vertically, and a wide diffusion in the horizontal direction - say about 120° - is thus achieved. In the present invention, for the purpose of ghost elimination, the sheet thickness might be 1 mm when the width of the lenslets is 0.1 mm, which gives rise to an angle of diffusion of about 9°. In embodiments using the tapered waveguide technology the direction of taper is typically vertical and for normal purposes one does not want to diffuse too much in the vertical (because that wastes light) , so the narrow angle of diffusion is quite satisfactory.

It would however be possible to run the axis of the wedge- shaped waveguides horizontally, and the slits vertically, in which case ghost elimination and diffusion can be performed by the same component having a big width/focal- length ratio, say with a much thinner sheet 1.

Other ways in which prisms might be made to condense light are by curving the first surface so that it acts like a lens, or by embossing either the first or second facet or both with a suitable hologram. Furthermore, instead of embossing the bottom of the sheet with prisms one could leave it smooth and replace the prisms with a series of curved mirrors internal to the sheet, all designed to deflect light from the critical angle to the normal, as shown in Figure 7. A graded-index coupling sheet 20 is preferably placed between waveguide and condenser sheet, to reduce losses.

Rays from a video projector diverge, and while the profile of WO 03/013151 ensures that the out-of-plane angle of all rays is the same just before they emerge from the wedge, the in-plane angle varies. Figure 8 shows how this can be corrected by curving the prisms in the plane to run in a generally arcuate shape optically centred on the projector (or camera) 30, so that after deflection all wanted rays are travelling perpendicular to the plane of the sheet. The slits of course follow the shape of the prisms. This concept is generally applicable to wedge-type displays, whether or not they use the light selector of the present invention. The invention is therefore also concerned in another aspect with transparent screens comprising means for deflecting rays incident at glancing angles on the front face of the screen through a given angle into the screen towards its normal, in which the deflecting means comprises elongate elements arranged generally in parallel but curved in the plane of the screen so as to divert rays, emanating from a point source and incident on the screen, uniformly in the said normal direction.

Figure 8 also shows an input or "expansion" waveguide 40 which allows the light to spread (or condense) laterally to cover the width of the tapered waveguide, which is much larger than that of the projector. This input waveguide, which is flat, can be folded under the main (tapered) waveguide for a compact layout.

When a flat projection system is to be used in reverse as a camera, diffusive elements 8 should be removed from the screen, but the remaining elements 1, 2 and 3 or 7 should be left in place. Light incident on the screen and passing through the slits 2 will be collimated by the lenslets 3 or curved facets 7 and bent by the curved facets 7 or prisms 1 so as to enter the tapered lightguide 4 at an angle, after which the ray will undergo total internal reflection and emerge at a unique angle at the thick end of the lightguide, giving rise to the captured image. Rays that would otherwise be partially reflected at the rear of the wedge 4 so as to cause a ghost image cannot enter the system because they are blocked by the slits 2.

Claims

1. A transparent screen comprising means (1; 10) for deflecting rays incident at glancing angles on the front face of the screen through a given angle into the screen towards its normal, means (3, 7; 10) for condensing the rays onto an array of points on the rear face of the screen, and means (2) for absorbing all rays except those originally incident on the front face at a glancing angle.

2. A screen as in claim 1, in which the deflecting means includes an array of mirrors (10) whose curvature is such as to condense light onto the array of points.

3. A screen as in claim 1, wherein the deflecting means includes a set of prisms on the front face of the screen, their axes perpendicular to the said angle of incidence.

4. A screen as in claim 3, wherein the condensing means is formed on or by one of the facets (7) of the prisms.

5. A screen as in any of claims 2 to 4, where in the prisms or mirrors are curved in such a manner as to ensure that rays diverging in a plane parallel to the screen are collimated after undergoing reflection.

6. A screen according to any preceding claim, wherein the absorbing means includes a set of slits (2) in opaque material on the rear of the screen.

7. A screen as in claim 6, wherein the array of slits is split into two or more layers (2a, 2b) so that light can pass through the slits in only one general direction.

8. A screen as in claim 6 or 7, wherein means (8) for diffusion are provided at each slit.

9. A light guide including a screen as in any preceding claim, whose front face is applied to one face of a tapered waveguide so as to extract light emerging from that face at a glancing angle.

10. A projector comprising a lightguide according to claim 9, and a video projector (30) adapted to inject an image into the thick end of the tapered waveguide.

11. A camera apparatus comprising a lightguide according to claim 9 and a small camera adapted to receive an image from the thick end of the waveguide.

12. A transparent screen comprising means (1; 10) for deflecting rays incident at glancing angles on the front face of the screen through a given angle into the screen towards its normal, in which the deflecting means comprises elongate elements arranged generally in parallel but curved so as to divert rays emanating from a point source uniformly in the said normal direction.