GB2354838A - Rear projection apparatus with Fresnel lens - Google Patents

Abstract

Rear projection apparatus (30) for displaying images (32), which apparatus (30) comprises a curved rear projection screen (34), at least one projector (36) for projecting the images (32) on to a rear surface (38) of the projection screen (34), and at least one fresnel lens (40) positioned between the projector (36) and the rear surface (38) of the projection screen (34). The projector (36) may be a polysilicon projector and can include a small exit pupil.

Description

2354838 REAR PROJECTION APPARATUS FOR DISPLAYING IMAGES This invention

relates to rear projection apparatus for displaying images.

Different types of1projection apparatus for displaying images from a projector, having a single exit pupil on to a rear surface of a curved rear projection screen are well known. Also well known are the problems that arise from such rear projection apparatus. One known problem is that of geometry errors. More specifically, because the image is being viewed away '!from the projection point at the projector, distortion correction is required in the image. Another known problem is that of luminance variation. An observer tends to see a different light level from the same point on the image as the observer moves around or, alternatively, the observer sees an "incorrect" light level from different parts of the image. The problems of the geometry errors and luminance variation occur because projected light rays from the projector are diverging away from the observer when they intersect with the curved rear projection screen, except of course for those rays that are on-axis with respect to the curved rear projection screen.

It is an aim of the present invention to obviate or reduce the above mentioned problems.

Accordingly, in one non-limiting embodiment of the present invention there is provided rear projection apparatus for displaying images, which apparatus comprises 2 a curved rear projection screen, at least one projector for projecting the images on to a rear surface of the projection screen, and at least one fresnel lens positioned between the projector and the rear surface of the projection screen.

By obviating or reducing the above mentioned known problems of the known rear projection apparatus, the rear projection apparatus of the present invention lends itself to the design of continuous, large field of view, bright, potentially high- resolution, low maintenance, compact rear projection apparatus, with no structures on the image side of the screen. Applications for such rear projection apparatus are popped-hatch, train (and other simulators where cab shadowing is a substantial problem) and small single/dual user visualisation systems.

The rear projection apparatus of the present invention is preferably one in which the fresnel lens is positioned away from the rear surface of the projection screen. if desired however, the fresnel lens may be positioned so that it is touching the rear surface of the projection screen.

The fresnel lens may have a focal length which is different from a theoretically required focal length, thereby to remove fresnel lens distortions and inherent geometric errors. Alternatively, the fresnel lens may have a barrel distortion which is equal and opposite to distortions from the projection screen. The barrel distortion then ensures that there is no requirement for 3 the fresnel lens to solve the distortion problem, although the luminance variation, will then still be present and will need to be addressed separately.

The rear projection apparatus of the present invention may be one in which the f resnel lens is such that it has a plurality of rings with each ring havin g a prism angle calculated to direct all rays from the projector towards a design eye point of the apparatus.

The fresnel lens may be configured to direct light rays from the projector some distance behind a design eye point of the apparatus.

Usually, the projector will have a single exit pupil.

The projector may have a small exit pupil and hence a large depth focus. An example of such a projector is a polysilicon projector.

The projector may be a fixed matrix projector, for example a liquid crystal device or a digital micromirror device fixed matrix projector.

The apparatus of the present invention may include a plurality of the project,ors and a plurality of the fresnel lenses. In this case, there will usually be one of the fresnel lenses for each one of the projectors.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which:

Figure 1 shows known rear projection apparatus for displaying images; 4 Figure 2 shows an example of incurred image distortions obtained with the rear projection apparatus shown in Figure 1; Figure 3 shows back projection gain lobe effects; Figure 4 shows the operation of a fresnel lens; Figure 5 shows the action of a fresnel lens when. illuminating away from focal length; and Figure 6 shows rear projection apparatus in accordance with the present invention.

Referring to Figure 1, there is shown known rear projection apparatus 2 for displaying images 4. The apparatus 2 comprises a curved rear projection screen 6, and a projector 8 for projecting the images 4 on to a rear surface 10 of the projection screen.6.

One of the problems afforded by the apparatus 2 is that of geometry errors. Because the images 4 are being viewed away from a projection point 12 provided by the projector 6, distortion correction is required in the images 4. In typical rear projection apparatus, the distortion correction would be applied in the projector 1. However, in the present rear projection apparatus of the present invention, it may be preferred to utilise a fixed matrix projector such for example as a liquid crystal device projector or a digital micromirror device projector. Such fixed projectors do not inherently have a distortion correction capability, although it is possible to apply distortion correction to the image by digitally re-mapping, but this in itself has undesirable consequences. The resulting image utilising a fixed matrix projector has a pin-cushion distortionli which is shown in Figure 2. The severity of the disto Ttion is dependent upon the image channel field of view,, and the screen radius. With the apparatus of the present invention which includes a fresnel lens, the severity of the distortion is also dependent upon the focal length of the fresnel lens. The distortion problem is significant for back projection apparatus having a single projector and therefore a single channel projection, and the problem increases when a plurality of 10 i projectors are employed which use channels which are tiled together to achieve a larger field of view.

Another known problem is that of luminance variation and this known problem::will now be discussed with reference to Figure 3.

In Figure 3, there is shown a projector 6 which is projecting on to a flat back projec tion screen 14. Images 16 from the projector 6 cause back projection gain lobes shown as gain lobes 18, 20. An observer is shown schematically as observer 22.

Ideally, the light distribution characteristics of the back projector screen 14 would be lambertian, that is the observed light level at any one point of the image 16 would be the same irrespective of viewing location of the observer 22. However, the reality is that all back projection screens have a gain lobe 18, 20 that is not 6 lambertian. The result of this is that the observer 22 sees a different light level from the same point on the image 16 as the observer 22 moves around or, alternatively, the observer 22 sees an "incorrect" light level from different parts of the image 16. This is shown in Figure 3 where the arrow length within the gain lobe 18 represents the relative intensity that would be observed from that direction. It will be seen that the longer the arrow, the brighter the image. For the on-axis light rays 24, the observed light level is good. However, as the light rays get more off- axis, the observed light level decreases in accordance with the screen gain lobe 18, 20. As can be seen from Figure 3, the gain lobes 18, 20 are 11centred" on their incident ray angle. The observed light level decrease is indicated in Figure 3 by the shorter length arrow towards the observer 22 in the gain lobe 20, this gain lobe 20 being at the edge of the image channel 25 as shown. If the same principles are applied to a curved back projection screen, it can be seen that the used component of the gain lobe is even worse, f or the channel edges. For example, gain lobes can be visualised around the rays forming the channel edges in Figure 1.

The above mentioned problems of geometry errors and luminance variation occur because the projected rays are diverging away from the observer when the 'intersect with the screen, except for those rays that are on- axis. Conceptually, the ideal environment would be a projection 7 device that, instead of projecting from a point source outwards, projected from a 2-dimensional plane into a point at the observer 22. This would effectively create a,logical display plane at the projector 6 that the observer 22 directly views. The present invention is based on a technique that emulates exactly this conceptually ideal environment. More specifically, the present invention utilises a fresnel lenb positioned between a projector and a screen.

It has been stated above that Figure 3 shows the main lobe effect with a flat screen, and that a curved screen would give an even greater gain lobe effect. In f act there is another ef f ect, which causes reduced luminance at the channel edge when projecting on to the outside of a curved screen. When moving from the channel centre to the edge, the throw distance becomes aggressively greater for the curve screen than for the corresponding flat screen. The inverse square law thus dictates that the illumination will be correspondingly less towards the edge than for the flat screen.

In addition, when moving from the channel centre to the edge, the illumination becomes progressively more oblique on the curved screen compared to the flat screen, again causing a reduction in luminance. There would thus be a very severe luminance fall-off if projecting on to the outside of a curved screen but the use of the fresnel lens compensates for these luminance effects, in addition to the 8 gain lobe effect. The luminance uniformity would be similar to projecting on to the inside of the curved surface.

Referring now to Figure 4, there is shown the operation of a fresnel lens 26. The fresnel lens 26 is a flat lens that is typically used in an illumination path to control the direction of light. Figure 4 shows how, if a point light source 28 is placed at the focal length 'If" of the fresnel lens 26 and radiates light, the direction of all light rays beyond the fresnel lens 26 will be parallel.

The fresnel lens 26 refracts rays of light by an angle 9 based on the height away from the fresnel lens centre "h" and the fresnel lens focal length Ilf 11. Mathematically, this can be expressed as the following equation:

9 = tan -1(h) f Referring now to Figure 5, there is shown the same f resnel lens 26 which is utilised in Figure 4. However, the point light source 28 has been placed further away from the fresnel lens 26. The fresnel lens 26 thus acts to focus the light to a point in order to satisfy the above mentioned equation.

The arrangement shown in Figure 5 provides a mechanism for orienting light towards a point in space. Now consider the case where the point light source 28 is a projector, and an observer is located where the light rays are 9 focused, that is at a point where the rays converge off to the right as shown in Figure 5. If a diffusing back projection surface is located immediately after the fresnel lens 26 such that an image is formed, then the following statements are conformed to.

1. The observer will see a uniformly illuminated image irrespective of back projection gain lobe, assuming that the projected image is uniformly illuminated. This is because each pixel of the image is on-axis to the observer as described above with reference to luminance variation.

2. The image will be geometrically correct irrespective of the shape of the back projection surface. This is because the f,resnel lens-is creating a virtual display plane such that, assuming that the image is generated with the appropriate logical display plane, there are no geometric distortions.

The fresnel lens characteristic can be chosen to illuminate barrel distortions inherent in many of the lenses fitted to fixed matrix projectors.

It will be appreciated from the above description that Figure 4 shows rays emanating from the focal point of a fresnel lens, whilst Figure 5 shows-the same lens with rays emanating from a point displaced backwards from the focal point. It has been mentioned above that the rays will be focused to a point. In fact, if the fresnel lens is designed to give exactly parallel light when used in the situation in Figure 4, then there will be some aberration in Figure 5, so that the rays will not all converge to a point. For many applications of rear projection apparatus of the present invention this may well be acceptable. However, if required, the fresnel lens may be designed with the prism angle of each ring calculated to direct all rays from the projection point towards the eye point.

Referring now to Figure 6, there is shown rear projection apparatus 3.0 which is in accordance with the present invention and which is for displaying images 32. The apparatus 30 comprises a curved rear projection screen 34, a projector 36 for projecting the images 32 on to a rear surface 38 of the projection screen 34, and a fresnel lens 40 positioned between the projector 36 and the rear surface 38 of the projection screen 34. As can be seen from Figure 6, the fresnel lens 40 is positioned away from the rear surface 38 of the projection screen 34.

The fresnel lens may be such that its pitch is chosen so as not to "beat" with the pixel structure or degrade resolution. Imperically finer would appear to be better.

The physical size of the projection screen 34 may be limited by the ability to purchase and manufacture large fresnel lenses 40 of the required specification.

If the fresnel lens 40 is placed against the rear surface 38 of the projection screen 34 at the tangent, then there will be no optical overlap available at all. In practice it would probably be better to locate the fresnel lens 40 slightly away from the projection screen 34 as shown in order to allowfor a small blend. This will also provide room for supporting structures etc.

If the fresnel lens 40 is not located physically against the projection screen 34, then there is a focus implication. The severity of this will depend upon the channel configuration and the distance between the fresnel lens 40 and the projection screen 34. It can be seen from Figure 6 that there will be a f ocus variation on any one channel due to the fr esnel lens 40 being a different distance from the projection screen 34 across the channel 42.

By choosing a focal length for the fresnel lens 40 that is different fro m that required for the ideal situation, it is possible to remove the lens distortions as well as the inherent geometric errors. Lens barrelling actually helps the distortions. If a lens were designed that had a barrel distortion equal and opposite to the projection screen distortions, there would be no requirement for the fresnel lens to solve the distortion problem, although the luminance variation would still need to be addressed.

The rear projection apparatus 30 may be such that the projection screen 34 may take any complex shape, assuming it is curved. Thus the projection screen 34 may be a spherical projection screen or a cylindrical projection screen. As stated, geometric errors in the displayed image 12 may be corrected for. Luminance roll-off and screen gain effects may be largely corrected for. Multiple projected images may be edge-aligned, or blended together. Multiple projectors and multiple fresnel lenses may be employed such that they are tiled with additional overlap regions to form a larger image on the rear surface 38 of the projection screen 34.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, f or example, the fresnel lens directs rays from the projection point exactly towards the design eye point, and if adjacent fresnel lenses are butted together, then the edge of each fresnel lens will act as a blending mask for its channel. This will result in a blend at the projection screen, but it will be very narrow since the blending mask is closed to the screen. Locating the fresnel lens away from the screen will help to a small extent but the blend may still only be a few pixels. In order to obtain a sizeable overlap, for example tens of pixels, plus space for support structure at the edges of the f resnel lens, it may be desirable to conf igure the f resnel lens to direct the rays towards a point some distance behind the design eye point. This will compromise the distortion correction feature, giving rise to some pin cushion distortion. This could compensate for an equal amount of barrel distortion in the projection 13 lens, but projection lenses generally have pin cushion distortions so that the effects will add.

With regard to focus implications, the LCD projection lenses are generally designed to form an image on a flat screen. If the fresnel lens is placed in tangent-contact with the screen, then the projector is focused at the centre of the display. This will imply that the image will be formed on the flat fresnel lens. There will therefore be some focus error at the edges of the projection screen. This error may be less than would be the case if the projector illuminated the projection screen directly without a fresnel lens. Thus the fresnel lens does reduce the focus problem. A polysilicon projector has a small exit pupil and hence a large depth of focus, so this may give acceptable focus. However, a small exit pupil will also imply a small blend region. With projectors having larger exit pupils, focus is likely to be more of a problem.

The major benefits of the present invention are inherent distortion correction permitting fixed matrix devices to be used without digital remapping, and good luminance unif ormity. These advantages outweigh possible risk areas of channel overlap/blending and f ocus.

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Claims (13)

1. Rear projection apparatus for displaying images, which apparatus comprises a curved rear projection screen, at least one projector for projecting the images on to a rear surface of the projection screen, and at least one fresnel lens positioned between the projector and the rear surface of the projection screen.

2. Rear projection apparatus according to claim 1 in which the fresnel lens is positioned away from the rear surface of the projection screen.

3. Rear projection apparatus according to claim 1 or claim 2 in which the f resnel lens has a f ocal length which is different from a theoretically correct required focal length, thereby to remove fresnel lens distortions and inherent geometric errors.

4. Rear projection apparatus according to claim 1 or claim 2 in which the fresnel lens has a barrel distortion which is equal and opposite to distortions from the projection screen.

S. Rear projection apparatus according to any one of the preceding claims in which the fresnel lens is such that it has a plurality of rings with each ring having a prism angle calculated to direct all rays from the projector towards a design eye point of the apparatus.

6. Rear projection apparatus according to any one of the preceding claims in which the fresnel lens is configured to direct light rays from the projector some distance behind a design eye point of the apparatus.

7. Rear projection apparatus according to any one of the preceding claims in which the projector lens has a single exit pupil.

8. Rear projection apparatus according to claim 7 in which the projector has a small exit pupil and hence a large depth of focus.

9. Rear projection apparatus according to claim 8 in which the projector is a polysilicon projector.

10. Rear projection apparatus according to any one of the preceding claims in which the projector is a fixed matrix projector.

11. Rear projection apparatus according to any one of the preceding claims and including a plurality of the projectors and a plurality of the fresnel lenses.

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12. Rear projection apparatus according to claim 11 in which there is one of the fresnel lenses for each one of the projectors.

13. Rear projection apparatus for displaying images, substantially as herein described with reference to Figure 6 of the accompanying drawings.