Optics: image projectors – Cabinet encloses projector and one side of screen
Patent
1996-10-04
1999-04-27
Dowling, William
Optics: image projectors
Cabinet encloses projector and one side of screen
353 94, 353119, 353 72, G03B 2114
Patent
active
058971920
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a rear projection module for displaying an image on a large-area projection screen, comprising a light transmissive picture generator for depicting the image on a reduced scale, an illumination unit for the transillumination of the light transmissive picture generator and a projection unit for the magnified imaging onto the projection screen of the image depicted by the light transmissive picture generator.
Such rear projection modules are widely used where a large image is to be displayed. The image originals in the picture generator can be video images, computer images or images of another kind. By a large image are understood images which typically have display screen diagonals of more than 0.5 m. Common fields of application for such rear projection modules are data display terminals which are to be viewed by several persons simultaneously, as is the case with conferences or presentations. Large-image rear projection is in wide use in particular in modern supervisory control centers.
The light transmissive picture generator is typically a light transmissive liquid crystal display unit. It can be constructed as a black-and-white or preferably as a coloured liquid crystal display. According to the current state of the art in the matter of liquid crystal display units there are two classes, which differ in size. The smaller liquid crystal display units, which are for example manufactured from polysilicon with sizes of the screen diagonal of about 2 inches, have the disadvantage that the so-called pixel aperture is relatively small. The pixel aperture is a measure of the area fraction with which the liquid crystal display unit is translucent. This, combined with the high luminous intensity required to achieve a bright projected image, leads to barely controllable heat dissipation problems and disadvantageously high surface temperatures.
These disadvantages are overcome if larger-area liquid crystal display units having diagonals of 6 to 20 inches, which are manufactured from amorphous silicon according to the present state of the art, are used. The pixel aperture is greater with these units. In addition the surface temperature is also diminished as the result of the reduced luminous intensity, so that long service lives are obtained even with high luminous fluxes. In the context of the invention there are preferred according to the current technology liquid crystal display units with diagonals of around 10 inches. It is generally the case with optical imagings that the technical difficulties in achieving a fully satisfactory imaging also increase with the size of the image. It is astonishing, therefore, that the high quality requirements in the area of application according to the invention can be fulfilled with such picture generator sizes.
A known illumination unit consists of a lamp, a reflector and a condenser. Metal-halide short-arc lamps are preferably used.
The projection unit for the magnified imaging onto a projection screen of the image depicted by a light transmissive picture generator always includes a projecting lens. Occasionally the picture generator is also related to the projection unit.
Known projection screens for rear projection modules possess a lenticular sheet and a support sheet, each of which has a light-input-side face facing the projection unit, a light-output-side face lying opposite the latter and a peripheral surface connecting the input face to the output face. Between the input face and the output face of the projection screen a Fresnel structure is disposed. The Fresnel structure can be formed for example on the light-input-side face of the lenticular sheet, the light-output-side face of the support sheet or on a separate Fresnel sheet. The construction of the projection screen from two or three sheets is governed by static and optical considerations. The support sheet serves as a mechanical support for the projection screen as a whole.
The Fresnel structure refracts the incident light for the illumination of the following lenticular sheet. It is th
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