The invention relates to an image projection arrangement, comprising a radiation source, an image display system having at least one image display panel for generating an image to be displayed and wherein the direction of polarisation of the beam originating from the source is modulated in accordance with the image information, a projection lens system for projecting the image produced by the image display system onto a projection panel, a polarisation-sensitive beam splitter being arranged in the path of the beam coming from the source for producing two mutually perpendicular-polarised sub-beams which are each intended to be modulated with image information.
The concept of image projection arrangement must be understood in a very general sense and comprises an arrangement for displaying, for example, a video image, a graphic image, numerical information or a combination thereof. The images may be both monochrome and colour images. In the latter case, the display system may have three colour channels for, for example, the primary colours red, green and blue, each channel including a display panel.
Such an image projection arrangement for a colour image is disclosed in the U.S. Pat. No. 4,127,322. The display panels of the prior art arrangement are reflective light valves having as the active, or image producing, element a layer of liquid crystalline material of the so-called nematic type. This layer locally changes the polarisation direction of an incident light beam, in dependence on the image information. For that purpose the light beam must be linearly polarised, for which a polarisation-sensitive beam splitter is arranged in the path of the beam originating from the light source. This beam splitter splits the light beam into two, mutually perpendicular-polarised sub-beams. Only one of these beams is transmitted to a display system, so that approximately half of the light of the radiation source reaches this display system.
So as to make a more efficient use of the light from the source, the U.S. Pat. No. 4,127,322 proposes the use of a second display system. Therein the first display system is exposed to the sub-beam polarised in a first direction and the second display system to the sub-beam polarised in the second direction. After modulation by the display systems the beams are combined by the same polarisation-sensitive beam splitter which also effected splitting according to polarisation direction. Thus, in principle, 100% of the light from the radiation source is used for the image projection.
In the arrangement disclosed in the U.S. Pat. No. 4,127,322, the number of display valves is twice the number in more conventional arrangements. In addition, each display valve forms part of a complex, relatively large and expensive system. In this system an image is generated by means of a cathode-ray tube. The light beam emanating from this tube is incident onto a photo-conducting layer in which a charge pattern is produced in accordance with the image on the cathode-ray tube. As a result thereof, an electric field is formed, also in accordance with the image on the cathode-ray tube, across a layer of liquid crystalline material disposed between the photo-conductive layer and a second, counter, electrode. The varying electric field produces a variation of the birefringence within the liquid crystalline layer and consequently local differences in the shift of the polarisation direction of a projection beam incident on this layer. To enable proper operation this system must be provided with yet a number of additional layers. The arrangement disclosed in the U.S. Pat. No. 4,127,322 is intended for professional applications, and, because of its complex structure, size and cost price is not so suitable for consumer purposes.
More suitable for consumer, and other, applications are the so-called matrix controlled display panels having a layer of liquid crystalline material between two electrodes. In the case of a passively controlled display panel, both electrodes are distributed into rows and columns, and in the case of an actively controlled display panel a matrix of electronic drive circuits are provided on one of the electrodes. In both cases the panel is divided by the electrode matrix into a large number of image elements. The electrode matrix is controlled by an electronic signal, for example a video signal. An image projection arrangement comprising display panels of this type is less complicated, cheaper and of smaller bulk than the arrangement disclosed in the U.S. Pat. No. 4,127,322. However, due to the construction of the display panels, the useful luminous flux is small. In, for example, the case of an actively controlled display panel, only approximately 10% of the light from the source is transmitted to the projection lens system via the display panel.