1. Technical Field
This invention relates to light valve projectors and, more particularly, to filters for blending off-state and on-state illumination levels for a projection system which employs multiple light valve projectors to form an overlapping, composite image.
2. Discussion
Conventional projection systems have employed two (or more) light valve projectors to form a composite image. In the case of two projector systems, the images are positioned side-by-side or one on top of the other. Projector systems can also include an array of projectors. When two light valve projectors are used, one light valve projector projects a first portion of an image. A second light valve projector projects a second portion of the image. The first and second light valve projectors are positioned such that the first and second images are projected onto the screen adjacent to each other. As can be appreciated, it is difficult to align the two (or more) light valve projectors exactly and therefore undesirable seams between the first and second images are often apparent to the viewer.
To improve the appearance and continuity of the composite image, the first and second light valve projectors are conventionally positioned such that the first image slightly overlaps the second image to a form a seamless composite image. When the projectors are partially overlapped, the overlap region contains the illumination level for both projectors. The overlap region has a greater image intensity because the brightness levels are added together. To alleviate this problem, conventional light valve projection systems employ brightness adjustment electronics to eliminate the excessive brightness in the overlap region. For example, see U.S. Pat. Nos. 4,974,073 and 5,136,390 which discloses an electronic smoothing device for providing seamless composite images from multiple light valve projectors.
These electronic smoothing devices, however, cannot correct overlap errors at off-state levels without significantly reducing contrast. The off-state (or black level) of a light valve projector still contains some illumination or brightness as opposed to CRT projection systems where the brightness of the phosphor can be turned off completely. When two light valve projectors are partially overlapped to create a seamless composite image, the overlap region contains the off-state illumination level of both projectors. The illuminated areas from the first and second light valve projectors add together and a bright stripe is produced in the overlap region.
The black or off-state illumination level is the lowest light level that can be achieved by a light valve projector. Therefore, it is not possible to lower this off-state level further by electronic signal correction according to the above-identified patents. The only other solution which can be accomplished using electronic signal correction is to raise the black level of the non-overlapping regions to a level equal to the illumination level in the overlapping region. This electronic correction is not desirable since it significantly reduces the overall contrast ratio of the light valve projection system and cannot be used for applications such as night simulation where high contrast is required (in other words, maximum dark black levels).
Therefore, a method and apparatus for blending off-state illumination levels of a light valve projection system with multiple light valves to form seamless, composite images having maximum contrast is desirable.