1. Technical Field
The invention concerns improvements in the peak brightness of projection displays, and more particularly to controlling peak brightness by selectively controlling uniformity of screen illumination.
2. Description of the Related Art
Rear projection television (RPTV) engines based on LCD, LCOS or DLP imagers, have an illumination system that performs some very important steps to ensure adequate light throughput. The first one consists in a format conversion. The circular beam of light collected by a reflector is not suitable for illuminating a square imager as too much light would be lost. For this reason, fly-eye lenses and rectangular light pipes are commonly used. These lenses convert the light beam footprint format from circular to rectangular. The second step performed by such RPTV illumination systems is to achieve a very uniform illumination of the imager. This is in fact a natural ability of the light pipe and fly-eye lens. These elements are by principle able to provide a very uniform illumination. This is sound and desirable for data projections where uniform fields are needed for presentations as most of the time the slides have large uniform areas. Data projectors can also place a substantial number of lumens on the screen, since there is less need to cut off the yellow peak of light's spectrum for achieving a very good white balance.
On the other hand, illumination intended to be used for video applications must have an excellent white color and, as they need to be much cheaper, the imagers used tends to be smaller than in data projectors. Hence, due to all these constraints, the light output is more limited. But, video content does not need to have a screen brightness uniformity of 90 to 100% like data projection, and it would be profitable for a TV application, if the center peak brightness of the set could be significantly improved by sacrificing to the uniformity, without losing light power. Light engines that are presently limited to use in RPTVs of smaller screen sizes (40″ to 46″) because of their limited light output, could be used in bigger screen sized cabinets (50-52″) by improving the peak brightness.