The relatively simple new NIST tool---dubbed a stray-light elimination tube---improves measurements of the contrast and sharpness of images produced with digital projectors. It also may help reduce errors in assessing other projection systems.
Digital cinema should offer movie goers sharper, brighter pictures, but digital projectors currently are expensive and complex. Engineers from the Digital Cinema Lab, a project of the Entertainment Technology Center (ETC), are using the NIST-developed device as one of many tools to evaluate the performance of these new projectors. Part of the University of Southern California, ETC is funded by major studios and other organizations.
The tool, which costs under $100 to construct, is a simple tube made of glossy black plastic with cone-shaped inserts. Light measurements are taken at one end of the tube, while the other end is aimed toward the projector. The cones have a hole in the center (typically 2 inches) so that only those rays arriving directly from the projector reach the light detector, while stray, ambient lighting is filtered out. The device is typically about two feet long and is attached to a tripod or other adjustable, stable mount so that it can be moved to take multiple readings from images projected on a screen. Traditional methods for judging the light output of projectors may introduce error rates of 40 percent or more because ambient light is inadvertently included in the measurement.