Equipment and standards for objectively measuring the real-world performance of digital movie projectors are just now coming into being, as engineers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) collaborate with the Entertainment Technology (ET) Center at the University of Southern California (USC; Los Angeles, CA) and the Digital Cinema Initiative (DCI) film industry consortium to apply optical science to cinema concerns. One major concern in translating precise optical measurements into the cinema environment has been the management of stray light.
Performance specifications for electronic front-projection systems that display their images on remote screens are normally made in ideal darkroom conditions with black walls, floors, and ceilings, no reflective objects, and even a black screen, according to Paul Boynton, an engineer in the Flat Panel Display Laboratory at NIST. The ET Center and DCI, however, need to perform objective projector-performance evaluations in the same movie-house environments in which expert film viewers offer subjective impressions of cinema images.
So Boynton and colleagues have come up with tools for managing stray light and associated problems such as veiling glare, that allow objective projector assessments outside the darkroom environment.1 The ET Center is adapting those tools for cinema-specific measuring systems. For example, a stray-light-elimination tube (SLET) developed at NIST essentially simulates darkroom conditions at the point of measurement. It consists of a 61-cm-long tube with a 15-cm inner diameter and a glossy black exterior and interior. Projected light enters one end of the tube and falls upon a light meter at the other end. Glossy black truncated cones or frustums within the tube serve to direct stray light out of the measurement path.
To apply the SLET technology to a 20-ft-high movie screen mounted 5 ft above the ground, the team at the ET Center mounted the device on a camera crane, which enables them to position it anywhere they want in physical space, while also controlling pan and tilt (see figure). They used laser guidance to aim the SLET—which was placed at the screen—precisely back into the center of the projector beam usually originating in a projection booth toward the back of the theater. Test images provided by NIST allowed evaluation of parameters such as contrast ratio and modulation-transfer function.
The ET Center is also collaborating with an architectural surveying group at USC, which uses laser-based tools to evaluate the borders of the projected image at the screen to see how closely they approach a perfect rectangular shape.