REAL D Technology
"I am the chief scientist at REAL D and developed the REAL D system that is installed in more than 1000 theatres worldwide. There are several performance issues that deserve some clarification. This system uses an electronic polarization modulator (called a Z screen) at the projector which alternates between left and right circular polarization, and matching circular polarization glasses.
1. All the modulation systems (REAL D, or shutter glasses, or spectral division) have similar efficiency, which is driven by the fact that the projector output is time multiplexed between eyes (a 50% hit) and each system further divides the light through either polarization or color spectrum division - reducing an additional 50% of the light. This leaves theoretically 25% of the light available for each eye. In addition, more light is lost because the polarization or color splitting isn't perfectly efficient.
2. Polarization systems (linear and circular) require a silver screen. Silver screens have one significant benefit - they have gain, and will deliver more light back to the audience. A 2.4 gain silver screen will deliver an image twice as bright as a 1.2 gain silver screen. Light distribution from silver screens is indeed not as broad as a matte white screen, but the light distribution is significantly better on current screens than it was 5 or 10 years ago. (half gain full angle of 52 degrees). Correct curvature of the silver screen helps to deliver an even illumination to the audience. The silver screen requires less light to light it to the equivalent level of luminance, thus giving the opportunity to use less power or light a larger screen (or light it brighter) than a white screen. (In practice for a 40' screen, it results in electrical and lamp cost saving of >$1000 per year, and the electricity savings alone reduce the amount of CO2 produced annually by 3 tons per screen.)
3. Silver screens look a little different. They induce a slight shift in white (less than .005 in x and y). This is accurately corrected in digital projection through projector calibration so the color of intended images is correct. Silver screens manage light scatter in the theatre better than white, resulting in slightly higher contrast and slightly higher saturation in the image as viewed by the audience. (almost the difference between vision and vision premiere film.)
4. Ghosting does differ from system to system. The actual physical leakage through the system is higher in circular polarized systems than other systems - in practice is between 50:1 and 100:1 in the REAL D system. This can be mitigated by ghost pre-correction where the ghosting between left and right images is predicted by a model and pre-corrected for in the projected image so the image as seen by the eye is ghost free. In practice this moves the ghost image to 300:1 to 500:1 in most projected images on the REAL D system, leaving them visually ghost free.
5. Circular polarization allows the viewer to tip his head without inducing ghosting, and provides a significant advantage. Eyewear using high quality circular polarizers is sufficiently inexpensive (<$1.00) that it does not need to be collected and re-used. That said, refurbishment programs are in process to re-use eyewear which is refurbished at a central location, relieving the exhibitor of the need to manage the inventory (as is required with shutter glasses and color separation systems.)"
By Matt Cowan, CML-3D
