3D: Brighter is Better

In the May issue of Large Display Report, we covered a summary of a demonstration run by Inter-Society of Digital Cinema Forum (ISDCF) last March. The demonstration had originally been closed and targeted at studio executives but details leaked out, as could be expected. This test involved 3D content mastered or re-mastered for showing at different luminance levels. 4.5 Ft. lamberts served as a baseline since that is the specified luminance of 3D content in the theaters. The material was also re-mastered for showing at higher luminance levels. The different versions of the material were then shown at 4.5, 6, 10 and 14 Ft. Lamberts. 14 Ft. Lamberts is the normal theatrical luminance for 2D material. Video material in the home is typically shown at still higher levels, from 30 Ft. Lamberts on up.

Why is this important? The way the eye perceives color changes rapidly as a function of light level at these very low luminance levels. 14 Ft. Lamberts was chosen as the standard for 2D cinema for two reasons. First, it is well within the range of film projectors with reasonable sized lamps. Second, and perhaps more importantly, at this low light level, the eye does not normally perceive the flicker produced by the 48 Hz field rate of double-flashed film. While digital cinema is more commonly triple flashed to 72 Hz, eliminating flicker as a problem, it continues to use the 14 Ft. Lambert standard. This allows digital and film theaters to have a similar "look and feel," an important property to the movie industry.

3D digital cinema projectors are very inefficient compared to the same projector showing 2D content, 14% efficiency is a typical value. By using a larger lamp and a high gain screen, the current generation of 3D digital cinema projectors can make about 4.5 Ft. Lamberts on the screen but not 14 Ft. Lamberts. In general, 3D presentations have been in the smaller auditoriums of a multiplex, since it is easier to achieve 4.5 Ft. Lamberts on a small screen than a large screen.

Since human perception of color is different at 4.5 and 14 Ft. Lamberts, studios need to color correct their 3D movies twice: once for 2D screens and once for 3D screens. This is an expensive process and studios would like to eliminate the second color correction.

There are two approaches to doing this. First, they could color correct for some intermediate light level, say 6 or 10 Ft. Lamberts, and use that color correction at both the higher 2D and the lower 3D light levels. The ISDCF demo showed that this might actually be an acceptable path.

The other approach would be to increase the light level on 3D cinema screens to a value closer to the 2D light level. From a studio’s point of view, this would be ideal: only one master would be required and the 3D experience would then match the 2D experience, with the addition of depth.

Theater owners would not like this, however. Current digital cinema projectors strain to achieve 4.5 Ft. Lamberts, especially on larger screens. Lamps must be run at maximum power, leading to high electric bills and short lamp life. High-gain screens help but there is an upper limit to acceptable gain. Going to a high enough gain screen to achieve 14 Ft. Lamberts in 3D means that only the seats on the theater centerline will get an acceptable image. When brighter 3D projectors are available, theater owners want to put them into larger auditoriums with more seats, not make brighter images on the same screens.

Some famous large-screen theaters are embracing projector technology that will allow 3D material to be shown on their large screens. For example, NEC announced yesterday that a single NC2500S-A projector will be used in Clearview’s 1,169 seat Ziegfeld theater in New York City to produce 3D on the Ziegfeld’s 15.8M (52') screen. This tops the normal 12M upper limit on screen size for a single projector.

This higher throughput of the NC2500S-A is enabled by the higher bandwidth electronics driving the DLP imagers. In the past, bandwidth issues limited DLP cinema projectors to using a 1628 x 880 subset of the 2K imager. The higher-bandwidth electronics allow use of 1998 x 1080 pixels, the same as is normally used in 2D projection. Being able to illuminate this larger image area leads to about a 30% increase in light output compared to previous 3D single projector installations.

Another famous theater, the 1300 seat Empire Theater in London, will be using a dual-projector 3D installation from Dolby to produce an image on its 20M (66') screen. This setup consists of two Barco DP-3000 DLP cinema projectors. Passive color filters are used both at the projectors and on the audience’s glasses.

Expanded versions of the Ziegfeld and Empire theater stories will be published in the upcoming issue of LDR, as will other details that emerge on high-brightness 3D cinema projection.

By Matt Brennesholtz, DisplayDaily