American Cinematographer - April 2011 - 54

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Optical Filtration and 3-D

Left: Element Technica’s Neutron rig. The beamsplitter configuration allows narrow and overlapping interaxial distances not possible in a true side-by-side arrangement. Above: This diagram illustrates the mounting position of the quarter-wave retarder at the front of the BX-3 mirror box.

projector or display and the glasses, this is called an “active” system. Linear polarizers have long been popular for separating stereo views because they can isolate a certain vibratory direction of light to the exclusion of others. Thus, light passing through a vertically oriented polarizer will not pass through a horizontally oriented one. As long as the projected image of the right and left views are oriented the same as the right and left eyeglass lenses, respectively, the 3-D illusion will remain intact. Theatrical projection can be accomplished by superimposing both images on the screen. Because the glasses will allow only the proper view to each eye without actively being in sync with the projector, this is called a “passive” system. Variations of the above methods can improve certain image characteristics. For instance, if you are tilting your head sideways to gulp popcorn while using linearly polarized glasses, you will begin to lose the sense of depth. Circular polarizers solve that by effectively “corkscrewing” the polarization axis so that it can function in all directions. The drawback is slightly lower polarization efficiency, which can add a touch of “ghosting.”
54 April 2011

RealD is a key supplier of 3-D systems that make use of either active shutter LCD or polarized optics that best suit the application. There is not just one technology for all situations. Theatrical screens, like those made by Stewart and Harkness, traditionally have been coated with the familiar white surface. Several 3-D systems can make use of these, but those using polarization require a different reflective surface, one that retains the polarization properties. For this purpose, special silvery screens coated with reflective materials such as fine aluminum flakes provide higher gain (a greater percentage of the illuminating light is reflected and at a more focused viewing angle). The additional brightness is a welcome assist when light is already being lost to filtration in the optical system. The use of polarizers on the projector will reduce screen brightness, but the polarizers in the eyewear compensates for some of this loss. You might think these would reduce brightness even further, but by absorbing much of the generally unpolarized ambient light from within the theater, the eyewear polarizers actually transmit a relatively higher percentage of the
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axis-matched light from the screen. The result is a brighter reflected image from the screen, compared to the darker theater interior. New efforts utilize interference coatings to break the spectrum into smaller segments, with fascinating results. Dolby, known for its sound systems, uses interference filters to split each of the red, green and blue bands into two distinct sections. A different band for each color is included for each eye view, providing full-color imaging; the two sets do not overlap, thereby eliminating ghosting. Dolby has made further improvements. One example: interference coatings on flat surfaces change light transmission and visual color as the viewing angle changes. In typical eyeglass lenses, this means that as you look anywhere but straight through the lens, this effect can create enough of a color mismatch to disrupt the sense of depth. Instead, Dolby has created “shaped” spherical lenses, which keep the viewing angle consistent regardless of where you’re looking. A while back, Panavision developed a breakthrough in the use of interference filters by fitting them inside the camera lens, where they can see only collimated light, thus eliminating the

American Cinematographer - April 2011

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