American Cinematographer - April 2011 - 58

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Optical Filtration and 3-D

Dolby and 3M have partnered to introduce 3-D glasses (below) with new, multi-layer, optical film lenses that use spectral separation technology and boast a lighter weight with advanced ambient-light management. Right: This spectraltransmission chart illustrates how the glasses separate left- and right-eye views by using two mutually exclusive tricolor sets to create full-color stereo imaging.

almost perpendicular. At the centerline [of the lens view] we’re looking through 150 percent of the thickness of the glass, roughly 41⁄2mm, because we’re looking through it at an angle. When you look through the top, you’re not quite at 3mm … and when you look through the bottom, and this gets worse [diagonally] out towards the corners … you’re looking through twice the glass, say 6mm.” In other words, there is a variable, wedge-angle-induced optical “distortion” created simply by the necessary rig geometry. This is made more difficult because the vertical camera, seeing only the reflection from the mirror surface,
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experiences none of that wedge angle. And yet it is of critical importance that the two images match in all ways except in point of view. You can see where this is heading. It gets worse. The image-forming light that is reflected toward the vertical camera strikes the mirror at varying angles in both horizontal and vertical directions. In discussion with Jason Goodman, founder and CEO of 21st Century 3D, who has been building and using custom 3-D cameras for more than 15 years, it came out that this range of angles produces variable polarization characteristics in the reflected image not seen by the direct camera. It’s bad enough that the reflected light is partially polarized and the direct light is not. For every object in the scene that reflects polarized light — windows, water and sky, for example — this creates an unacceptable difference between the two views, disrupting the 3-D experience for the viewer.
American Cinematographer

However, there is a solution. As Goodman notes on his innovation, “The use of a quarter-wave retarder in front of the whole … system would take any of this incoming linearly polarized light and convert it to circularly polarized light and … greatly mitigate the problem.” The retarder depolarizes the light entering the mirror box, eliminating the detrimental polarizationinduced effects described above. It gets even more complicated, for instance, when you have to mount the cameras so that their shutters move in different directions. Schneider Optics, a prominent manufacturer of a broad range of optical filters for imaging, is now offering large filters for mounting in front of the mirror box (as described above) in a range that includes quarter-wave retarder plates as well as other useful filter effects. Because beamsplitters are the major “filter” involved in production, I

American Cinematographer - April 2011

Table of Contents for the Digital Edition of American Cinematographer - April 2011