Passive 3D Projection – Part 1
This Blog starts a discussion on passive 3D projection systems. I expect this discussion will continue over several future blogs. For this initial blog I present some of the fundamentals of passive 3D projection. This is intended to prepare the reader for more in-depth discussions of the alternative technologies and descriptions of do-it-yourself (i.e., DIY) dual projector passive 3D projection setups.
Passive 3D Projection Basics
As with all 3D video systems, a passive 3D projection system must provide the means to project both the right and left (i.e., stereoscopic) image streams in such a manner that simple passive (i.e., with no active electronic components) 3D glasses are capable of separating the right from the left image streams. In order to achieve this some form of manipulation of the projected light is required to differentiate the projected light associated with the right versus the left image streams. At the highest level, there are basically two competing approaches, as discussed below, for differentiating the right from the left images projected light.
Polarized 3D Passive Projection
The most common approach for passive 3D is to project polarized light using a different orientation for the polarization of the projected light associated with the right versus left images. The passive glasses worn by the viewers then have lenses that are simple polarizing filters with each lens’ orientation matched to the orientation of the polarization of the light emitted by the projector(s). Two basic types of polarization can be used.
(a). The simplest approach uses linear polarizing filters on the projector(s) as well as with the 3D glasses. In this case the orientation of the polarization for one image stream (e.g., right images) is rotated 90 degrees from the other image stream (e.g., left images). The two most common orientations used for passive 3D projection are to either have one filter oriented for vertical polarization and the second filter oriented for horizontal polarization, or to have one filter oriented +45 degrees from vertical and the other filter oriented –45 degrees. This latter orientation is used by the IMAX passive 3D projection system.
(b). The alternative approach uses circular polarizing filters on the projector(s) as well as with the 3D glasses. In this case one image stream is right-hand circular polarized while the other image stream is left-hand circular polarized.
With either linear or circular polarization approaches the polarizing filter used for a given lens on the glasses will only pass light having the same orientation of that filter and will block the light having the opposite polarization. Thus if this technique were perfect (which it is not), the right eye only see the right image stream and the left eye would only see the left image stream. In the real world there will be some amount of the light intended of the other eye that is leaked through and when this occurs the result will be 3D crosstalk/ghosting.
In order for polarization based passive 3D systems to work correctly, the projection screen must retain the polarization of the light being emitted from the projector. Any loss of the polarization will result in 3D crosstalk/ghosting. In general, special screen materials must be used for such projection systems and such screens typically have a silver colored reflective surface. Several of the major projection screen manufacturers, such as Stewart and Da-lite, offer screen materials specifically designed to retain to a high degree (over 99%) of the polarization.. Unfortunately such silver screen materials are generally less than ideal for normal 2D projection as they frequently have issues with “hot spotting” and in some cases may introduce objectionable grain and/or sparklies in the image.
3D Passive Projection using Color Bandpass Filtering
The second major approach for passive 3D projectors takes advantage of color spectrum of the projected light. Full color projected images are created by projecting the red, green and blue (RGB) components of the image. The full spectrum of visible colors can be created by combining these three primary colors (i.e., RGB) in different proportions. Certain passive 3D projection systems separate the right from left images by using narrow bandpass optical interference filters incorporating a small offset to the exact wavelength (or frequency) used for the right image’s primary colors versus the left image’s primary colors. This technique is sometimes called “wavelength multiplexing”. The viewer’s 3D glasses then must have lenses using complementary narrow bandpass interference filters. The following illustration depicts the use such a system for 3D.
The leading developer of this technology is the German company Infitec GmbH and the Infitec system is used by Dolby Laboratories for their passive “Dolby 3D” projection system that is used in some commercial cinemas (installed in 260 theaters as of 2010). The Infitec system was the result of a research project at DaimlerChrysler AG. The advantage of the Infitec system over the use of polarization as the basis for a passive 3D system is the Infitec system can be used with conventional projection screens instead of requiring the use of a silver screen. Thus the same projection screen that is well suited for 2D projector can be used for 3D projection with the Infitec passive 3D system.
An informative presentation explaining linear and circular polarization of light for 3D as well as the Infitec system can be found HERE (a pdf file).
My next Blog will start the discussion of passive 3D projection systems that use polarization.