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Passive 3D Projection - Part 7

This post continues the series of blogs discussing do-it-yourself (diy) passive 3D projection systems that use two conventional front projectors.  In my previous blog (i.e., Part 6 of this series), I completed (at least for the time being) the discussion on projectors suitable for use as part of a passive 3D projection system that uses polarized light as the means to separate the right from the left images that make up the stereoscopic image pair that creates the 3D effect.  This new blog wraps up the current discussion on passive 3D projection systems that use polarized light. As I noted in my previous blogs in this series, some of the information used for this discussion is based on information provided by Rob Stewart who has experimented with a number of do-it-yourself dual projector alternatives for passive 3D. Shown below is an illustration (repeated from an earlier blog in this series) showing the functional components that make up a passive 3D projection system. The six previous blogs in this series have covered all of the functional elements shown in the above functional diagram, except for the projection screen.  For this blog I will recap some of the information presented in my earlier blogs (from last year) on projection screens intended for use with passive polarized 3D projection systems and also discuss the potential role for video processing.

Projection Screens for Passive Polarized 3D

Back in August 2011, I posted a two part series of blogs on “Screens for 3D Projection.”  Later I posted some information on new “silver screen” materials from Da-Lite and Stewart.  Below is an update from some of the material presented in earlier blogs. Retention of Polarized Light – This characteristic of projection screens is only important for 3D projection and even then it only applies for screens used with 3D projection systems that project polarized light. With passive 3D projection systems that rely on polarized light (as being discussed in this current series of blogs), retaining the polarization of the projected light is absolutely essential.  This is because such 3D projection systems use polarized light, with different orientations, to separate the left and right images and the viewer must wear polarized 3D eyeglasses (sometime called passive 3D glasses since they contain no electronics).  The light may either be linear polarized, where the polarization orientation for the right vs. left images is offset by 90 degrees (e.g., vertical vs. horizontal -OR- vertical +45 degrees vs. vertical –45 degrees).  This technique is frequently seen for 3D presentations in digital IMAX theaters. The other polarization approach is to use circular polarization with an opposite direction for the polarization orientation used the right vs. left images (i.e., right hand circular vs. left hand circular polarization).  This is the technique is used by RealD for 3D presentations in commercial digital cinemas.  Only projection screens that retain very near to 100% of the polarization are suitable for use with such passive 3D projection systems.

Screens for use with Passive Polarized 3D Projection Systems

For screens intended for use with passive polarized 3D projection systems (i.e., that rely on polarization to separate the right from the left images), the screen must retain close to 100% of the polarization otherwise visible 3D cross-talk or ghosting will occur. This generally means that you must use a ‘Silver Screen’ material made specifically for use with such passive 3D projection systems.  For such ‘silver screens,’ manufacturers frequently publish an “extinction rate” or “extinction ratio” which is a measure of the ability of a given screen material to retain the polarization of the projected light. This value is normally measured by projecting a highly polarized light onto the screen material then using a light meter to measure the light level thru a polarizing filter that is oriented the same as the projected light then measured again thru the polarizing filter oriented opposite to that of the projected light. Thus, for example, if a manufacturer specifies a screen to have an extinction ratio of 100 that means that when viewed thru a polarizing filter with the matching orientation the light passing thru the filter will be 100 times more than when the filter has the opposite orientation. In this example, that means the 3D cross-talk image will be 1% as bright as the main image that is intended for that eye. The best extinction ratio I have seen being specified from the major consumer screen manufactures (e.g., Stewart or Da-Lite) in on the order of 150, which would produce a cross-talk image that is 0.67% as bright as the main image or stated another way approx. 99.3% of the polarization is retained for the main image. Most manufacturers normally only specify a extinction ratio for screens intended for use with polarized 3D projection systems and the specified value for extinction ratio normally applies to linear polarization (i.e., extinction ratio for circular polarization is typically lower).  See my qualification further down in this blog on the reality of the above values. Several manufacturers offer traditional ‘silver screen’ fabrics specifically intended for 3D projection.  Below are examples:

Dalite 3D Virtual Grey

Dalite Silver Matte

Stewart Silver 3D

Harkness Stagelite Stereo

Harkness Spectral 240

Some tests by hobbyists indicate that the Harkness Spectral 240 and the Stewart Silver 3D screen materials do the best job of the above screen materials in retaining linear polarization and as a result should produce the least visible 3D crosstalk/ghosting.  Harkness claims that their Spectral 240 is “the most popular 3D screen surface in the world.”  It is widely used in commercial cinemas and the screen fabric itself can be purchased for building your own home projection screen (e.g., stretched over your diy frame).  Da-lite will also sell at least some of their screen fabrics that can be used for do-it-yourself screen construction projects. Some manufacturers realizing the growing consumer interest in 3D have over the past year introduced new screen materials claimed to be better suited for both 2D and passive 3D projection. Such screen materials, while not optimal for either 2D or 3D, have the potential of providing an acceptable single screen solution for some consumers building a passive 3D capable home theater.  Traditionally a “silver screen” material is used passive polarized 3D projection. Such screen materials do have a silver color, rather than white or grey, and tend to have fairly high gain due to their reflective nature.  While such screens are necessary for use with passive polarized 3D projection systems, they tend to be less than ideal for viewing normal 2D video material.  This is because such silver screens have a tendency to have visible hot spotting, narrow viewing angles, and frequently introduce increased grain in the image.  Recently both Stewart and Da-Lite have introduced new silver screen materials that are marketed as suitable for use with both passive 3D as well as 2D projection.  The Da-Lite material is called Silver Lite 2.5TM and has a rated gain of 2.5, a half gain viewing angle of 25 degrees and is specified to maintain 99.3% of the polarization.  The Stewart material is called “Silver 5D” (as in 2D + 3D = 5D), has a rated gain of 2.0 with a half gain viewing angle of 30 degrees and is specified to have an extinction ratio of 140:1 (reference: Stewart White Paper on the Silver 5D screen material).  An extinction ratio of 140:1 corresponds to the screen maintaining 99.3% of the polarization.  Thus both the new Stewart and Da-lite silver screen materials are rated to maintain 99.3% of linear polarization.  Generally silver screen materials do not retain as much polarization when circular polarization is being used.  While 99.3% is a fairly high value, that still leaves 0.7% of the reflected light coming from the screen as becoming randomly polarized. The values for extinction ratio claimed by the screen manufacturers generally assume that the light coming from the projector (with external filter as necessary) is perfectly (i.e., 100%) polarized and in reality this will not be the case.  It is the unpolarized, or randomly polarized, light that becomes the source for 3D crosstalk/ghosting.  Since neither the polarizing filters used with projectors nor the polarized 3D glasses are ever perfect in rejecting all light having the opposite polarization, the net result (i.e., with the contribution of these filters, glasses and the screen) can be expected to be a crosstalk level of perhaps 1% or a little higher (probably significantly higher if circular polarization is being used by the 3D projection system). I used a screen sample of the “Silver Lite” and the “Silver 5D” screen materials to do a quick look at how they performed for 2D projection.  I projected a 100% white test pattern on to the screen samples and also used a Stewart StudioTek 130 sample for reference.  It appeared that both of these silver screen materials were equally bright on axis and based on a quick measurement both appeared to have a similar peak gain of over 2.0 on axis. Also both of the silver screen materials displayed increased grain (or a fine coarse texture) in the image as compared to the either the Studiotek 130 sample and also as compared to my main screen which uses a white matte screen material with an actual gain of about 1.2. The following composite photo shows portions of the same projected 2D image taken with the matte white screen material across the top, the Da-Lite “Silver Lite” material at the bottom left and the Stewart “5D” screen material at the bottom right. The top image from this composite was taken with a longer exposure time so as to provide an image closer to the brightness of the lower photos, i.e., for the higher gain silver screen samples. The level of grain introduced into the image with the silver screen materials is not shown very well by these photos as it was more obvious when viewing in person. Some evaluations by hobbyists have reported that these new screen materials intended for both 3D and 2D projection do not retain polarization as well as the best of the more traditional silver screen materials (e.g., Harkness Spectral 240) specifically designed for 3D projection. Silver screens have a reputation for not being well suited for conventional 2D projection due to a tendency to have “hot spotting” and a limited viewing angle.  The hot spotting limitation can be partially addressed by mounting the projector near the longest possible projector-to-screen throw distance (i.e., using minimum zoom) for the given screen size. Since silver screens are inferior to most conventional screen material for 2D viewing, some consumers using a passive 3D projection system have installed two screens with the silver screen used only for 3D viewing.  One typical arrangement for this is to use a wall mounted, fixed frame screen equipped with a silver fabric for 3D viewing plus also mounting an electric drop down screen, using a conventional screen material (e.g., matte white), such that it will drop down just in front of the fixed frame screen. Screen Size and Gain - As with any video projection system (i.e., 2D or 3D, single projector or dual projector) the screen size and gain need to be selected to match you viewing environment and the projector's light output (specified in lumens).  However, given the need for retaining polarization, the available range of screen gain will be restricted.  See my previous blogs on screen characteristics.  HERE is a good starting point.

Video Processing for Passive 3D Projection Systems

Some of the projector and external filter combinations required to created a dual projector passive polarized 3D projection system can introduce color shifts in the projected images.  This is can be especially the case when LCD (or LCoS) projectors are being used where the projector’s internal polarizing filters result in the orientation of one of the 3 primary colors being rotated 90 degrees from the other two primary colors.  Also some 3D glasses may introduce an additional color shift in the 3D image as it reaches the viewer’s eyes.  You may be able to correct for such color shifts if the projectors that are being used offer a flexible set of user accessible adjustments for color temperature, grey scale and color saturation/hue (i.e., a color management system).  If the projector’s do not have these internal adjustments available, an external video processor(s) can be added between the signal source and the projectors. For this blog I’m not going to get into the details of doing a color and grey scale calibration (perhaps the subject for a future blog), but if the projectors being used include this capability, then it may be possible to create a user memory setting that corrects for the color and grey scale errors that are being introduced by the above mentioned elements within a passive 3D system.  Even if you do not have the hardware and software tools, or the technical knowledge, to attempt to do a formal calibration yourself, you may be able to correct for gross errors in the image.  Many projectors offer a color temperature adjustment, such as allowing you select from 5500K, 6000K, 6500K, 7000K, etc.  The HDTV standard calls for 6500K color temperature and selecting a different value when projecting 3D video may provide a more accurate image by helping to correct for color shifts within the projection system.  Going one step further you may be able to adjust (by sight and without instrumentation) the Red, Blue, and/or Green Gain Controls to correct for large color shifts (however be certain to note the original settings so they be restored if needed).  Many projector’s offer a few user memory locations for storing your own custom settings and this is where you will want to store the settings you come up with for 3D viewing, without altering your existing settings used for normal 2D viewing.  All of these controls adjust the entire image and cannot correct for non-uniformity of color as can be introduced by some projector and external filter combinations (as discussed in my previous blogs in this series).  If the 3D images from both projectors show a very similar color shift and if the projectors you are using do not include user accessible controls that can be used to correct for this color shift, then it may be possible to insert a single video processor, such as some models from DVDO or Lumagen, just after the video source and before the HDMI video splitter.  If the color shifts are different or if only the right or left image has a color shift issue then an external video processor could be inserted between the HDMI video splitter and the projector(s) whose image needs to be corrected.

The Role of Frame Interpolation

In recent years several projector manufacturers have begun to include a feature for inserting additional artificially generated video frames between the original video frames.  HDTV and projector manufactures may describe this feature using such terms as “Smooth Motion”, “Creative Frame Interpolation”, “Frame Creation”, “Motion Flow”, “Clear Scan”, etc. (HERE is a Wikipedia article on the subject). These new frames are generated by a processing chip in the projector using an interpolation algorithm that analyses the previous and next video frames and produces appropriate intermediate frame(s).  In some cases a single new interpolated frame is inserted between original frames and with more aggressive use several new frames are generated.  For example, commercials movies contain only 24 unique frames per second and if the displayed image is refreshed at 120 times per second (i.e, 120 Hz), then up to a maximum of four interpolated frames could be inserted between each original frame (i.e., for 5 total frames each displayed for 1/120 sec. during each 1/24 second interval). Many dedicated home theater owners, and film lovers, object to the use of frame interpolation when viewing 2D movies as it’s use tends to create a live video, “soap opera” look.  The effect becomes progressively more severe when more than one interpolated frame is inserted between each original frame.  Some of these same critical viewers may however, on occasion use frame interpolation for watching live sports events.  Even many of those that do not like the effect of frame interpolation for 2D movies, feel that it does offer some benefits for viewing 3D movies (I fall into that category of viewer).  The primary goal of a well made 3D movie it to create a window into a virtual world and I find the low frame rate of commercial movies (i.e., 24 frames per second) cannot present the realistic motion smoothness that we experience in the real world.  Having the movie producers use a higher frame rate for the creation of their movies would be the most optimum solution while frame interpolation provides an after-the-fact solution (but certainly not a perfect solution).  Peter Jackson in shooting he upcoming 3D feature “The Hobbit” at the increased frame rate of 48 frames per second and James Cameron has indicated that future Avatar sequels will also be shot at higher frame rates.  If you feel that increase motion smoothness is important to your 3D viewing experience then you should consider using projectors that offer the frame interpolation feature. _____________________

My next blog will discuss using “Wavelength Multiplexing”, instead of polarization, for creating a dual projector passive 3D system.

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