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 blogs of this series the discussion has focused on passive 3D projection system that use 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 starts the discussion on the use of dual projector system that uses color bandpass filtering (i.e.,  “wavelength multiplexing”), instead of polarization, as the means to separate the right from the left images.  In simply terms, wavelength multiplexing, uses two projectors equipped with filters that pass only very narrow bands of colors within the visible spectrum with one of the filters passing a set of colors that is slightly offset in wavelength from the set of colors passed by the filter being used on the second projector.  The viewers then must wear passive 3D glasses whose lenses are filters that pass narrow bands of the visible spectrum that matches those being used on the two projectors. The technique of color bandpass filtering was briefly discussed in Part 1 of this series of blogs on dual projector passive 3D system and is expanded on below and with the next blog in this series. Shown below is an illustration (repeated from earlier blogs in this series) showing the functional components that make up a passive 3D projection system.

 

  Part 3 of the blogs in this series discussion covered all of the functional elements shown in the above diagram, to the left of the projectors and that earlier discussion equally applies in the context of this new blog.  Therefore the signal path from the 3D source device up to the input to the two projectors need not depend of whether polarization or color bandpass filtering is be used as the means to enable the 3D capability.  The considerations for selecting a projector to use in a dual projector passive 3D setup, as previously discussed  in Part 3 and Part 4 of this series of blogs, are also similar, but not identical, for a 3D passive projection system using color bandpass filters as compared to those 3D projection systems using polarizing filters. 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 diy dual projector alternatives for passive 3D.  

Color Bandpass Filtering Overview

The 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.  There are two different passive 3D projection systems available to the hobbyist that separate the right from left images by using optical interference filters that pass 3 or more narrow bands of wavelengths to create the full color image.  These optical filters incorporate 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. One of the  leading developers 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 second type of color bandpass filters available to the hobbyist is being sold as the Omega 3D system and uses the same type of filters developed for the Panavision 3D system.  Runco uses the Panavision 3D system on their mega-expensive dual projector passive 3D system, Model D-113d, that I saw demo’ed at the CEDIA Expo 2011 (Septerber 2011).  In concept, the approach used by Panavision/Omega 3D is similar to that used by Dolby/Infitec, but each filter passes 5 narrow bands of spectrum, rather than 3 as used by Infitec.  These two technologies will be discussed in some detail below and in my next blog.  

Selecting the Projectors

For this current discussion I am assuming that two projectors will be purchased and will therefore the following discussion is intended to steer the reader to selecting projectors that are well suited for this application.

1. Select two identical projectors.  There are several reasons for this advice.  First you will want to have similar image brightness and image characteristics such as contrast, gray scale, black level, etc.  Second, different projector models will have different delays between when the signal is input into the projector and when the resulting image is displayed on the screen.  This delay comes from the video processing being done within the projector and it is important to have the displayed right and left images that make up the 3D pair closed synchronized in time.

2. When installing two projectors for a passive 3D setup the projected images should have near perfect alignment.  While pixel perfect alignment is not absolutely essential for an acceptable 3D presentation, the closer to pixel perfect alignment that can be achieved the better.  If the two projectors are to be located one above the other (i.e., stacked vertically) and are located directly back from the center of the screen (along the screen’s right-to-left center-line), then having a vertical lens shift feature on the projector will be necessary to achieve optimum alignment of the two projected images.  If the projectors are to be placed beside each other, or if a vertically stacked pair are located off of the screen’s center-line, then having a horizontal lens shift feature will be necessary to achieve optimum alignment of the two images.  Also note that projectors with lens shift may have a limited range of adjustment and this can constrain where the projectors can be mounted in order to achieve correct image geometry on the screen (the same as with a single projector 2D installation).  So my bottom line advice is when selecting projectors for a dual projector 3D passive setup, choose models with lens shift.  Some DIY hobbyists have used projector’s without lens shift, but the results will be less than ideal.

3. Select projectors that you will be satisfied with for 2D projection.  Let’s face it, you will probably be watching a lot more 2D video in your home theater than 3D.  So select projectors that will provide the level of 2D performance you will be satisfied with (i.e., for such factors as contrast ratio, black levels, color accuracy, gray scale accuracy, etc.) and which have the mounting flexibility needed for your specific home theater installation.  This latter point involves selecting a projector with adequate zoom ratio, throw distance, lens shift adjustment range, and lumens of light output appropriate for your specific home theater setup (e.g., screen size and gain).  However, when you include the additional requirements and constraints for use within a dual projector 3D passive setup into the set of requirements, this will likely rule out some projector models that you might otherwise consider for a single projector 2D installation.

4. Unlike with the polarized passive 3D projection systems, discussed in prior blogs in this series, polarization of the light within the projector makes no difference with systems using color bandpass filters.  As a result, projectors that do not have consistent orientation for the polarization of the three primary colors can be used just as well with color bandpass filters as can other projectors having no polarization or fully aligned polarization for their projected light.

5. Color bandpass filters, both those used with each of the projectors and those used for the lenses of the 3D glasses, have a lot of light loss.  The total light reaching the viewer’s eyes will be less than 20% (may be much less) as much as using the same projector and screen for viewing normal 2D programs.  Therefore, you should select projectors capable of substantially higher lumens of light output than what would be needed if 3D were not a consideration.

 

Light Loss with Color Bandpass Filters

Any color bandpass filtering system used for 3D can have a lot of light loss.  The specific magnitude of light loss and the impact on the image colors (if any) can depend on the spectrum of the light being output by the projector’s lamp.  Originally such color bandpass filters were developed for the commercial cinema market where projectors use lamps filled with Xenon gas.  Pictured below is the lamp used in one of the most popular projectors used for Digital Cinema.

 

The spectrum of the light produced by a Xenon arc lamp is different from that produced by a Ultra High Performance (sometimes referred as Ultra High Pressure or simply UHP) arc lamp, using mercury vapor instead of Xenon.  The UHP lamps are most commonly used in consumer-level video projectors.  The following figure shows a color spectrum plot for a typical Xenon lamp and also for a typical UHP lamp. Since color bandpass filters rely on passing a set of very narrow bands of color, of slightly different wavelengths for the right vs. left images, the overall efficiency for the projector and filter combination will depend on the amount of energy being emitted by the lamp, and as modified by any internal filters used within the projector, at the specific wavelengths of light that are being passed by the bandpass filter.  

Infitec/Dolby 3D Technology

The Infitec filter system (the Infitec web site is HERE) 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 could be used for 3D projection with the Infitec passive 3D system.  However, since there is a lot of light loss with the Infitec system, a higher gain screen may be appropriate as a means to overcome some of the light loss when viewing 3D. Each of the Infitec filters pass 3 narrow band of spectrum as illustrated in the following figure.

 

As mentioned above, the Infitec filter system can have a lot of light loss and according to measurements performed by one hobbyist using such filters and 3D glasses in combination with a consumer video projector (i.e., using a UHP lamp) the light loss was approx. 94%, or the light level reaching the viewer’s eyes through the 3D glasses was only about 6% as much as viewing 2D when using the same projector operated in the same operating mode.  If you note wavelengths being passed by the Infitec filters in the above figure and compare this to the light spectrum from a typical UHP lamp (as shown in the earlier spectrum plot) you will see that for the specific wavelengths associated with the green and red components being passed through the Infitec filter fall where the typical UHP lamp’s output is much lower than for the blue component.  This is an issue with UHP lamps whose light output vs. wavelength has a lot of variation, with much more variation over the visible spectrum as compared to Xenon lamps (as seen in the earlier spectrum plot) which have much more uniform output over the visible spectrum. While the Infitec filter system can provide good separation of the right and left images as seen by the viewer’s right and left eyes, and thus low 3D crosstalk/ghosting, the light loss with this filter system make it not very practical for use with the vast majority of consumer video projectors (i.e., using UHP lamps). ________________________________

My next blog will continue this discussion with a look and mini-test of the Omega 3D/Panavision filter system for use with a passive 3D projection system.