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.
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.
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.
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). ________________________________