I have done a few tests to demonstrate the effect of using a screen that retains polarization vs. one that doesn't. The goal of these tests was determine for HT owners using a LCoS projector (i.e., that puts out polarized light but does not use polarization to separate the right and left images), how much real increase in image brightness for 3D viewing results from using a screen that retains some significant polarization. In order to get this benefit the viewers must be wearing liquid crystal shutter 3D glasses using the same polarization orientation as used by the projector.
My test setup is using my ceiling mounted JVC RS40 projector that is mounted (lens center) 7 ft. above the floor and 13.5 ft. from the screen. I have an electric screen which I rolled up half way then attached 3 sample screen materials at a time hanging from the bottom bar on my electric screen. This resulted in the center of each sample screen fabric being about 62" above the floor. I used two different variations on the combination of sample fabrics (see below) and did tests from two different viewing positions. Both positions were centered directly in front of the screen sample. Since one of the samples in all cases was a Da-lite High Power fabric (gain 2.4 version) which is retro-reflective, I did one set of measurements with the camera at 60 inches above the floor (or 24 below the level of the projector) and directly under the projector to show a case where the HP fabric is providing close to its nominal gain. I also did a second series of tests with the camera located 12 ft. from the screen samples and 40 inches above the floor, which would be near a normal seated viewing position.
A few qualifications are in order.
- The screen sample materials are all from Da-lite and are attached to paper cards. Some of these materials would normally be used with a tensioned screen and as a result my sample shows some uneven image brightest due to not hanging perfectly flat.
- Also I had my camera in auto-focus mode and some of the photos are somewhat out of focus, but you can still see the relative brightness between the 3 screen materials shown in each photo.
- The exposure varies some between photos, but this should not be an issue since what you should be looking at is the relative difference in brightness for the 3 screen samples shown within any given photo.
- Tests were conducted projecting a white test image and three tests were made. First-representing 2D viewing with no glasses; Second-thru a polarizing filter oriented to match he projector representing viewing thru 3D glasses with the correct orientation; Third-thru a polarizing filter oriented to not match the projector representing viewing thru 3D glass with the wrong (shifted 90 degrees) orientation.
- To verify that the results reported below are fully applicable to actually viewing 3D program thru shutter glasses, I played one of my Blu-ray 3D discs and viewed the image thru my JVC shutter glasses. The results matched what I had observed by viewing (and photographing) thru a linear polarized filter.
SERIES 1 - TESTS
- In both series of tests the left screen sample is the High Gain (gain 2.4) sample and the center sample is the Matte White (gain 1.0) sample. Neither of these screen fabrics maintain any significant level of polarization. In both series the right sample is a Da-Lite screen material that is known to maintain a significant amount of the polarization. As a result the left and center screen materials show no noticeable increase or decrease in image brightness when viewed thru 3D glasses have the same vs. opposite orientation from the polarization as the projector. However, the right screen samples do show a significant brightening and dimming of the image when using 3D glasses with the correct vs. wrong polarization orientation.
Screen samples: High Power (2.4) - Matte White (1.0) - Cinema Vision (1.3)
SERIES 2 - TESTS
Photo #1 - seated position - 2D viewing (no 3D glasses)
Photo #2 - seated position - 3D viewing with glasses w/correct polarization
Photo #3 - seated position - 3D viewing with glasses w/wrong polarization
Photo #4 - 60 inch high position - 2D viewing (no glasses)
Photo #5 - 60 inch high position - 3D viewing with glasses w/correct polarization
Photo #6 - 60 inch high position - 3D viewing with glasses w/wrong polarization
Screen samples: High Power (2.4) - Matte White (1.0) - Video Spectra (1.5)
Photo #7 - seated position - 2D viewing (no 3D glasses)
Photo #8 - seated position - 3D viewing with glasses w/correct polarization
Photo #9 - seated position - 3D viewing with glasses w/wrong polarization
Photo #10 - 60 inch high position - 2D viewing (no glasses)
Photo #11 - 60 inch high position - 3D viewing with glasses w/correct polarization
Photo #12 - 60 inch high position - 3D viewing with glasses w/wrong polarization
When viewing from the 60" high position the Da-Lite High Power (2.4) produced the brightest image for 2D viewing (without 3D glasses). This projector-screen-viewer geometry is well suited for using the High Power screen fabric and would be equivalent geometry to having the projector mounted at approx. 60 to 64 inches above the floor and viewing from a normal seated position. Using the screen gain calculator develop by AVS Forum member FLBoy (see link in Post #1), the expected screen gain with the High Power screen material using this projector/screen/viewer geometry is 2.1 (lowing the projector position to just above eye-leve would be required to get the maximum gain from the High Power screen).
When viewing from the 60" high and screen center position the Da-lite High Power still has the brightest image for 3D viewing using shutter glasses with the same polarization orientation as the projector, but the Da-Lite Video Spectra (1.5) was a close second as it showed significant benefit from its ability to retain polarization. However, the High Power has faster fall-off in image brightness as the viewer moves off of the screen center-line as compared to the other screen materials that were tested.
When viewing from the seated position (approx. 40" above floor level) the High Power, Cinema Vision and the Video Spectra were all brighter for 2D viewing (without shutter glasses) than the gain 1.0 matte white material. The High Power screen material appeared similar to the Video Spectra (gain 1.5). Using the Screen Gain Calculator developed by AVS Forum member FLBoy the expected gain with this projector/screen/viewer geometry is 1.76.
When viewing from the seated position (40" above floor level) the Da-Lite Video Spectra (1.5) has a noticeably brighter image than the other screen materials tested for 3D viewing when using shutter glasses with the same polarization orientation as the projector. The Cinema Vision (1.3), while noticeably not a bright as the Video Spectra 1.5 did show benefits from its ability to retain polarization and was the 2nd brightest for 3D viewing from the seated position.CONCLUSIONS
If you can mount the projector approx. 24 inches, or less, above the viewer's normal eye level then the Da-Lite High Power is a good choice. However, for ceiling mounted projectors that are well above the viewer's eye-level (in my case about 44 to 48 inches above my seated eye level), using a anglar reflective that retains a significant level of polarization, such as the Da-Lite Video Spectra 1.5, can produce a substantially brighter image for 3D viewing thru 3D shutter glasses.
For screens that do retain any significant level of polarization, the 3D glasses must have the same orientation for the polarization as that used by the 3D projector (i.e., for the JVC 3D projectors use the JVC glasses and not the Xpand universal glasses (which have the opposite orientation).