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Author Topic: DLP projectors and rainbow effect Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-03-2009 07:15 PM                     That is a completely different sort of rainbow. You are merely seeing the unfocused three DMDs...you'll never see that on a focused image. The rainbow effect from single chip DLPs will appear on all focused images...particularly ones that have your eyes moving and even more emphasized on B&W images. This comes from the fact that the colors are presented in a time-shifted fashion due to the color wheel. The CP2000-ZX, like all DCinema projectors are 3-chip DLPs and present all three colors at the same time. Steve  |  IP: Logged Julio Roberto Jedi Master Film Handler Posts: 938 From: Madrid, Madrid, Spain Registered: Oct 2008 posted 10-03-2009 09:45 PM                    Well ... to say all 3 colors "arrive" at the same time, is probably not entirely correct for this type of subtle effect, as the DLP are basically pulse-width binary modulator devices (unlike the Lcos, which are continous liquid modulators even if digitally driven) and the intensity of each color is timed to the amount of time the micro-mirror stays in the "on" position compared to the "off" position. Of course, to render a particular color, the mirrors must be on-and-off a different amount of time for each primary color, thus, i.e., a 10% red, 70% green, 20% blue color would mean that the red DLP chip remains "on" for ~10% of the time (and off the rest of the time), 70% of the time for the green and 20% for the blue. For a (incredibly small) split of a second, the colors are subtetly off-phase while the chips complete the PWM scan cycle. Our brains then time-fuse the pulses of on-off light into a continuous "gray scale" for each primary color. You know: our brains become a cheap (free) digital-to-analog converter I don't know if this effect is visible by the human eye at all (happens too fast and only on colors with significant difference in components). If your retina moves too fast (i.e. move the eye with respect to the imager, or like moving the projector of effectively moving a reflected image off a mirror), then one part of the retina would've collected so-many red photons on a red-predominant color while the blue and green chips had not had the chance to reflect enough amount of light (on-cycle) to make-up the correct shade when they "landed" in a different part of the retina (since the "head" or the "image" moved). Again, this is suppossed to happen so fast that, at worst, could only happen on colors with over 50% of difference in intensity between one of the 3 primaries for less than 288th of a second, perhaps an order of magnitude less depending on how the DLP's are driven (i.e. bitsplitting is used), so I don't think it should be visible as a rainbow, but perhaps as a slight artifact (slight color fringe) at worst. So the (d)effect is (theoretically) neglible, but it's not "perfect" by the time-based nature of the DLP to produce different shades of "gray", which are different for each individual color. With DLP, there is always SOME (miniscule?) time-based artifacts. But the effects noticed here are probably a lens aberration/optical, specially if watching the image through viewing glasses (and even more if reflected from non-first-surface mirrors), as all the light is not perfectly color-collimated. Hell, all I have to do is watch my computer monitors through my viewing glasses at an angle (i.e. not straight on 90º) and I can see the subpixels of the LCD going in different directions ...  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 10-05-2009 09:37 PM                     Don't forget that at least on Christie projectors the Red (I believe) is just a tad bit out of focus. This is not a design flaw in the prism but just the way physical constraints made things happen in the prism itself. So it is technically impossible for all three colors to be in focus at the same time!! You can set them up individually to be in focus when you look at them but never will you see all three in focus at once... Mark  |  IP: Logged

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