DLP Front projection contrast and clipping question - AVS Forum
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post #1 of 4 Old 12-20-2012, 05:08 AM - Thread Starter
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I have a question for those how have been calibrating font projectors for a long time. Does the change in spectral quality of a lamp as it ages effect the contrast setting and point of clipping? For discussion purpose lets assume DLP, 2point white balance. Gray scale has been set to the extent of the the controls with minimal tail chasing. If this same process was executed every 1000 hours over the life of the lamp, would one expect to see a drop in the contrast setting to prevent clipping as you reach and exceed the 3000 hours of lamp life. I know the lamp will get dim, and in my case red has always been the first to clip. However, this time around , (+3000 hours) to prevent visible clipping (blue discoloration), contrast has had to be turned down considerately from where it has historically been set. When one thinks about how the DLP system work, this would seem to be normal if the light source just does not have the red that is needed, but just wanted some verification.
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post #2 of 4 Old 12-21-2012, 02:57 PM - Thread Starter
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post #3 of 4 Old 12-22-2012, 01:19 PM
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Yes, the spectral output of the lamp changes as it ages. So does the total light output -- a lamp's peak illumination can drop by 50% or even more when you compare new vs. aged output. When you re-do calibration periodically, you need to address EVERYHTING again... grayscale, gamma, and color. Any given type of projection lamp will generally lose output in similar ways, but when you see different projectors and different lamps, you tend to see differing amounts of drift (different colors drift by different amounts).

You may or may not have to change the Contrast setting over the life of the lamp... it all depends on the lamp, optics, filters and how the projector processes images - and perhaps most important of all, how much "headroom" you had when the lamp was new. For example, if your projector screen combo had to have everything set to "max" settings (iris opened all the way, Contrast set quite high, "high" lamp mode, etc.) and you just barely hit 16 fL (or maybe you can't even get to 16 fL), you'd be more likely to see drift that would require more adjustment and more light loss as the lamp ages. If your system has PLENTY of light with a new lamp and you were able to use "low" lamp mode and a mid or smaller iris opening (or automatic iris) and you could run contrast quite a bit lower than the max settings, you have a lot of things you can do as the lamp ages to keep the light output more consistent (like opening the iris more, going to "high" lamp mode at some point, raising Contrast, etc.). Running into clipping or other big issues would be less likely when the projector/screen combo has plenty of extra light when the lamp was new.

Any given type of lamp is likely to have similar losses as it ages... you may find your lamps lose red as they age... that's fairly common. But a small variation in the lamp could cause green or blue to drop more than other colors during aging... or 2 colors could drop while the 3rd color stays fairly robust. So you can't just assume you need to tweak red higher or green & blue lower as the lamp ages, you really have to have measurements to guide you. Calibration MAY get trickier with an older/aged lamp also... particularly if you are trying to maintain 16 fL at the screen. You might have to make a lot of changes to keep d65 accurate, keep color accurate, and keep gamma (essentially grayscale luminance) accurate. Any and all calibration and light output controls are open to being used as the lamp ages.

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post #4 of 4 Old 12-22-2012, 01:56 PM - Thread Starter
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Thanks Doug! Still have lots of light on a tiny 2.8 HP screen with lamp on eco and irs closed.. 20fl so no issues with output. Just found it unsettling with similar 2 point setting I would need to make an 8 click decrease in contrast to prevent discoloration. However, the image is excellent and gamma is flatter than it has ever been.
Since I posted this, I tied setting the BR to RGB standard and the projector displays WTW (never had that ability on my old BR so things were clipped at 235) and I can see that this contrast setting is correct.
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