In most cases the correct brightness and contrast settings would not change at all as the lamp ages. That may seem puzzling at first, but it's true.
If the PJ has IRIS controls or a lamp power selection, those are what should be used to bump up the light output as the lamp ages. White balance will also need touching up. I would recommend at least one touch up after the initial calibration for the life of the lamp.
In the case of Sony SXRD models, I've seen abnormal drift and deterioration that could not be explained by lamp aging, so I have to assume the light engine can drift significantly on them as well as the lamp. Those would need more frequent touch ups.
I tend to start with the "Low" lamp mode (assuming the projector/screen combo produces sufficient light in "low" lamp mode, of course) and increase Contrast over time to compensate for the lamp getting dimmer. At some point you either run out of Contrast setting range to make the projector any brighter, or the projector begins misbehaving at higher Contrast settings... so when I reach the practical limit for Contrast in the projector, I will switch to "high" lamp mode, and bring Contrast down again to get a reasonable amount of light on the screen.
Keep in mind that the lamp will likely age faster once you have selected the "high" lamp mode, so that's (when you select "high" mode) about the time you want to order your next projection lamp if you don't keep a spare for "emergencies".
If you start with 16 fL for 100% white... how often should you check 100% white to see if you should increase Contrast to compensate for the aging lamp? Well, it's up to you... you could drive yourself nuts and check every 100 hours. Or you could re-adjust every few hundred hours.... just depends on how much hassle it is to get all setup for measurements and what your tolerance for that is. Most lamps drop off enough over 400 hours that you might find your self bumping Contrast up a little.
Then there's the issue of how the spectral distribution of the lamp changes as the lamp ages... chances are very high that the specta of the lamp changes a fair bit as the lamp ages and that will make your grayscale drift and your colors may drift also. So while you are checking Lumiance to see if it has dropped too low (and probably raised it a few clicks), you may as well re-measure grayscale and make appropriate tweaks, then do the CMS measurements and adjustments again also. Chances are the lamp is going to drift towards yellow as it ages so 100% white isn't your only concern as the lamp ages.
And all this lamp stuff makes me wonder why LED light sources for projectors haven't become massively dominant in the last couple of years. I calibrated an LED-powered projector several years ago... 3 at least. I think. It was bright enough and calibrated nearly perfectly with the available controls. But manufacturers are not abandoning expensive projection lamps even though today's LEDs are even brighter than those available just 3 or 4 years ago. And there's always the old trick of using 2 or 3 green LEDs for each 1 red and 1 blue LED in order to make green as bright as it needs to be (roughly 72% of the light we see is green light). How much better would you feel about your projector if you were going to get 50,000 to 100,000 hours of viewing before you need to think about replacing the light source?
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