Originally Posted by gregr
Gamma A is almost exactly the same as Gamma 2.4 (i.e. about 2.3 from 30%-40% and up - see my comments above about the 1.8-2.6 gamma values) but then rolls off to nearly 2.1 at 5%. So it does indeed increase the brightness of the darkest details.
re: Grayscale adjustment. You always want to use the maximum green possible (i.e. that you can adjust to D65 at around 75% luma) because that will maximize your peak brightness and the contrast ratio. So set green at max and then adjust the red and blue to try to get D65 at 75%. You may have to reduce green by a notch or two else you may not have enough red or blue. I think you will find the RS20 grayscale is very flat from 75% to 100%, but I don't let 100% go above 2 dE (LUV) for any of my projector measurements. That is visually very good, and it ensures my contrast ratio and brightness measurements can be sensibly compared between projector reviews.
At the other end of the grayscale, don't try to get D65 at 30% or 20% if it causes the error to blow up at 10%. Try to keep the dE (LUV) deviation at 2 or less down to around 40% and only gradually creep up at 10%. I've seen people try to set 30% or 20% to D65 on some projectors, and let 10% jump to 10 dE or higher. You will get much better picture results if you keep 10% to 5 dE or less even if 20% or 30% rise to 2 or 3 dE. Of course, on the RS20 you can use the custom gamma function to achieve even better results. My RS20 was 5 dE at 10% and within 2 dE from 40%-100% right out of the box.
Thanks very much for this Greg, it's extremely useful. A few quick questions though: do you use gamma reference to compute dE for greyscale? In HCFR, my dE looks like 10 at 10IRE but if I check "use gamma reference to compute grescale dE" it goes down to less than two. I haven't really fine-tuned my greyscale, but we have established (led by Mark Petersen) that on the rs20, pushing some [edit: offsets] up has a detrimental effect on black levels. Even one notch up on red, for example. really affects black levels badly.
So when one ends up - after having done a quick adjustment of gains and offsets like I did for my 3rd attempt - with a greyscale that looks like the one attached, with a 10IRE which does probably have a 10 or more dE in order to get the rest of the greyscale flat, what do you do to lower this dE at 10IRE:
1) compromise black levels and use positive offsets to get a better dE at 10IRE
2) keep offsets as they are and use gamma controls to lower the dE at 10IRE.
3) do nothing to priviledge black levels and compromise color accuracy?
Which leads me to the obvious last question: if we use gamma controls to lower dE at lower IRE (5-20), therefore using positive [edit: adjustments] at some control points, does it have exactly the same detrimental effect as using positive offsets? I have checked the effect with the offsets with a luxmeter and it's there, but I haven't checked with the gamma adjustments.
Here were my color temp settings to get this greyscale:
Custom color temp (new lamp)
Gain R = -30
Gain G = 0
Green was low at the top end, so gains were quite straightforward, although I made the mistake of making sure I got d65 at 100IRE (old habit as there is no gamma control point at 100IRE) while I was calibrating at 75% stim for the first time, so I should have paid more attention to D65 at 75%. For offsets, this was as close as I could get by using negative values only. I was doing offsets at 30 IRE as it's the lowest reliable point for the i1pro and I decided not to use the trained d2 for this attempt as I was concentrating on gamut work.