Hi Lenny,
Your math is correct, there are 64k possibilities (separations?). As to the photoshop, I think if you are in 16-bit mode the tone curve is presented to the user as 0 to 255 they are expanding it to 16-bit internally and I see your point that in so doing one has only a crude control over the curve that is 16-bit with an 8-bit control. If they allowed user input of a curve one would generate the desired curve to perfection, but that would be a 64k table to fill. Ouch. My only suggestion is be very precise with the curve tool in PS. Maybe someone can suggest a better suggestion.
I checked the Hamamatsu site concerning the PMT's spec's. I don't have the PMT number used in our Premiers but a PMT I looked at had the following features. If one took dark current as a "step" and maximum current as the max limit as the number of steps, then you ended up with a PMT that had 20,000 steps. Interesting result (matched Haddon's number). A more interesting was take the minimal value the PMT could sense and use that as step size. To my way of thinking this is the proper value to use. The PMT then had 6,000,000 steps. In otherwords, a 22-bit DAC could realistically be used to capture a color channel. That would yield 66-bit RGB. No way to print that :-)
_ .. --
Tim
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