> What is the meaning of an RGB value divided by a time ? Nothing says
> for instance that the sensitivity of the detectors is the same for all
> colors so that even for a picture that looks perfectly grey, you would get
> completely different values for the quotients.
I was thinking of the 'normalized' exposure times that have been
corrected for these hardware issues (not the the physical exposure
times given in say milliseconds). So a R:G:B of 1:1:1 should leave
gray gray.
> I'm just curious to understand in what application that division makes sense.
The main use is for scanning negatives. For negatives, density of blue
is ~3x that of red, and that of green is ~2x that of red (hence the
orange color of the negative mask). So if you scan with 1:1:1 exposure
time, the green channel will only use 1/2 of it's full range. In other
words, you have lost one bit of what is basically your luminance. The
blue channel is filled for only 1/3 and this poor blue definition can
also be very visible to the human eye.
All this can be solved by scanning with a 1:2:3 exposure so that the
full range of all channels is utilized.
-- -- Ewald
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