Hi Yulia,
> From: "Tyukhova, Yulia" <[email protected]>
> Date: March 2, 2012 9:14:15 AM PST
>
> Greg,
>
> I've been checking histograms with CANON Digital Photo Professional. I was
> checking L channel, I guess I need to make sure RGB values are below 200 as
> well. Are there any suggestions on this issue?
Photosphere or Photoshop can give you an RGB histogram (along with max. values)
from an 8-bit image.
> Martin,
>
> That is a very interesting suggestion! I guess I can see the difference
> between the measurements of two light sources.
>
> >> ..the photopic sensitivity curve V(lambda) currently used is incorrect. It
> >> underestimates blue wavelengths quite a bit.
> >> You should really use a CCD camera with a photopic filter to get better
> >> values..
>
> You say that V(lambda) underestimates the values. But doesn't CCD camera's
> photopic filter have the same response curve? So, it would have the same
> mistake.
Typical cameras do not follow the CIE standard observer curves for a number of
reasons. Instead, they use red, green and blue bandpass filters and a color
matrix optimized for color reproduction using some set of patches. It's an
under-constrained problem, and different makers will optimize their color
transform matrix differently. This is why dcraw tends to be more reliable --
Dave Coffin always derives his CTM the same way and doesn't bias it towards one
set of colors or another.
Even so, Martin is correct that highly saturated colors, such as those produced
by typical LEDs, will often cause problems for reproduction and luminance
estimation. There is no easy fix for this, unfortunately.
Best,
-Greg
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