Re: L* versus rods
Re: L* versus rods
- Subject: Re: L* versus rods
- From: Ken Fleisher <email@hidden>
- Date: Mon, 05 Dec 2005 20:33:07 -0500
My message does not seem to have gone through, so I'm re-sending. I
apologize if this ends up as a double-post...
On Dec 5, 2005, at 5:53 PM, eugene appert wrote:
Then what exactly does the CIE lab model represent?
Briefly, CIELAB is an attempt to quantify color in a space where equal
changes in L*, a*, or b*, will result in equal perceptual color
differences. Because of Weber's Law, the "just-noticeable-difference",
or JND, as "measured on the physical scale were not equal as the
stimulus intensity increased, but rather increased in proportion to the
stimulus intensity." (Color Appearance Models, 2nd ed., by Mark
Farichild, page 38).
What this means, for example, is that as something gets higher in
luminance, it requires a greater increase in luminance before a
difference will be "just noticeable". So something that is very dark
might require a very small change in luminance before the change is
detected, but something that starts very bright will require a much
larger change in luminance before it can be detected.
This might be easier to think of in terms of a different perception. If
you are holding something that weighs 50 grams and you add another 50
grams to it, you will certainly be able to feel the addition of the
extra weight. But if you start out holding 20 Kg, it is unlikely that
you will be able to detect the addition of an extra 50 grams.
CIELAB is an attempt to make judgment of color differences easier to
quantify. A change of 5 L* units is supposed to be perceptually the
same whether L* 10 is changing to L* 15, or L* 80 is changing to L*85.
Those 5 units should be "approximately" perceptually equal everyplace
on the scale.
That is generally what the CIELAB model was designed to represent.
Ken Fleisher
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