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General color questions

 
 
ronviers@gmail.com
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      07-27-2006
Hi,
I have been trying to nail down some terms and concepts.
Do Value, Brightness, and Luminance all refer to the same thing - that
is, the distance away from the tip of the color cone?

If they are referring to the same thing then I have the following three
questions:
Should Brightness be reserved for the Adobe RGB and sRGB spaces?
Should Luminance reserved for the LAB space?
Is Value reserved for conversations about general color theory?

Then I have these questions:
What is the difference between Luminosity and Luminance?

What is the difference between a perceived brightness distribution and
a brightness distribution?

Is color tone the same thing as cast?

My Canon manual defines its histogram as a graph indicating brightness.
So what kind of histogram is it? Would it be a Luminance, Luminosity,
RGB or composite RGB histogram?

Why do changes in saturation have such a weird effect on the
'Colors' histogram in Photoshop while in RGB mode? Shouldn't the
color ratios stay the same - making the effect simple?

Thanks,
Ron

 
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stauffer@usfamily.net
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      07-27-2006

http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
> Hi,
> I have been trying to nail down some terms and concepts.
> Do Value, Brightness, and Luminance all refer to the same thing - that
> is, the distance away from the tip of the color cone?
>
> If they are referring to the same thing then I have the following three
> questions:
> Should Brightness be reserved for the Adobe RGB and sRGB spaces?
> Should Luminance reserved for the LAB space?
> Is Value reserved for conversations about general color theory?
>
> Then I have these questions:
> What is the difference between Luminosity and Luminance?
>
> What is the difference between a perceived brightness distribution and
> a brightness distribution?
>



Value has no unique meaning in optical engineering. Both brightness
and luminance may have a couple of meanings. The meanings may be
different in modern digital photography than their meaning in
internationally agreed on radiometry and photometry.

Luminosity does have a well defined photometric definition, while
luminance has a couple of definitions.

One difference is whether you consider absolute values or normalized
values. In many discussions in digital photography the term luminance
can be a normalized value, as it is in video usage.

> Thanks,
> Ron


 
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ronviers@gmail.com
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      07-27-2006

> Value has no unique meaning in optical engineering.


I was sure I had heard about Value, other than from the HSV color
space. So I dug out my color book (water color) and sure enough there
it was. Not only was value there but also I was able to pinpoint the
source of my ongoing confusion about colors. I found it by looking at
how a value scale is created. To create a value scale you add more and
more of a hue starting with white. Fine, but the problem was that
adding more of a hue to white had been my working definition of
saturation - even though others had given me the correct definition
of saturation. So I read the author's definition of Saturation where
she explains:
Saturation is the amount of color perceived when compared to the amount
of white or black.
You can see that it would be easy to confuse this definition with the
correct definition of value.
So I just want to ask this one more question so I am 100% positive that
I understand the definition of saturation.
If I have a color cone sitting on my desk, with the black tip sitting
on the desk and the cone pointing up and I extend a line from a point
at the center of the cone, parallel to the desk, to the outer edge of
the cone. Then all points on that line differ only in saturation. The
hue and brightness would remain constant all along that line.
Would that be accurate?
Thank you for helping me get this cleared up,
Ron

 
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