Color model, brightness?

1. Aug 25, 2010

posixbar

Forgive me if this is the incorrect section of the forum for a question like this.

If I already have used the CIE Chromaticity diagram to plot a color with dominant wavelength of 600 nm and maximal purity, how should I proceed to describe a color with half the intensity of said color -- i.e. a color with half the brightness of a dominant wavelength of 600 nm with maximal purity? What model, and how?
Obviously I can't use the CIE Chromaticity diagram as it doesn't describe luminance, what type of diagram should be used?

2. Aug 25, 2010

Dr Lots-o'watts

If you don't change the color, then the color is the same. Am I wrong?

Seems like you're only dimming the light.

3. Aug 26, 2010

posixbar

I think that would depend on your definition of a color, if the definition includes the attribute brightness, then a color with a different brightness would implicitly have to be a different color.

However my question is simply, what does a color with half the brightness of a dominant wavelength of 600 nm with maximal purity look like, i.e. what does it look like in a digram -- and what type of diagram/model should I use in the first place?
This is rather confusing to me as as far as I understand 600 nm and maximal purity does not specify anything regarding the brightness -- then when I don't even know the brightness, how can I know what half the brightness would be? Is there some way I can calculate what the brightness must be based on this information alone?

4. Aug 26, 2010

Dr Lots-o'watts

Working in laser physics, I tend to correlate color with wavelength. But I suppose that certain applications must associate color with both wavelength and brightness.

5. Aug 26, 2010

posixbar

I would imagine that in lasers particularly the brightness is very crucial, e.g. a laser class III wouldn't be that bright, but a class IV of the same frequency would be much brighter?

6. Aug 26, 2010

diazona

Yeah, but physicists consider brightness to be a separate attribute from color, unlike people who work in digital art or computer graphics. The concept of a color space as it's used in computing doesn't really hold much meaning in physics.

I think you might be better off asking at a computer graphics forum.

7. Aug 26, 2010

mugaliens

It doesn't take much within the visible spectrum to burn one's retina, and permanently. Thus, the use of the word "brightness" is largely amiss.

Instead, please refer to the much more common terms of frequency and power (watts).

8. Aug 28, 2010

Redbelly98

Staff Emeritus
In addition to the 2 coordinates of the CIE diagram, you have a third variable: the intensity. This gives a total of 3 variables that fully describe what is going on, as should be the case for the human visual system.

It's an alternative coordinate system to, for example, the RGB system used in digital media. Much like spherical coordinates are an alternative to rectangular coordinates.

9. Aug 28, 2010

posixbar

I know, but since the intensity is unknown for the first color, wouldn't this make the intensity parameter also be unknown, except that it should be exactly half of whatever the first one is?
I don't think there is any way to calculate intensity from wavelength and purity as the only information?

10. Aug 28, 2010

diazona

Right. Intensity as a physical concept is independent of wavelength. You would need to know the intensity of the first color, or something equivalent like beam power or photon rate.

11. Aug 29, 2010

Redbelly98

Staff Emeritus
What diazona said.

I was not claiming that intensity depends on wavelength or color. Quite the opposite: it is a third, independent variable.

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