Color and visible light in general

[JeremyL]

I'm trying to get a better understanding of color and visible light in general and see how every single visible color is formed. Does this sound right?

The sun sends us white light which is a mixture of all the monochromatic colors that our eyes are sensitive to and when we mix both sides of the monochromatic spectrum, we get the extra spectral purples. We can then mix all of the monochromatic colors/extra spectral purples with with white to create less saturated colors. And Black is simply what we see when there is no light present at all. Does this explain how every single color is made? What about mixing the monochromatic colors/extra spectral purples with different grays or even black?

 

[mgb_phys]

The sun sends us white light which is a mixture of all the monochromatic colors that our eyes are sensitive to

Roughly yes

and when we mix both sides of the monochromatic spectrum, we get the extra spectral purples.

Yes - it's important to understand that this is a psychological effect in your brain. The light itself doesn't mix to make purple - the red and blue light is still separate.
Each spectral color triggers different (red/blue sensitive) cells in your eye - your brain decides that x% red trigger and y% blue is 'purple'

There is a good explanation here http://en.wikipedia.org/wiki/Color_vision

 

[Bob S]

"Purple" really isn't a color. It is the visual response to white light (all visible wavelengths between 4000 and 6500 Angstroms) that is missing wavelengths between about 4500 and 5500 Angstroms (green). This is why purple is sometimes referred to as "minus green".
Bob S

 

[mgb_phys]

"extra spectral purples" is the general term for any mixed color, ie not on the edge of the CIE diagram

 

[Q_Goest]

"Purple" really isn't a color. It is the visual response to white light (all visible wavelengths between 4000 and 6500 Angstroms) that is missing wavelengths between about 4500 and 5500 Angstroms (green). This is why purple is sometimes referred to as "minus green".
Bob S

Cool... I heard there's an issue with brown too. What's that all about?

 

[mgb_phys]

Brown is just orange seen in low light.
It doesn't appear as a color in CIE space on it's own - you need orange/yellow and low intensity.

 

[JeremyL]

So does the chromatic color spectrum plus the extra spectral purples make up every fully saturated color that the human eye can see? Why are all of these colors mixed with white such as in the CIE diagram?

 

[mgb_phys]

Roughly the X and Y are the responses of the two types of color sensor cells in your eye (there are really three but two are very similar)
So if 40% of X cells triggered and 60% of the Y you just lookup the corresponding coordinate (0.4,0.6) on the chart and see that you are looking at green-yellow

The colors around the edge of the diagram are pure monochromatic colors from the sun.
The white patch in the middle of the diagram is roughly where the eye is most senstive and the sun's output is highest (because we evolved to be sensitive to this).

To get a real color you also have to take brightness into account, so some colors will look different under different lighting, this is how you get colors like brown.

 

[JeremyL]

So if every single color or chromaticity formed by taking the chromatic colors plus the extra spectral colors and then mixing each of them with white?

Isn't that what the CIE diagram is doing?

 

[jobyts]

Brown is just orange seen in low light.

Is my skin supposed to look orange, on a bright sunny day?
Did you mean shape?

 

[mgb_phys]

 

[DrGreg]

Wow, that is a pretty amazing optical illusion.

Just in case other readers haven't appreciated the significance of this, one circle looks brown, the other orange. But in fact both are exactly the same colour (RGB 209,134,0). Not only that, the two squares surrounding each circle are the same colour too (RGB 112,112,112)! If you don't believe it, copy the picture into any graphical painting program and use the "eye-dropper" tool (which is what I had to do).

 

[mgb_phys]

If you don't believe it, copy the picture into any graphical painting program and use the "eye-dropper" tool (which is what I had to do).

Or just mask off the sides of the picture with your fingers so you only see the two orange circles.

 

[JeremyL]

Is gray light simply white light at a lower brightness?

 

[mgb_phys]

Yes,