Mixing Colors: Understand Red, Blue, Green & White

[nhmllr]

I know about the different frequencies of light producing different colors, red at the bottom, violet at the top.

But why do colors mix? Why can you make every color with red, blue, and green like in a computer screen? And why do all colors together make white?

Thanks

 

[DrGreg]

Within your eye, you have cells that sensitive to either red, blue or green light. The mixing of colours is an optical illusion: to your eye, a mixture of red and green light looks identical to pure yellow light, because your eye cells are stimulated in the same way in both cases.

 

[Disinterred]

That is awesome.

So we are all "colour blind" when it comes to mixtures of R,G,B ?

 

[sophiecentaur]

The sensors are very broad band and their responses very much overlap. Rather than a Red, Green and Blue analysis, there is a 'high end, mid range and low end' analysis. Any particular colour (which may consist of light with bands of wavelengths in all parts of the spectrum and not necessarily just one wavelength) may produce outputs from all three groups of sensors. Various combinations of wavelengths can produce the same apparent colour (a metameric match).

The light from the sun, when reflecting off a broadband reflecting surface (like snow) contains pretty much equal levels of all wavelengths. We call this 'white' and it stimulates the three sensors equally. Using three colours of light (from TV phosphors or even coloured spotlights) you can also stimulate the sensors equally. So, not knowing any better, your brain will interpret this as 'white', which is the best it can do. This is lucky for TV system designers. Just imagine if we had a more complex analysis system for our colour vision. We could have ended up needing RGB plus PQR and S channels.

 

[sophiecentaur]

That is awesome.

So we are all "colour blind" when it comes to mixtures of R,G,B ?

We are all 'colour-foolable'.

 

[gillwill]

"Within your eye, you have cells that sensitive to either red, blue or green light..."

Maybe it's just an issue of semantics but the "either" part of that statement is confusing and seems to suggest that human's eyes are sensitive to just those three colors of light. I don't know for certain myself, therefore this post:

However, I wonder sometimes if too many people (particularly in these optics related topics regarding color) are presenting information as comprehensive fact, that is actually abstract and limited knowledge based on technological devices they're accustomed to, such as TVs, computer displays and digital camera sensors; and how those devices utilize different bands of visible light, which in most cases is exclusively red, green, and blue.

If the human eye is only sensitive to red, green and blue, what happens if yellow light (570-590 nm) shines into the eye, orange light (590 - 620 nm), violet (380-450 nm)? Is it a fact that the human eye can only have perceptions of these colors via RGB mixtures?

 

[I like Serena]

Maybe it's just an issue of semantics but the "either" part of that statement is confusing and seems to suggest that human's eyes are sensitive to just those three colors of light. I don't know for certain myself, therefore this post:

However, I wonder sometimes if too many people (particularly in these optics related topics regarding color) are presenting information as comprehensive fact, that is actually abstract and limited knowledge based on technological devices they're accustomed to, such as TVs, computer displays and digital camera sensors; and how those devices utilize different bands of visible light, which in most cases is exclusively red, green, and blue.

If the human eye is only sensitive to red, green and blue, what happens if yellow light (570-590 nm) shines into the eye, orange light (590 - 620 nm), violet (380-450 nm)? Is it a fact that the human eye can only have perceptions of these colors via RGB mixtures?

The human eye (normally) has 3 types of cone cells, which are sensitive to the 3 colors although there is a large overlap. This gives us trichromatic vision.
Different people will have cone cells with different response curves, which explains why different people can see the same colors differently.
Apparently there also have been reports of people with 4 types of cone cells.

See wikipedia for more (factual!) details:
http://en.wikipedia.org/wiki/Cone_cell

[edit]Btw, it's quite true that someone may miss represent information as facts here on PF, but I found that with enough people watching and commenting, any miss representations will be ferreted out! [/edit]

 

[gillwill]

Thanks for the info, and I'm sure there is seldom deliberate misrepresentation, but not just here, elsewhere on the web (perhaps more so), particularly in references to RGB, it's so often presented as if those are the only bands of visible light. For example in documentation regarding image sensors and RGB filters, it would be stated that a sensor of only one of those colors captures 1/3 of the incident light (e.g from the sun), however, wouldn't it really be much less, considering violet, orange, and yellow light wavelengths also are not acquired?

Also it begs the question, if the human eye can only sense red, green, and blue wavelengths why are the bands of yellow, orange and violet termed "visible" light?

 

[I like Serena]

For example in documentation regarding image sensors and RGB filters, it would be stated that a sensor of only one of those colors captures 1/3 of the incident light (e.g from the sun), however, wouldn't it really be much less, considering violet, orange, and yellow light wavelengths also are not acquired?

A green sensor would not only capture the green band, but also the red and yellow bands, just to a lesser degree.
But it would not capture 1/3 of all incident light from the sun. It would probably only capture 1/3 of the incident light in the visible spectrum.
Part of the specification of a sensor is what the bottom and top wavelengths are that it captures.

Also it begs the question, if the human eye can only sense red, green, and blue wavelengths why are the bands of yellow, orange and violet termed "visible" light?

Yellow is a band between red and green. The human eye is most sensitive to red and green. Since those receptors are both triggered, yellow appears to us even brighter than red or green.

 

[Hurkyl]

If the human eye is only sensitive to red, green and blue...

A vacuous question -- each type of cone is sensitive to a wide spectrum of wavelengths.

what happens if yellow light (570-590 nm) shines into the eye

Each type of cone responds to the light.

The response happens to be the same as the response to an appropriate mixture of red and green.


You may be interested to take a wikiwalk from the starting points
http://en.wikipedia.org/wiki/CIE_1931_color_space
http://en.wikipedia.org/wiki/Gamut

Also, you may be interested in
http://en.wikipedia.org/wiki/Tetrachromat#Possibility_of_human_tetrachromats

 

[sophiecentaur]

Nearly everyone here is confusing what goes on during the Analysis of the light and how you can Synthesise the perceived colour with TV etc.
You might as well say that the sequential images that give us perceived motion imply that we sample what we see with our eyes in exactly the same way.
Both these systems rely on our 'approximate' senses.
Colour television has broadband analysis and narrowband synthesis.

Edit: Narrow band synthesis allows us to see 'saturated' colours on TV.

 

[Drakkith]

See this: http://en.wikipedia.org/wiki/File:Cones_SMJ2_E.svg (Can't attach it here in the thread, it gives me an invalid file type)

Anyways, the S,M, and L stand for Short, Medium and Long, referring to which wavelength the cones are most sensative too. As you can see each cone type is sensative to a broad band of wavelengths and peaks in a certain area.

So let's say you have light with a wavelength of 480 nm. From the chart, this falls almost in the middle of the overlap of all 3 cone types. This light will stimulate ALL 3 types of cones to about the amount, but it will not appear as bright as say 550 nm light would sense your cones are much less sensative to it.

 

[Drakkith]

Son of a... I had a question on what makes the different cones respond to different light and I JUST found this on the wikipedia page that I must have missed earlier lol. I made a post on it a few months back. Nothing like finding the answer to your own question waaay later.

Structurally, cone cells have a cone-like shape at one end where a pigment filters incoming light, giving them their different response curves.

It's so obvious now!

 

[sophiecentaur]

The thing about our colour vision is that it is extremely subtle in some respects - appreciation of very slight differences in greens and, in particular, skin tones, which is clearly, a huge evolutionary advantage for nutrition and social interaction (blushing etx.). And yet you can so easily fool the brain into believing it 'sees' a colour which is the same as that of a well known object by stimulating it with just the right amount of three narow band primaries. The ultimate 'fooling' is when you give someone red and green light from phosphors which have absolutely no yellow in them and that person will not be able to tell the difference between what you are showing them and some Spectral Sodium Yellow, which has just one wavelength - neither green nor red.
RGB rules.