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Right now, my understanding is that, a mixture of photons of many different frequencies is perceived as white by your eye. While no photons at all, is perceived as black. And photons with the blue frequency only cause you to see blue, etc.

My question is, how is the "brightness" controlled? I think it has to do with how much blue photons are coming at your eye, a low amount will be dark blue, a high amount will be... a lighter blue. But then I think, to get light blue, isn't it a mixture of mostly blue photons with white light (photons of all frequencies) to produce a blueish white or a light blue?

Also, when colors combine to produce different colors, is there any photon combining that exists or is it because your eyes see mixtures of photons and not photons themselves?

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The eye is able to perceive colors that may be described by 3 parameters, R,G,B - the amount of red,green,blue photons per second. The frequencies that contribute to R,G,B don't have to be exact - there are bell curves around the maximal frequencies - but they're projected to some values of R,G,B. A particular ratio between R,G,B looks like grey/white color of different brightness that increases with R,G,B. –  Luboš Motl Mar 28 '13 at 19:01
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1) Yes. You're right with the perception of white and black. Regarding other colors, it depends on the energy of photon which excites your cone and rods.

2) The brightness in physical (cosmological to be specific) terms, is differentiated into luminosity and flux. Both are quite comparable. Luminosity is the amount of energy emitted per second ($J/s$ or simply, Watts) from the source whereas flux is the power received per unit area $(Wm^{-2})$. Both are related simply by using the surface area of a sphere of radius $r$, which is probably the distance from the source and the relation is $L=F(4\pi r^2)$. This flux tells you how much energy is emitted as a function of time which explicitly shows the amount of photons impinging on your retina.

When colors combine to produce different colors, is there any photon combining that exists or is it because your eyes see mixtures of photons and not photons themselves?

For now, there's a difference. Photons don't combine. Instead (as I've previously mentioned), different photons excite your photo-receptors at different or same time periods. Based on the excitation, the color is observed by you. On the other hand (if those were assumed to have wave character), they can constructively or destructively interfere to produce additive or subtractive color. Now, to other question.

To get light blue, isn't it a mixture of mostly blue photons with white light (photons of all frequencies) to produce a blueish white or a light blue?

This does produce light blue. But, the flux factor produces greyish blue and not whitish blue. Now as white light contains all the frequencies, you'll perceive all in the same way but with several blue-ly energized photons along with it. Imagine this to be the surface area of a white circular object with some blue spots in it. At a comparable distance, you can see it as whitish blue (your favorite)...


If you're still confused of brightness, Wiki has a nice quote on it...

"Brightness" was formerly used as a synonym for the photometric term "luminance" and (incorrectly) for the radiometric term "radiance". In RGB color space, brightness can be thought of as the arithmetic mean of R, G & B color coordinates (although some components make the light seem brighter than others, which again, may be compensated by some display systems automatically)

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So when you say "greyish blue" is caused by the flux factor, do you mean to say that no matter how many blue photons hit your eye per second, the maximum brightness perceived will not have any "white" to it, and probably the brightest it could be would be solid blue? Therefore, blue photons could produce a range of color of black to solid blue depending on the flux of the blue photons that hit your eye? If that's the case then what happens if you raised the the flux to an extreme? Is there a cut off point where even if u add more flux you still see the same solid blue color? –  Dan Webster Mar 28 '13 at 19:32
    
@Dan: Not really. If blue photons are only perceived, how come there be any white photons? When I meant the flux factor, I really mentioned the depth of blue color. I think you're confusing this with the "brightness & contrast" in televisions which commonly use "whitish blue" as a brighter blue. That's totally different. If the flux is increased to its maximum, it's just closer to luminosity in range. Imagine what would happen if you look at a blue bulb far away & when you stick to it..? ;-) –  Waffle's Crazy Peanut Mar 28 '13 at 19:42
    
Well in that comment, is my description wrong? I can't imagine anything brighter than a solid color like solid blue without adding white light to make it light blue. –  Dan Webster Mar 28 '13 at 19:48
    
You say that brightness is the difference between the blue bulb being far away or when you are close to it. When you are further away less blue photons hit your eye because there is less flux so you perceive a darker blue. I guess my confusion is when you are very close to a high luminosity blue light, will the brightest color possible to perceive be a solid blue color? –  Dan Webster Mar 28 '13 at 19:55
    
What do you mean by "flux factor". –  Peter Shor Mar 28 '13 at 20:37
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Perception of color and brightness is very complicated. You brain performs all sorts of normalization and prediction of color based on what it thinks is correct. The same is true for brightness estimation. This leads to huge number of surprising optical illusions. Here is one based on shadows.

No answer here will be able to cover all of the biological and cognitive processes that occur to allow color vision. It's also important to realize that color is an illusion created by our brains. Photons have an energy proportional to their frequency. A "red" photon is just a photon with a specific frequency.

The basic gist of color vision comes from understanding thethe receptors in the eye. Humans have four receptors. Rod cells are light/dark sensors, not color sensors and they're sensitive to a wide range of frequencies. Rod cells aren't used for color perception. We also have three color-sensitive cells called cone cells. Cone cells are also sensitive to a wide range of frequencies but each type of cone cell has sensitivity centered at a different frequency.

Photons entering your eye have a probability of triggering a rod or cone cell proportional to the frequency of the photo relative to the peak absorption frequency of the cell it strikes. Photons of the "red" frequency have a low chance of triggering a green-sensitive cone cell but a high probability of triggering a red-sensitive cone cell.

All color you see is your brain's interpretation of the relative rates the three cone cells in your eye that light triggers.

The brightness of light that you perceive is just a measure of the rate at which cells are being triggered. More photons means more cells being triggered means a perception of brighter light.

In your question about dark blue versus light blue, there are two things your brain is measuring. To perceive the light as blue, blue cone cells have to be triggered more than the other two cone cells. The brightness of the blue light is your brain's measure of both the absolute rate that all of your cone cells are being triggered as well as the relative rate blue cells are being triggers compared to your other cone cells. If they're all being triggered frequently but blue slightly more then you'll perceive a blue-white color. If blue cone cells are triggered much more frequently than other cells you'll perceive that as a darker more saturated blue.

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Rod cells have nothing to do with daytime vision; they only contribute to vision in nighttime. Walk from a bright room into a dark room. You won't be able to see anything until your rods start working. If rod cells contributed to daytime vision, we would all be tetrachromats. –  Peter Shor Mar 28 '13 at 20:24
    
You're right, I thought they were used in intensity / brightness detection but it seems they aren't. Editing now. –  Brandon Enright Mar 28 '13 at 20:42
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