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?
Best Answer
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.
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.
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...