The RGB color model is closely modeled after the physical phenomenon of light. In a linear RGB color space, you can average two colors to get the same result as if you had physically averaged those colors in the real world by combining light.
However, Lab is modeled after the subjective human perception of light and color. Lab is divorced from physical interpretations. Averaging colors in Lab does not have any real physical interpretation.
"The RGB color model is closely modeled after the physical phenomenon of light."
Not actually true. RGB is modeled after the mechanism by which humans perceive colors (using red, blue, and green photoreceptors).
If you mix Red and Blue light, you still only have Red and Blue light, but the human perception system will perceive purple light, even though there is no EM radiation at the 'purple' frequency.
Not actually true. RGB is modeled after the mechanism by which humans perceive colors (using red, blue, and green photoreceptors).
This is exactly the misconception I was talking about when I said that RGB is modeled after physical processes and not after human perception. There is no such thing as a red, green, or blue photoreceptor. There are three types of cones: L, M, and S; there is also scotopic vision with its own response curve. The L, M, and S cones respond to a gamut which cannot be reproduced with any RGB system that uses real primaries. Or, put another way, no RGB system with real primaries can specify all the colors we see.
Instead, RGB is a simulation of a physical system which uses three light sources: red, green, and blue. That's all it is. For example, imagine that you have three LEDs, or three phosphors, or three lasers. It doesn't matter. The point is that RGB simulates these kinds of physical systems, not the systems in the human eye.
On the other hand, Lab and XYZ systems are modeled after the perception of light. The experiments which lead to the creation of the XYZ color system had subject participants match the color output of an RGB system with that of a monochromatic light source. This experiment allowed us to use a well understood color space (RGB) based on a physical process to test how human vision worked.
It's not like this question has a "right" answer, because we are describing different models as "perceptual" or "physical".
The same phenomenon happens in audio. The vast majority of musical synthesizers either model the perception of sound or the physical production of sound. For example, you can model a piano as a physical system with a vibrating string, or you can model it as a subjective phenomenon, constructing a frequency spectrum that sounds similar using FM synthesis, subtractive synthesis, additive synthesis, et cetera--none of which correspond in any meaningful way to the piano itself, we're really trying to trick the ear.
In the same way, I see RGB as a simplified physical model rather than a perceptual model, because it does correspond rather closely to physical reality, and it corresponds somewhat poorly to subjective reality. You can construct RGB as a simplification from a continuous spectrum model of radiation, all you have to pick the spectrum of your primaries. From there, you can use the RGB model in your physical simulations, such as ray tracing and photon mapping. Lab color does not work well for ray tracers because it does not correspond to physical reality: it is fairly nonlinear, and the coordinate system is awkward.
Likewise, RGB is a poor model for subjective perception (compared to Lab or XYZ) because its gamut is limited, and differences in RGB space do not correspond well to differences in subjective qualities. You can see how awkward RGB is for perceptual modeling whenever you use a color picker. It is frustrating to try and construct a pleasing palette of colors by dragging around RGB sliders, or even HSV/HSL sliders, because the model is so far from subjective perception that doing something conceptually straightforward, such as altering hue or matching luminosity, requires fiddling about.
In short, the description of RGB as a "physical model" is because we use it for physical simulations, as well as for working with hardware such as monitors and cameras. My description of Lab, CIECAM, XYZ, etc. as perceptial models are because we use those for modeling the subjective perception of color.
There's no receptor that actually peaks at red, though - it's more like we have indigo, yellowish-green, and greenish-yellow receptors, usually called S, M and L respectively.
But, if you mixed (0, 0, 255) blue and (255, 255, 0) yellow, you get (127, 127, 127) grey. But blue and yellow doesn't make grey in the physical world.
Well, it could only ever be an approximation because you cannot make pure white light using two wavelengths of light in the real world. Waves of blue light and yellow light (AKAIK) would not be perceived as white by the eye.
In RGB, (127, 127, 0) is not a "pure" yellow, it is just the color you get when you mix red and green primaries. The human eye is actually rather poor at spectrophotometry, which is why colorimetry and spectrophotometry are distinct subjects.
However, when you say "pure white light"... there is no such thing! White light is a combination of different wavelengths of light. You can indeed make light that looks white (and therefore is white) by combining monochromatic yellow and monochromatic blue light sources. Amazing! Even though it will look white, objects illuminated by this unnatural light source will have unexpected colors. This is quantified by the CRI of a light source. Incandescent lights are defined to have a CRI of 100, but this is just a convention: the blackbody spectrum is quite common. If you grew up under fluorescent lamps, colors under the light of day might seem quite unusual.
Why not? Yellow light ~580nm stimulates both long and medium ("red" and "green") photoreceptors in the eye. Blue light below 480nm stimulates the short "blue" photoreceptors, so you should see white. https://en.wikipedia.org/wiki/Color_vision#Physiology_of_col...
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Ah, that's problematic.
The RGB color model is closely modeled after the physical phenomenon of light. In a linear RGB color space, you can average two colors to get the same result as if you had physically averaged those colors in the real world by combining light.
However, Lab is modeled after the subjective human perception of light and color. Lab is divorced from physical interpretations. Averaging colors in Lab does not have any real physical interpretation.
"The RGB color model is closely modeled after the physical phenomenon of light."
Not actually true. RGB is modeled after the mechanism by which humans perceive colors (using red, blue, and green photoreceptors).
If you mix Red and Blue light, you still only have Red and Blue light, but the human perception system will perceive purple light, even though there is no EM radiation at the 'purple' frequency.
This is exactly the misconception I was talking about when I said that RGB is modeled after physical processes and not after human perception. There is no such thing as a red, green, or blue photoreceptor. There are three types of cones: L, M, and S; there is also scotopic vision with its own response curve. The L, M, and S cones respond to a gamut which cannot be reproduced with any RGB system that uses real primaries. Or, put another way, no RGB system with real primaries can specify all the colors we see.
Instead, RGB is a simulation of a physical system which uses three light sources: red, green, and blue. That's all it is. For example, imagine that you have three LEDs, or three phosphors, or three lasers. It doesn't matter. The point is that RGB simulates these kinds of physical systems, not the systems in the human eye.
On the other hand, Lab and XYZ systems are modeled after the perception of light. The experiments which lead to the creation of the XYZ color system had subject participants match the color output of an RGB system with that of a monochromatic light source. This experiment allowed us to use a well understood color space (RGB) based on a physical process to test how human vision worked.
I clearly misunderstood what you mean by "the physical phenomenon of light".
You seem to mean: "a common engineering method of displaying colors to the human perceptual system"
while I thought you meant: "the physical properties of the visible band of the electromagnetic spectrum"
It's not like this question has a "right" answer, because we are describing different models as "perceptual" or "physical".
The same phenomenon happens in audio. The vast majority of musical synthesizers either model the perception of sound or the physical production of sound. For example, you can model a piano as a physical system with a vibrating string, or you can model it as a subjective phenomenon, constructing a frequency spectrum that sounds similar using FM synthesis, subtractive synthesis, additive synthesis, et cetera--none of which correspond in any meaningful way to the piano itself, we're really trying to trick the ear.
In the same way, I see RGB as a simplified physical model rather than a perceptual model, because it does correspond rather closely to physical reality, and it corresponds somewhat poorly to subjective reality. You can construct RGB as a simplification from a continuous spectrum model of radiation, all you have to pick the spectrum of your primaries. From there, you can use the RGB model in your physical simulations, such as ray tracing and photon mapping. Lab color does not work well for ray tracers because it does not correspond to physical reality: it is fairly nonlinear, and the coordinate system is awkward.
Likewise, RGB is a poor model for subjective perception (compared to Lab or XYZ) because its gamut is limited, and differences in RGB space do not correspond well to differences in subjective qualities. You can see how awkward RGB is for perceptual modeling whenever you use a color picker. It is frustrating to try and construct a pleasing palette of colors by dragging around RGB sliders, or even HSV/HSL sliders, because the model is so far from subjective perception that doing something conceptually straightforward, such as altering hue or matching luminosity, requires fiddling about.
In short, the description of RGB as a "physical model" is because we use it for physical simulations, as well as for working with hardware such as monitors and cameras. My description of Lab, CIECAM, XYZ, etc. as perceptial models are because we use those for modeling the subjective perception of color.
There's no receptor that actually peaks at red, though - it's more like we have indigo, yellowish-green, and greenish-yellow receptors, usually called S, M and L respectively.
http://commons.wikimedia.org/wiki/File:1416_Color_Sensitivit... (note the sensitivities are normalized - in actuality the blue receptors are much less sensitive than the other two)
But, if you mixed (0, 0, 255) blue and (255, 255, 0) yellow, you get (127, 127, 127) grey. But blue and yellow doesn't make grey in the physical world.
Yes it does, at least with additive mixing.
Well, it could only ever be an approximation because you cannot make pure white light using two wavelengths of light in the real world. Waves of blue light and yellow light (AKAIK) would not be perceived as white by the eye.
In RGB, (127, 127, 0) is not a "pure" yellow, it is just the color you get when you mix red and green primaries. The human eye is actually rather poor at spectrophotometry, which is why colorimetry and spectrophotometry are distinct subjects.
However, when you say "pure white light"... there is no such thing! White light is a combination of different wavelengths of light. You can indeed make light that looks white (and therefore is white) by combining monochromatic yellow and monochromatic blue light sources. Amazing! Even though it will look white, objects illuminated by this unnatural light source will have unexpected colors. This is quantified by the CRI of a light source. Incandescent lights are defined to have a CRI of 100, but this is just a convention: the blackbody spectrum is quite common. If you grew up under fluorescent lamps, colors under the light of day might seem quite unusual.
Why not? Yellow light ~580nm stimulates both long and medium ("red" and "green") photoreceptors in the eye. Blue light below 480nm stimulates the short "blue" photoreceptors, so you should see white. https://en.wikipedia.org/wiki/Color_vision#Physiology_of_col...