Human color vision

The last remaining component to specify color scientifically is the observer, us humans. We talked about how colors in nature can be described with their distribution and intensity over the visible spectrum with spectral diagrams. Human vision and it's sensitivity to colors can be described similar and there are more factors to our vision that influence our color perception.

1. Trichromacy

Let's start with some history again, Thomas Young, a physicist and polymath, proposed in 1801 - “the retina of the human eye contains three "kinds of fibers", each sensitive to different wavelengths of light”.

Young's assumption was that light is too fine and that there are to many graduations of color, to have an infinite amount of receptors for each color in the retina. So he proposed that we perceive light and its colored wavelength filtered by three receptors sensitive to different wavelengths of the light. Herrmann von Helmhotz and James Clerk Maxwell expanded on Young’s idea in the late 1800s. The theory is now called Young-Helmholtz theory or trichromatic theory. 
The conclusion of trichromacy is, humans see colors with three different color receptors.

In the 1980's Young's idea was experimentally verified, with a microscope. There were two kinds of photoreceptors visible. One looked like a cone (responsible for our color vision) the other like a rod so they were named like that, cones and rods, science can sometimes be really simple.

Since we have three different cone types, we are trichromats, dogs for example have only two types of color receptors for blue and yellow light. There are types of shrimp that have more than twelve color receptors, must be quite a trippy experience.

2. Cones

The cones are responsible for our color vision, they are located in the retina. As discussed, we have three kinds of cones and they are sensitive to different but overlapping ranges of wavelength in the visible spectrum, short, medium and long.

Our cone response to the wavelength of light can be plotted in a Spectral Response Function with the wavelength in X and the sensitivity of our cones to those wavelengths in Y, the diagram above normalizes the cone sensitivity to 1. 
This is very much the same function as we saw earlier for light or material colors.

As you can see in the image above:
Long wavelength cones: are sensitive to a wavelength peak of 580nm, correlating with red.
Medium wavelength cones: are sensitive to a wavelength peak of 530nm, correlating with green/yellow.
Short wavelength cones: are sensitive to a wavelength peak of 330nm, correlating with blue.

There are roughly six million cones in our retina and the ratio of L, M and S cones is 6:3:1. The distribution of the cones mixed with their sensitivity is the reason why we are most sensitive to the yellow/green wavelength of light and the least sensitive to blue. 

You can see in the image above, that the blue cones are not located in the retina center and are more at its edges. That fact and its discovery lead to a change of the CIE Standard Observer in 1976, but more to that later.

The cone signal is pooled, not each single cone is connected to the optical nerve but an area of cones is combined to create the signal.

To sum it up, we have three kinds of cones and they operate in color ranges that are very much red, green and blue. Our cones essentially simplify the spectral colors into three sensations and without that fact we would not be able to replicate nature on our displays that easily.

Digital cameras, mimicking the human sensitivity to light, have the most noise in the blue channel because they are also the least sensitive to the blue wavelength.

As you can see in the image above digital cameras are as close to the human sensitivity to color as they technically can.

3. Metamerism

As a result of our color vision, where LMS cones simplify the color spectrum to three sensations, there are many combinations of light sources and colored objects that result in the same color (three sensations) for us, but they are actually different spectral colors. If we would have less coarse color receptors, let’s say one for each broader color wavelength range, like those fancy shrimp, the effect would not exist.

Metamerism is the name for that effect, two colors/lights are metamers if they have different spectral distributions but the same visual appearance.
The good thing about metamerism is that you can replicate a spectral color with different cone sensations, at least good if you want to replicate what you saw in nature on a display, because nature does not have to be present for that.

To cite wikipedia: "The basis for nearly all commercially available color image reproduction processes such as photography, television, printing, and digital imaging, is the ability to make metameric color matches".

It is really a very important take away, trichromacy and metamerism are the basis of our color reproduction.
We can only reproduce the wide range of colors and scenes we see in nature on our very limited displays, because metamerism allows us to replicate the same color that we saw in nature with different colors on our displays. We don’t need the scene lights and objects present.

Or in other words, we don't need daylight and a blue forget-me-not in our room, a display and three colored filters backlit by LEDs can reproduce the same visual sensation for us, luckily.

What is great for us in computer graphics, can be the bane of product designers, as you can see in the image below. Product designers that have to make sure their product colors are consistent under different light sources, so they have to find metameric color matches under different light sources. 

To cite Wikipedia here: “Using materials that are metameric color matches rather than spectral color matches is a significant problem in industries where color matching or color tolerances are important.”

4. Where do all the colors go by night? - Rods

So where do all the colors go by night, if there is not much light present, we hardly see any color?
Rods, the second kind of photreceptive cells, are responsible for our dim light or night vision, they can not discern colors but they are 16 times more sensitive to luminance than our cones. When it is getting dark we are literally turning color blind, because our rods are taking over. 

With waning light there is a transition between cones and rods, we still see color but they are getting less vibrant and saturated until they fade away with the rods taking fully over. This phase is called mesopic vision.


5. Summary
Humans are trichromats, we see colors with three receptors that are sensitive to red, green and blue light, the cones.
The fact that we are trichromats and thanks to metamerism, we can recreate all visible colors with three base colors, we don’t need all of the spectral colors present.