December 7, 2015

Whatever we look at, whatever we do, our perception of color is based on a mixture of three basic shades. The printer, by mixing three colored inks in different quantities, generates all the colors we see – these are the colors we see on everything from labels, stickers and images to products, books, films.

So why are the three primary colors highly important to how we see and create the color? In fact, the number three does not really matter, and primaries do not exist in the natural world at all. It is just lights of millions of waves of different lengths that are reflected off all objects and surfaces that surround us, and our eyes can only see a tiny part of that.

Why is the true physical variety of light inaccessible to the human eye and limited only to the visible range? There are obviously three primary colors, but how does this relate to the previous thesis?

 If you can visually sense the ripening of fruit from green to ripe by the difference in color, then your eye is able to distinguish between the wavelengths of light that are reflected by the peel of the fruit as it ripens. So you can, without yet smelling and flavoring it, decide whether the fruit is edible! Animals have developed visual systems designed to recognize different wavelengths in this useful spectrum of light, and they benefit from this information (fruit lovers in the first place).

When you know this, you want to assume that the existence of three primary colors in our color vision can be explained by the fact that each of our cones sees a primary color (blue, green, or red) and then mixes the signals in a certain way to create the final color perception. To be honest, this is not completely accurate, because the cone does not see color at all. It is totally color blind and it is a proven fact. The key to understanding why all the colors we see can be made of the three primary colors is to understand how cones work and how several types of cones work together to overcome this color blindness.

Why are cones actually color blind and how does it work?

The millions of rods and cones that capture light entering the eye and trigger the vision process are light-sensitive cells that densely fill the retina at the back of the eye.

It is important to note that the light-absorbing opsin molecules found in cones are not equally sensitive to all wavelengths of light in the visible spectrum. When we look at light with different wavelengths, the probability of absorbing the light packet (or photon) and triggering a signal to the brain is different. For example, an S-shaped cone reacts to visible light with the shortest wavelengths, and such waves are associated with the perception of indigo and violet colors.

In the image below, you can see a curve that describes how the sensitivity of the S-cone to light is directly related to the wavelength of light on the retina. Looking at this, we can also see that the cone is most sensitive to light with a wavelength of about 455 nanometers. This means that when the opsin molecules in this cone are stimulated by that particular wavelength, they will most likely absorb light and produce a neurochemical signal to the brain.

But here a serious dilemma can arise before the cone. What if we first stimulate it with 455 nm light, to which the cone is most sensitive, and then twice as much light at 470 nm, but to which the cone is half as weak? In both cases, the cone outputs the same signal. The brain will have no way of knowing which wavelength of light was responsible for activating this cone when it receives these identical signals, and it sees the two light sources as identical as well! Which is a phenomenon called metamerism.

Many (hundreds or more) different wavelengths can mimic each other, no matter which cone we are looking at. Then they, just by changing the intensity, make the cone generate the same signal. The brain, each time the cone sends it a different signal, will never be able to see if it was light of a different wavelength or if it just became more or less intense. Thus, the cone is actually color blind, since its signal does not tell you anything about the wavelength.

Why are two types of cones the minimum required for color vision?

The color blindness of the two types of cones that the retina contains becomes less problematic, while each type of cone is sensitive to different parts of the visible spectrum. This is because the light of each wavelength causes the two types of cones to generate different signals, as we can see below.

What happens if the lights of two different wavelengths meet the eye? These waves trick the first cone to generate an identical signal, the second cone generates different signals for such lights. Thus, we can easily tell the difference between when 1 packet of 455 nm light was fed into the eye, and when it was given twice as much light of 470 nm (taking for this the ratio of the two signals – how much stronger is the signal produced by the cone 1, compared with a cone 2).

Since the retina contains special cells that calculate the ratios of signals produced by different types of cones (1), it is the visual system that recognizes light with different wavelengths. If you would like to explore this issue in more detail, see the image below. It actually shows how the signal ratio can be computed using two hypothetical cone types (yellow and green).

Thus, it turns out that the main source of information that the brain must help it determine the wavelengths of light stimulating the eye is the ratio of the cone signals that are transmitted to the brain. The brain can create a basic form of color vision thanks to this ability to discriminate between different wavelengths, which is otherwise not possible with one type of cone.

At the heart of our ability to distinguish and sense millions of shades is a three-cone system known as the trichromatic (from the Greek Tria – three and khrĂ´ma – color). The brain can thus distinguish between light with one wavelength and mixtures with two wavelengths, which confuse the visual system, which has only two cones. However, if we consider the fundamental limitations of tricolor vision, we realize that the presence of three primary colors becomes apparent.

Can our brain be tricked? A mixture of just three wavelengths is enough for him to perceive them as being identical to one wavelength.

The dual cone system treats multiple single wavelengths and 2+ wavelength mixtures as identical. Likewise, a three-cone system is equally bewildered by mixtures of three wavelengths. From this, it can be concluded that for all wavelengths of visible light, there are several mixtures with three wavelengths. They produce exactly the same signal ratio in our three types of cones.

In light of this, our brains are unable to distinguish light mixtures that are more complex than three wavelengths. Thus, mixing three colors is sufficient to reproduce all visible shades.

We, humans, create a certain world for each other when we draw, print and display on screens. In such a world, only three primary colors are used. And we could not even imagine what a huge variety of light mixtures, consisting of many wavelengths, reflecting surfaces in the natural world.

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