Where to Find the Colors Your Screen Can’t Show You – Ryan Moulton's Articles

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There are colors that I want to show you, but I can’t. They exist in the real world. You probably saw some of them today, but I can’t show them to you on a screen. A digital photograph can’t capture them, and your screen can’t display them. No game you’ve ever played has contained them. Unless you have specialized equipment, they are entirely absent from the digital world.

Most of them are cyans. On screens we live a life starved of cyans. It is shocking when you see one in person. They seem unfamiliar and intense in an otherworldly way. I want you to experience that, but again, I can’t show them to you. Instead, I have to show you how to find them in the real world.

“You sound like a crazy person, what are you talking about?”

(If colorspaces and the CIE chromaticity diagram are already familiar to you, you can skip to the next section.)

Light is made up of wavelengths, and its collection of wavelengths is called its spectrum. Your eyes have three different kinds of cone cells for seeing color, each of which respond differently to different wavelengths. Importantly, the cells in your eyes do not register what wavelength they are seeing. They can only respond, or not, with a certain intensity. Everything your brain figures out about the color of the world comes from contrasting the intensity of the responses of those cells.

Essentially all your cone cells can do is yell at your brain. Each of the cells wakes up and yells at your brain at a different volume, and that’s it. All your brain has available to work with to see color is how loud each of those cells are yelling, and has to reconstruct the whole rainbow from that alone.

A direct consequence of this is that any two spectra that make your cones all yell with the same pattern are indistinguishable to your brain. Even if the spectra contain entirely different wavelengths of light, to you they will look the same color. You don’t actually see light, not directly. You see how loud your cone cells yell.

Suppose color screens didn’t exist, and you were trying to design one for the very first time. The fact that we only have three different cones would seem very convenient. If you can figure out how to manipulate each of those three different cones independently, then your screen can make any human who looks at it see any color that a human can see. It doesn’t matter if it doesn’t show the real light spectra of real objects. All that matters is that the screen manipulates human cone cells, and can make them yell at human brains at different volumes. If you can do that, you’ve solved the whole problem. You might notice the suspicious coincidence between three cone cells and three primary colors. This is not a coincidence.

In 1931, CIE, (International Commission on Illumination) set out to characterize the whole space of human color vision. They produced this graph.

The outer rim of this graph shows every individual wavelength of light that humans can see. In the space enclosed by that rim are all the colors that can be produced with mixtures of those wavelengths. The points in this graph combine linearly, so if a color is in between two wavelengths, you can make that color by mixing those two wavelengths.

On this map they marked three wavelengths of light to be primary colors, and any color inside the triangle of those primary colors can be made by mixing them. The goal of these primary colors is to yank around your cone cells, and they picked these three because each of them yanks around one cone more than it yanks around the other two cones. This gives you pretty good control over a person’s eyes. You can almost make them see any color, but not quite.

Right away you see the problem. There’s a whole giant lobe of green/cyan/blue that can’t be made by mixing the primaries they chose. The green and blue primaries make one of your cones yell more than they’re supposed to. You can see this clearly on a chart of how to mix the primaries to make each wavelength. To make cyans that are cyan enough to be the most cyan thing we can see, you’d need to have negative red. Negative red doesn’t exist.

But wait, it gets worse. To make isolated pure wavelengths of light, CIE used prisms to scatter the light, followed by narrow slits to select a tiny band of a pure wavelength, a device called a monochromator. This is necessarily a big heavy bit of equipment that wastes most of its light, not something you would want to carry around in your pocket for a screen. When it came time to invent color TV, they didn’t use monochromators, they used phosphors. Phosphors don’t glow at pure wavelengths, so there was no physical way to push the primary colors on color TV to the edge of the chromaticity graph. Due to the limits of the phosphors they could make, we ended up with this.

That is, frankly, just not a lot of color. We have a much wider variety of light making technology...