In my bedroom, I have an old incandescent light bulb. It gives off a yellowish-white light. Downstairs in the kitchen is a fluorescent light fixture. It gives off white light. To the human eye, the colors aren’t that different. They’re both whitish. They’re both bright. The only difference is that there’s a little more yellow in the incandescent light and a tiny touch of blue in the fluorescent. They’re both more white than anything else.
But that’s just the limitations of the human eye (Stupid human eyes. What are they good for? Apart from…well…everything.) From an optics point of view, the colors couldn’t be more different. As you may already know, the color of light depends on its wavelength. The shortest-wavelength light most people can see is violet, which has a wavelength of around 400 nanometers. The longest wavelength most people can see is red, which has a wavelength of about 800 nanometers. In between these, you get the whole ROY G. BIV rainbow. And, with the aid of a prism or a diffraction grating, you can separate the wavelengths of a light source into a spectrum. These devices refract (or diffract) light of different wavelengths to slightly different degrees, meaning they spread out in space so that we can see the intensities of the different colors with our own eyes. Here’s the spectrum of my incandescent lamp (taken with a bad camera and a diffraction grating):
(See that tiny blur of magenta at the far right? I couldn’t see that with my own eye. As it turns out, most cameras are sensitive to near-infrared light, which is invisible to most humans. Try this at home: take a standard TV remote, point the LED at the front at a camera, and press a button. You’ll see the LED blinking purple, even though its light is invisible to the eye.)
Notice how there are no breaks or bright spots in this rainbow. That’s because an incandescent lamp generates light by heating a little curlicue of tungsten to very high temperatures (around 5000 Kelvin), and most hot objects emit a so-called blackbody spectrum. No matter what it’s made of, most hot objects emit the same blackbody spectrum when they’re at the same temperature. The blackbody spectrum for a 5000-Kelvin object looks like this:
Notice the peak in that spectrum. Every blackbody spectrum has a peak, and each temperature peaks at a particular wavelength. The hotter the object, the shorter the wavelength, which is why very hot metal stops looking orange and starts looking white.
Here, on the other hand, is the spectrum of the fluorescent light:
This is not a blackbody spectrum. It’s got all kinds of gaps and bright spots and other weird shit going on. If an 8-year-old drew this in art class and told me it was a blackbody spectrum, I would give him an F. I’d make a bad art teacher.
But the reason this isn’t a blackbody spectrum is that fluorescent bulbs produce light in an entirely different way than incandescent bulbs. In fluorescent bulbs, most of the light is not given off by a hot substance, but rather by ionized mercury vapor with an electric current running through it. Mercury, like all elements, likes to produce light at certain particular wavelengths when it’s electrically excited (which sounds like the opening to bad slash fiction), and these show up as “spectral lines,” which are those bright bands you see in the picture. (There’s some blurring in those lines in part because my spectroscope is literally made out of tinfoil and a paper-towel tube, and in part because mercury bulbs are coated in a phosphor to absorb the skin-damaging ultraviolet light they put out and turn it into more useful colors that don’t give people cancer.)
Those spectra are really different. The colors, though, don’t look all that different to my eye. They’re just white with different tints. And that’s because an eye is not a spectroscope. Here’s (approximately) what the spectrum of an incandescent lamp and a fluorescent lamp look like (respectively):
But here’s (again, approximately, and again, incandescent and fluorescent respectively) what the human eye sees:
(In reality, the human eye is not equally sensitive to red, green, and blue: it’s most sensitive to green and least sensitive to blue, with red in the middle. Also, the wavelengths the eye “sees” as red, green, and blue aren’t this evenly spaced.)
The human retina contains two kinds of light-sensing cells, called rods and cones, as I mentioned in my previous article. They both have a round spherical end and a long tubular end. The tubular end contains a pancake-like stack of folded-over membranes covered in light-sensitive pigments. In rod cells, the tube is cylindrical and the membrane is covered with a pigment called rhodopsin, which is a protein containing a derivative of Vitamin A. Rods are only sensitive to a single color band, but they’re very sensitive, and they’re most important in low light and peripheral vision (which is why Vitamin A deficiency can lead to night-blindness and, if it’s bad enough, day-blindness, too). In cone cells, the tube is (unsurprisingly) conical, and its membrane can contain one of three different pigments, called photopsins. The individual photopsins are called erythrolabe, chlorolabe, and cyanolabe, which sound like the names of really weird pornstars, but are, in fact, the pigments sensitive to reddish light, greenish light, and bluish light, respectively.
This means that, with some caveats, the human eye receives color in almost the same way a computer screen produces it. A single pixel on a computer screen produces color by varying the amounts of red, green, and blue light it emits. You can specify a computer color using just three numbers, one for red, one for green, and one for blue.
This slightly nauseating salmon color corresponds to (255,154,93) (on a scale from 0 to 255). The human eye does roughly the same thing, but in reverse: it lumps all the light in the red band into the first number, all the light in the green band into the second number, and all the light in the blue band into the third number.
And that’s only if you have ordinary eyes. Some people are missing one of the cone photopigments, and are colorblind, unable to distinguish between, for instance, red and green. To a person missing the green photopigment, the nauseating salmon color might look like this:
(I’d just like to point out that the name of this image file is “Colorblind Salmon,” which would, of course, make a good name for a band.)
You could say that ordinary people have three-dimensional color vision: each color they see is determined by how much their red, green, and blue cones are activated, in much the same way (255,154,93) gives you our previous salmon color. A person with no green receptor, on the other hand, has two-dimensional color vision, and they might see the salmon color as (255,93) (which I’ve tried to recreate using the color (255,255,93); it’s actually quite hard to simulate colorblindness for non-colorblind people). There are different kinds of colorblindness. Some people are missing their red receptor, some are missing their green receptor, some are missing their blue receptor. All these people see two-dimensional color. Some are missing two color receptors, and for them, the world is (sort of) black-and-white. They see one-dimensional color. A very small number of people are missing all three color receptors, and have true black-and-white vision, but they also have other visual problems, because the cone cells are not just responsible for color vision, but for acute vision in normal light. They also see one-dimensional color.
Now here’s the cool part, but before I get to that, here’s a picture of something:
Although it looks like a decapitated lobster, this creature is actually one of the coolest crustaceans in all of existence: the mantis shrimp. Mantis shrimps are remarkably intelligent, extremely powerful (some kinds have built-in spring-loaded hammer-claws that can hit with enough force to crack a fucking mussel shell), and most important for the purposes of this article, they have some of the most complex and interesting eyes in the animal kingdom.
Ordinary human beings have three photo-pigments and three-dimensional color vision. Mantis shrimps have twelve photo-pigments and therefore twelve-dimensional color vision. A human being sees an incandescent lamp and a fluorescent lamp like this:
But a mantis shrimp might see them like this (again, incandescent on top, fluorescent on the bottom, which sounds like a very weird dessert):
The difference between the two is much clearer in this graph. Indeed, with high-dimensional color like this, a mantis shrimp would be able to tell the difference in color between a leaf, a perfect copy of that leaf made of dyed plastic, an picture of that leaf printed with an inkjet printer, a picture of that leaf taken with color film, and a picture of that leaf shown on a computer monitor, and that’s even if it only saw a tiny swatch of the leaf, so that it couldn’t tell what was a picture and what wasn’t. It could tell green algae apart from green paint of the same color (according to human eyes).
Just like it’s difficult for a human being to imagine what four-dimensional space would be like, it’s hard for us to imagine what four-dimensional color would be like. Our brains are hard-wired to see three colors, because there’s no reason for them to do anything else: they’ve only got three different photopigments to work with. And just like twelve-dimensional space, twelve-dimensional color is pretty much impossible to imagine. But we can imagine all the cool shit we could do with hypercolor vision.
Since the spectrum of light reflected by an object not only tells you its color, but tells you a lot about its composition, hypercolor vision would be incredibly useful. Let’s imagine that, rather than being limited to three different color receptors, you had a hundred, meaning you had hundred-dimensional color vision. Instead of the blocky bar-graphs of red-green-blue color vision, each “pixel” in your eye would essentially see a complete spectrum. And if you could see like this, the world would look fucking weird. How weird? Well, here’s a list of things you’d be able to do that most people can’t:
Recognize minerals and other substances by color alone.
See that the blue part of a butane flame contains sparse spectral lines, while the orange part is a smooth blackbody spectrum.
Tell when you’d spilled white yogurt on a white shirt.
Really enjoy flowers, which I imagine would look like psychedelic abstract art.
Recognize counterfeit money.
Detect art forgeries.
Predict the fertility of land and plants using nothing but your eyes.
Measure the temperature of glowing-hot objects just by looking at them.
Recognize when a flame, a plasma, a star, a planet, or any other bright object contains elements with strong spectral lines, like sodium, titanium oxide, or mercury.
Understand why plants are green (it’s because the sun is partly greenish, but we can’t see it because of our limited color resolution and because our eyes are biased in their color sensitivity).
Measure air quality and ozone concentration just by looking up at the sky.
Spend hours staring at rainbows (actually, normal people can do that too…)
But you wanna know the best part of all this? This isn’t just a thought experiment. There exist eyes that can see like this. They don’t belong to humans (obviously), or even mantis shrimps. They’re electronic, and they’re called hyperspectral imagers. They do exactly what I was talking about before: they create an image where each pixel doesn’t just have a red, green, and blue value, but rather has a whole high-resolution spectrum attached to it. They do this very much the same way I took the pictures of those spectra above: they pass the image of the object they’re looking at through a narrow slit, send that light through a prism or a diffraction grating, and then detect the brightness of the refracted or diffracted light with an image sensor. By scanning the slit across the image in question, a hyperspectral camera can get pictures with enormously high-dimensional color, and they can do all the things I mentioned above: tell similar-looking materials apart at a glance, detect forgeries and hidden writing, measure chemical composition and temperature, and all around see things that human eyes just can’t detect. And these cameras exist right now, and can be bought by people (as long as they have more money than me).
Sometimes, reality is kind of a drag. Other times, though, it can be awesome.