In this photography stack exchange question about why we encode pictures in RGB, several answers talk about the fact that our visual receptors are trichromatic, which is the inspiration for the RGB system. Not only is it the inspiration, it appears that we use RGB to encode colors explicitly because we have trichromatic vision.

It's my understanding that digital screens are composed of pixels, where each pixel is composed of three colors: red, green and blue. To form a color, each pixel is turned on precisely to the proportion specified by the RGB value. Since this happens at such a small scale (from far away), the colors mix and we get end up with a brand new color.

My question is this: If we use this scheme to trick our brain into thinking we're seeing a different color as opposed to using specific wavelengths and amplitudes, would an alien with a different visual perception system be able to see the colors in our digital screens? If another being had a tetrachromatic visual perception system or a true-wavelength perception system, would they perceive the same colors from our trichromatic screens?

In other words, is our mixing of the three colors actually producing this new color, or are we just tricking ourselves to think that it is?

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    $\begingroup$ I see two issues: the number of colors you can distinguish, and the resolution of individual pixels "tricking" the eye into seeing a solid color. Your last two paragraphs conflate these distinctions. A third issue might be Persistence of Vision, since our screens refresh at a certain rate that might look like slow flashes to a fly. $\endgroup$ – wetcircuit Jun 6 '17 at 16:35
  • $\begingroup$ I generally didn't understand the issue, so I don't know how resolution would play a role. Motot explained to me how I was conflating the two issues. I didn't expect the possibility that you would be able to see more colors, but I guess I see how that makes sense! $\endgroup$ – Brian Woodbury Jun 6 '17 at 16:40
  • $\begingroup$ Depending on their evolutionary pressures, it's possible their vision is primarily in the ultra violet range. In that case, their vision would be more similar to insects and likely would be unable up read a standard monitor. If we knew of this, however, I'm sure we'd be able to craft monitors that spread their emissions into the UV band. $\endgroup$ – Isaac Kotlicky Jun 6 '17 at 18:15
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    $\begingroup$ "Would aliens with different visual perception be able to read our screens?" - off topic, but I sometimes wonder what life would be like if you were some kind of variant human where glass wasn't transparent for you. (e.g. you "saw by sonar", or glass changed "polarization" so you couldn't see it and then light bounced inside a room and changed back so rooms were still lit). All windows, TVs, computers, smartphones would be opaque rectangles. Rooms and cars would be very obviously the completely closed boxes they are but don't normally feel like. People using screens and windows? Bizarre. $\endgroup$ – TessellatingHeckler Jun 7 '17 at 4:15
  • $\begingroup$ @TessellatingHeckler It's been quite a while now since computer screens (including TVs, but possibly excluding smartphone screens) were commonly made of glass. The most common way to manufacture computer screens today is almost certainly using plastic. Also keep in mind that every light source which is encased in glass (which is plenty of them) would become completely dark if glass was fully opaque. $\endgroup$ – user Jun 7 '17 at 6:46

If another being had a quadchromatic visual perception system or a true-wavelength perception system, would they perceive the same colors from our trichromatic screens?

Of course not. Tetrachromacy is real, can occur in humans, and she indeed seen differently:

In 2010, after 20 years of study of women with four types of cones (non-functional tetrachromats), neuroscientist Dr. Gabriele Jordan identified a woman (subject cDa29) who could detect a greater variety of colors than trichromats could, corresponding with a functional tetrachromat (or true tetrachromat).

(From Wikipedia)

To answer your main question:

Would aliens with different visual perception be able to read our screens?

Most probably yes. As you can notice, most of the information in our screens is coded by brightness-darkness, not actual colours. So if they can register wavelengths in our visible spectrum, they will be able to read our letters and navigate our websites.

Cells are responsive in quite wide bands, for example in birds:

If aliens are somehow similar, then no matter where exactly the peak will be, they will be able to register light or no light. Of course, the further away from our peaks, the more different would computer image look from the real world one - because computer screen only emulates the parts of the real-world look we, humans, notice, and not the full spectrum. Look for emission spectrums and absorbtion spectrums of various light sources and items to see them. Item with flat absorbtion spectrum will look gray / white to both us and them. Item that reflects similar amounts of our "peak" frequencies but different amounts where our cells are not so sensitive will look white-ish for us, but not for them. And so on.

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    $\begingroup$ "the further away from our peaks, the more different would computer image look from the real world one" I sense a certain anthropocentricism in this statement. Surely the aliens' opinion about what constitutes "the real world" deserves equal consideration as that of mere humans? $\endgroup$ – user Jun 6 '17 at 20:09
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    $\begingroup$ @MichaelKjörling computer screen is 100% anthropocentric in the way it operates. That's the point. $\endgroup$ – Mołot Jun 6 '17 at 20:13
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    $\begingroup$ Yes, but what's the "real world"? Who's to say that an alien species doesn't occupy the real world? Heck, what's to say a bee doesn't occupy the same real world as us? Of course with different sensitivity to color the image will look different -- just like how the image will probably look different to someone with far sharper or less acute eyesight -- but said being will still experience the same electromagnetic emissions, just in a (likely) different way. $\endgroup$ – user Jun 6 '17 at 20:19
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    $\begingroup$ @Michael emission spectrum of the image on the screen is not the same as reflected spectrum of real objects. It only looks the same for us because screen is anthropogenic in the way it works. Is it clear enough when described like that? $\endgroup$ – Mołot Jun 6 '17 at 20:29
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    $\begingroup$ @MichaelKjörling: I think the difference in interpretation here is whether "the real-world one" refers to an "ordinary" human perception of the screen, or the aliens' perception of a physical scene the screen is supposed to portray. $\endgroup$ – user2357112 supports Monica Jun 6 '17 at 20:30

Sensations and physical quantities

Color is a sensation: it exists in the mind; it is not a physical quantity, that is, it does not exist in nature. Since color is a sensation, all measurements of color are made with reference to a hypothetical "standard observer"; the color discrimination ability of most men with normal color vision is somewhat poorer than standard observer's, that of most women is somewhat better. Importantly, what is measured by a color measurement does not necessarily match what is perceived by the brain -- the measurement is related to the sensation of color only for extended objects, that is, objects which subtend a large part of the visual field; for smaller objects other mental mechanisms come into play which make a mess of the "objective" measurement. For illuminating examples, see professor Akiyoshi Kitaoka's illusion pages, for example, page 13.

For extended objects (objects which occupy a large part of the visual field), the sensation of color is related to the physical quantity spectral density of light; the relationship between the spectral density of the light and the perceived color is complicated but predictable using empirically measured formulas. For smaller objects the perceived color cannot be determined from the power spectrum of the light coming from those objects; only by taking into account the entire scene can the color be predicted (approximately), and there are no good formulas.

RGB is not enough and cannot be

The International Commission on Illumination (CIE, Commission internationale de l'éclairage) carried out an extensive set of experiments which determined that the standard observer can match any given color by varying three parameters. (Essentially, the test subjects had to match the color of a light source with the color of another light source which could be modified by turning three knobs.) Mathematical calculations showed that combining three abstract light sources is enough to match any visible color; unfortunately, the three abstract base colors which define the CIE 1931 XYZ color space are non-physical, that is, they cannot exist. (They are a red much redder than the reddest visible red, a blue very much bluer than the bluest visible blue and a green a little greener than the greenest visible green.)

In practice, we either accept than any three visible fundamental colors will be able to reproduce only a part of the visible colors, or, if we truly want to reproduce a larger part of the visible color, we accept that we need more than three fundamental colors. For example, high-quality color reproduction on paper is done in hexachromatic processes; for color reproduction on screen advanced televisions use four base colors.

In particular, the commonly used sRGB color space can reproduce less than half of the visible colors, which is understandable given that its primary colors are chosen so that they match the colors of the phosphors available for color TV screens in the 1950s. In particular, the green primary of sRGB is very poor; sRGB simply cannot reproduce luminous saturated greens.

sRGB chromaticity diagram

CIE 1931 xy chromaticity diagram showing the gamut of the sRGB color space and location of the primaries. By Spigget, made available of Wikipedia under CC BY-SA 3.0.

(There exist computer monitors and television sets which can reproduce a wider gamut than sRGB; but the problem is that (1) they are very expensive and (2) the overwhelming majority of visual media are encoded in sRGB. Look for "wide gamut" monitors, which are not the same as "deep color" monitors. Even monitors which can faithfully reproduce more than 95% of the sRGB gamut are quite expensive.)

So how come we can use sRGB?

Remember that color is not a physical quantity, but a sensation which exists in the mind. The perception of color is seldom absolute; in most practical situations it is the color contrast which counts. As a consequence, cameras and image manipulation software which use the sRGB color space cheat by mapping the visible colors to the smaller representable gamut. As a commonly encountered representation in the user interface, you may seen some color printer drivers offer a choice of intents when reproducing colors; common choices include pictures (mapping all colors to the device gamut) and presentations (clipping colors to the device gamut).

How would an alien perceive our screens?

All the preceding discussion intended to convey that the perception of color is different for different humans, no aliens needed. But to come back to the question: we are obviously unable to picture the sensations of an alien which has a color vision system with more primary stimuli than ours; but we can make an attempt at an analogy.

All user interface designers are taught that colorblindness is a thing, and that about 10% of human males have less-than-standard color vision; for this reason, user interfaces should always be checked for usability by colorblind people, and there are countless software programs which attempt to simulate what a colorblind person sees.

A beach scene in natural colors and simulated red-green color blindness

A beach scene in natural colors and simulated red-green color blindness. Own work by AlexP, simulated color blindness made using the Color blindness simulator from Etre.

We can intuitively imagine that for the multi-chromat alien our screens will have a similar relationship to a full-alien-color image as the simulated colorblindness images have to our full-human-color pictures. This is a gross simplification, obviously. One immediate example of why this is an oversimplification: imagine that the alien sees yellow as a primary color, whereas humans cannot -- we cannot distinguish between a mixture of red and green and monochromatic yellow; to the alien, all yellows in the picture will appear as weird color shifts.

But would they be able to read the screen?

Well, that really depends on what part of the electromagnetic spectrum they perceive as visible light. If their visible light overlaps with ours then yes, they will be able to read black text on white (or white text on black), although for them the white may not be white but some color. They may even be able to read some color combinations which are hostile to a human reader, such as the infamous magenta text on green background...

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    $\begingroup$ Note that modern monitors are available which have a wider gamut than sRGB. It's not uncommon to find monitors that natively use Adobe RGB in the professional space (heck, my mid-range digital camera from 2008 can be set to write out JPEGs using Adobe RGB if desired, the alternative being sRGB and of course raw/monochromatic sensor data dumps), and there are monitors available that use 10-bit color depths per subpixel instead of the more common 8-bit depths; and of course, even monitors that advertise 3 times 8-bit sRGB might in practice be more like 3 x 6-7 bit sRGB. $\endgroup$ – user Jun 6 '17 at 20:15
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    $\begingroup$ @MichaelKjörling: You are of course right, AdobeRGB has a much wider gamut than sRGB; so wide that it absolutely requires more than 8 bits per component. 10-bit color depth does not extend the gamut, it just makes available finer distinctions. (Due to the built-in gamma corrections of sRGB, which was designed so that it would match old CRT color screens, banding may be visible when using 8 bits per pixel.) On the other hand, a monitor which is able to show an extended color gamut cannot do anything with image files which are already encoded in sRGB. I could have expanded the answer tenfold... $\endgroup$ – AlexP Jun 6 '17 at 20:20
  • $\begingroup$ Oh yes, this is a huge subject (and far, far, far too large for the comment space). I just wanted to point out that sRGB is not the end-all be-all even of computer monitors, which your answer made it sound like, and that wider gamut hardware is available in the real world, today, if you want it and are willing to pay the bit (you see what I did here, don't you?) of price premium. Of course I ran out of space in the margin, so the wonderful proof will have to be left as an exercise for the reader... $\endgroup$ – user Jun 6 '17 at 20:28
  • $\begingroup$ That's much better! $\endgroup$ – user Jun 6 '17 at 20:32
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    $\begingroup$ @BrianWoodbury: Begin with Barry Maund's article "Color" in the Stanford Encyclopedia of Philosophy and with the article on color in Wikipedia. Then follow the links and bibliographies. $\endgroup$ – AlexP Jun 7 '17 at 16:24

Looking at Earth and the animals that we share this planet with, it has already been shown that other animals with different color perspective, like octopuses, dogs, cats and others can observe television screens and react to what happens on them:

  • Dogs have been seen chasing after virtual balls a baseball pitcher throws at the screen.
  • Cats have been seen trying to catch mice or birds that appear on a monitor.
  • Octopuses have been seen in lab environments to be able to interpret images shown on a monitor and respond to them.
  • During WW2, a series of tests was run using pigeons in bombs tapping on monitors to guide the bombs to their destination.

Now, these aren't aliens, but they do have different visual perception to humans. The main requirement is that the species evolved in an environment where the most prevalent light is in the human-visible light spectrum. An alien that grew up perceiving mainly infrared or ultraviolet light would be less likely to see our monitors, simply because those monitors are designed to have uniform output in the other spectrums.

  • $\begingroup$ octopus is not from Latin! So don't try and use a latin plural mechanism just because it ends in -us! $\endgroup$ – JDługosz Jun 8 '17 at 13:54

The previous answers explain the physics well that's behind color vision. Based on these physical principles, I would like to elaborate on the interesting cases where the aliens cannot read screen content that humans could, as the question title suggests.

Trivial case: Their eyes are no responsive to our wavelength spectrum

As mentioned in previous posts, they are obviously not able to read our screens if their eyes are not responsive to our screens' spectrum. We can for example envision a life form that lives in the energy-rich surroundings of a black hole. They would probably have optical receptors that are suited for that wavelength range, and not so much for our optical wavelengths.

The more interesting case arises when their color vision (let's assume they also have multichromatic vision) is sensitive in our optical region.

Black hole optical spectrum Black hole optical spectrum. Image from: http://astrophysics.fi/index.php?p=bhc_descr. The optical spectrum that's visible for humans is around 2 * 10^-3 eV in that graph

Flatbands: Equal luminosity is invisible!

So let's stick to our example of aliens living close to black holes. To have eyes that are sensitive to our visible light spectrum, they need extremely broadband eyes. It's thus quite probable that our visible light spectrum is so thin that it looks like just one single color.

The same holds if our visible spectrum is at the very border of their visible spectrum - only one receptor would be receptive, and they would see everything in the same color, much as we perceive all red colors above 630nm as the same red.

Now coming back to computer screens, this would mean that they cannot distinguish between colors that have equal luminosity but different chroma (for us). So if we wanted to hide our computer screen content from their eyes, we'd have to display everything with different colors with equal luminosity. That would make it tiresome for humans to read, but it could work!

High dynamic range eyes: Shadings are invisible!

We can make a similar argument for eyes with a very high sensitivity range for the visible receptors. If they live very close to a star, their eyes may be equipped for much higher light intensities than our eyes. They may also have a vision for lower intensities, however that vision might not be as well equipped as our vision to distinguish between small intensity differences. Gray text on white background may look just dark gray for them.


To add to previous answers…

In principle, if they share the same range of visual frequencies, they will be able to see the screen.

To some extent the colors will work even if they have different primaries. That is, chromatic colors (those on the edge of the horseshoe diagram or in a rainbow) will be simulated by any choice of pure frequencies on either side of it. If they have a small number of primaries too, they'll get the right idea. If they have many distinct primaries with narrow band filters, it will not work and they’ll perceive different un-natural colors instead.

For the purples, all bets are off.

Which different frequencies offer distinct perceptions may be very different from us. Consider a map coded with colors in the range of frequences that appear red-orange-yellow. The cold and hot marks on the map are utterly distinct, as red and yellow are different colors to us. Now the same range of values transposed up a bit would have the same frequency difference between high and low but it all looks blue to us, and we have poor discrimination of blues.

Someone might choose two widely different frquencies and suppose that they must be different colors, for anyone. But our vision forms a color wheel with purple looping back from violot to red. A species with more primaries might have a couple different distinct loops of color perception, or worse, they don’t normally loop for broad-band pigments in real life but are fooled by monochromatic primaries used in our displays.

So, there's lots of room for it to mostly work but have unexpected and surprising problems in specific instances.


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