Why is the visible light spectrum different to a hue wheel?

Question

The following problem has bugged me for a while, ever since I noticed it. On the Visible Spectrum Wikipedia, the following is the visible spectrum:

Now, in Photoshop, or really any colour picker, the hue slider looks something like this:

Or sometimes this:

I noticed that in both of these, the colour loops back to red. Why is this? I believe that this doesn't happen on the visible spectrum. The visible spectrum goes from a violet-ish colour to a maroon-ish colour, with a whole range in between. But where does the magenta colour from the hue slider fit in?

I take it that it is possible to have a purely yellow object, or a purely teal one, as it is on the colour spectrum, yet are magenta things and pink flowers inherently reflecting multiple wavelengths of light, from opposite ends of our particular viewing spectrum? All of this seems awfully odd to me, so I was hoping someone might be able to clear it up.

Google "color space" or see How does light combine to make new colours? — mmesser314, Aug 25 '23 at 03:44
A theory I have is that there's a sense in which it couldn't have been anything other than a wheel. A connected graph on 3 vertices (rgb) is either a triangle (same topology as a circle) or a line. Topologically there are only really two choices if we want color perception to be continuous. And I'd argue that the topology of a line doesn't really make sense if we can only perceive one color at a time (i.e. we don't perceive things as being blue and red). It seems natural that we would identify opposite ends of the spectrum to make sense of colors that are blue and red but not green. — Charles Hudgins, Aug 25 '23 at 11:40
In addition to the answers already given, keep in mind that the color wheel is not a physics tool, it’s an artistic tool. It helps the artistically-challenged such as myself to find colors that most people feel “look nice” together. I can get two complementary colors, three equidistant over the circle, or perhaps three close together, but reasonably equally spaced (aka the artist’s definition of “monochromatic” ;)). My wife likes those pictures better than when I make color decisions without this crutch ;) — W_vH, Aug 25 '23 at 19:33
"But where does the magenta colour from the hue slider fit in?" - nowhere. Magenta/pink does not have a corresponding frequency. It's a visual sensation you get when both the red cones and the blue cones are activated in your eyes. That's why it loops back - because of how your eyes work, and because your brain bridges the gap. — Filip Milovanović, Aug 26 '23 at 10:06
Tymon, the (usual standard) "color wheel" is not "real", it's just a convention and has no real connection to "wavelengths of light". — Fattie, Aug 26 '23 at 17:13
Don't you think that broadly the first two, straight-line charts feel counter-intuitive?
The spectrum of visible light is a straight line, stretching at either end into other wavelengths that mere humans can't 'see' as for instance infra-red or ultra violet but please note the vitally different meanings of 'infra…' and 'ultra…'

In the real world of print or projection, how is it not helpful to imagine that the extreme right of the spectrum loops straight back to the extreme left? — Robbie Goodwin, Aug 26 '23 at 18:16
@RobbieGoodwin - The question is not about the convention or the helpfulness of the color wheel, the question is why are there no purples in the visible light spectrum and how come it's possible to create the loop in the first place. — Filip Milovanović, Aug 27 '23 at 01:51
@FilipMilovanović You might be right but where did that interpretation come from?
Where did you get the idea either that there no purples in the visible spectrum… which could only be true if none of could see purple? Can you yourself see purple, or not?

How it's possible to create the loop depends on what you mean by 'the loop' which I won't presume to guess.

To you, what does '(create) the loop' mean, exactly? — Robbie Goodwin, Aug 27 '23 at 18:10
@RobbieGoodwin If you find this unclear read my (other) comment above and the accepted answer below. By "the loop" I mean the same thing as you in your "the extreme right of the spectrum loops straight back to the extreme left". I didn't say that purples are invisible, just that they don't appear in the rainbow. There is no "purple frequency" in the spectrum of the visible light - and yet a combination of frequencies can make us see purple. — Filip Milovanović, Aug 27 '23 at 20:05
@FilipMilovanović Thanks and where did you get the idea that purple had anything to do with the Question?
Did you not ask: '… why are there no purples in the visible light spectrum…'? Without detailed explanation, how could that not mean 'purple is not visible'?

Did you not ask how it was possible to create the loop in the first place? If by 'the loop' you do mean 'the extreme right of the spectrum loops… back to the extreme left…' what was there to Ask? — Robbie Goodwin, Aug 27 '23 at 20:53
@RobbieGoodwin "where did you get the idea that purple had anything to do with the Question?" - from the question (the different colors between violet and red they are asking about can be collectively termed 'the purples'). "Did you not ask" - No, I did not. I did not ask anything at all, I was simply summarizing what the original question was about, since you missed the point. — Filip Milovanović, Aug 27 '23 at 23:31
I really think you're mixing up at least two and prolly three quite different things while throughout, I've been asking clarification. Enough, though. — Robbie Goodwin, Aug 28 '23 at 21:10

Vaelus · Accepted Answer · 2023-08-26T22:45:03.037

23

The monochromatic spectrum and the hue wheel are different 1D paths through 3D color space

Since human color vision involves three types of cone cells, all the colors a human can see can be represented as a region of 3D space.

The most straightforward of these spaces is LMS space (long, medium, and short wavelength), where the coordinates of a point in space represent the stimulation of each type of cone cell. LMS space can be transformed with a linear transformation to the more convenient XYZ space. The axes of XYZ space were chosen to have useful properties, but for our purposes, it just produces nicer looking plots. Note that not all points in LMS space or XYZ space represent colors humans can see; for instance, a cone can't be stimulated by a negative amount.

The below "chromaticity diagram" is a plot of a 2D slice of XYZ color space in the plane $X + Y + Z = 1$.

The colored region of the plot, label "visible region", contains points in the space representing colors humans can see, while the non-colored region contains points representing physically impossible cone stimulations. The top edge of the colored region corresponds to monochromatic colors. The circled vertices of the triangle labeled "sRGB region" represent the red, green, and blue primary colors used by an sRGB monitor, and the edges and interior of the triangle are linear combinations of the primaries. Note that only colors in the sRGB region can be reproduced by an sRGB monitor, so colors outside the region in the diagram are clamped to nearby sRGB colors.

Finally, the answer: the visible light spectrum is the non-closed path along the top edge of the colored region, and the hue wheel is some closed path through the colored region. The exact path of the hue wheel varies, but it is often chosen to be the perimeter of the triangle defined by the three sRGB primaries.

Notice that the bottom edge of this triangle approaches the bottom edge of the colored region, which does not represent monochromatic colors, but instead combinations of red and blue, giving magenta.

edited Aug 26 '23 at 22:45

answered Aug 25 '23 at 14:09

Vaelus

593

5

And there's a name for the line along the bottom of the space (which is nearly the same as the line along the bottom of the triangle, if your primaries are decent): the line of purples. That's what brings you back to red, and magenta lives approximately in the center of it. – hobbs Aug 25 '23 at 18:04
Something I find helpful in understanding the chromacity diagram here: this diagram was constructed empirically. They asked people to distinguish between nearby pairs of colors and found that if they plotted the data in that particular way, the distance between any two colors corresponded to how different they were (how well people could distinguish them)\ – Cort Ammon Aug 25 '23 at 18:22
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@CortAmmon not really, the XYZ (CIE 1931) space doesn't encode perceptual distance. Its main objective is to make all the color matching functions nonnegative (which you can't achieve with any choice of "possible" RGB primaries). CIELUV is an actual attempt to make the color space perceptually uniform. – Ruslan Aug 25 '23 at 18:24
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@Vaelus "The circled vertices of the triangle represent the monochromatic red, green, and blue primary colors used by an sRGB monitor" — this is wrong, the triangle corresponds to CIE RGB, not sRGB. The latter represents a much smaller portion of the visible gamut. – Ruslan Aug 25 '23 at 18:27
@Ruslan Thanks, I'll update the diagram and wording when I have time over the weekend. – Vaelus Aug 25 '23 at 21:55
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While chromacity diagrams are very useful to better understand color, remember that you are most likely looking at these pictures displyed on an RGB screen, which can't properly render the colors outside that little RGB gamut triangle. The same problem applies to the monochromatic color charts. You'll have to experience those colors AFK to really see the difference. – jkej Aug 25 '23 at 22:45
Except that they're not 1D anything… they're 2D paths – Robbie Goodwin Aug 27 '23 at 21:53
@RobbieGoodwin Here, 1D means that each point on the path can be uniquely identified by one parameter, say $t$, despite being embedded in 3D space. In other words, X, Y, and Z of points on the path are all function of the single parameter $t$. – Vaelus Aug 27 '23 at 22:08
Oh, right. Sorry. – Robbie Goodwin Aug 27 '23 at 22:13

Matt Hanson · Answer 2 · 2023-08-25T02:50:43.370

10

The reason that color wheels are so common is that when certain colors are absorbed by materials, and thus not effectively reflected back towards a human's eyes, humans tend to perceive the material as being the "complementary color" on the opposite side of the color wheel to the color absorbed. This, as you rightly pointed out, has nothing at all to do with the actual electromagnetic frequencies of the light itself. Rather, it has to do with the peculiarities of how sensitive the human eye's distinct cell receptors are to different frequencies of electromagnetic spectrum. The three cell types, represented by the three distinct curves on this graph, have the color perception sensitivities shown.

The total obtained by adding up the various contributions of these curves looks like the following figure.

It just so happens that when you absorb, e.g. red light, that the majority of the intensity remaining will tend to be of greener color than anticipated, making you perceive the complementary color green. Likewise, when green colors are absorbed, the remaining colors are mostly red, making you perceive the complementary color red. The color wheel just neatly organizes this otherwise highly unintuitive information for us.

Rather than display the raw electromagnetic spectral values of colors, which is far more useful to a physicist, color wheels show us the relationships between colors as humans actually perceive them, which is far more useful to an artist, for instance. Of course, the fact that these are continuous spectra and the fact that materials can absorb to different degrees over wide varieties of wavelengths can make the actual prediction of optical properties of materials quite difficult a priori. This is just a useful heuristic for simple examples.

edited Aug 25 '23 at 02:50

answered Aug 25 '23 at 01:00

Matt Hanson

2,550

3

Note also that hue typically consists of composite colours whereas any given point on the spectrum that physicists use is essentially monochromatic. – Rodney Aug 25 '23 at 11:06
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Thank you for saying more clearly what I was trying to express at the end there! – Matt Hanson Aug 25 '23 at 11:31
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I think this does not answer the question. First, the sensation of colour comes from light directly, there's no need to explain it using objects, materials, or absorption: just look at a light source directly. Second, this talks about "complementary colours" without explaining first why there is a magenta, purple, etc. colour when there's no spectral magenta, purple, etc. – Pablo H Aug 25 '23 at 14:02
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I was explaining the preponderance of color wheels as devices for depicting color relations, not the existence of color. To do the latter we would just appeal to the specific electromagnetic frequencies of light. To your point about colors apparently derived from combinations of monochromatic light frequencies, that is just a quirk of the way human perception works. Sometimes the color perceived isn’t quite as simple as just which color is most dominant. Magenta is a good example for that purpose. But nonetheless, the color wheel is a perceptual tool to help us make this connection. – Matt Hanson Aug 25 '23 at 14:32
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@MattHanson Except, the colour wheel is more psychological / neurobiological than this "composite" explanation you give. Filter the red frequencies from a white colour source, and I wouldn't call the resulting colour green, but cyan. Filter the peak (green-ish) frequencies of sunlight, and you get a purplish colour (not red). This isn't why humans perceive red-green and yellow-blue as complementary pairs. – wizzwizz4 Aug 25 '23 at 14:53
I think the answer would be clearer if, instead of the red/green example, it was something akin to "When you absorb intense light, e.g. green, then the green cones "fatigue" and dim. When you then look away, the green cones are still dimmed, but blue+red are still "full", and our brain perceives this red+blue as a "purple". These opposite colors give the psychological impression of a continuous color wheel, with purple connecting blue and red. See https://en.wikipedia.org/wiki/Impossible_color#Chimerical_colors" – Mooing Duck Aug 25 '23 at 16:59
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I am not disputing the complex psychological features of color perception. Your comments actually serve to nicely enhance my fairly surface-level answer, which I was trying to emphasize was only a basic glancing look at the topic. The question was about the reason that there is a color wheel at all rather than a linear electromagnetic spectrum. I believe my answer at least motivated the distinction in terms of perceptual categories, though of course more detail can always be given and is welcomed. – Matt Hanson Aug 25 '23 at 19:22
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@MattHanson, just to make it more fun: when mixing light (not paint) the color wheel changes. There’s a nice example in this page: https://www.diyphotography.net/avoid-colour-banding-using-coloured-gels/. I have that open on my phone whenever I’m in the studio ;) – W_vH Aug 25 '23 at 19:25
I've never actually seen the additive and subtractive color wheels side by side before, so thanks for sharing something new! Color perception is just wild and wacky sometimes, and I am always amazed at just how aggressively it defies simple explanations. See the answer by Vaelus to see what I mean since he gets really into the hardcore perceptual colormap space idea. – Matt Hanson Aug 25 '23 at 19:27

score 6 · Answer 3 · answered Aug 25 '23 at 18:50

The reason for your confusion is that color words are used in two incompatible ways.

Humans with standard trichromatic vision have three types of cones. The outputs of the three types of cone can be seen as coordinates in a three-dimensional space of colors. Any beam of light has a single color, which is a function of the cone response it produces if you shine it into someone's eye. Magenta light exists; otherwise we wouldn't see that color.
Unfortunately, people also use color names like red and blue for single frequencies of light. They call a light beam whose Fourier spectrum has one narrow peak "monochromatic", and they say that a light beam with two peaks in the Fourier spectrum has two colors, even though you will see only one color when you look at it, and that color is not either of the two colors they assign to it.

This leads to endless confusion, including the idea that magenta is not a real color (just because there isn't any single frequency of light that can be given the name magenta in the second scheme).

In the rest of this answer I'll use color words only in the first sense.

Practically every colored object in nature, whether it's teal or magenta, reflects (scatters) a wide range of frequencies of light. The color of the scattered light is no less "pure" for all that.

Teal and magenta do differ in that teal can be produced by light with a single local maximum in its Fourier spectrum, while magenta needs two local maxima, or at least a local minimum in the middle of the visible range. That's quite different, though, from saying that teal is a single frequency and magenta is two frequencies. They aren't one frequency or two frequencies; they are colors.

We evolved color vision to see ripe fruit and saber-toothed tigers, not rainbows (probably). In terms of Fourier spectra, a rainbow is simple, but perceptually, it's a sort of random walk through the 3D color space. It could make it around the whole hue wheel, but happens not to—this is more or less an accident of how the cone pigments work. It could stop and reverse direction on the hue wheel, and I think it actually does do so at the long-wavelength end, but the colors are so dim there that it's hard to see.

Beyond that, see Vaelus's answer—but where it says "monochromatic", think "Fourier component".

I really like that you brought in the ideal examples of Fourier decompositions monochromatic light and contrasted that with colors that are composed of mixtures despite being perceptually pure. — Matt Hanson, Aug 25 '23 at 19:25

Why is the visible light spectrum different to a hue wheel?

3 Answers3

The monochromatic spectrum and the hue wheel are different 1D paths through 3D color space

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