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This mapping of wavelength to RGB value is very misleading: in particular, when moving to higher and higher frequencies, blue does not wrap around to red creati

This mapping of wavelength to RGB value is very misleading: in particular, when moving to higher and higher frequencies, blue does not wrap around to red creating purple. The perceptual color space that we are used to working in by mixing pigments or various frequencies of light is independent of light frequency. Unlike all other colors in the rainbow, there is no single frequency that will activate both the red and blue cones within your eye to create purple; if you had a single frequency between blue and red, it would look green. Instead, you must have two disjoint frequencies both stimulating the red and blue cones simultaneously (note that this is not strictly true, as for some people, the red cones do have some response down in the blue range, but the amount of red response within the blue region is extremely weak, and will not give the sensation of purple that is shown in articles such as this one). This is why rainbows do not have purple at the edge, and why prisms do not create purple; it is a "non-physical" color (I made that term up, but it hopefully makes sense given my explanation above). For a more physically-sound mapping of wavelength to color, see the CIE XYZ color system [1]. In particular, in Figure 2, the "pure wavelengths" are given as points along the outside edge of the curved surface, and as you can see, purple does not exist along that outside edge. For those interested in simulation of color, I highly recommend Jiahao Chen's old notebook "The Colors of Chemistry" [2], which goes through a lot of these concepts, and accurately simulates things like the color of solutions just from knowing their chemical composition. Truly fascinating stuff. [1] http://www.fourmilab.ch/documents/specrend/ http://www.fourmilab.ch/documents/specrend/ [2] https://github.com/jiahao/ijulia-notebooks/blob/master/2014-06-09-the-colors-of-chemistry.ipynb https://github.com/jiahao/ijulia-notebooks/blob/master/2014-...

mncharity 7y ago
Apropos fourmilab, just be warned, years ago the site had errors around whitepoint handling, some later fixed, some clearly not. That Figure 2 has the whitepoint label "W+" very offset from the rendered whitepoint (the Y crossing). My very fuzzy recollection is it was this specrend code that was behind the oops of a "The Universe is green!" NYTimes front-page(?) article - someone's research generated spectra, and for PR they derived a color using off-the-web code, but didn't recognize the bogosity of the output value. Red-shifted faces. The Wikipedia discussion of violet[1] currently seems plausible - yay. Unlike the "Black-body radiation" article,[2] which again has a CIE chromaticity diagram with a blackbody curve going through yellow and missing white, which is indicative of broken whitepoint math. Oh well. > This mapping of wavelength to RGB value is very misleading As is so much science education content. It can be fun to imagine how different science learning might be if content wasn't so broken. [1] https://en.wikipedia.org/wiki/Violet_(color) https://en.wikipedia.org/wiki/Violet_(color) [2] https://en.wikipedia.org/wiki/Black-body_radiation https://en.wikipedia.org/wiki/Black-body_radiation
ta1234567890 7y ago
Thank you for expressing your concern about color. About purple, recently I've been looking at pictures of rainbows, and one in particular caught my attention, it shows several (I believe 5) concentric rainbows. The order of the colors alternate on each rainbow, so of the outermost rainbow has red on the outside, the one right next to it has blue on the outside and red on the inside. Then if you look at the intersections or the space in between rainbows, it looks purple. What makes that space look purple? And what makes the color order alternate?
jacobolus 7y ago
Here was my best-effort rendering in 2010, https://en.wikipedia.org/wiki/File:Jacobolus_spectrum.svg https://en.wikipedia.org/wiki/File:Jacobolus_spectrum.svg

mncharity 7y ago

Apropos fourmilab, just be warned, years ago the site had errors around whitepoint handling, some later fixed, some clearly not. That Figure 2 has the whitepoint label "W+" very offset from the rendered whitepoint (the Y crossing). My very fuzzy recollection is it was this specrend code that was behind the oops of a "The Universe is green!" NYTimes front-page(?) article - someone's research generated spectra, and for PR they derived a color using off-the-web code, but didn't recognize the bogosity of the output value. Red-shifted faces. The Wikipedia discussion of violet[1] currently seems plausible - yay. Unlike the "Black-body radiation" article,[2] which again has a CIE chromaticity diagram with a blackbody curve going through yellow and missing white, which is indicative of broken whitepoint math. Oh well. > This mapping of wavelength to RGB value is very misleading As is so much science education content. It can be fun to imagine how different science learning might be if content wasn't so broken. [1] https://en.wikipedia.org/wiki/Violet_(color) https://en.wikipedia.org/wiki/Violet_(color) [2] https://en.wikipedia.org/wiki/Black-body_radiation https://en.wikipedia.org/wiki/Black-body_radiation

ta1234567890 7y ago

Thank you for expressing your concern about color. About purple, recently I've been looking at pictures of rainbows, and one in particular caught my attention, it shows several (I believe 5) concentric rainbows. The order of the colors alternate on each rainbow, so of the outermost rainbow has red on the outside, the one right next to it has blue on the outside and red on the inside. Then if you look at the intersections or the space in between rainbows, it looks purple. What makes that space look purple? And what makes the color order alternate?

jacobolus 7y ago

Here was my best-effort rendering in 2010, https://en.wikipedia.org/wiki/File:Jacobolus_spectrum.svg https://en.wikipedia.org/wiki/File:Jacobolus_spectrum.svg