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Paraphrasing from here:

A purely monochromatic 575nm wavelength light would be "perceived" as yellow, as would a light that has equal components in red and green (but no yellow). However, the actual waveform for the latter would be different than a pure yellow waveform.

However, we could, theoretically, use a Fourier Analysis to determine the frequencies at which the second light (red/green) has peaks in intensity. I am wondering if there would be any such case where an FFT would mislead us in how that light is perceived by most humans. For example, maybe you input some waveform to an FFT and the results show peak intensities in the red and green wavelengths, but for some reason that light is actually perceived as blue or something... discounting variances in the perception of color between humans (talking general case here).

Qmechanic
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codecitrus
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2 Answers2

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The Fourier transform does tell you the mix of frequencies in the light. From that, you can calculate the relative stimulations of the three types of cones in your eye. That mix tells you the amount of red, green, and blue you perceive, and hence the color. See Conversion formula from spectrophotometer readings to any standard color space profile?

But color happens in the mind. Gotchas are always possible:

  • Some people are colorblind. They only have two types of color receptors—or, rarely, just one.
  • You can saturate the receptors and see after-image colors.
  • Colors in the surrounding area affect perception. See What is Gray, from a physics POV? for an example.
Community
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mmesser314
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Spectral analysis of light is the basis of precision colour matching - although the problem in reality is made very much worse by the diffuse reflection component of real surface textures and fluorescence of some dyes. Spectral analysis is equivalent to taking the FT of the incoming waveform just implemented differently. Giving measured intensity as a function of wavelength.

Colorimetry has a long history - more than you probably ever want to know available here Basic Principles of Colour Measurement and Colour Matching of Textiles and Apparels

The best examples of objects that exhibit brain confusing colour behaviour are the natural gemstone Alexandrite (chrysoberyl) and the neodymium doped glass used by glass blowers to block the sodium D lines (effectively a narrow band stop filter against yellow light). The latter also found use in early colour TVs to improve the blue phosphor. In both cases the perceived colour of the material depends strongly on the colour temperature of the "white" light used to illuminate it. Such objects are rare but do occur both in nature and manufactured filters.

Looking through Nd doped glasses in particular can produce out of gamut colours in the brains perception of colour which although normally thought of as RGB is actually YGB raw sensor data with the red channel computed by the brain as Y-G for spotting ripe fruit. The effect is exaggerated red/green colour saturation. It even works on photography and such filters are sold to enhance fall colours.

Martin Brown
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