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Essay on Color, Para. 2, reality in physics (continued)
2.2 Colorless Light
Some people call white, black, and the various shades of grey "colors". But with a narrower meaning, these are completely unsaturated shades, which I call "colorless".
In the spectral domain, these shades can be characterized by a constant (or nearly constant) value all over the visible wavelengts:
But of course we can easily betray our eyes to seeing "white". Nearly all the blackbody radiation characteristics shown in fig. 1.3.4-b of para.
1.3.4, color temperature
will deliver "white" light. Only upon direct attention (or comparison) you will notice a hue being yellowsh, blueish or neutral.
And even worse: The eye simply cannot detect spectral distributions to be of even shape. It can only notice that all 3 cone types are stimulated to the same amount. So, even the "3-band" fluorescent lamp
will deliver pure "white" light!
"Colorless" is not easy to define since the eye is specialized on detecting color differences. And since it does this with but three colored receptors. Anyway, the following three statements will hold:
* Equal energy spectra are white.
* Whatever stimulates all three cone types equally is white.
* Spatial distribution has strong impact on color reception. An illuminant that fills the entire scene will in many cases be regarded as being white. No matter how rough actually the spectral characteristics may look.