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Old December 22nd, 2017, 12:30 PM
Doug Kerr Doug Kerr is offline
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Join Date: May 2006
Location: Alamogordo, New Mexico, USA
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Part 2
The human visual system

The part of the human retina with which we are concerned here comprises numerous cone receptors of three different types (for a human with "normal" color vision). The types have different spectral responses, the functions that describe how components of the light on the retina are "weighted" by wavelength to arrive at the cone's output value.
It is a situation almost exactly parallel to that of the sensor systems we encounter in today's digital photography.
As a consequence, the human color sensation is three-dimensional (in the mathematical, not geometric, sense). That means that if we could describe it numerically, we would need three numbers to describe any given sensation. But of course we can't describe the sensation numerically in any precise way.

Describing the color of light

Thus it might seem that the color of a light beam (the property that determines what color sensation it is expected to evoke in a human observer) should also be able to be described, numerically, in a system of three values. And elbaorate tests by color scientists confirmed that conjecture.

Then was thus a need for a standardized system of such three values to numerically describe the color of an instance of light. In 1931, the CIE (The International Commission on Illumination, the acronym coming from its French name) published such a system. In it, the color of an instance of light (and recall that color here includes the property of luminance, or "brightness") is described by a set of three values, represented by the symbols X, Y, and Z.

Because a system of, n values can be visualized in terms of a plot in n-dimensional space, the values of the variables can be thought of as "coordinates" of a point in that space. Thus the three values X, Y, and Z are often spoken of as the coordinates of a certain color.

In any case, light whose color is describable by certain values of X, Y, and Z is expected to evoke a consistent color sensation among human viewers.


The color of an instance of light (meaning, the color sensation it is expected to evoke in a human observer) is determined by the spectrum of the light. It turns out that, for the light to have any given color (that is, for us to expect it to evoke a certain color sensation in a human) it can have any of an infinity of spectrums. This situation is called metamerism.

So we can (at the very least) imagine that light of two particular spectrums will evoke the same color sensation in human viewers.

This is the time to mention that, even for humans having "normal" visual systems, the responses to light with different spectrums may differ slightly. Thus light with spectrum A and light with spectrum B might appear identical to Alice, but not quite identical to to Bob. Thus, all the scientific work I speak of, and the standards for describing the color of light, bear the concept of "for an average human viewer". The tests that established the premises for various measurements all involved large cadres of subjects.

[To be continued]
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