1.2.1 Psychophysics

Colorimetry:

Light is ELECTROMAGNETIC ENERGY in the 400nm to 700nm wavelength, where one nm (nanometer) is $1/10^{-6}$mm. Pure or MONOCHROMATIC LIGHT is perceived as one color in the range violet-indigo-blue-green-yellow-orange-red of the rainbow.

The amount of energy present at each wavelength is represented by the SPECTRAL-ENERGY-DISTRIBUTION of the light. For example, the spectral-energy-distributions of sunlight (what we call white light) is:

Figure: Energy distribution of day light

Many different distributions produce the same color impression (and are called METAMERS). Thus one can quantify light by:

• DOMINANT WAVELENGTH (color we see), which corresponds to hue,
• EXCITATION PURITY (proportion of pure light of dominant wavelength to white light), which corresponds to saturation, and
• LUMINANCE which corresponds to intensity.

Note however, that the dominant wavelength is not necessarily that with largest spectral component.

How is color determined by the human eye? The retina seems to have 3 kind of color sensors (called CONES) with approximative peak sensitivity at red, green and blue. This is the TRI-STIMULUS THEORY of color perception.

Figure: The cones and rods in the human retina
From:www.cs.fit.edu/.../index.html

The spectral response function $R$, $G$ and $B$ of this cones to monochromatic light is following:

Figure: Spectral responses of the cones to monochromatic light at given wavelength

Response to blue light is less strong. Note that the sensitivity peaks of $R$ and $G$ are in the yellow range.

LUMINOUS-EFFICIENCY function (the eye's response to light of constant luminance at given wavelength) corresponds to the sum of the curves above (compare this with the energy distribution of sunlight):

Figure: Luminous efficiency function

So the idea is, that every by the human eye distinguishable color can be produced by a additive mixture of red, green and blue. And this is the principle of color CRTs. The amount of R-G-B needed to match in the observers eye a monochromatic color of constant luminance has been experimentally determined as:

Figure: Color mixing functions

Note, that negative values of red are necessary in the range from 450nm to 525nm (which means that this amount of red has to be added to the given color in order that this new color can be matched by the described values of $G$ and $B$). A consequence is, that certain colors CAN NOT be produced on a CRT by R-G-B-mixes.

The human eye can distinguish hundreds of thousands of colors when shown side by side, see:

This sensitivity to color differences is dependent on the wave length. Approximately 128 fully saturated hues can be distinguished. The eye is less sensitive to hue-changes in less saturated light. Its sensitivity to changes in saturation is greater at the extreme end of spectrum (where there are approximately 23 steps). In contrast, at 575nm only 16 saturations steps can be distinguished.