TriStimulus

Our eyes have three different types of color sensitive receptors, called cone cells. Therefore, we need to have three values to describe a color sensation. These three values are called the tristimulus (trī stim' yə ləs) values of a color. The values are a measurement of the relative intensities of each light source additive primary color needed for our eyes to sense that specific color. The tristimulus values are usually given in the X, Y and Z values of the CIE color space.

The Eye

The spectral sensitivity of the
three types of cones, S, M and L.

The three types of cone cells are named:

• Short (S) for the short wavelengths
• Middle (M) for the medium wavelenghts
• Long (L) for the long wavelengths

OK, not the most creative names, but they are quite easy to remember.

The three types of cones are also known as blue, green or red receptors. They are able to receive three different wavelength of light:

• Short - Blue
• Medium - Green
• Long - Red

That is why we need three parameters (tristimulus values) to describe a color. The tristimulus values measure the relative brightness of each primary color needed to stimulate the three color receptors of the eye to create the sensation of seeing a certain color.

The Standard Observer

Because different people are more or less sensitive to various colors, researchers developed what is called the standard observer. This is a measure of the average sensitivity of a healthy person to light.

The Human Gamut

The gamut chromicity chart displays or maps on a single chart all of the colors the standard observer can see. It looks something like this. Note, this is a computer representation of the gamut chart and the colors are not correct, since a computer monitor cannot make all of the colors in the gamut.

The gamut chromicity chart showing all
the colors the standard observer can see

All Colors Cannot Be Generated

A color can be produced by many different combinations of source colors in various amounts. This is a perceptual effect called metamerism. No matter what combination of source colors are used to produce a given color, the color will always have the same tristimulus values.

But you cannot produce all colors of the gamut of human vision on a computer monitor.  The tristimulus values are limited by a triangle of values such as is shown in this diagram which represents the gamut of CRT colors.  The corners of the triangle represent the primary colors of a CRT which depend on the colors of the phosphors of the monitor.

Because the gamut chart is convex, it is not possible to produce every single color which we are capable of seeing. Put in geometric terms, it is not possible to find three points within the gamut which include the whole gamut.

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