Updated: 9/10/2019

EXPLANATION OF THE DIAGRAM

(for more depth see instructional modules m227, m228, m229)

Introduction

This program presents an introduction to the Chromaticity Diagram for the sRGB color-specification standard which has been adopted by the W3C and by other organizations.
There is a short version of this page available here.

The Photons

All visible light consists of photons that can be absorbed in the retina of the eye causing vision nerve signals to head toward the brain.
Although a photon is a quantum particle, each photon has a wave-like probability amplitude with a measurable wavelength (for a short illustrative lecture see this, skipping the ads if you want, and use your browser's "back" arrow to return).The wavelengths of visible photons constitute a continuous set of numbers in the range 380-780nm.
Each unique photon wavelength corresponds to a unique color so the infinite number of different photon wavelengths means there is an infinite number of available basic visible-light colors. All other visible-light colors are mixtures of photon colors.

The Photon Emitters

On a W3C-standard (sRGB) screen each screen pixel has three standard emitters which produce the light emitted from that pixel.
At a screen pixel, each of the three emitters emits its single designed color at a currently-assigned intensity. The pixels are made sufficiently small so that an observing eye does not resolve a pixel's three emitters singly. Then the single observed color from a whole pixel is the sum of the colors from its three emitters.
The intensities of the three individual emitters at a screen pixel are called the R, the G, and the B (red, green, and blue) values at that pixel. The three sRGB values are expressed as integers in the range (0-255). The RGB values for the pixel at the position of the yellow cursor are shown in boxes in the upper left of our Chromaticity Diagram.
You can see the colors of the individual emitters by selecting among the three small square red, green, blue icons on the left side of our Chromaticity Diagram.
There are some accompanying photos of an sRGB screen where the magnification is sufficient to resolve individual emmiters for emission of the each of the three RGB colors and and then for the emission of white from the screen.
One need not know what mixtures of photons the screen manufacturer uses to produce each of the three sRGB-specified emitter colors.
If two of the emitters of a pixel are turned off, the color of the pixel is at a vertex of the triangle in the Chromaticity Diagram. If only one emitter is turned off, the color of the pixel is at a point on a leg of the triangle. If no emitter is turned off, the color is within the triangle. Colors which are outside the triangle cannot be produced by a 3-emitter screen.
When the image to be emitted contains a color which is outside the RGB emission triangle, sRGB assigns the emission to a specified spot on a nearby triangle leg. The erroneous emitted color may be silentlycorrected by the observer's brain, at least to some extent, providing the image contains several objects (for more details see Module 228).

The Photon Absorbers

In the sRGB model the photons from the screen are presumed to travel to the eye of an sRGB "Standard Observer" where they are absorbed in the retina.
At each pixel in the retina there are three types of photon absorbers called cones because of their shape.
Absorption of incoming photons in a cone results in a nerve signal heading toward the brain. Determination of the coding of these neural signals is a highly active research field with important application goals (see this description and use your browser's "back" arrow to return).
In the Chromaticity Diagram the x-coordinate of a color is the fractional response of the eye's x-cones, the cones more responsive to longer wavelength photons (for more details see Module 227).
The y-coordinate of a color is the fractional response of the y-cones, the cones more responsive to medium wavelength photons.
A color's z-coordinate from the shorter-wavelength-responding z-cones is not shown because xyz are fractional responses so z is just unity minus x minus y.
The xy coordinates of some photon wavelengths are shown as yellow dots on our Chromaticity Diagram.
All colors interior to the yellow-dot locus are mixtures of the photon colors on the locus.
The yellow-dot photon-colors locus contains the infinite number of true basic colors in nature. None of the photon colors can be produced by mixing any combination of other colors but all other colors are mixtures of photon colors.
Although there are an infinite number of basic colors, limitations in the eye-nerve-brain system's powers of discrimination reduces the number of observably different basic colors to a large but finite number.

Connecting Colors and Their Descriptors

To the immediate right of our Chromaticity Diagram is a color-display pane which has all of its pixels at the RGB values of the pixel at the yellow cursor position.
The color-display pane is surrounded by a gray border which is part of the sRGB specification for observing a color.
The WSI numbers shown in the table accompanying the Diagram are our choice of descriptors for a color's hue, saturation and intensity. These values are measured along an imagined straight line on the Diagram drawn from the white point to the color point and on to the photon wavelength locus. The hue value, W, is the wavelength of the photon at the end of the imaginary line. The saturation value, S, is the fraction of the line that is between the white point and the color point. The intensity value, I, is the largest of the color's RGB values, expressed as a fraction of 255.

Notes

The sRGB standard is generally used by those whose interest is in light which is going directly from screen to eye. It is not used as much by, for example, artists, restorers of paintings and paint manufacturers.
Other definitions of hue are in general use. Our "hue" is sometimes called a color's "dominant wavelength."