In addition to device-dependent color spaces, there are also device- independent color spaces. These color spaces encompass all of human vision. The most common is called CIELAB (or L*a*b; often written as LAB, although technically the * should be used). Back in 1931, the CIE (Commission Internationale de L’Éclairage, also known as International Commission on Illumination), a group or color scientists, conducted a series of experiments and tests on humans to determine how they perceive color. The tests involved showing groups of volunteers a sample color under very controlled conditions whereby each subject adjusted the intensity of red, green, and blue lights until the mix of the three matched the sample color. This allowed the CIE to specify precisely the stimulus response of the human eye.
The CIE came up with the term standard observer to describe a hypothetical average human viewer and his or her response to color. Furthermore, the results of these tests produced a mathematical model of a color space formulated not on any real-world device, but rather on how we humans (the standard observer) actually perceive color. This core color model is called CIE XYZ (1931). This is the color model from which all other device-independent color models are created. Like the RGB color model with three additive primaries, CIE XYZ uses three spectrally defined imaginary primaries: X, Y, and Z. These X, Y, and Z primaries may be combined to describe all colors visible to the standard observer. Also in 1931, a synthetic space called CIE xyY was created, which itself is derived from CIE XYZ. In 1976, CIELAB and CIELUV were added to the mix of these device-independent color spaces. The CIELAB color space is a synthetic, theoretical color space derived from XYZ. Unlike the original, CIELAB has the advantage of being perceptually uniform (sort of . . .). That simply means that a move of equal value in any direction at any point within the color space produces a similar perceived change to the standard observer.
The XYZ color space is based on three quantities or stimuli. The geek term for describing this is tristimulus values (three stimuli). Technically the term tristimulus values refers to the XYZ values of the original CIE XYZ color model although you will often hear people describe tristimulus values when defining a color in RGB or CMY (or using any three values). This is incorrect. Since our aim is to keep the color-geek-speak to a minimum, it’s not important to know the differences in the various CIE constructed color models, but rather to recognize that a color space such as CIELAB is based on how we see color. What you should keep in mind here is that using a set of three values, any color can be specified exactly and mapped in three-dimensional space to show its location in reference to all other colors. This can be useful! There are no capture or output devices that directly reproduce CIELAB; however, this color space allows us to translate any color from one device to another.
Chromaticity Values and the Chromaticity Diagram: The CIE XYZ color space represents color using three imaginary primaries defined as X,Y, and Z—imaginary because the color of these primaries doesn’t correspond to a real-world light source. This is a three-dimensional color space. The CIE also defined a method in which chromaticity can be plotted in two dimensions (x,y). In this color space, the third component is luminance (Y). They named this CIE xyY. This color space allows you to plot hue and saturation, independent of luminance, two- dimensionally on a two-dimensional graph called the CIE Chromaticity Diagram. The x and y values used to plot a color on this diagram are referred to as the chromaticity coor- dinates or sometimes, chromaticity values.
IF you look closely at the diagram in the article, around the horseshoe shaped plot, you’ll see numeric values which are the range of frequencies from approximately 400nm to 700nm that is visible to the eye.