In the late 1920s William David Wright and John Guild independently conducted a series of color matching experiments that mapped out all the colors the average human (meaning the average of the humans in the experiments) can see. In 1931 color scientists used the results of the Wright and Guild experiments to create the 1931 CIE XYZ color space (“XYZ” for short).
To visualize XYZ, think of a three-dimensional cartesian coordinate system (high school algebra) with axes labelled X, Y, and Z. In the XYZ color space, Y corresponds to relative luminance; Y also carries color information related to the eye’s “M” (yellow-green) cone response. X and Z carry additional information about how the cones in the human eye respond to light waves of varying frequencies.
Theoretically, the XYZ axes go off to infinity in both the positive and negative direction. However, not every set of coordinates in XYZ space corresponds to a color that the average human can see. XYZ coordinates that are outside the locus of colors mapped by the color matching experiments that led to the creation of the XYZ color space are called imaginary colors. XYZ coordinates that are inside the locus of colors mapped by the color matching experiments are called real colors.
Not every being sees color exactly like the hypothetical average human. For example, birds, bees, dogs, and humans with nonstandard color perception don’t see the same colors in the same way as the average human. However, for purposes of the digital darkroom, the colors that are seen by any being with non-standard color perception are neither real nor imaginary. Here’s why:
As mentioned in the first section of this article, light waves of different frequencies are out there in the world, but color happens in the eye and brain. One could do (and I’m sure color scientists have done) color matching experiments with human tetrachromats, with color-blind humans, and perhaps even with birds, bees, dogs, and etc. But the resulting “tetrachromat-XYZ” color space (or “color-blind-XYZ” color space, or “bird-XYZ” color space) wouldn’t be the same as the “average humans only” 1931 CIE XYZ color space. These alternative color spaces would have their own sets of real and imaginary colors.
To summarize, if a flower reflects it (“it” being that complex phenomenon we call light) and a bee sees it, of course it’s real for the bee. And if a painting reflects it and a human tetrachromat sees it, it’s real for the tetrachromat. But as far as the 1931 CIE XYZ color space that we use in the digital darkroom is concerned, these “nonstandard color perception” colors aren’t real and aren’t imaginary, rather they simply weren’t measured during the color matching experiments that led to the creation of the XYZ color space.
The CIE 1931 color spaces were the first defined quantitative links between physical pure colors (i.e. wavelengths) in the electromagnetic visible spectrum, and physiological perceived colors in human color vision.
The mathematical relationships that define these color spaces are essential tools for color management, important when dealing with color inks, illuminated displays, and recording devices such as digital cameras.
The XYZ color space is the basis of everything that relates to color in a color-managed image editing application.