CIE 1931 Color Space

Language of Color

People who work with color need away to communicate so that when they want a certain color they will get it. That can only happen if that color has a specific definition which will not change. People need to be able to say, "I want this color," and be confident that they will get it because everyone agrees on what that color is.

The need for a language of color was answered with the CIE Color System which gave specific values to colors. Matching colors to the CIE Color System results in a unique combination of tristimulus values that define that color.

Unlike many of the other color systems you will see, not all of the colors that are defined by the color system are actually visible. In fact, it is this property that makes this Color system so valuable, since it encompasses the complte gamut of human vision.

The final result, is a color system that real value, since it allows color professionals to accurately and consistently define a color. I tis this feature, for example, that makes it possible for a TV advertiser, for example, to say, "I want these colors in the logo," and know that the colors will be exactly right.

Standard Observer

Before a system of defining light source colors could be developed, it was necessary to know how people perceive color. In the 1920s, W. David Wright and John Guild who each worked independently did pioneering work on how people perceive color. Their results were tabulated and combined in order to come up with the Standard Observer which defines how the average person sees colors.

What Wright and Guild did was have their subjects look through holes in a box at two colored dots. The source of the color was colored light beams. One dot was a fixed color and could not be changed, whereas the other one could be changed by manipulating knobs to combine the three primary light colors of red, green and blue. Their task was to combine the three colors to match the fixed color.

It was found that the same color could be created by many different combinations of three colors. This is called metamerism.

CIE 1931 Color Space or CIE XYZ

In 1931, the International Commission on Illumination or CIE, which is the abbreviation for its French name, Commission internationale de l'éclairage, established the first system for scientifically defining light colors or additive colors.

The CIE used the data produced by Wright and Guild to develope the CIE Color System or CIE XYZ color space. It was the first widely accepted, international standard way of defining color and is still the "gold standard" today.

CIE XYZ values, that is, the tristimulus values or the measure of cone cell activation of the three types of color sensing cells in the retina of our eye, are normally defined between 0% and 100%. But sometimes because of how the mathematics work out, you'll see numbers above 100%.

The CIE XYZ used the values of X, Y and Z and was designed so that the value of Y was a measurement of the brightness of a color. The values of X, Y and Z were used to derive the parameters of x and y to be able to plot the colors on the CIE gamut chart (or CIE xyY or chromisity chart.) See below.

The CIE XYZ color space cannot be represented as a solid as easily as the RGB color cube. But it would probably be fairly similar to the RGB color cube with noticable color distortions.

The CIE XYZ color space is only one model. Many other color space models exist today. In fact, numerous version of the same model exist. But the CIE XYZ model is special for two reasons:

• It was based on direct measurements of the human eye, specifically the three cone cell receptors, and
  
  
• It is the basis and gold standard which is used to describe many other systems.

The CIE Gamut Chart or CIE xyY or Chromaticity Chart

The CIE gamut chart is a two dimensional picture of the three dimensional CIE XYZ color space. The picture is distorted because it is forced onto a two dimensional space. But it is easier to see and understand then the CIE XYZ and is useful for color mixing because all of the colors can be easily seen.

CIE Gamut Chart, CIE xyY or Chromaticity Chart

The CIE gamut chart displays all the chromaticities which an average person can see. Chromaticity is the aspect of color which is determined by its dominant wave length and purity. In plain language, it is the color we see. When all the chromaticities are displayed, it is called the gamut of human vision. When plotted on a two dimensional piece of paper, the gamut chart forms an ellipsoid.

The spectral locus is the name of the top curved edge. Here we find the monochromatic colors, that is, those colors which are made up of only one color, not created by mixing colors. The spectral locus shows the monochromatic wavelength of light of those colors in nanometers. A nanometer is one billionth of a meter.

In the CIE gamut chart, colors become less saturated as they move toward the center of the diagram. White, or the place of equal frequency energy distribution, is located at a point where the value of 1/3 x and 1/3 y intersect.

The straight line on the bottom is called line of purples. Even though it is on the border, these colors have no monochromatic definition. They can only be formed by mixing different proportions of the two colors in the corners, at either end of the line of purples. This line connects the endpoints of the visible spectrum.

The usefulness of the CIE gamut chart is akin to the usefulness of a map. You can find and calculate points on this color map using only two values, even though we need three values to define a color. This is similar to the way latitude and longitude define points on the three dimensional earth using just two values. Furthermore, this chart is useful because it shows which color can be made when you mix colors. We'll get to this in a moment.

Prior to this standard being developed, people had a hard time accurately communicating a color for these important reasons.

• People don't see colors exactly the same way.
  
  
• The lighting, or the color temperature of the light source, can vary the color of an object substantially.
  
  
• Metamarism. Objects that clearly have different spectral power distributions when measured with a spectometer, will appear to be exactly the same color to people.

CIE 1931 Gamut Chart Drawbacks

Although this CIE XYZ gamut chart is still useful today, it does have some drawbacks, two in particular. First, the field of view is small, only about 2 degrees. In later versions of the CIE gamut chart (the CIE 1960, CIE 1964 and CIE 1976), the field of view has been expanded to 10 degrees. There are formulas which allow you to convert between the different color systems.

Secondly, it is non-linear. That is to say, the distance between two points on the CIE 1931 gamut chart, A and B, is different from the degree of difference between the two colors found at points A and B. Since this chart was developed, other charts (namely, CIELuv and CIELab) have corrected this problem. In those charts, it is possible to find the correct proportion of the source colors needed to create a desired color by determining the distance from the desired color to the source colors.

CIE Lab

As you can see in this gamut chart, A to B is similar to C to D, but perceptually, A and B are almost the same color whereas C and D are very different.

CIE Lab color space uses lightness (L), red-greenness (a) and yellow-blueness (b) as its tristimulus values. It made adjustments to the CIE XYZ color space to make it perceptually linear so that as you move across the chart, the color changes at a constant rate. 

To illustrate this point, we'll use the CIE gamut chart which is decidedly non-linear. As you can see from this gamut chart, the color changes very little between points A and B, but there is a significant color change between the points C and D.

The CIE Lab chart corrected that problem, reorganizing the colors into a perceptually uniform color space and making it possible to find the correct proportion of the source colors needed to create a desired color by determining the distance from the desired color to the source colors. In other words, the visual "difference" between two points on the chart will appear to be visually similar, regardless of where you started. Contrast that to the CIE gamut chart, where one unit of adjustment near the tip of the tongue (the greens) makes almost no difference, whereas a similar adjustment in the blues results in a significant color change.

A simple rendition of the CIE Lab color space.

Great for Digital Image Manipulation

This property of CIE Lab (CIELAB) makes it useful in digital image manipulation. If you want to color shift an image, you want all of the colors in the image to shift to the same degree. It is for this reason that many color image processing software programs will convert colors into the CIE Lab color space to perform image manipulations.

Getting White Right

A limitation of CIE Lab is that the values are relative to the white point which can change. This is a good example of why, then, we need different color spaces, since each color space has it's special properties that make it useful.