CIE 1931 Color Space

I would like to introduce a color blindness test based on the confusion lines of the CIE 1931 color space (also known as CIE XYZ color space). Because the topic is not the easiest one and needs some explanation I would like to split it into three parts. In this first of three parts I will introduce the CIE 1931 color space. In part two and three which will follow next week I will talk about the confusion lines of the CIE 1931 color space and last but not least the color blindness test based on the CIE 1931 color space confusion lines.

CIE 1931 Color Space

CIE 1931 Color Space

The acronym CIE stands for International Commission on Illumination which is the international authority on light, illumination, color, and color spaces. And 1931 is the year of birth of this specific color space.

Where it all began

Let us go back into history and have a look at where it all began. Based on the research on wavelengths and colors of Thomas Young at the turn of the 19th century, Hermann von Helmholtz developed around fifty years later a color theory. He stated, that the human eye has three different types of color receptors (cones) and that every color we perceive is a mixture of signals of those three types of cones, which roughly reflect the three different colors red, green and blue.

In the roaring twenties of the last century two scientists (W. David Wright, John Guild) took up this theory and independently made some experiments. To find out more about the three-color-theory a setup with four colored lights on two different sides was used.

  • On the left side a test color was projected by a light source.
  • On the right side the observer had three adjustable light sources (red, green and blue).

Now each observer (also called CIE standard observer) had the task to adjust the three lights accordingly, that the color on the right hand side was exactly the same as the test color on the other side. After many tests with a lot of test persons the results were mathematically analyzed. This produced three different curves of intensity for each light source to mix all colors of the color spectrum.

Funnily enough not every color could be matched and sometimes some red had to be added to the test color to get a correct match. This was also taken into account of the mathematical equations and resulted in a red curve including negative values. These three curves were standardized and are called the CIE RGB color matching functions r, g and b.

CIE RGB to CIE 1931

Because mathematicians don’t like negative numbers if they can change it, the commission changed it. Based on the CIE RGB functions and their corresponding values, new functions were calculated. Those new functions called x, y and z had to fulfill a list of conditions. Some of them were:

  • The new functions must be everywhere greater or equal to zero.
  • The y function describes only the luminosity.
  • The white point is, where x = y = z = 1/3.

All this put together produced the well known CIE XYZ color space which is also known as CIE 1931 color space. This color space aims to describe all visible colors to the human eye and can be shown as a three dimensional cube.

Projection

Because three dimensional objects can’t be illustrated very well a two dimensional representation had to be found. The Y parameter of the so-called tristimulus values X, Y and Z is a measure of the brightness. This helped to easily calculate the new chromaticity values x and y by the following rules:

  • x = X / (X + Y + Z)
  • y = Y / (X + Y + Z)

The corresponding chromaticity diagram is shown in the above picture. The outer curved line is called spectral locus and corresponds to the well known color spectrum, shown with corresponding wavelengths. The straight line on the lower part between blue and red is called purple line. This line relates to all colors which can only be mixed up by blue and red which are not part of the color spectrum.

In the next part I will talk about the confusion lines of the CIE 1931 color spectrum. And this leads us to the final part about the color blindness test based on those confusion lines.