CIE 1931

Mapping Human Vision

In 1931, the Commission Internationale de l’éclairage (International Commission on Illumination), or CIE, undertook a series of experiments to create a mathematical model of human color perception. The goal was to map every color the average person can see and assign it a specific set of coordinates. The result was the CIE 1931 XYZ color space, a groundbreaking achievement that remains the bedrock of color science to this day.

This model is a ‘device-independent’ standard. It doesn’t describe the colors a monitor can produce or a camera can capture; it describes the colors a human can see. This makes it the ultimate, objective reference point for all other color systems.

The xy Chromaticity Diagram

While the full XYZ color space is three-dimensional (with Y representing luminance, or brightness), it is most often represented by its two-dimensional projection: the CIE 1931 xy chromaticity diagram. This iconic, horseshoe-shaped chart is a map of all possible hues and saturations.

• The curved outer boundary, known as the spectral locus, represents the most saturated, pure monochromatic colors of the visible spectrum, from deep violet around 400nm to red around 700nm.
• The straight line connecting the two ends is the line of purples, representing non-spectral colors that can only be created by mixing red and blue light.
• The area inside the horseshoe encompasses every color perceptible to the human eye.
• A white point, representing pure white, is located in the center.

Its Foundational Role in Film and Television

The CIE 1931 diagram is the universal map upon which all modern color gamuts are defined. Its importance in the film industry cannot be overstated:

• Defining Standards: Color space standards like Rec. 709, DCI-P3, and Rec. 2020 are defined by the precise xy coordinates of their red, green, and blue primaries on the CIE 1931 diagram. This allows engineers worldwide to build cameras and displays that adhere to the exact same standard.
• Measurement and Calibration: When a professional monitor is calibrated, a probe measures the color patches displayed on the screen and plots their xy coordinates on the diagram to see how accurately they match the target standard.
• Gamut Visualization: The diagram provides an immediate, intuitive way to compare the size of different gamuts. By plotting the triangles for Rec. 709 and DCI-P3 on the same chart, one can instantly see how many more colors the theatrical P3 standard can reproduce.