CIE 1931 color space

CIE 1931 color space
Name	CIE 1931 color space
Caption	The CIE 1931 xy chromaticity diagram, showing the spectral locus and the Planckian locus.
Creator	International Commission on Illumination
Created	1931
Based on	Color matching experiments by John Guild and William David Wright
Transform to	CIE XYZ color space
Related	CIE 1960 color space, CIE 1976 color space, CIELAB color space

CIE 1931 color space. The CIE 1931 color space, defined by the International Commission on Illumination (CIE), is the first mathematically defined and internationally standardized color space. It provides a quantitative link between distributions of wavelengths in the visible spectrum and physiologically perceived colors in human color vision. This foundational system forms the basis for most modern colorimetry and is used to specify colors in fields ranging from lighting and display manufacturing to industrial design and scientific research.

Overview

The system is built upon the CIE 1931 standard colorimetric observer, a set of three color-matching functions derived from experiments conducted by John Guild and William David Wright. These functions mathematically define the tristimulus values X, Y, and Z, which represent the amounts of three imaginary primary lights required to match a color. The Y tristimulus value is specifically designed to correspond to the luminance of a color, a critical photometric quantity. This framework allows any color stimulus to be precisely located within a three-dimensional space, enabling objective color specification independent of device or medium.

Development and historical context

The development was driven by the need for an international standard to unify disparate color measurement practices used in industries like textiles and printing. It was formally adopted at the CIE session in Cambridge, England, in 1931. The work built upon foundational theories of color vision, including the Young–Helmholtz theory of trichromacy and the empirical data from the color matching experiments. Key figures in its establishment included delegates from national bodies like the National Physical Laboratory (United Kingdom) and the Optical Society of America. The system superseded earlier, less precise models such as those proposed by the Munsell color system.

Technical description

The core of the system is the CIE XYZ color space, a non-physical, mathematical space where the tristimulus values are always positive. The color-matching functions, often denoted as x̄(λ), ȳ(λ), and z̄(λ), are tabulated across the visible spectrum and are fundamental to calculating tristimulus values for any given spectral power distribution. The transformation from these values to more perceptually uniform spaces, such as the later CIELUV color space, is a common procedure. The mathematics of the system are formally defined in CIE publications like CIE 15:2004.

Chromaticity diagram

A pivotal derivative is the two-dimensional CIE 1931 chromaticity diagram, created by projecting the XYZ space onto the normalized plane defined by chromaticity coordinates x and y. This diagram plots the spectral locus (the colors of monochromatic light) and the purple line connecting the spectrum ends. The Planckian locus, representing the color of a black body radiator at different temperatures, is also plotted, which is crucial for correlated color temperature specification. The area within this horseshoe-shaped diagram represents all chromaticities perceivable by the standard observer, though it is not perceptually uniform.

Use and applications

The system is ubiquitously used for color specification in international standards, including those from the International Organization for Standardization (ISO) and the International Telecommunication Union (ITU). It underpins color management in RGB color space definitions for devices like cathode-ray tubes, liquid-crystal displays, and digital cinema projectors. Industries such as automotive paint manufacturing, printing, and light-emitting diode (LED) production rely on it for quality control. It is also the reference for defining color gamuts, like those in the sRGB and Adobe RGB standards.

Limitations and subsequent models

A major limitation is its significant perceptual non-uniformity, where equal geometric distances on the chromaticity diagram do not correspond to equal perceived color differences. This led the CIE to recommend more uniform spaces, such as the CIE 1964 color space for a 10-degree observer, the CIE 1976 (u', v') diagram, and the three-dimensional CIELAB color space. Despite these advancements, the CIE 1931 system remains the fundamental reference, and its standard observer is still used for colorimetric calculations where the field of view is approximately 2 degrees, as defined in the original experiments.

Category:Color space