Photometry Light Measurement

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CIE Overview (continued)

Human eye response research opens the door to photometry and colorimetry

The CIE or Commission internationale de l'clairage (International Commission on Illumination) is the international authority on matters involving light, illumination, and color. In an attempt to solve the problem of human color perception, the CIE in 1931 developed one of the very first mathematically defined color spaces, the CIE XYZ color space.

The human eye contains receptors for short, middle, and long wavelengths, also referred to as blue, green, and red respectively, and therefore a color sensation would have to be defined as three parameters. A specific method for assigning three numbers, also known as tristimulus values, to represent each color, is called a color space.

Based on direct human eye measurement experiments performed in the 1920's, an RGB color matching model of human vision was developed and from this the CIE XYZ color space was derived.

The central color matching function in calculating the CIE XYZ color space was the photopic luminosity function which represents the typical human eye response under normal lighting conditions and is referred to as the standard observer.

Scotopic luminosity represents the typical human eye response in dark conditions where no color perception exists and mesopic luminosity is a combination of the photopic and scotopic functions where minimal color perception exists and represents the human eye response in dimly lit conditions.

Having developed a basic color space based on a standard observer, we could then quantify the perceptual brightness of a light by direct measurement with devices that have the same response to light as that of the standard observer in a science now called photometry. Photometric light measurement is widely used in many industries to specify, monitor, and control the brightness of task, street, safety, automotive, and aerospace lighting to name a few.

It is important to note that the CIE XYZ color space can only describe colors of light itself and not objects since the perceived color of an object will depend on the surface and color properties of the object itself, that of nearby objects, and the ambient and direct lighting properties, but it did serve as the foundation for colorimetry, the science of color measurement or reproduction.

Many additional color models, measurement techniques and instruments have developed subsequent to this groundbreaking research to objectively reproduce color in a wide variety of analog and digital media.