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LEDs Test Method: LED Color Measurement 1

1 CIE1931-XYZ Chromaticity System

Color is caused by the feeling of radiation sources or by the reflections in the role of the results of the human eye ray of light. Therefore, color not only depends on the light stimulation, but also on the human visual characteristics. The measurement and standard relating to colors should comply with the observed result of human eyes. However, the color characteristics of human eyes have more or less differences for different observers. Therefore, we need to determine a group of primary color materials to match the color spectrum to represent the average color visual characteristics of human eyes for the use of colorimetric measuring and calculation.

In 1931, CIE (international lighting committee) adopted the assumptive three-primary colors (X), (Y), (Z) on the base of RGB systems [each represents the red, green and blue], and established the CIE-1931 chromaticity diagram, at the same time, CIE standardized the three- primary colors data of all sorts of colors that can match the spectrum, and determined the CIE1931-XYZ standard colorimetric system.

CIE1931-XYZ colorimetric system is the colorimetric foundation. Any color can be represented by the chromaticity coordinates x,y and stimulus value y in the CIE1931-XYZ chromatic diagram.

CIE1931-XYZ chromatic diagram

In this system, chromatic coordinate

chromatic coordinate

In this system, X, Y, Z is the three stimulus value of the color. Their values represent the proportions of the three-primary colors. The white light of the equal spectrum is composed of the same numbers of X, Y, Z

2 Calculation of the Chromaticity Coordinates

If the spectrum power distribution is known, calculate the chromaticity and the parameters of LED device according to the following formula:

color measurement 1

Due to the factual difficulty to figure out P (λ) in the mathematical expression, we often get the approximate points by summation. Here, Y means the light intensity of the light source. K is the regulation factor, which can be got by adjusting the value Y of the illuminant or light source to 100, namely:

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