Colorimetry Knowledge

Colorimetry is a part of photometric research. It is a science that studies the rules of human perception of color and color measurement technology.

The human eye can see colors because visible light enters the human eye, causing visual physiological and psychological feelings. Therefore, the basis for human eyes to see colors is light; this light can be the light emitted by the object itself, such as the light of the sun, the light emitted by a lamp, or the light emitted by the display screen; this light can also be the light reflected by the object, such as an apple Red and blue clothes, these objects do not emit light themselves, but will reflect light. The color of the former self-luminous object is only related to the spectrum of its own radiation, while the color of the latter non-luminous object is not only related to its own surface reflectivity, but also related to the lighting source. For example, white clothes also look red under red light.

Whether it is a self-illuminating object or a non-luminous object, the color seen by the human eye is related to its radiation or reflection spectrum. The spectrum that the human eye can see is limited to 380-780nm, which is called the visible spectrum. The visible spectrum is a small segment of the radiation spectrum, as shown in the image to the left. Photometry studies the visible light band.

Let's take a look at the spectra of common lighting sources. In the picture on the right, you can see that the energy in the yellow and red parts of the incandescent light spectrum is obviously stronger than that of blue and green, so it appears yellowish; the LED cool white spectrum has a bluish color due to its large blue component, so later There is a warm white LED light source that enhances the red part.The colored light we see every day is a mixture of monochromatic light at various visible wavelengths.


 CIE standard colorimetric system and color matching experiments

Colorimetry is a science that studies the laws of human color vision, color measurement theory and technology. It is a comprehensive science developed in the last century and based on physical optics, visual physiology, visual psychology, psychophysics and other disciplines. .
Based on the research and experiments of some color scientists, the International Commission on Illumination (CIE) stipulated a standard colorimetric system, called the CIE standard colorimetric system. This system is a basic component of modern colorimetry and the basis for colorimetric calculations.

The CIE standard colorimetry system is established based on color matching experiments. It is a color mixing system that uses the number of three primary colors that make up each color to quantitatively express colors, instead of using the three color appearance characteristics of brightness, hue, and saturation. Degree (chroma) is a measure of magnitude to represent a color.The picture on the left is a schematic diagram of the CIE 1931 color matching experiment. First, any color to be matched is given, and through human eye observation, the proportion of light of the three primary colors of red, green, and blue is found, and the number of the three primary colors of the color is calculated.

The CIE1931-XYZ system established by CIE in 1931 has become the most widely used chromaticity space still in use today. As shown on the right, in this chromaticity system theory, any color can be represented by XYZ tristimulus values, and XYZ can be converted into a Yxy coordinate system, where Y is both brightness and the human eye’s sensitivity to green. x and y represent the proportions of X and Y respectively in (X+Y+Z), so that chromaticity can be represented by a two-dimensional space such as xy. The center point (0.3333, 0.3333) of the chromaticity diagram is white, surrounded by colors. The color saturation is approximately higher toward the edge; the edge points represent spectral colors of different wavelengths.