In 1664, Newton used a shuttle lens to disperse the white sunlight into a spectrum of different tones, laying the physical foundation of light color. In 1860, Maxwell used different intensities of red, yellow, and green light to match white light to various colors. The light of color laid the foundation of trichromatic colorimetry. On this basis, the International Commission on Illumination established the CIE colorimetric system in 1931 and continued to improve it. Nowadays, the CIE chromaticity system has been widely used to quantitatively express the color of light.
Color is inseparable from lighting. Objects can show color only under light, and the color of light has a great influence on people’s psychology. Under the illumination of different light sources, the same object will show different colors. For example, green leaves are bright green under the illumination of green light, and they are almost black under the illumination of red light. It can be seen that the light source plays an important role in the appearance of the color of the illuminated object. The ability of the light source to fully display the color of the illuminated object when illuminating the object is called the color rendering of the light source.
The color rendering of the light source to the color of the object, that is, the fidelity of the color of the object under the illumination of the light source. The color rendering index of the light source is indicated by the color rendering index, which indicates that the color of the object under the light is better than the color of the reference light (sunshine). The deviation can fully reflect the color characteristics of the light source. The light source with high color rendering has better color performance, and the colors that people see are closer to natural colors. The light source with low color rendering has poor color performance and the color deviation seen is also greater.
There are high and low color rendering properties. The key lies in the characteristics of light. The wavelength of visible light is within the range of 380~780nm, which is the range of red, orange, yellow, green, blue, blue, and violet light seen in the spectrum. In the light emitted by the light source, the ratio of the various colors contained in the light is similar to that of natural light, and the colors seen by the eyes are more realistic.
The color rendering ability of the light source to the object is compared with the appearance color of the object under the same color temperature or reference light source (incandescent lamp). The spectral range of the light emitted determines the light color of the light source, but there are many, a few of the same light colors. Even if only two monochromatic light waves are synthesized, the color rendering of each color is quite different. Light sources with the same light color will have different spectral compositions, and light sources with a wider spectral composition can provide better color rendering quality. When there is little or lack of the main wave reflected by the object under the reference light source in the light source spectrum, the color will produce obvious color shift. The greater the colorshift, the worse the color rendering of the light source. There are two types of color rendering:
(1) Faithful color development. Faithful color rendering can correctly express the original color of the substance. To achieve true color rendering, a light source with a high color rendering index (Ra) must be used, and its value is close to 100, which has the best color rendering.
(2) The effect is colored. To sharply emphasize a specific color, you can use additive colors to enhance the color rendering effect. Using a low color temperature light source to illuminate can make the red more vivid; using a medium color temperature light source to illuminate, so that the blue has a cool feeling; using a high color temperature light source to illuminate, make the object feel cold.
Both sunlight and incandescent lamps radiate a continuous spectrum, which includes various colors such as red, orange, yellow, green, cyan, blue, and purple within the wavelength range of visible light (380~760mm). The object shows its true color under the sunlight and incandescent lamp, but when the object is illuminated by the discontinuous spectrum gas discharge lamp, the color will be distorted to different degrees. In order to quantitatively evaluate the color rendering of the light source, the concept of color rendering index is introduced. Based on the standard light source, the color rendering index is set to 100, and the color rendering index of the other light sources is less than 100. The color rendering index is represented by Ra. The greater the value of Ra, the better the color rendering of the light source. In the International Lighting Association, the color rendering index is generally divided into five categories, as shown in the table 1.
The color rendering index of the light source indicates the color reproduction performance of the light source, that is, the so-called color discrimination performance under the lamp. In order to display the color of the illuminated object naturally and truly, the color rendering index of the light source should be greater than 80%. The color rendering index of the incandescent lamp is defined as 100, which is regarded as an ideal reference light source.