Lighting color rendering index

Typical values of correlated color temperature (CCT) and color rendering index (CRl) for some common electric light sources. 

Figure 2. The color-rendering index (CRI), of similarity to daylight-rendering, dependence upon choice of triad of spectral lights to form white light of daylight-color. Wavelengths of two components are fixed at their peaks, and the wavelength of the third component is varied. Optimum combination appears in Figure 3.

The ability of a light source to correctly represent colors of objects is its color rendering index (CRI), and with the CCT it suffices to help colorists select an equivalent source. Established by the CIE this standard rates lamps as to their color matching ability. A CRI value of 100 indicates that a source truly represents all colors of the CIE standard. Sources with CRIs of 70 to 80 are considered good and those with values of 80+ are rated as excellent. 

Fluorescent lamps are rated in catalogs by two indices, their correlated color temperature (CCT) and color-rendering index (CRI). The CCT is the temperature of a black body whose chromaticity most nearly matches that of the subject light source. Because a fluorescent lamp only approximates a "black body," it is called correlated to distinguish it from actual. The CRI on the other hand is a subjective method developed by the CIE in which eight test colors are viewed under the test and a reference lamp(s) and its ability to reproduce the test colors numerically rated. Both numbers are not absolute but useful in selecting lamps for color rendering applications. 

Apart from the color point, there is another important lamp characteristic, viz. the color rendition. This property depends on the spectral enei distribution of the emitted light. It is characterized by comparing the color points of a set of test colors under illumination with the lamp to be tested and with a black body radiator. The color rendering index (CRl) equals 100 if the color points are the same under illumination with both sources. Under illumination with a lamp with low CRI, an object does not appear natural to the human eye. 

Color Rendering or Rendition - A measure of the ability of a light source to show colors, based on a color rendering index. 

How effectively shonld yonr light source render colors The higher the color rendering index (CRI) of a lamp, the better individual colors will be rendered. The CRI compares how closely a light sonrce will render colors to a standard.The standard is usually an incandescent lamp, which is, by definition, a black body and has a CRI of 100, the highest rating (Table 5.40). Metal halide lamps nsnally have a CRI between 65 and 85. By comparison, high-pressure sodium lamps have a CRI of 22. 

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