Colour Rendering Index
The Colour Rendering Index (Ra or CRI) denotes a photometric quantity which describes the quality of the colour rendition of light sources of the same colour temperature.
As a reference for the evaluation of the reproduction quality is used up to a colour temperature of 5000K, the light is emitted by a black body temperature of the corresponding color. Above 5000K it is referenced against a daylight-like spectral distribution. For example, the colour reproduction of a household light bulb (which is in good approximation, a black radiator) the spectrum of a black body with a temperature of 2700K used as a reference for the calculation. For a fluorescent lamp with the light colour 865 (865 for a colour rendering index greater than 80, 865 for a colour temperature of 6500K) however, the spectrum of daylight illuminant CIE 1931 color space D65 is used.
The colour rendering index is not dependent on a specific colour temperature. Any light source which simulates the spectrum of a black body of the same colour temperature in the range of visible wavelengths perfectly achieves a colour rendering index of 100. Theoretically it is possible to synthesize a light source (eg five light-emitting diodes of different colors) with a totally different spectral distribution than that of a black body radiator which achieves a colour rendering index of 100th. A light bulb (with clear glass) has a Ra of nearly 100 with excellent color rendering properties. Fluorescent lamps reach values between 70 and 90. Light sources that emit light in only one single wavelength, such as low pressure sodium vapor lamps, allow no differentiation of colours and accordingly have a very low or even negative Ra value.
As white LEDs in the lighting become more and more important, there are efforts by the LED manufacturer to bring the CRI as close as possible to 100 to achieve a natural colour reproduction. By combining multiple chips values between 85 and 95 can be reached at colour temperatures of 2700-3000K. At intermediate color temperatures (4000-5000K) values of 75 to 85 are nevertheless still typical. The light output is somewhat better than for the lower colour temperatures.