The Color RenderingIndex (CRI), is the color rendering index (Ra) used to characterize light sources. It is an index of the naturalness of color. The larger the color rendering index, referred to as the CRI or Ra value, the more natural colors are rendered and the more pleasant they are perceived. The magnitude of the Ra value can range from 0 to 100 and is decisive for the color rendering of illuminated objects.
The color rendering index is based on a color reference scale based on eight pastel colors. It forms the mean value from the measured color scale values. It is a comparative value with which the lighting spectrum of incandescent lamps, fluorescent lamps, energy-saving lamps and light-emitting diodes can be compared.
Sunlight has a color rendering index of 100. There is a classification for the color rendering index; thus, an Ra value of 95 is classified as excellent, one of 90 as fair, and one of 80 as poor. The color rendering index is different from the color temperature, which distinguishes between cool and warm light.
In addition to sunlight, which has an Ra index of 100, energy-saving lamps, for example, have an Ra value of about 80, metal halide lamps have a CRI value of 60 to 70, and white light-emitting diodes, WLEDs, have a CRI value of 80. An increase in the CRI value of WLEDs is achieved with "warm white" LEDs, which use two phosphors. These LEDs, which work with High Color Rendering( HCR), achieve CRI values of 94. The more narrow-band and thus monochromatic the light radiation is, the lower the CRI value.
The color rendering index is specified according to DIN 6169 in six quality levels defined by the Commission Internationale d`Eclairage( CIE) with the classifications already mentioned. In most cases, the color temperature is also specified with the color rendering index.
In addition to the color rendering index, the color quality scale( CQS) and the feeling of contrast index( FCI) are two other characteristic values for the subjective perception of color in the illumination of objects. Another concept that focuses on the well-being of people and their adaptation to the normal course of the day and year is Human Centric Lighting( HCL).