Color Rendering Index 

The general color rendering index Ra (reference index general) was introduced to objectively identify the color rendering properties of a light source (English designation CRI). The color rendering index designates the measure of the correspondence of the seen body color with its appearance under the respective reference light source.

To determine the Ra value, the color shifts of 8 standardized test colors are determined, which result when the test colors are illuminated with the light source to be tested or with the reference source. The smaller the deviation, the better the color rendering property of the tested light source. A light source with Ra = 100 optimally reproduces all colors as under the reference light source.

The lower the Ra value, the less well the colors are reproduced. For example, the reference light source for daylight white light sources is natural daylight with a color temperature of 6500 K, which corresponds to an overcast sky without sunlight. For light sources with a color temperature < 5 000 K, the reference illuminant is the Planckian illuminant.

Lamps with a color rendering index lower than 80 should not be used indoors where people work or stay for long periods of time. Exceptions are permitted for special lighting tasks that require lamps with high luminous efficacy and thus low color rendering for economic reasons, e.g. high-pressure sodium lamps.

This applies, for example, to the lighting of high halls. However, in these cases, suitable measures must be taken to ensure higher color rendering at fixed and permanently manned workplaces and where safety colors must be recognized without error (see also ISO 3864 "Safety colors and safety signs - Part 1: Safety signs at workplaces and in public areas - Design principles").

In order to identify the light color and color rendering of lamps in a generally understandable way in addition to the typical manufacturer designations, a manufacturer-neutral color designation consisting of three digits has been introduced internationally. For example, the designation 840 means a fluorescent lamp with a color rendering index of 80 to 89 and a color temperature of 4000 K, which corresponds to the light color neutral white.