CRI is short for Color Rendering Index. The indication R_a index or index Ra means general reference index. The “a” stands for “general”. It is a so-called photometric quantity, which makes the color rendering of different light sources comparable. A unit does not have the measure.
The CRI value characterizes each light source, how well it can reproduce the colors compared to the sun. The higher the CRI or Ra value, the more natural and pleasant colors are rendered or perceived.
Typical values are:
Light bulb up to 100
Halogen lamp up to 98
White LEDs 70-95
Fluorescent lamp 50-90
High pressure mercury lamp 45-50
Metal halide lamp 60-95
Sodium vapor high pressure lamp 18-30
According to DIN 6169, 14 test colors are specified. For this one measures the secondary spectra with the reference and the test light source. The deviations result in 14 reference values (Ri). To determine the general color rendering index, the arithmetic mean of the first eight indices is determined. So you add up the eight numerical values and divide the result by eight.
The number of reference colors used in the color rendering index has been increased to 14 and later to 15. The more colors are included in the calculation of the index, the more precisely it describes the quality of the light. Since an average of the different Ri values is determined, even a high quality light source may be at a loss when playing individual colors.
Our lights mainly use LEDs with> = CRI 90.