CRI - Colour Rendering Index
In order to evaluate the quality of light sources objectively, the internationally recognised colour rendering index Ra (English CRI) was defined. The method of evaluation is described in the international standard CIE 13.3 of 1995. This colour rendering evaluation is valid worldwide. The test light source is compared with a reference light source using 15 selected test colours. The evaluation bandwidth is between a maximum of plus 100 and minus 60. The more accurately the test light source reproduces the test colour, the higher or better the colour rendering value. The mean value of the first 8 test colours gives the colour rendering index Ra. The average Ra value is divided into: moderately good colour rendering for Ra 70-80, e.g. for street lighting; good colour rendering for Ra 80-90, e.g. for lighting workplaces, and very good colour rendering for Ra 90-100, e.g. for museums and high-quality retail applications. In order to further specify the quality of the colour rendering, a typical Ra value of Ra (type) 85, for example, within a bandwidth of Ra ≥ 80 to 90, can also be specified. Due to the requirements of the EN 12464-1 lighting standard, only the Ra value is usually taken into account. The values for the saturated colours R9 - R12 are often meagre. Therefore, for retail applications, the value for saturated red, the R9 value, is also specified. The results for the test colours R9 – R15 give detailed information about the colour rendering of saturated colours (R9 – R12), skin colour (R13), leaf green (R14) and Asian skin colour (R15).
The averaged value of all 15 test colour results is expressed by the Re value. The Re value is used to assess high quality light at Ra > 90. Especially in high-end retail and for medical examinations, the Re value is more meaningful than the Ra value.