Colour rendering

Colour rendering is also important. This is the ability of a light source to show surface colours as they should be, usually in comparison to a tungsten or daylight source. Lamps with poor colour rendering will distort some colours.

Colour rendering is usually assessed by the CIE colour rendering index (CRI or Ra); a number between 0 and 100, where lower values indicate poor colour rendering and higher ones good colour rendering. CRI only works for approximately white sources.
Guidance in the CIBSE Code recommends lamps with CRI between 90 and 100 for inspection and colour matching. Where accurate colour judgement is required, such as in offices and shops, lamps with CRI over 80 are recommended.
Researchers at NIST in the USA have suggested that the colour rendering index is less suitable for LED spectra, especially in the way saturated colours like red and yellow are dealt with. They have proposed an alternative colour quality scale (CQS).


The binning of LEDs is a practice used by LED manufacturers to manage the variation of LED performance that is inherent in the mass production. LEDs of similar appearance to the human eye are put in different boxes, or bins. LEDs from the ‘best’ bins are more expensive. Additionally LED manufacturers often operate slightly different binning policies from each other.


Ceramic metal halide (CMH) lamps combine high output with great colour rendering and good efficiency. They produce light by passing an electric arc through a mixture of gases. In a metal halide lamp, the compact arc tube contains a high-pressure mixture of argon, mercury, and a variety of metal halides.