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).

UGR and clare

The UGR method (Unified Glare Rating) is an international index presented by CIE, the international lighting research body, in its publication 117 and is used to evaluate and limit the psychological direct glare from luminaires. Contrary to previous methods where the glare was rated using the luminance values of a single luminaire, this method calculates the glare of the entire lighting installation at a defined observer position.

Colour gamut

Colour gamut is a measure of how colourful a scene will appear under a given light source. Sources with a high colour gamut generally make objects appear brighter and more colourful.