Light and colour determine our sense of wellbeing and shape the atmosphere of a room by their warmth or coolness. Correct colour rendering is an important task for good lighting, including artificial lighting.

The colour rendering rating of a lamp indicates the effect its light has on coloured objects. Light sources have different colour rendering properties – and do not always render the colours of viewed objects correctly. Under the light of certain lamps, for instance, a face may look pale or vegetables unappetising.

Optimal: R_a 100 index Colour

rendering is rated by the Ra index. Based on frequently found test colours, it indicates how naturally those colours are rendered. As a general rule, the lower the index, the less well the surface colours of illuminated objects are rendered. The best colour rendering index is R_a = 100; for indoor use, the Ra index of lamps should be no lower than 80.

"Stored" visual standards

In everyday life, we come across a range of surface colours which can differ in appearance depending on how they are illuminated but for which we have "stored visual standards" that override the effect of lighting. For example, we have a stored impression of the colour of human skin in daylight. If artificial light lacks a spectral colour or if some of the colours are exaggerated (e.g. under incandescent light), skin will appear different but, because of our experience, will still be considered "natural". In the case of coloured materials for which no stored standards exist, however, completely different colours may be perceived.