The colour rendering property of a lamp indicates the effect its light has on the appearance of coloured objects. This is rated by reference to the R_a index, which indicates how natural colours appear under a lamp's light.

The colour rendering index is based on eight frequently found test colours. R_a = 100 is the best rating; the lower the index value the poorer the colour rendering properties. Lamps with a R_a index of less than 80 should not be used in interiors where people work or are present for lengthy periods.