According to BRE’s The essential guide to retail lighting:
Colour appearance refers to the ‘…visual sensation correlated with the ‘warmth’ or ‘coolness’ of the light emitted by a lamp. The metric used to characterise the colour appearance of the light emitted by a light source is the correlated colour temperature (CCT), expressed in Kelvin (K). Warm white light is produced by lamps having a colour temperature below 3,000 K (reddish hues), whereas 4,000 K and above (bluish) is cool and cold white light. The colour appearance is quite separate from the colour rendering of the lamp.’
Colour rendering is ‘The ability of a light source to show surface colours as they should be, usually in comparison with a tungsten or daylight source. Measured on the colour rendering index (CRI) scale. A value of 0 means it is impossible to discern colours at all, and a score of 100 means no colour distortion. For most indoor lighting applications a value of at least 80 is recommended.’
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