The CRI (Colour Rendering Index) is the measure of a light source’s ability to reflect the trueness of colours compared to incandescent or daylight.
It is calculated from the differences in chromaticity of eight colours which are a reference point of the standardised level of the perception of the colour by the human eye known as the CIE (Colourimetric Parameters)
The CRI scale is between 0-100. The lower the CRI, the more unnatural the colours appear when illuminated in comparison to their natural lighted reference. The higher the number, the closer the representation to daylight. Daylight has a colour rendering index of 100, whilst electric lamps cover the entire range from 0 (low-pressure sodium lamps) to almost 100 (tungsten halogen lamps). The higher the CRI, the better the light source renders a wide range of colours in the visible spectrum.
Excellent colour rendering properties are achieved by electric lamps with a CRI over 90. But in general, a CRI above 80 is considered sufficient for accurate colour judgement in most internal spaces, such as offices and shops. Electric lamps with a lower CRI may be less satisfactory for interiors but can be very suitable for external lighting applications.
Colours are compared to a reference illuminant of their kelvin measure known as the correlated colour temperature (CCT); the smaller the average difference in chromaticity, the higher the CRI.