Since 1931, when the first system of measuring colour rendering was formalised by the CIE (Commission Internationale de l’Eclairage = International Commission on Illumination), the lighting industry has been able to communicate the quality of its light to specifiers and end users alike.
The Colour Rendering Index (CRI or Ra) is a quantitative measure, which rates a light source’s ability to reproduce the colours of objects. In order to objectively compare the colour rendering properties of any light source, the CIE’s standardised measuring method operates on a scale from 0 to 100 (poor to excellent). The colour change of 14 standard colours is calculated when an object is exposed to a specific light source and then this is compared to a reference illuminant of the same colour temperature (a black body* is used for colour temperatures up to 5000K and daylight above that). The CRI for a pair of light sources can only be compared if they have the same colour temperature.
The CRI scale is chosen so that an ideal black body source, such as incandescent or halogen lamps, is by definition a CRI rating of 100. For light sources emitting a discrete spectrum, like LED and discharge lamps, the CRI can be anywhere between 0 - 100. As a rule of thumb, the more the spectrum is filled at all wavelengths (380 – 760nm), the better the colour rendering properties of the source, however a high CRI measurement intrinsically means lower efficacy (as less efficient wavelengths are also represented in the spectrum).
* A black body is a theoretical object that absorbs all incident electromagnetic radiation and due to its ability to absorb at all wavelengths, is the best possible emitter of thermal radiation. It radiates a continuous spectrum that depends on the body's temperature.