Architecture: April 2013

The colour rendering of a light source is an indicator for its ability of realistically reproduce the colour of an object.
Following the CIE (International Lighting Commission), colour rendering is given as an index between 0 and 100, where lower values indicate poor colour rendering and higher ones good colour rendering. The colour rendering of a light source is compared to daylight if its CCT is >5000K and to a black body (i.e. a source that produces a continuous spectrum) otherwise.

Comparing the colour appearance under different light sources (left);
Test swatches under different light (right)

To make a comparison of the colour rendering qualities of light sources easier, colour rendering groups have been defined:

The CIE colour rendering groups
Group	R_a	Importance	Typical application
1A	90...100	accurate colour matching	Galleries, medical examinations, colour mixing
1B	80...90	accurate colour judgement	Home, hotels, offices, schools
2	60...80	moderate colour rendering	Industry, offices, schools
3	40...60	accurate colour rendering is of little importance	Industry, sports halls
4	20...40	accurate colour rendering is of no importance	Traffic lighting

Some tasks such as colour matching in the printing industry have high demands in accurate colour rendering and require special attention from the lighting designer. For normal offices, however, the colour rendering group will be 1B or 2, which is easily achived with normal fluoresent lamps.

Colour rendering index for different light sources
Light source	Colour rendering group
Incandescent	1A
Metal halide	1A ... 2
Fluorescent	1A ... 3
High pressure sodium	1B ... 4
Low pressure sodium	4

The reason for lamps with a poor colour rendering such as high and low pressure sodium being used at all is their high efficacy. They output more light per Watts of electrical power than lamps that provide a good colour rendering. The luminous efficacy is given as the ratio of lamp lumens/lamp watts. It is generally true that lamps with high wattages are more efficient than their low-wattage counterparts.