Colour rendering is more difficult to define but in common use is the Colour Rendering Index (CRI) or Ra. This indicates the amount of colour distortion of a number of specified colour samples (see Figure 10.6 page 6) compared with a standard such as an incandescent lamp. The scale is 0-100 where the higher the value the better the colour rendering.

Note that chromaticity and colour rendering are independent of each other and therefore both performance characteristics need to be considered separately.

However these numerical systems have limitations and visual assessment of the light source illuminating the object is the best form of colour judgement.

Chromaticity refers to the visual "warmth" or "coolness" of colour. CIE chromaticity diagram is a chart with x, y axes to locate every colour.

A mixture of all colours appears white in the centre of the diagram that is roughly triangular in shape with the primary colours red, green and blue forming the corners.

The diagram can be used to compare different colours and will indicate the relative appearance by their positions.

Chromaticity - Kelvin Scale
Temperature chromaticity defines incandescent light source colour in Kelvins (K). As objects get hotter and start to give off light (become incandescent), the colour of the emitted light goes from deep red, red, to yellow, to white, to blue-white and this can be measured on Kelvin temperature scale as chromaticity (see heavy line on Figure 10.5, page 6).

Non incandescent light sources (all discharge lamps including fluorescent) are given Correlated Colour Temperature (CCT) values - as if they emitted light at a Kelvin temperature of their rating, but this is only an approximation and of limited value when comparing the colour appearance of light sources.

Colour Rendering Index (CRI) is an indication of how well a light source renders the colour of objects in a natural or familiar way. The system uses a pallet of pastel colours to visually compare colour shift to an "ideal" light source with the same Colour Temperature. An "ideal" light source has CRI of 100.

High CRI numbers indicate light sources which will make people and objects look natural and normal. But CRI alone does not indicate whether a lamp is suitable for a given task. For example both incandescent lamps ands natural daylight have CRI of 99+, but incandescent is weak in blue. Daylight may be too cold for an intimate restaurant setting as at low lighting levels acceptable "white" light becomes warmer in appearance i.e. a higher red content.

CRI comparison is valid only when comparing sources with the same chromaticity - light sources with different chromaticities may have similar CRI rating but will render colours differently. CRI is not a perfect measure of colour, but is the only internationally recognised indicator of colour rendering ability.

The table below gives the various colour values for some different lamps together with a subjective assessment of their performance.

Fluorescent Lamp Colours
The light from fluorescent lamps depends upon the phosphor: