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Talking Photometry: A Guide to Colour Rendering


Natural colour rendering provided by tungsten lamp on left; poor colour rendering provided by red-tinted lamp on right

Colour rendering, or colour rendering index (CRI for short) is an important metric which describes the quality of illumination produced by a light source. For those developing energy efficient lighting based on LEDs, the holy grail is to achieve the perfect balance between luminous flux (the amount of light, in lumens), luminous efficacy (the electrical to optical conversion efficiency, in lumens per Watt), colour temperature (the shade of white light, in Kelvin) and colour rendering.

To understand the importance of colour rendering, consider the case of low pressure sodium discharge lamps which are commonly used in street lights. Such lamps are popular because they produce the highest luminous efficacy (up to 200 lumens per Watt, compared with about 20 lumens per Watt for an ordinary incandescent lamp), which in turn makes them much cheaper to run. However the quality of the illumination is very poor - a low pressure sodium lamp produces near-monochromatic light of an orange hue (around 590nm).

Any monochromatic (single colour) light source will inevitably provide very poor colour rendering – an orange lamp will only ever render orange colours reasonably. All other colours will look washed out or just grey when illuminated with orange light. To put this another way, why does a leaf appear green under sunlight illumination? Because sunlight contains light of green wavelengths and the leaf preferentially reflects green, hence the leaf appears green. Given that street lights are employed in situations where the human eye is most likely dark adapted, and remembering that under low ambient illumination the human eye possesses a scotopic response and hence retains no ability to discern colour, the poor colour rendering of a sodium streetlamp is not a show-stopper. However, for general illumination, it is important that light sources possess good colour rendering so as to ensure that objects appear natural. Often this is for one’s comfort and convenience, other times it can be a safety issue (imagine confusing the red and blue wires when wiring a plug).

The colour rendering index is the metric used in the colorimetry of LEDs and light sources in general which defines how well colours are rendered by different sources of illumination compared to a defined standard light source, or “illuminant”. The system in use today is that defined in the CIE publication number 13.3 (1995). This defines 14 “special” colour rendering indices (Ri where i = 1-14) which define the colour rendering of the test light source when used to illuminate 14 standard colours. For lamps with correlated colour temperatures of up to 5000 Kelvin, the reference illuminant used is a blackbody source (an incandescent lamp is an example of a blackbody radiator), while for colour temperatures of greater than 5000 Kelvin, the reference light source is a CIE daylight illuminant. The so-called “general” colour rendering index (Ra) is the average of the first eight special colour rendering indices (which correspond to non-saturated colours). The general colour rendering index (Ra) scale varies from zero to 100. For general illumination, an Ra of 80 is considered the minimum acceptable, while for certain safety critical jobs or for those tasks which rely upon precise colour matching, an Ra of 95+ may be necessary. By definition, a tungsten bulb (which is a blackbody source) will have an Ra of 100.

Photometric Testing serves the lighting industry by providing a service for almost any application in measuring the colour and brightness of LEDs, luminaires, lamps and displays - including the measurement of colour rendering and colour rendering as a function of angle.