What is the Reference Illuminant?

Over the past few months I’ve had a manufacturer, a sales rep, and a lighting designer all tell me they think CRI compares a light source to daylight.  When I tried to correct one of them the reply was an acknowledgment that an incandescent source is normally used, but daylight can be used, too. Given that the lighting industry has been using CRI since 1965, all three should have known better.  On the assumption that they’re not alone in their misunderstanding, let’s talk about reference light sources.

The International Commission on Illumination (CIE for Commission Internationale de l’Eclairage) and the Illuminating Engineering Society (IES) both define color rendering as, “The effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.”  In other words, we evaluate the color rendering of a given light source by comparing it to another light source.  The other light source is called the reference illuminant.  

In 1965 the CIE published CIE 13 Method of Measuring and Specifying Colour Rendering Properties of Light Sources.  The current version is CIE 13.3-1995.  Its General Color Rendering Index, Ra, is usually referred to as CRI. CIE 13.3 says, “the reference illuminant for light sources with correlated colour temperatures below 5000 K shall be a Planckian radiator, and from 5000 K one of a series of spectral power distributions of phases of daylight.”  

So, the reference illuminant must have the same color temperature or correlated color temperature (CCT) as the light source being tested. For all light sources with a CCT below 5000 K we use the spectrum of a Plankian, or blackbody, radiator.  For all light sources with a CCT of 5000 K or above we use a CIE model of daylight, again at the same CCT as the light source we’re testing.

Regular readers of this blog know that I’m not a fan of CRI, greatly preferring the increased accuracy and depth of information provided by ANSI/IES TM-30 IES Method for Evaluating Light Source Color Rendition.  What about TM-30’s reference light source?  It’s nearly the same.  For CCTs of 4000 K and below it’s a Plankian radiator.  For CCTs of 5000 K and above, it’s the CIE model of daylight.  TM-30 avoids CRI’s sudden jump between reference illuminants by using a graduated blend of Plankian radiator and daylight over the range of 4001 to 4999 K. 

CIE 224 Color Fidelity Index for Accurate Scientific Use is identical to TM-30’s Fidelity Index (Rf) and uses the same reference light sources.