Over the past few months I\u2019ve had a manufacturer, a sales rep, and a lighting designer all tell me they think CRI compares a light source to daylight.\u00a0\u00a0When 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.\u00a0\u00a0On the assumption that they\u2019re not alone in their misunderstanding, let\u2019s talk about reference light sources.<\/p>\n\n\n\n
The International Commission on Illumination (CIE for Commission Internationale de l\u2019Eclairage) and the Illuminating Engineering Society (IES) both define color rendering as, \u201cThe effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a reference illuminant.\u201d\u00a0\u00a0In other words, we evaluate the color rendering of a given light source by comparing it to another light source.\u00a0\u00a0The other light source is called the reference illuminant.\u00a0\u00a0<\/p>\n\n\n\n
In 1965 the CIE published CIE 13\u00a0Method of Measuring and Specifying Colour Rendering Properties of Light Sources<\/em>.\u00a0\u00a0The current version is CIE 13.3-1995.\u00a0\u00a0Its General Color Rendering Index, R<\/em>a<\/sub>, is usually referred to as CRI. CIE 13.3 says, \u201cthe 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.\u201d\u00a0\u00a0<\/p>\n\n\n\n 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.\u00a0\u00a0For 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\u2019re testing.<\/p>\n\n\n\n Regular readers of this blog know that I\u2019m 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<\/em>.\u00a0\u00a0What about TM-30\u2019s reference light source?\u00a0\u00a0It\u2019s nearly the same.\u00a0\u00a0For CCTs of 4000 K and below it\u2019s a Plankian radiator.\u00a0\u00a0For CCTs of 5000 K and above, it\u2019s the CIE model of daylight.\u00a0\u00a0TM-30 avoids CRI\u2019s sudden jump between reference illuminants by using a graduated blend of Plankian radiator and daylight over the range of 4001 to 4999 K.\u00a0<\/p>\n\n\n\n