Color Rendering Index

In this installment we will discuss CRI or the color rendering index. 

LED CRI Defined

The Color Rendering Index (CRI) is the relative ability of a light source to replicate colors generated by a reference light source of the same color temperature. It is measured as a number between 0 and 100. 

LED CRI Explained

This is often used to compare a light source ability to replicate the sun. In this case the sun is the reference light source. In layman's terms a CRI of 80 means that the LED light bulb is replicating 80% of the visible color spectrum that the sun would produce at the same color temperature. A CRI of 100%, taken at a color temperature of noon day sunlight will exactly reproduce the colors found on a sunny day at noon. A CRI of 50% taken at noon day sun will partially reproduce the colors when compared to that of a normal noon day. 

If you have ever watched a sunset you will notice how the colors change from a purer white to more yellow and red. This color change is typical of the changes in color temperature from higher numbers to lower numbers. If the reference light source used for CRI is at noon day sun a 100% CRI will result in a whiter light, if the reference source used is at sun set, at 100%CRI wil result in a light with more yellow and red. If you were to measure the light of sunset to a reference of noon day sun, the sunset light would only produce a CRI of 60% or less: less than 60% of the light spectrum of noon day light is present in the light of a sunset, hence the shift to more yellows and reds. 

Examples of Color Temperature and CRI

Light Source	Color Temperature	Color Rendering Index
Candle	1700k	100 CRI
High Pressure Sodium	2100k	25 CRI
Incandescent	2700k	100 CRI
Tungsten Halogen	3200k	95 CRI
Cool White	4200k	62 CRI
Clear Metal Halide	5500k	60 CRI
Natural Sunlight	5000-6000k	100 CRI
Daylight Bulb	6400k	80 CRI

Watch the fine print

Some marketers use differing color temperatures as a reference to attain 100% CRI, when the actual CRI would be much less than 100% if noon day sun light was used as a reference. 

Many products marketed as 'natural sunlight' light sources are far from noon day sun. 

For the Scientist

If this has not confused you try this Wikipedia definition: 

"The Color Rendering Index (CRI) (sometimes called Color Rendition Index), is a measure of the ability of a light source to reproduce the colors of various objects being lit by the source. It is a method devised by the International Commission on Illumination (CIE). The best possible rendition of colors is specified by a CRI of one hundred, while the very poorest rendition is specified by a CRI of zero. For a source like a low-pressure sodium vapor lamp, which is monochromatic, the CRI is nearly zero, but for a source like an incandescent light bulb, which emits essentially black body radiation, it is nearly one hundred. The CRI is measured by comparing the color rendering of the test source to that of a "perfect" source which is generally a black body radiator, except for sources with color temperatures above 5000K, in which case a simulated daylight (e.g. D65) is used. For example, a standard "cool white" fluorescent lamp will have a CRI near 63. Newer "triphosphor" fluorescent lamps often claim a CRI of 80 to 90. 

CRI is a quantitatively measurable index, not a subjective one. A reference source, such as black body radiation, is defined as having a CRI of 100 (this is why incandescent lamps have that rating, as they are, in effect, almost blackbody radiators), and the test source with the same color temperature is compared against this. Both sources are used to illuminate eight standard samples. The perceived colors under the reference and test illumination (measured in the CIE 1931 color space) are compared using a standard formula, and averaged over the number of samples taken (usually eight) to get the final CRI. Because eight samples are usually used, manufacturers use the prefix "octo-" on their high-CRI lamps."