For years now, most lamp or luminaire cut sheets will include a CRI number. Usually this number varies from 50-100 depending on the source. Obviously, one would think that a 99 is “better” that 55, but what does this index number represent?
“CRI” stands for color rendering index. And the number represents the degree of fidelity in which the lamp renders colors when compared to a reference source. There reference source is a theoretical “black body radiator”, but that sounds a bit complicated. For sake of simplicity, the black body radiator is an incandescent lamp. Therefore incandescent lamps have, by definition, a CRI of 100.
The image below indicates the original 8 reference swatches that were originally used to calculate the CRI. Basically, the source being tested will show the swatches with some degree of variance with regards to the reference source ( incandescent). Basically the average vale of the variances determines the color rendering index.
CRI Standard Colors
Now just a quick look at this method you can see major issues:
- Only 8 colors
- Color are mostly pastel
- No red swatches
- An “average” value does not indicate much
When we think about it a bit. A single average value of the deviations does not indicate much. Sure, in general we know that a CRI above 90 would be great, But we can see how two lamps with an equal CRI of 85 may act very differently in the same environment, This is because they may vary from a reference source in different way, but average out t the same number.
But this method worked in general, And the limitations it has today were due to the lack of resources available in the 1960’s, when this method was developed.
In recent years, there have been some further modifications to the method. Adding 6 more color samples, being one of those modifications.
Over the years, these limitations have been observed and the IES has introduced the TM-30 method. There a a few different enhancements in this method:
- 99 reference colors (rather than 8-14)
- Measures of gamut (in order to measure color saturation changes)
- Introduces a fidelity measurement of skin
- Graphical illustration of result (in addition to numerical indexes)
The following results are for a High Pressure Sodium Source. The middle graphic represents the relative distribution of the 99 reference colors. The Rf is the average values of the deviations among the 99 colors. As you would expect from an HPS source, it s a low 32. The Rg is the relative gamut, meaning the relative saturation of the source The 61 represents a very de-saturated environment due to the HPS source.
Both these measurements are represented in the vector graph. The black circle represents the reference source, while the red continuous line represents the HPS source. The deviations are represented in the vectors. As you can expect, the HPS source lacks green and red colors. The Rf, skin value represents the fidelity of skin under the test source. The 34 represents a very low fidelity of skin under an HPS source.
In contrast, you can observe the same results from a ceramic metal halide source below.
As expected, The CMH source has better indices for color saturation, fidelity and skin fidelity than the HPS source. But the graphical representations give more information as to what colors are lacking.
In summary, the traditional CRI method only resulted in one index number (Ra) that provided limited information to the user.
The new TM-30 method proposed by the IES provides three index numbers to compare skin (Rf,skin) general fidelity (Rf) and saturation (Rg) and a graphical representation of the results in order to better comprehend the performance of the light source.
The new method presents an opportunity for the user to see more information and make more informed decisions when selecting lamps.