IES Light Source Color Rendition (IES TM-30-15)

The Illuminating Engineering Society TM-30-15 is a system of several related measures and (optional) graphical displays that can be used together to effectively evaluate and communicate a light source’s color rendering properties.While similar to the CRI in overall intent and method, it uses a much larger set of 99 selected test colors, uses a modern color difference metric based on CIECAM02, and evaluates both Color Fidelity and Gamut.

TM-30-15 Readout

A spectral instrument is needed to compute the TM-30-15, and for the value to be valid, the illuminant must be within a certain tolerance of the black body or Daylight color locus, and the TM-30-15 text will be shown in Orange if this is not the case. If the color is extremely unlike an illuminant, then the TM-30-15 may return Failed. You may decide to use the TM-30-15 valuess even if they are shown in Orange, if you judge the illuminant to be close enough to the illuminant locus to be useful.

The four measurements displayed by TM-30-15 are:

Fidelity Index Rf
Analogous to CIE Ra (CRI). Characterizes the average color shift of the 99 CES to characterize the overall level of similarity between
the test source and reference illuminant. Values range from 0 to 100,  with 100 being maximum fidelity.
Gamut Index Rg
Compares the area enclosed by the average chromaticity coordinates in each of 16 hue bins to characterize the average saturation level of the test source compared to the reference illuminant. A neutral score is 100, with values greater than 100 indicating an increase in saturation and values less than 100 indicating a decrease in saturation. The range in values grows as fidelity decreases.

Correlated Color Temperature CCT

This is the temperature in degrees Kelvin of the black body radiator that has the closest color in the (obsolete) perceptual CIE 1960 UCS. This is used in TMS-30-15 to determine the color temperature of the reference Black Body or Daylight illuminant. A Black Body reference is used up to 4500K, and a Daylight reference is used above 5500K, with a blended reference used between these two color temperatures.

Delta u,v Duv

This is the distance in CIE 1960 UCS uv value ("Duv") from the chomaticity of the reference illuminant to the measured sample. A tolerance of 0.0054 Duv is regarded as satisfactory in many lighting standards, corresponding to roughly 7-8 CIEDE2000 (Note that the MacAdam Ellipse "step" number correlates with the Delta E 2000. This is not a coincidence.) A positive value indicates that the measurement is above the black body locus (green direction), while a negative value is below it (purple direction). Note that this value will differ to that returned in traditional Correlated Color Temperature measurement, due to the latter being a distance to the Black Body locus rather than blended Black Body & Daylight locus.

For more information about TM-30-15, please consult the IES website.

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