TM-30-15: IES Method for Evaluating Light Source Color Rendition

Color rending metrics assign a rating to light sources using pre-defined test color samples. These metrics are designed to compare a sources’ ability to accurately render color.

In 1974 the CRI standard that we use today gave us our color rendition standard. Today TM-30-15 is a new method to evaluate color rendition; it includes several measures and graphics to evaluate the Fidelity (Rf) and Gamut (Rg) of a source when compared to a reference light source – a tungsten halogen source. Unlike the Color Rendering Index (CRI) which only considers the average accuracy of the 15 limited color samples, TM-30 widens the comparative color samples and offers a more comprehensive way to evaluate overall rendition.

The Color Fidelity Index (RF)

TM-30 Evaluates Color Spaces And 99 Color Samples

The Fidelity Index of TM-30 uses the average value of 99 color samples; CRI, however, only uses 8 to generate the Ra value. Why does this matter? Well, there are many more than 8 colors within the visible spectrum, so using such a limited sample set to try and describe color rendering performance means that the reproduction of a subject’s visual appearance may vary significantly under the artificial source when compared to natural daylight. An Rf value of 100 means that the test source renders the color samples in exactly the same way as the reference source and therefore is very close to natural daylight.

CRI Colors
TM30 CRI Colors
CRI vs TM-30-15

The Color Gamut Index (RG)

The color Gamut Index, or Rg, represents the average saturation shift of the source compared to the reference illuminant. Values greater than 100 can be visualized as an increase in average saturation, whereas values less than 100 represent a decrease. Find great examples of manipulating the Gamut with ALPHABET Vibrancy or Beauty CCT. Vibrancy Series makes objects look more vivid. Imagine the first time you saw a HDTV. Whites, reds and blues pop and become rich and saturated. See Vector index below for an illustration.

TM30 RGB Desaturated

Desaturated RG < 100

TM30 RGB Original

Original RG < 100

TM30 RGB Saturated

Saturated RG < 100

The Color Vector Index

In addition to the two indices, Color Vector Graphics are used to visually highlight gamut changes. The reference light source is represented by the black circle, deviation outside of this region shows an increase in saturation for the given color (Think ALPHABET VIBRANCY Series or BEAUTY Series for deviation outside), whereas a shift within, shows a saturation decrease for the given hue. The accompanying arrows between reference and source depict the gamut vector shift.

TM30 Color Vector

TM30 Color Vector

TM30 Color Distortion

TM30 Color Distortion