IES TM-30-18 Method for Evaluating Light Source Color Rendition: Industry Knowledge and Testing Solutions

The IES TM-30-18 color quality evaluation method is currently the most advanced color rendering index evaluation standard, using a dual-index evaluation system with Rf (Color Fidelity Index) and Rg (Color Gamut Index). This article provides a detailed introduction to the TM-30 color rendering index evaluation method, the meaning of Rf and Rg parameters, and testing methods. The test interface for the IES TM-30-18 color quality evaluation method is shown below. This report was measured by Yiming Technology's integrating sphere spectrometer testing system.

On October 9, 2018, the Illuminating Engineering Society (IES) of North America released a new version of the light source color quality evaluation standard: IES TM-30-18 Method for Evaluating Light Source Color Rendition, which partially updated the previous version. Compared to the version released in 2015, TM-30-18 made the following three modifications:

1. Adjusted the spectral reflectance calculation function for color samples in the ranges less than 400nm and greater than 700nm.
2. Adjusted the transition region of the reference light source, changing the applicable color temperature range from 4500K-5500K to 4000K-5000K.
3. Modified the evaluation formula for color fidelity Rf, changing the calculation scale factor from 7.54 to 6.73.

In April 2017, the International Commission on Illumination (CIE) published a technical report "CIE 224:2017 Colour Fidelity Index for accurate scientific use", which defined the color fidelity index Rf. This index is based on the fidelity index calculation in IES TM-30-15, but has slight differences in the extrapolation method for color evaluation samples, the transition region of the reference light source, and the scale factor for fidelity calculation. With the release of TM-30-18, these two documents have been harmonized, which is an important step towards globalization of color fidelity measurement.

The general color rendering index Ra used by CIE to evaluate color quality is used to describe the visual experience of various light source technologies. However, IES believes that Ra only considers one aspect of color quality and cannot fully express human perception of color, cannot establish the integrity of visual experience, and is flawed in accurately reproducing light source colors. To this end, IES established a Color Metrics Task Group in March 2013 to study improved measures for characterizing light source color rendering properties, in order to develop an alternative system to CRI that better serves the lighting industry and its stakeholders. On May 18, 2015, IES officially released the normative document IES TM-30-15 for evaluating light source color rendering properties, using two indicators, color fidelity index Rf (Fidelity Index) and color gamut index Rg (Gamut Index), to jointly evaluate color quality, which is significantly different from CIE's CRI system. Compared to CIE's original color rendering index evaluation system, the IES TM-30 evaluation method has the following 5 major changes:

I. TM-30 Dual-Index Color Rendering Index Evaluation System: Rf and Rg

TM-30 uses two indicators, fidelity Rf and gamut Rg, to jointly evaluate color quality, forming a unique dual color rendering index evaluation system. Fidelity Rf is used to characterize the light source's color rendering index CRI (Color Rendering Index), which is the similarity of standard colors under the test light source compared to under the reference light source, with values from 0 to 100, where higher values indicate better color fidelity. The Rf color rendering index is one of the core parameters in the TM-30 standard. This indicator is similar to the general color rendering index Ra used in CIE13.3-1995 for evaluating CRI, both are used to characterize average color fidelity, but TM-30 uses 99 new color samples and newly developed calculation methods. IES believes that the Rf color rendering index can provide more statistically reliable data and be more accurate in color reproduction.

• Rf=100 is the maximum value, indicating no color difference from natural light source colors, with realistic color effects.
• Rf=0 is the minimum value, indicating the maximum color difference from natural light source colors, with distorted color effects.

Color gamut Rg is another important color rendering index parameter in the TM-30 standard, used to characterize the light source's gamut area index GAI (Gamut Area Index), which is the saturation degree of standard colors under the test light source compared to the reference light source, with an index of 100 indicating moderate saturation. The Rg color rendering index works together with the Rf color rendering index to form a complete TM-30 color quality evaluation system.

• Rg=100 indicates that the light source's saturation is the same as natural light, with moderate color saturation.
• Rg>100 indicates that the light source can increase color saturation, making objects appear more vivid and vibrant.
• Rg<100 indicates that color saturation decreases under the test light source, with insufficient color saturation, making objects appear dull and lifeless.

II. 99 Standard Colors Used in TM-30 Color Rendering Index Evaluation

CIE's Ra selected 8 Munsell color samples as standard colors. These colors have medium lightness and color saturation, which can fully represent common colors in indoor lighting, but are not representative of high-saturation colors that exist in outdoor lighting. Since the colors used are insufficient to cover all visible light wavelengths, luminaire manufacturers can optimize their light source's spectral power distribution (SPD) to obtain higher measurement values.

Compared to Ra's only 8 standard color samples, the TM-30 system uses as many as 99 standard color samples. These samples are no longer Munsell color cards, but are selected from 105,000 real object colors, including 7 major categories: natural objects, skin tones, textiles, paints, plastics, printing materials, and color systems, covering various colors from saturated to unsaturated, from bright to dark, commonly found in our daily lives. Most importantly, these 99 standard colors have equal sensitivity to all wavelengths, making it difficult for manufacturers to improve measurement values by adjusting spectral distribution.

TM-30 provides spectral reflectance calculation functions for different color samples as well as calculation formulas and guidance for Rf and Rg color rendering index values, and measurement results can be calculated through auxiliary software. These 99 standard colors ensure the comprehensiveness and accuracy of TM-30 color rendering index evaluation.

III. Selection of More Reasonable Reference Light Source

Since the reference light source used by CRI uses blackbody radiation below 5000K and natural light model above 5000K, there is a discontinuity problem at 5000K. The TM-30 system uses a mixed spectrum of blackbody radiation and natural light model as the reference light source in the 4000K-5000K range. In addition, both the Rf color rendering index and Rg color rendering index in the TM-30 standard use a reference light source with the same color temperature as the test light source, thereby overcoming an important drawback of GAI and making TM-30 color rendering index evaluation more scientifically reasonable.

IV. Adoption of More Uniform Color Space

TM-30 adopts the modern color space CAM02-UCS (CIE Color Appearance Model 02), which has improved uniformity and more accurate color adaptation equations, as well as an optimized set of color samples representing real objects.

V. New Color Vector Graphic

Since the Rf color rendering index and Rg color rendering index are calculated based on averages, they can only comprehensively evaluate the average color rendering ability of light sources for various colors, and cannot be used to judge the saturation degree of a specific color. When there is a need for color rendering ability for certain specific colors, the TM-30 system also provides a Color Vector Graphic, which directly displays the color shift and saturation changes of specific colors under the test light source in graphical form, whether they are duller or more vivid, which is an important supplement to the Rf and Rg color rendering indices.

The black circle represents the reference light source (sunlight), composed of 16 color categories of the reference light source, and the red circle represents the test light source. Any red line inside the black circle indicates that these colors of the test light source are duller compared to the reference light source, and any red line outside the black circle indicates that these colors of the test light source are oversaturated compared to the reference light source. When the red and black circles perfectly overlap, it indicates that these colors of the test light source and reference light source are the same, with no difference in color rendering properties between the two light sources.

VI. Summary

IES recognizes that color specifications depend on various conditions, including but not limited to visual requirements, interaction with furniture and decorative materials, age, and color preferences. Therefore, IES recommends that lighting professionals use the more accurate and complete color reproduction evaluation method TM-30-18, refer to various measurement methods in the standard, and develop applicable specifications according to various spectral and application characteristics to achieve a transition as soon as possible. The TM-30 standard, through the two color rendering index parameters Rf and Rg, provides a more scientific and comprehensive color quality evaluation method for the lighting industry compared to the traditional color rendering index Ra.

As a professional LED lighting testing equipment manufacturer, Yiming Technology deeply understands the importance of the IES TM-30-18 standard and has developed the SPM5000 series integrating sphere spectrometer testing system, which can accurately measure color rendering index parameters required by the TM-30-18 standard such as Rf color rendering index and Rg color rendering index, providing standardized TM-30 color rendering index testing solutions for enterprises. This system fully complies with TM-30 standard requirements and can help lighting enterprises accurately assess the color rendering index and color quality of light sources, providing reliable basis for product quality control and jointly promoting the standardized development of the lighting industry.