Specifying Color Rendering via IES TM-30-24 Metrics

Professional guide to applying IES TM-30-18 color metrics, explaining how to utilize the fidelity index Rf and gamut index Rg for spectral analysis.

Illumination Pros Editorial

May 22, 2024 10 min read

For decades, the lighting industry relied on the traditional Color Rendering Index (CRI) to evaluate a light source’s ability to accurately render colors. However, as solid-state lighting (LED) matured, the limitations of the CRI system—based on only eight pastel samples—became problematic. The Illuminating Engineering Society (IES) addressed this by introducing IES TM-30-18 color metrics, now officially superseded by ANSI/IES TM-30-24, establishing a modern standard for color rendition evaluation.

TM-30-24 shifts the specification paradigm by providing comprehensive metrics derived directly from the spectral power distribution (SPD) of a light source, rather than a single flawed average. This guide details how lighting engineers can interpret the color fidelity index ($R_f$) and the color gamut index ($R_g$) to achieve precise color rendering compared to traditional CRI metrics. By understanding these indices, specifiers gain unprecedented control over visual environments.

The Limitations of Traditional CRI

To understand the necessity of ANSI/IES TM-30-24, it is crucial to recognize the shortcomings of traditional CRI. The traditional $R_a$ metric averages the color appearance shifts of eight Munsell color samples when illuminated by a test source compared to a reference source (blackbody radiator or daylight model, depending on CCT).

Limited Sample Set: The eight samples (R1-R8) are low-chroma (desaturated) pastels. They do not effectively represent saturated colors, skin tones, or modern synthetic materials. A light source can achieve a high $R_a$ while rendering deep reds (R9) or saturated blues poorly.
Averaging Metric: $R_a$ is an average. A source might render seven of the eight samples perfectly and fail completely on the eighth, yet still achieve an ostensibly high average score.
Fidelity Only: CRI is strictly a fidelity metric. It only measures how closely a source matches a reference. It cannot describe how the color shifts (e.g., whether a color becomes more saturated or less saturated, or shifts in hue).

The ANSI/IES TM-30-24 Color Rendition System

ANSI/IES TM-30-24 resolves these limitations by utilizing an entirely new calculation engine and an expanded, scientifically rigorous sample set.

The 99 Color Evaluation Samples (CES)

Instead of eight pastels, TM-30 uses 99 Color Evaluation Samples (CES). These samples were carefully selected from a database of over 100,000 spectral reflectance measurements of real-world objects, including skin tones, textiles, paints, plastics, and printed materials.

The CES are uniformly distributed across the three-dimensional color space (hue, value, and chroma), ensuring that a light source’s SPD is evaluated across all visible wavelengths and color representations. This makes it virtually impossible to “game” the TM-30 metrics by tuning a light source’s SPD to spike at specific wavelengths, a common practice used to artificially inflate CRI scores.

The Core Metrics: $R_f$ and $R_g$

TM-30 establishes two primary, high-level indices: the Color Fidelity Index ($R_f$) and the Color Gamut Index ($R_g$).

Color Fidelity Index ($R_f$)

The $R_f$ metric is TM-30’s analog to CRI ($R_a$). It quantifies the average color shift of the 99 CES when illuminated by the test source compared to the reference illuminant.

Scale: 0 to 100.
Interpretation: An $R_f$ of 100 indicates perfect fidelity; the test source renders all 99 samples exactly as the reference source would.
Comparison to CRI: While functionally similar to $R_a$, $R_f$ is significantly more accurate due to the 99 CES. An $R_f$ value is often slightly lower than a corresponding $R_a$ value for the same LED source, as $R_f$ penalizes spectral deficiencies that $R_a$ misses.

Color Gamut Index ($R_g$)

The $R_g$ metric provides information entirely missing from the CRI system. It quantifies the average change in saturation (chroma) of the 99 CES compared to the reference illuminant.

Scale: Typically ranges from 60 to 140.
Interpretation:
- $R_g$ = 100: The test source produces the same average saturation as the reference source.
- $R_g$ > 100: The test source, on average, increases saturation (oversaturates or “pops” colors).
- $R_g$ < 100: The test source, on average, decreases saturation (desaturates or washes out colors).

The Color Vector Graphic (CVG)

While $R_f$ and $R_g$ provide excellent top-level averages, they do not tell the whole story. An $R_g$ of 105 indicates an overall 5% increase in gamut area, but it doesn’t specify which colors are saturated (e.g., are the reds popping, or the greens?).

To address this, TM-30 includes the Color Vector Graphic (CVG). The CVG is a two-dimensional plot that visually represents hue and saturation shifts across 16 hue bins.

The Reference Circle: A black circle represents the reference illuminant (perfect fidelity and neutral saturation).
The Test Polygon: A red polygon represents the test source.
- If the red line extends outside the black circle in a specific hue bin, those colors are oversaturated.
- If the red line pulls inside the black circle, those colors are desaturated.
- If the red line shifts rotationally along the circumference, those colors experience a hue shift.

The CVG allows lighting designers to visually assess exactly how an SPD will render specific color ranges, enabling highly tailored lighting specifications for demanding applications like retail, art galleries, or healthcare.

Specifying with TM-30-24: Application Targets

Because TM-30 provides a multi-dimensional assessment of color, specifying “good” color rendering is no longer as simple as requiring “80 CRI.” The ideal TM-30 specification depends entirely on the application’s visual requirements. Is the goal strict, clinical accuracy, or enhanced, visually appealing vibrancy?

The IES provides guidance on target values based on design intent.

TM-30 Specification Annex Targets

Design Intent	Primary Goal	Recommended $R_f$ Target	Recommended $R_g$ Target	Application Examples
Fidelity (Tier F1)	Match reference illuminant exactly.	$\ge$ 95	$\ge$ 98	Healthcare diagnostics, color matching, fine art galleries.
Preference (Tier P1)	Enhance visual appeal, slightly oversaturate warm tones.	$\ge$ 78	$\ge$ 95	High-end retail, hospitality, grocery (produce/meat), residential.
Vividness (Tier V1)	Maximize vividness and color saturation.	$\ge$ 75	$\ge$ 100	Accent lighting, specialized displays.

Spectral Power Distribution (SPD) Control

Understanding $R_f$ and $R_g$ ultimately relies on understanding the underlying Spectral Power Distribution. LED manufacturers can manipulate the phosphor mix in white LEDs to intentionally alter the SPD, thereby tuning the TM-30 metrics.

For example, a typical standard-efficiency 80 CRI LED often has a distinct “cyan gap” (a drop in spectral energy around 480nm-500nm) and a narrow red peak. This results in an acceptable $R_a$ but a mediocre $R_f$ (often around 78-82) and an $R_g$ below 100, indicating slight desaturation, particularly in deep reds (which correlates with a low R9 value in the CRI system).

Conversely, a premium “high-fidelity” LED will utilize a broader, more continuous phosphor emission. This fills the cyan gap and extends the red emission further into the 650nm+ range. This specific SPD manipulation directly results in an $R_f$ > 90 and an $R_g$ near 100, providing excellent color rendering for critical applications.

For retail applications aiming for “Preference,” manufacturers might intentionally design an SPD with slight spectral peaks in the red and green regions, coupled with a slight suppression in the yellow region. This engineered spectrum yields an $R_f$ in the mid-80s but pushes the $R_g$ up to 105-110, creating the vibrant, high-contrast environment preferred in commercial merchandising.

Practical Implementation and Specification Workflows

Transitioning a firm’s specification practices from traditional CRI to the comprehensive TM-30 framework requires deliberate workflow adjustments. While the technical superiority of TM-30 is clear, its adoption necessitates education for both the design team and the end clients, who are often accustomed to the simplicity of a single $R_a$ number.

Educating the Client

The first hurdle in implementation is often client communication. When presenting lighting mock-ups or specifications, designers must translate the technical metrics of $R_f$ and $R_g$ into practical, visual outcomes.

Instead of presenting a spreadsheet of numbers, it is highly effective to utilize the Color Vector Graphic (CVG) as a visual communication tool. By overlaying the CVG of a proposed luminaire against the reference circle, designers can explicitly show why a particular fixture was chosen. For example, in a high-end retail environment, pointing to the CVG’s red polygon extending slightly outward in the red/orange hue bins provides tangible evidence that the specified lighting will make merchandise appear more vibrant and appealing to customers.

Furthermore, physical mock-ups remain indispensable. However, the mock-up process should now be informed by TM-30 data. If a client prefers the appearance of “Option A” over “Option B,” reviewing the TM-30 reports for both options allows the design team to quantify that preference (e.g., “The client prefers the slight oversaturation indicated by the $R_g$ of 106 in Option A”). This quantified preference can then become the basis for the final specification across the entire project.

Structuring the Written Specification

A robust lighting specification must be precise to prevent inferior substitutions during the bidding and procurement phases. A vague requirement for “high color quality” or even simply “TM-30 compliant” is insufficient.

A strong specification utilizing TM-30 should include explicit target ranges based on the design intent, referencing the IES Annex targets discussed previously.

Example Specification Language (Fidelity Focus):

“Luminaires shall utilize LED light sources evaluated in accordance with ANSI/IES TM-30. The Color Fidelity Index ($R_f$) shall be greater than or equal to 92. The Color Gamut Index ($R_g$) shall be between 98 and 102. The manufacturer must provide a complete TM-30 report, including the Color Vector Graphic, from an accredited independent testing laboratory.”

Example Specification Language (Preference Focus):

“Luminaires shall utilize LED light sources evaluated in accordance with ANSI/IES TM-30. The Color Fidelity Index ($R_f$) shall be greater than or equal to 85. The Color Gamut Index ($R_g$) shall be between 102 and 108. The Color Vector Graphic shall demonstrate no significant desaturation (inward shift) in hue bins 1 through 4 (reds and oranges).”

By explicitly defining acceptable ranges for both indices, the specifier establishes a quantifiable baseline that protects the integrity of the design while still allowing for competitive bidding among manufacturers whose products meet the rigorous spectral requirements.

The Future of Color Rendering Standards

While ANSI/IES TM-30 represents the current state-of-the-art in North America, color science is continuously evolving. The lighting industry is increasingly focusing not just on how colors appear, but on how the spectral composition of light affects human physiology and well-being.

Future iterations of color metrics may integrate non-visual spectral requirements, such as those related to circadian entrainment (e.g., Equivalent Melanopic Lux, or EML). As the understanding of Human-Centric Lighting (HCL) deepens, specifiers may soon be required to balance strict TM-30 color fidelity targets against specific spectral energy requirements designed to promote alertness or support sleep cycles.

Furthermore, international harmonization remains an ongoing process. While TM-30 is an ANSI/IES standard, the International Commission on Illumination (CIE) has also published CIE 224:2017 (Colour Fidelity Index for accurate scientific use), which is mathematically similar to the $R_f$ metric in TM-30. As global lighting markets continue to integrate, specifiers working on international projects must be fluent in both the IES and CIE frameworks, understanding their similarities and subtle methodological differences.

Conclusion

ANSI/IES TM-30-24 represents a fundamental advancement in how the lighting industry evaluates and specifies color rendering. By moving beyond the simplistic, flawed average of traditional CRI and adopting a multi-dimensional approach based on 99 real-world samples ($R_f$, $R_g$, and the CVG), specifiers gain unprecedented control over the visual environment.

While standard $R_a$ may remain on basic data sheets for legacy reasons, professional lighting designers and engineers must master TM-30 metrics. Understanding how a luminaire’s underlying Spectral Power Distribution dictates its Fidelity Index and Gamut Index is critical for delivering tailored, high-quality lighting solutions across diverse architectural and commercial applications.

Frequently Asked Questions

What is the difference between CRI and TM-30 Rf?

CRI (Ra) is an outdated average based on 8 pastel color samples. TM-30 Rf (Fidelity Index) is a highly accurate average based on 99 diverse, real-world color samples, strictly measuring color shifts.

What does a TM-30 Rg value over 100 mean?

A Gamut Index (Rg) greater than 100 indicates that the light source, on average, oversaturates colors compared to the reference illuminant, making them appear more vibrant or intense.

Can a light source have a high CRI but a low TM-30 Rf?

Yes. Because CRI uses only 8 low-chroma samples, a light source can be spectrally tuned to score a high Ra while rendering saturated colors poorly, resulting in a significantly lower TM-30 Rf score.

How do I use the TM-30 Color Vector Graphic (CVG)?

The CVG visually maps hue and saturation shifts across 16 color bins. If the test source’s red plot extends beyond the reference circle in a bin, those specific colors are oversaturated.