Beyond CRI: Evaluating Light Source Color Rendition Using ANSI/IES TM-30-24
Fidelity index (R_f), gamut index (R_g), and local chroma shifts across architectural materials.
Because the traditional CIE Color Rendering Index (R_a) is calculated from only eight moderately saturated test samples, a high R_a does not by itself characterize the fidelity or chroma behavior of highly saturated colors such as the red sample represented by R_9. Using ANSI/IES TM-30-24, we examine light-source color rendition using 99 standardized Color Evaluation Samples (CES) designed to provide broad coverage of object colors. We evaluate the Fidelity Index (R_f), Gamut Index (R_g), 16-hue-bin Color Vector Graphics (CVG), and quantified chromatic shifts across natural architectural materials including Roman travertine, white oak, and terracotta.
1. Methodological Context of CIE R_a and Historical Limitations
Established in 1965 and updated in 1974, the CIE General Color Rendering Index (R_a) assesses color difference across eight standardized pastel test color samples (TCS 01 to TCS 08) relative to a reference blackbody radiator or daylight phase. Saturated colors, including saturated deep red (TCS 09, R_9), were designated as special color rendering indices and excluded from the arithmetic mean of R_a.
Consequently, early phosphor-converted white LEDs could achieve acceptable R_a ratings while exhibiting substantial spectral deficiencies in the long-wavelength red region. For architectural materials rich in natural red and warm pigments—such as terracotta, timber grain, and human skin—such spectral deficiencies result in a muted appearance that is not captured by R_a alone. While R_9 provides a useful supplementary indicator of saturated-red rendering, it should be interpreted alongside TM-30 fidelity and hue-specific measures rather than as a complete red-rendering characterization on its own.
2. The Two-Dimensional Matrix: Fidelity (R_f), Gamut (R_g), and Hue Vector Graphics
ANSI/IES TM-30-24 evaluates light-source color rendition using 99 standardized Color Evaluation Samples (CES) designed to provide broad coverage of object colors. The method decomposes rendition into two primary metrics: the Fidelity Index (R_f, 0–100 scale), quantifying chromatic similarity to reference illuminants, and the Gamut Index (R_g, typically 60–140), quantifying relative average saturation across 16 hue bins.
Crucially, R_g is an overall average: a light source with R_g ≈ 100 may exhibit balanced average saturation while locally desaturating reds and over-saturating greens. TM-30 resolves this ambiguity through 16-hue-bin Color Vector Graphics (CVG) and local chroma shift values (R_cs,h1 to R_cs,h16). In architectural material specification, Phaos targets controlled chromatic neutrality (R_g ≈ 99–103) to preserve authentic material fidelity, rather than maximizing artificial vividness.
For high-fidelity architectural illumination, Phaos engineering targets designate R_f ≥ 95, R_g between 99 and 103, and R_9 ≥ 90 at 3000K nominal CCT. These are Phaos design targets rather than universal TM-30 requirements; other applications (e.g. retail vs. museum conservation) may purposefully specify higher or lower gamut indices depending on design intent.
3. Architectural Material Reflectance and Quantified Color Shifts
To demonstrate real-world architectural consequence, the spectral power distribution S(λ) is convolved with measured spectral reflectance curves of natural interior materials: Roman travertine stone (S_stone(λ)), European white oak (S_wood(λ)), and Tuscan terracotta tile (S_terra(λ)). Calculating chromatic coordinates in CIECAM02-UCS confirms that high-fidelity spectral tuning (R_f ≥ 95, R_cs,h1 ≥ -1%) preserves natural material chroma and warm undertones without introducing graying or artificial hue drift.
ANSI/IES TM-30-24 derives the overall Fidelity Index R_f from the mean color difference of the 99 standardized Color Evaluation Samples (CES) in CAM02-UCS space using the prescribed non-linear scaling transformation and bounding algorithms.
R_g calculates the ratio of the polygon area formed by the 16 hue bins in the CAM02-UCS (a', b') plane relative to reference. Local chroma shift R_cs,hj quantifies the fractional increase or decrease in saturation for hue bin j.
Spectroradiometric TM-30-24 Characterization & Material Shifts
Laboratory spectroradiometer measurement [Instrument Systems CAS 140D, 350–800 nm, 3000K reference module at nominal drive, expanded measurement uncertainty ±1.0 with coverage factor k = 2]. TM-30-20 retained where historical comparability is required; current characterization follows ANSI/IES TM-30-24.
Measure of how closely the appearance of 99 standardized Color Evaluation Samples under the test source matches their appearance under a reference illuminant of the same CCT.
Measure of average color saturation across 16 hue bins relative to the reference illuminant. R_g > 100 indicates average saturation increase; R_g < 100 indicates average desaturation.
Relative chroma shift in Hue Bin 1 (red hues). Positive values indicate red saturation enhancement; negative values indicate red desaturation.
Supplementary individual color rendering index corresponding to test sample TCS 09 (saturated deep red, ~660 nm), useful as a cross-reference indicator.
- ANSI/IES TM-30-24: IES Method for Evaluating Light Source Color Rendition. Illuminating Engineering Society, New York. (2024)
- ANSI/IES TM-30-20: IES Method for Evaluating Light Source Color Rendition. Illuminating Engineering Society, New York. (2020)
- CIE 13.3-1995: Method of Measuring and Specifying Colour Rendering Properties of Light Sources. International Commission on Illumination. (1995)
- David, A., et al. (2015). Development of the IES method for evaluating the color rendition of light sources. LEUKOS, 11(4), 169-198. (2015)
Luminaires Embodying this Optical Principle
STYLOS O-12
Deep-recessed micro-aperture darklight pinhole luminaire
CYLINDRO X-40
Miniature precision surface-mounted & track projector
CONVEX C-25
Sub-miniature plaster-in concealed cove and millwork profile