How to Interpret Advanced Color Rendering Metrics: R9 (red), Rf (fidelity), Rg (gamut)

How faithfully a light source accurately delivers colors of objects to the human eye is critical for applications such as museums, art galleries, high end retail and other specialized uses.

With the adoption of LED light sources the importance for the buyer and specifier understanding color rendering metrics has increased.

Color Rendering Index (CRI)

Traditionally, the lighting industry has used the Color Rendering Index (CRI) as the metric - a single number from 70 to 95 (100 is the daylight reference). This metric was developed by the International Commission on Illumination (CIE) in 1965 and revised in 1974. Until LEDs, CRI was used primarily for fluorescent and HID light sources (incandescent and halogen lamps have a CRI of 100).

For general lighting applications, a CRI of 80 to 90 was usually sufficient. When superior color rendering was important for an application, buyers looked for lamps providing a CRI of 92 or higher.

Beginning in the early 2000s, coinciding with the increased adoption of LEDs, specifiers realized the "single number" CRI metric was missing subtle variations in the spectral output that influenced the actual human visual perception of color rendered by the lamps. New metrics were required.

R9

The traditional CRI metric includes the average of eight pastel colors - designated as R1 to R8. R9 is not part of traditional CRI. R9 measures how well a light source renders strong, saturated red colors. It evaluates deep red tones (important for skin, food, wood, and fabrics). R9 values range from 0 to 100.

Because the CRI number does not capture this important factor, it's possible for two lamps – both with a CRI of 90 – to have different R9 values: Lamp A - R9 of 15, Lamp B - R9 of 95. If the application requires natural skin tones and vibrant reds in fabrics, Lamp B is the better choice, but CRI alone does not distinguish between the two lamps.

Eventually manufacturers started testing for R9 and adding the value to their specs along with CRI.

TM-30 - Rf & Rg

In 2015 the Illuminating Engineering Society (IES) responded to the need for a more robust method of testing the color rendering capabilities of lamps with the development of TM-30 - a color rendering test method to replace CRI. It included a fidelity index (Rf) and a gamut index (Rg).

Rf (Fidelity Index)

Rf, the "fidelity index" (scale is 0-100), measures how accurately colors appear when illuminated by a lamp compared to daylight - the same method as CRI. However, CRI uses 8 color samples and TM-30 uses 99. The samples include a diverse range of colors, including skin tones, fabrics, and, unlike the CRI, more saturated colors. The result is a more accurate assessment of the color rendering capabilities of a lamp than provided by CRI.

Rg (Gamut Index)

In addition, TM-30 includes Rg, a gamut index with a scale centered at 100. Rg measures how saturated colors appear compared to a reference light. A value of 100 indicates that same overall saturation as the reference light - a balanced rendering. A value <100 means the colors appear more muted. A value >100 indicates over saturation.

Rf & Rg Together

Together Rf & Rg values provide the specifier or buyer with a highly accurate, nuanced indication of the color rendering capabilities of a lamp. They are designed to be used together, as a single number can not tell the whole story. A particular light source could have perfect fidelity but washed out colors, or vice versa.

The diagram above is from a lamp manufacturer's spec sheet. The highlighted areas show a set of all color rendering metrics. This gives the specifer and buyer a complete understanding of how the lamp will render the color of objects.

Some manufacturers, as shown above, will still provide a traditional CRI value in their spec sheet because it is a single number easily understood and in long time use in the industry. However the TM-30 metrics have superseded CRI and should be available from any manufacturer offering lamps for color critical applications.

Whitness Index?

Some manufacturers, most notably Soraa, also include a non-standard metric called Rw (whiteness index). This is designed to evalute how a light source renders white objects, including fabric, paper, and walls. It measures whether whites appear neutral, bright, and clean, or whether they take on a tinted appearance, looking more yellowish or bluish. This whiteness index can be used alongside the TM-30 metrics, R9, or CRI (if that is all that is available). However, most manufacturers do not specify a whitness index.