3500k light source with good color fidelity (Rf 83) and near neutral gamut (Rg 98), resulting in decreased saturation in red and blue-green, and increased saturation in yellow-green and blue.
Courtesy PNNL 3500k light source with good color fidelity (Rf 83) and near neutral gamut (Rg 98), resulting in decreased saturation in red and blue-green, and increased saturation in yellow-green and blue.


3500k light source with moderate color fidelity (Rf 66) and low gamut (Rg 79), resulting in significant decreased saturation in red and green.
Courtesy PNNL 3500k light source with moderate color fidelity (Rf 66) and low gamut (Rg 79), resulting in significant decreased saturation in red and green.

The much maligned Color Rendering Index (CRI) has proved an inadequate tool for professional designers fixed on a discerning consideration of light-source color rendering capabilities. Although useful, color fidelity as defined in the CRI system is, at its best, only a part of what is needed to adequately assess color rendering. At its worse, it is a misleading indicator of color rendering. The Illuminating Engineering Society’s (IES) TM-30 color fidelity metric (Rf) is a significant improvement over CRI. Its use of 99 color samples, a more-uniform color space, and calculations more fitting and expressive of real-world conditions are confident steps forward. However, taken singularly, this advance alone does not provide the comprehensive insight needed to fully consider color rendering. TM-30’s addition of a color gamut metric (Rg), however, delivers an accounting of a light source’s saturation. When used with the procedure’s color distortion icon (two, shown)—a saturation “fingerprint” of sorts—an understanding of how a light source will reveal color is heightened.

With the emergence of solid-state lighting, we now have the capability to manipulate a source’s spectral content in a manner that was not previously practical. With the arrival of multicolor emitter arrays and other blended LED source options, white light and its color rendering abilities can be structured to a specific project need. Manufacturers can use TM-30 as a tool to develop new sources, further increasing options for designers and end users.

There is still much to learn about the potential impacts, both purposeful and unintended, of spectral shaping. Diligence must be exercised so as not to alter the spectrum in a way that expresses an intellectual dishonesty in revealing color reality. Just how that boundary is defined is open to spirited debate.

Glossary
Color Fidelity: Enumerates the accuracy with which the color appearances of illuminated surfaces and objects under a given test source match their appearances under a reference illuminant.

Color Gamut: Enumerates the average increase or decrease of the chroma of surfaces and objects when viewed under a given test source relative to when viewed under a reference illuminant.

Color Graphic Icon: Provides a visual representation of hue and saturation changes occurring in surfaces and objects when viewed under a given test source relative to when viewed under a reference illuminant.

Saturation: The colorfulness of a stimulus relative to its own brightness.

Randy Burkett, FIALD, FIES, is the president and design principal of Randy Burkett Lighting Design, in St. Louis.

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