TM-30-18, IES Method for Evaluating light Quality

The IES has recently issued TM-30-18, IES Method for Evaluating Light Source Color Rendition. This Technical Memorandum (TM) documents a comprehensive method for evaluating light source color rendition. It is a revision of IES TM-30-15 containing three modifications that achieve consistency with the International Commission on Illumination’s (CIE) recent publication CIE 224:2017. CIE 224:2017 defines the General Color Fidelity Index (Rf), which is based on the Fidelity Index (Rf) calculation within IES TM-30-15, but with a few minor changes. These changes involve the extrapolation method used for the color evaluation samples (CES), the transition zone for the reference illuminant, and the scaling factor for fidelity calculations. These modifications cause only minor differences in the rank order of light sources’ Rf values, and only have a small effect on the values for typical light sources. With the issuance of TM-30-18, the two measures have been harmonized so that the CIE 224:2017 Rf and the newly revised IES TM-30-18 Rf are identical, which is a key step towards broader acceptance of the index. This is a first step towards globalizing a new color fidelity measure, and IES continues to work with international lighting organizations to advance this work.

The IES TM-30-18 Fidelity Index, Rf, is analogous to the familiar CIE 13.3-1995 General Color Rendering Index Ra (colloquially, “CRI”). Both measures are intended to characterize average color fidelity, but the IES maintains that Rf is superior. IES Rf uses a modern color space (CAM02-UCS)—with improved uniformity and a more accurate chromatic adaptation transformation—and an optimized set of color samples representative of real-world objects, making it especially relevant when average color fidelity is an important consideration. The IES recommends that lighting professionals transition to IES Rf and acknowledges the usefulness of publishing CIE Ra values alongside Rf during the transition period.

IES TM-30-18 comprises a core system of calculations that quantify how a test source differs from a specified reference illuminant of the same correlated color temperature. Based on this core model, the calculation of two overall average measures (color fidelity and gamut area), three sets of average measures for each of the 16 hue-angle bins, and 99 sample-specific measures are defined, along with a graphical representation of color rendition. The measures and graphics of IES TM-30-18 are used to convey multiple aspects of a light source’s color rendition characteristics. This allows the lighting manufacturer and/or specifier, in collaboration with the design team, owner, and other stakeholders, to assess suitability for different lighting applications.

It is often important to include color rendition in specifications to ensure acceptable color quality. Most lighting specifications currently rely solely on average color fidelity—specifically CIE Ra. While replacing CIE Ra with IES Rf is an improvement, average color fidelity is only one aspect of color rendition, and therefore cannot characterize the totality of the visual experience. The IES recommends that the practice of creating specifications based solely on average color fidelity be reevaluated on a case-by-case basis to determine whether it is sufficient now that a more accurate, more complete method for evaluating color rendition (IES TM-30-18) is available. IES also recognizes that color specifications depend on a variety of conditions, including but not limited to visual needs, interaction with furnishings and finishes, age, and color preference. Therefore, a one-size-fits-all specification of color rendition is discouraged. IES recommends that specifications be based on various spectral and application characteristics, utilizing multiple measures from IES TM-30-18. Recommendations for manufacturers’ data reporting are provided in Annex D of IES TM-30-18.

IES TM-30-18 does not include guidance or suggestions for determining appropriate performance criteria or recommended values. While some research on this topic has been published, a formal Recommended Practice does not yet exist. The IES supports continued research on recommendations and application-specific criteria. IES application committees and other consensus-based groups are encouraged to incorporate IES TM-30-18 into new guidance documents as experimental and experiential evidence are built.

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