A recent survey of lighting designers and specifiers by the National Lighting Product Information Program showed that for retail applications, light source color properties are considered more important than any other light source criterion, including energy efficiency.

To define light source color properties, the lighting industry predominantly relies on two metrics, correlated color temperature (CCT), commonly used as an indication of the apparent “warmth” or “coolness” of the light emitted by a source, and color rendering index (CRI), an indication of the light source’s ability to make illuminated objects appear natural.

However, these two metrics, developed in the last century, are facing increased challenges and criticisms as new types of light sources, particularly LEDs, become more prevalent in the market.

In order to help retail lighting designers better understand CCT and CRI and choose the best lighting product for specific retail applications, the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) has published a new volume in its ASSIST recommends series.

The first issue, “Guide to Light and Color in Retail Merchandising,” provides a background on CCT and CRI, including their advantages and drawbacks, and discusses how they may be augmented for better use in retail merchandising. The second issue, “Recommendations for Specifying Color Properties of Light Sources for Retail Merchandising,” recommends two-metric approaches for specifying light sources to achieve desired color appearance of the illumination as well as good color rendering in retail applications.

For example, in order to meet the expectations for good color rendering in retail applications, ASSIST advises using the well-established CRI along with another metric called gamut area index (GAI). GAI represents the relative separation of object colors illuminated by a light source; the greater the GAI, the greater the apparent saturation or vividness of the object colors, according to Mark Rea, Ph.D., director of Rensselaer Polytechnic Institute’s Lighting Research Center and co-author of the publication.

“Broadly speaking, CRI is a measure of how ‘natural’ an object appears, and GAI is a measure of how ‘vivid’ the colors appear,” said Rea. “LRC experiments show that light sources which balance both CRI and GAI are generally preferred over ones that have only high CRI or only high GAI.”

Practical, step-by-step methods are included in the publication for the two-metric color rendering approach described here, as well as a two-metric approach for achieving consistent results in desired color appearance of the illumination.

“The rationale and new methods provided in this ASSIST volume should lead to light source specification that most closely represents a designer’s intentions,” said Rea.

The LRC recently held the first in what will be a series of roundtable discussions, including color-rendering mock-ups and experiments, with practicing lighting designers and specifiers to introduce the two-metric approach to color rendering and obtain feedback. The first session was recently held with a group of high-end retail lighting designers who agreed that this new approach was a step forward over the reliance on CRI alone and would be a useful tool when trying to narrow down light source options from a large selection of products.

The two new ASSIST recommends are available for free download from the ASSIST web site here.