In a demonstration performed at Rensselaer’s Lighting Research Center, observers were asked to view a still life of fresh fruit illuminated by different undisclosed light sources. They were then asked to comment on the appearance of the fruit and on their preferences. Three out of the four sources were considered acceptable, the favorite being an RGB white LED, followed by an incandescent lamp. Observers commented favorably on the vividness of the colors of the fruit under the LED source, while the incandescent scored high in naturalness.
The interesting part of the experiment was the characteristics of each light source. The incandescent had a CRI of 100, the maximum. The LED source only had a CRI of 40, way below the generally accepted range of good CRI for a light source. How is it possible that observers preferred the source with the lower CRI over a source with a CRI of 100?
To answer the question, first we must look at how we define color rendering. Color rendering is not just CRI. CRI is the color rendering index, and is not always reflective of a source’s ability to render color. Sources with the same CRI can have different spectral power distribution (SPD), and therefore render colors differently. For example, two sources, both with a CRI of 85, might have different short and long wavelengths, so that one source will render reds better, while the other source will render blues more vividly.
In order to define what makes up good color rendering, lighting designers must take into account the SPD of a source, the objects that are being illuminated, and the perception of the viewer.
Because CRI has limited ability to predict a person’s perception, another factor must be taken into account to determine a good luminous environment. That factor is called Gamut Area Index, or GAI. GAI is determined by plotting the chromaticity values of the eight color swatches used in CRI definition for a light source. The area of the polygon created by this plot is that source’s GAI, so the larger the area, the higher the GAI. High GAI is characteristic of a source with good color discrimination and saturation of colors, or vividness. Unlike CRI, GAI can be over 100, but this usually means colors appear oversaturated, and observer’s preference declines. In the experiment mentioned earlier, the preferred LED source had a GAI of 80, while the incandescent source had a GAI of only 40.
Examples of gamut area indices for various light sources. Each point of the polygon
represents the chromaticity of one of the eight color swatches used in traditional CRI
definition. The larger the area of the polygon, the higher the GAI of the source.
To ensure a good lighting design, designers should choose sources with high CRI and high GAI. The combination of naturalness provided by high CRI and vividness provided by high GAI ensure high viewer satisfaction for warm and cool sources, both at high and low levels of illumination, for either general illumination or accent lighting.
To achieve the best lighting design, one must also take the application into account. The question “what is being illuminated?” is critically important when choosing a light source. If the object being illuminated is red, you would want to specify a source that will render reds more vividly. But if the application is unknown, or will change over time, choosing a source with high CRI (85 or greater) and a high GAI (80-100) is generally the best approach.
Image credits: Alicia Miksic (1), Lighting Research Center (2)