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Spectral Power Distribution – Effective Concepts LLC
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Sep 142014

One small issue with LEDs is they have a different quality of light. We have measurements that worked well with Incandescent, HIDs, and Fluorescent lamps. CRI (Color Rendering Index) which gave us a relative measure of quality and CCT which gave us a lamp’s color temperature. But these measurements were based on factors that are no longer in play in the world of solid state lighting.

In the July-August 2014 issue of Architectural Lighting, Alice Liao writes about the issues facing the consumer of quality lighting and the issues facing the professional user of lighting.

“Both CRI and CCT are derived through rote mathematic simulation rather than through empirical measurement. CRI testing is calculated on a computing device using a source’s spectral power distribution (SPD), a diagram that depicts the radiant energy a source emits at different wavelengths of visible light—wavelengths of 380 to 780 nanometers—and the spectral reflectance of each color chip. CCT is also computed from the source’s SPD.”

After Lias discusses the short comings of these measurements, we’re introduced to the Color Quality Scale (CQS). Developed by the National Institute of Standards and Technology. The CQS tests with a broader range of colors, higher chromas and deeper saturation.

“CQS also factors in extreme color temperature, which impairs a source’s ability to render color, and takes a root-mean-square of the color shifts of all 15 test colors rather than an average. This ensures that poor performance on a few samples is given proper weight.”

I am hopeful the IES Color Metrics Task Group will come up with a simple scale for consumers who are already confused about LEDs. They should also develop a set of ratings for Professionals, whose requirements for light require more data not less. For those interested in this important topic, I highly recommend this article.

The CIE chormaticity diagrams map perceived color.

The CIE chormaticity diagrams map perceived color. Lightness, the third dimension of the color space, is not shown in these two-dimensional graphs. The CIE created the 1960 Uniform Chromaticity Scale (UCS) to reduce the limitations of the 1931 system; it has since been updated by the 1976 UCS. The Planckian, or black body, locus—shown by the curved lines within the filled areas—indicates the color that a black body radiator emits within each chromaticity diagram as it is heated up.
Credit: U.S. Department of Energy

Jan 012009

Color Temperature

The correlated color temperature of a light source is expressed in Kelvins (K) and is a means of describing the appearance
or chromaticity of the source itself. It describes the apparent whiteness of the lamp. Sources having a low correlated color temperature (2700K to 3400K) are said to be “warm” in color such as Warm White, Warm White Deluxe, and Designer 3000K (D30) and Designer “800” 3000K (D830). (Most incandescent lamps have a color temperature between 2700K and 3000K.) Fluorescent sources having higher correlated color temperature (4100K to 6300K) such as Cool White, Cool White Deluxe, Daylight Designer “800” 4100K (D841), or Designer 4100K (D41) are said to be “cool” in color. The Designer Series and OCTRON Lamps having a correlated color temperature of 3500K are considered “mid-range” and provide excellent color rendition while not being either “warm” or “cool.” The correlated color temperature of the light source contributes greatly to the overall visual appearance of the lighted space.

Color Temperature Scale

Color Temperature Chart Deg K Lamp Type
8500 North Light/ Blue Sky
6500 Daylight Fluorescent Lamp
6000 Clear Mercury Lamp
4500 Clear Metal Halide Lamp
4000 Cool White Fluorescent Lamp
3000 CAPSYLITE Tungsten Halogen Lamp
Warm White Fluorescent Lamp
2500 40 Watt Incandescent Lamp
2000 High Pressure Sodium
1800 A Candle
The color temperature scale assigns a numeric value to the
color appearance of a a light source ranging from orange (warm
light) to blue (cool light).

Color Rendition Index (CRI)

Color Rendering Index (CRI) is an international numbering system from 0-100 which indicates the relative color rendering quality of a light source when compared to a standard reference source of the same correlated color temperature. It expresses the degree to which colors will appear ‘familiar’ or “natural” under the light source being selected. In general, the higher the CRI number, the better the color rendering properties of the light source in question. The color rendering index of any two sources should only be compared if those sources have the same correlated color temperature.

Color temperature figures, SPD curves and CRI ratings provide helpful information, but they are not perfect. Color temperature, for instance, fails to indicate anything about how a given light source will render colors. two light sources with similar color temperatures will look the same but may have slightly different color properties. In general, a high CRI figure means a light source will render colors well. Since CRI is based on an average of eight different colors, a light source with a high CRI will not necessarily reproduce every color equally well. Used in conjunction, however, all these measurements can provide excellent benchmarks for the comparison of light sources.

Spectral Power Distribution Curve

SPD Curve CRI Lamp Type Notes
Noontime Sunlight The SPD of sunlight show it to be an evenly balanced light source with all wavelengths of visible light present in nearly equal quantities.
95+ Incandescent The SPD curve of a standard incandescent lamp shows little radiation in the lower end of the spectrum. This explains why the lamp type tends not to render blues very well.
82 OCTRON® 4100° deg K Fluorescent This curve show how an improved triphosphor coating produces even more pronounced energy bands in the primary colors, further improving both color rendering and efficiency
75 Cool White Fluorescent This curve, typical of standard fluorescent lamp, shows how the addition of a halophosphor coating smooths out the spectral energy spikes of a gaseous discharge light source.
75 MetalArc® Metal Halide The spectral energy from metal halide lamps is relatively even, but there are some distinct gaps. For most application, these gaps are not significant, allowing these lamps to render colors suprisingly well.
21 Lumalux® High Pressure Sodium The SPD curve for an HPS lamp show an intense concentration of spectral energy in the yellow portion of the spectrum. It should be obvious that his lamp is unable to render colors accurately.

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