Jan 102010

Since I started selling lighting systems in 1991, I’ve been interested in light: quantity, quality, and efficiency. Of these three quality is the hardest to quantitize. We have been stuck with the Color Rendering Index (CRI) since 1964 when the Commission Internationale de l’Eclairage (CIE) came up with the idea of comparing each lamp to an ideal source. What many didn’t realize is this idealized source isn’t the same for every lamp. This makes CRI readings of limited use. Jeff Robins writing for Architectural Lighting (January 2010) reports on a new standard being developed by the National Institute of Standards and Technology (NIST). The NIST is trying to overcome the limitation of 14 pigment samples and the way the human sees light- it varies with the illumination level and color temperature.

Since 2006, the National Institute of Standards and Technology (NIST) has been developing a new metric, the color quality scale (CQS), that determines color performance using a method different from the CRI. When completed, the NIST will propose it as the new international standard. A different color space is used, and a new set of 15 reflective color samples, highly saturated and taken from the Munsell color system, replaces the 14 CRI samples and defines the difference between the test lamp and its reference. The NIST claims this should overcome hue and saturation shifts left out of the CRI calculation. This penalizes lamps of extreme CCTs, which frequently exhibit poor color quality. In the end, what will be familiar to users of the CRI is the CQS span, which will range from zero to 100. – (The Color Rendering Debate)

Side note:
This article also talks about the history of lighting as it relates to the CIE and the development of the CRI standard.

The CIE did not, however, solve the problem of color rendering. At the time, the issue was a small one. Remember, this was 1931; the lighting industry was dominated by the incandescent lamp, and color rendering had not yet been identified as an issue. The first lamp source other than incandescent, the mercury vapor lamp, was not available commercially until 1933. Fluorescent (1) and sodium discharge lamps(2) wouldn’t follow until later that decade, and metal halide wouldn’t come along until the late 1950s.

From the 1930s to the early 1960s, lamp quality improved significantly, especially in products that featured fluorescents, which, because of their lumen efficiency and reduced energy needs, became the lamp of choice for most commercial interior applications. The retail market was particularly keen to use the most energy-efficient means to light its products in an attractive manner.

  1. Albert W. Hull of GE’s Schenectady Research Laboratory filed for a patent on this invention in 1927, which was issued in 1931.Sales of “fluorescent lumiline lamps” commenced in 1938. By 1951 more light was produced in the United States by fluorescent lamps than by incandescent lamps. -excerpts from Wikipedia
  2. General Electric’s History of Light: Timeline

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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.