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 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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