Lighting Research Center Lighting Research Center
    Volume 9 Issue 1
June 2006    
average rated life - The number of hours at which half of a large group of product samples fail under standard test conditions. Rated life is a median value; any lamp or group of lamps may vary from the published rated life. ballast factor (BF) - The ratio of the light output of a fluorescent lamp or lamps operated on a ballast to the light output of the lamp(s) operated on a standard (reference) ballast. Ballast factor depends on both the ballast and the lamp type; a single ballast can have several ballast factors depending on lamp type. color rendering index (CRI) - A rating index commonly used to represent how well a light source renders the colors of objects that it illuminates. For a CRI value of 100, the maximum value, the colors of objects can be expected to be seen as they would appear under an incandescent or daylight spectrum of the same correlated color temperature (CCT). Sources with CRI values less than 50 are generally regarded as rendering colors poorly, that is, colors may appear unnatural. rapid start - A method of starting fluorescent lamps in which the electrodes are heated prior to starting, using a starter that is an integral part of the ballast. Heating the electrodes before starting the lamps reduces the voltage required to strike the electric arc between the electrodes. A rapid-start system starts smoothly, without flashing. initial light output - A lamp's light output, in lumens, after 100 hours of seasoning. instant start - A method of starting fluorescent lamps in which the voltage that is applied across the electrodes to strike the electric arc is up to twice as high as it is with other starting methods. The higher voltage is necessary because the electrodes are not heated prior to starting. This method starts the lamps without flashing. It is more energy efficient than rapid or preheat starting, but results in greater wear on the electrodes during starting. The life of instant-start lamps that are switched on and off frequently may be reduced by as much as 25 percent relative to rapid-start operation. However, for longer burning cycles (such as 12 hours per start), there may be no difference in lamp life for different starting methods. correlated color temperature (CCT) - A specification for white light sources used to describe the dominant color tone along the dimension from warm (yellows and reds) to cool (blue). Lamps with a CCT rating below 3200 K are usually considered warm sources, whereas those with a CCT above 4000 K usually considered cool in appearance. Temperatures in between are considered neutral in appearance. Technically, CCT extends the practice of using temperature, in kelvins (K), for specifying the spectrum of light sources other than blackbody radiators. Incandescent lamps and daylight closely approximate the spectra of black body radiators at different temperatures and can be designated by the corresponding temperature of a blackbody radiator. The spectra of fluorescent and LED sources, however, differ substantially from black body radiators yet they can have a color appearance similar to a blackbody radiator of a particular temperature as given by CCT. mean light output - Light output typically evaluated at 40% of rated lamp life. In combination with initial light output, mean light output may be used to estimate lamp lumen depreciation. lamp efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt (LPW). color rendering - A general expression for the effect of a light source on the color appearance of objects in conscious or subconscious comparison with their color appearance under a reference light source. system efficacy - Also referred to as relative system efficacy, system efficacy is a measurement of a system'’s ability to convert electricity into light. Measured in lumens per watt (LPW), system efficacy is the ratio of the light output (in lumens) to the active power (in watts). lumen maintenance - The ability of a lamp to retain its light output over time. Greater lumen maintenance means a lamp will remain brighter longer. The opposite of lumen maintenance is lumen depreciation, which represents the reduction of lumen output over time. Lamp lumen depreciation factor (LLD) is commonly used as a multiplier to the initial lumen rating in illuminance calculations to compensate for the lumen depreciation. The LLD factor is a dimensionless value between 0 and 1. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. chromaticity - The dominant or complementary wavelength and purity aspects of the color taken together, or of the aspects specified by the chromaticity coordinates of the color taken together. It describes the properties of light related to hue and saturation, but not luminance (brightness). full-spectrum color index (FSCI) - A mathematical transformation of full-spectrum index into a zero to 100 scale, where the resulting values are directly comparable to color rendering index. An equal energy spectrum is defined as having an FSCI value of 100, a “standard warm white” fluorescent lamp has an FSCI value of 50, and a monochromatic light source (e.g., low pressure sodium) has an FSCI value of 0. gamut area - A measure of color rendering based upon volume in color space. It is the range of colors achievable on a given color reproduction medium (or present in an image on that medium) under a given set of viewing conditions. CIE - Abbreviated as CIE from its French title Commission Internationale de l'Eclairage, the International Commission on Illumination is a technical, scientific, and cultural organization devoted to international cooperation and exchange of information among its member countries on matters relating to the science and art of lighting. standard deviation - A measure of the average distance of a set of data points from their mean. A set of data points that are all close to their mean will have a smaller standard deviation than a set of points that are further from their mean. RE90 - Designation referring to lamps that use rare-earth phosphors and have color-rendering index values equal to or greater than 90. RE80 HLO, LL - An RE80 lamp with additional enhancements of high light output (HLO) and/or long life (LL). RE80 - Designation referring to lamps that use rare-earth phosphors and have color-rendering index values of 80-89. RE70 - Designation referring to lamps that use rare-earth phosphors and have color-rendering index values of 70-79. combined uncertainty - Combined uncertainty is calculated by finding the sum of the squares of sample random variability (standard deviation) and laboratory measurement uncertainty and taking the square root of that sum. MacAdam ellipse - Researcher David L. MacAdam showed that a just noticeable difference (JND) in the colors of two lights placed side-by-side was about three times the standard deviation associated with making color matches between a reference light and a test light (MacAdam 1942, Wyszecki and Stiles 1982). These JNDs form an elliptical pattern of "constant discriminability" in a chromaticity space, centered on the chromaticity of a reference light, known as MacAdam ellipse.

Appendix B: Color rendering of T8 fluorescent lamps

The most widely used metric in the lighting industry, and the one recommended by the CIE, is the color rendering index (CRI). CRI is based on the differences in color predicted by the CIE colorimetric system of the light emitted from eight standard color samples when illuminated by the test light source and a reference light source of the same correlated color temperature (CCT). These differences are averaged to produce a single number, the CIE General CRI, which ranges from zero to 100. A cool-white T12 fluorescent lamp using halophosphate phosphors has a CRI of 62. Current T8 fluorescent lamps using rare-earth phosphors have CRI values in the 70s and 80s. Some specialty fluorescent lamps, such as those for museums and deli cases, have CRI values in the 90s, but this typically comes with a sacrifice in lamp efficacy.

In order to render subtle differences in the perceived color of objects, a light source must provide radiant power at those particular wavelengths for which the objects have slight differences in spectral reflectance. An equal energy spectrum is an imaginary spectrum that has the same radiant power at all wavelengths; therefore, it would be expected to render colors well. The full-spectrum color index (FSCI) is a measure of how much a light source's spectrum deviates from an equal energy spectrum. FSCI values range from zero to 100; an equal energy spectrum has the maximum FSCI value of 100.

A third color-rendering metric is gamut area (GA). GA is a polygon-shaped area enclosed by the chromaticity coordinates of the eight CIE standard color samples when illuminated by a given light source. The chromaticities are plotted in CIE 1976 color space. Generally, object colors will appear more vivid, or saturated, when illuminated by a light source with a larger GA. The gamut areas for the T8 fluorescent lamps tested for this publication have been scaled so that an equal energy spectrum has a GA of 100. This scaling facilitates comparisons between GA, CRI, and FSCI.

The NLPIP report Lighting Answers: Light Sources and Color introduced a new method to represent values of CRI, FSCI, and GA along with lamp efficacy. Figure App-1 illustrates this method with data for the T8 fluorescent lamps tested by NLPIP, which are of types RE80 and RE80 HLO, LL. The light gray lines in the figure show the values of CRI, FSCI, and GA for an equal energy spectrum. The tri-color bars in the figure show the average values of CRI, FSCI, and GA for all lamps tested, while the gray-and-black bar shows the average value of lamp efficacy. The gold error bars show the standard deviations in each of these measured values. These error bars show that there are only small variations in CRI, FSCI, and GA for these models of T8 fluorescent lamps.

Figure App-1. Color and efficacy values for RE80 and RE80 HLO, LL T8 fluorescent lamps tested by NLPIP

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