What is color stability?

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Volume 8 Issue 1	October 2004

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. 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. efficacy - The ratio of the light output of a lamp (lumens) to its active power (watts), expressed as lumens per watt. spectral power distribution (SPD) - A representation of the radiant power emitted by a light source as a function of wavelength. blackbody radiator - A temperature radiator of uniform temperature whose radiant output in all parts of the spectrum is the maximum obtainable from any temperature radiator at the same temperature. Such a radiator is called a blackbody because it absorbs all the radiant energy that falls upon it. All other temperature radiators can be classed as non-blackbodies. Non-blackbodies radiate less in some or all wavelength intervals than a blackbody of the same size and the same temperature. 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). color appearance - The resultant color perception that includes the effects of spectrum, background contrast, chromatic adaptation, color constancy, brightness, size and saturation. color consistency - The measure of how close in color appearance random samples of a lamp or source tend to be. color matching - The action of making a color appear the same as a given color. Often used as a method of evaluating the ability of a light source to render colors faithfully. color stability - The ability of a lamp or light source to maintain its color rendering and color appearance properties over its life. The color properties of some discharge light sources may tend to shift over the life of the lamp. full-spectrum index (FSI) - A mathematical measure of how much a light source's spectrum deviates from an equal energy spectrum, based on the slope of its cumulative spectrum. 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. hue - The attribute of a light source or illuminated object that determines whether it is red, yellow, green, blue, or the like. isotemperature - A set of coordinates within which all points have the same temperature. In a color space diagram, isotemperature lines represent lights with identical correlated color temperatures. metamers - Lights of the same color but of different spectral power distribution. photopic - Vision mediated essentially or exclusively by the cones. It is generally associated with adaptation to a luminance of at least 3.4 cd/m². primary - Any one of three lights in terms of which a color is specified by giving the amount of each required to match it by additive combination.

Color properties of lamps may change over the life of those lamps, even when they are manufactured with consistent correlated color tempatures (CCTs). Color stability describes the ability of a light source to maintain its color properties over time. As shown in Table 1, survey respondents rated color stability as high as CCT, the second most important criterion for lighting specifiers when selecting light sources.

Figure 11. Color stability of CFLs

Figure 11 illustrates the average shift in CIE 1976 color space of CFLs from two different manufacturers, A and B, along with the ANSI 2700 K 4-step MacAdam ellipse. The chromaticity coordinates of 100 lamps produced by 10 manufacturers were measured in NLPIP's laboratory. The data from the two manufacturers shown in Figure 11 were chosen to illustrate the extremes in measured color shift between 100 and 2400 hours of operation. This time period represents 40% of rated lamp life. Compact fluorescent lamps (CFLs) from manufacturer A exhibited a much larger shift in the average chromaticity of 10 lamps over time than those from manufacturer B. For both manufacturers the movement in average chromaticity is less than a 4-step MacAdam ellipse, suggesting that color stability over time is quite good for CFLs, despite the wide variation among different lamps shown in Figure 10.

Color stability is substantially different for metal halide lamps than for CFLs. Figure 12 presents color stability measurements for both probe-start and pulse-start metal halide lamps from two manufacturers. The data were chosen to illustrate the measured color shift over 8000 hours of operation, 40% of rated lamp life. The color shift over time for both manufacturers is much larger than a 4-step MacAdam ellipse, except for the pulse-start lamps from manufacturer B. Therefore, over their lifespan, these metal halide lamps will shift in color much more than typical CFLs. So, as noted by respondents to NLPIP's survey (Table 1), lamp type can play an important role in defining a light source's color characteristics.

Figure 12. Color stability of metal halide lamps