What is color consistency?

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Volume 8 Issue 1	October 2004

Color rendering index (CRI) - A measure of the degree of color shift that objects undergo when illuminated by a lamp, compared with those same objects when illuminated by a reference source of comparable correlated color temperature (CCT). A CRI of 100 represents the maximum value. A lower CRI value indicates that some colors may appear unnatural when illuminated by the lamp. Incandescent lamps have a CRI above 95. The cool white fluorescent lamp has a CRI of 62; fluorescent lamps containing rare-earth phosphors are available with CRI values of 80 and above. Correlated color temperature (CCT) - A specification of the apparent color of a light source relative to the color appearance of an ideal incandescent source held at a particular temperature and measured on the Kelvin (K) scale. The CCT rating for a lamp is a general indication of the warmth or coolness of its appearance. As CCT increases, the appearance of the source shifts from reddish white toward bluish white; therefore, the higher the color temperature, the cooler the color appearance. 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. 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 consistency refers to the average amount of variation in chromaticity among a batch of supposedly identical lamp samples. Generally speaking, the more complicated the physics and chemistry of the light source, the more difficult it is to manufacture with consistent color properties. This is why consistency is a problem for discharge light sources, particularly metal halide. Different samples from the same batch of metal halide lamps may exhibit different chromaticities. To limit this variation, the lighting industry uses a color consistency system based on MacAdam ellipses (Wyszecki and Stiles 1982).

People frequently exhibit some error when attempting to match the colors of two lights. This implies that two lights of slightly, but measurably, different chromaticities may not be detected as different by everyone. A number of researchers, most notably D.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). MacAdam also argued that these JNDs form an elliptical pattern of "constant discriminability" in chromaticity space, centered on the chromaticity of a reference light. Thus, a human observer should not reliably detect a color difference between a reference light having a chromaticity at the center of the ellipse and any other light within the elliptical pattern of constant discriminability. Because the elliptical pattern was defined in terms of three standard deviations when making color matches it has been termed the 3-step MacAdam ellipse.

For reasons that may include variations in manufacturing processes over time and limitations in human perception in practical applications compared to a laboratory setting, industry standards for color consistency are more liberal than the 3-step MacAdam ellipse. The American National Standards Institute (ANSI) has formalized six, 4-step MacAdam ellipses, centered on specific chromaticity coordinates, for T8, T10, T12 and some CFL fluorescent lamps (ANSI 2000); these are shown in Figure 9. The International Electrotechnical Commission (IEC) standard (IEC 2002) specifies six, 5-step MacAdam ellipses as color consistency criteria for double-capped fluorescent lamps. According to these standards the chromaticity of fluorescent lamps of a stated CCT must also fall within the color consistency criterion associated with that CCT. Requiring a color consistency criterion together with the stated CCT eliminates the problem of lamps with identical CCT values having vastly different color appearances, such as the lamps illustrated by Points A and B in Figure 8 ( discussed in "What is correlated color temperature?").

Figure 9. ANSI specifications on the chromaticity tolerance for fluorescent lamps

Loose adherences to CCT and color consistency criteria currently plague the compact fluorescent lamp (CFL) market in North America. Figure 10 shows the chromaticity coordinates of many CFLs that are claimed to meet the requirements of the ENERGY STAR^® program from several different manufacturers. It also shows the ANSI 2700 K 4-step MacAdam ellipse and the IEC 2700 K 5-step MacAdam ellipse. To meet the ENERGY STAR specification, the CFLs' CCT must be between 2700 K and 3000 K. If it is not, the lamp packaging must clearly designate the color temperature and color tone of the product in terms of "cool" or "warm" (ENERGY STAR 2003). The chromaticities of many CFLs in Figure 10 lie outside the ANSI and IEC color consistency criterion (ANSI 2001; IEC 2002). This results in noticeable differences in the color of light emitted by these lamps. Their scatter exceeds even the more lenient ENERGY STAR color consistency criterion, defined by the region between the 3000 K and 2700 K CCT isotemperature lines.

Figure 10. Chromaticity coordinates of 375 CFLs (meeting ENERGY STAR program requirements) from different manufacturers

Three factors create the variation in chromaticities shown in Figure 10. Some manufacturers have different target points within the ENERGY STAR zone, so there may be perceived color variation in the lamps produced by any two manufacturers. A given manufacturer may also have different target points for different lamp wattages. For example, the same manufacturer may have a 13 W lamp that produces a noticeable color difference from its 26 W lamp with the same CCT rating. Finally, some manufacturers have difficulty controlling the color of the lamps they produce, so different lamps of the same wattage from the same manufacturer may produce noticeably different colors of light.