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.
What is correlated color temperature?

The spectral power distribution (SPD) of a blackbody radiator can be completely determined from its absolute, or color temperature in Kelvin (K). Correlated color temperature (CCT) is a measure of light source color appearance defined by the proximity of the light source's chromaticity coordinates to the blackbody locus, as a single number rather than the two required to specify a chromaticity. Practical light sources of different SPD but identical chromaticities will also have identical CCTs. Six isotemperature lines are plotted in the CIE 1976 chromaticity diagram in Figure 8. The CCT of a light source can be determined by extending an isotemperature line from the blackbody locus out to the chromaticity coordinates of the source. For example, Point A in figure 8 represents a light source with chromaticity coordinates of (0.24, 0.59). This point lies on the 3000 K isotemperature line, thus the light source has a CCT of 3000 K.

Figure 8. The CIE 1976 chromaticity diagram with six isotemperature lines used by manufacturers to represent light emitted by commercially available fluorescent lamps

Since it is a single number, CCT is simpler to communicate than chromaticity or SPD, leading the lighting industry to accept CCT as a shorthand means of reporting the color appearance of "white" light emitted from electric light sources. CCT values of most commercially available light sources usually range from 2700 K to 6500 K. CCT values are intended by the lighting industry to give specifiers a general indication of the apparent "warmth" or "coolness" of the light emitted by the source. According to lighting industry convention, lamps with low CCT values (2700 K to 3000 K) provide light that appears "warm," while lamps having high CCT values (4000 K to 6500 K) provide light that appears "cool." This convention may have been established because non-electric light sources with low CCTs, such as fire, connote warmth. However, this industry convention may be confusing to many people because the higher the CCT of the lamp, the "cooler" the light appears.

Another weakness of CCT is illustrated in Figure 8 by points A and B, representing two light sources with the same CCT (3000 K). Although lights A and B have exactly the same CCT they have very different chromaticities and will look very different to the eye. The light emitted by source A will look greenish-white, while the light emitted by source B will look purplish-white. To address the potential problem of lamps with the same CCT having a different color appearance, the lighting industry utilizes a color tolerance system in conjunction with CCT to specify color consistency.

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