What is the relationship between color rendering and light levels?

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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.

In general, both light level and lamp color rendering play crucial roles in color perception. At low light levels, good color rendering is difficult regardless of the light source used. At high light levels, nearly every light source will provide some level of color rendering (excluding monochromatic lights such as low-pressure sodium). Even a light source with very low color rendering values can perform better at high light levels than a light source with high color rendering values at low light levels.

For example, high-pressure sodium lamps score poorly on all three color rendering metrics (color rendering index (CRI), gamut area (GA) and full-spectrum color index (FSCI). However, they have very high luminous efficacy. In an application where lighting power loads must be kept low, this lamp type, which can provide higher illuminance levels with less power, may provide adequate color rendering. To produce enough light to enable good color rendering, other light sources might exceed the power restrictions.

Figure 16 shows color naming accuracy for objects viewed individually under different light sources and at different light levels. In the experiment, people were asked to name the color appearance of 20 color chips in terms of their primary and secondary hues. For example, when a person was shown a violet chip, a correct answer would have been "Blue is the primary hue; red is the secondary hue." The average percentage of correct responses for all 20 color chips is shown on the vertical axis in the figure for seven light sources at four light levels (blind chance is 25%). The black arrows show one example where an incandescent lamp with a CRI of 100 performs worse at 0.1 cd/m² than a HPS lamp with a CRI of 22 at 10 cd/m² (Deng et al. 2004). This example illustrates that at high enough light levels, even a light source with a very low CRI can render colors better than a lamp with a CRI of 100 can at lower light levels.

Figure 16. Color naming accuracy of light sources at different light levels

Source: Deng et al. 2004