Two Ways to Look at Light
There are two systems of measurement commonly used to describe the color properties of a light source: “color temperature,” which expresses the color appearance of the light itself, and “color rendering index” (CRI), which suggests how an object illuminated by that light will appear in relation to its appearance under other common light sources. Both can be extremely valuable in evaluating and specifying light sources, but it is important to understand their limitations.
Color Temperature–the Appearance of Light
The color temperature of a light source is a numerical measurement of its color appearance. It is based on the principle that any object will emit light if it is heated to a high enough temperature, and that the color of that light will shift in a predictable manner as the temperature is increased. The system is based on the color changes of a theoretical “blackbody radiator” as it is heated from a cold black to a white hot state.
With increased temperature, the blackbody would shift gradually from red to orange to yellow to white and, finally, to blue white. A light source’s color temperature, then, is the temperature, measured in degrees kelvin, expressed in kelvin (K), at which the color of the blackbody would exactly match the color of the light source. For many light sources an exact match cannot be achieved. In such cases, the closest possible match is made, and the color is described as correlated color temperature. An OCTRON® T8 fluorescent lamp with a color temperature rating of 4100K, for example, has a color appearance similar to that of a blackbody heated to 4,100 kelvin (3827°Celsius, 6920° Fahrenheit).
Warm vs. Cool–the Psychology of Light
Some people find it confusing that low color temperature light sources are called “warm” while those with higher temperatures are referred to as “cool.” In fact, these descriptions have nothing to do with the temperature of the blackbody radiator but refer to the way color groups are perceived—the psychological impact of lighting. Colors and light sources from the blue end of the spectrum are referred to as cool, and those toward the red/ orange/yellow side of the spectrum are described as warm.
How Light Affects the Colors of Objects
Color rendering index (CRI) is a system derived from visual experiments. It assesses the impact of different light sources on the perceived color of objects and surfaces. The first step is to determine the color temperature of the light source being rated. Next, each of eight standard color samples is illuminated—first by the light source and then by a light from a blackbody matched to the same color temperature. If none of the samples changes in color appearance, the light source is given a CRI rating of 100. Any changes in color appearance which do occur result in a lower rating. The CRI decreases as the average change in the color appearance of the eight samples increases. Any CRI rating of 80 or above is normally considered high and indicates that the source has good color properties.
Color Temperature and CRI–Useful References
Color temperature and CRI provide some helpful information, but they are not perfect. Color temperature, for instance, fails to indicate anything about how a given light source will render colors. For example, imagine two “cool” light sources with similar color temperatures and color appearances. Suppose light source A produces fairly uniform energy, Suppose light source B, which looks the same, produces a similar spectrum except with almost no light in the red. Red objects which appear natural under light source A will therefore look dull and colorless under light source B even though both lights have the same color temperature.
In general, a high CRI figure means a light source will render colors well. However, since CRI figures are calculated for light sources of a specific color temperature, it is not valid to compare a 2700K, 82 CRI light source to one of 3500K, 85 CRI. In addition, remember that CRI is an average of eight different colors. This means that a light source with a high CRI will tend to render the broad range of colors well, but it is not a guarantee that any specific color will appear natural. Used in conjunction, however, color temperature and CRI can provide excellent benchmarks for the comparison of light sources.