Describing the Color Properties of Light

Whether natural or artificial, different types of lighting possess unique characteristics that impact color. Most of these characteristics are determined by the specific wavelengths and intensities of light generated by the light source.

There are three concepts typically used to describe the color properties of light: correlated color temperature (CCT), color rendering index (CRI), and spectral power distribution (SPD) graphs. CCT is used to describe the color of a white light source, while CRI helps predict how well a light source will render colors when compared to an ideal light source, such as daylight. SPD graphs can help predict the CRI of a given light source. Let us take a close look at each of these concepts.

Correlated Color Temperature (CCT)

The CCT scale measures the color temperature of light. More technically, this scale describes the color changes that occur when a “blackbody radiator” is heated. A blackbody radiator is an ideal but hypothetical physical body that absorbs all electromagnetic radiation that strikes it.

A real-world example can help us better understand this concept. Think of the color changes metal undergoes when it is heated. It first glows red, then orange and yellow. As the temperature rises, the color shifts to “white hot” and finally to blue.

CCT describes these changes by correlating a given color—whether red or white hot—to its temperature as measured on the Kelvin scale. The Kelvin scale starts with absolute zero, or minus 273 degrees Celsius. The CCT of a candle is about 1850 K, while the CCT of light on an overcast day is 6500 K. Most artificial lighting ranges between 2500 and 5000 K.

The higher the temperature, the bluer or “cooler” the light; conversely, the lower the temperature, the “warmer” the light. Note that coolness and warmth are subjective terms that describe how we experience light. This can be confusing, as warmer light has a lower CCT than cooler light.

Daylight, illuminating this scene through a window, renders color accurately because it contains all the wavelengths of the visible spectrum. Note the very “warm” spots of candlelight, which has a CCT of about 1850 K

Light Throughout the Day and Seasons

Because daylight is made up of sunlight that is filtered through the atmosphere, the color and intensity of daylight can change significantly throughout the day, impacting how we perceive color.

Think of the “golden light” of early morning and late afternoon in contrast to the bright white light of high noon. Morning and evening light ranges between 2000 and 3000 K, while daylight ranges from 5500 to 10,000 K. For comparison, a “soft white” incandescent bulb measures 2700 K.

Clouds can also affect the quantity and quality of light that reaches our eyes. Light changes seasonally too as the sun’s path changes. This has important implications when selecting colors for rooms with different orientations or for projects located in the higher latitudes. Such projects will see more dramatic changes in the quantity and quality of light as the seasons change.

Color Rendering Index (CRI)

Another important metric used when evaluating light sources is the CRI. This standard measures the ability of an artificial white light source to render color accurately. CRI is measured on a scale of 0 to 100. The nearer to 100, the better the light source is at rendering colors accurately. A CRI above 80 is adequate for most interior spaces. Lower CRI values indicate that some colors may appear unnatural when illuminated by the lamp.

CCT and CRI are two distinct measurements. Two light sources can have an identical CCT but a different CRI, with important implications for how the light source will impact paint and other colors.

Remember, the CCT rating tells us about the color of white light emitted by the bulb. The CRI rating tells us how well that same bulb renders color. To understand the difference between CCT and CRI, think about a soft white incandescent bulb with its CCT of 2700 K. This bulb has a perfect CRI rating of 100—in fact, most incandescents have high CRI ratings of 95 or more. An LED lamp with the same CCT of 2700 K may have a much lower CRI of around 80.

To understand why, let us consider the SPD of a given light source. An SPD graph shows the power (strength) of each wavelength of light across the visible spectrum produced by a particular light source. This characteristic is important in determining how a light source renders colors.

In general, broad spectrum lighting—lighting that contains a more or less uniform quantity of all wavelengths—will provide the most uniform rendition of all colors. Daylight is considered an ideal light source because it is relatively balanced across the visible spectrum. Narrow spectrum lighting will make certain colors look monochromatic or less colorful.

The higher the light source’s CRI, the more of the spectrum it will illuminate.

Compared to other artificial light sources, incandescent lamps energize a broad range of wavelengths. Let us go back to our soft white incandescent bulb with its perfect CRI of 100. This bulb produces much more energy at the red end of the spectrum, resulting in the warm light that so many people find appealing. (On the other hand, these lamps produce very little radiant energy in the short wavelength end of the spectrum; consequently, they do not render blues very well.)

Our warm white LED, with its CCT of 2700 K and CRI of 80, produces energy in the middle of the visible spectrum. Whites are crisp and colors vivid, but they do not render reds very well.

Limitations of CCT and CRI

CCT and CRI provide some helpful information, but they are not perfect. CCT, for instance, fails to tell us anything about how a given light source renders colors. As we saw in the example above, two light sources with the same CCT can have significantly different CRI ratings, with important implications for how paint and other colors will appear when illuminated.

Another challenge is comparing light sources with different CCTs but similar CRIs. In general, a light source with a high CRI will render colors well, but this figure is based on an average and does not guarantee that a specific color will appear the way it does in daylight.

Still, when used together, CCT and CRI can provide excellent benchmarks for the comparison of light sources.