Lighting choices can affect how we perceive spaces, even the colors of objects and surfaces. As a result, lamp color characteristics are a critical factor in a wide range of spaces, from high-end retail, where merchandise must be presented as vibrant and in its true colors, to offices, where good lighting renders faces naturally and facilitates interaction. Although almost all architectural lighting uses white light, white is available in many hues and color rendering abilities, providing a palette of choices for lighting spaces.
This article describes the science of color vision, popular metrics used to evaluate and predict the color performance of lighting products, and guidelines that can support successful application.
How people see color
Visible light is energy that produces a visual sensation in the human eye, which we call sight. Light can be reflected, transmitted or absorbed. For the eye to perceive an object, visible light must fall on the object and be reflected (or transmitted) to the eye.
Visible light resides along the 380–760 nanometer band on the electromagnetic spectrum. The size of the wavelengths determines the color of the light, from violet to red. Put it all together, and you have white light. Shine white light through a prism, and you get a rainbow.
For the human eye to see an object as being a certain color, that color must be present both in the object and the light falling on it. An apple is red because its skin is chemically oriented to absorb all color wavelengths except red, which it reflects to the eye. If we were to filter red out of the light, the apple would appear gray or black.
The eye contains receptors called rods, which are receptive to quantity of light, and cones, which are receptive to colors. As light levels decrease, the rods gradually take over from the cones, and the eye’s peak luminance sensitivity shifts toward the blue end of the color spectrum. This gives us the Purkinje effect, where a rose appears as rich red against dull green leaves under noon sunlight and dark red or black with relatively bright leaves at dusk. Under extremely low light levels, such as moonlight, people become virtually color blind.
White light with a balanced color spectrum makes colors appear more natural and vibrant. However, three colors are critical ingredients of white light: red, green and blue (RGB), which can combine to form almost any other color, including white. A light source with balanced red, green and blue wavelengths should, therefore, provide good color appearance.
The presence of blue wavelengths, in particular, may affect visual clarity. Researchers at the Lawrence Berkeley National Laboratory found that tasks are easier to see under the light of high-color-rendering lamps rich in blue wavelengths, and the light is perceived as brighter. This research has led to some experimentation with combining blue-rich general lighting with low light levels as a method to save energy.
Lighting color metrics
Lighting designers rely on numerical measurements to evaluate, compare and predict performance of lighting products for a given application. The most popular color quality metrics are color temperature and color rendering index (CRI).
Color temperature describes the color appearance of the light, whether it is visually warm, neutral or cool. Imagine a block of iron heated until it begins to glow dull red, then orange, then yellow and so on until it appears bluish-white. At any time during the heating, we could measure the block’s temperature and assign the value in kelvins (K) to the corresponding color. Because incandescent lamps are blackbody radiators—i.e., they contain an element that heats until it glows—we call this measurement color temperature. Because fluorescent, light-emitting diode (LED) and high-intensity discharge (HID) lamps do not use a heated element, the value is approximated as correlated color temperature (CCT).
Light sources are generally classified as visually warm (about <3,000K), which appear yellowish-white; visually neutral (about 3,500K), which appear white; or visually cool (about >4,000K), which appear bluish-white (see Figure 1). Warm light sources are saturated in red and orange wavelengths, enriching the appearance of warmer color tones in a space (see Figure 2). Sunrise and candlelight are very warm—about 2,000–2,500K. Cool light sources are saturated in blue and green, enriching cooler color tones. Daylight is very cool—about 5,000–10,000+K.
While color temperature is useful, it does not tell us if a lamp renders colors naturally. Two lamps with a similar color temperature, for example, could render the color of an object differently due to different concentrations of color wavelengths in their output. One tool is the spectral power distribution (SPD) graph, produced by the lamp manufacturer, and it reveals relative intensities of various wavelengths for a given lamp; a balanced color output, particularly RGB, generally indicates good color rendering ability. Another tool, more popularly used due to its relative simplicity, is the lamp’s CRI rating.
CRI is calculated based on how closely a lamp renders a set of eight standard color samples compared to a reference light source with the same color temperature (see Figure 3). For lamps up to 5,000K, the reference source is a blackbody radiator with an assumed 100 CRI; for lamps more than 5,000K, it is a mathematical model of daylight. The more deviation from the reference source, the lower the CRI for the given color. The overall CRI for the lamp is the average of CRI values for each of the eight test colors. Generally, a higher CRI means the lamp is better overall at color rendering (see Figure 4). Halogen lamps and daylight have a CRI of 100 with incandescent lamps coming in a close second.
CRI has its limitations, though. It is only meaningful when comparing lamps with the same color temperature. Because it is an average of eight measurements, it does not guarantee that the given lamp will render each individual color as well as the overall rating suggests. Lamps with imbalanced SPD tend to render certain colors poorly regardless of CRI ratings. For example, despite a high CRI for the incandescent lamp, blues and greens may appear relatively dull under its light because its output is relatively deficient in these wavelengths. At extreme color temperatures below 1,360K or above 5,000K, the problem of unbalanced SPD becomes more pronounced.
Another issue that is gaining in importance is the fact that the test colors are pastels, largely leaving out an additional six test colors, four of which are pastels, notably R9, a saturated red commonly found in retail environments. As a result, lamps competing with halogen (R9 of 70+), such as ceramic metal halide, are now being offered with R9 values up to 40+, promising good color rendering of saturated reds for display lighting.
With the advent of LED lighting, this issue has taken on even more importance, as some LEDs render saturated colors very well but not pastels, resulting in a CRI rating lower than if all 14 colors were included in the test. To address the problem, the National Institute of Standards and Technology developed a new metric called the color quality scale (CQS), which is based on a new set of 15 saturated colors. CQS has been touted as more accurate than CRI, particularly in regard to LED sources, and has been endorsed by the Department of Energy. The lighting design community currently is vetting it.
Light and color by design
Lighting color choices can have psychological and physiological effects. A recent study involving 500 people tasting white Riesling wines under fluorescent lamps producing red, blue, green or white light found that the tasters generally rated the wine’s quality higher when they drank it in a room with red or blue light versus green or white. They found the test wine much sweeter and fruitier when sampled in a room lighted by fluorescent lamps with a warmer white color tone and were willing to pay more for it.
In some commercial building applications, color is not at all important, meaning it can be sacrificed for efficiency or other benefits offered by choices such as high- and low-pressure sodium. Low-pressure sodium lamps are particularly poor in terms of color; this source is actually monochromatic, rendering everything yellow, gray or black.
White light is preferred in most architectural lighting applications, and with the advent of advanced phosphors for fluorescent lamps, the choice of hues has expanded dramatically. As LED general lighting grows in adoption, building owners may also enjoy the potential to change color temperature to match an event or the typical daylight cycle (see Figure 5).
Practical and psychological considerations affect color choice. From a practical standpoint, we want the lighting to emphasize rather than dull the dominant color scheme, support color contrasts that make tasks more visible, and call out safety marking and instructions. A space with a lot of reds, browns and yellows, for example, might be best suited for warm-color lamps, while a space with blues, grays and greens might be best suited for cool-color lamps. Some applications, involving special tasks such as color matching and paint inspection, require special performance. For these tasks, 5,000K, high-CRI sources are recommended. That being said, warm or cool are again not the only choices; neutral white (about 3,500K), in fact, is most popular for public workspaces, such as offices. Generally, it is advisable to maintain a single color temperature for all lamps in the space and, in particular, avoid mixing cool and warm colors. Note that daylight does not mix well with warm light sources, and its own color quality may become distorted by the color transmission quality of the glass.
Aside from these practical considerations, for the most part, color temperature choice is psychologically motivated. In the lighting community, there is a prevailing idea that, in cooler northern climates, people tend to prefer warmer light sources, while in warmer southern climates, people prefer cooler light sources. At home, people tend to prefer warmer sources, while at the office, they prefer neutral or cool sources. Informal research conducted in the 1940s by A.A. Kruithof suggests that people are more accepting of warm color temperatures at lower light levels. Additionally, another study (Quellman and Boyce, 2002) suggests that people, regardless of race, prefer neutral white light (3,000–4,100K, with most preferring 3,500K) as rendering skin tones most pleasantly; very warm (2,850K) and very cool (5,000K) sources were least favorably viewed by study participants.
Regarding CRI, higher generally is better, with 80+ being optimal for most applications, and 90+ being desirable for high-end retail and similar applications. Note that after some retrofits from older cool-white T12 lamps to triphosphor T8 lamps, some occupants may have the impression that people look pinker than they usually do and should be educated that it is due to the superior ability of the triphosphor T8 lamps to render skin tones naturally.
When choosing color and CRI for a given lamp, recognize the ability of the lamp to offer good lamp-to-lamp consistency, maintain its color performance over time, and how dimming may affect its color performance. Standard quartz metal halide lamps, for example, experience an up to 300K color shift toward the warmer end of the color spectrum as they age, while CRI remains stable. As these lamps are replaced over time in a large system, poor lamp-to-lamp color consistency may develop unless the system is group-relamped. During dimming to 50 percent lamp power, color shifts to cool, while CRI can drop from 65 to 45. To learn more about a given product, consult with its manufacturer.
Quality is in the eye of the beholder
Ultimately, in recognition of the limitations of the ability of color temperature and CRI to truly predict how a given light source will perform within a given space, there is no substitute for seeing for oneself and developing a learned eye. As a result, it is generally recommended to obtain samples, install them in a space mockup or similar application, and observe firsthand how well the source renders colors, including skin tones. It is particularly important for evaluating LED lighting products, where low-CRI products may provide better performance than one might expect.
By understanding color and its application in light, electrical contractors can engage their customers in a conversation that goes beyond energy and help them maximize the benefit of their lighting system.
DILOUIE, L.C., a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at www.zinginc.com.