Both natural and artificial lighting have a profound impact on how we perceive color. This course will provide the reader with a foundational understanding of how we perceive light and color and of the three characteristics—correlated color temperature, color rendering index, and spectral power distribution—which describe the quality and color of light. This course will also include a brief overview on the composition of paint and colorants, and explain how qualities such as sheen impact how the selected color is perceived in the space. Finally, this course will explore how light creates more than just visual effects; it also has biological and psychological effects that can impact the health and wellbeing of occupants. Included in this content will be research on design psychology as a basis for making informed color choices for clients, in addition to writing paint specifications that help avoid color discrepancies and confusion.
Photo courtesy of Benjamin Moore & Co.
The type of light that illuminates a room or surface greatly impacts how we perceive the color.
THE SCIENCE OF LIGHT
It might seem an obvious statement, but without light, there would be no color. Less obvious, perhaps, is the impact that certain types of light have on the colors we perceive. Think about how a human face appears when illuminated by early morning light. No matter what the color of skin, it will appear warm and soft. That same face in the midday sun will be marked with harsh, contrasting shadows, and the skin tones will appear “cooler.” Now imagine how that same face appears when bathed in candlelight versus a conventional fluorescent fixture. While candlelight is kind to most of us, under the fluorescent fixture, human skin often takes on a sallow, greenish cast.
The colors we perceive are determined in part by the quality and quantity of the light that illuminates them, whether natural, artificial, or a combination of both.
For millennia, humans relied solely on the sun or firelight. Today, of course, artificial lighting illuminates our world, day and night. We lived with the incandescent, or Edison bulb, for over 100 years. Today, several types of artificial lighting systems, each with its unique characteristics and impacts on color, illuminate our buildings. Light-emitting diodes, or LEDs, are quickly replacing other types of lighting in both new and existing buildings.
Because light has such a profound impact on color perception, design professionals should have a foundational understanding of how lighting choices will impact paint color and finish selections. To do so, we must first look at the nature of light itself.
The Electromagnetic Spectrum
Light is composed of many wavelengths of energy. The full range of these frequencies is called the electromagnetic spectrum. Only a small portion of these wavelengths are visible to the eye. This narrow band is known as the visible light spectrum, and it includes all the colors of the rainbow.
Photo courtesy of Benjamin Moore & Co.
Additive mixing (left) involves the mixing of light where the resulting color will become lighter and eventually white. Subtractive mixing (right) involves the mixing of substances and results in a darkening of colors.
The visible light spectrum includes wavelengths ranging from 780 nanometers down to 390 nanometers (nm). For comparison, UV light ranges from 10 to 400 nm, and X-rays are shorter still. Specific wavelengths correspond to specific colors. Reds have the longest wavelength and range from 650 to 700 nm. “Cooler” colors have shorter wavelengths. The violets have the shortest wavelengths of all and range from 390 to 430 nm.
Warmer, longer wavelengths are easier to see in dim light, while cooler colors tend to disappear. This concept is important to remember when we discuss the impact of light on paint color.
Two Ways of Making Colors with Light
When a beam of light is projected onto an object, the object will absorb some wavelengths while reflecting others. For instance, a red object absorbs all but the red wavelengths. However, it’s not always that simple. For example, an object may appear yellow if the object absorbs blue light but reflects green and red light, because when these wavelengths mix, they create yellow.
You can create different colors by mixing light in what are called the “primary wavelengths” of red, green, and blue. This is called additive color mixing.
Mixing red and green produces yellow, while mixing red and blue yields magenta, and mixing green and blue yields cyan. Mixing all three primaries creates white. Color televisions, computers, and other displays rely on additive color mixing to create the full range of colors.
Another way to create colors is through subtractive mixing. You likely recall the thrill of using tempera paints to make new colors—for instance, mixing blue and yellow to create green. This is subtractive color mixing. Most materials, including dyes, paints, colorants, and inks, rely on this process.
Most design professionals are familiar with the color wheel. Based on the primaries of red, yellow, and blue, this tool uses subtractive color mixing to create a plethora of secondary and tertiary colors. White and black can be added to create a nearly limitless number of tints and hues. The printing process, which relies on inks, uses slightly different primaries of cyan, magenta, and yellow, but this is still a subtractive process which relies on the mixing of substances, not on the mixing of light.
Photo courtesy of Benjamin Moore & Co.
How the Eye (and Brain) See
Whether color is created through additive or subtractive color mixing, the way we see and process color is the same. To better understand the relationship between color and light, let us first recall the basic biology of the human eye and how the brain interprets information received by it.
The eye works very much like a camera. A transparent cornea allows light to enter the eye, and the iris controls the amount of light entering through the pupil. The cornea and lens work together to focus the image on the retina, which is located at the back of the eye.
The retina—comparable to the film, sticking with the camera analogy—is lined with light-sensitive receptors called rods and cones. These photoreceptors convert light into electrical signals, which are transmitted to the brain via the optic nerve. The visual cortex of the brain converts the image impulses into objects.
The human eye has far more rods than cones—about 125 million rods to 7 million cones. Each type of receptor plays a distinctive role in vision. (The retina actually contains a third type of receptor, which does not play a role in vision, but which is essential for regulating the body’s circadian rhythms.)
Rods primarily perceive light and dark, and they are more sensitive to dim light than cones. Hence, rods are especially important for night vision. Rods are far more sensitive than cones, and better at detecting motion.
Cones detect colored light. The human eye contains three types of cones: red, green, and blue. Each is sensitive to a different range of wavelengths. Many cones are located in the fovea, a small pit in the back of the eye that helps us perceive sharpness and detail.
The color of the objects we see is largely due to the way the objects interact with light, and how that light is ultimately reflected or transmitted to our eyes. Using the example of the yellow object above, if an object transmits green and red wavelengths, the green and red cones are stimulated and we see the object as yellow.
Photo courtesy of Benjamin Moore & Co.
Describing 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’s take a close look at each of these concepts.
Photo courtesy of Benjamin Moore & Co.
Correlated Color Temperature (CCT)
The Correlated Color Temperature (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.
Correlated Color Temperature, sometimes just called color temperature, 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 warmth and coolness are subjective terms which describe how we experience light. This can be confusing, as warmer light has a lower CCT than cooler light!
Light Throughout the Day and Throughout the 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 10,000K. 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 Color Rendering Index, or 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. Correlated color temperature and CRI are two distinct measurements. Two light sources can have an identical color temperature 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’s consider the Spectral Power Distribution of a given light source. A Spectral Power Distribution 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 color. 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’s 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 don’t render reds very well.
Photo courtesy of Benjamin Moore & Co.
Limitations of CCT and CRI
Color temperature and CRI provide some helpful information, but they are not perfect. Color temperature, 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 color temperature 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 color temperatures 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, color temperature and CRI can provide excellent benchmarks for the comparison of light sources.
ARTIFICIAL LIGHTING AND COLOR
Depending on the type and age of a building, you may encounter a range of artificial lighting systems, from incandescent and halogen to fluorescent and LEDs. Often buildings will include a combination of lighting types and will supplement artificial lighting with daylighting.
You likely understand intuitively that certain colors look more appealing when illuminated by certain types of artificial lighting compared to others. These differences can largely be explained by the differences in color temperature, color rendering index, and efficacy (efficiency) of these light sources. Understanding these characteristics can help design professionals select the ideal paint color for both residential and commercial projects. It can also help designers take into account how artificial lighting systems and color impact occupants’ well-being and productivity.
Photo courtesy of Benjamin Moore & Co.
Incandescent Lighting
The incandescent, or Edison bulb, has been with us for over 100 years, with the basic technology unchanged. These lamps produce light via a tungsten wire filament that is placed inside a glass bulb. When an electric current is passed through the filament, resistance in the filament creates heat, and the bulb glows. As we saw earlier, the spectral power distribution graph for incandescent lighting shows that there is much more energy toward the red end of the spectrum, which explains why incandescent bulbs produce such warm light.
Incandescent lighting has a CCT of about 2700 K and its CRI is 100, meaning incandescent lamps are very good at rendering color. Unfortunately, these lamps are highly inefficient. About 90 percent of the energy is lost as heat, and their life expectancy is short. Consequently, incandescent lamps are being rapidly phased out in the name of energy efficiency. The United States has set efficiency standards that, in effect, preclude the manufacture or importation of certain incandescent lamps. As of this writing, a second tier of even more stringent standards has been delayed.
