INTRODUCTION
A major lamp manufacturer once ran an ad campaign showing spectacular scenes of nature with the headline, "Sure, it's a great view. But it's the lighting that makes it work." To be able to see depends on light; in turn, what we see depends on how the environment is lighted. And seeing, as they say, is believing.

What many people do not realize is that even how we see colors depends on lighting. Perception of color depends on three things: the human eye, the electromagnetic composition of ambient light, and the chemical composition of the object being lighted. For this reason, that shirt we buy at the store may look a different shade of color at home.

Designers must understand lighting's relationship to color to specify lighting systems that meet the space's design goals. In this article, we will discuss how light is made up of colors, the metrics used to express the color characteristics of light sources, and tips on integrating lighting and color in interior applications.

Color & Light: Basic Principles
Visible light is represented on the electromagnetic spectrum as a narrow band of energy wavelengths. In turn, this narrow band of wavelengths, which together comprise "white light," is comprised of a spectrum of colors, from violet at 380 nanometers to red at 620-760 nanometers (see Figure 1). We can see this principle in action in the rainbow, in which sunlight is refracted by droplets of moisture in the air; we can also shine a beam of white light through a glass prism to make a rainbow of colors appear on the other side (see Figure 2).

Figure 1.  Light, a portion of the electromagnetic spectrum, in turn is made up of colors. Courtesy: Architectural Lighting Magazine

Figure 2.  Isaac Newton determined that light is made up of colors by refracting light through a glass prism to produce the color spectrum. Courtesy: Architectural Lighting Magazine

Humans cannot see "visible light," however. Even if we switch on a flashlight in a dark space, the beam of light we see is actually light reflected from millions of dust particles floating in the air. Light is simply the vehicle through which we can see objects; therefore, we see objects only when light is reflected or emitted from (or through) them.

How We See Color
All objects are chemically oriented to absorb certain wavelengths of light and reflect others. The wavelengths that are reflected combine to produce a color that is perceived by the human eye.

A red object such as a tomato struck by white visible light, for example, will absorb most of the energy except a segment of red wavelengths, which are reflected (see Figure 3). A pure black object absorbs all visible light energy, while a pure white object reflects all of it.

With lighting, we can affect how colors are perceived; creating a visual system that includes the eye, the colored object and the light source. As lighting designers, we can control the color characteristics of the light source, empowering us to create a variety of effects to meet the goals of a broad range of applications.

Figure 3.  Objects are chemically oriented to absorb certain wavelengths and reflect others, as in the case of a tomato, lettuce and butter. Courtesy: Architectural Lighting Magazine

COLOR METRICS
Metrics are used to understand, analyze and predict the performance of an object or system. To understand how a lamp's light will affect the color of objects in the space, three methods are used, including color temperature, color rendering and a tool called the light source's spectral power distribution.

Color Temperature
Color temperature, expressed on the Kelvin scale (K), is the color appearance of the lamp itself and the light it produces.

Imagine a block of steel that is steadily heated until it glows first orange, then yellow and so on until it becomes "white hot." At any time during the heating, we could measure the temperature of the metal in Kelvins (Celsius + 273) and assign that value to the color being produced. This is the theoretical foundation behind color temperature.

For incandescent lamps, the color temperature is a "true" value; for fluorescent and high-intensity discharge (HID) lamps, the value is approximate and is therefore called correlated color temperature. In the industry, "color temperature" and "correlated color temperature" are often used interchangeably. The color temperature of lamps makes them visually "warm," "neutral" or "cool" light sources. Generally speaking, the lower the temperature is, the warmer the source, and vice versa.

It's important to remember this because it seems counterintuitive -- we want to believe that bluer light sources have a low or "cooler" color temperature, and that yellow light sources have a high or "warmer" color temperature, but the exact opposite is the case.

Lamps with a lower color temperature (3500K or less) have a warm or red-yellow/orangish-white appearance. The light is saturated in red and orange wavelengths, bringing out warmer object colors such as red and orange more richly.

Lamps with a mid-range color temperature (3500K to 4100K) have a neutral or white appearance. The light is more balanced in its color wavelengths.

Lamps with a higher color temperature (4100K or higher) have a cool or bluish-white appearance.

Summer sunlight at noon on a clear day has a very cool appearance at about 5500K. The light is saturated in green and blue wavelengths, bringing out cooler object colors such as green and blue more richly. The color characteristics of daylight, however, are variable, as in the early morning and late afternoon the light is warm in color appearance. Daylight is therefore a dynamic light source whose color qualities change throughout the day.

The vast majority of electric light sources in the architectural market are considered "white" light; some are just a bit warmer or cooler than others. There are specialty lamps on the market that give off more saturated colors; the light source can be specified to emit light in a basic color. Fluorescent and incandescent lamps are available in red, blue, green and yellow (or gold). Metal halide lamps are available in blue, green, yellow and pink.

Figure 4.  Typical Color Temperature Ratings for Common Lamp Types

Figure 5.  Objects under a warm light source (left), a neutral source (middle) and a cool source (right). Courtesy: Osram Sylvania, Inc.

Color Rendering
Color rendering, expressed as a rating from 0 to 100 on the Color Rendering Index (CRI), describes how a light source makes the color of an object appear to human eyes and how well subtle variations in color shades are revealed. The higher the CRI rating is, the better its color rendering ability.

Imagine two objects, one red, one blue, which are lighted by a cool light source with a low CRI. The red object appears muted while the blue object appears a rich blue. Now take out the lamp and put in a cool light source with a high CRI. The blue object still appears a rich blue, but the red object appears more like its true color.

A common misconception is that color temperature and color rendering both describe the same properties of the lamp. Again, color temperature describes the color appearance of the light source and the light emitted from it. Color rendering describes how well the light renders colors in objects.

However, the two metrics are interconnected: To compare the CRI ratings for any two given lamps, they must have the same color temperature for the comparison to have any meaning.

Figure 6.  Typical CRI Values for Common Lamp Types

Standard incandescent lamps have a CRI rating of 100, although they are also the most inefficient. Fluorescent lamps are in the range of 52 to 95, depending on the lamp. Advances in phosphor technology have enabled fluorescent and HID lamps to advance greatly in color rendering.

A common misconception is that all fluorescent lamps are neutral or cool in color appearance and do not have very good color-rendering ability. This is largely due to the fact that historically the "cool white" fluorescent lamp was the industry standard. It has a cool cooler (4200K) and poor CRI rating (62). This is simply no longer the case. Fluorescent lamps are available that offer a warm color temperature, and numerous compact fluorescent lamps are offered with a 2700K color temperature to emulate standard incandescent lamps. Regarding color, a wide variety of fluorescent lamps (T12, T8, T5, etc.), using rare-earth tri-phosphor technology, offer superior color rendition (as high as 95) and a wide range of color temperature choices (from 2700K to 5000K and higher).

Spectral Power Distribution
Spectral power distribution shows the visible light spectrum and the wavelength composition for the light from the lamp (see Figure 7). The spikes indicate that the light is stronger in revealing certain colors. Incandescent lamps are sources that provide a continuous spectral power distribution versus a spiked or discontinuous power distribution. For fluorescent sources, phosphors are chosen to match the natural peaks of cones in the eye. Tri-phosphor lamps possess color-mixing properties (red, blue and green) to render virtually any color well.

Figure 7.  Spectral Power Distribution Curve for a 4100K fluorescent lamp with a triphosphor (red, blue, green) coating to improve color rendering. Courtesy: Osram Sylvania, Inc.

Design Considerations
Good daylighting strategies enable controlled penetration of daylight indoors. Daylight is very cool in color appearance for most of the day except during the early morning and late afternoon, when it becomes warm in appearance.

In many indoor spaces (and outdoors at night), we must rely on electric light sources to produce the light that determines how we perceive the space. Few lamps produce pure white light. For this reason, the intended effect of the most ambitious interior design scheme can be dulled if an improper light source is selected. Again, how you see it depends on how you light it. Fortunately, there are many choices and strategies available to meet the needs of almost any application, from lighting a static office to a kinetic dance club to mimicking the dynamic cycle of days and seasons at zoos for the benefits of animals.

COLOR APPLICATION
The design goals always determine the characteristics of the lighting system. However, below are general guidelines for specifying lamp color characteristics. A broad variety of options are available to produce lighting effects from the simple to the theatrical, with some applications presenting extraordinary opportunities for the lighting designer to be artistic. However, greater complexity requires greater time, budget and attention to detail; in these applications, it may be desirable to mock up the installation to test the system.

1.  Establish Desired Color Temperature:  One of the most important factors to consider is the psychological impact of various light sources. Warm light sources are generally preferred for the home, restaurants, hospitality and high-end retail applications to create a sense of warmth and comfort, while neutral and cool light sources are generally preferred for high-activity areas such as offices, schools, supermarkets and similar applications to create a sense of alertness.

A good example is to compare two restaurants -- one a fast-food restaurant and one a high-end restaurant. In the fast-food place, where the food is relatively cheap, the lighting is typically cool fluorescent, providing an environment for the function of eating, enticing us to eat our meal quickly, get up and make way for the next customer. In the high-end restaurant, where the food is more expensive and we are purchasing the experience as much as the food, the lighting will most often be warmer incandescent and/or even simple candles, creating pools of intimacy, warmth and comfort, inviting us to stay as long as we like to enjoy the experience.

Another consideration regarding color temperature is research that suggests that cooler light sources, saturated in blue wavelengths, appear to enhance visual clarity and brightness perception at lower light levels.

2.  Decide Importance of Color Rendering:  Once color temperature is established, we must decide the importance of rendering the colors of objects across the spectrum accurately. In general, in spaces that are occupied for long periods, whether it be for work or recreation, highest-color-rendering sources would be practical. The costs associated with worker productivity far outstrip the incremental costs of upgraded lighting.

Table 1. The influence of color temperature on mood and lighting applications. Courtesy: Philips Lighting Company

3.  Determine Color Scheme of Space:  One of the first environmental questions is whether daylight is available. Daylight offers dynamic color characteristics that change during the cycle of the day and season. For most of the day, daylight is a very cool light source with excellent color rendering. Daylighting's dynamic qualities, while difficult to control, are actually part of its great appeal to office workers, who generally desire a connection to the outdoors and patterned variability in an otherwise monotonous, uniform environment. Therefore, it may be advantageous to optimize daylighting in the space depending on its design goals.

The color scheme of the space also determines selection of the light source. Suppose we have a room with heavy red accents. A warmer source to reveal these reds as rich as possible may be desirable. Conversely, cooler sources work well with blues, greens and other cool colors. And again, specifying a high-color-rendering light source may be desirable to enhance the predominant color scheme but also properly render other colors in the space.

4.  Specify Color Effects:  Lighting designers can create color effects with light in three ways:

• Light sources.  A light source with the most desirable color properties (color temperature, CRI) should be specified. The light source can also be specified to emit light in a basic color. Fluorescent and incandescent lamps are available in red, blue, green and yellow (or gold). Metal halide lamps are available in blue, green, yellow and pink. Some new light sources such as LEDs can be specified in a wide range of colors.

• Filters.  For applications requiring a theatrical flair, a filter, such as a gel, dichroic filter or other medium, can be used to create static and dynamic color effects. This device filters out "unwanted" color wavelengths, enabling only desired color wavelengths to pass through. Filters can be controlled with color wheels, scrollers and sophisticated DMX systems, enabling automatic/timed (and, if desired, programmable) movement of filters in front of the light source, resulting in the dynamic change of color.

• Reflection.  White light can be reflected off of nearby surfaces that are painted a desired color; these surfaces act as a color filter, absorbing all color wavelengths except the reflected color, which is emitted into the space saturated with these wavelengths.