As the world transitions away from ordinary incandescent lighting and toward more energy-efficient light sources, the topic of light-source color temperature has suddenly become more important. Engineers and product designers who once took color temperature for granted now find they must educate themselves about how people perceive different color temperatures and what kind of effects to expect from various lighting technologies.
Fortunately, a little knowledge about color temperature can go a long way when designing or using illumination products. Light sources have two important color specifications: Correlated Color Temperature (CCT) and Color Rendering Index (CRI).
CCT is the temperature (in degrees Kelvin) of a blackbody radiator that would most nearly approximate the color of the light source. As a blackbody (or a steel bar) is heated to higher temperatures, it begins to glow: first reddish-orange at 800°K, then orange at 2,000°K, orangeish-white at 3,000°K (the temperature of an incandescent lamp filament), yellowish-white at 4,000°K, white at 5,200°K, and bluish-white at 6,000°K and above. The CCT of a fluorescent or LED lamp refers only to the color of the lamp light, not to the lamp temperature, which is much lower.
Lamp manufacturers produce both 5,000°K and 5,500°K lamps, and the difference in color appearance between them is negligible; all lamps between 5,000°K and 5,500°K appear white.
For rooms that lack any incandescent lighting, lighting experts recommend using lamps that produce white light (CCT of 5,000°K to 5,500°K). For rooms lit with some incandescent lighting, the best compromise between white and the color of incandescent is a CCT of around 4,000°K.
The best way to whiten the light in a room that is lit by 3,000°K fluorescent lamps is to replace all the 3,000°K lamps with 5,000°K lamps at the same time. However, facilities that spot-replace lamps as they burn out would be better off using 4,000°K lamps as replacements until all the 3,000°K lamps are gone. After that, replace burned-out 4,000°K lamps with 5,000°K lamps. This will minimize the color differences between lamps.
To produce a full-spectrum fluorescent lamp, manufacturers combine several phosphors in the right proportion so light emits fairly equally in all parts of the visible spectrum. This results in a white light with a high CRI.
The CRI is a number, expressed as a percentage, that indicates the ability of a light source to show different colors accurately compared with a standard source. The higher the CRI, the better colors look under the light. Warm White, Cool White, and Daylight are old technology and have CRIs of around 60. Full-spectrum lamps have the best CRIs, in the 90s. High-CRI compact fluorescent lamps (CFLs) are less efficient than lower-CRI lamps, but they are still twice as efficient as incandescents.
Neutral white light CCT
Sometimes a source of neutral white light would be preferable to a source of higher or lower CCT. However, the CCT of neutral white light is not widely publicized. Consequently, those who select light sources might prefer neutral white light, but not know what CCT to select.
Neutral white light is white light that has no color tint. It can be full-spectrum light, consisting of equal amounts of light from all parts of the visible spectrum, or it can comprise two or more different wavelengths of light that together appear as neutral white.
One can figure out the CCT of neutral white light from widely available reference material. On the 1931 CIE Chromaticity Diagram, pictured in Fig. 4-4 of the IESNA Lighting Handbook, full-spectrum white light is the point x = y = 0.333, and in Fig. 4-4 is labeled “Equal energy.” [Rea 2000]
The color of a blackbody radiator as a function of temperature, called a planckian locus, is represented in Figure 4-4 in the Handbook by the curved line that starts in the Red corner, passes through the White area in the middle, and ends in the Bluish-White area at ∞.
To derive the CCT of neutral-white light, first note that Figure 4-13 of the IESNA Lighting Handbook shows lines of constant CCT in the central area of the Chromaticity Diagram. So it is evident that full-spectrum white light has a CCT, but what exactly is that CCT?
Starting with the IES Lighting Handbook, 5th edition, the section “Color Temperature” on p 5-11 states that a blackbody radiator would be “white (neutral) at about 5000°K.” [Kaufman 1972] Also, several fluorescent lamp manufacturers produce full-spectrum white lamps that have CCTs ranging from 5000 to 5500°K. The color differences of these full-spectrum lamps are slight. All CCTs in this range appear to be quite close to neutral white.
However, the CCT of full-spectrum white light can be narrowed from this range. The blackbody temperature that would be closest to full-spectrum white light would be the one having the flattest planckian distribution curve across the visible spectrum, 400 - 700 nm. Figure 4-14 of the IESNA Lighting Handbook, which plots a family of such planckian distribution curves, shows that the flattest curve would be between the 5000 and 6000°K curves. Interpolating between these two curves shows that the flattest curve would be at about 5200°K.
From p G-38 of the IESNA Lighting Handbook, the CCT of the flattest planckian curve can be more accurately determined by using the first principal corollary of the Wien displacement law,
λmT = b,
where λm is the wavelength at which the maximum spectral radiance occurs, µ; T is the absolute temperature of the blackbody, K; and b is 2.8978 × 10-3 m × K. To obtain the flattest curve across the visible spectrum, set λm = 0.555 µ, the wavelength of maximum photopic sensitivity. The curve is flat at 0.555 µ and tapers off slightly towards the red and violet ends. Then solve for T.
T = b/λm
This gives T = (2897.8/0.555)°K
Therefore, T = 5220°K (to three significant figures). This is the CCT of full-spectrum white light.
Two light sources that have different spectrums, but the same color appearance, have the same chromaticity diagram x and y coordinates, and the same CCT. Consequently, the CCT of any non-full-spectrum neutral white light would also be 5220°K. For most situations, two significant figures would be sufficient, giving T = 5200°K.
The line on the chromaticity diagram that designates a CCT of 5200°K extends from a greenish-white through neutral white to a purplish-white. Consequently, specifying a CCT of 5200°K by itself does not ensure the light will be neutral white. However, also specifying that the light be on the black body curve does ensure the light will be neutral white.
In a room lit by lamps of a CCT other than neutral white, chromatic adaptation (basically, the way humans perceive an object to have the same color despite being lit by different-colored light sources) undercompensates somewhat for the color of the lamps. Therefore, chromatic adaptation is not complete; it is only partial, and the perceived colors of objects are affected. For example, under a 3000°K lamp, blue objects appear as navy; whereas under a 5000°K lamp, blue objects appear a vivid blue. Consequently, the CCT of neutral white light remains an important reference point in spite of chromatic adaptation.
Kaufman J, editor. 1972. The IES Lighting Handbook. 5th ed. New York: The Illuminating Engineering Society.
Rea M, editor. 2000. The IESNA Lighting Handbook. 9th ed. New York: The Illuminating Engineering Society of North America.