Color temperature
Color temperature is a parameter describing the color of a visible light source by comparing it to the color of light emitted by an idealized opaque, non-reflective body. The temperature of the ideal emitter that matches the color most closely is defined as the color temperature of the original visible light source. The color temperature scale describes only the color of light emitted by a light source, which may actually be at a different temperature.
Color temperature has applications in lighting, photography, videography, publishing, manufacturing, astrophysics, and other fields. In practice, color temperature is most meaningful for light sources that correspond somewhat closely to the color of some black body, i.e., light in a range going from red to orange to yellow to white to bluish white. Although the concept of correlated color temperature extends the definition to any visible light, the color temperature of a green or a purple light rarely is useful information. Color temperature is conventionally expressed in kelvins, using the symbol K, which are units for absolute temperature.
This is distinct from how color temperatures over 5000 K are called "cool colors", while lower color temperatures are called "warm colors", exactly the opposite of black-body radiation. "Warm" and "cool" in this context is with respect to a traditional aesthetic association of color to warmth or coolness, not a reference to physical black body temperature. By the hue-heat hypothesis, low color temperatures psychologically evoke warmth, while high color temperatures evoke coolness. The spectral peak of warm-colored light is closer to infrared, and most natural warm-colored light sources emit significant infrared radiation. The fact that "warm" lighting in this sense actually has a "cooler" color temperature often leads to confusion.
Categorizing different lighting
The color temperature of the electromagnetic radiation emitted from an ideal black body is defined as its surface temperature in kelvins, or alternatively in micro reciprocal degrees. This permits the definition of a standard by which light sources are compared.To the extent that a hot surface emits thermal radiation but is not an ideal black-body radiator, the color temperature of the light is not the actual temperature of the surface. An incandescent lamp's light is thermal radiation, and the bulb approximates an ideal black-body radiator, so its color temperature is essentially the temperature of the filament. Thus a relatively low temperature emits a dull red and a high temperature emits the almost white of the traditional incandescent light bulb. Metal workers are able to judge the temperature of hot metals by their color, from dark red to orange-white and then white.
Many other light sources, such as fluorescent lamps, or light emitting diodes emit light primarily by processes other than thermal radiation. This means that the emitted radiation does not follow the form of a black-body spectrum. These sources are assigned what is known as a correlated color temperature. CCT is the color temperature of a black-body radiator which to human color perception most closely matches the light from the lamp. Because such an approximation is not required for incandescent light, the CCT for an incandescent light is simply its unadjusted temperature, derived from comparison to a black-body radiator.
The Sun
The Sun closely approximates a black-body radiator. The effective temperature, defined by the total radiative power per square unit, is 5,772 K. The color temperature of sunlight above the atmosphere is about 5,900 K.The Sun may appear red, orange, yellow, or white from Earth, depending on its position in the sky. The changing color of the Sun over the course of the day is mainly a result of the scattering of sunlight and is not due to changes in black-body radiation. Rayleigh scattering of sunlight by Earth's atmosphere causes the blue color of the sky, which tends to scatter blue light more than red light.
Some daylight in the early morning and late afternoon has a lower color temperature due to increased scattering of shorter-wavelength sunlight by atmospheric particulates – an optical phenomenon called the Tyndall effect.
Daylight has a spectrum similar to that of a black body with a correlated color temperature of 6,500 K or 5,500 K.
For colors based on black-body theory, blue occurs at higher temperatures, whereas red occurs at lower temperatures. This is the opposite of the cultural associations attributed to colors, in which "red" is "hot", and "blue" is "cold".
Applications
Lighting
For lighting building interiors, it is often important to take into account the color temperature of illumination. A warmer light is often used in public areas to promote relaxation, while a cooler light is used to enhance concentration, for example in schools and offices.CCT dimming for LED technology is regarded as a difficult task, since binning, age and temperature drift effects of LEDs change the actual color value output. Here feedback loop systems are used, for example with color sensors, to actively monitor and control the color output of multiple color mixing LEDs.
Aquaculture
In fishkeeping, color temperature has different functions and foci in the various branches.- In freshwater aquaria, color temperature is generally of concern only for producing a more attractive display. Lights tend to be designed to produce an attractive spectrum, sometimes with secondary attention paid to keeping the plants in the aquaria alive.
- In a saltwater/reef aquarium, color temperature is an essential part of tank health. Within about 400 to 3000 nanometers, light of shorter wavelength can penetrate deeper into water than longer wavelengths, providing essential energy sources to the algae hosted in coral. This is equivalent to an increase of color temperature with water depth in this spectral range. Because coral typically live in shallow water and receive intense, direct tropical sunlight, the focus was once on simulating this situation with 6500 K lights.
Digital photography
Photographic film
Photographic emulsion film does not respond to lighting color identically to the human retina or visual perception. An object that appears to the observer to be white may turn out to be very blue or orange in a photograph. The color balance may need to be corrected during printing to achieve a neutral color print. The extent of this correction is limited since color film normally has three layers sensitive to different colors and when used under the "wrong" light source, every layer may not respond proportionally, giving odd color casts in the shadows, although the mid-tones may have been correctly white-balanced under the enlarger. Light sources with discontinuous spectra, such as fluorescent tubes, cannot be fully corrected in printing either, since one of the layers may barely have recorded an image at all.Photographic film is made for specific light sources, and, used properly, will create a neutral color print. Matching the sensitivity of the film to the color temperature of the light source is one way to balance color. If tungsten film is used indoors with incandescent lamps, the yellowish-orange light of the tungsten incandescent lamps will appear as white in the photograph. Color negative film is almost always daylight-balanced, since it is assumed that color can be adjusted in printing. Color transparency film, being the final artefact in the process, has to be matched to the light source or filters must be used to correct color.
Filters on a camera lens, or color gels over the light source may be used to correct color balance. When shooting with a bluish light source such as on an overcast day, in the shade, in window light, or if using tungsten film with white or blue light, a yellowish-orange filter will correct this. For shooting with daylight film under warmer light sources such as sunsets, candlelight or tungsten lighting, a bluish filter may be used. More-subtle filters are needed to correct for the difference between, say 3200 K and 3400 K tungsten lamps or to correct for the slightly blue cast of some flash tubes, which may be 6000 K.
If there is more than one light source with varied color temperatures, one way to balance the color is to use daylight film and place color-correcting gel filters over each light source.
Photographers sometimes use color temperature meters. These are usually designed to read only two regions along the visible spectrum ; more expensive ones read three regions. However, they are ineffective with sources such as fluorescent or discharge lamps, whose light varies in color and may be harder to correct for. Because this light is often greenish, a magenta filter may correct it. More sophisticated colorimetry tools can be used if such meters are lacking.
Desktop publishing
In the desktop publishing industry, it is important to know a monitor's color temperature. Color matching software, such as Apple's ColorSync Utility for MacOS, measures a monitor's color temperature and then adjusts its settings accordingly. This enables on-screen color to more closely match printed color. Common monitor color temperatures, along with matching standard illuminants in parentheses, are as follows:- 5000 K
- 5500 K
- 6500 K
- 7500 K
- 9300 K
Digital cameras, web graphics, DVDs, etc., are normally designed for a 6500 K color temperature. The sRGB standard commonly used for images on the Internet stipulates a 6500 K display white point.
Microsoft Windows prior to Windows 10 are use sRGB as default display color space, and use 6500 K as default display color temperature. Windows 10 1607 have supports for high dynamic range. Windows 11 22H2 have supports for Auto Color Management which further optimized for OLED monitors by reading EDID data.
Apple iOS, iPadOS and macOS use sRGB and DCI-P3 as default display color spaces.