Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, manufacturing, and other fields. The color temperature of a light source is determined by comparing its chromaticity with that of an ideal black-body radiator. The temperature (usually measured in kelvin, K) at which the heated black-body radiator matches the color of the light source is that source's color temperature; for a black body source, it is directly related to Planck's law and Wien's displacement law.
Higher color temperatures (5,000 K or more) are cool (blueish white) colors, and lower color temperatures (2,700–3,000 K) warm (yellowish white through red) colors.
Categorizing different lighting
Because it is the standard against which other light sources are compared, the color temperature of the thermal radiation from an ideal black body radiator is defined as equal to its surface temperature in kelvin, or alternatively in mired (micro-reciprocal degrees kelvin). For source other than ideal black bodies, the color temperature of the thermal radiation emitted from it may differ from its actual surface temperature. In an incandescent light bulb the light is of thermal origin and is very close to that of an ideal black-body radiator.
However, many other light sources, such as fluorescent lamps, emit light primarily by processes other than raising the temperature of a body. This means 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). 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 the comparison to a black body radiator.
The Sun
As the sun crosses the sky, it may appear to be red, orange, yellow or white depending on its position. The changing color of the sun over the course of the day is mainly a result of scattering of light, and is unrelated to black body radiation. The blue color of the sky is caused by Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red.
Daylight has a spectrum similar to that of a black body. In professions involving color reproduction, such as photography and publishing, daylight is often approximated using standard illuminant D50 or D65, as recommended by the CIE.
For colors based on the black body, blue occurs at higher temperatures, while red actually at lower, cooler, temperatures. This is the opposite of the cultural associations that colors have taken on, with "red" as "hot", and "blue" as "cold". The traditional associations come from a variety of sources, such as water and ice appearing blue, while heated metal and fire are of a reddish hue. However, the redness of these heat sources comes precisely from the fact that red is the coolest of the visible colors, the first color emitted as heat increases.
Color temperature applications
In digital photography, color temperature is sometimes used interchangeably with white balance , which allow a remapping of color values to simulate variations in ambient color temperature. Most digital cameras and RAW image software provide presets simulating specific ambient values (e.g., sunny, cloudy, tungsten, etc.) while others allow explicit entry of white balance values in degrees Kelvin. These settings vary color values along the blue–yellow axis, while some software includes additional controls (sometimes labeled tint ) adding the magenta–green axis.
Film photography
Film sometimes appears to exaggerate the color of the light, since it does not adapt to lighting color as our visual perception does. An object that appears to the eye to be white may turn out to look very blue or orange in a photograph. The color balance may need to be corrected while shooting or while printing to achieve a neutral color print.
Film is made for specific light sources (most commonly daylight film and tungsten film), 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 bulbs will appear as white (3,200 K) in the photograph.
Filters on a camera lens, or color gels over the light source(s) may also be used to correct color balance. When shooting with a bluish light (high color temperature) 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 (calibrated to 5,600 K) under warmer (low color temperature) light sources such as sunsets, candle light or tungsten lighting, a bluish (e.g., #80A) filter may be used.
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. Color temperature meters are usually designed to read only two regions along the visible spectrum (red and blue); more expensive ones read three regions (red, green, and blue). 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 it is often greenish, a magenta filter may correct it. More sophisticated colorimetry tools can be used where such meters are lacking.
Desktop publishing
In the desktop publishing industry, it is important to know your monitor’s color temperature. Color matching software, such as ColorSync will measure a monitor's color temperature and then adjust 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:
- 5,000 K (D50)
- 5,500 K (D55)
- 6,500 K (D65)
- 7,500 K (D75)
- 9,300 K.
Note: D50 is not a color temperature. It is scientific shorthand for the daylight spectrum at about 5,000 K. (Similar defintition for D55, D65 and D75.) Designations such as D50 are used to help classify color temperatures of light tables and viewing booths. When viewing a color slide at a light table, it is important that the light be balanced properly so that the colors are not shifted towards the red or blue.
Digital cameras, web graphics, DVDs, etc. are normally designed for a 6,500 K color temperature. The sRGB standard commonly used for images on the internet stipulates (among other things) a 6,500 K display whitepoint.
TV, video, and digital still cameras
The NTSC and PAL TV norms call for a compliant TV screen to display an electrically black and white signal (minimal color saturation) at a color temperature of 6,500 K. On many actual sets, however, especially older or lower-quality units, there is a very noticeable deviation from this requirement.
Most video and digital still cameras can adjust for color temperature by zooming into a white or neutral colored object and setting the manual "white balance" (telling the camera that "this object is white"); the camera then shows true white as white and adjusts all the other colors accordingly. White-balancing is necessary especially when indoors under fluorescent lighting and when moving the camera from one lighting situation to another. Most cameras also have an automatic white balance function that attempts to determine the color of the light and correct accordingly. While these settings were once unreliable, they are much improved in today's digital cameras, and will produce an accurate white balance in a wide variety of lighting situations.
Artistic application via control of color temperature
Experimentation with color temperature is obvious in many Stanley Kubrick films; for instance in Eyes Wide Shut the light coming in from a window was almost always conspicuously blue, whereas the light from lamps on end tables was fairly orange. Indoor lights typically give off a yellow hue; fluorescent and natural lighting tends to be more blue.
Video camera operators can also white-balance objects which aren't white, downplaying the color of the object used for white-balancing. For instance, they can bring more warmth into a picture by white-balancing off something light blue, such as faded blue denim; in this way white-balancing can serve in place of a filter or lighting gel when those aren't available.
Cinematographers do not "white balance" in the same way as video camera operators; they can use techniques such as filters, choice of film stock, pre-flashing, and after shooting, color grading
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