Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, 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 kelvins (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 (5000 K or more) are "cool" (green–blue) colors, and lower color temperatures (2700–3000 K) "warm" (yellow–red) colors.
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).[1] 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 sunAs 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 not caused by black-body radiation, but rather to
Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red. This phenomenon has nothing to do with the properties of a
black body.
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 is the "hotter" color, while red is actually the "cooler" color. 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.
In photography and image processing, color balance is the global adjustment of the intensities of the colors (typically red, green, and blue primary colors). An
important goal of this adjustment is to render specific colors – particularly neutral colors – correctly; hence, the general method is sometimes called gray balance,
neutral balance, or white balance. Color balance changes the overall mixture of colors in an image and is used for color correction; generalized versions of color
balance are used to get colors other than neutrals to also appear correct or pleasing.
Image data acquired by sensors – either film or electronic image sensors – must be transformed from the acquired values to new values that are appropriate for color
reproduction or display. Several aspects of the acquisition and display process make such color correction essential – including the fact that the acquisition sensors
do not match the sensors in the human eye, that the properties of the display medium must be accounted for, and that the ambient viewing conditions of the acquisition
differ from the display viewing conditions.
The color balance operations in popular image editing applications usually operate directly on the red, green, and blue channel pixel values, without respect to
any color sensing or reproduction model. In shooting film, color balance is typically achieved by using color correction filters over the lights or on the camera lens
White Balance
Most digital cameras have a means to select a color correction based on the type of scene illumination, using either manual illuminant selection, or automatic white
balance (AWB), or custom white balance. The algorithm that performs this analysis performs generalized color balancing, known as illuminant adaptation or chromatic
adaptation.
Many methods are used to achieve color balancing. Setting a button on a camera is a way for the user to indicate to the processor the nature of the scene lighting.
Another option on some cameras is a button which one may press when the camera is pointed at a white card or other neutral object. This "custom white balance" step
captures an image of the ambient light, and this information is helpful in controlling color balance.
There is a large literature on how one might estimate the ambient illumination from the camera data and then use this information to transform the image data. A variety
of algorithms have been proposed, and the quality of these have been debated. A few examples and examination of the references therein will lead the reader to many
others. Examples are Retinex, an artificial neural network or a Bayesian method.
temperature of a light source is determined by comparing its chromaticity with that of an ideal black-body radiator. The temperature (usually measured in kelvins (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 (5000 K or more) are "cool" (green–blue) colors, and lower color temperatures (2700–3000 K) "warm" (yellow–red) colors.
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).[1] 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 sunAs 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 not caused by black-body radiation, but rather to
Rayleigh scattering of the sunlight from the atmosphere, which tends to scatter blue light more than red. This phenomenon has nothing to do with the properties of a
black body.
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 is the "hotter" color, while red is actually the "cooler" color. 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.
In photography and image processing, color balance is the global adjustment of the intensities of the colors (typically red, green, and blue primary colors). An
important goal of this adjustment is to render specific colors – particularly neutral colors – correctly; hence, the general method is sometimes called gray balance,
neutral balance, or white balance. Color balance changes the overall mixture of colors in an image and is used for color correction; generalized versions of color
balance are used to get colors other than neutrals to also appear correct or pleasing.
Image data acquired by sensors – either film or electronic image sensors – must be transformed from the acquired values to new values that are appropriate for color
reproduction or display. Several aspects of the acquisition and display process make such color correction essential – including the fact that the acquisition sensors
do not match the sensors in the human eye, that the properties of the display medium must be accounted for, and that the ambient viewing conditions of the acquisition
differ from the display viewing conditions.
The color balance operations in popular image editing applications usually operate directly on the red, green, and blue channel pixel values, without respect to
any color sensing or reproduction model. In shooting film, color balance is typically achieved by using color correction filters over the lights or on the camera lens
White Balance
Most digital cameras have a means to select a color correction based on the type of scene illumination, using either manual illuminant selection, or automatic white
balance (AWB), or custom white balance. The algorithm that performs this analysis performs generalized color balancing, known as illuminant adaptation or chromatic
adaptation.
Many methods are used to achieve color balancing. Setting a button on a camera is a way for the user to indicate to the processor the nature of the scene lighting.
Another option on some cameras is a button which one may press when the camera is pointed at a white card or other neutral object. This "custom white balance" step
captures an image of the ambient light, and this information is helpful in controlling color balance.
There is a large literature on how one might estimate the ambient illumination from the camera data and then use this information to transform the image data. A variety
of algorithms have been proposed, and the quality of these have been debated. A few examples and examination of the references therein will lead the reader to many
others. Examples are Retinex, an artificial neural network or a Bayesian method.
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