Color temperature

"White light" is commonly described by its color temperature. A traditional incandescent light source's color temperature is determined by comparing its hue with an increasingly heated black-body radiator. When the hue of the lamp matches the black-body radiator then that lamp is rated with the temperature in kelvins when the black-body radiator matches that lamp.


Categorizing different lighting

The color temperature of a black-body radiator is the actual surface temperature. All other lights, since they are but comparisons to a black-body radiator, should be referred to as having a correlated color temperature (CCT). Incandescent lighting can be described by its CCT. The CCTs of incandescent light sources are determined by comparing the light's color appearance to that of a heated black-body radiator. As the black-body is heated, eventually at a certain point the lamp should match the black-body radiator in color.Whatever the black-body radiator temperature is at that moment is the CCT of this particular light bulb. However, the units of measure used are not degrees Fahrenheit, but kelvins. Lighting is rated using the Kelvin temperature scale, named after the 19th-century British physicist Lord Kelvin.

Depending on the time of day, as the sun crosses the sky, it may appear to be red, orange, yellow, white, or blue. The changing colors of the sky as the day passes each have a CCT too. It will always match a color produced by that black-body radiator at some temperature, measured in kelvins.
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Increasing hues of the Planckian locus

Here are some more common examples:

  • 1200 K: a candle
  • 2800 K: tungsten lamp (ordinary household bulb), sunrise and sunset
  • 3000 K: studio lamps, photofloods,
  • 5000 K: electronic flash, average daylight. A designation of D50 stands for "Daylight 5000K" and is the most common standard for professional light booths for photography, graphic arts, and other purposes.
  • 6000 K: bright midday sun
  • 7000 K: lightly overcast sky
  • 8000 K: hazy sky
  • 10,000 K: heavily overcast sky

"Color temperature" is sometimes used loosely to mean "white balance" or "white point". Notice that color temperature has only one degree of freedom, whereas white balance has two (R-Y and B-Y).

Color temperature applications

Film photography

It is important to match the color sensitivity of your film to the color temperature of your light source. Use tungsten film while photographing indoors with incandescent lamps; the yellow light of the tungsten bulbs will appears as pure white in the prints or slides once the film is processed.

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 your monitor's color temperature and then adjust your monitor’s settings accordingly. This enables on-screen color to more closely match printed color. Common monitor color temperatures are as follows:

5000K (D50), 5500K (D55), 6500K (D65), 7500K (D75) and 9300K.

Designations such as D50 are used to 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 the colors are not shifted towards the red or blue.

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 6500K. On many actual sets however, especially older and/or cheaper ones, there is a very noticeable deviation from this requirement of the standard.

Most video and digital still cameras can adjust for color temperature by zooming into a white object and setting the white balance (telling the camera "this object is white"); the camera then shows true white as white and adjusts all the other colors accordingly. White-balancing is necessary especially indoors under fluorescent lighting and when moving the camera from one lighting situation to another. The setting called "Auto white balance" is not recommended for optimum quality video or stills.

Artistic application via control of color temperature

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The house above appears a light cream during the midday, but seems a bluish white here in the dim light before full sunrise. Note the different color temperature of the sunrise in the background.

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 (both by exposure at the labs, and also digitally, where digital film processes are used). Cinematographers also work closely with set designers and lighting crews to achieve their desired effects.

Correlated color temperature

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CIE (1964) xy chromaticity diagram including the Planckian locus, with temperatures indicated. Wavelengths of monochromatic light are shown in blue. The lines crossing the Planckian locus are lines of constant correlated color temperature.

The kelvin system for lamp description worked fine in the past, when most light bulbs were incandescent. They plotted directly on the Planckian locus of the CIExy color model. Their chromaticity coordinates made them mathematically land on that curve in color space where all the "real" color temperatures are defined by Planck's law of black body radiation, with no deviations. Since fluorescent lighting is not incandescent, it presented a new challenge. Fluorescent lamps are made using myriad combinations of phosphors and gasses. The illumination they produce almost never plots, in color space, directly on the Planckian locus, as do all incandescent sources. The kelvin scale assumes that the chromaticity coordinates in question will fall on the locus. If it does not, then the use of the kelvin scale would not work.

How then can you describe the quality of light from these new fluorescent lamps? The method used is called the "correlated color temperature", which a mathematical method of assigning a color temperature to a color near, but not on, the Planckian locus. The above plot shows lines crossing the Planckian locus for which the correlated color temperature is the same. Nevertheless, the colors are not the same, and the method gives only an approximate description of a non-blackbody color. Due to this shortcoming, the rated CCT of any fluorescent tube is unreliable.

A number of color spaces have been developed in which the difference between two colors may be estimated by the distance between them on a chromaticity diagram. Thes include the 1960 CIELuv (which is now outdated) and the 1976 CIELu'v' and CIELab spaces. On a chromaticity diagram for which distances specify color distances, the best estimate of the color temperature of any point will be the color temperature of the point on the Planckian locus closest to that point. Although it is outdated, the CIE specifies distances in the 1960 CIELuv chromaticity space to define correlated color temperature.

Photographers often use color temperature meters. Color temperature meters by design read only two regions along the visible spectrum (red & blue) or some expensive ones read three regions (Red, Green & Blue). They are almost useless under fluorescent light. There are general guidelines and some specific filters recommended to obtain optimum quality under such frustrating circumstances.

Color rendering index

Main article: Color rendering index

The CIE developed a newer model for describing and rating light sources, called the color rendering index, which is a mathematical formula describing how well a light source's illumination of eight sample patches compares to the illumination provided by a reference source. The index provides a number up to 100 for ideal light.

Spectral power distribution plot

The spectral power distributions provided by many manufacturers may have been produced using 10 nanometre increments or more on their spectroradiometer. The result is what would seem to be a smoother (fuller spectrum) power distribution than the lamp actually has. 2nm increments are mandatory for taking measurements of fluorescent lights. Here is an example of just how different an incandescent lamp's SPD graphs compared to a fluorescent lamp:

Recommendations for those without the expensive equipment

Only those with expensive spectrophotometers and spectroradiometers can obtain accurate data. Those without these tools should take the time to review the lamp's specifications, and if they seem good, then the eyes are one's best tool.

New mathematical indices are being proposed which look hopeful. There are at least 3 new ways to categorize lamps more accurately. They are not in use as of yet, but they look promising. One is referred to as the color rendering capacity (CRC).


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External links

Film- and video-related

it:Temperatura di colore ja:色温度 pl:Temperatura barwowa


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