Color Temperature

The CIE 1931 x,y chromaticity space, also showing the chromaticities of black-body light sources of various temperatures, and lines of constant correlated color temperature

Color temperature is a characteristic of visible light that has important applications in photography, videography, publishing and other fields. The color temperature of a light source is determined by comparing its chromaticity with a theoretical, heated black-body radiator. The temperature (in kelvin) 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.

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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. For bodies 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, do not primarily emit light because of the temperature of the source and the emitted radiation do not follow the form of a black-body spectrum, and are assigned what is known as a correlated color temperature (CCT). CCT is the color temperature of a black body radiator which in the perception of the human eye 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.

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 refraction and, to a lesser extent, scattering of light, and is unrelated to black body radiation.

Even when the sun is low over the horizon, we can estimate its apparent color temperature and correct it to compute its effective temperature. So, even if the sun looks red, and showing an apparent color temperature of 2500 K, a calculation can demonstrate that its effective temperature is in reality close to 5770 K.

The blue color of the sky is not due to 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.

The colors of 5000 K and 6500 K black bodies are close to the colors of the standard illumininants called respectively D50 and D65, which are used in professions working with color reproduction (photographers, publishers, etc.).

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. To see this, observe that while incandescent bulbs glow a reddish to yellowish color throughout their lifetimes, when one blows out, the flash of light is noticeably bluish - the filament is hotter when it burns out, as evidenced by the scorch mark often left on the glass.

"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).

In photography, an alternative numerical measure used is the mired (micro reciprocal degrees). Color temperatures and mireds are convertible to each other via a simple formula (see the mired page for details of the computations, and the reasons for the use of the alternative unit).

Color temperature applications

Film photography

Film sometimes appears to exaggerate the color of the light, since it does not adapt to lighting color as our eyes do. 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 color 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 (3200 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 5600 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.

Fluorescent light varies in color and may be harder to correct for. Because it is often greenish, a magenta filter may correct it.

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). They are almost useless under light from fluorescent or discharge lamps. There are general guidelines and some specific filters recommended to obtain improved quality under such circumstances.

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 are as follows:

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 that the colors are not shifted towards the red or blue.

Digital cameras, web graphics, and DVDs etc. are normally designed for a 6500 K color temperature & indeed the sRGB standard stipulates (among other things) a 6500 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 6500K. On many actual sets however, especially older and/or cheaper ones, 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. The setting called "Auto white balance" is not recommended for optimum quality video or stills.

Artistic application via control of color temperature

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.

For artists, most pigments and papers have a cool or warm cast, as the human eye can detect even a minute amount of saturation. Gray mixed with yellow, orange or red is a "warm gray". Green, blue, or purple, create "cool grays". Note that this sense of "temperature" is the reverse of temperature in kelvin; bluer is described as "cooler" even though it corresponds to a higher-temperature blackbody.

Correlated color temperature

CIE (1931) 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.

Incandescent lamps are well described by their temperature on the Kelvin scale, because as nearly black body radiators, their chromaticity coordinates land directly on the Planckian locus of the CIE 1931 (x, y) chromaticity diagram. Fluorescent lighting is not incandescent and presents a new challenge. Fluorescent lamps are made using myriad combinations of phosphors and gases. The illumination that they produce is almost never described by a point in color space that lies on the Planckian locus.

Difficulties arise when trying to describe the quality of light from a source other than a black body radiator, such as a fluorescent or high-intensity discharge lamp. The method used is called the "correlated color temperature" (CCT), which assigns a color temperature to light near the Planckian locus, but not on it. 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 specification of a particular color. Due to this shortcoming, the rated CCT of a fluorescent or discharge lamp does not precisely specify its color.

Color spaces have been developed in which the distance on a chromaticity diagram may estimate the difference between two colors. These include the CIE 1960 uv chromaticity diagram and the CIE 1976 u'v' chromaticity diagram. On a chromaticity diagram for which distances specify color differences, 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 the 1960 uv specification has been mostly obsoleted by the 1976 u'v' (CIELUV) system,^[1] the CIE specifies distances in the 1960 uv chromaticity space to define and calculate the correlated color temperature.^[2]^[3] The isothermal lines on these Uniform Chromaticity Scale spaces are perpendicular to the Planckian locus. Once the isothermal lines are calculated, they can be translated back to CIE xyY, as shown on the diagram above, where they are no longer perpendicular to the Planckian locus.

The distance from the locus (i.e., degree of departure from a black body) is traditionally indicated in units of

Δ u v

; positive for points above the locus. This concept of distance has evolved to become Delta E, which continues to be used today.

Color rendering index

The CIE developed a newer model for describing and rating light sources, called the color rendering index (CRI), 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. This index is useful in determining the suitability of illuminating spaces occupied by humans, since there are adverse health effects of over-illumination by artificial lights or by mismatch of natural light sources.

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. Increments of 2 nm 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.

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