Color temperature

An object's color temperature is the temperature of a black-body radiator with a perceived colour the same as that of the light source.

For example, colour CRT monitors emit light with a complex spectrum, composed of the output from three colour phosphors, but the balance of their intensities can be adjusted so that the overall balance of the emitted spectrum, as judged by the human eye, can match a number of different colour temperatures.

A higher colour temperature corresponds to a bluer light, a lower colour temperature to a yellower light. This can be seen clearly by comparing halogen lights to standard filament bulb lights which operate at a lower temperature.

Daylight has a colour temperature of around 5500 K. Standard tungsten filament bulb lamps have a colour temperature of around 3200 K.

'Colour temperature' is sometimes used loosely to mean 'white balance'. Notice that colour temperature has only one degree of freedom, whereas white balance has two (R-Y and B-Y): for example, there is no temperature at which a black-body radiator has a purplish hue.

Most video 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 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.

Artistic use 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 to 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.

Compare with brightness temperature.

External link

• See Charles Poynton's Color FAQ for more details.