Color temperature

Colour temperature is a term used to describe the colour of light sources. It is the temperature of a black-body radiator with a perceived colour the same as the light source.

For example, colour CRT monitors emit light with a complex spectrum, composed of the output from three colour phosphor?s, 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.

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.

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.

Cinematographer?s can also white-balance to objects which aren't white, downplaying the color of the object used for white-balancing. For instance, cinematographers can bring more warmth into a picture by white-balancing off something light blue, like faded blue jeans; in this way white-balancing can serve in place of a filter or lighting gel when those aren't available.

External link:

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

/Talk