Colour temperature
Definition
Colour temperature is the colour value emitted by a light source, expressed in Kelvins.
Introduction
Different light sources produce different coloured light. These different colours can be expressed using a number, and this number is known as the colour temperature.
Image source
5500 K is the colour temperature of average noon daylight (i.e., midday sun). Colour film is typically balanced to this colour temperature, which means that, when shooting film in optimal conditions, the image will record white objects as white and all of the colours will appear natural.
This video by Phillips Lighting provides a good explanation of colour temperature:
As explained in the video:
When a solid body is heated, it emits visible radiation. The colour of this light is determined by the temperature of the object. This dependency is used to quantify the white tone of lamps. This so called "colour temperature" is used to objectively describe the white tone of a light source and is expressed in Kelvins, the standard unit of temperature.
This video provides a longer and more in-depth overview of the science and psychology behind colour temperature:
Related terms
References
"What is colour temperature?" Photographymad.com. http://www.photographymad.com/pages/view/what-is-colour-temperature.
Wikipedia contributors, "Color temperature," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Color_temperature&oldid=817900849 (accessed January 12, 2018).
http://lowel.tiffen.com/edu/color_temperature_and_rendering_demystified.html