Correlated colour temperature (CCT) describes the warm-to-cool appearance of white light, expressed in kelvin (K). It relates a source’s colour to the nearest point on the colour path of an ideal heated radiator.
What it means on set
Lower values generally look warmer and higher values cooler when compared under the same viewing conditions. Around 3200K is a common tungsten reference; 5600K is a common daylight reference. Actual daylight changes with weather, time and the balance of direct sun and sky.
The camera’s white balance determines how that light appears in the recorded image. A 3200K source photographed at a 5600K white balance will normally look warm; a 5600K source photographed at 3200K will normally look cool.
What CCT does not tell you
Two fixtures at the same CCT can still differ in green–magenta tint, spectral output and the way they reproduce skin or fabrics. Matching the number on their displays is a starting point, not a guarantee of a visual match.
Practical example
For an interview beside a window, start by matching your key light to the daylight reaching the subject, then check the image or measure the light and adjust tint as needed.
Related reading
Lux · Types of lights explained · Duv · Bi-Colour · Colour Mixing
References
CIE: correlated colour temperature · Aputure: CCT and green–magenta controls