Colour Gel
A translucent coloured sheet placed over a light to change the colour of its output.
Measured in
stops (absorption)
In depth
Gels serve two quite different purposes: correction, matching one source to another so a scene renders neutral, and effect, deliberately colouring part of the frame. Saturated effect gels absorb a lot of light — a deep blue can cost two stops or more — which must be accounted for when balancing several heads.
Examples in practice
A deep red gel on a background light turns a grey backdrop crimson while the key light keeps the subject's skin neutral.