gelA colour gel or colour filter, or a lighting gel or simply gel, is a transparent coloured material that is used in theatre, event production, photography, videography and cinematography to colour light and for colour correction.
Modern gels are thin sheets of polycarbonate or polyester, placed in front of a lighting fixture in the path of the beam. Gels have a limited life, especially in saturated colours. The colour will fade or even melt, depending upon the energy absorption of the colour, and the sheet will have to be replaced. In permanent installations and some theatrical uses, coloured glass filters or dichroic filters are being used. The main drawbacks are additional expense and a more limited selection.