Gel

A gel (short for gelatin filter, though today made of heat-resistant plastics like polyester or polycarbonate) is a transparent, colored sheet used in lighting design. It is placed directly in front of a fixture's lens to color the beam of light. While it is an effective way to cast a solid, vibrant color over a metal gobo projection, gels will naturally fade or degrade over time due to the intense heat of the lamp and will eventually need to be replaced.