Their purpose is to filter the light according to the specific properties of the gel being used. Normally they would be clipped right in front of the light, but that is not always the case. It is not unusual to cover the windows of an interior location set with color correction gel, to adjust the sunlight passing thru it so it matches the lights being used inside.
Colour correction/ conversion gels
The most common uses are to convert the color temperature of tungsten light toward daylight.
CTB- Correcting tungsten to match daylight. The camera is set to a daylight color balance, so the room looks correct and the lights color is too warm. CTB gel corrects the lights to the room and camera setting.
CTO-Correcting daylight to match tungsten. The camera is set to a tungsten color balance, so the lights look correct, and room looks too cool. CTO gel on the windows correct the daylight to the lights and camera setting.
Natural Density Gels
These are grey in appearance and reduce the output of a light without diffusing the light or affecting the color temperature. This is preferred to electric dimming of the light which will shift its color warmer.
ND-Cutting light level. The light we are adding is too strong for the ambient levels in the room. Neutral Density cuts our light levels to match the room.
Colour Gels
Sometimes referred to as theatrical gels to set them apart from color correction gels. These are used to allow color lighting accents, and unnatural effects. There is an enormous number of colors available from several manufacturers.
There is a loss of intensity when using gels. With deep colors, heavy correction or dense diffusion, the loss can be appreciable. You can be creative with gels by covering only a portion of the light beam with color or natural density. Gels are also considered expendables as over time, gels will become brittle, colors will fade, and the degree of correction will change.
There is a loss of intensity when using gels. With deep colors, heavy correction or dense diffusion, the loss can be appreciable. You can be creative with gels by covering only a portion of the light beam with color or natural density. Gels are also considered expendables as over time, gels will become brittle, colors will fade, and the degree of correction will change.