Gels, an Introduction...
Gels are sheets of heat resistant polyester that are placed between a light source and the subject being lit. 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.

Gels fall into four general categories:

orange dot Color Correction or conversion gels. The most common uses are to convert the color temperature of tungsten light toward daylight. You may find them referred to as CTB (Color Temperature Blue). They are available in Intensities from 1/8th to Double, or Extra Blue, strengths. CTO (Color Temperature Orange) is used to convert daylight light sources toward the color temperature of tungsten lights in similar degrees. There are also a number of gels that convert the numerous types of common fluorescent lights to mix better with tungsten or daylight sources. Color Correction gels should not be confused with theatrical color gels which color the light for dramatic results.

Note - there can be a creative advantage in using a weaker strength blue gel than may be called for to balance tungsten to existing daylight on a set. For example, if a full blue would be the correct strength, using 1/2 blue will create a light that is a little warmer in color and may be more flattering to your subject if they have a pale skin color.

orange dot Neutral Density (ND). Grey in appearance, they reduce the output of a light in progressive steps from 1/2 of a lens stop to 4 full stops without diffusing the light or affecting the color temperature. This is preferred to electric dimming of the light which will shift its color warmer.

orange dot Diffusion Gel is available in various densities from nearly clear to a dense milky white that diffuses the light without changing the color temperature. There is a loss of output as the density (thickness) increases. Adding diffusion makes the light softer as it makes the size of the light source larger. If you stack sheets of diffusion together it doesn't make the light softer than a single sheet would, but drops the output of the light another step without changing its color, so it can be used in a similar manner as Neutral Density. Using heavy diffusion in front of the barndoors, in a frame or clipped around them, will cancel the cutting effect of the barndoor. Any cutting that needs to be done will have to be done with flagging methods. Diffusion is not just available in gel material, but also in what is called "spun", a woven synthetic cloth-like material that is heat resistant and also comes in varying densities.

orange dot Colors. 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.

Remember, 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, diffusion or ND.

Gels are considered expendables. Over time, gels will become brittle, colors will fade, and the degree of correction will change.

ctb gels
CTB Gels

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 gels
CTO Gels

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.

ND gels
Neutral Density Gels

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.

diffusion gels
Diffusion (Frost) Gels

close up frost
Diffusing / softening light The light source we chose had a quality that was harder than we wanted. Diffusion gel softened the output.

colored gels
Color / Theatrical Gels

colored gel Wide Shot
no colored background
colored background
Color accents or dramatic effect. Our shot was well lit, but lacked visual interest. We added a colored gel to the background light and closed in the barn doors to make the beam look like a slash.

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