Before knowing the different uses of gels, it’s probably good to know what a gel is.
A color gel or color filter, also known as lighting gel or simply gel, is a transparent colored material that is used in theater, event production, photography, videography and cinematography to color light and for color correction.
The three types of lighting gels
In terms of gels (the colored pieces of plastic you see fixed in front of film and theatre lights) there are three general categories of use:
These gels are strictly used to soften a light. We’ll do another article on diffusion gels in the future, but today we want to go over the uses of colored gels.
These are used simply to create colored light for artistic effect. They are available in literally any shade of the rainbow and can be used however a lighting designer thinks up to create a unique visual experience for the viewer. There’s no difference between using gels this way, or using colored lights in a display, in a shop, in a nightclub, etc.
Color Temperature Adjustment Gels
On the other hand, the gels used on large film and television lights generally fall into this latter category. The average person on the street doesn’t realize that “white light” (as perceived by humans) actually comes in a wide variety of colors.
Even over the course of the day, basic sunlight actually varies in measurable color quite a bit.
Our eyes are very good at automatically adjusting for this discrepancy – however photograph and moving picture cameras are not. Anyone who has taken even a couple of photos on their phone has likely seen the effect of daylight seeming overly “blue” or inside light making everything seem “too orange”.
Streetlights, fluorescent lights, and stadium lights can often veer into the purples and greens when you view them through a camera. Often these results are very unpleasing.
You can even see this with your eyes if you spend a long time in a darkened room (like a movie theatre) and then go outside (or vice versa) it takes time for our brains to “recalculate” what colors should look like.
White Balance with camera
Thankfully, even simple cameras today have extremely advanced “white balance” settings, by which we can tell the camera what type of lighting condition we are in, and it can try to figure out what spectrum of light should be perceived as “white” (and then deduce what other colors should be in relation to that). If you set your camera’s white balance to “daylight” what you’re really doing is telling the camera you would like to see light at around 6,500K (color is measured in light in Kelvin) as “white” and shift all the other colors to compensate. If you set it to “indoor” the camera will assume that “white” is closer to 3,200K… etc.
This also isn’t just a feature of digital cameras, manual film and photography cameras could get the same effect by putting a piece of colored glass either in front of the lens to shift all the colors coming into the camera, or in front of the film being exposed. This way you could accomplish the same result without digital technology (although the digital gives you far more control and options, obviously).
White Balance with gels
So if we can adjust white balance in-camera why do we still need to put all this stuff in front of lights? Because while the camera can be set to whatever white balance value we want – we need to make sure that the temperature value of all the lights that will be hitting our subject, or scene, match that expected value.
As stated above, if white “daylight” is 6,500K and white light from a tungsten lightbulb (like in a big industrial film light) is about 3,200K and we are using both to light a scene – we’re going to get two very different colors (in fact that’s exactly what’s happening in the photo above – the left is pure daylight, the right is pure tungsten light).
If you wanted the example image up top to appear “even” you could either make the daylight “more orange” so it matches the indoor light (perhaps by covering up the entire window with a large orange gel). Alternately, you could make the inside light “more blue” to match the daylight (through putting blue gel in front of your light sources, or using special daylight-balanced lightbulbs). Then once you have the scene either “all blue” or “all orange” it’s much simpler to adjust the white balance (either while shooting, or in post production) to get a pleasing, evenly colored, image.
This is why the two most common colors of film gel you will see on film and television sets are CTO (Color Temperature Orange) and CTB (Color Temperature Blue). There are set values for these basic gels, and then you can get all kinds of variants to match particular light sources (for example you could get a 1/4 CTO if you only wanted a little orange, or a double CTB if you wanted a LOT of blue).
This is obviously just the tip of the iceberg of a very complicated field. Modern film and photographic lights are now getting into using color-adjustable LED lights, or having different bulbs pre-calibrated to different color temperatures. There are special “film” bulbs you can put into lighting fixtures (like lamps and chandeliers) so you can film them without adjustment, and of course you can then start to use color temperature filters for artistic reasons as well.