An overview of using gels with lighting.
In the previous installment of this column, I covered the concepts of color temperature, the Kelvin scale, and white balance. In this column, I'm moving on to three related subjects: changing the color of light from one temperature to another, fixing light sources that have too much green or too much magenta, and adding color to lights. In nearly all cases, you use gels to effect these changes.
Gels are sheets of polyester that have been dyed specific colors. They're called gels from the early days of theater lighting, when they were made with colored gelatin dried into thin sheets. The cheapest form of theatrical gels aren't appropriate for a video or film shoot because the gels are usually mounted close to the light (and heat) source, and inexpensive gels will fry pretty fast. For that reason, the type of gels used in video and film are known as "tough" gels. Excellent tough gels are manufactured by a number of companies, the best known of which are Great American Market (GAM) (www.gamonline.com), Lee Filters (www.leefilters.com), and Rosco (www.rosco.com).
This chart shows the transmission characteristics of a Rosco full CTB gel. Image courtesy of Rosco, Inc. (www.rosco.com).
The three types of gels
Three basic categories of gels are used in video and film work: color-conversion gels, color-correction gels, and color-effect gels. Color-conversion gels are carefully designed to convert one specific color temperature of light into another temperature. Because the two basic designations for color temperature in video and film are 3,200 K (incandescent, indoor) and 5,600 K (sunlight, exterior), conversion gels are designed around those two temperatures. A gel that will change a quartz incandescent instrument from yellowish-orange 3,200 K to bluish 5,600 K is known as a "Color Temperature Blue" gel, or CTB gel. Conversely, a gel that is meant to convert sunlight to match a quartz light is known as a "Color Temperature Orange" gel, or CTO gel. We live in a less-than-perfect world where actual color temperatures vary a lot, so these gels are available in several densities or strengths, usually 1/4, 1/2, and full.
It's important at this point to understand how gels work. Gels are optical filters that act by absorbing certain wavelengths (colors) and transmitting others freely. A CTB gel transmits blue frequencies but absorbs red, orange, and yellow frequencies. A gel works by removing certain colors. It's a common misconception that gels add color to the light. Gels can't add color. If the desired frequency isn't present in the light in the first place, or is present in low amplitude, a gel won't add more of the color. The sodium vapor lights used in parking lots, for example, emit only a narrow frequency of orangish light. Placing a CTB gel over one of these will not produce blue light; it will absorb the orange and pass almost no light.
A classic situation in which you would use color-conversion gels is when you're interviewing a subject indoors in front of a large window. The blue sunlight streaming in the window is pretty much a given, so you use a full CTB gel on your incandescent instruments to make them match the color of the sunlight. The only problem with this approach, of course, is that a full CTB gel eats up a lot of light--nearly two-thirds of the light output! So an alternate solution would be to use a large sheet of CTO gel on the window itself to convert the very bright sunlight to orange to match your incandescent instruments. Rosco and others make large sheets for this purpose, known as window gels.
The author clips a CTB gel to a fresnel to match it to the
incoming sunlight. Image courtesy of John Jackman.
Color-correction gels are designed to fix lights that have too much green, usually fluorescent lights. Cheap fluorescent tubes, especially older ones, make people look very sick on video because they have very little red and a huge spike of green on the color spectrum. To fix this, you can put a minus-green (magenta) gel over the light to absorb the excess green wavelengths. The gel, though it looks magenta, doesn't add magenta to the light--it just removes green. The resulting light will still be a bit deficient in magenta tones, so you'll need to boost the magenta either in the camera or in post. If you are using incandescent instruments with the corrected "cool white" tubes in an office, however, this may make the added incandescent lights look too magenta. Hence, the opposite number, a plus-green gel, is used over the incandescent to suck some of the magenta out of its light output. These gels are used less frequently now that many fluorescents have improved color rendering characteristics.
With both color-conversion and color-correction gels, the ultimate goal is to create light that the camera will see as white. Both types of gels are applied to the lights before the manual white balance is set on the camera. If properly balanced, the on-camera image won't look magenta, blue, orange, or green--the light will look uniformly white.
What if you want to put some color in your pictures? That's where color-effect gels come in. These are gels that are used to color the light intentionally, either subtly (a touch of warm amber) or dramatically (strong primary red or blue). It's important to apply these gels to the light after the white balance is set on the camera, otherwise the camera will be struggling to make your color effect look white, too--which will just botch up the lights that should look white.
Typical uses for color-effect gels would be setting mood, a la the original Star Trek series, or for special effects, such as simulating fire or the green glow of a radar screen. Strong primary and secondary colors are often referred to as "party color" gels by gaffers.
It's common to use color-conversion gels for color effects--all you do is apply them after the white balance is set. For example, it's common to use a 1/2 CTO gel to suggest warm interior light, or to use 1/2 CTB to suggest light coming from a window. Often lighting designers will use 1/4 CTO gels on lights just to give a bit of added warmth to the lighting. Blue tints are used to suggest moonlight for night scenes.
For a good overview on how to light night scenes, read my article "Lighting the Darkness" (http://www.dv.com/news/news_item.jhtml?LookupId=/xml/feature/2002/jackman1002, Oct. '02 DV).
A kissin' cousin to the gel is the neutral-density filter. This is just a "gray" gel that absorbs all frequencies equally. As with the ND filter on a camera, it reduces the light level without changing the color of the light at all. Of course, you can use an electronic dimmer on the light, but this will usually shift the color toward orange as the light dims. A neutral-density gel can cut the light output without shifting the color. There are other methods of doing this, such as scrims (screens that you insert in the gel holder) or nets (black gauzy fabric on a frame placed in front of the light), but neutral-density gels are a handy option to have, and they're easier to pack than big nets!
The Rev. John Jackman has been shooting video since Jimmy Carter was president. He's the author of Lighting for Digital Video & Television (CMP Books, 2004) and is moderator of The Craft of Lighting forum and column at DV.com. You can reach him there with your questions.