Gels control the color of the light. Gels (or color filters) are thin sheets of transparent colored plastic, usually dyed polyester.
There are three major gel manufacturers: Rosco in Connecticut, GAM in Hollywood, and Lee Filters in the UK. For film and video, gels break down into two categories: color conversion or correction gels and color effect gels.
One point that is very important: not all gels have equal tolerance of heat. Cheap theatrical gels will literally shrivel under the heat of larger instruments. It’s important to use gels with high-temperature tolerance known as “tough” gels.
Color conversion gels are precisely calibrated to change one light color temperature to another, say to change a 3200-K quartz fresnel to 5500 K to match sunlight coming in a window. These are intended to force the color temperature of the light to match the white balance setting of the camera. Lights using color conversion gels typically will appear to be white on camera; though they are also used to create color effects in certain situations as well. Color conversion gels change light along the yellow-to-blue range.
Color correction gels add or subtract green component. This is necessary when dealing with typical fluorescent lights, which often have a strong green spike in their spectrum emission. If you have to shoot in an office or a factory that is lit with hundreds of “cool white” fluorescents that cannot be turned off and you need to also use a quartz key on the subject, you may need to add some green to the quartz light to match. The picture will be consistent, but will have a bit of a greenish cast since camera white balance really cannot remove the excessive green component of these lamps. You can cover the fluorescents with a magenta tone gel known as a “minus green.” The simplest method is to lay large sheets of gel in the tray or grille under the tubes. Of course, covering a large number of big fluorescent fixtures with lots of minus green gel is not always practical. In that case, you can also use a camera lens filter (such as the Tiffen FL-B®) of the same magenta tone to subtract green from the entire scene. However, these often subtract so much light from the scene that they create an exposure problem in video. Another solution is to just gel the lights over the people you are actually focusing on, and let the background and other areas run a bit green.
On the other hand, color effect gels are intended to change the on-camera appearance of the light. These are used when a strong color tint is needed. Tints like bastard amber are known as colorizing gels, whereas strong primary colors are known as party colors. To simulate the effect of summer sunlight coming through a canopy of leaves, for instance, you might use a green gel such as Roscolux #86 (Pea Green) on the base key light and then Roscolux #07 (Pale Yellow) on an accent light with a cookie over it to create a dappled pattern. However, the camera white balance would be calibrated to 3200 K so that these colors would appear as a color effect on the video (if you white balance with color effect gels on the lights, the color effect will vanish or diminish). In many cases, a strong blue filter will be used to suggest moonlight. In other cases, you may want to tint a scene or part of a scene with a strong color for dramatic reasons.
Using color conversion gels is an essential science for video and television production. Depending on the type of video you produce, you may never (or rarely) use a “party color,” but you will certainly have to use color conversion on a regular basis. If you’re doing straightforward video work and just want a basic set of color conversion/correction gels, Rosco has a package called the Cinegel Sampler that contains what you will need.
Color conversion and correction gels primary break down into four types:
• BLUE for converting tungsten to daylight: known as Color Temperature Blue, or CTB
• ORANGE for converting daylight to tungsten: known as Color Temperature Orange, or CTO
• GREEN for correcting tungsten to fluorescent: known as Plus Green
• MAGENTA for fluorescent correction: known as Minus Green
Each type has a range of tints for different purposes. These are generally referred to by quarters—for instance, “Full Color Temperature Blue,” “¾ CTB,” “½ CTB,” or “¼ CTB.” Each tint will change color temperature a specific amount.
This can all be pretty confusing at first and the confusion is aggravated by inconsistencies between brands and even ranges within a single brand. As you may have noticed, the rules aren’t consistent through the ranges. Rosco’s Full CTB converts from 3200 K to 5500 K, while their Full CTO converts 5500 K to 2900 K. Lee and GAM use slightly different temperatures for CTB and CTO. To add to the confusion, the fractional gels aren’t linear—in other words, a double layer of Rosco ½ CTB will not be quite the same as a single layer of their Full CTB. Sorry, you just have to get used to it. It isn’t as confusing as women’s sizes in department stores, but that’s not saying much, is it?
Typically, you will fall into using a few specific gels depending on what lights you use most, so it’s not long until this becomes second nature. For most videographers who have a quartz light kit, the most common use will be to match sunlight coming in a window. This presents the largest variation, since the actual color temperature of sunlight varies dramatically through the year, through the day, and depending on weather conditions. To do this properly with a pro camera, perform a manual white balance on a white card illuminated only by the sunlight. The camera will display in the viewfinder what color temperature it is balanced to. For instance, on a clear day near noon it may read 4800 K. Your quartz lights will be somewhere near 3200 K, though to be sure, you might perform the same operation with the white card illuminated only by the quartz lights. This situation will call for a ¾ CTB.
Large sheets of conversion gel material are available to use over windows to convert sunlight to 3200 K. GAM makes a product called WindowGrip that has a low-tack adhesive already applied for this use. Special material to correct the daylight temperature of computer monitors and television screens is also available.
It is a common misconception that gels add color to light, but this is the opposite of reality. It is important to understand that gels are light transmission filters. They do not add color to light; rather, they subtract it or filter it out. In other words, a green gel actually works by absorbing the opposite color (red), but freely transmitting green. The gel turns apparently white light green by removing other nongreen components. The precise manner in which a particular gel does this is plotted on a Spectral Energy Distribution (SED) curve. This displays the entire rainbow of discrete colors, and plots the filter’s ability to pass or block each color.
The graphs above shows two SED curves for two different Rosco gels. The first one, #389 Chroma Green, passes 80% of the energy at 500 nanometers (nm), but nearly eliminates all other frequencies. Its overall transmission rating is 40%. The second, #367 Slate Blue, has a more complex signature. It passes 460 nm (medium blue) and above 700 nm (red), but sharply curtails 600 nm ( yellow) and reduces 400 nm (UV blue) to 40%. The overall light transmission is 20%. ND (neutral density) gels have a neutral tint that passes all frequencies equally, and these are used to reduce light level from an instrument or through a window, without changing its color or quality.
Gels can be used on common instruments in a couple of different ways. Most fresnels and open-faced instruments come with gel holders that can slip in behind the barn door assembly. You might want to mount a commonly used gel (for instance, a full or ½ CTB) in a gel frame for frequent use. However, in most cases, gaffers will simply clip sheets of gel onto the barn doors of an instrument using wooden clothespins, often referred to in the business as C-47s. Don’t use anything but old-fashioned wooden spring clothespins—the plastic ones will melt in short order! Other instruments such as large softbanks may need to have a large sheet clipped inside the front diffusion material.
Jason J. Tomaric is an Emmy-winning director and cinematographer in Los Angeles, and produces the online filmmaking resource, FilmSkills.com. FilmSkills uses dozens of instructional videos from hundreds of working film industry experts to enhance students’ learning experience.