A gel is a thin, colored, translucent sheet placed over a light source to tint its output. The name comes from the original gelatin-based dyes used in theater lighting; modern gels are polycarbonate or polyester films that resist heat and fading. They are sold in standardized sheets (Rosco and Lee being the dominant brands) and identified by name and number, such as Rosco 3407 Full CTO or Lee 201 Full CTB.
The two most common categories are color temperature correction and creative color. CTO (color temperature orange) warms a flash from its native daylight balance of roughly 5500K toward tungsten at 3200K, which is how photographers match a strobe to interior practical lights or to a sunset. CTB (color temperature blue) does the opposite, cooling a tungsten source to match daylight. They come in graduated strengths (1/8, 1/4, 1/2, Full, Double) that correspond to specific Mired shifts, letting you mix flash and ambient without a color clash.
Correction gels also include plusgreen and minusgreen, which compensate for the tint shift of fluorescent and some LED sources. A flash through a fluorescent-heavy office space looks magenta against a green ambient unless plusgreen is added to the flash or minusgreen is added in post. Setting white balance for the gelled flash brings the ambient into the same neutral tone, and the room stops looking sick.
Creative gels exist in dozens of colors and are used to paint backgrounds, separate subjects from environments, or imply a source the viewer cannot see (a neon sign, a sodium streetlight, a TV screen). A common portrait setup pairs a clean key on the face with a heavily gelled rim or background light in a contrasting color, giving cinematic separation without adding any actual practicals to the scene. Magenta on the background with a warm key light remains a signature look.
Gels reduce light output. A Full CTO cuts roughly two-thirds of a stop, and saturated colors like Congo Blue can absorb four stops or more. Photographers compensate by opening the aperture, raising power, or moving the head closer, keeping the inverse square law in mind. Cheap kits of small gel snippets that clip to a speedlight handle 90% of on-location work; bigger sheets cut to fit a softbox front, a strobe reflector, or a window are used in the studio.
Common mistakes include letting the gel sag against a hot bulb (which warps it and shifts its color), buying random colored cellophane from a craft store (which is not photographically calibrated and shifts unpredictably), and double-gelling without realizing that two 1/2 CTOs are not the same as a Full CTO because the spectral curves stack nonlinearly. A small swatchbook of named, numbered gels is one of the cheapest and most expressive additions to a lighting kit.