Gel

Gel

noun | Camera & Optics

A sheet of coloured or optically treated transparent material -- typically heat-resistant polyester film -- placed in front of a lighting instrument to change the colour, quality, or intensity of the light it produces. Gels are used to correct colour temperature mismatches between different light sources, to add creative colour to a lighting setup, to reduce light intensity, or to apply diffusion. They are among the most versatile and widely used consumables in a professional lighting kit.

Quick Reference

The Explanation: How & Why

Gels perform four primary functions in production lighting:

Colour correction: Converting the colour temperature of one light source to match another. The most common correction gels are CTB (Colour Temperature Blue, which cools a warm tungsten source to match daylight) and CTO (Colour Temperature Orange, which warms a cool daylight-balanced source to match tungsten or the warm tone of sunset). These are available in full, half, quarter, and eighth grades. A full CTB over a tungsten 3200K source shifts it to approximately 5600K (daylight balance). A full CTO over an HMI 5600K source shifts it to approximately 3200K. Correction gels allow the camera to be set to a single white balance when the scene contains multiple light sources of different colour temperatures.

Creative colour: Adding colour to a lighting setup for aesthetic purposes -- a deep blue back light, a warm amber fill, a cool green practical. Unlike correction gels, which aim to neutralise colour temperature differences, creative gels add colour intentionally. Lee Filters and Rosco produce hundreds of named creative gel colours: Congo Blue (Lee 181), Bastard Amber (Lee 162), Urban Sodium (Lee 761), Cosmetic Peach (Rosco 339). Each has a specific colour character and light transmission percentage.

Neutral density (ND) reduction: ND gels reduce light intensity without changing colour -- equivalent to neutral density filters but applied to the source rather than the camera lens. Used when a specific instrument is too bright for the desired exposure without being moved or dimmed.

Diffusion: Diffusion gels scatter light to soften shadows. These are categorised separately under the term "diffusion" but are physically the same gel format applied in the same way.

Gels are applied by clipping, taping, or sliding them into gel frames that mount on the front of lighting instruments. Heat resistance is a critical property for gels placed close to hot tungsten or HMI sources -- polyester gels designed for this use can withstand temperatures up to approximately 120 degrees Celsius. LED sources run cooler and extend gel life significantly.

Historical Context & Origin

Lighting gel in its modern form evolved from earlier theatrical tradition. Theatrical colour filters were originally made from dyed gelatin -- hence the term "gel," which persists even though modern gels are almost universally polyester. Gelatin filters were fragile, moisture-sensitive, and short-lived, but they established the practice of modifying light colour through filtration. The development of dyed polyester film as a gel substrate in the 1960s and 1970s, pioneered by Lee Filters (founded 1967 in the UK) and Rosco Laboratories (expanding into film gel from theatrical products in the 1960s), produced a material that was far more durable, heat-resistant, and colour-consistent. Lee and Rosco both developed comprehensive numbered cataloguing systems for their gel ranges -- the Lee number and Rosco number for a given gel are part of standard gaffer vocabulary on professional sets worldwide.

How It's Used in Practice

Scenario 1 -- Mixed Light Correction (Gaffer): An interior scene is lit by a combination of tungsten practical lamps (3200K) and HMI instruments (5600K) brought in by the gaffer. The DP wants to set the camera to a single 3200K white balance to match the warm practicals. The gaffer covers each HMI with full CTO gel, converting the HMI output from 5600K to approximately 3200K. All sources now match the tungsten balance; the camera is set once and not adjusted between shots.

Scenario 2 -- Creative Night Exterior (DP / Gaffer): For a night exterior scene intended to have the look of cool moonlight, the DP asks the gaffer to add half CTB and a touch of Rosco 60 (Mist Blue) to the key. The combination cools the source to approximately 7000K -- convincingly lunar in tone. The practical street lights in the background are warm sodium -- the contrast between the cool key and the warm practicals creates a colour separation that suggests a real urban night environment without being artificial.

Scenario 3 -- ND Gel on Window (Gaffer): The background window in an interior shot is 4 stops brighter than the foreground subject. The gaffer cannot reach the exterior of the window to add ND filter material to the glass. Instead, the gaffer tapes Lee 209 (0.6 ND, 2 stops reduction) to the interior of the window. The window is now only 2 stops brighter than the foreground -- within the camera's recordable range without gradient filters.

Usage Examples in Sentences

"Put a full CTO on the HMI -- we are white-balancing to 3200K for the tungsten practical look."

"The back light needs a touch of Congo Blue -- I want the rim to read cool against the warm key."

"Cut two stops of ND gel for the window; we cannot get outside to add a filter."

"The gaffer keeps a full gel book on cart -- every Lee and Rosco colour in standard, half, and quarter grades."

Common Confusions & Misuse

Gel vs. Diffusion Gel: Both come in the same physical format -- sheets of polyester film in similar sizes. Colour and correction gels modify the colour or intensity of the light without intentionally scattering it. Diffusion gels scatter the light to soften shadows. The confusion arises because both are called "gels" and are applied in the same way. On a lighting plan or gel order, they are listed separately: colour correction gels by name and grade, diffusion gels by type and density.

Gel vs. Camera Filter: A gel is placed in front of a light source to modify the light before it enters the scene. A camera filter is placed in front of the lens to modify the light after it has left the scene and before it enters the camera. Both can alter colour balance -- a 1/2 CTB gel on a tungsten source and a 1/2 CTB filter on the camera lens both shift the image cooler -- but they do so at different points in the optical chain and produce different results. Gels modify specific sources; camera filters modify the entire image uniformly.

Variations by Context

Related Terms

• Key Light -- The source most commonly modified by correction or creative gels
• Diffusion -- A category of gel used to soften light quality; physically similar to colour gels
• Gaffer -- The crew member who selects, cuts, and applies gel to lighting instruments
• Colour Temperature -- Gels shift the colour temperature of light sources to achieve the desired balance
• Backlighting -- Back lights are frequently gelled with creative colours (blue, amber) to add colour separation

See Also / Tools

The Lighting Power Calculator calculates the total power draw of your lighting setup, including all instruments being gelled. Note that ND gels reduce effective output and may require compensating by increasing instrument intensity or adjusting exposure settings.