Gel

Gel

noun | Camera & Optics

A sheet of transparent, heat-resistant polyester or polycarbonate film placed in front of a lighting instrument to modify the color, intensity, or quality of the light it produces. Gels correct color temperature mismatches between sources, add creative color to a setup, reduce light intensity without changing color (neutral density), or scatter light to soften shadows (diffusion). They are among the most consumed expendables in a professional lighting kit.

Quick Reference

The Explanation: How & Why

A gel solves a problem that no other tool on set solves as efficiently: modifying the output of a light source at the source itself, before the light enters the scene. This matters because every light in a scene may need different modification. A single camera filter affects the entire image uniformly, but a gel on one fixture affects only that fixture. When a scene contains 8 lights and only 2 need color correction, gels are the only practical solution.

The mechanism is spectral filtration. A gel absorbs certain wavelengths of light and transmits others. A full CTB (Color Temperature Blue) gel like Rosco 3202 absorbs warm (red/amber) wavelengths and transmits cool (blue) wavelengths, converting a 3200K tungsten source to approximately 5500K. The transmission percentage tells you how much light survives the filtration: Rosco 3202 Full Blue CTB transmits only 36% of the light (-1.5 stops), which means a 1000W tungsten lamp gelled with full CTB produces the equivalent of approximately 360W of daylight-balanced light. This transmission loss is why gaffers carry half, quarter, and eighth CTB grades: a half CTB (Rosco 3204) transmits 52% (-0.9 stops) and shifts 3200K to 4100K, giving a warmer daylight that preserves more output.

The significance of mired shift values: the kelvin scale is not linear for color correction. A half CTB shifts a 3200K source up by 900K (to 4100K), but the same half CTB shifts a 2600K source up by only 600K (to 3200K). The mired shift value is a constant that predicts the effect of a correction filter regardless of the source color temperature. Mired = 1,000,000 / K. Full CTB has a mired shift of -131. This is why gaffers calculate correction in mireds, not in kelvin.

A worked example: an interior scene has tungsten practical lamps at 3200K and an HMI window light at 5600K. The DP wants to shoot at 3200K white balance. The gaffer applies full CTO (Rosco 3407 or Lee 285) to the HMI, converting it from 5600K to approximately 3200K. The CTO transmits approximately 59% of the light (-0.8 stops), so the HMI output drops by roughly one stop. The gaffer compensates by opening the HMI's ballast output or adjusting camera exposure. All sources now match at 3200K, and the camera white balance stays fixed across all shots.

Historical Context & Origin

The term "gel" comes from the gelatin base used for theatrical color filters in the 19th and early 20th centuries. Gelatin-based filters were dyed, cast into sheets, and placed in front of stage lights. They were fragile, moisture-sensitive, and would char under the heat of tungsten halogen lamps that became standard in the 1960s.

Lee Filters was founded in 1967 in Andover, UK, when cinematographer David Holmes BSC approached the Lee Brothers to manufacture coated polyester color filters for film production. Oswald Morris BSC used Lee 164 Flame Red on Oliver! (1968), one of the first high-profile uses of the new polyester gel. Production was brought in-house in 1973.

Rosco Laboratories, founded in 1910, developed the Cinegel range in 1970 and received an Academy Award in 1974 for the Cinegel system. Rosco received a second Academy Award in 1998 for the CalColor range of calibrated camera colors. The Supergel line, made from body-colored polycarbonate, is the most heat-resistant filter available, with a softening point of 320 F (160 C) compared to polyester's 260 F (126 C). Both Lee and Rosco use numbered cataloguing systems that are standard vocabulary on professional sets worldwide: a gaffer asks for "Lee 201" or "Rosco 3407" by number, not by name.

How It's Used in Practice

Scenario 1 - Mixed Light Correction (Gaffer): An interior scene is lit by tungsten practical lamps (3200K) and an ARRI M18 HMI (5600K) punching through a window. The DP wants to shoot at 3200K white balance. The gaffer cuts a sheet of Rosco 3407 Full CTO to fit the M18's gel frame. The HMI output shifts from 5600K to approximately 3200K, with a transmission loss of about 0.8 stops. The gaffer bumps the HMI ballast from 70% to 85% output to compensate. All sources now match at 3200K. Camera white balance is set once and locked for the entire scene.

Scenario 2 - Creative Night Exterior (DP / Gaffer): For a night exterior intended to read as moonlight, the DP asks for a cool key at approximately 7000K. The gaffer applies half CTB (Rosco 3204, shifts 3200K to 4100K) plus a sheet of Rosco 60 (Mist Blue) to a 2.5K HMI on a 40-foot Condor. The combined effect cools the source to approximately 7000K with a slightly desaturated blue tint. The practical street lamps in the background are warm sodium (2200K). The 5000K color separation between the cool key and the warm practicals creates a color contrast that reads as natural urban night.

Scenario 3 - ND Gel on Window (Gaffer): An interior day scene has a background window reading 4 stops hotter than the subject at f/2.8. The gaffer cannot access the window exterior. They cut Lee 209 (0.6 ND, 2-stop reduction, 25% transmission) to the window's interior dimensions and tape it flat with gaffer tape. The window now reads 2 stops hotter than the subject, within the camera's dynamic range. Total material cost: approximately $8 per sheet. Time: 10 minutes.

Usage Examples in Sentences

"Put a full CTO on the M18. 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 color in full, half, and quarter grades."

Common Confusions & Misuse

Gel vs. Diffusion Gel: Both come in the same physical format: sheets of polyester film in 20x24 inch sheets or 48x25 foot rolls. Color and correction gels modify the color or intensity of light without intentionally scattering it. Diffusion gels scatter light to soften shadows and reduce contrast. The confusion arises because both are called "gels" and are applied the same way (in gel frames, clipped to barndoors, or taped to windows). On a lighting plan or expendables order, they are listed separately: correction gels by name and grade (e.g., "Rosco 3407 Full CTO x4"), diffusion gels by type and density (e.g., "Lee 410 Opal x6").

Gel vs. Camera Filter: A gel modifies light at the source, before it enters the scene. A camera filter modifies light at the lens, after it has left the scene. A 1/2 CTB gel on one tungsten fixture cools only that fixture. A 1/2 CTB camera filter cools the entire image uniformly. Gels allow per-source control; camera filters apply a global shift. Using a camera filter to correct one mismatched source will shift every other source in the frame in the same direction, which is usually not what you want.

CTB vs. CTO Direction: CTB (Color Temperature Blue) makes light cooler (raises kelvin). CTO (Color Temperature Orange) makes light warmer (lowers kelvin). A common mistake among beginners is applying CTO when they need CTB or vice versa. Memory aid: "Blue goes Up, Orange goes dOwn" in kelvin terms.

Variations by Context

Key People & Films

Rosco Laboratories received an Academy Award in 1974 for developing the Cinegel range, and a second in 1998 for the CalColor system. Lee Filters, founded by cinematographer David Holmes BSC in 1967, became the standard gel brand on UK and European productions. Oswald Morris BSC used Lee 164 Flame Red on Oliver! (1968), one of the first feature films to use the new polyester gel system. Vittorio Storaro, ASC, AIC collaborated with Rosco to develop The Storaro Selection, a curated range of color gels calibrated to his specific color philosophy, used on films like The Last Emperor (1987) and Bulworth (1998). On Blade Runner 2049 (2017), DP Roger Deakins and gaffer Bill O'Leary used custom gel combinations on ARRI SkyPanels to achieve the film's distinctive color-separated lighting, though the SkyPanel's built-in color mixing reduced the volume of physical gel needed compared to a tungsten-era production.

Equipment / Tools Reference

• [Rosco Cinegel](https://www.bhphotovideo.com/c/search?Ntt=rosco+cinegel&BI=24167&KBID=29577): Over 200 color, correction, and diffusion filters on polyester (PET) base. Softening point: 260 F (126 C). Standard sheets: 20in x 24in. Rolls: 48in x 25ft. Part numbers: 3202 (Full CTB), 3407 (Full CTO), 209 (0.6 ND).
• [Lee Filters Lighting Range](https://www.bhphotovideo.com/c/search?Ntt=lee+filters+lighting+gel&BI=24167&KBID=29577): Hundreds of numbered colors and corrections on polyester base. Standard sheets: 21in x 24in. Rolls: 48in x 25ft. Part numbers: 201 (Full CTB), 285 (Full CTO), 209 (0.6 ND).
• Rosco Supergel: Body-colored polycarbonate filters with superior heat resistance. Softening point: 320 F (160 C). Flame-retardant rated (M1, B1, C1). 24in wide rolls.
• Gel Frames: Metal or cardboard frames that hold gel sheets in front of lighting fixtures. Available in sizes matching standard fixture barndoors (6in, 8in, 10in, 18in).
• Gel Carts: Wheeled carts organized with labeled drawers for each gel type, typically stocked with full, half, and quarter grades of CTB, CTO, and ND plus key creative colors.

Standards & Specifications

Rosco Cinegel filters are manufactured on polyester (PET) base with a softening point of 260 F (126 C) and a melting point of 290 F (143 C), per Rosco's published specifications. Rosco Supergel filters are manufactured on polycarbonate base with a softening point of 320 F (160 C) and a melting point of 425 F (220 C), making them the most heat-resistant color filter available. Supergel has earned the German B1 flame-rating, French M1, Italian C1, and NFPA 701 classifications. Color correction gels are specified by mired shift value (a constant independent of source color temperature), not by kelvin shift alone. The Rosco Filter Facts document provides the complete mired shift and transmission data for every Cinegel correction filter. Full CTB (Rosco 3202) has a mired shift of -131 and transmits 36% of light. Full CTO (Rosco 3407) has a mired shift of +131 and transmits approximately 59% of light.

Common Questions / FAQ

Q: How long do gels last on hot lights?

A: On tungsten fixtures (3200K), a gel placed directly on the barndoors may last 4-12 hours of continuous use before fading or burning through. HMI fixtures run hotter and shorten gel life. LED fixtures run cool enough that gels last indefinitely. Using gel frames (which create an air gap between the gel and the fixture) extends life significantly on all sources.

Q: Do I still need gels with color-tunable LED fixtures?

A: For color correction, often not. Fixtures like the ARRI SkyPanel S60-C can dial any color temperature from 2800K to 10000K without gel. However, creative color gels, diffusion, and ND gels are still essential. Some DPs also prefer the specific color character of a physical gel over the LED's digital color mixing.

Q: What is the difference between CTB and "Plus Green" or "Minus Green"?

A: CTB shifts color temperature (kelvin). Plus Green and Minus Green correct for the green spike in fluorescent and LED sources. Plus Green (Rosco 3304) adds green to match fluorescent sources. Minus Green (Rosco 3315) removes green from fluorescent or LED sources. They solve a different problem than CTB/CTO.

Q: Can I layer multiple gels on one fixture?

A: Yes, but each layer reduces light output. Layering full CTB (36% transmission) with 0.6 ND (25% transmission) gives a combined transmission of approximately 9% (-3.5 stops). Mired shift values are additive when layering correction gels, which makes calculating combined correction shifts straightforward.

Related Terms

• Key Light - The source most commonly modified by correction or creative gels
• Diffusion - A category of gel that scatters light to soften shadows; physically similar to color gels but functionally different
• Gaffer - The crew member who selects, cuts, and applies gel to lighting instruments
• Color Temperature - Gels shift the color temperature of light sources to achieve a matched balance across all sources in a scene
• Backlighting - Back lights are frequently gelled with creative colors (blue, amber) to create color separation between subject and background
• CTB / CTO / CTS - The three primary color correction gel types: Blue (cools), Orange (warms), and Straw (warms with yellow bias)

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. For a deeper dive into color temperature and how correction gels interact with camera white balance, see the Color Temperature Explained blog post.

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