What CTB and CTO Actually Do

Every gel roll in a grip truck is solving the same underlying problem: two light sources on the same shot don’t share a color temperature. CTB stands for Color Temperature Blue and shifts a warmer, lower-Kelvin source (like a 3200K tungsten Fresnel) toward the cooler, higher-Kelvin end of the scale (daylight, around 5600K). CTO stands for Color Temperature Orange and does the opposite: it shifts a cooler source down toward tungsten warmth.

Think of the Kelvin scale as a dial. Tungsten household and studio fixtures sit around 3200K and read orange to a camera balanced for daylight. Daylight and most HMIs sit around 5600K and read blue to a camera balanced for tungsten. Gel physically absorbs part of the light spectrum to nudge a fixture’s output toward the opposite end of that dial, so it matches whatever the rest of the scene is doing.

Full, Half, and Quarter Strength

Both gels come in graduated strengths. A “full” gel does the whole conversion; fractional strengths (1/2, 1/4, 1/8) shift color partway, which is often more useful than a full correction when you just want to nudge a source rather than flip it entirely.

Common On-Set Uses

• Gelling a tungsten Fresnel with full CTB so it matches daylight spilling through a window
• Gelling an HMI, window, or daylight-balanced LED with CTO to match interior tungsten practicals
• 1/4 or 1/8 CTO on an interview key light to warm skin tone slightly without a full color shift
• Day-for-night work, where CTB is combined with underexposure and a cooler white balance
• Matching two LED panels from different manufacturers that shipped slightly off from each other

If you want to skip the math entirely, our CTO/CTB gel calculator converts between Kelvin values and gel strength directly.

Gel vs. Physically Changing the Fixture

Gel isn’t the only way to fix a color mismatch. You could swap a tungsten bulb for a daylight-balanced one, use a bi-color or RGB LED that dials in Kelvin electronically, or correct in-camera with white balance. Gel remains the standard for one reason: it works on any fixture instantly, without rewiring or replacing a lamp mid-shoot.

Placement: On the Fixture or On the Window

Gel can go directly on a light or across an entire window, and the choice changes the math. Gelling a window with a large CTO sheet converts all the daylight pouring through it in one pass, which is efficient when the window is a major light source and the interior is lit with tungsten or warm LED. Gelling individual fixtures is cheaper per light and more flexible, but has to be redone any time a fixture moves.

• Window gel: large rolls (48″ or wider), applied once, ideal for static interior setups
• Fixture gel: cut to size per light, faster to swap, better for setups that move between shots
• On big windows, some crews use rigid or roll CTO/CTB rather than clipping loose sheets, for a cleaner hold in wind or HVAC airflow

Buying Tips

• Rosco and Lee are the two standard gel brands used across most productions
• Sold in cut sheets (roughly 20″×24″) for small fixtures and in rolls for windows or large soft sources
• Use heat-resistant gel near HMIs and tungsten Fresnels — standard gel can bubble or melt against a hot lamp housing
• LED panels run cooler, so budget gel is usually fine there
• Keep a swatch book of small cut samples in the kit for quick reference and testing before committing a full sheet