The ‘Warming’ Gel 🧡
CTO stands for Color Temperature Orange. It is one ofthe most fundamental and commonly used tools in any lighting kit. Its purpose is purely corrective: to make a ‘cool’ (blueish) light source appear ‘warm’ (orange). The primary use case for CTO is to balance mixed lighting scenarios where the cinematographer needs to make different types of lights match each other.
The Science of Color Correction
The two primary standards for ‘white’ light in film are Daylight (approx. 5600K), the cool, blue light from the sun or an HMI lamp, and Tungsten (3200K), the warm, orange light from an incandescent bulb. A camera can only be white-balanced for one of these standards at a time.
A CTO gel is precisely calibrated to shift the color of daylight towards tungsten. If a scene is being shot indoors with tungsten lights, but there is also daylight contamination from a window, a gaffer would place a CTO gel on the window. This warms up the incoming daylight, making it match the color temperature of the interior tungsten lights. To the camera, all the light in the scene now appears to be the same color.
CTO Gel: Standardized Strengths
To allow for precise control, CTO gels (manufactured by companies like Lee Filters and Rosco) come in standardized densities:
- Full CTO: Converts 5600K daylight all the way down to 3200K tungsten.
- 1/2 (Half) CTO: Provides a half-correction, for a moderately warm look.
- 1/4 (Quarter) CTO: A subtle warming effect.
- 1/8 (Eighth) CTO: A very slight warming effect, often used on a key light to give skin tones a healthy glow.
While its main purpose is corrective, cinematographers frequently use lighter strengths like 1/4 or 1/2 CTO for purely creative reasons, adding warmth to a scene to evoke a feeling of comfort, nostalgia, or to simulate the ‘golden hour’ of late afternoon sun.