Blue color correction gel converting tungsten light (3,200K) to daylight (5,600K). Lee 201 and Rosco 3202 are industry standards, available in 1/4 to Full CTB densities.
Technical Details
Full correction CTB gels shift 3,200K to exactly 5,600K and are available in gradations of Full CTB, 3/4 CTB, 1/2 CTB, and 1/4 CTB. The gel is made of heat-resistant polyester with embedded cyan and blue dyes, capable of withstanding temperatures up to 150°C. Lee Filters 201 (Full CTB) and Rosco 3202 are considered industry standards. Spectral transmission shows characteristic absorption peaks at 580-700 nanometers in the yellow-red range.
History & Development
Lee Filters developed the first commercial CTB gel in 1971 in response to the growing need for color temperature correction when mixing artificial and daylight. Rosco followed in 1974 with their own variants. The introduction of HMI lights in the 1970s initially reduced the demand for CTB, but modern LED panels with variable color temperature have made CTB gels indispensable again, as not all LEDs are precisely calibrated.
Practical Use in Film
Roger Deakins extensively used CTB gels on "Blade Runner 2049" (2017) to seamlessly integrate tungsten practicals into daylight scenes. Standard workflow: Tungsten lights are fitted with CTB gel before being mixed as fill light with natural daylight. Disadvantage: Light loss requires stronger lamps or higher ISO values. Advantage: More cost-effective than complete lamp replacement and allows for precise intermediate tones through partial correction.
Comparison & Alternatives
CTO (Color Temperature Orange) gel works in reverse: converting daylight to artificial light. Modern LED panels with adjustable color temperature are increasingly replacing CTB gels, but cost 10-15 times more. ND filters only reduce light quantity without color correction. Minus Green filters correct fluorescent light, not color temperature. CTB remains the first choice for tungsten lamps and affordable LED panels without a bi-color function.