Correction Gel

Technical Details

Correction gels are divided into two main categories: CTO (Color Temperature Orange) and CTB (Color Temperature Blue). CTO gels lower the color temperature by 2900K (Full CTO), 1800K (3/4 CTO), 1300K (1/2 CTO), or 600K (1/4 CTO). CTB gels correspondingly increase the color temperature from 3200K to 5600K. The gels reduce light output by 0.3 to 0.6 stops depending on density. Plus green and minus green gels additionally correct magenta-green shifts that occur with fluorescent lamps.

History & Development

Lee Filters developed the first standardized correction gels for the film industry in 1967. Previously, cinematographers mixed various artificial light sources without color correction, leading to inconsistent skin tones. In 1974, Rosco introduced competing products. Digital technology from the 1990s onwards enabled precise color corrections in post-production, yet physical correction gels remain standard for controlled lighting conditions on set.

Practical Application in Film

Cinematographer Roger Deakins extensively used CTO corrections on LED panels in "Blade Runner 2049" (2017) to create warm 2700K interiors. For exterior shots, 1/4 CTO gels correct HMI lights from 5600K to 4300K for more natural skin tones during the Golden Hour. Gaffers mount gels directly onto lights or stretch them in separate frames. Disadvantage: Gels can melt at high temperatures, requiring them to be at least 30 cm away from halogen or Fresnel lights.

Comparison & Alternatives

Correction gels differ from effect gels by their neutral color shift without creative coloring. LED panels with variable color temperature are increasingly replacing gels, but offer less precise spectral distribution. Dichroic filters in modern luminaires allow for stepless color temperature adjustment without light loss. Correction gels remain indispensable for mixed light sources or when precise Kelvin values are required without electronic components.