Transparent polyester diffusion gel placed in front of lights to soften harsh illumination. Lee Filters and Rosco offer grades from 25–73% light transmission.
Technical Details
Modern diffusion gels are made of polyester film or acetate base with incorporated scattering particles or embossed surface structures. The thickness varies between 0.1mm and 0.5mm, with standard sheet sizes being 50x60cm and 100x120cm. Lee Filters categorizes its diffusion filters from Light Frost (Lee 200, 65% transmission) to Heavy Frost (Lee 253, 25% transmission). Rosco offers similar gradations with its Cinegel series, such as Tough White Diffusion (Rosco #216, 73% transmission). The gels can withstand temperatures up to 150°C and have an unlimited lifespan with LED lights, or approximately 100 burning hours with tungsten lights.
History & Development
Diffusion filters emerged in the 1930s as an evolution of tissue paper and frosted glass, which were already used in stage technology. Lee Filters introduced the first standardized gelatin filters for film in 1967, followed by Rosco in 1970 with the Cinegel line. The breakthrough came in 1985 with heat-resistant polyester films that could also be used with HMI lighting. Since 2010, manufacturers have been developing special LED-optimized gels with improved color rendition and longer lifespan.
Practical Use in Film
In "Her" (2013), DoP Hoyte van Hoytema used Heavy Diffusion in front of all window lights to enhance the melancholic mood. Emmanuel Lubezki diffused the available daylight in "The Revenant" (2015) through large Silk frames with 1/2 CTO Diffusion. Typical workflow: Diffusion gel is stretched in a filter frame and mounted 30-60cm in front of the light source. With soft lights, the lux output is reduced by 20-75%, but this creates even illumination without hotspots. Disadvantage: Light loss requires stronger base lighting.
Comparison & Alternatives
Unlike bouncing, where light is reflected, diffusion maintains the direction of light. Softboxes integrate diffusion directly into the fixture body, while separate gels allow for more flexible positioning. Silk frames with stretched diffusion are more mobile than filter gels in fixed frames. Modern LED panels with built-in diffusion are increasingly replacing the combination of a hard light source plus filter, but offer fewer adjustment options for light characteristics.