Thin ND filter gel applied to fixtures or windows; flexible and cuttable for precise light reduction.
Technical Details
Standard ND gels are 0.1mm to 0.3mm thick and consist of a polyester base material with carbon particles or metallic coatings. Common density levels include 0.3 ND (50% light reduction), 0.6 ND (25%), 0.9 ND (12.5%), 1.2 ND (6.25%), and 1.8 ND (1.56%). Spectral transmission varies between 400-700nm by a maximum of ±2%, keeping color shifts below critical thresholds. Lee Filters offers ND gels in rolls 762mm wide, while Rosco uses 610mm. Heat-resistant variants can withstand up to 120°C continuous load.
History & Development
Kodak developed the first photographic ND filters from optical glass for still cameras in 1952. Lee Filters launched flexible ND gels for film production in 1967, initially only in 0.9 ND strength. In 1974, Rosco expanded its range to include continuously variable ND gels. The introduction of Inconel-coated gels in 1989 significantly improved heat resistance. Since 2010, new sputtering coating processes have enabled extremely uniform density values with tolerances below 1%.
Practical Use in Film
In "Blade Runner 2049" (2017), Roger Deakins used ND 1.2 gels in front of tungsten sources to consistently work at T2.8 aperture during daylight shots. ND gels are standard when HMI lights need to be dimmed for night scenes, as electronic dimming causes color temperature shifts. The workflow requires precise light measurement after applying the gel, as manufacturer specifications can deviate by up to 0.2 stops. Reflections on the gel surface are a disadvantage, creating ghost images at unfavorable angles.
Comparison & Alternatives
Unlike CTO/CTB gels, ND material exclusively changes the amount of light, not the color temperature. Scrim fabric reduces light by fixed 0.5 or 1.0 stops, but offers no intermediate steps. Electronic dimmers operate steplessly but significantly shift Kelvin values with tungsten lamps. Variable ND filters on the lens allow continuous adjustment but exhibit color casts and vignetting at extreme settings. ND gels remain the first choice for critical color grading projects when spectral neutrality is crucial.