Double CTB

Overview

Double CTB (Color Temperature Blue) is a blue conversion gel used in color correction. It is stretched in front of a light fixture or placed in a filter frame to shift the color temperature of warm artificial light (incandescent/halogen) towards daylight. The "Double" refers to the density: while a Full CTB raises tungsten light (approx. 3200 K) to nominal daylight (approx. 5600 K), Double CTB corrects approximately twice as much and overcorrects significantly towards the cool end.

It is not a single product from one manufacturer but a standardized gel category produced by several suppliers. Common reference numbers include Lee 200 ("Double C.T. Blue"), Rosco/Cinegel 3220 ("Double Blue CTB"), and corresponding GAM CineFilters.

Technical Data

The gel is a deeply dyed, heat-resistant polyester material. Due to its high color density, it absorbs a lot of light. Values vary slightly depending on the manufacturer; typical published specifications:

The strong negative Mired shift illustrates the intense correction towards the cool end. The light loss is significant, in the range of about three stops – considerably more than with Full CTB. The nominally very high target Kelvin values are calculated figures and indicate that Double CTB is designed for overcorrection; in practice, the actual result depends on the source light and the chosen white balance.

On-Set Use

Double CTB is chosen when a Full CTB is insufficient. Typical scenarios:

• Mixed lighting situations where artificial light needs to be matched more closely to bright daylight (e.g., through windows) than a simple CTB can achieve.
• Camera fill or on-camera lighting for exterior daylight shoots where additional correction beyond Full CTB is necessary.
• Intentional cool effects (moonlight, night effect, cool lighting mood) where a distinctly blue look is desired.

Gels are available as individual sheets (commonly around 20x24 inches or 24x21 inches) and as rolls (typically 25 ft length). Due to the high light loss and heat generation, sufficient distance from the light source and adequate power reserves should be planned. Related gels in the same family include Full CTB (e.g., Lee 201) and Half CTB (e.g., Lee 202) for lesser degrees of correction.