Color Temperature Blue

Overview

Color Temperature Blue (short form CTB, also called "Full Blue" or "Daylight Blue") is a blue color conversion filter (gel) placed in front of a light fixture to raise the color temperature of a warm artificial light source (typically 3200 K incandescent/tungsten) towards daylight. It is the counterpart to the orange CTO (Color Temperature Orange), which corrects in the opposite direction.

CTB is among the most frequently used filters in the lighting and grip department. It is used to match tungsten light to incident daylight or to daylight sources (HMI, LED daylight), so that multiple light sources on set harmonize with each other. CTB is available as a full conversion (Full) and in several partial conversions (1/2, 1/3, or 1/4, 1/8) for finer tuning of daylight under different cloud conditions, voltage drop at a dimmer, or aged lamps.

Principle of Operation

The filter absorbs part of the long-wavelength (red/yellow) spectrum and allows comparatively more blue light through, thus increasing the correlated color temperature. The correction is specified in Mired Shift (negative value = increase in color temperature). The Mired Shift of a filter is constant, while the target Kelvin value achieved depends on the source's initial color temperature. With increasing density (from 1/8 to Full), the degree of correction increases, while the light transmission decreases, as the filter costs light output.

Technical Data

Standard values from the two market-leading manufacturers. Kelvin values refer to a 3200 K tungsten source.

Note: Rosco offers partial conversions as 1/2, 1/3, and 1/4, while LEE offers them as 1/2, 1/4, and 1/8 – therefore, the names are not directly equivalent between manufacturers.

On-Set Usage

• Matching Tungsten to Daylight: Full CTB in front of a tungsten fixture (e.g., Fresnel, Blonde, Redhead) to match it to daylight coming through a window or to HMI sources.
• Supplementing Window Light: Interior shots where artificial light should support the daylight in the scene without creating a warm color cast.
• Partial Conversion for Fine-Tuning: 1/4 or 1/8 CTB to make slightly too warm light appear "cooler," to compensate for dimmed (reddish) light, or to bring aged HMI/fluorescent lamps back towards the target daylight.

Filters are attached to the barn doors or stretched in frames in front of the lens; with powerful fixtures, sufficient distance and heat resistance (heat shields or high-temp versions) must be ensured, otherwise the filter will fade or melt. The light loss caused by CTB (especially Full CTB) must be considered in the lighting calculations.