CTB (colour temperature blue) is a blue correction filter used to raise a source’s correlated colour temperature. A common use is correcting a tungsten source toward a daylight appearance.
Conversion and light loss
The resulting CCT depends on the filter and starting source. LEE 201 Full C.T. Blue is specified to convert 3200 K tungsten to approximately 5700 K. Its published tungsten transmission is about 35%, so the correction also has a substantial effect on available light.
Fractional correction
Half, quarter and other strengths provide smaller shifts. These are not simple arithmetic fractions of the kelvin difference. Choose from manufacturer data and check the actual source, especially when correcting LED light with a different spectrum.
Practical example
To use a tungsten Fresnel with daylight-balanced sources, test CTB on the Fresnel and re-meter after fitting it. If output becomes insufficient, changing the lighting plan may be more practical than opening the camera aperture for the whole scene. Follow the fixture and filter mounting instructions so the material is used within its heat limits.
Related terms
CTO (Colour Temperature Orange) · CCT (Correlated Colour Temperature) · Lux