Last Updated 5 days ago
Filmmaking Dictionary: Letter C / CTB (Color Temperature Blue)
Definition
CTB (Color Temperature Blue) is a blue colour-correction filter used to shift a light toward a higher colour temperature. A common application is bringing tungsten light closer to daylight balance. It can also make a source deliberately cooler relative to the camera’s white balance.
CTB absorbs some of the light passing through it. It does not add blue output, and matching colour-temperature numbers alone does not guarantee that two different sources will render colours identically.
Full, Half and Eighth CTB
Full, half, quarter and eighth describe grades within a filter range. They are not fixed exposure losses or universal Kelvin additions.
- Full CTB: a stronger conversion commonly used to shift tungsten toward daylight. LEE 201, for example, is specified for a 3200K source to approximately 5700K.
- Half CTB: a smaller cooling shift than full CTB within the same range.
- Eighth CTB: a lighter correction used when only a small change is wanted.
Choose a product from its actual specification and test it on the source. “Daylight” itself varies, and a tungsten lamp changes colour when dimmed.
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Quarter CTB
Quarter CTB, written ¼ CTB, is a relatively light blue colour-correction grade. It is useful when a source needs a modest cooling adjustment rather than full tungsten-to-daylight conversion. LEE 203 Quarter CTB is specified at a mired shift of −35 and takes a nominal 3200K source to about 3600K.
That example does not mean quarter CTB always adds 400K. Colour-temperature changes are more usefully compared through mired shift, and the starting source affects the result. A 3600K result is still substantially warmer than a typical 5600K daylight reference.
Quarter CTB is also unrelated to a quarter-stop exposure change, quarter grid cloth or a quarter-spot beam adjustment.
Using CTB on Set
A tungsten unit beside an uncorrected window may look warm when the camera is balanced for daylight. CTB can bring that unit closer to the window light. Alternatively, the cinematographer may leave some warmth for a deliberate contrast.
Use a lighter grade when full correction removes more warmth than the scene needs. Keep the camera’s white-balance setting consistent while comparing gels; otherwise the camera may conceal the change you are trying to judge.
For a repeatable setup, record the fixture, dimmer level, gel manufacturer and product number. “A bit of blue” is difficult to reproduce when a later shot needs to match.
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Output, Heat and Matching
- Allow for transmission loss, especially with stronger CTB grades.
- Use the fixture’s approved filter holder or mounting method and observe heat clearances.
- Check gels for fading, damage or colour changes after use.
- Test the actual source and camera; a gel cannot supply wavelengths that the source does not produce.
- Treat green–magenta correction separately from a simple warm–cool adjustment.
A suitable adjustable-white LED may achieve the required colour temperature electronically. That does not make all sources interchangeable, and output or colour rendering can still change with the setting.
CTB Versus CTO
CTB shifts toward a cooler appearance at a fixed white balance. CTO shifts toward a warmer appearance. Neither is a neutral-density filter or a diffusion material.
See the lighting department guide for how colour matching fits into lighting preparation.
Related Terms
- CTO (Color Temperature Orange)
- Color Temperature
- 3200K Colour Temperature
- 5600K Colour Temperature
- Gel / Gels
- Fractional Diffusion, Gel and Filter Strengths
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