CTB (Color Temperature Blue) - HowToFilmSchool.com

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Definition

CTB (Color Temperature Blue) is a blue-colored color-correction gel placed on lighting fixtures to shift the color temperature of tungsten or warm light sources toward daylight. It is one of the most commonly used gels in film lighting and a standard tool in every grip/electric department’s kit.

Full CTB converts 3200K tungsten light to approximately 5600K daylight, allowing different light sources to match each other on camera.

What CTB Does

Lights on set rarely come from a single color temperature. Tungsten units produce warm orange light (around 3200K), while daylight and many HMIs/LEDs sit around 5600K or higher. CTB helps bridge this gap.

CTB shifts a light by:

Cooling warm tones
Balancing tungsten sources with daylight
Correcting for mixed lighting conditions
Matching sources for proper white balance and skin tones

Using CTB prevents mismatched color temperatures that can create muddy shadows, inconsistent skin tone, or unwanted color casts.

Common Strengths of CTB

CTB comes in multiple densities depending on how much cooling is needed:

Full CTB ? 3200K ? 5600K
½ CTB ? Warmer daylight balance or subtle cool shift
¼ CTB ? Mild cool tint; useful for balancing practicals
? CTB ? Very subtle color shift; often used cosmetically on faces or in fill light

Electrics and gaffers choose density based on the lighting scenario and how they plan to white-balance the camera.

When CTB Is Used

1. Balancing Tungsten to Daylight

Typical when mixing tungsten fresnels with daylight windows or HMIs.

2. Cooling Practicals

Practical lamps (household bulbs) can be gelled with ¼ or ½ CTB to avoid going overly orange in a daylight-balanced scene.

3. Creating Cool Stylistic Light

CTB isn’t only technical—directors of photography sometimes use it intentionally for a cooler, more stylized look.

4. Matching LED Sources

Many LEDs vary in spectral output; CTB can fine-tune color to match other units on set.

CTB vs. CTO

CTB (Color Temperature Blue): Warms ? Cool (Tungsten ? Daylight)
CTO (Color Temperature Orange): Cool ? Warm (Daylight ? Tungsten)

Both are essential for maintaining a consistent visual color environment.

Practical Considerations

CTB cuts light output, especially in full strength.
Heat from tungsten fixtures can fade or wrinkle gels—always clip and space them properly.
Modern RGBWW LEDs can dial in color temperatures without gels, but gels remain widely used on fresnels, HMIs, and practicals.

Related Terms

CTO – Color Temperature Orange, warms daylight to tungsten.
3200K / 5600K – Standard tungsten and daylight color temperatures.
Diffusion Gel – Softens the beam; often used with CTB.