CTB

CTB

noun | Lighting

Colour Temperature Blue — a family of colour correction gels manufactured primarily by Lee Filters and Rosco that raise the colour temperature of a warm light source by filtering its output through a calibrated blue medium. Full CTB converts a standard tungsten fixture (approximately 3200K) to approximate daylight balance (approximately 5600K). Available in full, half, quarter, and eighth strength increments, CTB gels allow precise colour temperature adjustment of tungsten sources when a production is shooting at daylight white balance or when tungsten and HMI sources must match in the same frame.

Quick Reference

The Explanation: How & Why

Tungsten light fixtures emit light at approximately 3200K — a warm, orange-biased colour that, when a camera is set to daylight white balance (5600K), renders as visibly orange. This is the correct rendering of the physics: tungsten is genuinely warmer than daylight. But in many production situations, a cinematographer wants a tungsten fixture to contribute to a daylight-balanced image without being visibly warm — matching an HMI light, a window, or the overall daylight balance of the production.

CTB gel solves this by filtering the tungsten output through a blue medium that absorbs the warm wavelengths and transmits a cooler, higher-colour-temperature light. The calibrated CTB families are designed so that each strength performs a known conversion:

• Full CTB: Converts 3200K to approximately 5600K — a complete daylight conversion
• 1/2 CTB: Converts 3200K to approximately 4300K — halfway between tungsten and daylight
• 1/4 CTB: Converts 3200K to approximately 3800K — a modest cool shift
• 1/8 CTB: Converts 3200K to approximately 3500K — a very subtle cool correction

When to use CTB:

The most common scenario is mixed lighting: HMI sources (daylight-balanced, 5600K) are positioned outside windows to simulate natural daylight, while tungsten practical fixtures and tungsten supplemental lights are used inside the set. If the camera white balance is set to 5600K for the HMI sources, the tungsten fixtures will read orange unless corrected with CTB. Full CTB on the tungsten sources converts them to match the HMI balance.

Light loss:

CTB gel absorbs some of the light passing through it, reducing the output of the fixture. Full CTB reduces light output by approximately two stops — a tungsten fixture with full CTB produces about a quarter of its original output. Half CTB costs approximately one stop; quarter CTB approximately half a stop. The DP must account for this light loss when calculating exposure, either by increasing the fixture's wattage, adjusting the camera's ISO, or opening the aperture.

Physical application:

CTB gels are cut to size and attached to the fixture's barn doors or gel frame using C47s (clothespegs). In hot tungsten fixtures, the gel must be held at sufficient distance from the lamp to avoid melting. On lower-temperature LED and fluorescent fixtures, this concern is reduced but the gel still degrades over time under prolonged heat.

Historical Context & Origin

CTB is a Lee Filters product designation that became the industry standard term for the entire category of cooling colour correction gels. Lee Filters, a UK-based professional lighting gel manufacturer, developed the CTB family as part of their colour correction range in the years following the company's founding in 1968. The specific conversion values of each CTB strength are based on the Kelvin scale and the spectral properties of the gel medium. Rosco Laboratories in the United States produces an equivalent range. The Lee designation "CTB" and Rosco's equivalent products are treated as interchangeable on professional sets.

How It's Used in Practice

Scenario 1 -- Interior Mixed Lighting (Gaffer / DP): An interior location is being lit with HMI sources through the windows (5600K) and tungsten fill lights inside. The DP sets the camera to 5600K white balance to match the HMI sources. The gaffer places full CTB on all the tungsten fill fixtures to convert them to daylight balance. The resulting image shows consistent colour across all light sources.

Scenario 2 -- Partial Conversion (Gaffer): A scene requires the feeling of a late afternoon interior — warm but not orange. The DP wants the tungsten practicals to register as slightly warm but not uncorrected. The gaffer uses half CTB on the practical-replacing fixtures, leaving them at approximately 4300K — noticeably warm but not the full orange of uncorrected tungsten.

Scenario 3 -- Exposure Compensation (DP): A 2K tungsten fresnel with full CTB is the key light for a setup. The gaffer reports that the full CTB is cutting approximately two stops of output. The DP adjusts: they open the aperture by one stop and raise the camera's ISO by one stop to recover the lost exposure, maintaining the T-stop for the depth of field needed for the shot.

Usage Examples in Sentences

"Full CTB on the 2K and the redhead. We are shooting at 5600 today — everything needs to match."

"Half CTB gives you that warm but not-orange quality. It is good for interiors that should feel lived-in."

"Full CTB costs you almost two stops. Factor that into your exposure before you commit to the setup."

"CTB is in the gel bag — Lee 201 or Rosco equivalent. They work the same."

Common Confusions & Misuse

CTB vs. CTO: CTB (blue) raises colour temperature — it cools a warm source. CTO (orange) lowers colour temperature — it warms a cool source. They perform opposite conversions. CTB is used on tungsten sources shooting on daylight balance; CTO is used on HMI or daylight sources shooting on tungsten balance or when a warm effect is required.

CTB vs. Generic Blue Gels: CTB is a calibrated colour correction gel with known conversion values (full CTB = 3200K to 5600K). Generic blue decorative or effect gels (such as Lee 183 Moonlight Blue) are not colour correction gels and do not perform a known Kelvin conversion. Using a decorative blue gel in place of CTB produces an uncalibrated, unpredictable result.

Related Terms

• CTO -- Colour Temperature Orange; the opposite conversion gel — warms a cool source
• CTS -- Colour Temperature Straw; a subtle warm tint gel, less extreme than full CTO
• Gel -- The broader category of which CTB is a specific colour-correction member
• White Balance -- The camera setting that CTB correction works in conjunction with; CTB is used when white balance is set to daylight and the source is tungsten
• Gaffer -- The lighting department head responsible for selecting and applying CTB gels to fixtures

See Also / Tools

The Shot List Generator is relevant to CTB planning — setups involving mixed lighting sources (HMI with tungsten) require CTB for colour consistency, and a detailed shot list showing interior vs. exterior lighting allows the gaffer to plan CTB requirements for each setup.