CTB

CTB

acronym | Lighting

Colour Temperature Blue (CTB) is 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 (3200K) to approximate daylight balance (5700K). 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 (Lee 201): Converts 3200K to approximately 5700K. Mired shift: -137. Transmission: 34%. Light loss: approximately 1.5 stops.
• 1/2 CTB (Lee 202): Converts 3200K to approximately 4300K. Mired shift: -68. Transmission: 59%. Light loss: approximately 0.8 stops.
• 1/4 CTB (Lee 203): Converts 3200K to approximately 3800K. Mired shift: -35. Transmission: 76%. Light loss: approximately 0.4 stops.
• 1/8 CTB (Lee 204): Converts 3200K to approximately 3500K. Mired shift: -18. Transmission: 87%. Light loss: approximately 0.2 stops.

The mired shift formula quantifies the conversion: Mired Shift = (1,000,000 / T2) - (1,000,000 / T1), where T1 is the original colour temperature and T2 is the target. Full CTB has a mired shift of -137, meaning it reduces the mired value by 137 units. Gaffers use mired values to combine gels for custom conversions.

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 (Lee 201) transmits only 34% of the light, costing approximately 1.5 stops. A 650W tungsten fixture with Full CTB effectively becomes a 220W daylight source. Half CTB transmits 59%, costing approximately 0.8 stops. Quarter CTB transmits 76%, costing approximately 0.4 stops. 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, founded in 1967 in Andover, England, developed the CTB family as part of their professional lighting gel range. The specific conversion values of each CTB strength are based on the Kelvin scale and the spectral properties of the gel medium. Rosco, founded in 1910 in New York, produces an equivalent range under its Cinegel system (R3202 Full Blue CTB), which earned an Academy Award for the product line. The Lee designation "CTB" and Rosco's equivalent products are treated as interchangeable on professional sets. The need for CTB arose with the development of colour film stock in the mid-20th century: tungsten-balanced film (Type B, 3200K) and daylight-balanced film (Type D, 5500K) required matching light sources to the stock. When shooting daylight-balanced film with tungsten fixtures, CTB gel converted the tungsten output to match the film stock.

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 (Lee 201) on all the tungsten fill fixtures to convert them to daylight balance. Each fixture loses approximately 1.5 stops of output (34% transmission). The gaffer compensates by using 1K fixtures instead of 650W units. 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 (Lee 202, -68 mired) on the practical-replacing fixtures, leaving them at approximately 4300K - noticeably warm but not the full orange of uncorrected tungsten. Transmission is 59%, costing only 0.8 stops.

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 1.5 stops of output (34% transmission). The DP adjusts: they open the aperture by one stop and raise the camera's ISO by one stop on the ARRI ALEXA Mini LF 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 a stop and a half. Factor that into your exposure before you commit to the setup."

"CTB is in the gel bag - Lee 201 or Rosco R3202. 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 5700K, -137 mired). 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.

Full CTB vs. Two Half CTBs: Stacking two Half CTB gels does not produce the same result as one Full CTB. Two Half CTBs (each -68 mired) produce a combined mired shift of -136, which is close to Full CTB (-137 mired), but the transmission is lower: two Half CTBs transmit approximately 35% combined (59% x 59%), compared to Full CTB at 34%. The results are nearly identical, but stacking is less efficient and increases heat absorption between the gel layers.

Variations by Context

Key People & Films

Cinematographer Roger Deakins used CTB on tungsten fixtures throughout Skyfall (2012) to match interior tungsten sources with daylight-balanced exterior shots in London. Cinematographer Rachel Morrison used Half CTB on tungsten fixtures in Mudbound (2017) to create a mixed-source naturalism that read as period-appropriate without full daylight conversion. Cinematographer Bradford Young used Quarter CTB on Arrival (2016) to subtly cool tungsten practicals, creating the clinical but warm scientific aesthetic that defined the film's visual language. Rosco earned an Academy Award for its Cinegel system, which includes the CTB family.

Equipment / Tools Reference

Lee Filters Technical Filters provide the full CTB range: Lee 201 Full CTB (-137 mired, 34% transmission), Lee 202 Half CTB (-68 mired, 59% transmission), Lee 203 Quarter CTB (-35 mired, 76% transmission), Lee 204 Eighth CTB (-18 mired, 87% transmission). Rosco Cinegel equivalents: R3202 Full Blue CTB (-131 mired, 36% transmission), R3203 Half Blue CTB, R3204 Quarter Blue CTB. Rosco's GAM equivalent chart cross-references all three manufacturers. The Lee Filters Colour Temperature Calculator computes the required gel for any source-to-target conversion. C-stands with gel frames hold CTB in front of fixtures; C47s (clothespegs) secure gel to barn doors; spring clamps attach gel to windows and practical fixtures.

Standards & Specifications

CTB gels do not have formal SMPTE or ITU standards. However, the mired shift system is a CIE (International Commission on Illumination) recognised method for quantifying colour temperature changes. Lee Filters publishes transmission data (Y%, x, y chromaticity coordinates, absorption, and mired shift) for every CTB gel in their technical filter catalogue, measured against both Daylight (Source C) and Tungsten (3200K) sources. Rosco publishes equivalent data in their FilterFacts catalogue. The mired shift formula (M = 1,000,000/T2 - 1,000,000/T1) is the standard calculation used by gaffers and DPs to determine CTB gel combinations for custom conversions. The Academy of Motion Picture Arts and Sciences awarded Rosco a Technical Oscar for the Cinegel system.

Common Questions / FAQ

Q: How much light does Full CTB cost?

A: Full CTB (Lee 201) transmits 34% of the light, costing approximately 1.5 stops. A 650W tungsten fixture with Full CTB effectively becomes a 220W daylight source. If you cannot afford the light loss, use Half CTB (0.8 stops) and adjust the camera white balance to 4300K as a compromise.

Q: Can I use CTB on LED fixtures?

A: Most modern bi-colour LED fixtures (ARRI SkyPanel S60, Astera Titan Tube, Aputure 600d) have built-in colour temperature control and do not require CTB. However, if you have a fixed-colour-temperature LED (e.g., a 3200K Aputure 300d), you can apply CTB to convert it to daylight balance just as you would with a tungsten fixture.

Q: What is the difference between Lee CTB and Rosco CTB?

A: The mired shift values differ slightly: Lee 201 Full CTB is -137 mired; Rosco R3202 Full Blue CTB is -131 mired. The transmission is also slightly different (Lee: 34%, Rosco: 36%). In practice, the difference is imperceptible on camera and the two are treated as interchangeable on professional sets.

Q: Why does CTB lose more light than CTO?

A: Full CTB transmits 34% of light; Full CTO transmits 55.4%. CTB absorbs a broader portion of the spectrum (the warm red and orange wavelengths) to achieve the blue shift. CTO absorbs primarily the blue wavelengths, which represent a smaller portion of the tungsten spectrum. The physics of spectral absorption means cooling a warm source always costs more light than warming a cool source.

Related Terms

• CTO - Colour Temperature Orange; the opposite conversion gel that warms a cool source
• CTS - Colour Temperature Straw; a yellow-biased alternative to CTO for warming daylight
• CTB, CTO, and CTS - The combined glossary entry covering all three gel families together
• Gel - The broader category of coloured lighting filters; CTB is a specific colour-correction gel type
• 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

Use the Colour Temperature Calculator to determine the exact CTB strength needed for any source-to-target colour temperature conversion. For planning lighting setups with mixed colour temperatures, the [Lighting Setup Planner] helps map which fixtures need CTB and which gel strength to order from the expendables list.