CTB, CTO, and CTS

CTB, CTO, and CTS

acronym | Lighting

CTB (Colour Temperature Blue), CTO (Colour Temperature Orange), and CTS (Colour Temperature Straw) are lighting gel families used to shift the colour temperature of a light source to match another source or to achieve a specific colour balance. CTB cools tungsten light (3200K) toward daylight (5700K). CTO warms daylight (6500K) toward tungsten (3200K). CTS warms daylight toward tungsten with a yellow bias rather than the reddish-orange shift of CTO. Each family is available in graded strengths: Full, Three-Quarter, Half, Quarter, and Eighth.

Quick Reference

The Explanation: How & Why

Every light source has a colour temperature measured in Kelvin (K). Tungsten fixtures like the ARRI 650 Plus Fresnel output 3200K. Daylight-balanced sources like HMI lamps and LED fixtures output 5500-6500K. When a scene mixes tungsten and daylight sources, the colour temperatures must be matched or the image will show inconsistent colour - one area appears orange, another appears blue.

CTB solves the problem of using tungsten lights in a daylight-balanced scene. A sheet of Full CTB (Lee 201) placed over a 3200K tungsten fixture shifts its output to approximately 5700K, matching photographic daylight. The cost is light loss: Full CTB transmits only 34% of the light, absorbing the rest. A 650W tungsten fixture with Full CTB effectively becomes a 220W daylight source.

CTO works in the opposite direction. A sheet of Full CTO (Lee 204) placed over a 6500K daylight source (like a window or HMI) shifts its output to 3200K, matching tungsten. Full CTO transmits 55.4% of the light - less lossy than CTB because the gel absorbs blue wavelengths rather than the broader spectrum.

CTS (Colour Temperature Straw) performs the same warming function as CTO but with a yellow-gold bias rather than the reddish-orange of CTO. Cinematographers often prefer CTS for skin tones, as the yellow shift produces a more natural warmth on Caucasian and East Asian skin. CTS was introduced by Rosco in the 1990s as an alternative to the redder CTO. Lee 441 Full CT Straw has a mired shift of +160, nearly identical to Lee 204 Full CTO at +159 - the difference is in the spectral character of the shift, not the magnitude.

The mired (micro reciprocal degree) shift value quantifies how much a gel changes colour temperature. The formula: Mired Shift = (1,000,000 / T2) - (1,000,000 / T1), where T1 is the original colour temperature and T2 is the target. Full CTB (Lee 201) has a mired shift of -137. Full CTO (Lee 204) has a mired shift of +159. Gaffers use mired shift values to combine gels for custom conversions that standard strengths do not provide.

All three gel families come in Full, Half, Quarter, and Eighth strengths. Full performs the complete conversion; Half performs roughly half; Quarter performs roughly a quarter. This gradation allows precise colour matching. CTB and CTO gels are heat-resistant polyester films placed in gel frames in front of fixtures. In hot tungsten fixtures, gels must be placed at sufficient distance from the lamp to avoid melting. Gels degrade under heat and should be replaced when they show discolouration.

Historical Context & Origin

Colour temperature correction gels originated 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. Early correction filters were glass or gelatin sheets placed over camera lenses or light fixtures. Lee Filters, founded in 1967 in Andover, England, pioneered the polyester-based lighting gel system that became the industry standard. Rosco, founded in 1910 in New York, developed its Cinegel system in the 1970s, earning an Academy Award for the product line. Rosco introduced CT Straw filters in the 1990s as an alternative to the redder CTO. The energy crisis of the 1970s drove the adoption of new light sources (HMI, CID, Xenon) that were neither standard tungsten nor daylight, requiring more nuanced correction gels and leading to the Plus Green/Minus Green families alongside CTB/CTO/CTS.

How It's Used in Practice

Scenario 1 - Daylight Interior (Gaffer): A gaffer needs to light a daylit interior with tungsten fixtures while the camera is white-balanced to 5600K. They apply Full CTB (Lee 201) to each ARRI 650W Plus Fresnel, shifting the output from 3200K to 5700K. Each fixture loses approximately 1.5 stops of light (transmission 34%). The gaffer compensates by using 1K fixtures instead of 650W units to maintain the target exposure at T4.0 on the ARRI ALEXA Mini LF.

Scenario 2 - Window Correction (Gaffer / DP): A scene is shot tungsten-balanced (3200K) but features large windows letting in 6500K daylight. The gaffer applies Full CTO (Rosco R3407) to the windows, shifting the incoming daylight to 3200K. The Rosco R3407 transmits 47% of the light, reducing the window's exposure by approximately 1 stop. The DP rates the scene at T2.8 on a Sony VENICE 2 to compensate for the reduced window light.

Scenario 3 - Mixed Source Matching (Gaffer): A practical scene uses an HMI (5600K) as the key light and a tungsten practical lamp (3200K) in the background. The gaffer applies Half CTO (Lee 205, +109 mired shift) to the HMI, shifting it to 3800K. They apply Quarter CTB (Lee 203, -35 mired shift) to the practical, shifting it to 3600K. The two sources now match within 200K, close enough for the camera's white balance to render both naturally.

Usage Examples in Sentences

"Put Full CTB on the tungsten units - the camera is balanced to 5600K and we need to match the daylight coming through the windows."

"Half CTO on the HMI will warm it enough to match the practicals without losing too much output."

"I prefer CTS over CTO for close-ups - the yellow bias reads more naturally on skin than the orange shift."

"Full CTB costs you a stop and a half of light. If you cannot afford the loss, use Half CTB and adjust the camera white balance to 4300K as a compromise."

Common Confusions & Misuse

CTB vs. CTO Direction: CTB cools light (shifts toward blue/daylight). CTO warms light (shifts toward orange/tungsten). A common mistake among beginners is applying CTB to warm a source or CTO to cool one. The mnemonic: Blue gels make light bluer (cooler); Orange gels make light oranger (warmer).

CTS vs. CTO: Both warm daylight toward tungsten. CTO produces a reddish-orange shift. CTS produces a yellowish-gold shift. The mired shift values are nearly identical (Lee 441 CTS: +160; Lee 204 CTO: +159). The difference is spectral, not magnitude. CTS is often preferred for skin tones; CTO is the standard for general daylight-to-tungsten conversion.

Full CTB vs. "Blue Gel": Not all blue gels are CTB. Rosco's NCB (New Colour Blue) series produces dramatic blue effects for creative lighting, not colour temperature correction. CTB is specifically calibrated to shift colour temperature along the Kelvin scale. A generic blue gel will shift the colour but not to a measurable target temperature.

Variations by Context

Key People & Films

Cinematographer Roger Deakins used CTS gels extensively on Blade Runner 2049 (2017) to achieve the warm amber practical looks throughout the Las Vegas sequences, preferring the yellow-gold bias of straw over the reddish shift of CTO. Cinematographer Emmanuel Lubezki used CTO on windows throughout The Revenant (2015) to match natural daylight to the tungsten-balanced interior firelight scenes. 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. Rosco earned an Academy Award for its Cinegel system, which includes the CTB, CTO, and CTS families.

Equipment / Tools Reference

Lee Filters Technical Filters provide the full CTB, CTO, and CTS range: Lee 201 Full CTB (-137 mired, 34% transmission), Lee 204 Full CTO (+159 mired, 55.4% transmission), Lee 441 Full CT Straw (+160 mired, 57.3% transmission). Rosco Cinegel equivalents: R3202 Full Blue CTB (-131 mired, 36% transmission), R3407 Roscosun CTO (+167 mired, 47% transmission), R3411 Full CT Straw. 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 the gels in front of fixtures; spring clamps secure gels to windows and practical fixtures.

Standards & Specifications

Colour temperature correction 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. The Academy of Motion Picture Arts and Sciences awarded Rosco a Technical Oscar for the Cinegel system. Lee Filters publishes transmission data (Y%, x, y chromaticity coordinates, absorption, and mired shift) for every 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 gel combinations for custom conversions.

Common Questions / FAQ

Q: How much light do I lose when using Full CTB?

A: Full CTB (Lee 201) transmits 34% of the light, costing approximately 1.5 stops. Full CTO (Lee 204) transmits 55.4%, costing approximately 1 stop. If you cannot afford the light loss, use Half or Quarter strength and adjust the camera white balance to a compromise colour temperature.

Q: What is the difference between CTO and CTS?

A: Both warm daylight to tungsten, but CTO produces a reddish-orange shift while CTS produces a yellowish-gold shift. Their mired shift values are nearly identical (CTO: +159, CTS: +160). CTS is often preferred for skin tones because the yellow bias reads more naturally. CTO is the standard for general daylight-to-tungsten conversion.

Q: Can I stack gels to get a stronger effect?

A: Yes. Two sheets of Full CTO (Lee 204) produce a mired shift of +318, shifting 6500K to approximately 2500K. However, each layer absorbs additional light - two Full CTOs transmit only about 30% of the original light. Stacking is common for extreme warming effects but requires compensating with higher-wattage fixtures.

Q: Do I need CTB if I am shooting with LED panels?

A: Most modern LED fixtures (ARRI SkyPanel S60, Astera Titan Tube, Aputure 600d) have built-in colour temperature control from 2700K to 6500K and do not require gels. CTB/CTO/CTS are primarily used on tungsten fixtures, HMI lamps, and for window correction. Some DPs still use Quarter CTO on LEDs for a subtle warming effect that the built-in controls do not replicate precisely.

Related Terms

• CTB - Colour Temperature Blue; the specific gel family that cools tungsten toward daylight
• CTO - Colour Temperature Orange; the specific gel family that warms daylight toward tungsten
• CTS - Colour Temperature Straw; the yellow-biased alternative to CTO for warming daylight
• Gel - The broader category of coloured lighting filters; CTB, CTO, and CTS are specific gel types
• Gaffer - The head of the lighting department who determines which gels to use and where
• Key Light - The primary light source in a scene; often the first fixture to receive colour temperature correction

See Also / Tools

Use the Colour Temperature Calculator to determine the exact gel 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 correction and which gels to order from the expendables list.

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