CTB, CTO, CTS

CTB, CTO, CTS

noun | Lighting

Three families of colour correction gels used in film and television lighting to shift the colour temperature of a light source:

• CTB (Colour Temperature Blue): A blue gel that raises the colour temperature of a warm source, converting tungsten light (approximately 3200K) toward daylight (approximately 5600K).
• CTO (Colour Temperature Orange): An orange gel that lowers the colour temperature of a cool source, converting daylight or HMI light toward tungsten colour balance.
• CTS (Colour Temperature Straw): A pale straw or amber gel that adds a subtle warm tint to a light source without fully converting it to tungsten balance.

These gels are available in full, half, quarter, and eighth strength variants, allowing precise colour balance control across a range of conversion needs.

Quick Reference

The Explanation: How & Why

Different light sources emit light at different colour temperatures — measured in Kelvin (K). Tungsten bulbs emit warm, orange-biased light at approximately 3200K. Daylight from the sky is cool, blue-biased light at approximately 5600K. HMI lights, which are commonly used in film production for their high output efficiency, emit at approximately 5600K, matching daylight.

When mixing light sources of different colour temperatures on the same set — for example, HMI lights through windows alongside tungsten fixtures inside — the mixed result is visually inconsistent. One source will appear orange and the other blue unless they are corrected to match. CTB and CTO gels solve this problem by converting one source to match the other.

CTB in practice:

A full CTB gel placed over a tungsten fixture converts it from approximately 3200K to approximately 5600K — matching daylight and HMI sources. Half CTB converts to approximately 4300K; quarter CTB to approximately 3800K. CTB gels are used when the production is shooting on daylight balance (white balance set to 5600K) but some light sources in the setup are tungsten. Rather than correcting in the camera's white balance (which would affect everything) or in post-production (which reduces latitude), the gel corrects the individual source.

CTO in practice:

CTO converts a cool, daylight-balanced source to a warm, tungsten-balanced one. It is used less frequently than CTB in standard production, but has important applications: warming an HMI to match tungsten practical lamps in a scene, creating a sunset or firelight effect by warming a daylight source, or converting the light coming through windows to match an interior tungsten balance. Full CTO converts 5600K to approximately 3200K; half and quarter CTO provide intermediate conversions.

CTS in practice:

CTS (Colour Temperature Straw) is a pale amber gel that adds warmth without performing a full colour temperature conversion. It is used to add a subtle golden quality to a source — warming skin tones, adding the quality of late afternoon light, or taking the edge off the cool blue of a heavily CTB-converted tungsten fixture that has become too clinical. CTS is a finishing gel rather than a conversion gel.

The full/half/quarter system:

All three gel families come in full, half, quarter, and eighth strengths. Full strength performs the complete conversion; half performs roughly half the conversion; quarter performs roughly a quarter. This gradation allows very precise colour matching — if a source needs to be only slightly warmer, a quarter CTO or even an eighth CTO provides the adjustment without overcorrecting.

Physical properties:

CTB and CTO gels are heat-resistant, translucent plastic films designed to be placed directly in front of or inside lighting fixtures. In hot tungsten fixtures, gels must be placed in gel frames or gel holders at a sufficient distance from the lamp to avoid melting. LED and fluorescent fixtures run cooler and are less demanding of gel heat resistance. Gels degrade over time under heat and should be replaced when they show discolouration or physical deterioration.

Historical Context & Origin

Colour correction gels were developed alongside the film industry's need to mix light sources and control colour balance. Lee Filters, founded in the UK in 1968, developed the CTB, CTO, and CTS designations as part of their professional lighting gel range, which became standard across the industry. Rosco Laboratories in the United States developed a parallel range with similar colour correction families. Both Lee and Rosco gels are industry-standard and their product codes are used as shorthand on call sheets and lighting plans worldwide.

How It's Used in Practice

Scenario 1 -- Interior/Exterior Mix (Gaffer / DP): A scene is set in an interior with large windows. HMI lights outside the windows provide daylight-balanced fill through the glass; tungsten practical lamps provide warm interior light. The DP decides to shoot on daylight balance (5600K white balance) and asks the gaffer to put full CTB on the tungsten fixtures used as fill inside, converting them to match the HMI balance. The practical tungsten lamps are left unconverted, reading as warm practicals — an intentional design choice.

Scenario 2 -- Sunset Effect (Gaffer / DP): A scene set at dusk requires warm orange light raking across the set. The gaffer places full CTO gels on a set of HMI lights positioned low and to the side of the set, converting their daylight-balanced output to a warm tungsten equivalent. The result reads as low sunset light without requiring actual outdoor photography at the specific time of day.

Scenario 3 -- Subtle Warmth (DP / Gaffer): A DP is unhappy with the slightly clinical quality of a daylight-balanced HMI setup on a close-up — the light is correct in terms of exposure and direction but feels slightly cold. The gaffer adds a quarter CTS gel to the key light. The minimal colour shift adds a barely perceptible warmth to the skin tone that the DP wants without meaningfully altering the colour temperature of the setup.

Usage Examples in Sentences

"Put full CTB on the tungsten redhead in the back. I need everything on this setup matching at 5600."

"Half CTO on the HMI coming through the door — I want it to read warm, like it is coming from a lamp inside, not from outside."

"CTS on the key — just a quarter. It is too blue right now and I want to warm the skin up slightly."

"The gel is getting hot in that fresnel. Check it is in a proper gel frame with enough clearance from the lamp."

Common Confusions & Misuse

CTB vs. Blue Gels Generally: CTB is a specifically calibrated colour correction gel designed to perform a known conversion of colour temperature. Decorative blue gels (such as Lee 201 Full CT Blue) are also blue but are not the same product and do not perform the same colour temperature conversion. The CTB family refers specifically to the colour-correction-calibrated gels.

CTO vs. Warm Amber Gels: Similarly, CTO is a calibrated conversion gel; general warm amber gels (such as Lee 205 Half CT Orange or Rosco equivalents) perform the conversion but may have slightly different spectral characteristics. The designations CTO and CTB refer specifically to the colour-correction-calibrated product lines from Lee and Rosco.

Variations by Context

Related Terms

• Gel -- The broader category of which CTB, CTO, and CTS are specific colour-correction members
• White Balance -- The camera setting that CTB and CTO gels are used in conjunction with to achieve colour consistency
• Key Light -- The primary light source that most often receives CTB or CTO correction in mixed-source setups
• Gaffer -- The lighting department head responsible for selecting and applying gels to fixtures
• Colour Temperature -- The underlying physical measurement (in Kelvin) that CTB and CTO are calibrated to adjust

See Also / Tools

The Shot List Generator is complemented by the gel selection process — when planning a shot requiring mixed light sources, the shot list notes which setups involve exterior daylight, interior tungsten practicals, or mixed sources, informing the gaffer's gel requirements before each setup.