CTO

CTO

noun | Lighting

Colour Temperature Orange — a family of colour correction gels manufactured primarily by Lee Filters and Rosco that lower the colour temperature of a cool light source by filtering its output through a calibrated orange medium. Full CTO converts a daylight-balanced source such as an HMI fixture (approximately 5600K) to approximate tungsten balance (approximately 3200K). Available in full, half, quarter, and eighth strength increments, CTO gels allow precise colour temperature adjustment of HMI and fluorescent sources when a production is shooting at tungsten white balance, or when a warm creative effect — sunset, firelight, interior lamp — is required.

Quick Reference

The Explanation: How & Why

HMI fixtures are the standard high-output location and studio lights for daylight-balanced production. They emit at approximately 5600K — matching daylight and making them ideal for use through windows, as exterior fill, or anywhere a cool, clean source is needed. But there are many situations where a cinematographer needs a warm light — interior practical lamp quality, sunset or golden hour effects, firelight — from a source that would otherwise read as daylight-cool.

CTO gel solves this by filtering the HMI or fluorescent output through an orange medium that absorbs the cool blue wavelengths and transmits warmer, lower-colour-temperature light. The calibrated CTO families perform known conversions:

• Full CTO: Converts 5600K to approximately 3200K — a complete tungsten conversion
• 1/2 CTO: Converts 5600K to approximately 4300K — halfway between daylight and tungsten
• 1/4 CTO: Converts 5600K to approximately 4800K — a modest warm shift
• 1/8 CTO: Converts 5600K to approximately 5100K — a very subtle warm correction

When to use CTO:

Tungsten white balance production: When a production is shooting on tungsten white balance (3200K), HMI lights will read as blue if uncorrected. Full CTO on the HMIs converts them to match the tungsten balance.

Warm creative effects: CTO is widely used to create warm-light effects — sunset quality, golden hour fill, the light of a practical interior lamp. Placing full or half CTO on an HMI and positioning it low and to the side of the subject creates a convincing warm-light quality that reads as sunset or firelight, regardless of actual time of day.

Matching practicals: In an interior scene with tungsten practical lamps (table lamps, floor lamps) as the motivated light source, supplemental HMI sources used for additional fill may need CTO to match the warm quality of the practicals.

Light loss:

CTO gel reduces the output of the fixture it is applied to, though typically less severely than CTB because the orange wavelengths it transmits are closer to the peak efficiency range of many sensors. Full CTO reduces output by approximately one to one-and-a-half stops, depending on the specific gel product and fixture type.

Physical application:

CTO gels are cut to size and attached to fixture barn doors or gel frames using C47s (clothespegs). They are handled identically to CTB gels in terms of physical application; the same heat resistance considerations apply for tungsten-adjacent fixtures.

Historical Context & Origin

CTO is a Lee Filters product designation that became the industry standard term for the warming colour correction gel category. Lee Filters developed the CTO family alongside CTB as part of their professional lighting gel range. The specific conversion values are based on the Kelvin scale and the spectral absorption properties of the orange medium. Rosco Laboratories produces an equivalent warming gel range. Both are used interchangeably on professional sets. The CTO gel is less frequently used than CTB in standard production because most contemporary production shoots at daylight balance, making CTB (for correcting tungsten sources) the more common need — but CTO is essential when warm effects are required.

How It's Used in Practice

Scenario 1 -- Sunset Effect (Gaffer / DP): A scene set at sunset requires the quality of low, warm, golden light. The production cannot rely on actual sunset timing. The gaffer positions two 2.5K HMI fixtures low and to the side of the set, places full CTO on each, and aims them at a low angle across the set. The result reads as warm, directional sunset light — convincing in quality and available on demand throughout the shooting day.

Scenario 2 -- Tungsten Balance Production (Gaffer): An older-style production chooses to shoot at tungsten white balance (3200K) to take advantage of tungsten practical lamps in the sets. Any HMI lights used as supplemental fill must receive full CTO to match the tungsten balance; otherwise they will read as blue against the warm practicals.

Scenario 3 -- Firelight Effect (Gaffer / DP): A scene around a campfire requires the quality of flickering warm light. The gaffer places a 1K fresnel with full CTO low, in front of the fire position, and has a grip wave a silk in front of it irregularly to create the flickering quality. The full CTO on the fresnel gives the light the warm, orange quality of real firelight while the silk-waving creates the movement.

Usage Examples in Sentences

"Full CTO on both HMIs. We are going for sunset quality and I do not want any blue in the key."

"Half CTO on the back HMI — I want it slightly warmer than the key but not fully converted."

"If you are shooting tungsten balance, everything HMI needs CTO. Everything."

"CTO is in the gel bag, right next to the CTB. They live together because you always need both."

Common Confusions & Misuse

CTO vs. CTB: CTO (orange) lowers colour temperature — it warms a cool source. CTB (blue) raises colour temperature — it cools a warm source. They perform opposite conversions. CTO is used on HMI or daylight sources when warm quality is needed; CTB is used on tungsten sources when daylight balance is needed. Confusing the two will produce the opposite of the intended result.

CTO vs. Warm Effect Gels: CTO is a calibrated colour correction gel with known conversion values (full CTO = 5600K to 3200K). Warm amber decorative or effect gels (such as Lee 205 Half CT Orange or Rosco Gold) may produce similar warm tones but are not calibrated to specific Kelvin conversions. For precise colour matching across multiple fixtures, calibrated CTO is preferred over decorative warm gels.

Related Terms

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

See Also / Tools

The Shot List Generator is relevant to CTO planning — shots requiring warm-light effects, sunset quality, or mixed tungsten/HMI setups all require CTO, and a detailed shot list helps the gaffer identify which setups will need it.