CTO

CTO

noun | Lighting

Color Temperature Orange (CTO) is a family of calibrated color correction gels manufactured by Lee Filters and Rosco that lower the color temperature of a cool light source by filtering its output through a calibrated orange medium. Full CTO converts a daylight-balanced source at approximately 6500K to tungsten balance at approximately 3200K. Available in full, half, quarter, and eighth strength increments, CTO gels allow precise color temperature adjustment of HMI and LED sources when a production shoots at tungsten white balance or when a warm creative effect is required.

Quick Reference

The Explanation: How & Why

HMI fixtures output at approximately 5600K to 6000K, matching daylight and making them ideal for exterior fill, window light, or any setup requiring a cool, clean source. But many situations demand warm light from a source that would otherwise read as daylight-cool: interior practical lamp quality, sunset or golden hour effects, firelight. A cinematographer facing this problem needs to shift the color temperature of the HMI or LED output downward without replacing the fixture.

CTO gel solves this by absorbing the cool blue wavelengths and transmitting warmer, lower-color-temperature light. Each strength performs a known conversion based on the Kelvin scale and the spectral absorption properties of the orange dye in the polyester substrate:

• Full CTO (Lee 204): Converts 6500K to approximately 3200K. On an HMI outputting 5600K, full CTO brings the source to approximately 2900K to 3000K, slightly warmer than nominal tungsten.
• 1/2 CTO (Lee 205): Converts 6500K to approximately 3800K. On a 5600K HMI, this lands near 3400K to 3500K.
• 1/4 CTO (Lee 206): Converts 6500K to approximately 4600K. A modest warm shift visible on a monitor but not extreme.
• 1/8 CTO (Lee 207): Converts 6500K to approximately 5200K. A very subtle warm correction, barely perceptible without a reference.

The light loss from CTO is typically 1 to 1.5 stops for full CTO, decreasing proportionally with weaker grades. This is less severe than full CTB (approximately 2 stops) because the orange wavelengths CTO transmits are closer to the peak sensitivity of many camera sensors. The DP must account for this loss when calculating exposure.

CTO gels are cut to size and attached to fixture barn doors or gel frames using C47s (clothespegs). On hot tungsten-adjacent fixtures, the gel must be held at sufficient distance from the lamp to avoid melting. LED fixtures run cooler and extend gel life significantly, though all gels degrade under prolonged heat and should be replaced when discoloration appears.

Historical Context & Origin

CTO is a Lee Filters product designation that became the industry standard term for the warming color correction gel category. Lee Filters, founded in 1967 in Andover, UK, 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-dyed polyester. Rosco Laboratories produces an equivalent warming gel range under the same CTO designation. Both manufacturers' products are used interchangeably on professional sets. The Lee Filters color temperature calculator provides exact conversion values for each gel grade across different source color temperatures. CTO is less frequently used than CTB in standard production because most contemporary productions shoot at daylight balance, making CTB the more common need. CTO is essential when warm effects or tungsten-balance matching is required.

How It's Used in Practice

Scenario 1 - Sunset Effect (Gaffer / DP): A scene set at sunset requires the quality of low, warm, directional golden light. The production cannot rely on actual sunset timing. The gaffer positions two ARRI M18 HMI fixtures (1800W each) low and to the side of the set, places Lee 204 Full CTO on each, and aims them at a 15-degree angle across the set. The result reads as warm, directional sunset light at approximately 3000K. The setup is available on demand throughout the shooting day, and the gaffer reports approximately 1.2 stops of light loss from the full CTO.

Scenario 2 - Tungsten Balance Production (Gaffer): A production shooting on an ARRI ALEXA Mini LF at 3200K white balance uses tungsten practical lamps in the set dressing. Any HMI lights used as supplemental fill must receive full CTO to match the tungsten balance. Without CTO, the HMIs will read as blue against the warm practicals. The gaffer pre-cuts Lee 204 for each HMI fixture on the lighting order and confirms the match on a calibrated monitor.

Scenario 3 - Firelight Effect (Gaffer / DP): A scene around a campfire requires flickering warm light at approximately 2000K to 2500K. The gaffer places a 1K fresnel with two layers of full CTO (doubling the warming effect) low and in front of the fire position. A grip waves a 4x4 silk frame in front of the fixture irregularly to create flickering movement. The double CTO pushes the color temperature below 3000K, into the deep orange range of real firelight.

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 color temperature. It warms a cool source. CTB (blue) raises color temperature. It cools a warm source. They perform opposite conversions. CTO goes on HMI or daylight sources when warm quality is needed. CTB goes on tungsten sources when daylight balance is needed. Confusing the two produces the opposite of the intended result. The mnemonic is simple: Orange warms, Blue cools.

CTO vs. Warm Effect Gels: CTO is a calibrated color correction gel with known conversion values (full CTO = 6500K to 3200K per Lee Filters specifications). Warm amber decorative or effect gels (such as Lee 105 Amber or Rosco Bastard Amber) may produce similar warm tones but are not calibrated to specific Kelvin conversions. For precise color matching across multiple fixtures, calibrated CTO is the correct choice. For a single fixture where exact Kelvin matching does not matter, a decorative warm gel may suffice.

Full CTO on 5600K vs. 6500K: Lee Filters calibrates CTO conversions using 6500K as the daylight reference. In practice, HMI fixtures output at approximately 5600K, not 6500K. Full CTO on a 5600K HMI will bring the source to approximately 2900K to 3000K, slightly warmer than the nominal 3200K tungsten target. This is usually acceptable, but DPs who need a precise 3200K match may combine 1/2 CTO with a small additional piece of 1/4 CTO to fine-tune the conversion.

Variations by Context

Key People & Films

Gordon Willis used warm tungsten-balanced lighting extensively in The Godfather (1972), creating the film's signature amber interiors. While Willis worked primarily with tungsten sources rather than CTO-corrected HMIs, his aesthetic established the visual standard that CTO-corrected HMIs replicate on modern productions. Roger Deakins used warm HMI correction on Blade Runner 2049 (2017) for the orange-tinged Las Vegas sequences, where CTO and additional warming gels on large HMI units created the post-apocalyptic heat quality. Conrad Hall's warm interior lighting in Road to Perdition (2002) relied on careful color temperature matching between practicals and supplemental sources, a workflow that CTO makes possible on daylight-balanced fixtures.

Equipment / Tools Reference

• Lee 204 Full CTO: The standard full CTO gel, converting 6500K to 3200K. Available in rolls and sheets.
• Rosco 3407 Full CTO: Rosco's equivalent full CTO product, used interchangeably with Lee 204 on professional sets.
• ARRI SkyPanel S60-C: A bi-color LED fixture with built-in color temperature control from 2900K to 10000K, eliminating the need for CTO gel in many setups.
• Astera Titan Tube: A wireless LED tube with full color temperature control, often used as a warm practical replacement without requiring CTO.

Standards & Specifications

Lee Filters publishes exact transmission values for each CTO grade on their product specification pages. Full CTO (Lee 204) has a Mired Shift value of +159, transmission Y of 55.4% measured against a daylight source, and absorption of 0.26. The Mired shift system (micro-reciprocal degrees) is the standard unit for color correction gel specification. A positive Mired shift indicates warming (lowering color temperature). The Academy Color Encoding System (ACES) provides a framework for managing color temperature conversions digitally, but physical CTO gel remains the standard for on-set light source correction.

Common Questions / FAQ

Q: How much light does full CTO cost?

A: Full CTO reduces light output by approximately 1 to 1.5 stops, depending on the fixture and gel manufacturer. Lee 204 Full CTO transmits approximately 55% of daylight-sourced light. This is less severe than full CTB, which transmits approximately 25% and costs approximately 2 stops.

Q: Can I use CTO on LED fixtures?

A: Yes, but many modern bi-color LED fixtures (ARRI SkyPanel, Astera Titan Tube, Litepanels Astra) have built-in color temperature control and do not need CTO. CTO on LED fixtures is necessary only for fixed-color LEDs or when the built-in warming range is insufficient. LED fixtures run cooler than tungsten or HMI, so CTO gel lasts longer on them.

Q: What is the difference between CTO and CTS (Color Temperature Straw)?

A: CTO produces a full orange warming conversion. CTS produces a warming conversion with a yellow bias rather than orange. Full CTS (Lee 441) converts 6500K to 3200K, the same nominal target as full CTO, but the resulting light is straw-colored rather than orange. CTS is often preferred for skin tone warmth. CTO is preferred for full tungsten matching.

Q: Do I need CTO if I am shooting RAW?

A: RAW recording captures full sensor data and allows color temperature adjustment in post, but CTO corrects the light source before it reaches the sensor. If an HMI reads too cool on set, CTO fixes the actual light. RAW adjustment shifts the entire image globally. CTO allows selective warming of individual fixtures while leaving others unaffected.

Related Terms

• CTB - Color Temperature Blue; the opposite conversion gel. Cools a warm source toward daylight.
• CTS - Color Temperature Straw; a warming gel with a yellow bias rather than orange. Less extreme than full CTO for subtle warmth.
• Gel - The broader category of which CTO is a specific color-correction member.
• White Balance - The camera setting that CTO correction works alongside. 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 and HMI setups all require CTO, and a detailed shot list helps the gaffer identify which setups will need it. For understanding how color temperature affects exposure, see the Exposure Triangle blog post.