CTS

CTS

noun | Lighting

Color Temperature Straw (CTS) is a family of color correction gels manufactured by Lee Filters and Rosco that add a yellow-biased warm tint to a light source. Unlike CTO, which produces an orange warming shift, CTS produces a straw-yellow warming shift with a similar mired shift value. Full CTS (Lee 441) converts 6500K daylight to 3200K tungsten with a yellow bias. Available in full, half, quarter, and eighth strengths, CTS gels are used for skin tone warming, late afternoon light effects, and subtle refinement of cool sources where the orange character of CTO would be too strong.

Quick Reference

The Explanation: How & Why

The CTS gel occupies a specific niche in the color correction gel family that neither CTB nor CTO fills: a warming correction with a yellow spectral character rather than an orange one. Many lighting setups that are technically correct in terms of color balance still feel slightly cold or clinical, particularly setups using HMI or LED sources at their native 5600K output. A small amount of warmth often makes the difference between an image that feels accurate and one that feels inviting. CTS delivers that warmth with a straw-yellow bias that reads more naturally on skin tones than the orange bias of CTO.

The Lee Filters CTS family performs known Kelvin-scale conversions with a yellow bias:

• Full CTS (Lee 441): Converts 6500K to 3200K with a yellow bias. Mired shift +160. Transmission 57.3% measured against daylight.
• 1/2 CTS (Lee 442): Converts 6500K to 4300K. A moderate warm shift with straw character.
• 1/4 CTS (Lee 443): Converts 6500K to 5100K. A subtle warm shift, barely perceptible without a reference.
• 1/8 CTS (Lee 444): Converts 6500K to 5700K. A very faint warm tint, essentially a finishing gel. Transmission 83.1%.

The mired shift of full CTS (+160) is nearly identical to full CTO (+159). Both perform the same nominal Kelvin conversion. The difference is spectral: CTO shifts the light toward orange-red, while CTS shifts toward yellow-gold. This spectral difference matters most in close-up work where skin tone rendering is visible. The yellow bias of CTS is generally more flattering on human skin than the orange bias of CTO, which is why many gaffers default to CTS for skin tone warming and reserve CTO for full tungsten matching or sunset/firelight effects.

CTS is typically used in three ways. First, subtle key light warmth: a daylight-balanced key at 5600K can read as slightly cool on skin tones in close-up. Adding 1/4 or 1/8 CTS warms the skin tone rendering without changing the apparent color temperature of the scene. Second, late afternoon quality: when a scene is set in late afternoon but shot at a neutral time of day, 1/4 CTS on the key suggests the warmth of late afternoon without the full orange conversion of CTO. Third, post-CTB refinement: when CTB has converted a tungsten source to daylight but the result feels too clinical, adding CTS on top brings the source back to a warmer neutral.

Light loss from CTS is minimal. Full CTS transmits 57.3% of daylight-sourced light, costing approximately 1 to 1.5 stops. Quarter CTS transmits 73% and costs less than half a stop. Eighth CTS transmits 83% and costs less than a quarter stop, making it essentially free warmth.

Historical Context & Origin

CTS is a Lee Filters product designation developed alongside the CTB and CTO families as part of their professional color correction gel range. The "straw" designation reflects the gel's pale yellow-amber appearance, the color of wheat straw. Lee Filters developed the CTS to address the gap between uncorrected daylight sources and the more substantial conversions of the CTO range. The Lee Filters technical filters catalogue lists the full CTS range with exact conversion values. Rosco produces an equivalent pale warm correction gel in their color correction range. Both are standard professional products carried in the gel bags of gaffers on most productions. The CTS range has gained prominence as LED fixtures have replaced tungsten in many lighting setups, because the yellow-biased warmth of CTS compensates for the clinical quality of LED light more naturally than the orange bias of CTO.

How It's Used in Practice

Scenario 1 - Skin Tone Warmth (Gaffer / DP): A close-up setup uses an ARRI M40 HMI (4000W) through a 4x4 Chimera as the key at 5600K. The DP looks at the monitor and finds the skin tones slightly cold, technically correct but not flattering. The gaffer adds Lee 443 Quarter CTS to the key. The monitor shows an immediate improvement in skin tone rendering, warmer and more natural, without a noticeable change to the scene's overall color temperature. The quarter CTS costs less than half a stop of light.

Scenario 2 - Afternoon Quality (DP / Gaffer): A scene set in late afternoon is shot mid-morning on a soundstage. The DP wants the light to have a slightly warmer, late-day quality without obviously looking like sunset. The gaffer adds Lee 443 Quarter CTS to the ARRI M18 HMI key coming through the window side of the set. The shift moves the key from 5600K to approximately 5100K. The audience will not notice the correction, but the image feels more like afternoon than morning.

Scenario 3 - Post-CTB Refinement (Gaffer): A 2K tungsten fresnel has received full CTB (Lee 201) to convert it to daylight balance. The result is technically correct at approximately 5600K but feels slightly too clinical. The gaffer removes the full CTB and substitutes Lee 205 Half CTB plus Lee 443 Quarter CTS, landing the fixture at approximately 4800K. The combination is close enough to match the HMI sources on camera but with a slightly warmer, less clinical quality. The CTS functions as a fine adjustment to the broader CTB correction.

Usage Examples in Sentences

"Give it Lee 443 on the key. Not CTO, just quarter straw. I want subtle warmth, not orange."

"CTS is what you use when the image is almost right but needs just a touch of warmth to live."

"Half CTB plus quarter CTS gets me to about 4800K. Good match for mixed sources where I do not want fully neutral."

"Quarter straw barely costs you any light. It is essentially free warmth."

Common Confusions & Misuse

CTS vs. CTO: Both are warming gels with nearly identical mired shift values (CTS +160, CTO +159). The difference is spectral character. CTO shifts light toward orange-red. CTS shifts light toward yellow-gold. Full CTS and full CTO both convert 6500K to 3200K, but the resulting color is visibly different side by side. CTS is generally more flattering on skin tones. CTO is better for full tungsten matching and sunset or firelight effects. Using CTO when CTS is needed gives the source an orange cast. Using CTS when CTO is needed gives it a yellow cast instead of the intended warm orange.

CTS vs. Decorative Amber Gels: Decorative amber gels (such as Lee 162 Bastard Amber, Rosco 339) produce visible warm tints designed to be noticeable as a color effect. CTS is a calibrated correction gel with known Kelvin conversion values. For creative warm effects where the color should be visible, a decorative amber gel is appropriate. For invisible color correction where the shift should be felt but not seen, CTS is the correct choice. The two categories serve different purposes and should not be substituted for each other.

CTS vs. 1/8 CTO: Both produce subtle warming. 1/8 CTO (Lee 207) converts 6500K to 5200K with an orange bias. 1/8 CTS (Lee 444) converts 6500K to 5700K with a yellow bias. The CTS shift is smaller and more yellow. The 1/8 CTO shift is larger and more orange. For the faintest possible warm correction, 1/8 CTS is the more subtle choice.

Variations by Context

Key People & Films

Roger Deakins used straw-toned warming on 1917 (2019) to maintain the warm, golden quality of the film's continuous-take aesthetic, where LED and HMI sources required careful color correction to match the practical torchlight and firelight motivation. Bradford Young used subtle warm correction gels on Arrival (2016) to give the interior scenes a naturalistic, slightly warm quality that avoided the clinical look of uncorrected LED sources. Hoyte van Hoytema's work on Tenet (2020) involved extensive color temperature matching between HMI and LED sources, where straw-toned gels provided the intermediate warmth needed to unify mixed sources on camera.

Equipment / Tools Reference

• Lee 441 Full CTS: The standard full straw gel, converting 6500K to 3200K with a yellow bias. Mired shift +160.
• Lee 443 Quarter CTS: The most commonly used CTS grade for skin tone warming. Converts 6500K to 5100K. Transmission 73%.
• Rosco 3408 Full CTS: Rosco's equivalent full straw product, used interchangeably with Lee 441.
• ARRI SkyPanel S60-C: A bi-color LED fixture with built-in color temperature control from 2900K to 10000K, plus green-magenta adjustment. Can replicate the CTS warming effect digitally without gel.

Standards & Specifications

Lee Filters publishes exact transmission values for each CTS grade on their product specification pages. Full CTS (Lee 441) has a Mired Shift value of +160, transmission Y of 57.3% measured against a daylight source, and absorption of 0.24. The mired shift system (micro-reciprocal degrees) is the standard unit for color correction gel specification. The nearly identical mired shift values of CTS (+160) and CTO (+159) confirm that both gels perform the same nominal Kelvin conversion. The spectral difference between them is in the chromaticity coordinates: CTS has x=0.426, y=0.407 while CTO has x=0.437, y=0.392, confirming the yellow-versus-orange spectral character.

Common Questions / FAQ

Q: When should I use CTS instead of CTO?

A: Use CTS when the warming needs to feel natural on skin tones, particularly in close-up work. Use CTO when you need a full tungsten match or a warm creative effect like sunset or firelight. CTS produces a yellow-gold warmth. CTO produces an orange-red warmth. Many gaffers default to CTS for subtle skin tone correction and CTO for dramatic warming.

Q: How much light does quarter CTS cost?

A: Quarter CTS (Lee 443) transmits approximately 73% of daylight-sourced light, costing less than half a stop. Eighth CTS (Lee 444) transmits 83%, costing less than a quarter stop. Full CTS transmits 57.3%, costing approximately 1 to 1.5 stops. The light loss from CTS is comparable to CTO at equivalent grades.

Q: Can CTS and CTO be combined?

A: Yes. Some gaffers combine CTS and CTO to achieve a warming effect that sits between the yellow character of CTS and the orange character of CTO. For example, 1/4 CTS plus 1/8 CTO produces a warm shift with a balanced yellow-orange character. This is more common on large-budget productions where the gel inventory supports fine-tuning.

Q: Is CTS still relevant with bi-color LED fixtures?

A: Yes, but less frequently. Bi-color LEDs like the ARRI SkyPanel can replicate the CTS warming effect digitally. However, CTS gel is still needed for HMI fixtures, fixed-color LEDs, and any situation where the built-in warming range of a bi-color LED is insufficient. CTS also provides a consistent reference that does not change if the LED fixture is reset.

Related Terms

• CTB - Color Temperature Blue; the cooling equivalent. Cools a warm source toward daylight.
• CTO - Color Temperature Orange; the stronger warming conversion gel family. CTS provides a yellow-biased alternative to CTO's orange character.
• Gel - The broader category of which CTS is a specific color-correction member.
• White Balance - The camera setting that CTS correction works alongside. CTS refines the warmth of a source relative to the white balance setting.
• Gaffer - The lighting department head responsible for selecting and applying CTS gels.

See Also / Tools

The Shot List Generator is relevant to CTS planning. Close-up shots that will be scrutinized for skin tone rendering are the primary candidates for CTS on the key light, and a shot list showing which setups involve significant close-up work helps the gaffer identify where CTS refinement is most worthwhile. For understanding how color temperature affects exposure, see the Exposure Triangle blog post.