CTS

CTS

noun | Lighting

Colour Temperature Straw — a pale straw or amber colour correction gel manufactured primarily by Lee Filters and Rosco that adds a subtle warm tint to a light source without performing the full Kelvin-scale conversion of a CTO gel. CTS sits at the lighter end of the warming gel spectrum — it shifts the colour temperature of a source gently toward warmth, improving skin tone rendering, adding the quality of late afternoon light, or taking a slight clinical edge off a cool HMI source. It is a finishing or refinement gel rather than a conversion gel, and is used when full or half CTO would over-correct the source.

Quick Reference

The Explanation: How & Why

The CTS gel occupies a specific niche in the colour correction gel family that neither CTB nor CTO fills: the very subtle warm correction that improves an image without noticeably changing its apparent colour temperature. Many lighting setups that are technically correct in terms of colour 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.

The CTS in practice:

CTS is typically used in one of three ways:

Subtle key light warmth: A daylight-balanced key light — an HMI or daylight-balanced LED — at 5600K can read as slightly cool on skin tones, particularly in close-up. Adding CTS to the key warms the skin tone rendering without changing the apparent colour temperature of the scene in a way that would be perceptible to an audience. The effect is felt rather than seen.

Late afternoon quality: When a scene is set in late afternoon light but is being shot at a neutral time of day with studio or HMI sources, CTS can suggest the warmth of late afternoon without the full orange conversion of CTO. The pale straw of CTS shifts the source toward the golden quality of afternoon light without obviously making it look sunset.

Correcting overcooled sources: When CTB has been used to convert a tungsten source to daylight balance, the result can sometimes feel slightly too blue — the CTB conversion has overcooled the source past the intended neutral point. Adding CTS on top of the CTB (or using slightly less CTB with CTS as a finishing gel) brings the source back to a warmer neutral.

CTS vs. CTO:

The distinction between CTS and CTO is one of degree. CTO performs a substantial conversion — even 1/4 CTO shifts colour temperature by several hundred Kelvin in a way that is visible as warmth to the naked eye. CTS performs a gentler shift that is typically invisible to untrained observation but registers in the emotional quality of the image. The choice between CTS and 1/4 CTO depends on how much warmth is needed — if the shift needs to be perceptible, 1/4 CTO; if it needs to be felt without being seen, CTS.

Light loss:

CTS is a pale gel and therefore absorbs very little light — the exposure loss from CTS is typically less than a quarter stop, making it a negligible consideration for exposure calculation.

Physical application:

CTS gels are applied identically to CTB and CTO — cut to size, clipped to barn doors or gel frames using C47s. Their pale colour makes them easy to distinguish from CTO in the gel bag by visual inspection. Like all colour correction gels, they degrade under prolonged heat exposure and should be replaced when discolouration or physical deterioration is observed.

Historical Context & Origin

CTS is a Lee Filters product designation developed alongside the CTB and CTO families as part of their professional colour correction gel range. The "straw" designation reflects the gel's pale yellow-amber appearance — the colour 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, providing a product for the common situation where a small warm correction is needed without the full impact of even 1/4 CTO. Rosco produces an equivalent pale warm correction gel in their colour correction range. Both are standard professional products carried in the gel bags of gaffers on most productions.

How It's Used in Practice

Scenario 1 -- Skin Tone Warmth (Gaffer / DP): A close-up setup uses a large HMI through a Chimera as the key light at 5600K. The DP looks at the monitor and finds the skin tones slightly cold — technically correct but not flattering. The gaffer adds CTS to the key light. The monitor shows an immediate improvement in skin tone rendering — warmer, more natural, more flattering — without a noticeable change to the scene's overall colour temperature.

Scenario 2 -- Afternoon Quality (DP / Gaffer): A scene set in late afternoon is being shot mid-morning. The DP wants the light to have a slightly warmer, late-day quality without obviously looking like a sunset. The gaffer adds CTS to the HMI key light coming through the window side of the set. The shift is subtle — the audience will not notice the correction, but the image feels more like afternoon than morning.

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

Usage Examples in Sentences

"The key is slightly cold. Give it a CTS — not CTO, just CTS. 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 CTS gets me to about 4800K. It is a good match for mixed sources where I do not want fully neutral."

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

Common Confusions & Misuse

CTS vs. CTO: CTO performs a substantial warm conversion — full CTO converts 5600K to 3200K; even 1/4 CTO is a noticeable shift. CTS performs a very subtle warm tint — typically 100-300K — that is felt rather than seen. Using CTO when CTS is needed over-warms the source; using CTS when CTO is needed under-warms it. The choice depends on how much warmth the situation requires.

CTS vs. Decorative Amber Gels: Decorative amber gels (such as Lee 205, Rosco Bastard Amber) produce visible warm tints that are designed to be noticeable as a colour effect. CTS is designed to be invisible — a subtle correction rather than an effect. For creative warm effects, a decorative amber or orange gel is more appropriate; for invisible colour correction, CTS is the right choice.

Variations by Context

Related Terms

• CTB -- Colour Temperature Blue; the cooling equivalent of CTS/CTO; cools a warm source toward daylight
• CTO -- Colour Temperature Orange; the stronger warming conversion gel family; CTS sits at the lighter end of the CTO family's warming range
• Gel -- The broader category of which CTS is a specific colour-correction member
• White Balance -- The camera setting that CTS correction works alongside; CTS is used to refine 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 of actors that will be scrutinised 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.