The ‘Cooling’ Gel 💙
CTB stands for Color Temperature Blue. It is the direct counterpart to CTO and is used to make a ‘warm’ (orange) light source appear ‘cool’ (blue). Its primary function is to balance mixed lighting situations by making tungsten-balanced lights match the color of daylight.
Corrective and Creative Applications
The most common technical use for a CTB gel is when shooting in a daylight-lit environment. If a filmmaker is shooting in a room with large windows and needs to add an artificial light (which is often a tungsten-based Fresnel), they would place a CTB gel over the tungsten light. This raises the color temperature of the warm 3200K lamp to match the cool 5600K daylight coming through the windows. With the camera white-balanced to daylight, both light sources will now appear as a natural, neutral white.
Like CTO, CTB also comes in standardized strengths for precise control:
- Full CTB: Converts 3200K tungsten all the way up to 5600K daylight.
- 1/2 (Half) CTB: A partial shift towards blue.
- 1/4 (Quarter) CTB: A subtle cooling effect.
- 1/8 (Eighth) CTB: A very slight cooling effect.
The ‘Moonlight’ Effect 🌕
Beyond simple correction, CTB is very frequently used for a creative purpose: simulating moonlight. The human eye perceives moonlight as having a cool, blueish tint. To create this effect for a night exterior scene, a cinematographer will often place a Full CTB gel on a powerful light source like an HMI or a large LED. Sometimes, they will combine the CTB with a touch of green gel (like Plus Green) to create a more stylized, cyan-colored moonlight look. This technique is a long-standing convention in cinematography for signaling to the audience that a scene is taking place at night.