Why Gel Strength Is Measured in Mireds, Not Kelvin

Color temperature in Kelvin isn’t a linear scale for how gels behave — the same gel shifts different Kelvin values by different amounts depending on where you start. Lighting professionals instead use mireds (micro reciprocal degrees, equal to 1,000,000 divided by the color temperature), because a given gel produces a roughly constant mired shift regardless of the starting temperature. That’s why gel strengths like “1/2 CTO” are rated in mired shift rather than a fixed Kelvin change.

CTO Warms, CTB Cools

CTO (color temperature orange) gels warm a light source, lowering its effective color temperature — useful for converting daylight or HMI sources to match tungsten. CTB (color temperature blue) gels do the opposite, cooling a light and raising its effective color temperature, commonly used to convert tungsten fixtures to match daylight. This calculator computes the mired shift between your source and target, and recommends the closest standard gel strength (or combination) to get there.

Why the Light Loss Numbers Are Approximate

Every gel absorbs some light to shift color, and stronger gels absorb more. The exact stop loss varies by manufacturer and even by batch, so the values here are typical published approximations meant for planning — always confirm the actual transmission of the gel you’re using, especially when exposure precision matters.

This tool is especially useful for:

• Gaffers and lighting techs matching color temperature between mixed light sources on set.
• Photographers gelling flashes to match ambient window or tungsten light.
• Anyone planning exposure around the light loss a gel will introduce.