Measured light intensity hitting a surface—in lux. Determines your ISO and aperture choices on set.
On set, you measure illuminance with a light meter—this is your tool to know how much light actually reaches the sensor. The value in lux directly determines whether you can work with your planned aperture settings or need to make adjustments. At 500 lux, you'll need a different ISO setting than at 5,000 lux, and this affects grain and dynamic range.
The practical significance lies in predictability: before the first take, you measure at the key light position, in the shadows, on the main character's face. This way, you immediately know if your HMI setup is sufficient or if you need additional Fresnels. For exterior shoots with sunlight, you often have 100,000+ lux—you can work with ND filters and use small apertures. In the studio or for available light scenes, you're often at 200–2,000 lux, which forces higher ISO or longer exposure times. Important: Illuminance is not the same as luminance—the latter measures how bright a surface appears, while illuminance only indicates how much light *falls on it*. A black, matte-painted set can have 2,000 lux of illuminance but looks dark.
In everyday editing, you'll hardly need this value, but when planning and calibrating your workflow—RAW overexposure, sensor performance in low light—you can't avoid it. Some DoPs work by a rule of thumb (e.g., at least 800 lux for native work without noise), others measure meticulously. Your light meter should be a reliable professional tool like your measuring tape—without it, you'll lose time justifying to the production manager why the night scene simply needs more light.