Light striking the surface directly — measured with incident light meter. Critical for accurate exposure independent of surface reflection.
On set, you measure incident light by pointing the light meter directly at the light source — the dome tells you how much energy is actually hitting the surface, regardless of whether it's white, black, or reflective. This is the crucial difference from subject brightness, which the cinematographer captures through the focusing screen or a reflective meter. You're interested in incident light when you want consistent exposure and don't want the surface properties of the set to mislead you — a black wall reflects less back, but the amount of light hitting it is identical to the white one next to it.
In practice, it's about reliability: you position the measuring probe so it faces the same direction as the camera, or even better, you measure directly on the talent or at the critical point in the frame. With multiple light sources — key light, fill, back — you measure each individually in incident light to understand the balance later. This gives you an objective basis for your lighting setup, especially important for large sets or when you need to match other scenes later and don't have time for endless test exposures. The reading becomes a documented benchmark that gaffers and the second camera team can rely on.
Practical Workflow: Start with an incident light measurement before the first take. Hold the dome parallel to the camera chip plane, not to the light source — this way you accurately capture what will hit the film plane. With HMI lamps or sunlight with diffusion, measure the value with and without flags to see how much light you're taking out. This is more reliable than looking at it and guessing. Especially in daylight exterior scenes where the sun is constantly shifting, incident light control becomes the baseline for your corrective measures — ND filter in or out, adjust reflector — everything is calculable.
The difference from reflective measurements (spot meter on lenses) becomes apparent in the edit: two takes of an actor against different backgrounds can be identically exposed in incident light, but the reflective reading will vary. Therefore, incident light is the more objective reference when you need to stabilize a look across multiple shooting days. It's not glamorous, but it's the language between you, the gaffer, and the colorist — and they speak numbers, not feelings.