Absolute luminous output of a light source or image area — measured in candelas or lux. Critical for contrast and whether your sensor clips.
On set, we talk about brightness when it comes to absolute luminosity – meaning how much light actually comes from a source or reaches a specific area in an image. This is not the same as contrast or color temperature. Brightness determines whether your camera can still see details in the shadows or if the highlights are already clipping into pure white.
Practically, this is measured in lux (illuminance on a surface) or candela (luminous intensity of the source itself). On set, we use a light meter – either an incident meter (measures incoming light) or a spot meter (measures reflected light from a specific point). If I measure a set key light with an incident meter and it shows 800 lux, then I know: that's my reference for this scene. Brightness dictates how long I need to expose and whether I need ND filters to stay within my desired aperture and frame rate.
The critical point: too much brightness clips. Modern cameras record in log formats and have a certain maximum output. If a window in the background is too bright, it will turn pure white – no detail, no recovery possible. That's why you work with flags and reflectors to control brightness while simultaneously filling shadows. A face at 2000 lux can mean clipping in the eyes if you don't dim it accordingly or expose the camera differently.
Also important: brightness and exposure are closely related, but not identical. You can shoot a very bright scene underexposed – then it will look dark, even though there's a lot of light. Conversely, you can overexpose with little light and get an overexposed, washed-out scene. A lot can be corrected in grading, but if you handle brightness cleverly on set, you'll save yourself headaches later. Use your waveform and histogram – they show you the brightness distribution pixel by pixel.