Measurement of light intensity on a surface — determines whether your camera has enough stops or you need to add light. Standard metric for exposure planning on set.
You measure an area on set and wonder: Is the light sufficient for my camera? This is precisely where luminance comes into play — candelas per square meter (cd/m²). It tells you how much light per unit area is hitting your sensor. Unlike illuminance (lux), which measures how much light a surface receives, this is about the light that emits from it — the luminance of a surface.
On set, you need this value for realistic planning. A monitor might show you 100 cd/m², a well-lit wall 500 cd/m², direct sunlight on concrete can reach 5,000 cd/m². These values help you understand and control exposure contrast. When filming a bright window next to a dark face, you quickly realize: the windowsill has 3,000 cd/m², the face only 200 cd/m². This is a contrast problem — you need to overexpose or reduce the bright area with ND filters or diffusion.
Practical application: You set your light meter to luminance mode or use your camera's histogram in zebra mode. Some DoPs work with a simple rule of thumb — below 10 cd/m² it gets critical for low-light material, between 100 and 1,000 cd/m² you are in a comfortable range for most modern cameras. Above 5,000 cd/m² you need neutral density filters to avoid appearing overexposed.
The distinction from illuminance (lux) is important: lux tells you how much light falls on a surface; candelas per square meter tells you how much it reflects. A white fabric under 1,000 lux can have 500 cd/m², a black one only 50 cd/m². This is why color grading also works on this basis — you are ultimately manipulating the luminance of different image elements to control the visual impression.