Photometrics
How much light a fixture puts on your talent, and the stop that gives you, before you get to set.
Manufacturer figures: 25,188 lux at 3 m (Medium lens).
Uses the inverse-square law from the lux at 3 m, which holds once you're a few times further away than the light is wide; very close to a big soft source, expect less than shown. Diffusion losses are typical figures, and a bounce depends on its size and distance. Check with a meter on the day.