Illuminance
The amount of light arriving on a surface per unit area, measured in lux, independent of what that surface does with it.
Formula / rule
E = Φ / A
Measured in
lux (lx)
In depth
Illuminance is what an incident meter measures, which is precisely why an incident reading is unaffected by whether the subject is a wedding dress or a dinner jacket. It falls off with the square of distance from a point source, which underpins every lighting-ratio calculation in the studio.
Examples in practice
Full sunlight delivers roughly 100,000 lux, an overcast day 1,000 lux, and a well-lit office about 400 lux.