Illuminance measures the amount of light falling on a surface. It is measured in lux (lx) or foot-candles (fc).
It supports lux (lx), foot-candles (fc), phot, and nox.
Lumens measure the total light output of a source. Lux measures how much of that light reaches a specific surface area (1 lux = 1 lumen per square metre).
A typical office requires 300-500 lux for comfortable working. Detailed tasks like drawing require 750-1000 lux.
Direct sunlight is approximately 100,000 lux. An overcast day is about 1,000 lux. A full moon provides about 0.1 lux.
1 foot-candle equals approximately 10.764 lux. Foot-candles are used in the US; lux is the SI standard used internationally.
Use a lux meter (light meter) to measure illuminance. Smartphone apps can also estimate lux using the phone's ambient light sensor.