The Lumen What it's for, and what it's not

One can generally see fine with enough illuminance, which is the luminous flux (unit: lumen) per area. The unit of illuminance is lux if the area is measured in sq meters, and footcandles (fc) if sq ft are used. Setting aside other aspects of the lit environment for the moment, a normally-sighted person can see very well over a very wide range of illuminance values. (For example this is often about 10,000 fc outdoors and about 50 fc in offices).