How much light is enough?

Illuminating Engineering Society publishes recommended light levels for specific visual tasks

Selected guidelines for work surface illumination:
Provided by the Green Lights program
TasksFootcandles
Average Reading & Writing50
Offices with Computer Screens 50(tasks)25 (room)
Hallways 10
Stockroom Storage 30
Loading and Unloading 20
High-Volume Retail 100
Low-Volume Retail 30