
22 Basic Principles of Light andVision
Each unit surface of the sphere is illuminated proportionally to the ux of
the light source and a small window collects and measures this luminous
ux. A unit like the lumen is dened as the total radiated power in watts
times the efcacy of the light bulb times 683 lm/W.
The surface of a sphere is 4π r
2
where r is the radius of the sphere, ...