Question

Your office has installed the incandescent bulb in a simple light socket hanging directly in the center of the room, with no diffuser or reflector. The light energy output of the library is 1500 lumens. The bulb filament is exactly 3 feet from the ceiling. The height of the room is 10 feet and your desk is 2 feet tall. The center of your desk is 3 feet from being directly under the light bulb. Your desk is 4'x3'x2'. The ceiling and walls around you are all painted a very dark color. Compute the illuminance falling on the exact center of the top of your desk.

Answer 1

Given, vertical depth of center of table from light source = 10-2-3 = 5 feet

table center offset from room center = 3 feet

Thus total distance of light source to table center =

5.83 ft = 1.778 m. thus

dark walls and ceiling give zero reflection and need not be accounted for calculation of illuminance at center of table top.

also for center of table top and light source, the angle can be given as

(It would be more convenient to understand when you go through the formulas for illuminance from a point source)

i.e Illuminance is 407 lux.