346 Electrical Power Systems Technology
Find: the total light output from the lighting system.
Solution:
- Find the total light output from the lamps.
3000 lumens per lamp × 16 luminaires ×
2 lamps per luminaire = 96,000 lumens- Find the total light output:
light output = total lumens × CD × DF
= 96,000 × 0.45 × 0.90
= 38,880 lumens
The distribution of light output onto work surfaces must also be con-
sidered. The CD and the DF are used to find the total light output of a
lighting system. A greater light output is required for larger areas. The
light output to a work surface is expressed as lumens per square foot, or
footcandles. A light meter is used to measure the quantity of light that
reaches a surface.
The following two formulas are useful for finding the required lu-
mens for a work area, or the light output available from a particular light-
ing system:
desired footcandles × room area in ft.
total lumens = —————————————————
CD × DF
and
total lumens × CD × DF
footcandles available = —————————————
room area in feet
Sample Problem:
Given: a lighting system with 12,000 total lumen output, luminaire
CD = 0.83 and DF = 0.85, is installed in a 600 sq. ft. room.
Find: the footcandles of light available on the work surface.
Solution:
lumens × CD × DF
FC = ————————
room area