Re: LED lights in shop

Posted by John T on November 15, 2015 at 15:46:49 from (216.249.74.3):

In Reply to: LED lights in shop posted by coshoo on November 15, 2015 at 11:14:27:

Its IMPOSSIBLE to accurately tell you how many fixtures/bulbs you may need without knowing a whole ton of facts such as ceiling height, height above work space you wish to illuminate, square feet, color and reflectance of ceiling and floor, lumens of light per fixture etc. etc. etc. One zonal cavity method formula that may help you is that Footcandles = Lumens x Coefficient of Utilization divided by Area. THE HARDEST PART IS TO CALCULATE THE CU as that depends on height and colors and reflectance etc. In an office with 8 ft white ceilings and good fixtures, the CU may be high like over 0.7, however in a dark floor and ceiling shop with high ceilings it may 0.4 or less.

As an example, if you wanted say 50 foot candles on your workbench (a decent light level),,,,,,,,,,had an area lets use 1000 sq ft (20 x 50),,,,,,,,,,,,,,each light bulb produced say 1500 lumens (like a 100 watt incandescent),,,,,,,,,,guess at CU is 0.5,,,,,,,,,,,, HOW MANY BULBS ARE NEEDED????

50 = X lumens x 0.5/1000,,,,,,,,,,,,50,000 = .5X,,,,,,,,,,,,,,,x = 100,000,,,,,,,,,,,if each bulb produced 1500 lumens,,,,,,,,,,,,,100,000/1500 = 67 light bulbs IF MY MATH IS RIGHT NO WARRANTY LOL

NOTE if the LED is rated 100 incandescent watt equivalent (but it uses far less watts) that could equal around 1500 lumens of light for each bulb. Still the CU depends on the fixture in which its enclosed and its color and reflectance and ceiling and floor and reflectance etc etc ITS EASY IF I HAD ALL THE DATA

TO GIVE YOU AN IDEA OF ILLUMINATION LEVELS: In a drafting room you may require say 100 footcandles of illumination, while an outdoor parking lot may have only 5 footcandles, while on a bright sunny summer day there could be 10,000 footcandles outdoors

DISCLAIMER It was nearly 40 years ago I attended a week long lighting seminar at GE's NELA Park in Cleveland Ohio and Ive been retired as an engineer like 24 years SO NO WARRANTY AND DONT ANYONE HAVE A CALF IF THIS IS ALL WRONG its just my best effort to provide some insight and theory of how to compute how many lights/fixtures/bulbs may be required but it takes the heights and colors and reflectance and other info to give an accurate answer WHICH THIS IS NOT

John T

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