Foot-Candles Formula

@coachlizschneider What’s the difference between lamp lumens and lumens per fixture? Can you explain the difference between the two formulas in the attached image? I missed this in our last workshop. Thank you!

1 Like

Hi @ctan & welcome to our ARE Community!

Please take advantage of our PPD practice quiz as well!
Practice Quizzes

1 Like

Hi Crystal! Thanks for following up with this.

The distinction is best understood when you think about the difference between a lamp and a fixture. In this case we they use the term lamp they really are referring to the bulb. And the fixture would be the entire light fixture which may have multiple bulbs or lamps. So when they say lamp lumens it is the number of lumens for a single lamp or bulb and when they say lumens per fixture this refers to the total number of lumen for the entire fixture which may have multiple lamps or bulbs.

As for the difference between the two formulas. The first is calculating how many foot-candles a single fixture is producing, while the second is calculating how many foot-candles are being produced by multiple fixtures.

I also want to point out that there is a typo in the second formula. Instead of lumens per fixture, it should read lamps per fixture. So the formula is:
Foot-candles = (lamp lumens) x (lamps per fixture) x (number of fixtures) x (CU) x(LF)/area in sqft
where CU = coefficient of utilization and LLF = Light Loss Factor

2 Likes

Thank you!

1 Like
6300 N. Northwest Highway P.O. Box 31846, Chicago, IL 60631-9998 / All rights reserved © 2020 Black Spectacles.