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ialm9

Calculating light spacing for very wide angle lights

ialm9
8 years ago

I'm in the middle of planning my lighting, and am wondering if someone can confirm how I'm doing a calculation.

I'm trying to figure out overhead light spacing for general light. I already am planning wall, table, and floor lights separately, so this is for ambient light -- basically if we want "high" light levels for a time.

Most light spacing guides I can find discuss recessed lighting, and they all seem to be assuming a maximum of 60 degree beam angle, which is common for incandescent bulbs.

However, with recent LED advances, some newer fixtures have much wider beam angles. For example, I'm looking at this flushmount fixture which has a beam angle of 117 degrees and an output of 2950 lumens, or here's another example of a fixture that has a beam angle of 110 degrees and an output of 1150 lumens.


So, my understanding is that the beam angle measures the point at which 50% of the light output is contained within it.

I have also read:

"The general rule for ambient or task lighting is to space recessed ceiling fixtures approximately the same distance apart as the beam spread at the work height, typically assumed to be 30 inches above the floor (36 inches for kitchen counters)."


So if I were to take the first fixture in my example, with a beam angle of 117 degrees, and a ceiling height of 9', subtracting 30" above the floor that means the light would be travelling downwards 6.5' to a surface.

So the beam spread radius would be:

tan (117/2) * 6.5' = 10.6'

so the diameter of the beam would be 10.6' * 2 = 21.2'.


So based on the recommended light spacing that means that these fixtures would be spaced 21.2' apart, assuming that is enough light of course (just one of these fixtures in a very large room would yield only about 4 footcandles average within this circle which is not very much by itself).


Do these calculations look correct? It seems as though with such wide angles you don't have to worry as much about spacing, as long as you can get enough lumens in the space, assuming I'm not misunderstanding something. Thanks!


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