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I'm trying to determine how many recessed lighting fixtures I'll need to light an area that is a 500 square foot rectangle, with 30 lumens per square foot (30fc). 30 lumens per square foot is what is recommended for the type of room in question, but the recommendation guides don't say what height those recommendations are intended for.

In my case, 30fc times 500 sqft would be 15,000fc total.

An R40 compact florescent lamp has a rating of 1150 initial lumens (at the fixture). Looking at photometric data though, it appears that this lamp produces 1fc at a 12' height with a 37' spread.

So, if 30fc is the goal for the room, do I want 12 fixtures, which will produce 30 lumens per square foot near the ceiling? Or 30 fixtures to produce 30 fc at the floor? Or something in between?

Or to put it more succinctly, when guides recommend that such and such a room should be x lumens, do they mean initial lumens at the fixture, or lumens at the floor, or lumens at the expected working height (like a counter top)?

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    Depends upon what the room is used for. An emergency exit path, for instance, would need to be continuously illuminated "at values of at least 1 ft-candle (10.8 lux), measured at the floor." NFPA 1(2009): 14.12.1.3. Task lighting (e.g., 1,500 lux for doing detailed work) could be measured at the location where the task is being performed. – Upnorth Aug 23 '17 at 20:51
  • CFLs light spheres. A 12' radius sphere has a 1810sf surface area, giving about 0.6 lumens/sf. If you're getting 1 lumen/sf, that's reflectors helping a bit. Why are you using that old junk that wastes so much light? Go LED, aim it where you want it. – Harper Aug 23 '17 at 22:37
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Where are we getting all of these lumen questions all of a sudden?

So let's start with your nomenclature, you start out with lumens but what you are looking for is footcandles, which you actually reference. Lumens are the amount of light created by a lamp. Footcandles are the light measurement on a flat plane.

By reading your third paragraph you are using an R40. So I am assuming you are using a recessed can. A recessed can is designed to direct light in a certain direction. So depending on the lamp and can it could be a wide flood, a flood, a narrow flood, a spotlight, a narrow spot or a very narrow spot. This doesn't include asymmetrical designs. Regardless you need to layout recessed lighting so you don't get dark spots or shadows in between bright light spots unless you're a fan of film noir motif.

Google the fixture manufacturer they should have a specification sheet. That sheet should have the fixtures photometrics. Then you can determine the footcandle level of one fixture. You're looking for 30 footcandles at task level (about 3' above the floor). Then look at the spacing of the fixture and determine the spread of the fixture and set your spacing that way.

But you can always do what I do. I start by Googling and contacting their tech help center or if they have a local manufacturer's rep I'll call and ask them to give you the spacing. If you give them the right information they can email you a rough plan and give you the right spacing and lamp.

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You're thinking of candela or lux.

Lumens are the absolute light output of the bulb in 360 degree sphere. If you are lighting the interior of a sphere, divide by the surface area of the sphere, and you have lumens per square foot.

However most of the time you want a wedge of light. Before LED, tough beans, all lights gave you a sphere, and reflectors were pretty lousy and wasted a lot of light. Now LED naturally emits a wedge narrow enough to direct it further with optics, which are near 100% efficient when clean. Again, take the LED lumens, the surface area it is striking, divide, there's your lumens per square foot.

If you are wasting light illuminating things you don't need to light, then you'll need to deduct that fraction of lumens.

The guides are trying to simplify it for you. You're turning it complicated again. What the guides are saying is if your 500sf room needs 30 lumens/sf, that's 15,000 lumens. Put 15,000 lumens of reasonable quality light in the ceiling, you should be done. If the light is wasteful, discount for that. So for instance a 4' fluorescent tube replacement LED is about 1500 lumens, so five 2-bulb fixtures, throw in 1-3 more as a fudge factor, 6-8 fixtures and yer done.

  • So long answer short, it's the initial lumens that should be used. But isn't there also a spacing requirement to ensure that the coverage is smooth without dark spots or bright spots? ie. if the fixture is 8' long, there should be no more than x feet from end of one fixture to the start of the next? And the side of one fixture should be y feet from the side of the next? – Nick Aug 24 '17 at 21:18
  • @Nick not really anything much beyond common sense. If you've actually looked at a fixture in the showroom, you know roughly where light is going to fall from it. Try to light up tops of desks and not walls, think about cubicle walls, bookshelves or partitions, and try not to cast areas into shadow... and you've put more thought into it than most people. – Harper Aug 24 '17 at 23:56
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The height matters only from the standpoint of light distribution: shadows and beam angle. Given the relatively small distances, we consider no loss of light from the height.

Given the amount of light and the room size, I suppose the OP is talking about a plant growing facility, not an office space.

In that case, you might want to think in terms of flux, because the lumen scale is weighted to account for the human eye response.

Place your lights high enough to prevent shading from the tall plants.

And please place your lights on a GFCI circuit. And protect them from the irrigation splashes and leaks.

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