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I'd like to add floodlights mounted to my house to light up my yard at night and I'm trying to figure out how bright a fixture I should buy. I've Googled a ton and found plenty of calculators to do the math, i.e. how high I'm mounting them and how much yard I'm trying to cover which results in this many foot-candles or lux. But those don't tell me anything about whether the results would make my lawn look dimly lit or like daylight.

Is there a set of rough guidelines or rules of thumb? I don't need to light it up like a night game at an athletic field but I want it brighter than just a security light. I need to be able to walk around with my dog and clearly see everything. I'd like to err on the side of more light, not less.

The lawn I'm trying to cover is 65x80 feet (about 20x25 meters) and I'd mount the lights about 20 feet high.

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    en.wikipedia.org/wiki/Lux There's a table in there that gives examples of how brightly a surface is illuminated based on the lux of that surface. Should be sufficient. Looks like 20-50 lux should be in your ballpark. – Chris M. Apr 26 '17 at 16:13
  • It's maybe not so much about lumens as distribution. Are you trying to light the yard from a single point? Is it centrally-located? – isherwood Apr 26 '17 at 17:11
  • @isherwood - yes, from a single point on the side of the house out towards the lawn so I know things won't be even. I think working out an average will have to do. – DaveBurns Apr 26 '17 at 17:14
  • ummm.. a dark-adapted eye requires very little light to be able to see quite clearly at night. Be nice and don't light the place up to the point your neighbors get pissed. – Carl Witthoft Apr 26 '17 at 18:49
  • Do you mind if I asked what you ended up with? I am currently having the same problem. Thanks. – Joanna Apr 18 '18 at 14:56
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I'm not sure whether we can find any charts for this, since it's very much a subjective thing. I can tell you about my experience...

My back yard is similar, but maybe a bit larger. It's probably 100' by 75'. On the rear gable of my house is a dual-bulb flood, which I've recently furnished with these 90 watt equivalent LED bulbs. They're as bright as the original bulbs and at only 15w. They light up about 1/3 of the yard to the level you describe.

Based on that, I strongly suggest a fixture that provides four sockets, each containing bulbs like mine. I think you'll be happy. Obviously, with a single point of origin, you'll have hard shadows behind yourself and any objects in the yard. That would take a different solution, though.

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