Anyone have a formula for....

[davejohnwright]

Anyone have a formula for loss of lumens due to distance? I found one that I'm not quite sure about.

davejohnwright Reviewed by davejohnwright on 07-08-2007. Anyone have a formula for.... Anyone have a formula for loss of lumens due to distance? I found one that I'm not quite sure about. Rating: 5

[HighTillIDie]

SCROG bitches

[davejohnwright]

When I take my light meter and measure sunlight intensity at noon on a clear summer day-----around 10,000 fc (footcandles). That's true wheather standing atop a tall building or on the ground.

18" directly below my hortilux 1K hps is also 10,000 fc------now another 18" farther below is only around 2500 fc-------yep about 1/4 from the previous point of measurement.

And yes Virginia----I've found that plant material exposed to intensities much higher than 10K fc start bleaching-------and not from heat.

So what this means to me is-------we have a very small envelope of useable light intensity from HID lighting.

Things to do:
1) keep your canopy flat or concave with tying branching down, trellising, or whatever.
2)light movers are great----- especially in the aero/hydro world where moving plants around is not practical.

cheers,

Alaric

Damn, that makes it sound only practical to have plants up to about 2.5 feet tall. That means my dreams of having 8 foot + tall plants is gonna require a ton of supplemental lighting on the lower branches to get em' big without sacrificing quality.
Back to the drawing board, and I gotta pick up a ton more flouros than I hoped. Fuck it, good things require the extra effort!
peace

[rhizome]

Morning Dave-

It's a function of the inverse square law. Here's the math, as we know you love numbers Sorry, couldn't seem to get a clearer thumbnail. ( Note- edited to include better thumbnail)
Chart looks bout right to me. Don't forget to allow for reflection- inverse square is only a clean calculation if there's no reflective light. If you were to build the space up and then sample w/ a light meter, you'd find that the math holds good until the plane where reflective intersects direct, @ which point they reinforce. You'll find smaller than predicted loss of intensity as you move lower in the space- still there, but the loss ceases to be purely porportional and becomes somewhat fractional, as the level of reflective has an inverse relationship with the level of direct.

Intensity i @ distance r=

[the image reaper]

here's the short, simple version, for us dumb guys: "the closest the plant can get to the light without heat damage, the fastest the plant grows"

[davejohnwright]

Thanks Rhizome, I'm trying to rethink how tall I can let plants get under my light without the bottom branches getting to little light.

[oldsanclem]

A good parabolic reflector helps

[davejohnwright]

Im gonna save a buck and use the hood I already have. Besides a 1000w puts off alot of heat, and I've never seen an air cooled parabolic.

[Weedhound]

Buy yourself a nice cheap handheld temp gun....tell you exactly how close you can put your light to keep leaf surfaces under 80 degress....which I am told is optimum.

PS...I'm told by the same source....(who knows his shit btw) that that is how the "pros" do it......with a light meter and temp gun which will tell them max watts per sq foot so I'm not sure there is "one" right formula for plants.

[oldsanclem]

Note one way to suck heat off a parabolic is to place alm. vent at the top of the fixture and use a bathroom 70 cfm to pump out the extra heat.
I used on with a suncircle with a 1000 watt MH and a 400 watt hps. The vent temp was about 15 degrees differance between room temp and venting temp.
The room was about 20% less time on a normal venting time.
I use a single bulb system now, as very little co2 get picked up by the vertical heat rising from the bulb. The tanks last about the same amount of time. A extra advantage to the heat pump is it warms up the grow room and clone space.