Help with the inverse square law....

Discussion in 'Growing Marijuana Indoors' started by HighOnTheHill, Jun 7, 2009.

1. Okay, light intensity fades @ the rate of 50% every foot right?

400w HPS = 50,000 initial lumens.

50,000/(1ft x 1ft) = 50,000 lumens

50,000/(2ft x 2 ft) = 12,500 lumens

50,000/(3ft x 3 ft) = 5,555 lumens

Okay so here's my question. If the light intensity fades @ 50% per foot, then wouldn't you be getting 12,500 lumens @ 1 foot?

Suppose I were to want to knock this down to inches? Little bit more specific, because when I arrange my lights, I go by inches away, not feet. I ran my light 8-10 inches during my 1st grow, but I'm thinking about going 14-18 inches away this time as the canopy would be cooler, BUT I'd like to know about how many lumens I'd be getting.

Anyone?

2. #2
Last edited by a moderator: Jun 7, 2009
Sweet. I found this but I'm not sure of its accuracy; its kinda contradicting with the inverse law...

It's only for HPS.

Edit: Are foot candles the same thing as lumens? I think so =/

Double edit: Okay this annoys me. I veg under 356 watts of CFL's which put out ~23,000 lumens....I keep my HPS light at about 10 inches from the canopy....

356w CFL's @ 2 " = ~20,000 lumens
400w HPS @ 10" = ~20,000 lumens....

And the HPS gives me heat problems? For fucks sake...

Can anyone tell me why my HPS is better for flowering than soft white CFL's?

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3. lumens are typically measures at 12", not from the bulb's surface.

so, if a bulb has 20000 lumens listed, thats averaging 20k lumens at 12 inches from the bulb. At 10 inches, the lumen rating would be higher, not lower.

also, the inverse square law isnt 50% per foot. its a function, in which the denominator is the unit of "inches squared". Thus, you can see that the relationship between lumens and distance is exponential, not linear.

for example. 1/(2^2) = 1/4 ; and 1/(4^2) = 1/8. 1/(5^2) = 1/25. see how its not linear from 2" to 4" and then to 5".

4. #4
Last edited by a moderator: Jun 8, 2009
Yes, I was hopng I'd have a reply to this. Okay if lumens are typically measured at 12" does this mean that the chart is wrong? It's saying something like 16k lumens @ 12" with a 400w HPS. The bulbs are rated @ 47-50k, yet that range requires being 6 inches from the bulb.

And wait, thats in INCHES?! I thought the inverse squared law was for measurements using feet? That's what I got out of the grow bible anyway. Or was that just an example?

I'm just trying to get my lighting "optimal." I'm finally ordering a REAL exhaust fan tomorrow, so I'm trying to figure out how close I WANT my light, and then take steps to assure I can get it that close.

Plus I'm keep some persoanl grow logs and I'm trying to LEARN from all of this. What works, what doesn't. I keep track of everything, how much I water/feed, how fast they grow, every deficiency/toxicity that I can find, everything, I just need to figure out how much light.

I need a PAR meter, but lumens/watts are going to have to work for now.

5. The problem is you are not applying an inverse square. Light intensity does not fade at the rate of 50% per foot. In fact, there is no measure of how much light intensity diminishes per foot because you need a difference in distance, and thus two distance measurements (before and after changing the distance) to calculate a difference.

Light intensity is inversely related to the square of the distance, that's straight from the laws of physics (if you have a problem with that you gotta get in touch with Isaac Newton). What that means is that if the distance is doubled then the light is diminished to 1/4 of the original intensity. Cut the distance in half and the light intensity is increased four-fold.

Lumens is not the same as foot-candles, lumens is a way of rating the light output of the bulbs but is not a measure of light intensity (because again, that is a function of both the strength of the light source and the distance). Move a 50,000 lumen bulb closer or farther away and it is still 50,000 lumens.

6. Sweet, I like you guys =D Okay REALLY drunk right now and just going to bed, but I thought I'd acknowledge the post, TRY to make sense of it, bookmark it and come back to it later.

Okay the inverse square law that I'm NOT applying - It was just a formula that I got out of the grow bible, or off someone off the forums somewhere, I couldn't tell you.

I'm with you on the first paragraph though, somewhat. My issue was, I think, that I was considering the bulb (0") the first "distance." Or did that not make any sense?

Actually I'm going to quit trying now, I have to keep reading over and over =/ Ugh, I aced HS physics, and failed University physics. Well I got a D or something =/ Go figure.

Can I just ask for a formula that works? Nice simple "plug the numbers in and solve it" formula?

Edit: I'd like to point out it took me 20-25 minutes to make this post =D Don't mind me. -bookmarks-