Grow Lights & The Inverse Square Law

[RainerRocks]

During my research I found this youtube video.

It was very helpful to me and thought it might help
other's.

Now that being said...this just covers the Inverse Square Law and
doesn't take into account size of grow area or how many plants an
if any foil or reflection is use.

It's just a quick short video that explains The Inverse Square Law which
is very easy to understand.

 

[cephalopoda]

good video

 

[RainerRocks]

good video

Thanks..I"m a newbie at this indoor grow thing so this video helped me understand the Law more clearly.

It was nice and simple compared to many others on youtube. I figured
there must be other's like me out ( Hahaha) there that's why I posted it.

 

[Papa]

well, the videographer has the concept . . . but his numbers and understanding are somewhat off.

moving the light from 0 to 2 inches away, he takes the 2 inches, squares it (4), and concludes that you have 1/4 the light.

that's like saying he moved the light away 50mm, squares it (2,500), and concludes that you now have 1/2,500 the amount of light.

instead of selecting arbitrary units of measurement, it's easier to understand that every time you double the distance from the light light source to the illuminated surface . . . you get only 25% of the light.

 

[RainerRocks]

Now I'm more confused...LOL

Isn't that what he was doing by doubling the distance and calculating
it would be 25% less ?

Thanks

 

[Papa]

'sorry for the confusion RainerRocks.

when learning this many years ago, i also found it helpful to see it visually, and we set up a demonstration with a light, tape measure, and light meter to learn it . . . sort of what this guy had in mind.

where he gets into trouble however . . . is by attempting to use units of measurement (arbitrary units, inches) instead of using the initial distance from the light source (we'll call it "r") as the unit of measurement. he then compounds the problem by beginning with the light at 0" . . . and the example gets worse after that.

let me try to provide a better example . . .
let's say the light source is 1 unit (inches, centimeters, thumb lengths, whatever) from the top of the plant. we will refer to this distance as "r" . . . . and lets say that this provides "Z" amount of light at the top of the plant.

if you were to double the distance of the light source to the top of the plant (to 2r), then the amount of light at the top of the plant would be Z divided by 2 squared . . . or (Z/2 x 2) or 1/4 the amount of light that we previously had.

to help out, we'll do it again with different numbers . . .
let's say the light source is 15 units (inches, centimeters, thumb lengths, whatever) from the top of the plant. we will refer to this distance as "R" . . . . and lets say that this provides "Y" amount of light at the top of the plant.

if you were to triple the distance of the light source to the top of the plant (to 45 units or 3R), then the amount of light at the top of the plant would be Y divided by 3 squared . . . or (Y/3 x 3) or 1/9 the amount of light that we previously had.

the video example doesn't work because starting with a distance of 0" . . . if you square 0, it's still 0. he just figured out that if he moved it 2" and used the 2" x 2" it got him to the 1/4 that he was looking for. if you were to use units other than inches for those same 2" (as i did in the previous post, mm), and keep everything else the same, you end up with a wildly different answer. that's a hint that somethings fishy.

here's a graphic that visually describes the inverse square law:

Papa

 

[sedate]

doesn't take into account size of grow area or how many plants an
if any foil or reflection is use.

Neither of these things has anything to do with it with it. Light reflected from a reflector will still be reduced by the same rate. If you are thinking a reflector somehow ameliorates this is not the case . . .

 

[green punk]

ameliorates.... Nice one Sedate. Sent me to the dictionary.

 

[pork]

reflector will actually take some light too...and the glass on the hood...but more or less...60% efficiency on a horizontal reflector would be epic, and hard to achieve that whole 60% at the canopy. where as with a vertical bulb, 60% is much easier to get to the plants, theoretically one could achieve close to 100% with a donut style config...

 

[sedate]

reflector will actually take some light too...and the glass on the hood...but more or less...60% efficiency on a horizontal reflector would be epic, and hard to achieve that whole 60% at the canopy. where as with a vertical bulb, 60% is much easier to get to the plants, theoretically one could achieve close to 100% with a donut style config...

Just to quibble - the inverse square law applies to the linear distance the light is traveling. Since it is hitting the reflector and traveling at another oblique angle (downward someway, we assume) we can just take the linear measurement of the two lines added together.

I mean even with 1000w bulbs, the light fall-off in the larger reflectors is noticeable at the end w/o the bulb . . .

The fact that reflectors are not 100% reflective is another matter though . .

And vertical lighting is - with anything other than very large plants, generally one of the more inefficient (in terms of grams-per-watt, not grams-per-plant) ways of growing . . donut configs never work that well without at least 3 vertical bulbs . . . not in my experience anyway

 

[pork]

never run a donut, but did just do a better gpw on 4 verticals in a stadium than my runs with 4 or 6 or 8 or even 10 horizontal hoods that i've done before...granted, i've also become a more skilled gardener in that over those years of runs...best I've done flat is just over .6 gpw, this last vert (1st with a stadium setup, done tress unsuccessfully (harder imo in the long run) i did .68 and that wasn't anywhere near an ideal run horizontal or vertical. yet still a better gpw than ever before, and i can see easy potential to get it up to at least 1 gpw this next run.