Distance from HID light in a 95% reflective Mylar tent

Hi all
I just bought a 2X4 Mylar tent and was wondering if the distance from the light to my plants will have much of an effect on the lumen count. The way I understand it, people want to put the lights as close as possible because the light is comming from a point source and the further the light is away from the plants, the more the "light rays" will diverge. In my situation it shouldnt matter. Even if i put the light ten feet above the plants, the only way the lumen count should be reduced is by 5% every time photons hit a wall. I have a 2X4 tent with a 400 hps.

My real question is, what do you think the change in lumen count would be going from 18" to 36" in such a tent. I would guess based on my theory that it would be less than 5% as opposed to maybe 50% or more in an open space

Your going from 18 inches away to 36 from bulb to top of plant you will be getting less than 1/3 of the power of the light. It's not the light we see, it's the light we don't see that counts. The lamp can only project the PAR about 3 feet, that's the stuff we don't see that makes plants grow.

If you put your light at the top of your tent your plants will stretch and than you have another problem.

i run my 400 at 6 inches above. a 400 only penetrates 24 inches.

12 inches is pushing it for light vs heat. any closer then 8" and ur doing more harm than good. after 12" inches, light intensity weakens exponentialy

You guys don't get my question. I don't think traveling through the air actually reduces the 'potency' of the light. Its just that the light gets more spread out. But the light has nowhere to go but down. So in this case sense 95% of the light is reflected off of each surface, it should not change the lumen count on the 8 square feet by very much. I know this is all theory on my part but if someone would explain why my reasoning is wrong that would be cool.

if u already have reasoning why ask the question. do waht u want

I just don't have anything to compare it to so I wont know if I'm full of shit for a while. Time is of the essence

I'm also interested in how shit works and just curious about this subject

@Dako: Light traveling any distance DOES reduce "potency". (It HAS somewhere to go, you can't reuse light.)

Light bouncing around the room because of mylar doesn't help that much.
I read somewhere that Mylar, in a closed space, adds about 2% usable light to the plants, but I have no source/citations for that. But forget my speculation, and read these facts:

All light dimishes exponentially, and any light reflected by the mylar is minimal (when talking about multible bounces of photons).


Read wikipedia, light fades REALLY fast (exponentially in fact!).

See this chart: http://www.breedbay.co.uk/gallery/data/588/HPS_chart.jpg

it has a lot of info, but notice this:

A 600 Watt HPS bulb at a distance of:
8 inches gives 65890 lumens.
only ONE inch more:
9 inches gives 52061 lumens.

That is a LOSS of 13000 lumens just for ONE INCH of extra distance!
And that is why light bouncing around the room because of mylar doesn't help that much.

yeah the light is going sideways towards the wall and than bounce back off the wall so that distance will determine the ligts intensity. They say that mylar reflect 98% of light if thats true so the light travels to the wall than loses 2% of remaining lumens and than however many more traveling to the plant.

Thanks everyone for the replies. I agree with what you all are saying but when I look up information on light (unrelated to growing) it seams that it comes in packets (quantum theory stuff) and that these packets release there energy in an all or nothing situation. So when a group of photons hits a wall, 5% will give up there energy but the rest should bounce with no effect on energy levels. Where the depreciation with distance comes in is that light spreads out causing less photons per area. but spreading shouldn't matter when it will just be reflected again anyways. I know I'm missing something but I don't know what it is. Maybe UV is absorbed by the air? but i though uv wasnt that important for growing. is the 95% reflection statistic bs? Ive been trying to find this answer in physics forums too but it is kindof grow specific and i dont have a backround in horticulture. I understant basic biology botany and physics but obviously there is a lot more going on here. So any technically inclined people that want to fill me in please do.
Krok
"A 600 Watt HPS bulb at a distance of:
8 inches gives 65890 lumens.
only ONE inch more:
9 inches gives 52061 lumens."
This is true but I'm talking about an enclosed mylar "box". If anyone has a light meter and a mylar grow tent I would love to hear what the depreciation is. Thanks

I guess even that wouldn't really help because the light meter can't detect all 180 degrees

@Dako:
Light is "absorbed" by the DISTANCE (travelled) itself!
Your qoute about "energy levels" doesn't make sense because of this, because the energy fades simply because of distance - and it fades incredibly fast.

Even if mylar reflected 100% there would still be a loss, after a couple bounces the light will have travelled so far that it is almost non-existent to the plant.

Remember that this is indoors. Outdoors in the sun, this distance-effect is not observed in same way by the plant, simply because the sun is so far away that a distance of 10 feet (say plant height) doesn't matter when the lights has travelled all the way from the sun!