Light Distance Vs. Inverse Square Law Discussion

Ok so we all know about the inverse square law when it comes to light? Basically the further away a light is the less powerful it is by an exponential amount. Well when a light is rated at lets say 1000 lumens that is telling you that at 12" or 1' that is how many lumens that bulb puts out. And at 2' it is 1/4 of that. Well it works the same in the other direction too! That same bulb will put out 4 times the light at half the distance. In other words at 6" your 1000 lumen light is now giving your plants 4000 lumens over a smaller area of coverage. Here is a link to calculate your actual lumens.
http://www.intl-lighttech.com/support/calculator/inverse_calc

bigv1976 said:

Ok so we all know about the inverse square law when it comes to light? Basically the further away a light is the less powerful it is by an exponential amount. Well when a light is rated at lets say 1000 lumens that is telling you that at 12" or 1' that is how many lumens that bulb puts out. And at 2' it is 1/4 of that. Well it works the same in the other direction too! That same bulb will put out 4 times the light at half the distance. In other words at 6" your 1000 lumen light is now giving your plants 4000 lumens over a smaller area of coverage. Here is a link to calculate your actual lumens.
http://www.intl-lighttech.com/support/calculator/inverse_calc

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I don't think thats correct. I believe lumens are an energy unit, and a bulb emits a certain amount. You then talk about lumens per unit of area.

A 10,000 lumen bulb emits 10,000 lumens. How many lumens per square foot is a function of the distance you are from that light source, but the best you could theoretically do is direct 100% of the light onto your plant, and get 10,000 lumens out of it. You cannot focus the light like that in the real world though, as it dissipates according to the inverse square law. You need to find that sweet spot that allows a high intensity of lumens per unit of area, but is still far enough away to not burn the plants.

One thing that seems to always be forgotten when inverse square law is mentioned is that the light doesn't disappear as it gets further from the bulb. It simply spreads out more, covering a greater area.
So while arguing that brightness drastically decreases the further away you are from the bulb, you could also argue that the amount of coverage drastically decreases the closer you get.
Though there is the factor that the further away from the bulb, a greater percentage of the light may be directed in directions other than the plants. So in that sense the light can be wasted more.
So one has to decide if they want say 4 times the amount of canopy by having the canopy 2 feet from the bulb rather than 1 foot, or 25% of the canopy with 4 times the brightness at 1 foot instead of 2 feet.

zeropercentthc said:

One thing that seems to always be forgotten when inverse square law is mentioned is that the light doesn't disappear as it gets further from the bulb. It simply spreads out more, covering a greater area.
So while arguing that brightness drastically decreases the further away you are from the bulb, you could also argue that the amount of coverage drastically decreases the closer you get.
Though there is the factor that the further away from the bulb, a greater percentage of the light may be directed in directions other than the plants. So in that sense the light can be wasted more.
So one has to decide if they want say 4 times the amount of canopy by having the canopy 2 feet from the bulb rather than 1 foot, or 25% of the canopy with 4 times the brightness at 1 foot instead of 2 feet.

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You are absolutely correct and you brought up the area covered which I thought I did but obviously didnt due to a new bong. My interest in this topic is in an HPS vs. T5 debate. I think this law greatly favors T5 tubes and even if lumes dont add up lets say a T5 tube is 5000 lumens at 12" at 2 inches it is over 125,000 lumens. Now say you have six evenly spread tubes and the light is directly over your canopy throughout the whole tent you could arguably say that a T5 light is giving your plants 125,000 lumens at 2". If I am wrong correct me. It is confusing to me.

IDK exactly how flouros are rated, because the numbers certainly don't add up. T5's barely put out 1000 lumens at 12", let alone 5,000. I measured my own, and the meter has to be practically touching the bulb, to get 5000 lumens, which tells me that they rate them at the bulbs surface. Sorta misleading, isn't it?

bigv1976 said:

You are absolutely correct and you brought up the area covered which I thought I did but obviously didnt due to a new bong. My interest in this topic is in an HPS vs. T5 debate. I think this law greatly favors T5 tubes and even if lumes dont add up lets say a T5 tube is 5000 lumens at 12" at 2 inches it is over 125,000 lumens. Now say you have six evenly spread tubes and the light is directly over your canopy throughout the whole tent you could arguably say that a T5 light is giving your plants 125,000 lumens at 2". If I am wrong correct me. It is confusing to me.

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Ohh I have no idea. Hehe. But with the tube, I suppose that light is spread out across the tube, so really close to the bulb you're only really close to that spot on the bulb. But T5s seem great for having a close even horizontal canopy. (SCROG would work well particularly I'm assuming)
I've heard that lumens don't add, but also read and seen with my own eyes test results that suggest that they do add. So I don't know. But why wouldn't they add? If they didn't add, wouldn't one bulb be just as bright as five bulbs? It doesn't make sense to me.

Here is some really interesting analysis for T5 vs HPS: https://www.rollitup.org/indoor-growing/58701-results-cfl-vs-t5-vs.html

With CFLs or T5s you have much more area that is close to a bulb than with one HPS.

And another factor with lower wattage HPS (250w or less) is that the ballasts use extra wattage. I don't know if this is common but I've heard of some lower wattage ballasts using obscene amounts of extra wattage - like a 250w ballast actually using 350 watts for example.

My planned setup is two 6 outlet power strips of CFLs for vegging and a 1000watt vertical HPS for flowering. The CFL fixtures involve: 3 socket to outlet plugs in each power strip, and a Y bulb socket adapter for each plug, making 6 CFLs for each power strip. 20-30 watt CFLs have relatively good lumen per watt ratio and can be found online for very low prices.

Again you guys are messing up units. Lumens is a measure of the brightness of light. It has absolutely nothing to do with distance. The lumen output is the lumen output is the lumen output. "10,000 lumens at 2"" doesn't really mean anything. What you mean is 10,000 lumens PER SQUARE UNIT OF AREA. You are going to get a set amount of lumens out of whatever lamp you choose, and if you increase the distance you don't always "waste" those lumens.

For example you have a 1000 watt hps light. It outputs 107,000 lumens. You have a reflector on it essentially reflecting 100% of your light below the light. If you put your light directly onto your plants so you illuminate a 1 foot by 1 foot area you will be getting 107,000 lumens/ft^2. That is too hot for your plants, you will toast them. You increase the distance to your plants, so you now illuminate a 2 foot by 2 foot area. Now you have 107,000 lumes/ 4 ft^2, or 26,750 lumens/ft^2. You get the exact same amount of lumens, but its spread out over 4 square feet instead of 1.

bigv1976 said:

Ok so we all know about the inverse square law when it comes to light? Basically the further away a light is the less powerful it is by an exponential amount. Well when a light is rated at lets say 1000 lumens that is telling you that at 12" or 1' that is how many lumens that bulb puts out. And at 2' it is 1/4 of that. Well it works the same in the other direction too! That same bulb will put out 4 times the light at half the distance. In other words at 6" your 1000 lumen light is now giving your plants 4000 lumens over a smaller area of coverage. Here is a link to calculate your actual lumens.
http://www.intl-lighttech.com/support/calculator/inverse_calc

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this is wrong dude, i dont think you have any ideaa what your talking about

BrandonT said:

this is wrong dude, i dont think you have any ideaa what your talking about

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You are an idiot bro. Do some reading before doing a 1 and done post. Ever heard of a light meter? See what happens when you get it closer to the bulb.

PS I just noticed that every one of your posts are pretty basic questions so why dont you explain this to me smart guy.

jawbrodt said:

IDK exactly how flouros are rated, because the numbers certainly don't add up. T5's barely put out 1000 lumens at 12", let alone 5,000. I measured my own, and the meter has to be practically touching the bulb, to get 5000 lumens, which tells me that they rate them at the bulbs surface. Sorta misleading, isn't it?

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You are correct. Fluorescents and HID bulbs are rated at or very near the bulb distance. Plus there is so much more to lighting. PAR ratings, ability to penetrate the canopy, temp in relation to distance from bulb and so on.

The inverse square law works fine for a light (or heat, or even sound) source that is radiating equally in all directions - a good example is the light from the sun.

However many of the light sources we use are CYLINDRICAL - they do not radiate equally in all directions by any means. In fact for an infinitely long cylindrical light source the relationship becomes linear inverse - not inverse square.

Complicated further by the fact that we commonly use a parabolic or spherical section reflector. For such a light the inverse square law will be far from rigidly applied.

bigv1976 said:

You are an idiot bro. Do some reading before doing a 1 and done post. Ever heard of a light meter? See what happens when you get it closer to the bulb.

PS I just noticed that every one of your posts are pretty basic questions so why dont you explain this to me smart guy.

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I still think there is a fundamental misunderstanding of how light and lumens work.

Well when a light is rated at lets say 1000 lumens that is telling you that at 12" or 1' that is how many lumens that bulb puts out. And at 2' it is 1/4 of that. Well it works the same in the other direction too!

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That is wrong. When a light is rated at 1000 lumens it is telling you it emits 1000 TOTAL LUMENS regardless of distance. You need to use this information to figure out how to position your lights and plants. A light meter is going to give you a lumens/area value. It can't possibly tell you the total lumens. It has no idea the total lumen out put of your light, but it can detect how much light is falling on the sensor pad (with a given area) and give you a lumen/area value.

The ability to penetrate the canopy is just a function of the inverse square law. The difference between ground level and the top of a 6' plant with regards to the sun is nothing. The 6 foot difference is practically nothing compared with the distance from the source (the sun). If you run the calculation for lumens/area given the 93M mile distance you will calculate the same thing for practical purposes. If you calculate it for a 1000 watt HPS you will find a much smaller difference between too much light and too little light. This band of acceptable lumen/area diminishes as the source gets fainter or smaller.

For example say you have 3 sources: a single cfl bulb, a 1000 watt hps, and the sun.

cfl = 1,400 lumens
hps = 107,000 lumens
sun = 6,840,000,000,000,000,000,000,000,000 lumens

Acceptable lumens/area - upper = 50,000 lumens/ft^2, lower = 10,000 lumens/ft^2

E=I/(d^2)

we have E and I for these sources so we can calculate d.

Using these values, the closest you can put a plant and not exceed 50,000 lumens/ft^2 is:
cfl - 0.17 feet
hps - 1.46 feet
sun - 3.7E11 feet or 70 million miles

The farthest you can put a plant and still get at least 10,000 lumens/ft^2

cfl - 0.37 feet
hps - 3.27 feet
sun - 8.3E11 feet or 156 million miles

as you can see you have a very small acceptable range for that cfl. The gradient of light intensity is huge. You have a total of 0.2 feet of workable distance. Anything outside that range is too much or too little. You can still get enough lumens, but only over a small area.

The hps though gives you a much larger range. You have almost a 2 foot vertical distance of acceptable light concentrations. So at those given distances you will get the appropriate amount of light, but it will necessarily be spread out over a larger area than the cfl.

The sun has a huge range because of the intensity and distance involved.

Note none of these values should be considered accurate or exact, they were for illustrative purposes.

I am thinking some more reading and less copying and pasting would help you out.

I've done plenty of reading and have a good grasp on light and physics in general, nothing there was copy-pasted. I can tell from this thread you do not have a fundamental understanding of it.

Okay thank you.