Inverse-Square 'LAW' of Light

[Gold123]

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yup..its the same with this picture that I posted earlier:

If you measure the end product you get the same intensity but spread out over the area. This thread is done!

You are showing a point source of light, no reflector hood
Different world than a grow with a reflector hood. If you just use a bare CFL hanging from a socket, then it would apply.

 

[CannabisSativa]

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You are showing a point source of light, no reflector hood
Different world than a grow with a reflector hood. If you just use a bare CFL hanging from a socket, then it would apply.

Yes I understand that, but the original point of the OP was that the charts were misleading, and let me explain why.

Let me bring back the example I posted earlier: This example was taken with a flashlight with a reflector...

In this first picture, the light is closer to the square sheet of paper.

Lets just say that if you measure the area where there is light and you get "200" (not dealing with units here, just using an easy number for this example.

Here we have the light raised up higher.

If you measure the light intensity of the ENTIRE area where the light is spread, it should be close to the original measure of 200.

That should be clear, but why are the charts misleading?

Let me explain why. Lets say I had a light meter to measure ONE point in the first pic and I get 1000 lumens.

Now if I raise my light up 2 inches, and measure that same spot it will read lower than the first reading. This is misleading because they are not measuring the entire area, just one spot so of course its going to lose intensity. The charts that they show are correct, but misleading. They should state that the light intensity lowers per area when the light is raised (This is the ISL Law).

If you had a light meter that could measure the ENTIRE area, then you should theoretically (in a perfect world) get the same measurement.

He is not trying to disprove this law...just simply stating that those charts are misleading and a lot of people read them wrong.

Let me show you the chart that was shown earlier:

Lets say someone has a 1000 watt lamp and decides to look at this chart...4 inches from the top you get 401,070. Now if you raise the light by two inches you get 178,254 . The grower will think "wait, I do not want to raise my lights because I am going to lose (401070 - 178254) 222, 816!" Like stated earlier, the measurements are taken at a single point.

The chart is not wrong, just misleading! The chart is also assuming that there are no reflector/walls (very important). Once you add those, then you are introducing new variables, but we are not talking about those variable here because the chart isn't talking about them.

READINGS ARE TAKING AT A SINGLE POINT, NOT OVER THE ENTIRE AREA

Another example to make sure its clear...but lets use something else.

Lets say I have 1 gallon of water and I pour it into a jar that is longer than its wide. If I measure the depth of the water I read something like 6 feet (the container is very tall and skinny). Now if I get a container that is a wider, then the measurement of the water depth is about 2 feet. Wait, did I just lose (6 - 2) 4 feet thus have less than a gallon of water? NO! You still have 1 gallon of water, its just spread out more.

I hope this is clear...and I hope everyone agrees with this concept...but let me just restate this again: READINGS ARE TAKEN AT A SINGLE POINT, NOT OVER THE ENTIRE AREA.

He is not trying to disprove this law...just simply stating that those charts are misleading and a lot of people read them wrong..

 

[GMT]

Excellent post CS.

 

[boroboro]

Long time no read.

Been skippin' all dat "cogitatin' to absurdity", 'cause, as you know, I'm a practical lizard and never let the amygdala do da drivin'.
I'm also a fair Lizard, with a sense of humor, so,
Here's some gas for the fire, brah.;

View attachment 11734

View attachment 11732

View attachment 11733

Sorry bout da angle, it's a very tight fit in dere.
Use it as you see fit.

Been fun with ya.

Weezard

Weezard for the win!

Thanks for the numbers. 253 / 709 = 36% light intensity at twice the distance, once you removed the plants and let a little reflection into the picture.

Definitely more than 25%, but definitely not a whole lot more.

I imagine that 36% would go up more with more efficient parabolic reflectors, spotlights, laser lighting, etc. I'm not planning to switch to lasers, though....

never let the amygdala do da drivin'.

 

[GMT]

I'm not planning to switch to lasers, though....

LMAO, spoil sport.

 

[boroboro]

Umm did you read ANYTHING, you dont want that number to go up!

Oh, I want light intensity to go down?

Sure as shit I didn't read the entire thread carefully. Seemed like you and him arguing over the inverse square law. Seems like he just posted measurements different from his earlier measurements that were right on the inverse square curve.

Seems like we don't really need 170+ posts to discuss the finer points of how light bounces around, when the quality of light and the plants' responses to it are likely far larger variables.

 

[GMT]

assuming that in the original measurement there was no reflection ie the light covered the canopy exactly with no
wall coverage, and you then raised the lights to 2x the distance, then by opening the doors, you auto lose 19%
of the possible light that you could have with 100% reflective walls. (calc = 25% of 75% of light.) Now we need
to know the reflectivity of the wall material. Any chance we can get a meter reading of the light at 20 cm, and
one of the light 10cm from the wall with a black card blocking out all of the light coming directly from the bulb,
and the meter reading on the opposite side of the card say 8cm from the wall, (2cm to allow for diagonal increase
in distance (guestimate)). That would give us the reflectivity percentage of the material used. Lets say for
arguements sake that you get a reflectivity in practice of 80%, and the set up we already imagined, and the initial
reading is 709, then by raising the light 2x you would expect (assuming the doors were closed) the direct light
to equal 177.25, and the reflected light to equal 531.75*0.8=425.4. That is assuming that the original 709 was
the average of all readings taken rather than a single reading and the new expected total measured light of
425.4+177.25=602.65 is the average accross the whole canopy rather than a single reading. ofcourse if the doors
are open then that needs to be reduced to 496 (assuming an average bounce of 1 or less). A reading of 253 would
suggest that 1 of the following is true:

1) the reflective surface is only 19% efficient ie reflecting 19% of the light hitting it
( 253-177.25=75.75 reflected light from 3 walls = 101 total reflected light when the doors are closed,
(total door closed reading = 101+177.25=278.25) 101/531.75*100 = 19%).

2) The average bounce count is more than 1 meaning that a number of photons are cancelling each other out

3) the reading was a spot reading rather than the average reading from a grid of the right proportions

4) the new theory is totally wrong and we need to start again from the beginning.


Either way, we know that the door closed reading of 278.25 is higher than the ISL predicted reading of 177.25
by 57%.


Therefore experimental data shows the ISL to be wrong in our circumstances.

sorry about the formatting of that, I did it in windows notepad before uploading as I didnt want to risk losing it.

 

[Gold123]

Suppose for example you are growing in a 4x4x8 tent, 600W light, reflector hood. If the light is raised to 2.5' the area under the light gets almost all the light because it is focused down (parabola). Now raise the light up to 5' most of the light still hits the area underneath and little hits the walls. Now raise it to 7' now some of the light is wider than the area underneath and there is more wall bounce.
My point being because we use a reflector hood the charts are off. Only if you place plants outside the illuminated area do you have more drop off. I agree with your readings and calculations, but because of growing in a confined space the situation is changed.
If you had a large open grow room many plants, multiple lights to cover the area then the charts would be more useful.

 

[magiccannabus]

I guess my whole argument in this thread has been that the chart shows light with no reflector, no walls. I don't think it was ever claiming to represent those factors. The ISL applies to all radiant energy. Yes this chart needs amended when factoring walls and reflectors in, but I just figure that is obvious. ISL still applies, but the math is enormously more complicated. Fields of light overlap in most grows too, so rough calculating the ISL is difficult. That still doesn't mean the light magically stops trying to spread out according to the ISL.

 

[CannabisSativa]

READ MY LAST POST EVERYONE.

This is part of my last post:
Lets say someone has a 1000 watt lamp and decides to look at this chart...4 inches from the top you get 401,070. Now if you raise the light by two inches you get 178,254 . The grower will think "wait, I do not want to raise my lights because I am going to lose (401070 - 178254) 222, 816!" Like stated earlier, the measurements are taken at a single point.

The chart is not wrong, just misleading! The chart is also assuming that there are no reflector/walls (very important). Once you add those, then you are introducing new variables, but we are not talking about those variable here because the chart isn't talking about them.

READINGS ARE TAKING AT A SINGLE POINT, NOT OVER THE ENTIRE AREA

Another example to make sure its clear...but lets use something else.

Lets say I have 1 gallon of water and I pour it into a jar that is longer than its wide. If I measure the depth of the water I read something like 6 feet (the container is very tall and skinny). Now if I get a container that is a wider, then the measurement of the water depth is about 2 feet. Wait, did I just lose (6 - 2) 4 feet thus have less than a gallon of water? NO! You still have 1 gallon of water, its just spread out more.

I hope this is clear...and I hope everyone agrees with this concept...but let me just restate this again: READINGS ARE TAKEN AT A SINGLE POINT, NOT OVER THE ENTIRE AREA.

He is not trying to disprove this law...just simply stating that those charts are misleading and a lot of people read them wrong..


---

It's VERY simple!

 

[CannabisSativa]

I guess my whole argument in this thread has been that the chart shows light with no reflector, no walls..

No, its that some people that read the chart and think like this:

Mr Grower: Ok, lets look at 'The Bible' by Jorge Cervantes so I can see where to put my light...hmm...what page ::flips through pages::

The Bible:

Here is what I present to you, Mr Grower!

Mr. Grower: Ok, I have a 1000 watt...let me take a peak...

Mr. Grower: HOLY SHIT! At 4 inches I have 401,070...but if I raise it up a couple of inches I am only going to have 178,254...thats a loss of 222, 186!

Now this is where its read incorrectly.

The total amount of intensity is not lost, but SPREAD AROUND. If you place more plant to cover the area where it was spread out, you will get the same intensity (again, in a perfect situation) The same if they have a reflector..it will still spread but not by much. Do you guys understand now? I don't know how simpler I can put it.

 

[Weezard]

Weezard for the win!

Thanks for the numbers. 253 / 709 = 36% light intensity at twice the distance, once you removed the plants and let a little reflection into the picture.

That is incorrect.
These readings are from a different light.
One with 30 degree lenses. and it should help SS make his point.
180 degree light is still a posterchild for the inverse square law.
But, a focussed array is not.

Definitely more than 25%, but definitely not a whole lot more.

I imagine that 36% would go up more with more efficient parabolic reflectors, spotlights, laser lighting, etc. I'm not planning to switch to lasers, though....

Then did the UFO

Got 7500 and 2950 at 12 and 24 inches
Looks like this knock-off uses lensing and that's what makes it work a treat.
If you use one light per plant that is.

Weezard

 

[GMT]

strange 57% above isl again, but with the doors open this time.

 

[Weezard]

strange 57% above isl again, but with the doors open this time.

All three times.
Not enough room for my butt and da meter.
And the lensed array only had 2 sides close by and 2 sides open.
Took the highest spot reading I could find at each level.
Not gettin what I expected from the reflectix on the wall.
But do get what I should with collimating lenses.

It's not science, but it did tell me what I wanted to know.

Anybody got an HPS with a reflector they'd care to measure and post?

Aloha,
Weezard

 

[greenhead]

Just a small comment about that chart - - you can get closer than 4" with a 150.

 

[CannabisSativa]

All three times.
Not enough room for my butt and da meter.
And the lensed array only had 2 sides close by and 2 sides open.
Took the highest spot reading I could find at each level.
Not gettin what I expected from the reflectix on the wall.
But do get what I should with collimating lenses.

It's not science, but it did tell me what I wanted to know.

Anybody got an HPS with a reflector they'd care to measure and post?

Aloha,
Weezard

I have a 400 MH/HPS that I would like to test...but I do not have a light meter. Weezard, where did you get yours and how much? I have seen some on ebay for like $50.

 

[Lt. Herb]

Jesus tap-dancin' Christ.

Shogun still got the Cult Against the ISL goin'? Pass out the kool-aid already and head for the mother ship...

 

[GMT]

Lt. thanks for the experimental data and the constructive comments, they really helped to move the thread along in a positive direction, you've been a big help to the thread.

 

[Lt. Herb]

Sorry GMT, let me rephrase as to be more constructive:

The ISL does not account for enclosed spaces.

There, I've summed up the entire thread, pretty good of me, eh? Didn't even take 13 words, much less 13 pages. Data and constructive comments were pages and pages ago, btw, where they were promptly ignored. Seems anything that doesn't fit into shogun's prism is promptly discarded as heretical and unclean.

I've already stated that the light reaching the canopy anywhere but directly from the bulb is greatly diminished. Mainly due to dispersion when reflecting (the point of reflection becomes a point source of light emitting in all directions, i.e. spread over a greater area, more ISL there) and loss of energy when the photon collides with a surface. Even with 80% reflectivity (which would be pretty phenomenal in a grow environment), a photon striking 6 surfaces has already lost more than 75% of it's energy, @90% reflectivity it's on the order of 13 surfaces before losing 75%, and these figures don't take dispersion into account at all. This means that the vast majority of the photons that do not travel directly from the bulb to the canopy will have a severely limited contribution to actual photosynthesis. Therefore, the greatest useable contribution of light in a grow room comes directly from the bulb to the canopy, through open space, which is exactly where the ISL shines the brightest.

 

[devilgoob]

weezard, as you measure from further away, arent other leds' light hitting that meter?