Grow light height

[Tazard]

The use of the term energy is just a simplistic term used for better interpretation. You do prove my point however. Intensity is correct, however it may be more accurate as density but it has been recommended to avoid the use of the term now as it conflicts with other fields of science. You are right that penetration is improved by the placement of light, this comes down to my description of how the distribution of light is more importent. However you have to consider the biology of the plant also to understand why getting the light closer is not importent, infact it can be counter productive. I can go into more detail if you like.

Also inverse square law does not apply to typically any grow enviroment. ISL is only valid when a light source acts like a point source, which can be determined geometrically by its interpretation of a point of view. This is actually quite vauge but this is due because ISL does not start to hold true until light starts to radiate in a isotropic or three dimensional behaviour. With any light source that is not acting like a point source to the observer, light will not distribute in a isotropic fashion at close distances (metres). This is practically any artificial light source we have. This is extended further by collimating the light with reflectors, focusing the light, preventing the light from diverging. Not to mention wall reflection as well.
So infact, light when considered in these enviroments will actually fall less than the ISL.
Theres also the matter of LED lights which extend the depreciation with light optics, this places a theoreticle position of the light source behind the physical location. Allowing light to fall off much less than that of even typical lights. This can allow deeper penetration. This is how lasers work, but that goes into a whole other bag of hurt, dealing with concepts such as reyleigh range.

I could go into much detail on plant biology with respect to light, but il keep it simple for now.
The compensation point for most plants is around 50um, this is the minimum level which plants require to survive. (We are talking small plants like windowsill types).
As light intensity increases, the quantum yeild (efficency of light acceptance) stays the same up until about 500um. This means that as the light increases from the compensation point to the compression point, the plant utilizes the photon energy at the same efficiency throughout this range. On a graph this would be a linear slope with respect to the increase in light intensity. Once the compression point is reached, quantum yeild decreases, this point appears to be around 500um. As light intensity increases past this point, carbon utilization depreciates, this is because the rate of intracellular concentration cannot keep up with photosynthesis. As intensity increases, quantum yeild decreases. This is further advanced by other conditions such as photoinhibition and photoprotection the higher the intensity is.
A good reference for a study done on cannabis photosynthetic rate is the 2008 chandra study.

So you see, getting the light as close as possible is not effective, a optimum distance is desired, one where quantum yeild is good and canaopy iniformity both horizontally and veritcally is acheived.
Personally i use 700uml or around 60,000lux for my light distance, this only slightly hinders the photosynthetic efficiency but allows for better canopy penetration. Like we discussed

I really want to write short articles on such topics, i just find myself too busy and too tired.

I hope this helps, let me know if you would like any help.

I would suggest here is where it went personal read last sentence and signature but it doesn't matter I apologize.

[D.A.A.S.69]

I enjoyed reading both of y'alls points of view , and I can tell both of y'all are probably really smart but that is so far over my head I ain't got a clue.I put mine in there close an if they like good and if they don't I move it up till they do like it. I never was good math, I just let the plants tell me what to do.

[DrPhoton]

As i said, iv ment no disrespect if any taken, i only care about the discussion at hand. My signature i would not have expected to intimidate people, my whole principle of learning comes from the idea that being wrong is not bad. Its an opportunity to learn something new.

Anyway hopefully we can come to a conclusion.
Here are various light sources i have taken the liberty of measuring, they were all taken at night. With no light interference and using a solid frame and measuring stick.

14" diameter LED light
12" 5000 lux
24" 1700 lux (ISL should be 1250)

1.4m x 0.2m florescent light fixture
12" 2600 lux
24" 1300 lux (ISL should be 650

10" diameter LED light
12" 3300 lux
24" 1000 lux (ISL should be 825)

5" diameter LED fixture
12" 4000 lux
24" 1100 lux (ISL should be 1000)

Incandescent shaped LED bulb with reflector
12" 1700 lux
24" 400 lux (ISL should be 425)

Incandescent shaped LED bulb without reflector
12" 1050 lux
24" 300 lux (ISL should be 262)

As you can see none of the results come to the results that should have been expected with ISL. You may also note that as the size of the light source got smaller, the closer the measurements came to that of ISL. This supports the idea that the size of the light source determines wether it is classified as a "point source"
I believe the initial low measurements of the Incandescent LED with the reflector was because of reflective losses, i retested with the reflector off and as you can see the loss was regained.
Even a standard sized and shaped light bulb doesnt surcome to ISL at close distances. As i had previously mentioned, it seems likely that around 1-2m it would start to follow the ISL rule.

Now the size of the point source is not the only denominator that dictates the "minimum inverse square law" Its also the geometry and anything else that prevents the light from diverging in all directions. Reflectors such as parabolic or box reflectors, simulate a larger point source but also collimate the light, preventing divergence.
For example, if you had a reflector with zero reflection loss that was 20m long, light will collimate and travel further until it reached the exit point. Where it would start diverging at a rate less that ISL. Eventually it will start to follow ISL, but due to the length of the reflector, its much further than a shorter type.
Now lets make the reflector even longer, say miles or kms. Now you have fibre optic transmission, just without the narrowband frequency.

Here is a discussion about the math involved for determining the distance where ISL starts to apply.
I have many more references if you like, if i am not explaining it enough i can go into more detail. However my original post is quite descriptive i feel.

https://physics.stackexchange.com/questions/253543/lights-inverse-square-law-does-it-require-a-minimum-distance-from-the-source

[DrPhoton]

It also appears that you seem to think that the decrease in intensity describes a loss or destruction of photons. Which is not the case, the reduction of intensity is because of the divergence of photons. As light spreads, the density decreases as it fills more space. However if you counted all the photons that have dispersed, you would get the same number that has left the source (excluding variables such as reflective losses). This is what a integrating sphere does, measuring light output in lumens, collecting light and reflecting all light energy to a sensor. Highly complicated process.

[Tazard]

So a 150w light should be raised to the level where the "cone of light" is greatest without exceeding the limits of the garden.

[DrPhoton]

Yes and no. What you ultimately want is the coverage or footprint to occupy the canopy. If the light footprint is allowed to project wide outside the canaopy area, you loose efficiency from reflective loss and non contact (missing the plant entirely).
If the footprint is too narrow, there is the issue of non cantact again. However the light is still being delivered to the plant. So still efficient.

This is of course going to depend on the reflectors projection angle, more narrow will require a higher hang, more wide a lower hang. The size of the reflector also plays a role. For a typical grow tent enviroment, the best type of light reflector is one that is large and wide with a very narrow reflection angle. Such as the evolution 8, magnum gen3, blockbuster 8. These allow you to not worry so much about footprint because they produce a more collimated light source, allowing you to narrow in on your desired height.

Then you have to address the issue of plant physiology, whats most efficient for the plant. As i had said in my earlier posts, Quantum yeild dictates the efficient use of light.

The chandra 2008 study shows us an aproximate range of this, with quantum yeild being linear up until about 500uml. With a steady decline starting between 500uml to 1500uml.

Saying all that, its not simple to narrow in on the best height. One must take into consideration of all the factors effecting photosynthesis.

Personally myself, i hang about 16" at 700uml. I get good coverage with this height and i also gain a little more in verticle uniformity (penetration) with the closer proximity compared to 500umls.