Grow light height

[DrPhoton]

I could not entirely grasp your analogy, i believe you understand it correctly (correct me if im wrong). However 50 pounds on one square inch, is the same pressure or weight as 50 pounds on 4 square inches. Its only that the pressure or weight is spread out over more space, you are effectively getting the same result. Just the surface area utilized is different. This describes somewhat about the total light capacity mostly dictating yeilds (excluding all other factors) When applying ISL with the analogy, measuring the same surface area weight or pressure (if that were possible) of the two different surface areas, would result in the larger surface area giving less pressure or weight of one square inch on a 4 square inch surface. However this is not a great analogy with ISL, it confuses things. The pressure or weight wouldnt be 1/4 as the desnity of the material does not change.
Point sources of gravitational force, electric field, light, sound or radiation obey the inverse square law. Other physical forces do not.

[DrPhoton]

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.

[Tazard]

DrPhoton What you said is correct however you were not considering that the inverse square law applies to intensity and not energy. It is Like the equivalence of like say you were putting 50 pounds on one square inch and then you put 50 pounds on 4 square inches the effect of pressure is much less. This case that would be the lumens or as you mentioned the intensity (pressure) of the light. Placing lights as close as possible will (lumen for lumen) increase canopy penetration although each individual light will have a smaller area of effect as it will be inversely proportional to the inverse square law.

[DrPhoton]

Getting the light as close as possible is not going to increase yeilds, infact it can decrease yeilds.
Whats more important is the distribution of light over the canopy. Your light output capacity is going to dictate your yeilds, the amount of photons leaving the light source is the same no matter where you place your light. Inverse square law is wrongly thought of as the dustruction of energy over distance, where as it is just a divergence of energy through space.

Read here for more info.

http://forum.growweedeasy.com/forum/growing-community/101595-how-high-do-i-keep-my-light

[LuckyAcres]

Depends on strains a lot, sativa can handle my lec and my led better than indica. Depends on the health of the plant and obviously the type of light. All my lights were lowered below manufacturers recommendations.

[Tazard]

I have seen manufacturer charts providing height for lights above plants. However, I have not noticed where anyone has addressed the lowering of the light below manufacturer recommendations. When I was growing in the past (1980's) during the veg stage we would set our lights a bit higher than we thought they should be and then watched the plant for optimal growth (very short nodal distances see photo) while monitoring the canopy temperatures or signs of light burn. This may not be effective if you're growing more than one strain under one light since some plants may be more photosensitive than others. I don't know about LED's.