Debate - Does the Inverse Square Law apply to grow lights?

[Medijuana]

So I've been wrestling with an idea lately about the inverse square law. It seems to be commonly agreed upon in this forum that the inverse square law is universally applicable to all grow light setups. However, I'm going to have to argue that the law is not applicable in most cases.

The inverse square law applies only to an isotropic radiator (or point-source) that is emitting energy (or light) equally in 3 dimensions. The exact opposite of an isotropic radiator is a laser. If you measured the light energy of a laser beam at 30 feet away from the source, you'd get essentially the same amount of energy measured at 10 feet (as long as you're measuring in a vacuum, but the inverse square law isn't measuring scatter anyway, as that's a different set of physics).

Now, I'm not at all arguing that grow lights act in any way similar to a laser, but, many grow lights are definitely not isotropic radiators, but are designed to focus their energy in the direction of the garden. This is the whole point of reflector arrays on traditional HPS and MH lights. I am proposing that many grow lights fall somewhere in between this spectrum, with lasers on one end and isotropic radiators on the other.

The reason I started thinking about the inverse square law in the first place is that I am growing with an LED grow light. By their essential structure, LEDs do not emit light equally in all directions, as the LED is attached to a silicone plate, and so one whole side of that 3D space will be blocked. Of course you wouldn't try to measure the light intensity from the wrong side of the plate, and I'm not saying that, but you do have to factor in where that light from the point source is ending up; it is either reflecting to the open side of that plate or some of it is converting to heat against the plate it is attached to. The most critical factor of why i do not think the inverse square law applies to my particular model of LED grow light has to do with the use of lenses to focus the LEDs' emissions. Even though there's no such thing as a 'perfect' lens, especially in this type of application, the light is no longer traveling equally in 3 dimensions, but is being focused into a certain area. This automatically takes it out of the full realm of the inverse square law, as we're no longer dealing with an isotropic radiator, but have now inched a bit closer to the laser side of the spectrum.

If this has been beaten to death elsewhere, I apologize, but all the recent mentions of the inverse square law seem to indicate that people are applying universally to all grow lights, and I wanted to open a discussion to see what others think.

 

[LexLuthor]

I think the inverse square law still applies to LED's because it is a light source that is being spread over a surface. I understand your saying that all of the lumens are within x amount of degrees from the source point unlike an HID which is releasing all of its lumens 360 degrees. Because of this there might be a lower fall off of light with LED's compared to HID's but the light still looses lumens per square meter the further the light is from the surface it is illuminating. Just because no light is coming from the back of the LED does not mean the Law is thrown out, its just like an HID that has a reflector, there is no light coming from the back of the reflector. The Law has to do more with the spread of light over a surface from the source point, not what the light is coming out of. thats just my 2 cents I could be wrong.

 

[Medijuana]

I know the weakest part of my argument is what happens as a result of the diode being attached to the silicone. However, I believe that the use of the lenses and how they change the way the light is moving through space does change the behavior of the light in terms of how the inverse square law as it is normally applied to the isotropic radiator. The law does still apply in isolation, as the lenses wouldn't change this effect by any appreciable value, but if you compared the same LED with a lens and with no lens, the energy values would be different. This is why I don't think it is accurate to think of applying the law universally as it is written for point sources of light, but that modifying factors in how grow lights are constructed and used definitely moves them away from the theoretical isotropic radiator.

 

[LexLuthor]

I get what your saying. The inverse square law says when the distance is doubled the light is cut down to 1/4 so the LED's might not loose the same amount of light as an HID but the light is weaker from 24" compared to 12", I just dont know if its cut by 1/4 or a different amount. Maybe its only 1/2, I honestly have no idea :confused: ...anybody else have an opinion??

 

[ttystikk]

In the case of light focused by lenses, the inverse square law still applies, but the lens affects the math by distance; the more focus, the longer the distance for exponential decrease. I would be interested to find out what that formula is because I'm working on advanced light distribution systems, myself. I'm betting that a careful look through a college level physics book on optics will provide some illumination on the subject.

To dig into the practical aspects for a moment, I'm working on a way to more efficiently utilize the light coming from an incandescent, HID or plasma source. These lights generally DO meet the criteria of equal shine in all directions. Since LED is focused through its lens, I decided not to pursue my light moving/edge shaping designs with these in place, because there would be no way to effectively get 'sideshine' from an LED array unless it was specifically designed that way. That specific design would end up looking a lot like the bottom half of a disco ball, with LEDs and lenses arrayed in a radiating pattern from a centerpoint in the middle of that ball. That LED array would have to be seriously high powered, as well, and as such it just didn't make sense to pursue it further.

Instead, I took the opposite tack; using isotropoid light sources- that is, close to isotropic, since even HPS bulbs have a short linear filament- I am trying to direct, spread, distribute and reflect light to maintain a certain essential minimum light level throughout the canopy within the grow space and also provide an intermittent high intensity of light to encourage powerful growth.

In a circular light mover situation, the moment you place the bulb off center, the opposite side develops what many have called a 'dark crescent', where the light is inadequate to maintain good levels of photosynthesis. This is commonly held up as 'proof' that light rotators won't work, since the plant is only getting sufficient light part of the time. I developed a special canopy edging that allows for a light multiplication effect all around the canopy, and especially within that 'dark crescent' area to avoid this problem.

The other side of the coin, very high light intensities, can be just as problematic. We've all seen light 'bleaching' where the plant seems to be burned by excess light. Most of the time of course, the plant is actually 'bleached' by excess heat and sometimes a lack of sufficient nutrients transported to the scene to properly convert all that light into useful compounds. But what if the light moves? In this case, the high intensity waxes and wanes, allowing the plant to recover in between blasts of high energy. This gives the plant time to cool and transport more nutrients to these sites, allowing for growth to occur and avoiding damage.

Lastly, nearly every plant large enough for the human eye to see has evolved a strategy to take advantage of the fact that the sun travels across the sky every day. This is why a Christmas tree shape or apically dominant plant growth strategy is so common. This shape takes advantage of the movement of the light source to more or less evenly light the entire plant. Cannabis is no different. Other plants grow tall fast and then spread out at the top, as this is an effective strategy for shading out competitors and thereby guaranteeing a larger share of water and nutrients for itself. When we top and train and SCRoG cannabis, this is essentially the shape we're trying to achieve.

Many people argue that any time the light level varies, it's automatically bad because the plant could supposedly handle the maximum light level constantly. What if this isn't the case? What if a plant can get too much light? It can certainly get too much light for its available conditions, as anyone who has seen lush grass growing on the north side of a tree in summer can attest; it's doing fine there, even as the rest of the lawn burns to a crisp!

It is these questions that I'm investigating by engineering a system of light rotator, specially designed reflector and canopy edging to reduce the amount of light lost, and thus maximize the percentage of light falling on the plants. Rather than fearing and diffusing the 'hot spot' I am instead embracing it, and moving it around the canopy so that its benefits can be shared by all, while its detrimental effects are nullified. The reflector is not made of a 'hammer finish' diffusing shape, but smooth. If left stationary, this thing would make nasty scorchmark patterns throughout the nearest part of the canopy! But since it moves, those effects disappear. Likewise, the canopy edging would be of limited value if the light didn't move for similar reasons as the reflector. Taken together, these elements work together to create a whole that is brighter overall and better focused on the canopy beneath than the sum of its parts alone might suggest.

Another part of this discussion is what happens when light hits a specular (think mirror smooth finish) versus a diffusion coating (think white walls of panda film). People love to argue with me that the diffusion coating is superior becuase it so obviously 'looks brighter'. I disagree; if it's so bright to you while you're standing outside the canopy, what you're really being blinded by is WASTED light! And therefore, wasted energy. Does a telescope's mirror use a diffusion coating? Of course not! So why should we trust that a diffusion coating is going to get us optimum light intensity, focused on our target- the leaves of our plants? Diffusion coatings are, if you think about it, just a crutch to work around the unnatural stationary position of an indoor light source. If the light moves, diffusion coatings become unnecessary. This will also increase light intensity. After all, another word for diffusion is 'scattering' and for my hard earned lighting dollar, I don't want scattering, I want focus!

 

[squiggly]

LED's follow a modified inverse-square law. The hint that it applies universally is in the name--Law. I know it seems like laws, principles, and theories are haphazardly named, but there is actually a fairly strict convention which dictates their being named as one or the other. To become a law, something has to be held up by series of essentially incontrovertible truths and many many many repeat experiments (we're talking decades, here). It is very difficult. Most "laws" that we have in the natural sciences have almost never been questioned--and there certainly hasn't been data to suggest than any of the laws have been broken since their acceptance in the community at-large (sometimes this can be a long process).

That is why you saw so much fanfare over neutrinos exceeding the speed of light. That would've thrown everything on it's head--and we don't even call relativity a law yet!

For more in depth on the modified inverse-square law, see this:

 

[Medijuana]

The other side of the coin, very high light intensities, can be just as problematic. We've all seen light 'bleaching' where the plant seems to be burned by excess light. Most of the time of course, the plant is actually 'bleached' by excess heat and sometimes a lack of sufficient nutrients transported to the scene to properly convert all that light into useful compounds. But what if the light moves? In this case, the high intensity waxes and wanes, allowing the plant to recover in between blasts of high energy. This gives the plant time to cool and transport more nutrients to these sites, allowing for growth to occur and avoiding damage.

I had to chuckle a bit when you mentioned the bleaching. I pruned this off yesterday morning, so it is a bit shriveled, but this leaf got about 9 inches from the light and did suffer some bleaching. Now this is an LED, and you can barely detect any trace of heat with your hand from that distance. This is true bleaching. I'm only pointing this out to help dissuade the naysayers on LEDs. The good ones are powerful as hell, and are capable of damaging the leaves by their intensity alone if they get too close. My tent temps have never gone above 82`F since I started this grow (they average in the high 70's with lights on).

 

[squiggly]

Just to clarify, I'm not anti-LED--I use LED's myself. Just trying to bring clarity to the topic at hand.

 

[Medijuana]

LED's follow a modified inverse-square law. The hint that it applies universally is in the name--Law. I know it seems like laws, principles, and theories are haphazardly named, but there is actually a fairly strict convention which dictates their being named as one or the other. To become a law, something has to be held up by series of essentially incontrovertible truths and many many many repeat experiments (we're talking decades, here). It is very difficult. Most "laws" that we have in the natural sciences have almost never been questioned--and there certainly hasn't been data to suggest than any of the laws have been broken since their acceptance in the community at-large (sometimes this can be a long process).

That is why you saw so much fanfare over neutrinos exceeding the speed of light. That would've thrown everything on it's head--and we don't even call relativity a law yet!

For more in depth on the modified inverse-square law, see this:

I actually specifically pointed this out regarding how the LEDs light is focused and not simply emitted equally into 3D space. I'm well aware of what it takes for science to name something a "Law", but the Law of Inverse Squares as it classically stands applies only to point sources of energy. When you redirect a point light source, whether using reflectors, lenses, or other means, you are no longer operating under the same classic law of inverse square, and this is all I am arguing. This is why I think we shouldn't blindly site this law when talking about light intensities. I imagine there are well established laws of optics that we could refer to that would better explain how light intensity improves when changing the direction of light, but I have no background in this, so I don't want to pretend like I know. However, we might find the growing community as a whole better served if we began looking into the actual laws that dictate how how lights work when we begin redirecting them towards our canopies.

 

[squiggly]

Well if we think of them as photons, or electrons as I prefer to.

Found the following two comments digging through the internet:

The typical LED has a built in reflector and lenses that direct the light in one direction. Inverse square still applies but it's as if the point source was originated somewhere far to the rear of the actual LED so the fall of is less per unit distance after the LED surface.

If the light spreads out over some space as it moves away from the source, it spreads over the same amount of space regardless of whether it's from a light-emitting diode or from some other kind of source.

When you're moving light through a lens or prism--you get some weird effects in terms of what is considered the source of the light. This leads into all types of weirdness like "real" images and "virtual" images and so on. Optics is a bit too twist-turny for me, can't ever seem to wrap my head around it just right.

The short and long is, yes LED's follow the inverse-square law--but not in necessarily the same way for the reasons you have been pointing out, the initial conditions are different. It doesn't invalidate the law, which is written for a specific initial set of conditions.

The "modified law" for LEDs is merely attempting to fix the inverse-square law by a factor that corresponds to that change in conditions.

 

[squiggly]

This might help you get to where you want to be with this, looks to have a plethora of information:

http://ledlight.osram-os.com/wp-content/uploads/2010/11/Gaggione-LED-Light-presentation2.pdf

 

[ttystikk]

...and as is often the case, as soon as I open my mouth and say something I think to be true, I read something by someone with established credentials that tells me I had it wrong. It turns out that light oversaturation alone can cause bleaching, independent of heat. Hence your LED scorched leaf, and some suspicious looking bleach issues on lower stories of some of my plants- they just hadn't hardened off to the light available when I bent things out and exposed them to direct light.

"Photosynthesis Maximized", by Dr. Lynette Morgan, page 70 of the March 2012 issue of Maximum Yield magazine, states that there is a light compensation level, at which plants stop growing because their cost of respiration equals the benefit from their photosynthesis, occurs at the lower boundary of adequate light intensity. Light saturation occurs when the plant is getting enough light that more will not result in more photosynthesis and therefore, more growth. Clearly, the grower wants to aim for the latter; as close as possible to light saturation without crossing beyond it and thereby wasting energy.

 

[ttystikk]

Well if we think of them as photons, or electrons as I prefer to.

Found the following two comments digging through the internet:

The typical LED has a built in reflector and lenses that direct the light in one direction. Inverse square still applies but it's as if the point source was originated somewhere far to the rear of the actual LED so the fall of is less per unit distance after the LED surface.​

If the light spreads out over some space as it moves away from the source, it spreads over the same amount of space regardless of whether it's from a light-emitting diode or from some other kind of source.​

When you're moving light through a lens or prism--you get some weird effects in terms of what is considered the source of the light. This leads into all types of weirdness like "real" images and "virtual" images and so on. Optics is a bit too twist-turny for me, can't ever seem to wrap my head around it just right.

The short and long is, yes LED's follow the inverse-square law--but not in necessarily the same way for the reasons you have been pointing out, the initial conditions are different. It doesn't invalidate the law, which is written for a specific initial set of conditions.

The "modified law" for LEDs is merely attempting to fix the inverse-square law by a factor that corresponds to that change in conditions.

That first quote makes perfect sense to me; the lens and mini reflector act as a focus, and thereby simply move the light output 'down' the inverse square law's intensity curve, where less intensity is lost per unit distance due to the focusing effect. The second one is also pretty straightforward; it still follows the same spreading pattern, regardless of focus. The level of focus merely determines the distance of the virtual focal point behind the focused light source, whereupon the inverse square law still applies.

This explains why LED light tends to fall off in intensity less than with HID or other isotropic non-focused light sources. This doesn't make it stronger; just gives it more penetration than its actual wattage draw might suggest. Good stuff.

 

[LexLuthor]

Sounds good fellas, looks like we all agree the light from an LED still loose is its strength just at a different rate than an HID due it's lens focusing the light.

 

[ttystikk]

Sounds good fellas, looks like we all agree the light from an LED still loose is its strength just at a different rate than an HID due it's lens focusing the light.

That's correct. Think of it like this; if for example you have a 1 watt LED with a focusing lens on it, a good way to think about it is as if the actual source of light is a 4 watt light one foot behind the actual location of the LED. Then the attenuation of light intensity follows the inverse square law pretty well. Different wattages of chips and different lenses will change that 'virtual distance' behind the actual location of the LED, but this illustrates the basic concept.

 

[LexLuthor]

Gotcha, I knew the LED light still followed the Inverse Square Law I just wasn't sure of the exact equation but you explained it good, now I understand it more.

 

[whazzup]

thanks for bringing up this very interesting subject.

the inverse square law has a few "buts": It is effective at 5 times the distance of the longest measurement of the light source. That explains why it is not applicable for long CFLs for example at close distance. If you add a reflector then it is even a different story, those calculations will not add up any more t close distance. Of course you have a lower intensity at a greater distance. In horticulture however we keep lamps much higher above the plants, but they overlap so in the end you have the same intensity as lamps at a close distance, but at a much better uniformity.

For lamps in reflectors we use calculation programs based on IES profiles, that have been measured by certified light labs.

As for bleaching: That also depends on the spectrum. I had a plasma light at about 10 cm above a plant at more than 2500 umol s-1 m-2, and it did not burn or bleach.

Here is a small video presentation I made of this experiment, in which I put a seed in 90 liters soil en just lit it by one plasma light, to show the potential and quality of this light. Of course you would never grow a plant this way, but look at this, then add 600-1000W HPS and you understand what plasma can do.

LEP, by definition, is a point source at about 1 cm distance from the cell.

 

[LexLuthor]

Why do all these sales reps try to thread jack??

 

[ttystikk]

Why do all these sales reps try to thread jack??

Dude. Really? Whazzap is a professional in the field and is sharing this funny thing called 'research.' You may have heard of it? Then again, perhaps you'd rather just go with 'gut feelings' and 'hearsay'...

While you're doing that, I'm taking notes. Guess which one of us will come out ahead?

 

[LexLuthor]

Dude. Really? Whazzap is a professional in the field and is sharing this funny thing called 'research.' You may have heard of it? Then again, perhaps you'd rather just go with 'gut feelings' and 'hearsay'...

While you're doing that, I'm taking notes. Guess which one of us will come out ahead?

I'm talking about THREAD JACKING, if anybody cares about plasma lights they can do there own research on Google...there is NO need for people to try and sell there shit on a thread that has nothing to do about that subject....so if anybody wants to know about these new plasma lights they can start a NEW thread about it or do there own research online.