Inverse square law

Discussion in 'Lighting' started by Swami, Aug 24, 2011.

  1. Yes, another in a long line of misunderstood facts regarding lighting. *sigh*

    First off, the inverse square law which states that light intensity decreases inversely proportional to the square of the distance from the light source, only applies to a 360 degree unfocused light source.

    Add a reflector or focusing lens and the formula changes drastically. Nor does this formula apply to LEDs whose beam angle generally ranges from 40 to 160 degrees. Yes, the light still spreads out, but at a slower rate.

    Hypothetical: place a light source inside a sphere with 100% reflectivity (not possible) and no atmospheric interference (also not possible) and the size of the sphere is largely irrelevant as all of the light would eventually reach your plant place anywhere inside the sphere.

    What is the point of this thought experiement? It is this: the idea that the light source must be right on top of the plants to be useful is not accurate. The more important idea is to utilize as much of the light from your source as possible using highly refelctive walls and proper plant placement; hence the efficacy of SOG, SCROG, V-Stadiums and similar type arrangements.
     
  2. #2 rhapsodyrcks, Aug 24, 2011
    Last edited by a moderator: Aug 24, 2011
    Inverse applies to non 360 lights. The light going down is directional. Further you get away from it the weaker it is. Thousands of grows validate that. Thats the basic notion of the inverse square law of light. "A object that is twice the distance from a point of source of light will receive a quarter of the illunination". Most people who understand this concept well are photographers.

    Reflective lighting off the sides only recaptures about 10% of the light. Depending how big your tent or grow area is that light ranges from useful to non useful. Put a plant in a big ass tent in the middle and the reflective light is not all that useful.

    All of this is easily verifiable with a light meter or a easy way is to look at his chart which is pretty accurate.

    Plant placement is very important I agree but so is how far your light is. That being said you can have perfect placement, reflective sides and the further you get away from that light the weaker it is in terms of grow ability. Looking at it visually might not give that appearance but grow spectrum is not visible via the human eye.

    Another way to validate that is get a big ass tent put a 400w HPS in it. You'll find that the plants under it grow fine plants along sides do not..why because the inverse square law of light is kicking in and the plants are not getting enough spectrum do to it falling off dramatically as the plants are further away from the light.

    http://forum.grasscity.com/attachme...-lights-plants-light_distance_chart_1__1_.jpg
     
  3. This is just wrong and is why I wrote this thread. Never understood why people who do not fully grok a subject feel the need to spread bad information. if your statement was correct then it would be impossible to bounce laser beams off of the moon.
     
  4. #4 toastybiz, Aug 25, 2011
    Last edited by a moderator: Aug 25, 2011
    [FONT=&quot]I am with Rhapsody on this one. Oh, and Newton, and Einstein, and Hawking...

    The inverse square law applies to the energy itself. If light is being emitted from a source, let's take a close look at the light coming from 1 of that dispersion. Once it is emitted from the source, the energy will dissipate according to the laws of physics regardless of what is going on in the other 359[/FONT][FONT=&quot].

    It is true that adding a reflector bounces more of the light back, so yes more light is coming to your plants than without a reflector, but all of the light is still subject to the inverse square law. In fact, that law states, "Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law." So in this statement, the bulb is the point source of the light. Even if that light is then bounced off a reflector, it was the bulb, not the reflector, that still was the single point source. In other words, adding a reflector doesn't change the underlying condition of how the light was emitted or what properties of physics it is subject to.


    If I'm following what you are saying, that would mean that the light could bounce around inside that sphere any number of times without ever dissipating? No.

    Scrogs, SOGs, stadiums etc are more efficient because you are putting more plant matter where the light can hit it. Yes, the light spreads out, that's one of the underlying principles of the inverse square law in the first place.
     
  5. Sorry to hear that first part, but you are not in agreement with Newton and Einstein.


    This is incorrect.

    You got 1 out of 3, but as I mentioned in my OP, some lights sources such as LEDs, do not spread their influence equally in all directions, hence the dispersion formula must be modified to account for the beam angle.
     
  6. You know, I may need to eat some humble pie here.

    After posting I got to thinking about your point, specifically when you said that we wouldn't be able to bounce lasers off the moon if the inverse square law applied. I couldn't get what one had to do with the other. At first.

    But the idea behind the inverse square law is that you take a finite amount of light and as it travels it spreads out to cover an increasingly larger area. A laser doesn't do that. Keep in mind, though, that a laser is not a naturally-occuring form of light, and in fact it is amplified (that's what the "a" stands for). With a laser, all light is sent parallel in a straight line, and indeed it should not dissipate (not due to the inverse square, anyway) because the area that it covers as it travels is the same. That's the principle behind the point you were making about bouncing lasers off the moon -- the light never spreads out, because if it did then it would never get reflected back here.

    LEDs share some characteristics of lasers but don't send all their light out in a single direction. The issue with whether the inverse square law applies or not has nothing to do with reflectors, it has to do with the source of the light itself.

    So I think academically you are right, however given light sources used in grow rooms I think the inverse square law would apply to all of them, but I need to look more into how that works with LEDs.
     
  7. #7 Swami, Aug 25, 2011
    Last edited by a moderator: Aug 25, 2011
    I am glad you are reconsidering. A refreshing change from much of the stubbornness here.

    Let's not get lost here. Physics and the math associated with it does not change for artificial or natural; physics recognizies no such distinction; nor does it care if it has been amplified or not.

    More or less correct, but let us REALLY understand what the inverse law is talking about. Photonic energy does not dissipate the further it goes out, it is just that less photons are hitting an object further away and more of the photons are missing the object. With me so far?

    Now if we take many of those photons that would be missing the object and redirect them with a relflector, then the inverse square law still applies as to photons hitting the object DIRECTLY from the source, but the photonic energy from the secondary source (the reflectors) must be added to get the real-world number.

    Look up my thread on my bucket lamp. I have out-vegged many, many grows using a single 23 watt CFL from 12" away vs, 2 to 4 each 32W CFLs placed 6" away. How is this possible? Because I minimized light spillage and wastage by containing it.
     
  8. #8 rhapsodyrcks, Aug 25, 2011
    Last edited by a moderator: Aug 25, 2011
    We bounce lasers off the moon with high power lasers off a mirror that was laid by the astronauts of the Apollo Moon missions. Mystery solved.

    The actual experiment was called the LLRE Laser Ranging Experiment. It continues to today as the reflector obviously is not going anywhere and lasers continue to improve.
     

  9. No you are wrong. Thats why I responded :) Go youtube yourself with a light meter which is cheap to obtain and tell me that the light does not get weaker the further you get form the source.

    This is something that photographers all know very well.
     
  10. Goerge Washington was the first president of the United States.

    Why are you telling me something I know? Is there some relevance? And what mystery was solved?
     

  11. Lasers are effected by the inverse square law. For very short distances no, but long ones yes.

    The laser that they bounce off the moon by the time it hits the reflector its about 7 kilometers in diameter and about 20 when it comes back to the earth.
     

  12. Is there any relevance to anything you have said?
     
  13. Photographers know what an F-stop is. So what? You are attempting to refute something that I did not claim for reasons unknown.

    The inverse square law does not apply to focused or narrow beam light sources. Fact.
     

  14. Thats somewhat actually not so correct depending how you look at it.

    George Washington was the first President of the United States under the Constitution we follow today. There were actually 7 1 year Presidents under the Articles of Confederation which existed before the Constitution was signed. He was voted into office unanimously by the Congress...that included George Washington.

    One of the cool things that Hanson did was made the forth Thursday of every November Thanksgiving. Its still the same today.
     

  15. No its not a fact. If you had a perfect laser the law would not hold. The beam diameter would not change you would have the same energy density at any spot in the beam.

    However perfect lasers do not exist. Every laser has beam divergence. Like I said NASAs laser is shot out into space to the moon. It starts out small but do to inverse when it hits the moon its about 7 kilometers in diameter.

    So I am right and you are wrong.
     

  16. Ah yes, the math changes at some arbitrary distance, eh? Brilliant!

    So, according to your silliness, a laser at 20 feet would have 1/400 the intensity of a laser at one foot away.

    The power of the laser light hitting the moon would then be 1/((238857 miles * 5280 feet) * (238857 miles * 5280 feet)) or 1/1.5e-18. Yes folks, 1/15,000,000,000,000,000,000. In other words, undetectable. I truly hope this is not what 'all photographers know'.

    Sorry - total fail.
     
  17. What kind and how many beans would you wager?

    The inverse square law only applies to a point source of light. All lights diverge (past a focal length if focused). All divergence does not follow the inverse square law; the amount of divergence depends on the beam angle.

    Really cannot make it any simpler.
     
  18. Putting the inverse law aside....Just from actual experince most growers will agree that the stronger and closer the light is to the subject the better the grow results and if your not using reflective surfaces your not getting the full benifit of the light....
     
  19. #19 Swami, Aug 25, 2011
    Last edited by a moderator: Aug 25, 2011
    And plants need water, too! :p

    BTW, nice grow log. :smoke:
     
  20. the inverse square law applies the light weakens with distance, but think about it like this. The light energy has inertia. the more light flowing the easier light can flow. the reflected light is flowing into the light from the side facing the plants. a light with a reflector has a higher intensity than without because some of the light going the wrong way is reflected into the direction of the other. in other words, light is decreasing with distance but, because of the reflector, you may have a little more intensity a little farther down than if you didnt have it. thus when applying the inverse square law to the equation you must take into account the extra reflected light energy because it is adding to the light given off by the lamp.
     

Share This Page