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January 16th, 2011, 08:20 AM   #1
Joined: Jan 2011

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How does the inverse square law of light effect light intens


OK. I now know how to work out the "dose" of red light treatment based on this formula:

(unit is a 633nm red LED unit - 70 mW/cm2)

So to work out the J/cm2 for on minute it is- (70x60)/1000 = 4.2 J/cm2

I guess this is based on the assumption that no light is lost from the illuminated area (so the diode is pretty much as close to the skin as possible) and doesn't take into account the lack of uniformity of LEDs. lets just say for argument that the distance is 1/2 inch. Am I right in assuming that if the distance is doubled to 1 inch, the result will be 4x less powerful, so around 1 J/cm2 per minute?


PS - I saw this on the article I was reading which I don't understand as well:

Energy (J) or energy density (J/cm2) is often used as an important descriptor of LLLT (low level light treatment) dose, but this neglects the fact that energy has two components, power and time, and it has been demonstrated that there is not necessarily reciprocity between them; in other words, if the power doubled and the time is halved then the same energy is delivered but a different biological response is often observed.
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