This concept is pure silliness and yet is bandied about as if it has merit. If there were such a thing, it would have a clear definition and a way to measure such, but it doesn't.
Let's take a single photon. It can travel the length of the universe unless obstructed, so it can probably travel 5 feet unless obstructed. The sun is the most intense light in the solar system, yet cannot make it to the floor of a thick rainforest or jungle.
So it is not merely distance nor intensity. Then what is penetration? Who knows? If it is merely lighting up lower leaves then a couple of well-placed low watt cfls must have awesome penetration.
If it is merely lighting up the lower leaves then two widely-spaced 250w HPS have better penetration than a single 1000w HPS.
Unless someone has a solid and measurable explanation, this 'concept' should be stricken from the cannabis lighting lexicon.
First off, you're comparing how light travels in a vacuum to how it travels through our atmosphere. Light can travel much, much further through space than it can through the air on our planet.
Secondly and much more importantly, you're not considering the size of the light-emitting substance. Some people will understand better after drawing what I'm saying to imagine. Imagine two separate light sources. Instead of the normal field of light, imagine each source two-dimensionally as a point with sixty rays equidistant from each other, like a clock with each of sixty hands pointing at a different minute. Each hand represents a ray of light.
Now imagine a two-dimensional plant centered under the two light sources. The closer you imagine the plant to the light sources, the more of those 120 rays will run into it. The further away you imagine the plant, the fewer rays will run into it. Also, the further apart the two light sources are, the fewer of those 120 rays hit the plant. This is exactly why distance from the lamp is so significant to light intensity. It's also why the light from many fluorescents doesn't reach very far; the light is emitted from a much larger area than with an HID, for example, and so has much more of this effect.
The light emitted by a smaller source will reach further than the same lumen output emitted over a larger area.
Edited by Jellyman, 09 July 2011 - 05:57 AM.