Sub-canopy Penetration (same lamp)
The next graphic shows a cross-sectioned top-to-bottom view of the canopies illustrated on the previous page (narrower canopies have been rescaled for clarity).

In the image on the right notice how the sweet spot light is shallow and subdued, and how suitable light doesn't penetrate the rest of the space as deeply as it does in the other images where the lamp isn't hung as high. The image on the left has good light penetration, but because the lamp is too low some of the best growth is damaged by heat or over-lighting the sweet spot. The image in the center avoids the damaged growth of a lamp that's too low and provides deeper penetration than a lamp that's too high. All three affect the harvest in one way or another, roll your mouse here to see how.

Sub-canopy Penetration (different lamps)
Sub-canopy penetration increases not only as the light source gets closer, but as the source becomes brighter as well. When we graphically represent four common lamps and hang them at a height so that their closest safe-points are at the same elevation we can more clearly see two things happening....

Note that each lamp, by being adjusted higher or lower, can produce the same foot-candle values at closest safe-point in the sweet spot. However, because a light source with a higher lumen output will project stronger intensity at a farther distance than one with a lower lumen output, the effective range of a smaller wattage lamp is lessened due to its lower lumen output. As a result, the shallower canopies and lower intensities of smaller lamps produce shorter and less robust colas than larger lamps, even when light intensity at their sweet spots are the same.

As the canopy top receives only direct unobstructed light, the sub-canopy receives much of its light after being shaded by growth above, or by adjacent growth in the light's diagonal path through the canopy. Shade somewhat reduces the effectiveness of a lamp's unobstructed range, thus the plant spacing and/or growth density dictated by one's growing style has something to say about how much of his lamp's penetration potential will be utilized. As illustrated above, denser canopies shorten a lamp's useful range.