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Old 06-10-2005, 02:23 PM
Ted Byers
 
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"Ray" wrote in message
...
I had considered the fact that there might be edge effects - some slight
diffraction around the edges of the openings, but I doubt it's significant.
My mind still wants to go back, though - if I have a white board and a
green board creating shadows, aren't the shadows the same?

Yes, the shadows would be the same.

However, don't forget that in the case of the shade cloth, you have the
pores that let some of the light through. What happens to that light? It
isn't all absorbed by the plants and fixtures in the greenhouse. Much of it
is reflected, and much of what is reflected will hit the inside surface of
the shade cloth. So the shade cloth is hit by light on both of its sides.

To see what I mean, you could try some ray tracing. Draw a cross section of
your greenhouse, with a solid floor and walls and roof that have gaps in the
line syou've used to represent them. Then, draw a series of parallel lines
hitting the greenhouse, representing incident light. Some of those will hit
the lines used to represent one of the walls or the roof. Those will either
end there or be reflected away from the greenhouse. Those that hit the
simulated pores will enter. Then continue those lines until they hit a
surface, and start drawing the path that would be taken by the reflected
light. Some of these will escape the greenhouse through the pores in the
shade cloth. Many, though, will hit the walls (shade cloth) and be
reflected back into the greenhouse. These paths will continue inside the
greenhouse until they happen to hit a pore, or until the light is absorbed
by something.

Energy is conserved. Light doesn't just enter the greenhouse and disappear.
Whatever isn't absorbed is reflected, and that reflected light must go
somewhere. The ray tracing exercise described allows one to see this, in
simplified form. A complicating factor is that any light absorbed is either
used by the plants in photosynthesis (converting electromagnetic energy into
chemical energy) or reradiated at a lower frequency (usually in the infrared
range), and this is reradiated in all directions. So, on average, half of
the light absorbed by the shade cloth will be radiated into the greenhouse
in the infrared, and half will be radiated out of the greenhouse.

Cheers,

Ted