Ah HAH! I had forgotten about the reflected light. Do you think it's
that significant?

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"Ted Byers" wrote in message
.. .

"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