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Old 06-10-2005, 11:15 AM
Ray
 
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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?

Likewise, I suppose it's possible that some light is transmitted through the
mesh material, in which case the color might be important, but again I doubt
it's 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
...
OK, I give up.

In shade cloth, the solid material blocks the light, and the openings let
it pass, right? As an opening is an opening is an opening, what
difference does it make what color the light-blocking part is?

The color we perceive is due to the difference between the light reflected
(or that passes through translucent material) and the light absorbed. If
you shine white light on a green material, we seen green because most
other frequencies in the visible spectrum are absorbed and it is primarily
green light that is reflected. In the context of a greenhouse, then, you
will have light of various frequencies bouncing around, reflected by
different surfaces. An opaque green cloth will absorb the frequencies
most useful to plants and reflect those frequencies that are least useful
to plants. This is why, for example, leaves appear green; they absorb red
and blue and reflect green. I could go on, but I am sure those interested
can find a text on plant physiology or biochemistry.

It may help to think about the ultimate fate of the light that enters
through the openings in the shade cloth. While sunlight is nowhere near
white, lets assume that the light entering the greenhouse through the
pores in the shadecloth is white. What happens to it once inside. It
bounces around, reflected by one surface or another until it either
escapes through another pore or is absorbed. Since the plants will be
absorbing red and blue frquencies and reflecting green, the spectrum will
increasingly include primarily green colors. Similarly, if the shade
cloth is green, any red or blue light hitting it will be absorbed (and
probably reradiated as infrared), while the light reflected will be green.

Does any of this make a significant difference? I don't know since I have
not looked at, or conducted, any experiments that would tell us. I would
expect that there may be some effect on rates of primary production, and
thermal effects, but only a series of controlled experiments will tell us
for sure. I am not sure this is even important, except for those who
design and test shade cloth, since for ordinary growers and breeders, the
recommendations of how to use shade cloth will probably largely be based
on experience of how well different plants perform with different amounts
of shade cloth in different regions.

I don't know if this is helpful, but it is fun to think about.

Cheers,

Ted

--
R.E. (Ted) Byers, Ph.D., Ed.D.
R & D Decision Support Solutions
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