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Old 07-10-2005, 03:43 AM
Ted Byers
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"Ray" wrote in message
Ah HAH! I had forgotten about the reflected light. Do you think it's
that significant?


I'll give a definitive "It depends!" ;-)

It all depends on the details of the geometry of the structure. If the
geometry of the greehouse, and the properties of the shade cloth, are such
that a large proportion of the light entering the greenhouse is ultimately
absorbed inside the greenhouse (meaning used by plants to produce new
tissue, or ultimately converted into heat), then this effect may well
overwhelm the contribution to plant production from light that has entered
the greenhouse and not yet been reflected. On the other hand, if the
combination of the geometry of the greenhouse and the properties of the
shade cloth are such that most of the reflected light exits the greenhouse,
then the effect is likely negligible.

The only way to know for sure if the effect is likely to be significant is
to construct a computer simulation; a numerical experiment if you will. The
physics, and the computational requirements for ray tracing, is simple
enough. The data management, though, would be challenging, since you'd have
to run a reasonably large number of rays, simulating a constant input of new
rays, and keep track of rays that have left the structure or been absorbed,
until the system approaches equilibrium (probably reached almost instantly
in the real world, but likely to take some time in the simulation). You can
then estimate the proportion of rays striking a given surface that have been
bouncing around the greenhouse before striking the surface in question (and
do this for a number of different frequencies, simultaneously, to get a
sense of the impact of this effect on the spectral quality of the light. Of
course, once such a simulation model has been constructed, it is easily
modified to examine the consequences of using black shade cloth rather than
white shade cloth (or any other color for that matter), both for the quality
of light reaching a given surface and for thermodynamic effects.

While I am a specialist in environmental modeling, I am not sure I want to
spend the months needed to produce an application to do this to the
standards I maintain for my own work; especially when I am working hard on
getting a new business established. If I was working at some university,
though, I'd certainly assign it to graduate students to give them a sense of
the application of science in the real world.



R.E. (Ted) Byers, Ph.D., Ed.D.
R & D Decision Support Solutions
Healthy Living Through Informed Decision Making