"Ray" wrote in message
...
Well Ted, I guess we can ignore light reflected from the plants, since
it's the wavelength they don't use. Then we have to speculate on the
reflectivity of the benches, floor, media, algae, moss, etc.

No, we can't ignore light reflected from the plants. The light reflected
from a plant appears green because there is more green light than anything
else, but that does not imply that there is no blue or red. Blue and red
light may well be present in the reflected light, but in equal values of the
plant looks green, or a bit more red if there is a reddish hue to the
leaves. From a scientific perspective, we can not say that all green light
is reflected by leaves, nor that all red and blue light is absorbed. All we
can say is that more green light is reflected by the leaves and more blue
and red light is absorbed. And of the light that enters the leaves, more of
the green light will pass right through while less blue or red light will
pass through. Most leaves are translucent; not opaque!

While I didn't mention it before, there is the question of light that passes
through the leaves. In most species, if you hold a leaf up to the light,
you can see light coming through the leaf, and this light is normally green,
and this happens even though you can not normally see anything on the other
side of the leaf. This will be greater or less depending on the species,
but the important point to remember is that there is no such thing as an
energy conversion process that is 100% efficiency. It is unusual to get
better than 5 to 10 % efficiency, although I suppose that with some
creativity, some engineer might come up with something better. In the
biological world, the vast majority of energy of light of any frequency is
either reflected or absorbed and converted to heat.

And you're right. If we are to be thorough, we'd have to consider
everything in the greenhouse. Consideration of allgae and live moss,
though, could be lumped together with the other plants in the greenhouse as
they'd have similar properties. As they'd all be using the same process of
photosynthesis, they'd all have very similar optical properties. Alas,
things get much more complicated as we consider them in more detail.

Here is a question that might help understand the implications of this. You
have two surfaces, both subject to the same white light. One surface
appears to be dark green and the other appears to be light green. Since
both are subject to the same white light, they both get the same amount of
green light. How, then does one appear lighter than the other?

Here is a tip, to help answer the above question: In computer graphics, one
can lighten the color green obtained from the brightest green setting for a
set of pixels by adding equal amounts of blue and red. You can break the
light coming from a pixel, in this context, into two components: one made of
pure green light and another made of white light. I use this routinely in
any computer graphics I add to my applications.

My guess is that it's insignificant compared to the incoming light.

--

I am not so sure. I will not say anything definitive, since I have not seen
reports of experiments designed to test it, nor have I done any myself.
However, theory suggests it could be significant and Reka has provided some
evidence that plants are affected by the color of netting used, in her case
for protection from hail, but the purpose of the netting is immaterial here.

Cheers,

Ted

--
R.E. (Ted) Byers, Ph.D., Ed.D.
R & D Decision Support Solutions
http://www.randddecisionsupportsolutions.com/
Healthy Living Through Informed Decision Making