[APD] Re: Minimum Light Threshold

["Bill D" <billinet at comcast_net>]

It seems obvious that plants won't grow until they receive a certain minimum
level of light energy, and it also seems reasonable that this minimum could
be different for different species.   The actual amount of energy needed
might depend on the spectrum of the light, the nutrients in the tank, and
numerous other things.

This thread is touching on the theory of limits, as described in Zeno's
First Paradox.  In that he postulated that if a person started a distance
from an object and, in a unit of time, moved exactly half of the distance
between himself and the object, he would never get there and hence was
motionless.   A good description is at

http://occawlonline.pearsoned.com/bookbind/pubbooks/thomas_awl/chapter1/medialib/custom3/topics/combin.htm

"Tipping point theory" might also describe this lighting phenomenon <g>.

Would the lux at the bottom of the 9 watt, 1 gallon tank would be a lot less
than that in a 40 watt, 20 gallon tank? If so, wouldn't that account for the
difference in plant growth?.  Lux is a direct measure of light energy
availability, unlike watts, which is at best an analog of that.  (Yes, by
now I know all of the practical arguments that favor watts per gallon over
lux, but if we talked in terms of lux, or "standard lux", we would be able
to focus on the important thing, available light energy, and not on the less
direct and inaccurate metric, watts.)

Bill



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