# Re: [APD] Re: Minimum Light Threshold

```IF we're gong to get this far into it, then we might a well
eschew lux since the variations in lux from bulb are not
covariant with how well they make plants frow. AT least
Ivo's work certainly indicates that. Plants can use a wide
range of frequencies and humans are sensitve to a narrow
range, centering around the area of he spectrum that is
least usefu for plants. Watts might actually be a better
measure than lux.

However, PAR reach the plants -- that would be better and
if one could assess the diffs between large and small tanks
for a given bulb, using lux, then one could generalize to
irradiation diffs between and large and small tanks for
lgiht at all wavelengths.

Of course, at some point, we will want to shine one of
htese lights into a white buck of clear water and decide on
the color ;-)

Scott H.
--- Bill D <billinet at comcast_net> wrote:

> 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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