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lighting issues

     A number of folks brought up some interesting lighting issues in the
last week.  much of this evolved from Steve's request for input regarding
indirect sunlight.   Here are a few observations of my own for whatever they
are worth:  
     Light levels on a cloudy day can be less that 1/20 of those in full
sunlight.  Last winter I set up a greenhouse system for coral culture here in
northeast Ohio (where we only see the sun three days a month throughout the
winter because of lake effect cloud cover).  Light levels at noon on a
typical grey, dreary day are only about 80 uE in a foot of water.  On a
sunny, cloudless day I measure 1500 uE. (For reference, full sunlight just
below the surface of an open body of water might be about 1800 uE.)  I'd be
interested in hearing how folks with greenhouses in Seattle, which is
supposed to have similar light conditions in winter, cope.
     Measurements I've taken in a couple of brightly lit aquaria confirm that
light levels about half of the noon maximum can nicely accommodate the light
requirements of some of the most light-hungry plants.
     The post about using a camera meter to measure light levels offers a
technique that is worth exploring.  I outlined a rough method of extending
this to underwater measurements in the February 1994 issue of FAMA.
 Conversions from footcandles or lux to PAR are also given.  PAR measurements
and/or conversion factors are probably only  extremely useful if you want to
compare tanks with different lighting systems or if you need to reference
scientific literature.  When I set up my first plant tank I  just ignored my
PAR meter and checked the FAQ to see what everyone else was doing  <G>.  

I wish the sun would come out...April is a long way off.
Pete Mohan