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Measuring light in tank

    I have an uncommon light set-up (diy aluminum canopy with raised 
flourescent fixtures, n.o. bulbs on ice cap electronic ballast), and I want 
to quantify the light reaching the plants.
    Using a photography light meter in a baggie, I took readings through it's 
white plastic diffusion filter (averages the whole field in front of it), and 
without (which reads a narrow spot in front of it).  Given, are readings 
(taken with sensor pointing straight up, at 50 asa setting), and their 
conversion to lux, according to table in Baensch' Aquarium Atlas V1.
    w/filter:       @ mid-water depth (7")- 2.8f-1/60sec.=2,400 lux
                    @ just below surface   - 2.8f-1/125sec.=4,800lux
    w/o filter:           mid-water           (7")- 5.6f-1/250sec.=38,000lux
                                 just below surface    -       -same-      
    Have visited George Booths' site where he describes readings with a 
lux-meter but don't know how that compares.  
    Can anyone help me evaluate these readings?
    Thank You Kindly,
    Zach K