Re: Aquatic Plants Digest V2 #533

In a message dated 97-02-20 21:22:02 EST, George wrote:

<<  All values are in Lux (lumens per square meter).
 |                                      3"            12"
 | Trichromatic with reflector         5400          1060
 | Trichromatic without reflector #1   5300          1100
 | Trichromatic without reflector #2   5400          1160 >>

This is interesting.  A number of years ago I tested a series of fluorescent
lamps including some of the internal reflector types.  These tended to be
brighter than "similar" types of lamps without reflectors, but some
non-refector bulbs were as bright.  

I used a PAR meter, which measures all wavelengths between 400 and 700
nanometers without bias.  Lux and footcandle meters are inherently biased
because they mimic the human eye's perception of light.  These meters "see"
green light well, as do we (556 nm green light is seen best), and are
relatively insensitive to blue and red light.  When you look at a plant bulb,
such as a Gro-Lux lamp, next to a cool white, the cool white LOOKS much
brighter...but its NOT.  Cool whites are designed for human work areas and
produce lots of green light, so they appear very bright to us.  Plant lights
produce mostly the blue and red lights that are important for photosynthesis
and photomorphogenesis (they are REALLY bright to the plant!"), but
relatively little green light (so they are NOT bright to us).  When I
compared cool whites with Gro-Lux bulbs, the two produced similar PAR
(Photosynthetically Active Radiation) values (the cw was about 5% brighter,
but the plant light was putting out most of it's light in wavelengths needed
by plants).  For anyone who is interested: Gro & Sho and Agrolites were
similar to Gro-Lux.  When measuring light with a Lux or Footcandle meter, it
is difficult to know what the significance of "brightness" really is.  A
"brighter" bulb may just produce more green light, which is nice for viewing
but less important for the plants.

Pete Mohan
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