[Prev][Next][Index]

Re: Aquatic Plants Digest V2 #539

In a message dated 97-02-23 07:25:49 EST, you write:

<<  Lux and footcandle meters are inherently biased
 > because they mimic the human eye's perception of light...
 
 That's all true, but in this instance, I'm not sure where you are going 
 with this.  If the reflective tape were gold-colored, then I would say 
 you have a point, but most of the reflectors I've seen probably have very 
 little differential losses across the PAR region.  If the reflector 
 doesn't affect the spectrum, then what George did is perfectly valid for 
 relative intensity comparisons on the same lamp with and without a
reflector. >>

Craig,
     There were a number of white paints with high reflectivity listed in
 the CRC handbook.  I've been trying to track them down since my post to see
what they are made of.  Will let everyone know if I find anything.

     Regarding PAR measurements:  you are correct that many reflectors do not
affect spectra.  However, the purpose of my note was to point out that ALL
uses of footcandle or lux meters (they look at the same thing using different
units)  to measure light for plants are problematic - like trying to record
high frequency bat vocalizations with a standard tape recorder.  Here's the
post again, and some conversions for PAR and footcandles for specific lamps:

"....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."

PAR is expressed in microeinsteins per second per square meter. (Now you can
forget that <G>)

For each lamp, 1 uE = X ftc.  Use these conversion estimates to compare  the
PAR output of various lamps.  This is the light actually produced, not just
what people see.
     Gro-Lux   X = 5.2
     Gro & Sho  X = 3.5
 The Gro & Sho lamp reads much dimmer  than the Gro-Lux using a footcandle
meter,  but Is nearly as bright when PAR is measured.  The Gro-Lux just
produces more light that we can see.
     Cool white (GE)  X =  7.3
     Deluxe warm white (Philips)  X = 5.6

Pete Mohan