Light measurement

[Jonathan B. Marder]

In article <3tecrh$od3 at hecate.umd.edu>,
   bwilliam at oyster.smcm.edu (Bill Williams) wrote:
>Andy's conversions are good, but only for sunlight.  The units are not
>directly convertible without knowing the spectrum of the light source.
>"Lux" and most of the other "old" light measurements (foot-candles,
>etc.), are based on the response of the human eye and so weight yellow
>photons particularly strongly.  

I didn't yet see Andy's post, but I own number (for fluorescence lamps) 
is that 1 micromol per sq. metre per second is roughly equivalent to 100 
lux

>Quantum meters (and plants, mostly!)
>try to weight all photons in the 400-600 nm range equally.  Power
>meters (W/m2) weight the photons depending on their energy, so blue
>ones count more than red ones.  See if you can borrow a quantum probe
>from somebody!

For the sake of corerctness, quantum meters usually measure from 
400 to ** 700 ** nm (I presume that Bill's range was mistyped). Note
that chlorophyll has a massive absorption peak between 600 and 700 nm, 
so it would be a mistake to consider only 400-600 nm.

Jonathan B. Marder                 '
Department of Agricultural Botany  |     Internet: MARDER at agri.huji.ac.il
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