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>this would imply a linear map between
>visible radiation and photosynthetic
effectiveness.  How do we back out the
>bias towards green in lumen figures?

There is a linear relationship between
total visible radiation (watts) and PAR.
The blue photons contain more energy
than red photons but are no more
effective at stimulating photosynthisis.
PAR sensors are very democratic and
count every photon the same. You can
also convert photopic light measurements
(lumens) into PAR or radiation values,
but the relationship is not linear and
you must know the exact frequency of
the light source in order to convert the
If one wanted to determine which light
source was the most efficient maybe it
would be possible to measure the
spectrum of reflected light coming off a
growing plant and subtract those values
from the spectra of the light source. It
sounds pretty complicated and I am not
sure the results would be valid but I
would love to try it.
I have my doubts about the usefulness of
PAR values as the it would seem to
indicate that the most efficient light
source would emit all it's light at  the
700 nanometer frequency. This seems
contrary to the the experience of most

Wayne Jones