Re: Light (fwd)

Rick Walker (
Tue, 31 Jan 1995 11:03:33 -0800

I'm forwarding the following for Kevin Reiling, whose original message
got inadvertantly sent to Carolyn Phelan (CP) in HP UK. For UK members,
please be careful with replying to CP mailing list mail. There is a bug
in HP's mailer so that all CP mail to the UK gets the wrong "Reply-To:"
header. If it says "", please change it to
"" before replying. We are wearing thin on Carolyn's
good graces. -- Thanks! -Rick Walker

--------------------- forwarded letter ---------------

Hope this helps,

1 foot candle (lumen ft-2)0 = 10.764 lux (lumen m-2)

However, these are photometric measurements based on energy arriving at the
plant and valid only for a given source (because of different output spectra)
blue light has more energy than red. Photosynthesis however works on the
number of photons arriving (9used at the red energy level)
used at the red energy level), to make most sense
light measurements for plants should be of the Photosynthetically active
radiation (PAR) 400-700 nm, and measured in u mols photons m-2 s-1, a
radiometric unit.

radiometric and photometric are not directly convertable
full sun approx 95 000 lux only 42% of which is PAR
mercury vapour (400 watts) approx 24 000 lux, 52% PAR

basically different spectral outputs mean comparisons between sources are
very approximate unless using a radiometric PAR measurement.

ps I know spectra are important for flower induction, dormancy etc, but
a photometric measurement is still of little use.

I apologise for the minilecture but it may be useful to someone, even if only
to allow useful investigation of your next lighting purchase.

Best wishes,
Dr Kevin Reiling, a silent participant to the list until now