What is the optimal wavelength of light for growing plants (cacti indoors in a northern illinois climate)?

Oz Oz at upthorpe.demon.co.uk
Wed Oct 15 07:47:55 EST 1997

In article <34447570.7E42676B at>, Jan Andersen <root at>

>Very interesting. It seems as if you may be right :-) Still, you don't give
>any numbers for watt input/lumen gained, and as far as I remember, this
>was the interesting thing - the industrial 'cold white' tubes won big here.

I checked the y-axis. The figures were the same for all the tubes. The
book is away now but from memory it was something like:
Absolute Power mw/nm/1000lumens.

Hmmm, you may be right. The problem is that a lumen is NOT frequency
independent. IIFC yellow/green (being the most sensitive to the *human*
eye) counts much more towards the total lumens than say red or blue. On
top of that the *energy* in a blue photon is 150% more than a red photon
so if you are talking about *energy* output then lumens is a useless
measure. If you are talking about plant requirements then it is really
bad, weighting the yellow/green highly when the plant rates it very low.

It is unlikely that the phosphors are very different in their
efficiencies so one can get some sort of measure of the distortion
between *energy* and *lumen* output from the following lumen outputs.
[Oz hauls out aged tome again.]

LUMEN outputs for a 6' 75W tube:

blue    1400
Green   5600
Gold    2900
Red      270 (not a missprint).

So using lumens as a measure is not good.

>A comment about the hi-tech greenhouse growers: they are of course
>highly competent in their trade. But this doesn't guarantee that they know
>much about this subject - 

Perhaps not. However these lamps are also used in univerity departments,
knee deep in physicists and biologists who are VERY aware of the
importance of spectral power distribution. Your comments do not apply to
them at all.

>So, I'm still not convinced by your arguments.

Yet, perhaps?

'Oz     "Is it better to seem ignorant and learn,
         - or seem wise and stay ignorant?"

More information about the Plantbio mailing list