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

Jim Metzger metzger.72 at OSU.EDU
Tue Oct 14 16:34:25 EST 1997

At 04:19 PM 10/14/97 +0100, you wrote:
>In article <344366CE.3295FDDB at>, Jan Andersen <root at>
>>2) All (nearly) flourescent tubes are made with mercury gas inside - this
>>ultraviolet light, which is converted to visible light by means of an emulsion
>>at the
>>inside of the tube - a 'gro-light' has in addition to that a filter that takes
>>away some
>>wavelengths, which by necessity means that a 'gro-light' tube is less
>>one without filter.
>This need not be the case. There are a very large number of fluorescent
>phosphors able to change UV to a wide variety of different colours. A
>conventional CRT colour monitor has three different ones, but there are
>very many. A *properly designed* gro-light should have high efficiency
>phosphors designed to match the plant requirements nearly exactly and
>require no filter.
>You also stated that chlorophyll did not require any red to function.
>This was the generally held view, however I am pretty sure that I
>remember a paper (long time ago) that showed higher efficiency when red
>was also included. IIRC chlorophyll activated by red light could absorb
>blues of a lower frequency than when not, widening the available
>spectrum that could produce the first stage in the relevent biochemical
>However I have some old tube spectral outputs. Let's see.
>efficiency (in strange units)
>        continuous spectrum     Peaks: output at wavelength
>        400   500   600   650   <<(wavelength nm)
>Grolux   50    70    10   700   200 at 410, 400 at 440, 250 at 550
>Daylight 20    70    70    70    70 at 380, 110 at 410, 260 at 430, 150 at 550
>Tropical 20    70    70    30   110 at 410, 230 at 430, 160 at 550
>So it looks to me as if grolux are more efficient, and appear to have
>had their spectrum carefully tailored. Given the hi-tech nature of
>modern greenhouse growers and labs that use them, I would be surprised
>if gro-lux type tubes do not offer significant advantages over
>conventional tubes.

It is true that the special spectral quality of gro-lux lamps is due to the
type of phosphors used.  However it is not true that gro-lux provides any
special advantage for plant growth.  Moreover, they are more expensive, and
provide less PAR per input watt than a standard cool white bulb (Cathey and
Campbell, Hort. Reviews, 1980).  It is also not true that "modern greenhouse
growers" use them (or any fluorescent bulb) to any significant extent
(except for inhibiting flowering of short day plants by use of night
breaks).  Fluorescent light fixtures block more light than they produce!
Jim Metzger
Department of Horticulture and Crop Science
The Ohio State University
2021 Coffey Road
Columbus OH 43210

Voice:	614-292-3854
FAX:	614-292-7162
E-mail:	metzger.72 at osu.edu

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