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TOWARDS THE LIGHT
B
ack in 1949, two scientists in
the United States – Borthwick
and Parker at the Plant Industry
Station in Beltsville, Maryland –
published results on the use of carbon-
arc lamps for growing Biloxi soybeans.
As they describe in their paper
Growth
and composition of Biloxi soybean grown
in a controlled environment with radiation
from different carbon-arc sources
(Plant
Physiology, 1949), the lamps burned
‘sunshine carbons’.
This was the tentative beginning of
using electric light to stimulate growth
of vegetation.
The carbon electrodes were cored
with certain materials that beaome
incandescent (cerium fluoride, for
example) thereby increasing the energy in
the visible spectrum, in particular towards
the blue-violet part.
They found that by combining the
arc lamps with incandescent lighting, a
sturdier plant developed because the
radiation then also provided near infra-red
energy. The ‘sunshine carbons’ peaked
at 280mμ (millimicrons or 10
-9
m), and
chlorophyll has maximum absorption at
that wavelength.
High-intensity discharge (HID) lamps
have sometimes been used in combination
with incandescent lighting to provide
longer wavelengths. The discharge lamps
sometimes use phosphor-coated mercury.
Fluorescent lamps are still being
used, such as the slimline T5. However,
developments in LED technology
is bringing great advantages to
greenhouse horticulture.
Despite the advances, replacing the sun
is no simple thing. Table 1 shows a spectral
break-up of the colours in sunlight.
LIGHTQUANTA,NOTLUX
The physics of Albert Einstein and Max
Planck in the equation E=
ࢎࢉ
/
ࣅ
underpins
atomic interaction with light – as particles.
Light quanta (packets of light, each
with a specific E energy according to its
wavelength
ɉ
and travelling at the speed
of light c) are absorbed, and that energy
is used (photosynthesis) for taking up
carbon dioxide from the atmosphere. This,
together with water in the plant, splits the
water into oxygen and hydrogen, with the
latter forming carbohydrates in the plant.
The interesting thing about these
quanta is that each one that is absorbed
is responsible for knocking off an electron
(part of the chemical process). So rather
than thinking in lux (light as a continuous
electromagnetic wave motion) we
conceive it as particles of energy.
Rather than describing the light output
in lumens or lux, a rather unfamiliar term
is used for horticultural purposes: the
mol and micromol. But there is no need to
study chemistry. These units indicate that
it is light quanta as described that do the
‘business’. The shorter the wavelength, the
higher the energy of a light quantum. A
violet colour quantum packs more punch
than a green one, and so on, as energy
diminishes the more the wavelength
moves towards red and then to infra-red.
So what is a mol? In chemistry it
is the weight in grams of 6 x 1,023
atoms of whatever element you care to
GROWING VEGETABLES OUT OF
SEASON, ISOLATED FROM THE
WEATHER AND WITH NO NEED
FOR ARABLE LAND MAY NOT BE A
UTOPIAN DREAM ANY MORE.
PHIL
KREVELD
INVESTIGATES.
GROW LIGHTS
E L EC TR I C AL CONNEC T I ON
SUMME R 20 1 6