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choose. Because a light quantum can ‘do
something’ to an atom in order to make
a reaction take place, it is a good idea to
skip lumens and lux, and to use the same
‘quantity number’ for light quanta.
Thus 1 mol of light quanta (in some
narrowly defined wavelength range)
is equal to 6 x 1,023 quanta, each one
of which basically can help a single
photosynthesis reaction to happen. The
micromol is one millionth of 6 x 1,023
(6 x 1,017) or still a very, very large number.
From a greenhouse aspect. people are
interested in the number of quanta per sec
per square metre.
IMPORTANCEOFCOLOUR
What finally happens in a plant leaf
depends on the spectral absorbance, and
that is determined by plant pigments
such as chlorophyll, the characteristic
green colour.
However, there are also the yellow,
orange and red colours of other pigments
such as carotenes, xanthophylls, etc.
In Figure 1 the typical response curves
for LEDs are shown as a contrast to high
pressure sodium (HPS) vapour lamps. Note:
a nanometre is also a mμ (milli-micron)
The yellow curve indicates the HPS
spectral response, and the LED curves are
shown in admixtures of red/blue light for
100% red (red trace), 85% red-15% blue
(mauve trace), and 70% red-30% blue (blue
trace). The total quantum flux emitted is
identical for all curves at 70micro mols/
sec/m
2
. The strong advantage for the use
of LEDs in horticultural applications is the
flexibility available in spectrum distribution
to suit particular plant varieties.
COMINGOFAGE
LEDs began to be used for horticulture
several decades ago.
One of the first practical applications
was the growing of lettuce using red LEDs
augmented by blue fluorescent tubes.
Using red light alone had the effect of
elongating seedlings. In general, longer
wavelengths tend to elongate plant stems
and thicken them.
However, the experimentation
required to produce satisfactory results
defies simple rules; rather each plant
species demands careful evaluation of
colour mixtures.
A distinction must be made between
greenhouse environments and entirely
artificial lighting.
Photo-periodic lighting (to induce
flowering out of season) supplemented
by natural daylight involves different
demands, and a great deal of scientific
literature deals with ‘light recipes’.
It has resulted in the development
of highly specialised LEDs such as the
ORBITEC ‘light engine’ – red, blue, green
LED cells and photo-diodes for feedback
and control purposes. This includes
detection of spaces between plants and
switching off needless illumination.
Much higher efficiency compared
with other forms of lighting (for LEDs it’s
about 80%) means less heat dissipation.
Some radiated heat is often required
for sturdy plant growth but it must be
carefully controlled. The relative coolness
of LEDs allows them to be employed as
intra-canopy light sources. This can be an
advantage when tall plants are grown.
TAILORING
Careful selection of LED lighting
is required due to the large variety
of applications
Practical issues include dissipation
of heat that is not always unwanted
but generally requires venting to
control temperature.
There are several suppliers, and it’s
no surprise that development of LED
technology is encouraged in colder climes.
Philips is a leading supplier and promotes
products with high efficacy. Efficacy is
specified as μmol/J (joule). The joule is of
course the watt-second, which brings the
specification back to μmol/sec/watt.
The practical embodiment of an LED
illumination system requires several
features, including a thermal design to
extract heat and an optical design that
maximises light availability.
Integrating driver circuitry also
simplifies installation, and modules
should be capable of being plugged into
the mains.
In the case of Philips products, the
shape of the modules has been chosen
to match standard 40x40mm C-profiles.
With the accompanying brackets,
installing a module is a matter of seconds.
Installation time is further reduced by
routing mains wires through the module,
allowing for cable-free installation.
What of the future? The Advanced
Life Support Crops Group at the NASA
Specialised Centre of Research and
Training in Advanced Life Support
(the ALS NSCORT) and the Orbital
Technologies Corporation (ORBITEC)
have engaged in a collaborative
research project.
This has led to the development of
efficient, reconfigurable LED lighting
systems that will support crop growth in
a crewed space habitat. The LED arrays
were based on light sources using printed
circuit red and blue LEDs, individually
tuneable for a range of photo-synthetic
photon fluxes and plant responses.
In 1962, when Nick Holonyak at GE
invented the first LED (emitting infra-red
radiation), who could have imagined that
one day the technology would play such
an important part in food production?
TABLE 1: The perfect lamp would be able to
completely mimic the spectral distribution
of sunlight. In practice no single lamp will
suffice, and that also applies to LEDs.
COLOUR REGION WAVELENGTH (Mμ)
Violet
380 - 435
Blue
435 - 500
Cyan
500 - 520
Green
520 - 565
Yellow
565 - 590
Orange
590 - 625
Red
625 - 740
BY
PHIL
KREVELD