 Although
artificial lighting produces an imperfect spectrum compared to the sun, it
does allow control of variables such as duration and intensity.
Equipment overview
Artificial lighting
requires the use of a ballast (used to ignite then regulate current to the
lamp), a shade or reflector (directs light towards the plants and usually
incorporates the lamp holder/ socket), a timer (controls when the lights
turn on and off), and the lamp itself (Fig 8.1). Depending on the
application, either HID or fluorescent lamps are typically used.
Lamp selection (Optimizing colour
spectrum)
Photosynthesis occurs
mainly within the visible light spectrum (wavelength range 400 to 700nm).
Within this range, 445nm (blue) and 650nm (red) are needed most.
HID:
High Intensity Discharge lamps (HID) are generally used for the vegetative
and flowering/ fruiting phases due to their high lumens per watt rating
(produce ~5 times as many lumens per watt than incandescent lamps). The
following types of HID are commonly used:
+
 Metal
Halide (MH): Produce light predominantly ‘blue’ in colour. This
is ideal for strong vegetative growth (e.g. larger leaves, thicker stems)
and a shorter, dense plant (i.e. shorter internodal spacing) which will
utilize light more efficiently.
+
High Pressure Sodium (HPS): Produce more ‘red’ light. This promotes
flower onset and production and is therefore more suited to the flowering/
fruiting phase.
Unfortunately, MH and HPS
lamps each require a different ballast and lamp holder. It is therefore
common practice to use a HPS lamp for both vegetative and flowering –
especially if plants have a very short vegetative phase. However, using
HPS throughout can promote tall and spindly growth in many species.
+
Blended light lamps: To save needing a separate lamp kit
(lamp, ballast and lamp holder) for both vegetative (MH) and flowering (HPS),
“blended light” lamps are available. These produce a more balanced
spectrum of ‘red’ and ‘blue’ light.
Fluorescent:
Fluorescent lamps produce less lumens per watt compared to HID lamps,
hence their use is limited to plants needing low to medium light
intensities e.g. seedlings/ clones, herbs, orchids, lettuce. They are
simpler to use than HID lamps because they generate less heat.
These are broadly
categorized as either ‘strip’ fluorescents (SL) or ‘compact’ fluorescents
(CFL) and are available in different wattages and colour temperatures.
Those of high colour temperature (known as “cool white”) are more ‘blue’
in colour and more suited to vegetative growth and development of
seedlings/ clones. Lower colour temperatures (“warm white”), are redder in
colour and therefore better for flowering. CFLs are generally available in
higher wattages than SLs and are therefore suitable for species needing
higher light intensities.
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