Agriculture Reference
In-Depth Information
(around 6%) emitting most of the energy in
the IR range (Baille, 1993); they are used to
control the photoperiod, or to complement
other lamps, as they can induce a morpho-
genic response.
Fluorescent lamps are more efficient
than the incandescent, with around 20%
efficiency of conversion into PAR (Baille,
1993). They usually produce white light,
although there are different types.
Fluorescent lamps are used effectively for
germination and during the initial stages of
growth in growth chambers (or rooms)
because they can be placed close to the
plants, but are rarely used in greenhouses,
as they are not compact and cause large
shadows, limiting the daytime radiation
(Hanan, 1998).
The high-intensity discharge lamps are
used when high-intensity radiation levels
are required. Among these types of lamps
we can find the mercury, the halogen, low
pressure sodium, high pressure sodium and
xenon (Baille, 1993). The ones with better
energy efficiency (of PAR conversion) are
the halogen lamps and the sodium (low and
high pressure) lamps, which reach efficien-
cies of 26-27% (Baille, 1993).
A primary aspect to consider is the
radiation emission spectrum of the lamp, in
the PAR interval, so that the emitted radi-
ation is as similar as possible to the PAR.
When selecting lamps for complemen-
tary illumination, the characteristics of
radiation emission of morphological signifi-
cance to plants (red light and far red light)
must be considered, besides their energy
efficiency (PAR conversion) and the prox-
imity of the emitted light spectrum with
that of the PAR. The most commonly used
are the high-pressure sodium lamps, with
400-450 W of power. They are usually
installed so that there is one lamp for up to
every 10 m
2
, and this provides an installed
power of 50 W m
−2
, and a useful PAR level
of 10 W m
−2
(Urban, 1997a). These lamps
are usually positioned at between 1.5 and
2 m high, in frames of 2.2 × 2.2 m and 3.2 ×
3.2 m, to cover between 5 and 10 m
2
per
lamp (Hanan, 1998; Photo 9.4).
The uniformity in the light distribu-
tion at plant level must be assessed and
the shadows caused by the lamps limited
as much as possible, with the aim of mini-
mizing any decrease in solar radiation
caused. Therefore, it is preferable to use
rectangular reflectors which provide a rec-
tangular light distribution, rather than
circular reflectors which distribute the
light in circular shapes hindering light
Photo 9.4.
Lamps for complementary artificial illumination.
