Agriculture Reference
In-Depth Information
When two bodies are at different tem-
peratures, separated by a permeable
medium, there is a net heat transfer from the
warm body to the cold body in the form of
radiation (Fig. 5.2). The energy received by
the surface of the cold body is divided in
three parts: (i) a fraction is reflected; (ii) a
second part is transmitted (crosses the body
without heating it); and (iii) a third fraction
is absorbed by the body and increases its
temperature (Fig. 3.3).
Bodies have reflection, transmission
and absorption properties for radiation
which vary with the received wavelength.
For each material the reflection coefficient
or reflectivity (
r
), the transmission coeffi-
cient or transmissivity (
t
) and the absorp-
tion coefficient or absorptivity (
a
) can be
defined.
For a certain wavelength the sum of the
three coefficients is equal to 1, or 100%,
depending on how the coefficients are
expressed (per unit or as a percentage), due
to the energy conservation principle.
A grey body is any body in which the
absorption coefficient is independent of the
wavelength of the incoming radiation. In
practice many materials can be treated as
grey bodies. A black body is any surface in
which the absorption coefficient is 1, what-
ever the wavelength (Rosenberg
et al
., 1983);
it is an ideal body.
The transmissivity and the reflectivity,
are quite dependent on the angle of
incidence of the radiation over the materi-
al's surface (see Chapter 4). The absorptivity
is less dependent on such an angle and is
more linked to the type of material and its
thickness (Seeman, 1974).
When the angle of incidence is not
known and we talk, generically, of these
coefficients, the data usually refer to trans-
missivity and reflectivity for zero angle of
incidence (rays perpendicular to the sur-
face), which correspond to the maximum
transmissivity and minimum reflectivity
(Figs 3.3 and Plate 5).
When the radiation is diffuse, it does
not have an angle of incidence as it comes
from all possible directions and this trans-
missivity of diffuse radiation is different
from the transmissivity of direct radiation.
Emissivity is the proportion (per unit)
of the total radiation emitted by a body at a
given temperature with respect to the one
emitted by a black body of the same surface
under the same conditions (see Appendix 1
section A.4.4). The absorption coefficient of
a material to radiation of a certain wave-
length is equal to its emissivity in this same
wavelength, according to Kirchoff's law
(Rosenberg
et al
., 1983).
The emissivity and the absorptivity, in
the same range of wavelength, have equal
values, but if a body receives solar radiation
and emits radiation in the IR band the
absorptivity for the solar radiation is differ-
ent from the emissivity for the IR radiation
(Table 5.1).
For non-metallic bodies, such as plant
leaves and white paint, the emissivity is
high (from 0.7 to 1) at normal crop tempera-
tures, whereas for metals, especially if they
have been polished, it is low (0.05-0.3) (Bot
and Van de Braak, 1995).
It is essential to know the spectral dis-
tribution characteristics of transmissivity of
a covering material if the plants which are
grown in the greenhouse have colours gen-
erated by anthocyanins (pigment with col-
ourations between red and violet), because
if the material is not transmissive within a
certain range of UV (290-360 nm) it may
prevent such colouration (Takakura, 1989),
as may happen in aubergine. Indeed the
lack of UV radiation, to which the eyes of
S
2
T
2
F
12
S
1
T
1
Fig. 5.2.
Heat transmission by radiation between
two bodies at different temperatures (
T
1 >
T
2)
(see text).
