given wavelength, the reflection coefficient is approximately constant for w
from 0 to 50°, in relation to the normal position, but as c increases from 50 to
90°, q(k) increases substantially due to specular reflection. Foliar transmission
coefficients remain constant between 0 and 50°, decreasing for incidence angles
between 50 and 90°. Since reflectivity and transmissivity variations coefficients
are complementary, the fraction of radiation absorbed by leaves is practically
constant for incidence angles below 80° (Monteith and Unsworth 1991);
(ii) The optical properties of plant canopy in that radiation interacts with plant
organs by absorption and dispersion. These processes depend on variables
such as leaf structure, leaf age, spectral distribution, etc. Table 6.6 shows the
spectral values for a typical green leaf.
In the short-wavelength ranges, radiation exchanges by biological materials are
determined by pigments that absorb radiation at wavelengths associated with
specific electron transitions (Monteith and Unsworth 1991).
The transmissivity and reflectivity coefficients for a green leaf can be assumed to
be equal to 0.1 between 0.4 and 0.7 lm, and 0.7 between 0.7 and 3 lm (Fig. 6.9).
As each of these spectral bands contains about half the total radiation in the visible
and near IR, it can be assumed that the overall coefficient of leaf transmission and
reflection is about 0.25 in this spectral range. Leaves have high absorbances of
about 0.9–1, in the range of thermal radiation, typical of the terrestrial
surface-atmosphere system. Under these conditions, energy dissipation via convection and transpiration is important for the regulation of leaf temperature.
Table 6.6 Average values of
reflectivity, transmissivity,
dispersion, and absorbance
for a green leaf (adapt. Ross
1975)
PAR
NIR
Short-wavelength
radiation
Reflectivity
0.09
0.51
0.30
Transmissivity
0.06
0.34
0.20
Dispersion
0.15
0.75
0.50
Absorbance
0.85
0.15
0.50
Fig. 6.9 Representative
diagram of transmissivity,
reflectivity, and absorbance
characteristic of a green leaf
in visible and near IR spectral
bands
198
6 Heat and Mass Transfer Processes
from 0 to 50°, in relation to the normal position, but as c increases from 50 to
90°, q(k) increases substantially due to specular reflection. Foliar transmission
coefficients remain constant between 0 and 50°, decreasing for incidence angles
between 50 and 90°. Since reflectivity and transmissivity variations coefficients
are complementary, the fraction of radiation absorbed by leaves is practically
constant for incidence angles below 80° (Monteith and Unsworth 1991);
(ii) The optical properties of plant canopy in that radiation interacts with plant
organs by absorption and dispersion. These processes depend on variables
such as leaf structure, leaf age, spectral distribution, etc. Table 6.6 shows the
spectral values for a typical green leaf.
In the short-wavelength ranges, radiation exchanges by biological materials are
determined by pigments that absorb radiation at wavelengths associated with
specific electron transitions (Monteith and Unsworth 1991).
The transmissivity and reflectivity coefficients for a green leaf can be assumed to
be equal to 0.1 between 0.4 and 0.7 lm, and 0.7 between 0.7 and 3 lm (Fig. 6.9).
As each of these spectral bands contains about half the total radiation in the visible
and near IR, it can be assumed that the overall coefficient of leaf transmission and
reflection is about 0.25 in this spectral range. Leaves have high absorbances of
about 0.9–1, in the range of thermal radiation, typical of the terrestrial
surface-atmosphere system. Under these conditions, energy dissipation via convection and transpiration is important for the regulation of leaf temperature.
Table 6.6 Average values of
reflectivity, transmissivity,
dispersion, and absorbance
for a green leaf (adapt. Ross
1975)
PAR
NIR
Short-wavelength
radiation
Reflectivity
0.09
0.51
0.30
Transmissivity
0.06
0.34
0.20
Dispersion
0.15
0.75
0.50
Absorbance
0.85
0.15
0.50
Fig. 6.9 Representative
diagram of transmissivity,
reflectivity, and absorbance
characteristic of a green leaf
in visible and near IR spectral
bands
198
6 Heat and Mass Transfer Processes