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(a)
(b)
(c)
θ
z
(d)
(e)
(f)
θ
z
Figure 2.9
Effect of the zenith angle
θ
z
of incoming radiation on the relectivity
of a structured surface: small zenith angle (top row) and high zenith angle (bottom
row). Left (
a
and
d
): effect of multiple internal relection. Middle (
b
and
e
): effect of
objects casting shadows on other objects. Right (
c
and
f
): effect of direct radiation
penetrating (or not) to the ground which has a different relectivity. Black lines on
objects indicate direct illumination (only direct radiation is considered). Relection
is assumed to be Lambertian.
penetrated into the canopy will eventually leave the canopy again: the largest part
of the radiation is trapped. At large zenith angles (
Figure 2.9d
) multiple relection
is limited to the upper part of the canopy only, and radiation can more easily escape
again. Thus multiple internal relections cause the relectivity of a canopy to be lower
at smaller zenith angle (high solar elevation).
Objects cast shadows on other objects. If the zenith angle is small (
•
Figure 2.9b
) only
few shadows occur: the upward facing parts, as well as a large part of the side walls
of the objects are directly lit and hence will be the source of high levels of relected
radiation. On the other hand, if the solar zenith angle is large (
Figure 2.9e
), a large
fraction of the surface of the objects is shadowed by other objects. Hence only a small
part of the surface relects radiation. Thus shadowing causes the relectivity of a can-
opy to be higher at higher radiation zenith angle (the opposite of the effect of multiple
relections).
The surface between the objects will have a different relectivity than the objects them-
•
selves. Hence, the relectivity of the canopy as a whole depends on the degree to which
the radiation can penetrate down to the ground. At small zenith angles the radiation will
have a higher probability to reach the surface than at large zenith angles (
Figure 2.9c
). It
will depend on the relectivity of the underlying surface whether the penetration of radi-
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