Delta-T Devices SunScan SS1 User Manual page 54

Diffuse light transmission (cosine corrected
sensor)
Transmission
fraction
Leaf Angle Distribution

diff (
Vertical

diff (
Spherical

diff (
Horizontal
Modelling the canopy transmission
Accounting for incomplete absorption of PAR by the canopy elements,
and scattering of light within the canopy is complicated. It is no longer
sufficient to consider only the vertical component of PAR (i.e. as
measured by a cosine-corrected sensor) because scattering effectively
transfers power between the horizontal and vertical components, so the
canopy changes the spatial distribution of power in the PAR flux as it is
transmitted down through the canopy and reflected back up.
A multi-stream computer model was set up to calculate these effects as
follows.
Assumptions



54  LAI theory
1
0.1
, 0 L )
, 1 L )
1000 L , )
0.01
0.001
0
Leaf Area Index
The canopy is an infinite, uniform, horizontal slab, with leaf elements
randomly distributed in proportion to the surface area of an ellipsoid,
as described by Campbell.
The incident light consists of a component from a point source at a
given zenith angle (the Direct Beam); and a Diffuse component of
equal intensity from every point in the sky (Uniform Overcast Sky).
The canopy either has sufficiently high LAI that light reflected back
from the ground below is negligible, or the reflectance of the ground
is similar to that of the canopy.
Horizontal
2 4
L
Vertical
Spherical
6 8
SS1 User Manual v3.3
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