Delta-T Devices SunScan SS1 User Manual page 49

LAI theory
In this section we shall explain as fully as we can how the SunScan
computes its readings of leaf area index, and what the main limitations
and provisos are in interpreting these for real canopies.
Ingredients of the LAI computation method
There are three broad areas contributing to the final result.
Geometric analysis
The first is the analysis of what happens to a ray of light passing through
the canopy. In order to do this, we have to make some general
assumptions about the canopy, i.e. uniformity, randomness and total
absorption by canopy elements. This was done by Campbell (1986) for a
beam of light from a single direction (the Direct solar beam) passing
through a canopy with a generalised ellipsoidal leaf angle distribution
function. This function allows a wide range of different canopy types to be
described by the value of a single parameter ELADP.
Wood then integrated Campbell's result over the whole sky to give a
description of the transmission of Diffuse light through the same canopy.
This is important because the transmission of Diffuse light is different, and
in reality there is usually a combination of both Direct and Diffuse
illumination. In particular, the analysis shows that Diffuse transmission is
strongly dependent on the leaf angle distribution, a point which has not
generally been recognised.
These functions are integrals which do not have direct analytical solutions,
so have to be solved numerically, and computable functions fitted to the
results. This has been done to a high degree of accuracy, improving on
Campbell's original approximation.
Incomplete absorption - more elaborate analysis
The above analysis based on black leaves is relatively straightforward.
However, real leaves also reflect or scatter some of the light that falls on
them. Typically, only about 85% of the incident light is absorbed. This
means that in reality, every leaf element in the canopy is re-emitting light,
as well as absorbing it, which makes the situation much more
complicated.
Because the direction of any particular light ray can be changed by
reflection or scattering, it means the spatial distribution of the light
changes through the canopy. Therefore it is no longer adequate to
consider just the vertical component of the light (as measured by a cosine
corrected sensor), the horizontal component must also be included. This
is why Wood's analysis also considers a hemispherical response sensor
(which measures both horizontal and vertical components).
SS1 User Manual v3.3
LAI theory  49