SunScan User Manual v 1.05
LAI theory
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In detail:
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The canopy is divided into horizontal layers of LAI 0.1
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Direct beam absorption by each layer is calculated using Campbell's equation. In
calculating the amount of absorbed light that is re-emitted, the total power in the
direct beam has to be used (i.e. as measured by an integrating hemisphere or a
cosine-corrected sensor perpendicular to the beam). The amount intercepted by
the layer is the difference between the absolute intensity above and below the
layer.
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Diffuse light intercepted by the layer is calculated in a similar way, taking into
account the incident Diffuse light, and the sum of re-emitted light from all other
layers, attenuated by the intervening layers. This is done for both downwelling
and upwelling Diffuse light. A fraction of the Diffuse light absorbed by the layer
is also re-emitted. Again, absolute rather than cosine-corrected intensity
measurements must be used.
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Both cosine-corrected and absolute light measurements are calculated for each
layer, and the model iterated until it converges. This has been done for a range of
different values of zenith angle, Direct/Diffuse ratios, Leaf Angle Distributions
and Absorptions.
Simpler functions have been found to approximate these results, and are used in the
SunData software when inverting transmitted fraction back to LAI. These are
described in detail in the next section.
The canopy model
