Delta-T Devices SunScan SS1 User Manual page 50

The relentless advance of computing power has made it possible to model
the situation in ways that were not feasible in the past. By integrating the
"black leaf" analysis into a computer model Wood has calculated the light
levels in the canopy across the whole range of canopy and incident light
parameters.
Equation fitting and inversion
The results of the computer modelling, while accurate, are not suitable for
use in a field instrument. It takes many minutes of processing on a fast PC
to calculate light transmission for any given conditions using the model,
and the earlier data collection terminal previously supplied with SunScan
was not a fast computer!. The model calculates values of light
transmission for a given LAI, whereas the SunScan measures light
transmission. This means that the functions have to be inverted to work
back to LAI, which is more difficult.
To give you immediate results in the field, computable functions have
been fitted to the model data, and it is these that are solved to give LAI to
reasonable accuracy from the parameters measured by the SunScan
system.
Note! Wood's SunScan equations are copyright, and you should not copy
them without written permission unless for purposes of scientific debate or
publication, in which case they should be fully acknowledged.
Theory versus reality
We believe that Wood's SunScan equations accurately reflect the
assumptions that the modelling is based on.
By far the largest uncertainties are bound to be caused by

the mismatch between the real canopy architecture and the
simplifying assumptions built into the fundamental analysis

to a lesser extent the uncertainty in the numerical values of ELADP
estimated for your canopy.
With these caveats, the values of LAI for your canopy, even if of uncertain
accuracy, will provide valid trends for a given canopy (e.g. canopy growth
in a season), and valid comparisons between different canopies of similar
architecture (e.g. trial plots of different cultivars of the same species). If
you are able to compare SunScan estimates with actual harvested
samples from time to time, this will enable you to calibrate out any
systematic errors due to your canopy not matching the SunScan
assumptions.
If you wish, you can force the SunScan calculations to be equivalent to
older, less sophisticated inversions by setting some of the parameters to
appropriate values. For example, setting ELADP to 1024 (horizontal
leaves) and Absorption to 1.0 will give you the simple Beer's law
inversion.
50  LAI theory
SS1 User Manual v3.3