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Appendix A: Design and Test Summary
BF5 User Manual v 2.3
A basic layout of 7 photodiodes on a hexagonal grid, covered by a
patterned hemispherical dome was chosen. The dome pattern was
generated by computer, using a specially designed evolutionary algorithm.
Calculation of Outputs
The shadow pattern consists of equal areas of black and clear bands. This
means that all of the photodiodes receive 50% of the Diffuse radiation,
sampled from all over the sky, and at least one photodiode receives only
this radiation. At least one photodiode also receives the full amount of
Direct radiation from the sun. Which particular photodiodes these are
depends on the position of the sun in the sky, but the fully exposed one is
always the photodiode which receives the most radiation, and the fully
shaded one the least. All the photodiodes are measured by the
electronics, individual calibration factors applied, and the maximum and
minimum of the seven readings are used. The maximum reading
represents the Direct rad half of the Diffuse radiation, the minimum
reading represents half of the Diffuse radiation. The outputs are calculated
as follows:
Total = Diffuse = MAX + MIN
Diffuse = 2 * MIN * 1.05
(the 1.05 factor compensates for calibration
variations between photodiodes, and also variability in the intensity
distribution of the diffuse sky)
If Diffuse > Total then Diffuse = Total (to make sure Diffuse is never more
than Total)
Direct = Total - Diffuse
The Total and Diffuse values are used for the instrument output.
Note: This analysis is independent of the spectral characteristics of the
individual photodiode sensors, or their spatial response.
Conversion to appropriate units
Molar units (
mol.m
-2
.s
-1
)
The photodiodes used in the BF5 have a spectral response that is close to
an Ideal PAR response (see spectral response graph in the Technical
Reference section). The BF5 is calibrated using a reference quantum
sensor, so calculates its output values in PAR units.
Energy units (W.m
-2
)
The relationship between PAR and Energy measurements under given
conditions depends on the spectral content of the light. In practise, the
spectral distribution of the Total radiation in most conditions is nearly
constant, so a single conversion factor is sufficient to give the Energy