A basic layout of 7 sensors 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 sensors receive 50% of the Diffuse radiation, sampled from all over the
sky, and at least one sensor receives only this radiation. At least one sensor also
receives the full amount of Direct radiation from the sun. Which particular sensors
these are depends on the position of the sun in the sky, but the fully exposed one is
always the sensor which receives the most radiation, and the fully shaded one the
least. All the sensors are measured by the electronics, 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:
Diffuse = 2 * MIN
Direct = MAX - MIN
Total = Diffuse = MAX + MIN
The Total and Diffuse values are used for the instrument output.
Note: This analysis is independent of the spectral characteristics of the individual
sensors, or their spatial response.
Calculations – SPN1 outputs
Let MAX and MIN be the largest and smallest thermopile reading of the seven
thermopiles, after being adjusted for any calibration factors (calibration is done in the
solar lamp integrating sphere against a transfer standard SPN1)
Then
TOTAL = MAX + MIN
DIFFUSE = 2 x MIN x 1.02 (the extra 2% takes away a small
systematic bias due to there being typically a 1%-2% spread
between sensors under identical lighting conditions).
IF (DIFFUSE > TOTAL) then DIFFUSE = TOTAL (a sanity check
as Diffuse obviously can’t ever be greater than Total in reality)
There is then a further correction due to the spectral response of the sensors giving a
different sensitivity to direct and diffuse light in most conditions:
DIRECT = (TOTAL - DIFFUSE) x 0.99
DIFFUSE = DIFFUSE x 1.14
TOTAL = DIFFUSE. (TOTAL & DIFFUSE are output)
The sunshine presence output is calculated using the ratio of Total and Diffuse:
SUNSHINE if TOTAL/DIFFUSE > RATIO AND TOTAL > 24 W.m
-2
SPN1 User Manual v1.0
Appendix A: Design and test summary
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