On 23/04/2018 10:24, Nick Gardner wrote:

The BL uses a simple solar irradiation measurement coupled with a
quite clever software code to determine a sunshine/no sunshine
output.


Are you sure that's how BL works? I thought that the later design tries
to emulate the 120W/sqm direct irradiance measurement via some simple
but well thought-out optics. BICBW

I think the 'clever software' you're referring to is just the
traditional way used by a number of different software packages to try
and estimate sunshine hours from global irradiance by setting some
arbitrary threshold above which measured irradiance as a % of maximum
theoretical radiance is classified as bright sunshine. It's a little
intricate calculating the theoretical max for time and day of year,
lat/long, timezone etc and not forgetting the equation of time but it's
basically just coding some well-defined trig. IIRC the only real gotcha
is remembering to allow for which quadrant the ATAN result ends up in.