"Bernard Burton"
I don't think that the KZ rcorder uses the same method as the R&D, Nick. The
latter uses a shadow post to generate a difference in output from a ring of
photo-diodes. The KZ uses '3 photo-diodes with specially designed diffusers
the determine when a threshold of 120 W per sq m is exceeded'
See also:
measuringtheweather.com/wp-content/uploads/2012/06/An-overview-of-sunshine-recorders-July-2012.pdf
by Stephen Burt.
Bernard Burton

Bernard, I've just read the link to Stephen B's article and he
says that the KZ works by having at least one of the three
photodiodes in the shade giving a voltage differential. This is a
similar principle to the R&D.

Stephen also writes in the book version of Measuring the Weather
that the KZ and the R&D are likely to give close results as they
use similar principles to determine whether the sun is shining or
not.

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

--
Otter Valley, Devon
20 m AMSL
http://www.ottervalleyweather.me.uk