[JT118]

DRN-301;

Global Net Radiometer; 300-80,000nm

1.4mV/kW/m2

   

The DRN 301 has a thermopile; facing ground (bare copper side)

   

and facing sun (black coated side).

   

Presumably long wavelength for the ground to sense radiation emitted by soil, and Short wavelength for the sun.

I think this used have a polythene "bubble" around either side of the thermopile which is kept inflated by a diaphragm pump

   

which pumps air through silica gel to dry it out to ensure bubble stays dry and clear.