The CSD 3 measures sunshine duration. Sunshine duration is defined by WMO as the time during which the direct solar radiation exceeds the level of 120 W/m².
It has no moving parts and uses 3 photo-diodes with specially designed diffusers to make an analogue calculation of when it is sunny. The output is switched high or low to indicate sunny or not sunny conditions. The calculated direct irradiance value is also available. CSD 3 operates from 12 VDC power and has built-in heaters to dissipate rain, snow and frost. These are normally switched externally but an optional internal thermostat control is available.