These sensors measure direct radiation from the sun and its corona.
The C300R model represents a low cost direct radiation data acquisition alternative with respect to traditional direct radiometers equipped with an expensive tracking mechanism. It supplies a direct radiation reading and heliophytic status (sunshine referred to a certain threshold). Measurement is made in the visible range, near infrared, to second class WMO pyrometric specifications. Once set up for the latitude and location, the sensor does not require seasonal positioning unless greater precision, accomplished by two annual adjustments, is required. The principle of measurement is the following: a series of sensors detect the illumination of a small sphere that reflects light received from the sky in a 90º annular beam and which has the location's equinoctial line as its equator. A rotating band periodically intercepts the sunbeam. For each rotation, the instrument determines the two radiation levels of the beam, with and without the direct action of the sun disc, and calculates the difference, which gives a good approximation to the direct radiation level. The instrument also supplies the sunshine status, defined as present when direct radiation is above 120 Wm² (WMO standard, 1981). The sensor has two actionable heaters: a continuous anti-condensation heater and a thermostatic one for defrosting. In conditions of darkness, the band is stopped and the sunshine status is set to "no". The instrument can be connected to acquisition units, graph recorders and elapsed time counters.