The LP SD18 sunshine duration sensor is based on the use of 16 placed sensors in such a way that, in the presence of the sun, at least one of the photodetectors receives the light directly from the sun (in addition to the diffuse component).
Sensors that are not directly illuminated by the sun, are used for the measurement of the scattered light that is subtracted from the sensor measurement that directly sees the sun to get the direct radiation.
The cylindrical glass protects the sensors and the internal circuits of the instrument from weather and at the same time ensures excellent transparency to sunlight.
To avoid the formation of condensation inside the instrument, the LP SD18 is provided with addition to the heating element, a cartridge that must be loaded with drying in colloidal silica (silica gel).