Measurement of solar energy input
A variety of instruments have been designed to measure the energy of solar radiations of all wavelengths as well as of a particular range of wavelengths. Pyrenometer (Figure is as shown below) measures the energy of sunlight of short wavelengths, indirect sunlight and scatrered. Skylight radiation. The receiver of radiation A has alternate black and white strips. These act as hot and cold thermwouple junctions respectively. This arrangement is enclosed in a spherical glass bulbwhich shields the receiving surface from disturbances by wind. But the glass container limits the wavelength response to the range of 280 nm to 3,000 nm.
When the receiver is placed in the sunlight the temperature of black strips, increases because it absorbs all radiations falling upon it. But the white strips remain cool as they reflect all incoming radiation. Thus if we connect a thermocouple between these ships, a thermo-emf develops as shown in the margin (Figure below). Galvanometer B (Figure shown below) shows a deflection indicating this emf. In this way the radiations absorbed by the receiver can be measured. Normally. the galvanometer is calibrated to give a nearly linear voltage response to incidentradiationlfluxes absorbed by the receiver.