Radiometer

Radiometer measures the flux of energy of all wavelengths received on a single surface of the receiver. There are also instruments that can measure the difference between the downward incident solar radiation and upward streams of reflected re-radiation and gives us the net value of radiation. This is called net radiometer (Fig shown below). It has two exposed surfaces one upward facing (x) and another downward facing (y). The principle of this instrument is similar to that of the pyrenometer. The receiver A is a long thin blackened metal strip which is painted white on either side. The white and blackened parts of this strip are connected to a thermocouple. Since the black receiver is not enclosed within a glass cover, it responds to all wavelengths. However, exposed receiver may not give us very accurate value because its surface may transfer heat to air by convection and conduction. Moreover, the moisture can also influence it. To avoid such disturbances tbe receiver is either ventilated at a constant rate or it is covered by suitable material which is transparent to visible as well as infrared radiations. At meteorological depamnent of India, Punc, solar radiation is measured by a thermoelectric pyrenometer. The sensor is made up of blackened c,opper constant thermopile. When exposed it gives rise to a thermoel'ectric current proportional to the incident radiation. The current is fed to a continuously recording millivoltmete