Instruments

Solar meter (Pyranometer)

The solar meter measures the irradiation of a flat surface (Watt/m2). The irradiation measured is the sum of the direct irradiation produced by the sun and the diffused irradiation (global irradiation). The principle behind the instrument is based on a thermopile sensor. The sensitive surface of the thermopile is coated with an opaque black varnish that prevents the solar meter from selecting particular wavelengths. The spectral field of the solar meter is determined by the transmission of two glass domes. The radiating energy is absorbed by the black surface of the thermopile creating a difference in temperature between the centre of the thermopile (hot junction) and the body of the thermopile (cold junction). The difference in temperature between the hot junction and cold junction is converted into a power difference thanks to the Seebeck effect. The instrument has two concentric domes to guarantee thermal isolation of the thermopile from the wind and to reduce the sensitivity to thermic irradiation.

The reference cell or sample cell

Sometimes a reference cell or sample cell is used. This is a little photovoltaic cell of which the precise characteristics are known. The cell is connected to a digital multimeter and gives a reading in volts mV. Proportional to the value of irradiance; irradiated by the sun and measured the electrical power, allows the calculation of the total irradiation value. It is less precise than the solar meter.

Sunshine recorder

The instrument that measures the duration of solar irradiation is the sunshine recorder, sensitive to direct irradiation, or in other words to visible sunlight which creates shadows, that with a minimum amount of light can vary between 70 and 280 W/m2.