Solar radiation measuring instruments

Solar radiation instruments convert incident radiation into a measurable signal, usually heat or voltage.

Pyranometer

Measures global radiation (beam + diffuse) on a horizontal surface; with a shade ring or disc it measures diffuse radiation only.

The Eppley type has a black and white sensing surface (thermopile) under two glass domes. Black parts absorb radiation and get hotter than the white parts, so the thermopile produces a small voltage (about 10 µV per W/m²) proportional to the radiation. The domes protect from wind and rain, and a desiccant keeps them dry. A photodiode type is cheaper but spectrally selective.

   glass domes
     ____
    /    \
   |B|W|B|W|  <- thermopile hot (black)
   |______|      and cold (white) junctions
   heat sink --> voltage out

Pyrheliometer

Measures beam (direct) radiation at normal incidence. A long collimating tube with a narrow field of view (about 5–6°) is kept pointed at the sun by a tracker. The radiation falls on a thermopile or a blackened receiver. In the Angstrom compensation type, one of two strips is shaded and heated electrically until the temperatures equal; the electrical power then equals the absorbed radiation. It is also the reference instrument for calibration.

Sunshine recorder

Measures the duration of bright sunshine (hours), not intensity. In the Campbell-Stokes recorder a solid glass sphere focuses the sun's rays onto a card strip; the burn trace length gives the sunshine hours. The data are used with Angstrom regression $\bar H/\bar H_0 = a + b\,(\bar n/\bar N)$ to estimate radiation where no pyranometer is available.