(Kees van den Bos, Hukseflux Thermal Sensors, Pyranometer geometry type LP02) I uploaded this file from http://en.wikipedia.org/wiki/Image:Lp02-manual-v02.gif where it had public license so I used the same license here on commons The solar radiation spectrum extends roughly from 300 to 2800 nm. It follows that a pyranometer should cover that spectrum with a spectral sensitivity that is as “flat” as possible. For a flux density or irradiance measurement it is required by definition that the response to “beam” radiation varies with the cosine of the angle of incidence; i.e. full response at when the solar radiation hits the sensor perpendicularly (normal to the surface, sun at zenith, 0 degrees angle of incidence), zero response when the sun is at the horizon (90 degrees angle of incidence, 90 degrees zenith angle), and 0.5 at 60 degrees angle of incidence. It follows from the definition that a pyranometer should have a so-called “directional response” or “cosine response” that is close to the ideal cosine characteristic.