Basic Shadowband Principle
When the band is in a parked position, the pyranometer measures global
horizontal irradiance. (GHI) When the band rotates it momentarily shades
the pyranometer, at which moment it measures diffuse horizontal irradiance.
(DHI) The direct horizontal irradiance is obtained by subtraction,
direct normal irradiance (DNI) by division by the cosine of the solar zenith
Photodiode vs Thermal
Most solar instruments absorb incoming energy with either a black body, producing heat in proportion to photon energy
, or a photodiode, producing current in proportion to photon count
. Photodiodes cannot measure energy flux, i.e. watts per square meter, unless some assumption is made about spectrum.
Research Tracker and Responsivity
We are able to directly observe these effects by comparing the silicon pyranometers used in shadowbands to thermopile reference instruments at our research & calibration station. The graph above shows responsivity for a typical clear day and several effects are present:
- The direct responsivity (red) increases for the red-biased light at sunset and sunrise as the sun is low in the sky and airmass is high;
- The diffuse responsivity (blue) is extremely low for blue skies;
- The global responsivity (green) is a combination of these and also shows the geometric characteristics of the LI200R.