Solar Radiation Measurements

Instruments for measuring solar radiation:
Two basic types of instruments are used in measurements of solar radiation. These are:
1. Pyranometer: An instrument used to measure global (direct and diffuse) solar radiation on a surface. This instrument can also be used to measure the diffuse radiation by blocking out the direct radiation with a shadow band.
2. Pyrheliometer: This instrument is used to measure only the direct solar radiation on a surface normal to the incident beam. It is generally used with a tracking mount to keep it aligned with the sun.

  
Solar radiation defining terms:
Diffuse radiation: Scattered solar radiation coming from the sky.
Direct or beam normal radiation: Part of solar radiation coming from the direction of the sun on a surface normal to the sun’s rays.
Equation of time: Correction factor in minutes, to account for the irregularity of the Earth’s motion around the sun.
Extraterrestrial solar radiation: Solar radiation outside Earth’s atmosphere.
Insolation: Incident solar radiation measured as W/m2 or Btu/hr-ft2.
Solar altitude angle: Angle between the solar rays and the horizontal plane.
Solar azimuth angle: Angle between the true south horizontal line and the horizontal projection of the sun’s rays.
Solar constant: Extraterrestrial solar radiation at the mean Earth-sun distance.
Solar declination: Angle between the Earth-sun line and a plane through the equator.