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Solar Radiation Measurements There are numerous types and components of solar radiation as shown in the list below.The following image shows the solar spectral intensity distribution at the top of the Earth's atmosphere and at the Earth's surface.Direct solar irradiance, also called the "direct solar irradiation" or the "direct normal irradiation (DNI)" - the solar irradiance on a surface held perpendicular to the sun's rays while blocking the diffuse sky radiation. Global solar irradiance, commonly called the "global radiation" - the solar irradiance on a horizontal surface which includes both direct sunrays and diffuse sky radiation.In addition, a portion of the incoming solar radiation is absorbed, reflected and reemitted by the atmosphere and the earth.Units are typically in W/m2.

Solar Radiation Measurements
There are numerous types and components of solar radiation as shown in the list below. Units are typically in W/m2.

Irradiance – the instantaneous quantity of solar radiant energy incident on a surface per unit area.
Diffuse solar irradiance – the solar irradiance on a horizontal surface due to only sky radiation. This does not include the direct solar irradiance, which is defined next.
Direct solar irradiance, also called the “direct solar irradiation” or the “direct normal irradiation (DNI)” – the solar irradiance on a surface held perpendicular to the sun’s rays while blocking the diffuse sky radiation.
Global solar irradiance, commonly called the “global radiation” – the solar irradiance on a horizontal surface which includes both direct sunrays and diffuse sky radiation.
Reflected solar irradiance – the short-wave radiation which is reflected upward from the Earth’s surface.
Net terrestrial radiation – the long-wave radiation coming off the Earth’s surface minus the upward radiation through a horizontal surface near the Earth’s surface.
Net total irradiance – the downward irradiance minus the upward radiation as measured over the entire spectrum.
In addition, a portion of the incoming solar radiation is absorbed, reflected and reemitted by the atmosphere and the earth. The following figure illustrates this concept.

The following image shows the solar spectral intensity distribution at the top of the Earth’s atmosphere and at the Earth’s surface. The solar irradiance received at the top of the Earth’s atmosphere is about 1367 W/m2; this number is normally termed the “solar constant”.