2 Solar Spectra
29
Fig. 2.9 Moonlight has an irradiance level that is about ten million times lower than direct sunlight.
Courtesy: Dji-Illustration, Neuchâtel
2.6 Irradiance and Irradiation
‘Irradiance’ is an instantaneous quantity describing the flux of solar radiation incident
on a surface with the unit W × m
−2 . Thus, irradiance is solar ‘power density’. The
irradiance value changes very much, in a given location, depending on the moment
of the day. The typical variation of solar irradiance in tropical regions on a clear
sunny day is a bell curve, as shown in Fig. 2.10.
By referring to ‘spectral irradiance’ one refers to the ‘spectral power density’.
Thus, the unit of spectral irradiance or spectral power density is W × m
−2
× nm
−1 .
Irradiation
7 is the integration of irradiance over a specified period of time (MJ ×
m
−2 per hour, day, week, month, year as the case may be). Thus, it is simply the ‘time
integral of irradiance’ or ‘energy’ received at a given location on earth. Irradiation
measures the solar radiation energy density usually in Wh × m
−2 .
7 Alternative terms used for irradiation are a) solar insolation b) radiant exposure. Although the
International Electrotechnical Vocabulary (IEV [11]) defines “irradiation” as “exposure to ionizing
radiation”, the term is routinely used in the PV community instead of “radiant exposure” (i.e.
the radiant energy incident on an element of the surface). An alternative term commonly used is
“insolation”.
29
Fig. 2.9 Moonlight has an irradiance level that is about ten million times lower than direct sunlight.
Courtesy: Dji-Illustration, Neuchâtel
2.6 Irradiance and Irradiation
‘Irradiance’ is an instantaneous quantity describing the flux of solar radiation incident
on a surface with the unit W × m
−2 . Thus, irradiance is solar ‘power density’. The
irradiance value changes very much, in a given location, depending on the moment
of the day. The typical variation of solar irradiance in tropical regions on a clear
sunny day is a bell curve, as shown in Fig. 2.10.
By referring to ‘spectral irradiance’ one refers to the ‘spectral power density’.
Thus, the unit of spectral irradiance or spectral power density is W × m
−2
× nm
−1 .
Irradiation
7 is the integration of irradiance over a specified period of time (MJ ×
m
−2 per hour, day, week, month, year as the case may be). Thus, it is simply the ‘time
integral of irradiance’ or ‘energy’ received at a given location on earth. Irradiation
measures the solar radiation energy density usually in Wh × m
−2 .
7 Alternative terms used for irradiation are a) solar insolation b) radiant exposure. Although the
International Electrotechnical Vocabulary (IEV [11]) defines “irradiation” as “exposure to ionizing
radiation”, the term is routinely used in the PV community instead of “radiant exposure” (i.e.
the radiant energy incident on an element of the surface). An alternative term commonly used is
“insolation”.