Page 18 - Applied Photovoltaics

temperatures, beyond the melting points of most metals, are required to shift the peak
emission to this range.

1.3 THE SUN AND ITS RADIATION
The sun is a hot sphere of gas heated by nuclear fusion reactions at its centre
(Quaschning, 2003). Internal temperatures reach a very warm 20 million K. As
indicated in Fig. 1.2, the intense radiation from the interior is absorbed by a layer of
hydrogen ions closer to the sun’s surface. Energy is transferred by convection through
this optical barrier and then re-radiated from the outer surface of the sun, the
photosphere. This emits radiation approximating that from a blackbody with a
temperature of nearly 6000 K, as shown in Fig. 1.3.

Figure 1.2. Regions in the sun’s interior.

Figure 1.3. The spectral irradiance from a blackbody at 6000 K (at the same
apparent diameter as the sun when viewed from earth); from the sun’s
photosphere as observed just outside earth’s atmosphere (AM0); and from the
sun’s photosphere after having passed through 1.5 times the thickness of earth’s
atmosphere (AM1.5G).

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