**1. Spectral characteristics of solar radiation**

Solar radiation is a radiant energy emitted by the Sun as a result of its nuclear fusion reactions. Spectral characteristics of solar radiation, both external to the Earth's atmosphere and at the ground, can be seen in **Figure 1**. Over 99% of the energy flux from the Sun is in the spectral region of 0.15–4 μm, with approximately 50% in the visible light region of 0.4–0.7 μm. The total amount of energy emitted by the Sun and received at the extremity of the Earth's atmosphere is constant, that is, 1370 W/m<sup>2</sup> /s. The amount of energy received per unit area of the

**Figure 1.** Spectral distribution of radiation intensity [1].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Earth's surface is 343 W/m<sup>2</sup> /s. The standard spectrum for space applications is referred to as AM<sup>0</sup> . It has an integrated power of 1366.1 W/m<sup>2</sup> . Two standards are defined for terrestrial use: (1) the AM1.5 global spectrum is designed for flat plate modules and has an integrated power of 1000 W/m<sup>2</sup> and (2) the AM1.5 direct spectrum is defined for solar concentrator work. It includes the direct beam from the Sun and the circumsolar component in a disk of 2.5° around the sun. The direct and circumsolar spectra have an integrated power density of 900 W/m<sup>2</sup> (**Figure 1**).
