1.10 Spectral Characteristics of Solar Radiation

The sun is the energy source used to detect reflective energy of ground surfaces in the visible and near infrared regions.

Sunlight will be absorbed and scattered by ozone, dust, aerosols, etc., during the transmission from outer space to the earths surface (see 1.11 and 1.12). Therefore, one has to study the basic characteristics of solar radiation.

The sun is considered as a black body with a temperature of 5,900 K. If the annual average of solar spectral irradiance is given by FeO( ), then the solar spectral irradiance Fe(l) in outer space at Julian day D, is given by the following formula.

Fe( ) = FeO( ){1 + cos (2 (D-3)/365)} where : 0.167 (eccentricity of the Earth orbit) : wavelength
D-3: shift due to January 3 as apogee and July 2 as perigee

The sun constant that is obtained by integrating the spectral irradiance for all wavelength regions is normally taken as 1.37Wm . Figure 1.10.1 shows four observation records of solar spectral irradiance. The values of the curves correspond to the value at the surface perpendicular to the normal direction of the sun light. To convert to the spectral irradiance per m on the Earth surface with a latitude of , multiply the following coefficient by the observed values in Figure 1.10.1. = (L0 / L) cos z cosz = sin sin + cos cos cos h
where z : solar zenith angle : declination
h : hour angle,
L : real distance between the sun and the earth
L0: average distance between the sun and the earth

The incident solar radiation at the earth's surface is very different to that at the top of the atmosphere due to atmospheric effects, as shown in Figure 1.10.2, which compares the solar spectral irradiance at the earth's surface to black body irradiance from a surface of temperature 5900 K.

The solar spectral irradiance at the earth's surface is influenced by the atmospheric conditions and the zenith angle of the sun. Beside the direct sunlight falling on a surface, there is another light source called sky radiation, diffuse radiation or skylight, which is produced by the scattering of the sunlight by atmospheric molecules and aerosols.

The skylight is about 10 percent of the direct sunlight when the sky is clear and the sun's elevation angle is about 50 degree. The skylight has a peak in its spectral characteristic curve at a wavelength of 0.45 m.   