Terrestrial Spectra
The spectrum of the solar radiation at the earth's surface
has several components (Fig. 2). Direct radiation comes
straight from the sun, diffuse radiation is scattered from the
sky and from the surroundings. Additional radiation reflected
from the surroundings (ground or sea) depends on the local
"albedo." The total ground radiation is called the global
radiation. The direction of the target surface must be defined
for global irradiance. For direct radiation the target surface
faces the incoming beam.