The solar irradiation spectrum above the atmosphere can be modeled by a black body radiation spectrum having a source temperature of 5900 K, with a peak irradiation located at about 500 nm wavelength. Physical measurement of the solar irradiance has also been performed using ground based and spaceborne sensors. After passing through the atmosphere, the solar irradiation spectrum at the ground is modulated by the atmospheric transmission windows . Solar Irradiation Spectra above the atmosphere and at sea-level. When solar radiation hits a target surface, it may be transmitted , absorbed or reflected .
Different materials reflect and absorb differently at different wavelengths.
The reflectance spectrum of a material is a plot of the fraction of radiation reflected as a function of the incident wavelength and serves as a unique signature for the material. In principle, a material can be identified from its spectral reflectance signature if the sensing system has sufficient spectral resolution to distinguish its spectrum from those of other materials. This premise provides the basis for multispectral remote sensing. The reflectance of clear water is generally low. However, the reflectance is maximum at the blue end of the spectrum and decreases as wavelength increases.