The transfer of heat energy by radiation based on electromagnetic waves has several unique characteristics. A medium is not required for radiant heat transfer. The amount of radiation and the quality of the radiation depend upon temperature. It is propagated at the speed of light (3 × 108 m/s) in vacuum. The wavelength range of thermal radiation is 0.1–100 µm. The Sun emits radiation in wavelengths between 0.1 µm and 3 µm. This portion of the spectrum is known as solar radiation.
Let Qr, Qa and Qt be the fractions of radiation energy Qi incident on a surface that are reflected, absorbed and transmitted, respectively as shown in Figure 9.1.
Figure 9.1 Incident radiation.
where ρ,α and τ are the reflectivity, ...