Radiation exchange at the target surface

The measurement of thermal IR radiation is the basis for noncontact temperature measurement and thermography. Thermal IR radiation leaving a surface (W) is called exitance or radiosity. It can be emitted from the surface, reflected off the surface, or transmitted through the surface. This is illustrated in Fig. 2.4. The total radiosity is equal to the sum of the emitted component (We), the reflected component (Wr) and the transmitted component (Wt). The surface temperature is related to We, the emitted component, only. 

REFLECTED RADIATION (WIr) EMITTED RADIATION (We) TRANSMIITED RADIATION (Wl) 

Thermal IR radiation impinging on a surface can be absorbed, reflected, or transmitted as illustrated in Fig. 2.5. Kirchhoff s law states that the sum of the three components is always equal to the received radiation (the percentage sum of the three components equals unity)  

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