3.1 Introduction and Terminology
When radiant flux is incident upon a surface or medium, three processes occur: reflection, absorption, and transmission. A fraction of the beam is reflected, another fraction is absorbed, and the remainder is transmitted. Transmittance Ï is the ratio of transmitted power to incident power. Reflectance Ï is the ratio of reflected power to incident power. Absorptance Î± is the ratio of absorbed power to incident power.
Figure 3.1 shows an ideal geometric case, where the transmitted and reflected components are either specular (regular, in the mirror direction) or diffuse (scattered into the hemisphere). Figure 3.2 shows the transmission and reflection for real surfaces. Both spectral and directional properties are important.