Specular and diffuse surfaces

The roughness and surface characteristics will determine the type and direction of reflection of incident radiation. A smooth surface will reflect incident energy at an angle complementary to the angle of incidence. This is called a specular reflector. A rough or structured surface will scatter or disperse some of the incident radiation. This is called a diffuse reflector. No perfectly specular or perfectly diffuse surface can exist in nature. All real surfaces have some diffusivity and some specularity. 

When making practical measurements, the specular or diffusing characteristics of a target surface are taken into effect by accounting for the emissivity of the surface. Emissivity is discussed as part of the detailed discussion of the characteristics of IR thermal radiation in Sec. 2.3. 

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S. Maruyama, Radiation Heat Transfer Between Arbitrary Three-Dimensional Bodies with Specular and Diffuse Surfaces, Num. Heat Transfer, 24, pp. 181-196,1993. 

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