Microtopographic Influence of Nominally Flat Metallic Surfaces upon Their Thermally Reflective Behaviour

Abstract

The total reflectivity as well as the spatial distribution of the reflected energy is influenced by the topography of the reflecting surface. It has been commonly, but mistakenly, assumed that the angular variation of thermal radiation reflected from real surfaces has a directional distribution that falls between the diffuse and specular limits. For roughened surfaces, maxima in the reflected, total, radiant-intensity vector distribution occur at viewing directions other than at the angle of specular reflection, even when the mean optical roughness (i.e. r.m.s. roughness divided by wavelength of the incident radiation) is less than unity. Off-specular phenomena occur if the product of the mean normalized absolute surface slope and the predominant radiation wavelength is smaller than about 12. The magnitude of the off-specular angle is then a function of the mean surface slope, the r.m.s. roughness, the predominant wavelength of the incident radiation and its angle of incidence.