ionic or partially ionic solids, or due to electronic transi­ tions, both intrinsic and impurity-induced. For most materials, a sufficiently wide spectral window exists be­ tween these two limits, where the material is transpar­ ent. In general, the absorption coefficient, in the long wavelength side of, but sufficiently away from, the fun­ damental absorption edge, is relatively structureless and has an exponential dependence on frequency. Recent evi­ dence suggests that in the short wavelength side of the one-phonon region, but beyond two- or three- phonon sin­ gularities, the absorption coefficient of both polar and nonpolar solids is also relatively structureless and de­ pends exponentially on frequency.