Visible and near infrared optical properties of small particle random composites : carbon black in KBr

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Abstract
Spectroscopic data obtained between 0.3 and 1.8 μ on composites constituted by carbon black particles in a KBr matrix are reported as functions of wavelength and carbon volume concentration ø. We show that effective medium theories account correctly for the experimental results at very low concentrations. At concentrations higher than a few percent, these theories fail to interpret the two striking experimental facts deduced from the results : the existence, at every wavelength, of a maximum in the concentration dependence of the reflectivity and the fact that the reflectivity does not depend on wavelength, that is on carbon reflectivity, up to ø = 0.40. These facts are tentatively attributed to percolative effects