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A conic pyroelectric detector is employed to measure the effective reflectances of powder beds of metals, SiC, WC, and graphite. The experimental results are compared with numerical solutions of the radiation transfer equation in a half-infinite homogeneous body with effective radiative properties. The effective albedo and phase function are theoretically estimated by the model of opaque particles with specularly or diffusely reflecting surfaces in the assumption of geometrical optics. The studied powder beds with particles of several tens of microns in diameter are generally described by the model where specular reflection by particle surfaces is assumed corresponding to refraction and extinction indices of materials in the dense form. The model does not apply to semitransparent particles of SiC. The powder beds of submicron particles of SiC and WC are highly agglomerated and consist of clusters of particles with diameters of several tens of microns. A submicron powder bed is found to be considerably less reflective than a micron-sized powder bed of the same material. This is described by the model where the agglomerates of submicron particles are treated as large particles with diffusely reflecting surfaces. The effective normal-hemispherical reflectance of a micron-sized powder bed correlates with the hemispherical reflectance of dense material. The observed differences between the experiment and the model can be explained by a systematic error at calibration, oxidation of materials, and diffraction effects.