The scattered light from dust-contaminated mirrors was measured at λ = 10.6 μm, and results are compared to that predicted by a modified Mie theory. The theory is in good agreement with experiment. The far-infrared measurements of the "clean" mirrors were, for angles beyond about 8 deg from specular, limited by the scatter from particulates and not the surface microroughness. Similar limitations can be experienced by other irregularities such as scratches, digs, and pinholes in coatings. It is shown that contaminant scatter dominance in the far-infrared requires only a few very small particles, so the necessity for good cleaning techniques and good clean room practice is quite evident.