The amount of light occluded by a fiber as it passes through a laser beam can be used as the basis for fiber-diameter measurement. This technique is analyzed with a two-dimensional rigorous model. The occlusion seen for dielectric fibers as a function of their diameter is highly oscillatory owing to interference between the light transmitted by the fiber and the rest of the diffracted field. Scalar diffraction theory is shown to be adequate in modeling this effect. The oscillation sets a limit to the accuracy of simple diameter measurement systems and is confirmed experimentally for glass fibers. However, wool fibers are found to be better treated as an absorbing material. The effect of beam polarization is investigated and found to be negligible for dielectric fibers but significant for metal fibers of small diameter.