Slowly rotating, cylindrical, diffusing drums have been widely used in laser refraction. A simple geometrical evaluation of the optical path lengths involved is used to derive expressions for the position of the plane of stationarity and the apparent speed of speckle motion as a function of the characteristics of the illuminating wavefront, the drum, the refraction of the observer, and other system parameters. With apparatus of known characteristics, determination of the apparent speckle velocity allows calculation of the ametropia of the observer.