US Patent 7460249 Measuring instrument of polygon-mirror motor

A measuring instrument of polygon mirror motors includes a first light source, a first photo detecting element for detecting a first reflected light beam, i.e. first measuring light beam emitted from the first light source and reflected from the polygon mirror, a second light source, a second photo detecting element for detecting a second reflected light beam, i.e. second measuring light beam emitted from the second light source and reflected from the polygon mirror; and a calculator for performing a calculation based on a time difference between an output from the first photo detecting element and an output from the second photo detecting element. The calculator outputs at least one of an eccentricity and a level difference between facets of the polygon mirror.