Patent ID: 7460249

Claim:
A measuring instrument for a polygon-mirror motor spinning with a polygon mirror having a plurality of facets fixed to an output shaft of the motor, said measuring instrument comprising: a first light source; a first photo detecting element for detecting, via a first slit, a first light beam reflected from the polygon mirror after having been emitted as a first measuring light beam from said first light source; a second light source; a second photo detecting element for detecting a second light beam reflected from the polygon mirror after having been emitted as a second measuring light beam from said second light source; a time difference detector for measuring a first time difference between an output signal from said first photo detecting element and an output signal from said second photo detecting element for each of the facets of the polygon mirror; and a calculator that calculates at least one of the eccentricity and the level difference between facets based on the output of the time-difference detector and a second time difference, which is a difference between the first time difference of two of the facets, and outputs the at least one of the eccentricity and the level difference between facets of the polygon mirror; and wherein said second photo detecting element outputs the output signal therefrom when the second measuring light beam is aligned with the second light beam on one straight line.