A laser printer includes a light source, a photosensitive drum, an oscillating mirror, a beam detector, and a control circuit. The light source shines a light beam towards the oscillating mirror, which reflects the light beam towards the photosensitive drum. The beam detector detects light reflected from the oscillating mirror. A reflected light beam hits the beam detector twice in each period of the oscillating mirror. The time interval between detected beams is used to determine the maximum mechanical deflection angle of the oscillating mirror. In an alternate embodiment, two beam detectors are used to detect light reflected from the oscillating mirror and determine its maximum mechanical deflection angle.