Light beam scanning apparatus with adjustable scanning speed and offset

Claim 1

A light beam scanning apparatus comprising: means for sweeping a light beam in response to a ramp signal; means for dividing the swept light beam into first and second portions, said first portion being directed onto a medium to be scanned; an optical grating disposed in the path of said second portion and extending in a direction of sweep of said second portion; light detecting means receiving light passing through said optical grating for producing a detection signal having a pulse for each time said second portion crosses a line of said grating; a reference oscillator for producing reference clock pulses having a frequency greater than a frequency of said pulses of said detection signal; a first counter for counting said reference clock pulses; a second counter for counting said pulses of said detection signal; and control means for enabling said first counter to count said reference clock pulses when said second counter has reached a first predetermined count of said pulses of said detection signal and disabling said first counter from counting said pulses of said detection signal when said second counter has reached a second predetermined count of said pulses of said detection signal, said first and second predetermined counts corresponding to boundaries of a reference region along said grating, and adjusting a slope of said ramp signal in response to a count reached by said first counter when said second counter has reached said second predetermined count to thereby maintain a scan speed of said beam substantially constant.