Optical tracking device employing scanning beams on symmetric reference

A method for optically tracking lateral movement of an object, comprising the steps of:

• selecting a reference mark on said object, said reference mark has at least two symmetric lines;
• scanning repeatedly a first probe beam perpendicular to a first symmetric line of said reference mark;
• producing a first reference signal synchronized with said first probe beam to represent a first reference position for each scan;
• scanning repeatedly a second probe beam perpendicular to a second symmetric line of said reference mark, wherein said second probe beam is scanned alternatively with said first probe beam;
• producing a second reference signal synchronized with said second probe beam to represent a second reference position for each scan of said second probe beam;
• detecting scattered light from said object to produce a scattered-light signal consisting of a sequence of symmetric-profile signals;
• separating said sequence of symmetric-profile signals between said first probe beam and said second probe beam, through synchronization with said first and said second reference signals;
• determining the timing center of each symmetric-profile signal to determine the instant positions of said first and second symmetric lines;
• recording a first initial position of said first symmetric line and a second initial position of said second symmetric line;
• comparing said instant position of said first symmetric line with said first initial position to produce a first output signal indicating the relative position change of said first symmetric line;
• and
• comparing said instant position of said second symmetric line with said second initial position to produce a second output signal indicating the relative position change of said second symmetric line.

An apparatus for tracking lateral movement of an eye pupil, comprising:

• a first probe beam projector to project and to scan a first probe beam repeatedly on said eye pupil, wherein said first probe beam is scanned perpendicularly to a first symmetric line of said eye pupil, and wherein said first probe beam projector also produces a first reference pulse for each scan of said first probe beam;
• a second probe beam projector to project and to scan a second probe beam repeatedly on said eye pupil, wherein said second probe beam is scanned alternatively with said first probe bean and perpendicularly to a second symmetric line of said eye pupil, and wherein said second probe beam projector also produces a second reference pulse for each scan of said second probe beam;
• a photo-detection means receiving scattered light from said eye pupil to produce a scattered-light signal consisting of a sequence of square-like pulses, of which each square-like pulse corresponds to a scan of said first or said second probe beam and indicates with the center of said square-like pulse the instant position of said first or second symmetric line correlated to said scan;
• and
• an analyzing electronics coupled to said first probe beam projector, said second probe beam projector, and said photo-detection means to generate a first output signal and a second output signal, wherein said first output signal indicates the position change of said first symmetric line of said eye pupil with respect to a first reference position so as to track movement of said eye pupil along the scanning direction of said first probe beam, and wherein said second output signal indicates the position change of said second symmetric line of said eye pupil with respect to a second reference position so as to track movement of said object along the scanning direction of said second probe beam.