Patent ID: 8755055

Claim:
A method for measuring a length of a gap between a rotating tool and a workpiece by irradiating the gap with a laser beam, and measuring the length of the gap based on a diffracted pattern of diffracted light diffracted through the gap without being interrupted, the method comprising: generating a pulsed laser beam having a width greater than the gap between the rotating tool and the workpiece oppositely disposed to the rotating tool; irradiating the gap between an edge of the workpiece and the rotating tool with the pulsed laser beam, the gap resulting from tilting an optical axis of the pulsed laser beam relative to a plane of the workpiece; emitting the pulsed laser beam in an identical angle range of the rotating tool during oscillation pulse ON durations, the laser beam having an oscillation pulse period that is one pulse per revolution or per an integer number of revolutions of the rotating tool; and detecting on a light receiving sensor a light which is irradiated toward the gap and which is diffracted through the gap without being interrupted, so as to measure the length of the gap.