Patent ID: 7477365

Claim:
A method for determining a geometric parameter of an optical spot of a light beam, the method comprising: providing a calibration target, the calibration target including a systematic variation in a parameter; measuring the calibration target with respect to the systematic variation using the light beam to obtain a plurality of measurements; and analyzing the measurements and the systematic variation to determine the geometric parameter of the optical spot; wherein in the case that the geometric parameter includes a size of the optical spot, the calibration target includes a first portion, a second portion and a third portion, a dimension of the first portion in a first axis systematically decreasing along a second axis, the second portion and the third portion extending along opposite sides of the first portion and being separated by the first portion in the first axis and including a material of different optical response than that of the first portion, and the measuring including scanning the light beam along the calibration target in the second axis to make measurements of the calibration target, and the analyzing including: relating a measurement to a position of the optical spot with respect to the first portion, second portion and the third portion of the calibration target to identify a reference scan position of the optical spot where the optical spot is marginally located within the first portion; and determining a size of the optical spot in the first axis based on a corresponding dimension of the first portion in the first axis, the identifying including: detecting a change of value in the measurements indicating that the optical spot covers a portion of the first portion and a portion of at least one of the second portion and the third portion; adjusting a relative position between the optical spot and the calibration target in the first axis to locate the optical spot within the first portion; and continuing the scanning, detecting, and adjusting until identifying the reference scan position; wherein in the case that the geometric parameter includes a level of symmetry of the optical spot, the calibration target including a gradient in a parameter along an axis, the gradient being a function of the axis, and the analyzing including analyzing differences in the measurements to determine the level of symmetry, the measuring including: locating the optical spot on a given point of the gradient; and scanning the optical spot from the given point along the gradient tin different directions to obtain the measurements; and the calibration target including one of: a continuous gradient from one end of the calibration target to another end thereof in the axis; or a switch point in a given point of the calibration target in the axis between two sub-gradients, one of the two sub-gradients being an ascending gradient and another of the two sub-gradient being a descending gradient.