Patent ID: 7309161

Claim:
A method of determining the magnification M ref for a line scan camera that transports a work piece to be imaged in orthogonal x and y axis directions while at a fixed height along a z axis normal to the x-y plane containing the x and y axes, the method comprising the steps of: (a) transporting at a fixed location z cal along the z axis and in the y axis direction a calibration target having opaque edges parallel to the x and y axes and both opaque edges in a plane parallel to the x-y plane; (b) while performing step (a), illuminating the calibration target with actinic radiation emanating in a generally uniform conical pattern from a point source and in a direction toward the calibration target and then further toward a multiple element detector of the actinic radiation disposed upon a detection plane parallel to the x-y plane, the conical pattern having an axis normal to the detection plane, and the multiple element detector having a plurality of x i detection elements responsive to the actinic radiation along a line parallel to the x axis; (c) while performing step (b), collecting and storing at regular intervals of transport motion in step (a) the plurality of detection element outputs Y j α@X i that, at successive locations y j Δy apart along the y axis, are the respective outputs α of an i th detection element x i of the multiple element detector having some location along the x axis within the detection plane; (d) while performing step (b), collecting and storing at regular intervals of transport motion in step (a) the plurality of detection element outputs X i α@Y j that, at successive locations Δx apart along the x axis, are the respective outputs α of each i th detection element x i of the multiple element detector having some location along the y axis within the detection plane; (e) selecting an arbitrary trial magnification value M i from among a range of possible magnification values; (f) while a trial magnification value M i is in effect: (f1) subsequent to each instance of step (e), for each z i in a range from a selected z min along the z axis and by steps of a Δz toward a selected z max along the z axis, z min <z cal <z max , reconstructing the image at the height z i ; (f2) subsequent to each instance of step (f1), inspecting the reconstructed image for M i for a value z x of z i that exhibits a sharp x axis edge and a value z y of z i that exhibits a sharp y axis edge; (f3) subsequent to each associated instances of steps (f1) and (f2), saving a value e i that is indicative of the difference between the associated z x and z y ; (f4) subsequent to steps (f1), (f2) and (f3), selecting an unused next value for M i until a selected number of different M i have been in effect; (g) fitting a function e=f (M) to the set of data {(e i ), (M i )}; and (h) finding the y intercept M j of e=f (M) and taking M j to be the value of M ref .