Method for finding a reference point

Claim 1

In conjunction with an apparatus including means for positioning an object and means for scanning the surface of said object to produce a signal indicative of the properties of the surface of said object along a predetermined scan path, a method for operating said apparatus so as to locate the signal from a first fiducial mark, having a width ±w 1 , which is separated by a distance D from a second fiducial mark, having a width ±w 2 , when said signal is corrupted by a noise signal, said method comprising the steps of: scanning the surface of said object along a path passing through said first and second fiducial marks thereby generating a signal M(x) indicative of the surface of said object along said path, wherein x is the distance along said path; convolving M(x) with a filter function F(x) to form an enhanced function E(x), wherein F(x+c)>0 for at least one value of x between -w 1 and +w 1 , F(x+c)=0 for x between D+w 1 and D-w 2 , and F(x+c)>0 for at least one value of x between D-w 2 and D+w 2 , where c is a constant; analyzing E(x) to determine the location the portion of E(x) resulting from said first fiducial mark, and moving said object to a position related to said first fiducial mark.