MACHINE VISION INSPECTION SYSTEM AND METHOD FOR OBTAINING AN IMAGE WITH AN EXTENDED DEPTH OF FIELD

摘要

A method for providing an extended depth of field (EDOF) image includes: Periodically modulating an imaging system focus position at a high frequency; using an image exposure comprising discrete image exposure increments acquired at discrete focus positions during an image integration time comprising a plurality of modulation periods of the focus position; and using strobe operations having controlled timings configured to define a set of evenly spaced focus positions for the image exposure increments. The timings are configured so that adjacent focus positions in the set are acquired at times that are separated by at least one reversal of the direction of change of the focus position during its periodic modulation. This solves practical timing problems that may otherwise prevent obtaining closely spaced discrete image exposure increments during high frequency focus modulation. Deconvolution operations may be used to improve clarity in the resulting EDOF image.

主权项

1. A method for operating an imaging system of a machine vision inspection system to provide at least one image that has a larger depth of field than the imaging system in a single focal position, the method comprising:
(a) placing a workpiece in a field of view of the machine vision inspection system; (b) periodically modulating a focus position of the imaging system without macroscopically adjusting the spacing between elements in the imaging system, wherein the focus position is periodically modulated over a plurality of focus positions along a focus axis direction in a focus range including a surface height of the workpiece, at a modulation frequency of at least 3 kHz; (c) exposing a first preliminary image using an image exposure comprising a plurality of discrete image exposure increments acquired at respective discrete focus positions during an image integration time comprising a plurality of periods of the periodically modulated focus position, wherein:
the plurality of discrete image exposure increments are each determined by a respective instance of an illumination source strobe operation, or a camera shutter strobe operation, that has a respective controlled timing that defines the discrete focus position of the corresponding discrete image exposure increment;the respective controlled timings are distributed over the plurality of periods of the periodically modulated focus position, and are configured to provide a set of discrete focus positions which are approximately evenly spaced along the focus axis direction; andthe respective controlled timings are furthermore configured so that for a plurality of adjacent pairs of discrete focus positions in the set, when a first controlled timing provides a first discrete focus position set of the adjacent pair, a second controlled timing that provides a second discrete focus position of the adjacent pair is controlled to have a delay relative to the first controlled timing such that the second controlled timing is controlled to occur after N reversals of the direction of change of the focus position during its periodic modulation following the first controlled timing, where N is at least 1; and (d) processing the first preliminary image to remove blurred image contributions occurring in the focus range during the image integration time to provide an extended depth of field (EDOF) image that is substantially focused throughout a larger depth of field than the imaging system provides at a single focal position.