Machine vision inspection system and method for obtaining an image with an extended depth of field

Patent metadata

Field	Value
Publication number	US-9726876-B2
Application number	US-201314092862-A
Country	US
Kind code	B2
Filing date	Nov 27, 2013
Priority date	Nov 27, 2013
Publication date	Aug 8, 2017
Grant date	Aug 8, 2017

Abstract

A method for operating an imaging system of a machine vision inspection system to provide an extended depth of field (EDOF) image. The method comprises (a) placing a workpiece in a field of view; (b) periodically modulating a focus position of the imaging system without macroscopically adjusting the spacing between elements in the imaging system, the focus position is periodically modulated over a plurality of positions along a focus axis direction in a focus range including a workpiece surface height; (c) exposing a first preliminary image during an image integration time while modulating the focus position in the focus range; 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 EDOF image that is focused throughout a larger depth of field than the imaging system provides at a single focal position.

First claim

The invention claimed is:

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 focal length of a variable focal length tunable acoustic gradient index of refraction (TAG) lens included in the imaging system in order to periodically modulate 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 frequency of at least 3 kHz;
(c) exposing a first preliminary image during an image integration time while modulating the focus position in the focus range, comprising exposing the first preliminary image over an exposure duration comprising at least five periods of the periodically modulated focus position and providing illumination having an intensity variation synchronized with the periodically modulated focus position, such that it differently influences the respective exposure contributions for different respective focus positions within the focus range of the periodically modulated focus position; 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.

2. The method of claim 1 , wherein processing the first preliminary image to remove blurred image contributions comprises deconvolving image data corresponding to the first preliminary image using a predetermined function that characterizes the imaging system, to provide the EDOF image.

3. The method of claim 1 , wherein:
the imaging system further comprises an optical filter located to receive and spatially filter preliminary image light from the TAG lens; and
in step (d), processing the first preliminary image to remove blurred image contributions comprises spatially filtering the preliminary image light using the optical filter, to provide the EDOF image based on light output by the optical filter.

4. The method of claim 1 , further comprising:
repeating steps (c) and (d) at least one time to provide a plurality of EDOF images that are substantially focused throughout a larger depth of field than the imaging system provides at a single focal position.

5. The method of claim 4 , further comprising displaying at least one of the plurality of EDOF images on a display of the machine vision inspection system.

6. The method of claim 4 , further comprising displaying at least some of the plurality of EDOF images as video frames on a display of the machine vision inspection system.

7. The method of claim 4 , further comprising making an adjustment to a control signal component related to a nominal center of the periodic modulation based on a user input, such that at least two of the plurality of EDOF images are focused over different focus ranges.

8. The method of claim 1 , further comprising:
(e) terminating the periodic modulating of the focus position to provide a fixed focus position for the imaging system;
(f) exposing a measurement image using the imaging system with the fixed focus position; and
(g) processing the measurement image to provide a measurement of the workpiece.

9. The method of claim 1 , wherein exposing a first preliminary image during the image integration time further comprises exposing the first preliminary image over an exposure duration corresponding to an integer number of periods of the periodically modulated focus position.

10. The method of claim 1 , wherein the intensity variation comprises an intensity on/off cycle synchronized with the periodically modulated focus position, wherein the intensity is on only during a more linear portion of the periodically modulated focus position.

11. The method of claim 10 , wherein the focus position is periodically modulated over a plurality of focus positions spanning at least 20 times the depth of field than the imaging system in a single focal position.

12. A machine vision inspection system having an imaging system and configured to provide at least one image of a workpiece that has a larger depth of field than the imaging system of the machine vision inspection system in a single focal position, the machine vision inspection system comprising:
the imaging system configured such that periodically modulating a focal length of a variable focal length tunable acoustic gradient index of refraction (TAG) lens included in the imaging system periodically modulates the imaging system focus position over a plurality of focus positions along a focus axis direction in a focus range including a surface height of the workpiece at a frequency of at least 3 kHz, without macroscopically adjusting the spacing between elements in the imaging system;
a controller configured to expose a first preliminary image during an image integration time while modulating the focus position in the focus range comprising exposing the first preliminary image over an exposure duration comprising at least five periods of the periodically modulated focus position and providing illumination having an intensity variation synchronized with the periodically modulated focus position, such that it differently influences the respective exposure contributions for different respective focus positions within the focus range of the periodically modulated focus position; and
a processing element configured to process 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.

13. The machine vision inspection system of claim 12 , wherein the intensity variation comprises an intensity on/off cycle synchronized with the periodically modulated focus position, wherein the intensity is on only during a more linear portion of the periodically modulated focus position.

14. The machine vision inspection system of claim 12 , wherein the processing element configured to process the first preliminary image to remove blurred image contributions comprises a signal processor configured to deconvolve image data corresponding to the first preliminary image using a predetermined function that characterizes the imaging system, to provide the EDOF image.

15. The machine vision inspection system of claim 12 , wherein:
the imaging system comprises an optical filter located to receive and spatially filter preliminary image light from the TAG lens; and
the processing element configured to process the first preliminary image comprises the optical filter arranged to receive and spatially filter the preliminary image light from the TAG lens and output resulting filtered image light to provide the EDOF image.

16. The machine vision inspection system of claim 12 , further comprising a display, wherein the machine vision inspection system is configured to display the provided EDOF image on the display of the machine vision inspection system.

17. The machine vision inspection system of claim 12 , wherein the

Assignees

• Mitutoyo Corp

Inventors

• Bryll Robert Kamil

Classifications

• G02B21/361

  Optical details, e.g. image relay to the camera or image sensor (G02B21/364 takes precedence; illumination details G02B21/06 and subgroups) · CPC title

• G02B21/367 Primary

  providing an output produced by processing a plurality of individual source images, e.g. image tiling, montage, composite images, depth sectioning, image comparison · CPC title

• G02B21/241

  Devices for focusing (focusing in general G02B7/28) · CPC title

• G02B27/0075

  with means for altering, e.g. increasing, the depth of field or depth of focus · CPC title