Points from focus operations using multiple light settings in a machine vision system

Patent metadata

Field	Value
Publication number	US-9060117-B2
Application number	US-201213725992-A
Country	US
Kind code	B2
Filing date	Dec 21, 2012
Priority date	Dec 23, 2011
Publication date	Jun 16, 2015
Grant date	Jun 16, 2015

Abstract

A method of automatically adjusting lighting conditions improves the results of points from focus (PFF) 3D reconstruction. Multiple lighting levels are automatically found based on brightness criteria and an image stack is taken at each lighting level. In some embodiments, the number of light levels and their respective light settings may be determined based on trial exposure images acquired at a single global focus height which is a best height for an entire region of interest, rather than the best focus height for just the darkest or brightest image pixels in a region of interest. The results of 3D reconstruction at each selected light level are combined using a Z-height quality metric. In one embodiment, the PFF data point Z-height value that is to be associated with an X-Y location is selected based on that PFF data point having the best corresponding Z-height quality metric value at that X-Y location.

First claim

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for operating a precision machine vision inspection system to determine a set of multi-point Z-height measurement data comprising focus-based Z-height measurements in a particular region of interest on a workpiece, the precision machine vision inspection system comprising:
an imaging portion including a camera;
a controllable lighting portion;
a focusing portion;
a control portion comprising an image processor;
a first measuring mode for performing multi-point focus-based Z-height measurements on a workpiece, comprising operations that determine the multi-point focus-based Z-height measurements for a plurality of subregions within a region of interest based on a single image stack acquired using the same lighting parameters for each image in that image stack;
a second measuring mode for performing multi-point focus-based Z-height measurements on a workpiece, comprising operations that determine the multi-point focus-based Z-height measurements for a plurality of subregions within a region of interest based on a plurality of image stacks, wherein a first image stack is acquired using darkness limiting lighting parameters that satisfy a darkness limiting criterion for image pixels in the region of interest and that are the same for each image in the first image stack, and a second image stack is acquired using brightness limiting lighting parameters that satisfy a brightness limiting criterion for image pixels in the region of interest and that are the same for each image in the second image stack;
a user interface including an image display and a graphical user interface (GUI); and
a multi-point Z-height measurement tool comprising:
the second measuring mode;
the brightness limiting criterion for image pixels in the region of interest;
the darkness limiting criterion for image pixels in the region of interest; and
a multi-point GUI element including a region of interest indicator; and

the method comprising:
performing operations of the machine vision inspection system comprising:
acquiring an image of the particular region of interest on a workpiece;
activating an instance of a multi-point Z-height measurement tool; and
defining a region of interest in the acquired image;

performing automatic operations of that instance of the multi-point Z-height measurement tool corresponding to the second measuring mode, comprising:
(a) operations that automatically focus the imaging portion at a global best focus height for the region of interest, wherein the global best focus height is determined based on an image stack acquired using a preliminary set of lighting parameters and based on a focus metric that is determined based on the entire region of interest;
(b) operations that analyze images acquired at the global best focus height and adjust the lighting parameters to determine brightness limiting lighting parameters that satisfy the brightness limiting criterion for image pixels in the region of interest, and
(c) operations that analyze images acquired at the global best focus height and adjust the lighting parameters to determine darkness limiting lighting parameters that satisfy the darkness limiting criterion for image pixels in the region of interest.

2. The method of claim 1 , wherein the method comprises performing the operations of the machine vision inspection system under the control of part program instructions during a run mode, and the method further comprises:
acquiring a plurality of image stacks including the region of interest, that plurality of image stacks comprising at least a first image stack acquired using the determined darkness limiting lighting parameters and a second image stack acquired using the determined brightness limiting lighting parameters; and
determining the multi-point focus-based Z-height measurements for each of a plurality of subregions within the region of interest based on the plurality of image stacks.

3. The method of claim 2 , wherein determining the multi-point focus-based Z-height measurements for each of a plurality of subregions comprises:
determining a Z-height quality metric for that subregion for each of the plurality of image stacks; and
determining the focus-based Z-height measurement for that subregion based on the one of the plurality of image stacks that provides the best Z-height quality metric.

4. The method of claim 2 , wherein the plurality of image stacks comprises the image stack acquired using the preliminary set of lighting parameters.

5. The method of claim 1 , wherein the method comprises performing the operations of the machine vision inspection system during a learn mode, wherein:
in the step of acquiring an image of the particular region of interest, the workpiece is a representative workpiece and the step comprises displaying the acquired image on the image display;
the step of activating an instance of a multi-point Z-height measurement tool comprises displaying its multi-point GUI element in the image display; and
the step of defining a region of interest in the acquired image comprises defining a region of interest in the displayed image by positioning the region of interest indicator of the multi-point GUI element to surround the particular region of interest.

6. The method of claim 5 , wherein the method further comprises recording part program instructions for that instance of the multi-point Z-height measurement tool that perform operations comprising:
acquiring a plurality of image stacks including the region of interest, that plurality of image stacks comprising at least a first image stack acquired using the determined darkness limiting lighting parameters and a second image stack acquired using the determined brightness limiting lighting parameters; and
determining the multi-point focus-based Z-height measurements for each of a plurality of subregions within the region of interest based on the plurality of image stacks, comprising for each subregion:
determining a Z-height quality metric for that subregion for each of the plurality of image stacks; and
determining the focus-based Z-height measurement for that subregion based on the one of the plurality of image stacks that provides the best Z-height quality metric.

7. The method of claim 6 , wherein the Z-height quality metric comprises a relationship between a representative peak height indicated by a focus peak determining data set derived from the plurality of image stacks, and a representative noise level or “noise height” indicated by the focus peak determining data set.

8. A method for operating a precision machine vision inspection system to determine a set of Z-height measurement data comprising focus-based Z-height measurements on a workpiece, the precision machine vision inspection system comprising a user interface, an imaging portion including a camera, a controllable lighting portion, a focusing portion, and a control portion comprising an image processor, the method comprising:
(a) operating the precision machine vision inspection system to define a region of interest for performing points-from-focus operations on the workpiece;
(b) operating the precision machine vision inspection system to define N respective lighting vectors associated with N image stacks comprising images including the region of interest and having different focus Z-heights in a defined Z-height range, where N is at least 2, and defining the N respective lighting vectors comprises;
(b1) defining a first respective lighting vector such that a first brightness criterion is satisfied wherein the brightest pixel locations in the region of interest include brightness values in the uppe

Assignees

• Mitutoyo Corp

Inventors

• Bryll Robert Kamil
• Campbell Shannon Roy

Classifications

• H04N23/673 Primary

  based on contrast or high frequency components of image signals, e.g. hill climbing method · CPC title

• H04N23/70 Primary

  Circuitry for compensating brightness variation in the scene · CPC title

• H04N5/23212 Primary

  Electricity · mapped topic