# US 10,120,163 B2 · Auto-focus method for a coordinate-measuring apparatus

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- Number: US 10,120,163 B2
- Document ID: US10120163B2
- Status: Granted
- Kind: B2
- Application number: 15/442,662
- Filed: 2017-02-25
- Published: 2017-08-31
- Granted: 2018-11-06
- Earliest claimed: 2016-02-25
- Assignees: Carl Zeiss Industrielle Messtechnik GmbH
- Applicants: Carl Zeiss Industrielle Messtechnik GmbH
- Inventors: Dominik Seitz; Nils Haverkamp; Christoph Kranz
- Web page: https://patentgrep.com/patent/US10120163B2

## CPC

- G02B 7/38: measured at different points on the optical axis {, e.g. focussing on two or more planes and comparing image data}
- G01B 11/0608: {Height gauges}
- G02B 7/282: {Autofocusing of zoom lenses}
- G06T 7/0004: {Industrial image inspection}
- G06T 7/571: from focus
- H04N 23/56: provided with illuminating means
- H04N 23/673: {based on contrast or high frequency components of image signals, e.g. hill climbing method}
- G01B 11/005: {coordinate measuring machines}
- G06T 2207/10016: Video; Image sequence
- G06T 2207/10148: Varying focus
- G06T 2207/30164: Workpiece; Machine component
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G06T: IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
- H04N: PICTORIAL COMMUNICATION, e.g. TELEVISION

## Abstract

A method is disclosed for ascertaining a focus image distance of an optical sensor, which is provided with a lens, of a coordinate-measuring apparatus onto a workpiece to be measured, wherein the optical sensor and/or the workpiece are movable in a Z direction such that a distance in the Z direction between the workpiece and the optical sensor is variable. Further, a corresponding coordinate-measuring apparatus and a computer program product are disclosed.

## Claims

### Claim 1

A method for ascertaining a focus image distance of an optical sensor of a coordinate-measuring apparatus with respect to a surface of a workpiece to be measured, the optical sensor being provided with a lens, at least one of the sensor and the workpiece being movable in a Z direction such that a distance in the Z direction between the workpiece and the optical sensor is variable, the method comprising a determination step and an approximation step;
the determination step including:
setting a first focal distance and a second focal distance;
recording a first image of a surface of the workpiece with the optical sensor;
assessing an intensity of the recorded first image of the surface of the workpiece at the first focal distance;
recording a second image of the surface of the workpiece with the optical sensor;
assessing the intensity of the second image of the surface of the workpiece at the second focal distance to determine therefrom a most probable location of a best focus in relation to the first focal distance and to the second focal distance; and
the approximation step including:
setting at least three different focal distances in a proximity of the most probable location of the best focus; and,
approximately determining a location of the best focus based on the intensities of the recorded images.

### Claim 2 (depends on 1)

The method of claim 1, further comprising a contrast evaluation step, the contrast evaluation step including:
determining the location of the best focus based on contrast criteria.

### Claim 3 (depends on 2)

The method of claim 2, comprising the determination step, the approximation step and the contrast evaluation step.

### Claim 4 (depends on 3)

The method of claim 3, wherein the approximation step is performed after the determination step.

### Claim 5 (depends on 3)

The method of claim 3, wherein the contrast evaluation step is performed after the approximation step.

### Claim 6 (depends on 1)

The method of claim 1, further comprising selecting a first difference distance between the first focal distance and the second focal distance in the determination step in accordance with
DAa=a*DOF,
with DAa being the difference distance, a being a factor, and DOF being a depth of field of the lens.

### Claim 7 (depends on 6)

The method of claim 6, wherein the factor a is in a range of 1<a<5.

### Claim 8 (depends on 6)

The method of claim 6, wherein the factor a is in a range of 2<a<4.

### Claim 9 (depends on 6)

The method of claim 6, wherein the factor a is 3.

### Claim 10 (depends on 1)

The method of claim 1, further comprising selecting a second difference distance between the at least three focal distances in the approximation step in accordance with
DAb=b*DOF,
with DAb being the largest possible difference distance between the at least three focal distances, b being a factor, and DOF being a depth of field of the lens.

### Claim 11 (depends on 10)

The method of claim 10, wherein the factor b is in a range of 0.25<b<2.

### Claim 12 (depends on 10)

The method of claim 10, wherein the factor b is in a range of 0.25<b<1.25.

### Claim 13 (depends on 10)

The method of claim 10, wherein the factor b is 1.

### Claim 14 (depends on 1)

The method of claim 1, wherein:
the lens is a zoom lens, and
the zoom lens is telecentric on both sides.

### Claim 15 (depends on 6)

The method of claim 6, wherein the depth of field of the lens is determined pursuant to
DOF
=
λ
(
NA
·
V
)
2
wherein DOF is the depth of field of the lens, V is the magnification of the lens, in particular an axial magnification, NA is the image-side numerical aperture at the magnification, and λ is a reference wavelength.

### Claim 16 (depends on 15)

The method of claim 15, wherein the reference wavelength is in a range of 400 nm≤λ≤650 nm.

### Claim 17 (depends on 16)

The method of claim 16, wherein the reference wavelength is 550 nm.

### Claim 18 (depends on 1)

A coordinate-measuring apparatus comprising:
an optical sensor; and
a control device configured for focusing the optical sensor, wherein the control device is configured for carrying out the method of claim 1.

### Claim 19 (depends on 18)

The coordinate-measuring apparatus of claim 18, further comprising:
an illumination system for generating a bright-field incident light illumination; and,
wherein at least parts of the lens are configured to be a part of the illumination system for subjecting the surface to be measured to illumination light.

### Claim 20

The coordinate-measuring apparatus of 19, wherein:
the illumination system emits light with wavelengths λ in a range of 400 nm≤λ≤650 nm, and
for at least one wavelength λ in said range for the illumination system for generating the bright-field incident light illumination on the surface to be measured, the lens has a same location of the best focus as for the imaging of the surface to be measured on the sensor.

### Claim 21 (depends on 1)

A non-transitory computer readable storage medium containing a computer program which has a program code for carrying out the method of claim 1, if the computer program is executed on a coordinate-measuring apparatus.