WO2013023787A1

WO2013023787A1 - Determining the position of sub-apertures on a test object during surface measurements on the test object

Info

Publication number: WO2013023787A1
Application number: PCT/EP2012/003492
Authority: WO
Inventors: Frank Elsmann
Original assignee: Schott Ag
Priority date: 2011-08-17
Filing date: 2012-08-16
Publication date: 2013-02-21
Also published as: DE102011111542A1

Abstract

The aim of the invention is to determine the position of sub-apertures on a test object. This is achieved by carrying out the following steps: a) generating a data set from a sub-aperture measurement of the test object (1) using a measuring device (2), b) calculating the coordinates of at least one position of the test object (1) or the interferometer (2) in a first at least two-dimensional, preferably three-dimensional coordinate system, and c) adding the coordinates obtained in step b) to the data set obtained in step a). A step a) of detecting at least one first two-dimensional image of a first field of view is additionally carried out using a test object comprising a surface with first and second markers (3) or an interferometer (2) comprising a surface with first and second markers (3), said field of view covering the first and second markers (3). The calculation in step b) is carried out on the basis of the positions of the first and second markers (3) on the first images by means of triangulation. The position determining process can be advantageously used in a stitching method. The invention also relates to a device adapted to the position determining process.

Description

Determination of subapertures on a test specimen for surface measurements on the specimen

description

The invention relates to a method for determining the position of subapertures in surface and

Transmission measurements of optical electromagnetic waves on the test piece, in particular a large optical element, with an optical measuring device, in particular an interferometer. The invention further relates to the method adapted computer programs and devices. The invention is generally applicable to property determinations with raster-like measurement of patches and joining the measurement of patches of the test piece to an overall image in a stitching process. Property determinations in particular are refractive index homogeneity measurements by interferometry or voltage measurement.

Requirements for the homogeneity of large lens systems, as they are used for example in astronomy, are getting higher. The patent US 4,594,003 describes a Fizeau interferometer for testing optical components.

Homogeneity measurements of large lens systems with lens diameters over 800 mm, eg with lens diameters from 1 to 1.5 m, such state-of-the-art interferometers do not permit this.

The so-called stitching process is known from photography. An overall picture is composed of several single pictures. By means of this as stitching it is possible to determine individual large parts of an object and to represent an exact overall picture of this object.

WO 2007/023042 A1 shows raster-type interferometric measurements. This is done by measuring sub-apertures, from which a total interferogram is obtained in a stitching process. The grid-like measurement of a test specimen thereby harbors sources of error due to a rotation or displacement of the raster-like detected measuring ranges or measuring surfaces against each other. In order to avoid this, corresponding overlaps of the measuring surfaces were used.

By marking by means of two crosses or also several crosses, the crosses can be brought to coincidence by the grid-like detected measuring areas, in order to reconstruct an overall interferogram. However, this procedure has the disadvantage that measurements can not be carried out in the area of the markings. Without marking, z. B. by means of two or more crosses are weak contrasts in the overlapping areas sources of error. Calculations for a stitching process to reconstruct an overall interferogram are complex, laborious, and error prone.

In an alternative solution, a highly accurate

Transfer and positioning system of the specimen used to reconstruct a Gesamtferferogramm. However, this requires a high expenditure on equipment. Although a corresponding solution with a defined movement of the test piece with an exact positioning system can be sufficiently accurate, such techniques are very cost-intensive, especially when very large parts, e.g. with diameters in the range of 1.6 m with a typically necessary accuracy of <0, 1 mm must be moved. Because of a much larger space requirements also require corresponding systems for standard interferometers a massive conversion of the existing transfer systems. Each interferometer requires a special solution.

WO 2007/023042 A1 describes a position determination for subaperture measurements. In this case, first a data set is generated from a subaperture measurement of a test object with an interferometer, then coordinates of a position of the test object in a first coordinate system are calculated and then the data set from the subaperture measurement is expanded by the coordinates obtained in the previous step. Coordinates of a further position of the test specimen in a further Subaperture measurement in the first coordinate system due to overlaps of the measuring ranges of

Subaperture measurements determined.

The object of the invention is to overcome disadvantages of the prior art. A further object of the invention is to enable a grid-like surface measurement which is superior in terms of reliability, equipment complexity and speed, in particular a measurement for optical elements or specimens.

The invention therefore relates to a method and a device having the features of the independent claims. The invention therefore relates in particular the La ^¬ give humor of sub-apertures in a receiving box and a measurement box, comprising the steps of a) generating a data set from a

Subaperture measurement of the test object with a measuring device, b) calculation of the coordinates of at least one position of the test object or of the interferometer in a first coordinate system and, if appropriate, conversion of these coordinates into coordinates of a second coordinate system, c) extension of the data set obtained from step a) by those in step b ) obtained coordinates. A preferred measuring device is z. Eg an interferometer. Triangulation methods for position determination in a coordinate system are known per se. For example, Wieland, Th., "Investigation of a Linear Direct Method for Camera Calibration", Vision and Voice Magazine 6 (1992), No. 1, pp. 31-36, ISSN: 0936-8469 describes stereotriangulation methods with CCD cameras. The methods find application in the automated manufacture of vehicles where computers are controlled by a 3-D survey. In this case, measuring points are arranged at certain common prominent points on the vehicle blank. Such devices are offered for example by the Institute of Optical Precision Engineering (IOF) in Jena (Germany) under the name Kolibri or the company GOM under the name ATOS. An overview of such methods is provided by the University of Bochum, Department of Surveying and Geoinformatics, Lennershofstraße 140, 44801 Bochum, DE, in the article by Heinz-Jürgen Przybilla entitled

"Stripe Projection - Fundamentals, Systems and Applications", which is also available on the Internet at the Hochschule Bochum at the following address: http: / / www. college bochum. de / fileadmin / media / fb_v / labore / photogramme- trie / Article / Publications / Przybilla / Strip Project. pdf.

Corresponding triangulation methods, stereotriangulation methods or devices used therefor and computer programs are applicable to the present invention. Using triangulation methods, the invention provides

- that a candidate with a surface with first and second markings or as a specimen is used as the specimen

Interferometer an interferometer having a surface with first and second markings is used

- That in a further step n) at least a first two-dimensional image from a first especially optical recording field is created, which detects the first and second marks of the test piece and / or measuring device and

in that the calculation in step b) consists of positions of the first and second markings on the first

Images are done by triangulation.

According to the invention, it is preferable for the recording field to be larger than the measuring field in which, in particular, physical properties of the test object are recorded or measured. In addition, it is preferred that the markings are arranged outside the measuring field.

Interferometers as shown in US Pat. No. 4,594,003 are applicable to the invention.

In particular, a distance between the surface with first and second markings and the camera due to a distance between a first and a second mark in step n) recorded image are calculated.

While interferometer measurement systems with

Stitching techniques of the prior art provide for accurate positioning position determinations on high, the present invention allows a speed in the combination of reliability, equipment cost and speed Survival ^¬ gene grid-like surface measurement through the use of cameras and triangulation for position determination.

The system of the first and second mark on the test piece or measuring device (eg interferometer) can also be combined with markings on a test fixture of the test object. In this way it is possible to determine the position of the specimen absolutely to the position of the measuring fixture or the interferometer. The minimum requirements for the number of markings in the recording field is two, ^{vorzugswei ¬} se at least three, which are not in the optical measuring field of the measuring device in particular.

The invention can be used wherever features are determined by scanning small areas and then assembling the partial results.

As a test specimen, a test specimen having a surface with first and second markings and / or as Interferometer an interferometer can be used with a surface with third and fourth markers or can also be an interferometer with a surface with first and second marks and a test piece with a surface with third and fourth marks are used.

In the first recording field, the third and fourth markings are detected, the method preferably further comprising the steps of: a '') detecting a second two-dimensional image from a second recording field which detects the third and fourth marks, wherein first and second recording sheets overlap, b ¹ ) calculation of the coordinates of at least one position of the third and fourth marks having surface in the second three-dimensional coordinate system by stereotriangulation, wherein in step b) a conversion of the coordinates in coordinates of the second three-dimensional coordinate system or a step b ^{1 1} ) the conversion of the coordinates calculated in step b ¹ ) into coordinates of the first three-dimensional coordinate system and the replacement of the coordinates obtained in step c) in the extended data set by these converted coordinates takes place. Raster-type measurements according to the invention preferably also include the steps d) output of the extended data record to a data processing unit,

e) displacing the specimen or the interferometer,

f) repeating steps a) to d) and optionally at least one repetition of steps e) and f).

It is also possible to displace the device under test and the interferometer, whereby position determinations of the device under test and the interferometer are provided by stereotriangulation.

In the grid-like measurements according to the invention, steps n) and n ") can be carried out with a camera, and the camera is aligned with the first recording field for step n). A reorientation of the camera to the second recording field or

Step n) and n '') take place with one camera each, preferably simultaneously.

In the grid-like surveys according to the invention, the first coordinate system is a three-dimensional coordinate system and the calculation in step b) is performed from positions of the first and second markings on the first and second images by stereotriangulation. The invention is particularly suitable for a test specimen in the form of an optical window, in particular with a diameter of more than 800 mm, preferably more than 1000 mm, in particular preferably between 1 and 1.5 m.

In this case, markings are preferably arranged in an edge region of a surface of the test object or optical element, which is cut through the optical axis of the lens, preferably an edge region centered on the optical axis, which does not exceed 10%, in particular not more than 5% thereof Surface occupies. In particular, areas are used for markings that are no longer needed in the lens in use and are often covered anyway by a lens mount.

Preferably, the markings on the outer surfaces of the specimen, which are not optically effective, attached.

According to the invention, a simple and reliable stitching method is provided. In this case, the generation of an overall interferogram is carried out by arranging subinterferograms from the extended data records of a method according to the invention in accordance with the coordinates of these extended data records obtained in step b) or b ¹ ) of the method. The total interferogram is thus reconstructed in a coordinate system from the sub-interferograms on the basis of positions of the measured subapertures in this coordinate system.

The invention also relates to a computer program, we ches control commands and algorithms for performing the method according to the invention contains d. Preferably, the method is automatically performed by the program durc.

The invention also relates to a

Subaperturmessvorrichtung which has a Datenverarbei ^¬ processing unit having such a computer program up. The device comprises

an interferometer with a beam path in which a measuring range of the interferometer is arranged,

a holder for arranging a test object in at least two positions in which the test object overlaps the measuring range of the interferometer,

- actuating means which are connected ferometer with the bracket or the intervention, for adjusting one of the at least two positions of the specimen, and at least one camera having a first Aufnahmebe ^¬ rich in a first camera position and a second recording area in a second Kamerapo ^¬ sition, the first and second reception areas detect markings on the specimen and / or the interferometer, and positioning means which are connected to the camera, for moving the camera from the first camera position to the second camera position or at least a first camera with a first recording ^¬ range and at least one second camera with a second receiving area, wherein the first and second receiving areas detect markings on the test piece and / or the interferometer.

The invention is used in principle in all the measuring systems in which the entire measurement field of a test ^¬ astride generated by measurement of particular physical properties and assembling faces. It is basically irrelevant whether the measuring system or the test object is moved. Conceivable are homogeneity measuring devices, stress birefringence measuring devices, defect detection systems or the like.

Typical physical properties are in particular optical properties such. B.

Surface homogeneities, refractive indices, stresses, in particular material tensions, as well as deviations and defects. Especially suitable is the method and the device for determining the refractive index homogeneity by means of interferometry and for refractive index measurement. In this case, a Gesamtinterferogramm is generated from the data sets of step c), in particular using the device according to the invention. The method according to the invention and the device are particularly suitable for measuring the refractive index homogeneity by interferometry or for voltage measurement.

The invention will be described below with reference to FIGS

Drawing illustrated by an embodiment which shows further aspects and advantages of the invention.

1 shows schematically an arrangement of interferometers, cameras, markings and test specimen according to a device according to the invention.

A subaperture measuring device according to the arrangement shown in FIG. 1 comprises an interferometer 2 with a beam path in which a measuring range 7 of the interferometer is arranged. The beam path passes through a socket 8 to a test piece 1 arranged in the beam path. As an example, an optical window is shown as the test piece 1.

The specimen 1 itself is clamped in a holder, not shown in the figure 1. Also not shown in Figure 1 adjusting means, which are connected to the holder, allow movement of the specimen in a plane orthogonal to the beam path. Parallel to this plane, the interferometer 2 has two markings 3. The adjusting means include a first linear motor directly connected to one end of the sample holder and a second linear motor having a direction of movement orthogonal to that of the first linear motor and connected to the first linear motor. First and second linear motor thus enable a controlled two-dimensional movement of the DUT holder.

The test specimen has in an edge region on the side facing the cameras surface two cruciform Markie ^¬ stanchions. 3

The subaperture measuring device further comprises a data processing unit, not shown in FIG. 1, and two cameras 5, which detect recording fields through their lenses 6. Each camera 5 has a recording field, within the limits of which 4 two markings 3 of the test object and at least two markings 3 of the interferometer 2 are detected. Both receptors capture two equal marks on the interferometer. The cameras are fixed relative to each other and firmly connected. They are attached to a tripod and aligned relative to the interferometer and the specimen in accordance with the above-described features of their receiving screens. You can also focus freehand in a fast shooting mode according to the features described above. The data processing unit in the form of a PC communicates via interfaces with the cameras, the linear motors and the interferometer. For outputting control signals and processing measurement signals via the interfaces, the data processing unit is equipped with a computer program for executing the following sequence of steps:

Sl: detecting at least one first zweidimensiona ^¬ le) len image from the photographic field of the cameras,

S2: acquiring a second two-dimensional image from the recording field of the other camera,

S3: generating a data record from a

Subaperture measurement of the specimen with the interferometer,

S: calculation of the coordinates of at least one position of the test object in a coordinate system,

S5: calculating the coordinates of at least one position of the surface having the third and fourth marks in a second three-dimensional coordinate system by stereotriangulation,

S6: extension of the data record obtained from step S3 by the coordinates obtained in step S4,

S7: conversion of the coordinates calculated in step S5 into coordinates of the first three-dimensional coordinate system,

S8: replacing the coordinates obtained in step S4 in the extended data set by the converted coordinates obtained in step S7, S9: storing the extended data record,

SlO: move the test object,

Sil: Repeat steps S1 to S9 and

S12: repetition of the steps S0 and S5 until data sets for a complete interferogram of the measured surface of the test object are obtained;

S13: Generation of a total interferogram from the extended data records by arranging

Subinterferograms from the extended data sets according to converted coordinates obtained in step S7 in these data sets,

S14: Output of the total interferogram to a screen.

Since suitable markings are applied at suitable points of the measuring device and on the test object (on the side edge or in the area outside the optically effective surface) and a 3D coordinate measuring system is used via two special cameras, the position of the test object can be absolutely determined by the markings Interferometer to avoid disadvantages of the prior art. For every

Subaperture measurement can be determined by an exact position. In this case, the test piece or interferometer can be rotated or moved. The position of the subapertures to each other can be passed to appropriate computer programs for stitching or measuring. By suitable positioning of the markings no measuring field is shaded. The

Positioning accuracy of 3D measuring systems is <0,1 mm, preferably <0, 05 mm and especially 0, 001 mm. Such cameras are usually calibrated on fixed markers. If you apply markings directly to the interferometer, the cameras can be absolutely calibrated in the room. The location of the cameras is freely selectable, as far as enough marks in the

Field of view. This allows this system to be used universally on any interferometer. It is adjusted to a suitable location of the measuring marks and the cameras.

It is also projected marks used ^¬ to.

There is a high positioning accuracy, z. B. <0.1 mm, preferably <0.05 mm and especially 0.001 mm, possible by a simple construction with a small footprint with little retooling of existing measuring systems.

* * * LIST OF REFERENCE NUMBERS

1 test object

2 interferometers

3 marks

4 boundaries of the recording field

5 camera

6 camera lens

7 Measuring range of the interferometer

8 version

* * *

Claims

Method for determining the position of subapertures in a measuring field on a test object, comprising the steps of a) generating a data record from a

Subaperture measurement of the test specimen (1) with a measuring device (2), b) calculation of the coordinates of at least one position of the specimen (1) or the measuring device (2) in a first at least two-dimensional, preferably three-dimensional coordinate system and optionally conversion of these coordinates into coordinates of a second Coordinate system,

c) extension of the data set obtained from step a) by the coordinates obtained in step b), characterized in that

the test object (1) has a surface with first and second markings (3) and / or the measuring device (2) has a surface with first and second markings (3), and

in that a further step n) creates at least one first two-dimensional image from a first recording field which detects the first and second markings (3), and that the calculation in step b) comprises positions of the first and second markings (3) on the first Images are done by triangulation. A method according to claim 1, characterized in that the subapertures include a Aufnähmeteld for determining the position of the specimens and a measuring field for determining or measuring physical properties of the specimen, wherein the receiving field is greater than the measuring field and wherein the markers (3). but are arranged outside of the measuring field in the recording field.

A method according to claim 1 or 2, characterized in that both the test piece (1) and the measuring device (2) each have two markings (3), so that third and fourth marks (3) are present, and that continue in the first recording field the third and fourth markings (3) are detected and the method further comprises the steps of: a ' ¹ ) detecting a second two-dimensional image from a second capture field that captures the third and fourth markers (3), with first and second capture layers overlapping , b ') calculating the coordinates of at least one position of the surface having the third and fourth markings (3) in the second three-dimensional coordinate system by stereotriangulation, wherein in step b) the coordinates are converted into coordinates of the second three-dimensional coordinate system or a step b'') the conversion of the coordinates calculated in step b ') into coordinates of the first three-dimensional coordinate system and the replacement of the coordinates obtained in step c) in the extended data set by these converted coordinates takes place.

The method of claim 1 to 3, comprising the additional steps

d) output of the extended data record to a data processing unit,

e) displacing the test piece (1) and / or the interferometer (2),

Method according to one of claims 4 to 6, wherein step n) and n ¹ ') are performed with a camera (5) and the camera (5) is aligned for step a) on the first recording field, the method further comprising a step a ') Realignment of the camera (5) on the second recording field

includes.

Method according to one of claims 4 to 6, wherein step n) and n ''), each with a camera (5), preferably be carried out simultaneously.

7. The method according to any one of claims 4 to 8, wherein the first coordinate system is a three-dimensional coordinate system and the calculation in step b) from positions of the first and second ^{Markierun ¬} conditions (3) on the first and second images by stereotriangulation.

8. The method according to any one of the preceding claims, wherein the test specimen (1) is an optical element, in particular with a diameter of about 800 mm, preferably over 1000 mm, in particular ^{vorzugswei ¬} se between 1 and 1.5 m.

9. The method according to claim 10, characterized in that markings (3) of a surface of the test object, which is cut through the optical axis of the test object, are arranged in an edge region, preferably an edge centered on the optical axis, which is not more than 10 %, in particular not more than 5% of this surface.

10. Subaperture measuring device, comprising - an interferometer (2) with a beam path in which a measuring range (7) of the interferometer (2) is arranged,

a holder for arranging a test piece (1) in at least two positions in which the test piece overlaps the measuring range of the interferometer (2),

- Adjusting means with the bracket or the

Interferometer (2) are connected, for adjusting one of the at least two positions of the specimen, and

a data processing unit, characterized in that the data processing unit comprises a computer program which comprises control commands and algorithms for carrying out the method according to claim 1 and the subaperture measuring device comprises at least one camera (5) having a first receiving area in a first camera position and a second receiving area in a second Camera position, wherein the first and second recording areas detect marks (3) on the test piece (1) and / or the interferometer (2), and

Positioning means, which are connected to the camera (5), for moving the camera (5) from the first camera position to the second camera position or the at least one first camera (5) with a first receiving area and at least one second camera (5) with a second receiving area, wherein the first and second receiving areas detect markings (3) on the test piece (1) and / or the interferometer (2).