CN111065884A - Method and device for optical surface measurement by means of a confocal sensor - Google Patents
Method and device for optical surface measurement by means of a confocal sensor Download PDFInfo
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- CN111065884A CN111065884A CN201880057005.4A CN201880057005A CN111065884A CN 111065884 A CN111065884 A CN 111065884A CN 201880057005 A CN201880057005 A CN 201880057005A CN 111065884 A CN111065884 A CN 111065884A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/24—Measuring arrangements characterised by the use of optical techniques for measuring contours or curvatures
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
- G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
- G01B11/0608—Height gauges
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Abstract
The invention relates to a method and a device for optically measuring technical surfaces by means of a confocal sensor, wherein the light of a light source (2) is directed at the surface (8) of a sample to be measured by means of an optical system (5, 16). According to the invention, the optical system (5, 16) comprises an electrically actuated adaptive optical system (7), wherein the focal point of the optical system (5, 16) is changed in the Z direction by an electrical signal (f (t)). The light reflected by the sample surface (8) is deflected onto at least one photoelectric sensor (10), wherein the sensor signal is measured in time by means of a detection device (11) and the time point of the signal maximum is determined. The detection device (11) derives the height Z of the surface (8) from the electrical signal at the time of the signal maximum.
Description
Technical Field
The invention relates to a method for optically measuring a technical surface by means of a confocal sensor, wherein the light of at least one light source is directed by means of an optical system at the surface of a sample to be measured.
The invention also relates to a confocal sensor for carrying out the method, having at least one light source whose light is directed by means of an optical system at the surface of the sample to be measured.
Background
In confocal measurement techniques, the light of a light source is usually focused on the surface to be measured by means of a confocal filter, a beam splitter and an optical system. In the prior art, either the measuring table (on which the sample is located) or the optical system is moved up and down in the Z direction and the evaluation is carried out at precisely the following times: at this point in time, the focal point impinges on the surface to be measured. This light is directed to the corresponding sensor through a confocal filter (e.g., an aperture plate). The sensor shows the maximum signal when the surface is exactly in focus. The exact Z-height of the surface can thus be determined.
This type of method is not suitable for providing a high measurement rate due to the inertia of the mass to be moved.
Therefore, in the course of further development, the fact that the mechanical components are always still functional, although scanning methods have been further developed, also imposes limitations on this method.
Such mechanical elements can be omitted in a method using a chromatic confocal sensor. A broadband spectrum of the light source (for example white light) is introduced via an optical system with a defined Dispersion (Dispersion) onto the sample surface. Due to the dispersion, chromatic longitudinal aberrations occur, whereby each "light color" can correspond to a defined Z position on the sample surface and thus the surface structure of the sample can be determined. I.e. no mechanical scanning in the Z-direction is required anymore.
In chromatic confocal sensors, the exact Z position (i.e. the surface structure) of the sample surface is conventionally determined by means of a spectrometer. The light reflected by the sample is analyzed spectroscopically, the dominant wavelength corresponding to the Z position of the sample. The spectrometer row used is capable of readout at multiple kHz data rates. Thereby enabling a fast color confocal sensor. However, the readout speed of the spectrometer line has its limits in the range of a few kHz and cannot easily be further improved.
Disclosure of Invention
The invention is therefore based on the task of: a method of the type mentioned at the outset is further developed, so that high measurement rates can be achieved.
The invention proceeds from a method of the type described at the outset in that the object is achieved by: the optical system comprises an adaptive optical system which is actuated electrically, wherein the focal point of the optical system is changed in the Z direction by an electrical signal and the light reflected back by the sample surface is deflected onto at least one photoelectric sensor, wherein the sensor signal is measured over time and the time and intensity of the signal maximum are determined and evaluated, wherein the height Z of the surface is derived from the electrical signal at the time of the signal maximum.
In particular, acousto-optic lenses, i.e. adjustable Acoustic Gradient index (TAG) lenses (see "High-speed variable imaging with a movable Acoustic Gradient index of diffraction lenses" by a. memrilod-Blondin, e.mcleod and c.b. arnold, op. lett.33, Band 18, pages 2146 to 2148, 2008) are suitable as adaptive optical systems according to the invention. Such TAG lenses consist of a liquid-filled, cylindrical cavity which is excited in the radial direction with acoustic energy. This causes a periodic modulation in the liquid, the refractive index changing accordingly, and the lens changing its focal length continuously periodically, more precisely at a frequency in the kHz to MHz range. The realisation of an optical system with a rapidly variable focal position along the optical axis is illustrated in the above-cited prior art.
However, high-speed focusing of this type by means of adaptive optical systems has not hitherto been used for rapid measurement of the surface structure of technical surfaces by means of confocal sensors.
According to the invention, a TAG lens can be used in the optical system of a confocal sensor, which TAG lens consists of a cylindrical piezoelectric body as a cavity, which cavity is filled with a liquid. The piezoelectric body is applied with an electric signal, and then, the focal position of the optical system is changed in the Z direction. For this purpose, the electrical signal can be generated by a function generator of a known type.
The following adaptive optical systems are generally applicable according to the invention: the adaptive optical system comprises at least one modulating element which, for changing the focus, converts an electrical signal into a change in the refractive index of the optically transmissive material of the modulating element by using the acousto-optic effect. Such a modulating element can be a TAG lens of the type previously described.
Due to the use of the adaptive optical system described, a purely optical "scan" in the Z-direction occurs between the optical system and the sample surface. The light falling on the sample surface is focused and reflected back longitudinally in the Z region and falls in the simplest case on a fast photodiode as a photosensor, by means of which the signal maximum is determined, wherein the time dependence of the focal position in the Z direction of the adaptive optical system is synchronized with the electronic detection device used, so that the temporal profile of the electrical signal (which is manipulated by the adaptive optical system) determines the focal position in the signal maximum and thus the height Z of the sample is deduced.
The electrical signal can be a high-frequency alternating voltage in the range from 1kHz to 10MHz, preferably from 10kHz to 1MHz, particularly preferably from 50kHz to 200kHz, with which the acousto-optic lens is loaded. The focal position is changed in the Z direction periodically and rapidly between the maximum and minimum values accordingly. Thus, the measurement time for each point on the sample surface is less than 1 microsecond.
The photoelectric sensor can be configured as a point sensor. However, according to the invention, provision is also made for: the light of the light source is split into a plurality of sub-beams and a multichannel sensor, for example a (row-like or matrix-like) array of photodiodes, is used as a detector.
It is likewise possible to use a plurality of light sources, wherein the single light beam reflected back by the light sources is detected in parallel by means of corresponding multi-channel photosensors.
The detection of the surface structure of the sample can be accelerated further by parallelization in such a way that a single beam simultaneously scans a plurality of mutually spaced points on the sample surface.
In case the aperture of the modulation element is sufficient, a unique modulation element (e.g. a unique TAG lens) may also be sufficient when performing parallel measurements with multiple sub-beams or a single beam. Alternatively, each of the sub-beams or the single beam can correspond to a separate modulation element (for example in the form of a plurality of TAG lenses arranged side by side).
In one possible embodiment of the method according to the invention, the detection device used for evaluating the temporally variable signal of the photosensor has an extremum value memory which follows the temporally variable signal until an extremum of the signal is reached, wherein, when the extremum is reached, a peak indicator signal is generated in each case, the time of the extremum is determined from the peak indicator signal and, in turn, the focal position in the Z direction corresponding to the maximum value of the signal is determined from the peak indicator signal. If the time profile of the signal has a plurality of (local) extrema, the (absolute) signal maximum corresponds to the peak indicator signal that was last generated during the period of change of the focal position. By means of this method, a plurality of (local) signal maxima can be detected, for example for determining the layer thickness distribution of a coating on the surface of a sample by the method according to the invention.
The invention is based on a confocal sensor of the type described in the opening paragraph, whereby the above-mentioned object is solved: the optical system comprises an electrically actuated adaptive optical system, wherein the focal point of the optical system is changed in the Z direction by an electrical signal of a function generator and the light reflected back by the sample surface is diverted onto at least one photoelectric sensor, wherein the sensor signal is measured temporally by means of a detection device and the time of the signal maximum is determined, wherein the detection device is designed to derive the height Z of the surface from the electrical signal at the time of the signal maximum.
The light of the light source is directed at the optical system after passing through a confocal filter (aperture plate/"Pinhole", Pinhole "), for example by means of a semi-permeable mirror or a beam splitter cube. The light reflected back by the optical system passes through a semi-permeable mirror onto the photosensor, wherein only the light that is important for the measurement is allowed to pass due to a further confocal filter connected upstream of the sensor. With such an arrangement, when the light of the light source is focused on the surface of the specimen based on the instantaneous focal position of the adaptive optical system, the light on the sensor becomes maximum. In the case where the focal position is periodically changed, the sensor signal exhibits a typical signal peak (confocal peak). Given that the focal position (which corresponds to the electrical signal at the relevant point in time) is known, the height of the sample at the respective measurement location can be determined from the point in time of the occurrence of the maximum value of this signal.
One possibility is to implement such a device optically integrated. Here, the light source, the photosensor, and the optical system are connected to each other by optical fibers.
It is particularly advantageous for the invention to use a laser as the light source. However, various other light sources are basically suitable for the method. In order to increase the measurement accuracy, light sources that are spectrally as narrow-band as possible should be used in order to minimize the measurement errors based on the unavoidable color aberrations of the optical system.
Extremely high measurement rates can be achieved when using the TAG lens described above. If necessary, more than 100000 (3D) measurement points per second and per measurement channel are reached. During the measurement of the surface structure, the sample to be examined is moved relative to the confocal sensor in the X/Y direction (i.e. transversely to the direction of the light beam directed at the sample surface) relative to the optical system, so that the surface is scanned in a raster-like manner here. An X/Y adjustment device of known type can be used for said movement. It is likewise possible to use a controllable deflection device for deflecting the light beam directed at the sample surface, in such a way that the sample surface is raster-like scanned. Suitable deflection devices, for example, which work together with movable mirrors, are known from the prior art.
In order to be able to evaluate extremely high measurement data rates and the signals associated therewith correspondingly quickly, a detection device of the type specified above is preferably used, which electronically evaluates the sensor signals, wherein the detection device has an extreme value memory, which follows the temporally variable signals until an extreme value of the signals is reached, wherein, when the extreme value is reached, in each case a peak indicator signal is generated, the time of the extreme value is determined from the peak indicator signal and, in turn, the instantaneous focus position of the optical system corresponding to the maximum value of the signal is determined from the peak indicator signal. The principle of operation of the detection device in the determination of the maximum value of the signal is described in patent application DE 102016100261, to which reference is made in its entirety.
Multiple signal evaluations can be carried out simultaneously in parallel, and the respective time signal can be detected in a multichannel manner and the measured maximum can be evaluated in a multichannel manner.
The high measurement rates achievable open up new fields of use for confocal measurement techniques. Surface inspection is carried out in the production process, in which the test specimen is moved at high feed speeds (for example, rolling of plates, drawing of films).
As in conventional use of measurement technology, the thickness of thin, transparent test specimens or transparent coatings can also be checked by means of the rapid embodiment according to the invention, provided that the upper and lower sides of the film/layer are located in the measurement region of the sensor. In this case, the light reflected on the sample becomes maximum at two different focal positions. The layer thickness can be deduced from the spatial separation of the focal positions.
Naturally, this is not a complete listing of fields of use.
Drawings
Next, embodiments of the present invention are explained in more detail based on the drawings. The figures show:
fig. 1 is a schematic illustration of a sensor arrangement according to the present invention.
Fig. 2 is a schematic illustration of a sensor arrangement according to the present invention in an optical fiber based embodiment.
Detailed Description
In fig. 1, a confocal sensor is shown and is provided with the reference numeral 1 in common. An important component of such a confocal sensor 1 is on the one hand a light source, preferably a suitable laser provided with reference numeral 2. The laser 2 sends its light through a confocal filter (pinhole) 3 via a beam splitter 4 (in the present embodiment, the beam splitter is a semi-transparent mirror) onto an optical system 5 consisting of an objective lens 6 and a TAG lens 7. The TAG lens is loaded by means of an electrical signal f (t) generated by the function generator 18. This results in: in the optically transmissive material of the TAG lens 7, the radial profile of the refractive index n corresponding to the electrical signal is a function of time due to the acousto-optic effect. The light is therefore focused and directed at the different heights in the Z direction in accordance with the signal f (t) at the drawn specimen 8. The focal position is preferably periodically in zminAnd zmaxSo that the focal spot can be scanned in the Z direction in rapid sequence. Alternatively, direct focusing by means of the TAG lens 7 alone is also possible, that is to say without the objective lens 6.
The light is reflected back by the surface, guided through the optical system 5, the semi-permeable mirror 4 through the further confocal filter (pinhole) 9 onto the photosensor 10 (which can be a single photodiode), the measured signal i (t) of which is detected and evaluated in time by means of the detection means 11. Resulting in an intensity distribution as shown in 11.
From the electrical signals f (t), exactly the same focal position in the Z direction is determined by means of the detection device 11: the focal position belongs to the signal maximum of the intensity curve. The height of the sample surface is derived therefrom. In order to increase the accuracy and improve the linearity, the correlation between the electrical signal f (t) and the focal position can additionally be calibrated.
Fig. 2 shows an optical fiber based variant 12 of the sensor arrangement according to the invention. Parts corresponding to each other are denoted by the same reference numerals as in fig. 1. The light source 2 is connected at its output via an optical fiber 13 to a fiber coupler 14. The fiber coupler is coupled to a measuring head 16, which includes an adaptive optical system in the form of a TAG lens 7 and an objective lens 6, by means of a further fiber section 15. The fiber coupler 14 is coupled via a further fiber section 17 to the photoelectric sensor 10, which thus receives the reflected light on the sample 8. The necessary decoupling and coupling optics for decoupling light from the fiber section 15 or for coupling light into the fiber section 15 are not shown.
Claims (21)
1. A method for the optical measurement of technical surfaces by means of a confocal sensor, in which the light of at least one light source is directed by means of an optical system at the surface of a sample to be measured,
the optical system comprises an electrically actuated adaptive optical system, wherein the focal point of the optical system is changed in the Z direction by an electrical signal and the light reflected back by the sample surface is deflected onto at least one photoelectric sensor, wherein the sensor signal is measured over time and the time and intensity of the signal maximum is determined and evaluated, wherein the height Z of the surface is derived from the electrical signal at the time of the signal maximum.
2. The method according to claim 1, characterized in that the adaptive optical system comprises at least one modulating element which, for changing the focus, converts the electrical signal into a refractive index change of a light-transmissive material of the modulating element by using an acousto-optic effect.
3. The method according to claim 2, characterized in that the modulating element comprises an acousto-optic lens, wherein the electrical signal is a high-frequency alternating voltage in the range of 1kHz to 10MHz, preferably 10kHz to 1MHz, particularly preferably 50kHz to 200kHz, with which alternating voltage the acousto-optic lens is loaded.
4. The method of claim 3, wherein the acousto-optic lens is a TAG lens.
5. Method according to any one of claims 1 to 4, characterized in that the light of the light source is split into a plurality of sub-beams, wherein the reflected sub-beams are detected in parallel by means of a multi-channel photosensor.
6. Method according to one of claims 1 to 4, characterized in that a plurality of light sources are used, wherein the reflected back single light beam of the light sources is detected in parallel by means of a multichannel photosensor.
7. Method according to claim 5 or 6, wherein each of said sub-beams or single beams respectively corresponds to a modulating element.
8. Method according to one of claims 1 to 7, characterized in that for determining the signal maximum, a temporally variable signal of the photosensor is analyzed electronically by means of a detection device, wherein the detection device has an extremum value memory which follows the temporally variable signal in each case until an extremum of the signal is reached, wherein, when the extremum is reached, in each case a peak indicator signal is generated, the time of the extremum is determined from the peak indicator signal and, in turn, a focal position in the Z direction corresponding to the signal maximum is determined from the peak indicator signal.
9. Confocal sensor for performing the method according to one of claims 1 to 6, having at least one light source (2) whose light is directed through an optical system (5, 16) at the surface of a sample (8) to be measured,
the optical system (5, 16) comprises an adaptive optical system (7) which is operated electrically, wherein the focal point of the optical system (5, 16) is changed in the Z direction by an electrical signal (f (t)) of a function generator (18) and the light reflected back by the sample surface (8) is deflected onto at least one photoelectric sensor (10), wherein the sensor signal is measured in time by means of a detection device (11) and the time of the signal maximum is determined, wherein the detection device (11) is designed to derive the height Z of the surface (5) from the electrical signal at the time of the signal maximum.
10. The confocal sensor of claim 9, wherein the photosensor (10) is a photodiode.
11. The confocal sensor according to claim 9 or 10, characterized in that the light of the light source (2) is split into a plurality of sub-beams by means of a beam splitter, wherein the reflected sub-beams are detected in parallel by means of a multichannel photosensor (10).
12. The confocal sensor according to claim 9 or 10, characterized in that a plurality of light sources (2) are provided, wherein the reflected back single light beam of the light sources (2) is detected in parallel by means of a multichannel photosensor (10).
13. The confocal sensor of any one of claims 9 to 12, wherein the light of the light source (2) is directed at the optical system (5, 16) by a beam splitter (4).
14. The confocal sensor of claim 13, wherein the beam splitter (4) is a semi-transparent mirror or a beam splitter cube.
15. The confocal sensor according to any one of claims 9 to 14, characterized in that a confocal filter (9) is connected upstream of the photosensor (10) and/or a confocal filter (3) is connected downstream of the light source (2).
16. The confocal sensor of any one of claims 9 to 15, wherein the light source (2), the photosensor (10) and the optical system (16) are interconnected by a fiber coupler (14) through optical fibers (13, 15, 17).
17. The confocal sensor of any one of claims 9 to 16, wherein the light source (2) is a laser.
18. The confocal sensor according to any one of claims 9 to 17, characterized in that the adaptive optical system (7) comprises at least one modulating element which, for changing the focus, converts the electrical signal (f (t)) into a refractive index change of the optically transmissive material of the modulating element by using the acousto-optic effect.
19. The confocal sensor according to claim 18, wherein the modulation element comprises an acousto-optic lens, wherein the electrical signal of the function generator (18) is a high frequency alternating voltage in the range of 1kHz to 10MHz, preferably 10kHz to 1MHz, particularly preferably 50kHz to 200 kHz.
20. The confocal sensor of claim 19, wherein the acousto-optic lens is a TAG lens.
21. The confocal sensor of claim 11 or 12 and any one of claims 18 to 20, wherein each of the sub-beams or single beams corresponds to a modulating element.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE102017116993 | 2017-07-27 | ||
| DE102017116993.8 | 2017-07-27 | ||
| DE102017130211.5A DE102017130211A1 (en) | 2017-07-27 | 2017-12-15 | Method and device for optical surface measurement with the aid of a confocal sensor |
| DE102017130211.5 | 2017-12-15 | ||
| PCT/EP2018/070439 WO2019020800A1 (en) | 2017-07-27 | 2018-07-27 | Method and device for optical surface measurement by means of a confocal sensor |
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| CN111065884A true CN111065884A (en) | 2020-04-24 |
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| CN201880057005.4A Pending CN111065884A (en) | 2017-07-27 | 2018-07-27 | Method and device for optical surface measurement by means of a confocal sensor |
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| Country | Link |
|---|---|
| EP (1) | EP3658850A1 (en) |
| CN (1) | CN111065884A (en) |
| DE (1) | DE102017130211A1 (en) |
| WO (1) | WO2019020800A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112797937A (en) * | 2021-03-31 | 2021-05-14 | 苏州天准科技股份有限公司 | Non-contact measuring equipment |
| CN113108696A (en) * | 2021-04-06 | 2021-07-13 | 合肥埃科光电科技有限公司 | Light source wavelength scanning spectrum confocal sensor |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2020221859A1 (en) | 2019-05-02 | 2020-11-05 | Inproq Optical Measurement Gmbh | Surface examination method and device |
| WO2020221818A1 (en) | 2019-05-02 | 2020-11-05 | Inproq Optical Measurement Gmbh | Gear inspection method and device |
| CN110715617A (en) * | 2019-11-25 | 2020-01-21 | 宁波五维检测科技有限公司 | Microcosmic 3D (three-dimensional) morphology multi-channel measuring device and method |
| DE102021117116B4 (en) * | 2021-07-02 | 2023-10-26 | Stiftung für Lasertechnologien in der Medizin und Meßtechnik an der Universität Ulm ILM, Körperschaft des öffentlichen Rechts | Measuring device, method and medical measuring device for the optical measurement of an object |
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| DE102016100261A1 (en) | 2016-01-08 | 2017-07-13 | Nanofocus Ag | Method for electronic analysis of a time-varying signal |
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2017
- 2017-12-15 DE DE102017130211.5A patent/DE102017130211A1/en active Pending
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2018
- 2018-07-27 EP EP18756156.8A patent/EP3658850A1/en active Pending
- 2018-07-27 WO PCT/EP2018/070439 patent/WO2019020800A1/en active Application Filing
- 2018-07-27 CN CN201880057005.4A patent/CN111065884A/en active Pending
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| Publication number | Priority date | Publication date | Assignee | Title |
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| CN112797937A (en) * | 2021-03-31 | 2021-05-14 | 苏州天准科技股份有限公司 | Non-contact measuring equipment |
| CN113108696A (en) * | 2021-04-06 | 2021-07-13 | 合肥埃科光电科技有限公司 | Light source wavelength scanning spectrum confocal sensor |
Also Published As
| Publication number | Publication date |
|---|---|
| EP3658850A1 (en) | 2020-06-03 |
| DE102017130211A1 (en) | 2019-01-31 |
| WO2019020800A1 (en) | 2019-01-31 |
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Application publication date: 20200424 |