DE19859781A1

DE19859781A1 - Method for out-of-plane deformation or vibration analysis measurement using electronic speckle pattern interferometry based on use of microstructured refracting optical elements

Info

Publication number: DE19859781A1
Application number: DE1998159781
Authority: DE
Inventors: Bernhard Lau; Valery Petrov
Original assignee: FACHHOCHSCHULE ULM
Current assignee: LAU, BERNHARD, PROF. DR., 89275 ELCHINGEN, DE
Priority date: 1998-12-23
Filing date: 1998-12-23
Publication date: 2000-06-29

Abstract

With the method object wave and reference waves are generated by the same Laser source. At least one is at an angle to the optical axis of the camera objective and would be outside its field of view if not redirected by the refracting element

Description

Vorgestellt wird ein Verfahren zur Verformungs- oder Schwingungsmessung mittels elektroni scher Speckle-Pattern-Interferometrie (ESPI), das auf Verformungen oder Schwingungen or thogonal zur Ebene des Prüflings anspricht ("Out-of-Plane"-Verformungen bzw. -Schwin gungen). Hierzu wird der Prüfling mit der kohärenten Strahlung eines Lasers beleuchtet. Gleichzeitig wird eine Referenzoberfläche mit kohärenter Strahlung derselben Laserquelle be leuchtet. Die das Objekt und die Referenzoberfläche beleuchtenden Wellen werden an diesen diffus oder gerichtet reflektiert und bilden ESPI-Objekt- und Referenzwelle, welche von einem Kameraobjektiv aufgenommen werden. Zumindest eine ESPI-Welle (Abb. 1) oder beide ESPI- Wellen (Abb. 2) verlaufen schräg zur optischen Achse des Kameraobjektives. Ein speziell mi krostrukturiertes brechendes optisches Element, das als Strahlvereiniger wirkt, wird in den Strahlengang gebracht, um beide Wellen auf die optische Achse des Objektivs und zum Kame rasensor zu führen (Abb. 1 und Abb. 2). Beide ESPI-Wellen können eine glatte oder eine diffu se Wellenfront haben; beide Wellen können durch Teilung der Wellenfront oder der Amplitude der Laserstrahlung generiert werden; sie können von einem Spiegel oder einer diffus reflektie renden Oberfläche hervorgebracht werden.A method for deformation or vibration measurement using electronic speckle pattern interferometry (ESPI) is presented, which responds to deformations or vibrations orthogonal to the level of the test object ("out-of-plane" deformations or vibrations). For this purpose, the test object is illuminated with the coherent radiation from a laser. At the same time, a reference surface is illuminated with coherent radiation from the same laser source. The waves illuminating the object and the reference surface are reflected diffusely or directionally on them and form ESPI object and reference waves, which are recorded by a camera lens. At least one ESPI wave ( Fig. 1) or both ESPI waves ( Fig. 2) run at an angle to the optical axis of the camera lens. A specially micro-structured refractive optical element, which acts as a beam combiner, is brought into the beam path to guide both waves onto the optical axis of the lens and to the camera sensor ( Fig. 1 and Fig. 2). Both ESPI waves can have a smooth or a diffuse wavefront; both waves can be generated by dividing the wavefront or the amplitude of the laser radiation; they can be produced by a mirror or a diffusely reflecting surface.

Zumindest ein Teil der ESPI-Objektwelle oder ein Teil der ESPI-Referenzwelle breitet sich schräg zur optischen Achse der Kamera aus und bleibt ohne mikrostrukturiertes brechendes Element außerhalb des Blickfeldes des Kameraobjektives. Wenn das mikrostrukturierte Ele ment jedoch vor das Kameraobjektiv plaziert wird, so führt es beide Wellen auf die optische Achse des Kameraobjektives, welches die Abbildung auf den Kamerasensor durchführt.At least part of the ESPI object wave or part of the ESPI reference wave is spreading obliquely to the optical axis of the camera and remains without microstructured refractive Element outside the field of view of the camera lens. If the micro-structured Ele However, if it is placed in front of the camera lens, it leads both waves to the optical one Axis of the camera lens, which performs the imaging on the camera sensor.

Das von Objekt- und Referenzwelle gebildete Speckle-Interferenzmuster wird von einem Ka merasensor aufgenommen, der die Specklemuster vor und nach einer Verformung des Objekts festhält. Durch Auswertung dieser Specklemuster mittels Bildverarbeitung ist es möglich, die Out-of-Plane-Verformung oder -Schwingung des Prüflings zu berechnen. Hierfür wird der Prüfling im unverformten und im verformten Zustand (entsprechend zwei verschiedenen Zu ständen der Verformung bzw. Schwingung) mit mehreren Specklemustern verschiedener rela tiver Phasenlage der beiden interferierenden Wellen (Objekt- und Referenzwelle) aufgenommen und ausgewertet (Phasenschrittmethode). Eine andere Möglichkeit zur Auswertung ist die Be rechnung und Darstellung des Speckle-Korrelationsbildes zweier Specklemuster, die dem un verforrrtten und dem verformten Zustand des Objektes entsprechen. Ein solches Speckle- Korrelationsbild kann auch mittels einer photographischen Kamera hergestellt werden. Die geläufigen Verfahren von Out-of-Plane-ESPI unterscheiden sich hauptsächlich in der Art der Referenzwelle (ebene oder gestreute Welle), wie diese erzeugt wird (Wellenfront- oder Amplitudenteilung) und wie sie zum Kamerasensor geführt wird (durch die Kameralinse oder dahinter). Aufgrund des begrenzten Auflösungsvermögens der üblichen Kamerasensoren (hauptsächlich CCD-Sensoren) muß der Winkel zwischen der Objekt- und der Referenzwelle auf der Sensoroberfläche ausreichend klein sein, um ein auflösbares Interferenzmuster zu er zeugen. Abb. 3 zeigt ein Beispiel für eine ESPI-Anordnung zur Verformungs- und Schwin gungsanalyse entsprechend dem Stand der Technik, bei der eine diffuse Referenzwelle durch Wellenfrontteilung und mittels eines holographisch-optischen Elements erzeugt wird und durch das Objektiv auf den Sensor trifft [1, 2].The speckle interference pattern formed by the object and reference wave is recorded by a camera sensor, which records the speckle pattern before and after the object is deformed. By evaluating these speckle patterns using image processing, it is possible to calculate the out-of-plane deformation or vibration of the test specimen. For this purpose, the test specimen is recorded and evaluated in the undeformed and in the deformed state (corresponding to two different states of the deformation or vibration) with several speckle patterns of different relative phase positions of the two interfering waves (object and reference wave) (phase step method). Another possibility for evaluation is the calculation and representation of the speckle correlation image of two speckle patterns, which correspond to the undrawn and the deformed state of the object. Such a speckle correlation image can also be produced using a photographic camera. The common methods of out-of-plane ESPI differ mainly in the type of reference wave (plane or scattered wave), how it is generated (wavefront or amplitude division) and how it is guided to the camera sensor (through the camera lens or behind it) . Due to the limited resolution of the usual camera sensors (mainly CCD sensors), the angle between the object and the reference wave on the sensor surface must be small enough to produce a resolvable interference pattern. Fig. 3 shows an example of an ESPI arrangement for deformation and vibration analysis according to the prior art, in which a diffuse reference wave is generated by wavefront division and by means of a holographic-optical element and hits the sensor through the lens [1, 2].

Bei dem vorgestellten Verfahren kann das mikrostrukturierte brechende optische Element ent weder eine periodische oder eine nichtperiodische Struktur besitzen. In beiden Fällen muß das brechende optische Element mindestens zwei Gruppen von Mikrostrukturen aufweisen, um die ESPI-Objektwelle und die ESPI-Referenzwelle exakt auf der optischen Achse in das Kamera objektiv zu leiten. Das mikrostrukturierte brechende optische Element kann beispielsweise aus zwei Gruppen von periodisch angeordneten linearen Strukturen bestehen, von denen je eine mit der ESPI-Objektwelle oder mit der ESPI-Referenzwelle arbeitet, abhängig vom Einfallswinkel und von Periode, Profil und Brechungsindex des Materials des mikrostrukturierten Elements. In Abb. 1 und 2 sind mikrorefraktive Bereiche dargestellt, die als Strahlvereiniger dienen. Zum Beispiel können solche Bereiche aus zwei Gruppen von Mikroprismen bestehen.In the method presented, the microstructured refractive optical element can have either a periodic or a non-periodic structure. In both cases, the refractive optical element must have at least two groups of microstructures in order to objectively guide the ESPI object wave and the ESPI reference wave into the camera on the optical axis. The microstructured refractive optical element can, for example, consist of two groups of periodically arranged linear structures, one of which works with the ESPI object wave or with the ESPI reference wave, depending on the angle of incidence and on the period, profile and refractive index of the material of the microstructured element. Fig. 1 and 2 show microrefractive areas that serve as beam converters. For example, such areas can consist of two groups of microprisms.

Das vorgestellte Verfahren erlaubt es, ESPI-Anordnungen zu aufzubauen, die sehr kompakt sind (besonders wenn Halbleiterlaser oder diodengepumpte Nd : YAG-Laser verwendet wer den), einfach zu bedienen sind und praktisch keine Justierung benötigen. Solche neuartige An ordnungen können an Kompaktheit nur mit den in [1, 2] beschriebenen oder mit faseroptischen verglichen werden. Letztere benötigen ziemlich komplizierte und zeitaufwendige Prozeduren zur Justierung und sind nicht einfach zu bedienen.The method presented allows ESPI arrangements to be built which are very compact (especially when using semiconductor lasers or diode pumped Nd: YAG lasers den), are easy to use and require practically no adjustment. Such novel types Orders can only be compact with the ones described in [1, 2] or with fiber optics be compared. The latter require fairly complicated and time-consuming procedures for adjustment and are not easy to use.

Infolge des sehr einfachen optischen Aufbaus der vorgestellten ESPI-Anordnungen wird die Laserstrahlung sehr gut ausgenutzt. Daher ist es möglich, in einer hell beleuchteten Umgebung bei Beleuchtungsstärken zu arbeiten, wie sie für industrielle Arbeitsplätze empfohlen werden (einige hundert lux), oder sogar bei deutlich höheren Werten. Dazu kann in der Kamera ein geeigneter Interferenzfilter verwendet werden, der der Laserwellenlänge angepaßt ist, um den inkohärenten Strahlungsanteil auf dem Kamerasensor zu begrenzen.Due to the very simple optical structure of the ESPI arrangements presented, the Laser radiation used very well. Therefore, it is possible in a brightly lit environment work at illuminance levels that are recommended for industrial workplaces (a few hundred lux), or even at significantly higher values. You can do this in the camera suitable interference filter can be used, which is adapted to the laser wavelength by the limit incoherent radiation component on the camera sensor.

Die Einführung von mikrostrukturierten brechenden optischen Elementen, die als Strahlverei niger dienen, ermöglicht die Auswertung der Differenz von Verformungs- und Schwingungs mustern eines Musterobjekts und eines gleichartigen Prüflings. Dieses innovative ESPI- Verfahren kann beispielsweise als Schnelltest von Produkten und Komponenten in Produkti onslinien der Mikroelektronik-Industrie angewandt werden.The introduction of microstructured refractive optical elements that act as a beam serve niger, enables the evaluation of the difference between deformation and vibration sample a sample object and a similar test object. This innovative ESPI The method can be used, for example, as a quick test of products and components in product lines of the microelectronics industry.

literature

1. V. Petrov, B. Lau, Electronic speckle pattern interferometry with a holographically genera ted reference wave. Optical Engineering 35, pp. 2363-2370, (1996).
2. B. Lau, P. Kuschnir, U. Schmid, V. Petrov, Application of combined method of electronic speckle pattern interferometry and holography to vibration analysis. 2nd Intern. Conf. on Vi bration Measurements by Laser Techniques, Ancona, 23-25 September 1996, Proc. SPIE 2868, pp. 346-351, (1996).1. V. Petrov, B. Lau, Electronic speckle pattern interferometry with a holographically generated reference wave. Optical Engineering 35, pp. 2363-2370, (1996).
2. B. Lau, P. Kuschnir, U. Schmid, V. Petrov, Application of combined method of electronic speckle pattern interferometry and holography to vibration analysis. 2nd intern. Conf. on Vi bration Measurements by Laser Techniques, Ancona, 23-25 September 1996, Proc. SPIE 2868, pp. 346-351, (1996).

Claims

1. Method for deformation or vibration measurement using electronic speckle pattern interferometry (ESPI), which responds to deformations or vibrations orthogonal to the level of the test object ("out-of-plane" speckle interferometry), characterized in that both ESPI waves - object and reference waves - are guided over a microstructured refractive optical element onto the optical axis of the camera lens and at least one of the two waves is thereby broken.

2. Method for "out-of-plane" measurement of deformations and vibrations using ESPI according to claim 1, characterized in that the mi Crostructed refractive optical element either periodic or non-periodic May have microstructures.

3. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 or 2, characterized in that the mi crostructed refractive optical element has at least two groups of microstructure ren has to match both ESPI waves (object and reference wave) exactly to the optical Steer axis.

4. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 to 3, characterized in that the mi Crostructed optical element from at least two groups of periodically arranged linear microstructures can exist.

5. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 to 4, characterized in that the mi Crostructed optical element can consist of matrix-like microstructures.

6. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 to 5, characterized in that at least one Part of the ESPI object wave or part of the ESPI reference wave at an angle to the optical one Axis of the camera runs and without the micro-structured refractive optical ele ment is outside the field of view of the camera lens. If the microstructure However, if the element is placed in front of the camera lens, it performs both waves the optical axis of the camera lens, which is the image on the camera sensor carries out.

7. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 to 6, characterized in that the ESPI Object wave preferably runs symmetrically to the ESPI reference wave.

8. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 to 7, characterized in that instead of Object and reference surface two similar objects can be used by one of which serves as a sample and the other as a test object. If both objects the difference between the deformation or vibration patterns is measured. When used in production lines, objects can be examined and with a sample object be compared.

9. Process for "out-of-plane" measurement of deformations and vibrations using ESPI according to at least one of claims 1 to 8, characterized in that both plane as well as diffusely scattered ESPI waves can be used.

10. Method for "out-of-plane" measurement of deformations and vibrations by means of ESPI according to at least one of claims 1 to 9, characterized in that the Waves illuminating the object and the reference surface from the same laser source generated and with the help of wavefront or amplitude division to the object and get to the reference surface.

11. Method for "out-of-plane" measurement of deformations and vibrations by means of ESPI according to at least one of claims 1 to 10, characterized in that the Evaluations of the deformations and vibrations using a video camera and one Image processing based on the phase step or speckle correlation method system.

12. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI, characterized in that both ESPI waves - object and reference waves - via a microstructured refractive optical element on the optical axis of the Ka are directed and at least one of the two shafts is broken becomes.

13. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to claim 12, characterized in that the microstructured refractive optical element serving as beam combiner either can have periodic or non-periodic microstructures.

14. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to at least one of claims 12 and 13, characterized in that the microstructured refractive optical element has at least two groups of microstructures around both ESPI waves (object and Reference shaft) exactly on the optical axis.

15. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 14, characterized ge indicates that the microstructured refractive optical element consists of at least two Groups of periodically arranged linear microstructures can exist.

16. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 15, characterized ge indicates that the microstructured refractive optical element consists of matrix-like Structures can exist.

17. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 16, characterized ge indicates that at least part of the ESPI object wave or the ESPI reference wave runs obliquely to the optical axis of the camera and without the microstructured refractive optical element located outside the field of view of the camera lens. However, if the microstructured element is placed in front of the camera lens it guides both waves onto the optical axis of the camera lens, which is the image performs on the camera sensor.

18. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 17, characterized ge indicates that the ESPI object wave is preferably symmetrical to the ESPI reference wave runs.

19. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 18, characterized ge indicates that instead of object and reference surface two similar objects can be used, one of which serves as a model object and the other as Test object. If both objects are loaded, the difference in the deformation or vibration pattern measured. Objects can be used in production lines examined and compared with a sample object.

20. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 19, characterized ge indicates that both flat and diffusely scattered ESPI waves are used can.

21. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to any one of claims 12 to 20, characterized ge indicates that the waves illuminating the object and the reference surface of same laser source are generated and with the help of wavefront or amplitudes division to the object and the reference surface.

22. Arrangement for "out-of-plane" measurement of deformations and vibrations by means of ESPI for performing the method according to one of claims 12 to 21, characterized ge indicates that the evaluations of the deformations and vibrations by means of a Video camera and one on the phase step or speckle correlation method machine vision system.