DE102004012426A1 - Low-coherence interferometric method and apparatus for light-optical scanning of surfaces - Google Patents

Abstract

Described is a low coherence interferometric device for optical scanning of a surface (19) of an object, in particular a concave surface (19), for example a borehole, with a short-coherence interferometer comprising a short-coherent measuring light source (30), a light emanating from it a beam splitter (13) dividing onto a measuring light beam (23) and a reference light beam (22), a reference reflector (16), a measuring optics (17) and a detector (35) to which the reflected measuring light beam (23) and the reference light beam (22) for detecting a low-coherence interference signal. The measuring light beam (23) is focused by the measuring optics (17), which has a numerical aperture of not more than 0.3, in a focusing area (27) extending in a scanning direction, within which a control and evaluation unit (6) Scanning position varies by a reference point and registered thereby a match of the scanning position with the surface (19) to obtain a distance information about a distance of the reference point of the surface (19) of the object. From the distance information and the position of the reference point, a location information about a light-reflecting point of the surface (19) is determined.

Description

The The invention relates to a low coherence interferometric Method and a device for the light-optical scanning of a surface of an object, in particular a curved one Surface, for example a borehole.

The Scanning a surface is usually done with sequential procedures, each for a specific Place the surface a distance information about their distance from the device (usually from its measuring head) and then the device positioned by means of a lateral displacement relative to the surface will that the Distance information for another part of the surface can be won. In other words, interval scans alternate (distance scans) and lateral shifts take place gradually the desired to gain detailed three-dimensional information about the course of the surface.

The The invention is specifically directed to low coherence interferometric methods and devices for Surface scanning. The distance scanning takes place by means of one of the measuring head of the device the surface to be examined directed measuring light beam. The direction of the light beam defines a transverse (in the Rule perpendicular) to the surface extending scan line, which in practice almost always runs straight, though in principle also a distance scan on a curved scan line is possible. The following will be without limitation The general reference to a scanning line. Their direction is also called "z-direction". This one Scanning in the longitudinal direction of the light beam, the term longitudinal scan is used hereinafter. In the English-language literature is for such procedures the name Low Coherence Distance Scan (LCDS) is used.

The Longitudinal scanning with LCDS method is based on the fact that light a low-coherent (spectral broadband emitting) light source in two light paths, namely a measuring light path, the surface to be scanned leads and a reference light path is divided and the two partial light paths be merged in such a way before hitting a detector, that she interfere with each other. For this purpose, the device contains an interferometer arrangement, the usual except the low-coherent Light source a beam splitter, a reference reflector and the Detector includes. The light paths between these elements form interferometer arms. The light of the light source passes through a light source arm the beam splitter and is split there. A first proportion of light is used as measuring light over a Object arm in the scanning direction blasted onto the surface to be examined, while a second portion of light as reference light via a reflector arm to the Reference reflector arrives. Both light components are reflected (the measuring light on the surface to be examined, the reference light on the reference reflector) and on the same Light path (object arm or reference arm) returned to the beam splitter. There they are summarized and over one Detection of the interferometer such a detector fed to that from the summary light ("detection light") when hitting the detector Interference signal generated, which provides information about the strength of the reflection of the measuring light in dependence of contains the respectively set longitudinal scanning position.

The Variation of the scanning position along the sampling line is usually done through change the relation of the lengths the reference light path and the Meßlichtweges. This will be the position changed on the scan line, for the the prerequisite for the interference of the measuring light and the reference light (viz that yourself the optical path lengths both light paths at most by the coherence length of the light source from each other distinguish) is. The current scanning position is the position in each case on the scanning line, for the the optical length of the measuring light path with the optical length of the reference light path (each from the beam splitting to the beam merge) match ( "Coherence condition"). In general, the reference light path thereby shortened or extended, that the reference mirror is moved in the direction of the reference light beam.

at the longitudinal scanning of a surface reaches the interference signal a maximum when the scanning position coincides with the position of the surface. This will produce the desired distance information for the Place the surface, on the measuring light beam directed, determined. Thereafter, a lateral shift takes place, around for another part of the surface to perform the distance scan. To speed up the process, it is of course possible with several parallel to the surface irradiated measuring light beams and thereby simultaneously in a longitudinal scanning step the longitudinal scan for several points of the surface perform. Also this is to scan a larger surface Method required, in which alternately a plurality of longitudinal scanning steps and lateral shifts take place.

From WO 03/073041 it is known to vary the scanning position by means of a variable wavelength selection device in the detection arm. This scanning is done without translationally moving parts and is therefore faster than the spatial displacement of a reference mirror. The device makes it possible, for example, to continuously monitor a film guided past a measuring head at high speed to see whether a desired layer thickness is maintained. WO 03/073041 and the documents cited therein contain further explanations on LCDS methods, to which reference is hereby made.

Around with a low-coherence interferometric Device precise measurements With a good signal yield to perform the measuring light on the examined Place the sample surface be focused. When scanning flat surfaces, for example when a film or a rotating disk is guided past a measuring head, the focus can be unchanged while the scan of the entire surface are maintained. at the scanning of curved surfaces however, the need to focus according to the contours the surface to be scanned to track and change.

In US Pat. No. 6,144,449 proposes the required match the focusing with the respective scanning position by means of focus correction means ensure the position at any time during the scanning process the focus in agreement is brought to the respective scanning position. The proposed However, focus correction means are expensive and / or have a large space requirement. They are therefore for many applications not or only partially suitable.

While known low-coherence interferometric methods and apparatus permit the scanning of surfaces with high precision. However, they are for scanning surfaces with heavy bumps, ie at least partially curved surfaces, largely unsuitable. Only the scanning of flat surfaces, such as a rotating disk ( US 5,473,431 ) or the mentioned, passed on a measuring head foil is possible with known low-coherence interferometric devices at high speed.

task The invention is to show a way, as with a low coherence interferometric Device one any, in particular (at least in sections) curved surface of a Object can be scanned faster.

This object is achieved by a low-coherence interferometric method for light-optical scanning of a surface of an object, in particular a curved surface, by means of a directed to the surface measuring light beam and a short-coherence Inter ferometers, a short-coherent measuring light source, a reference reflector, a measuring optics, a detector and a control and evaluation unit connected to the detector, in which different lateral scanning positions are set by movement of the measuring light beam relative to the surface, in which the measuring light beam impinges on different light-reflecting points of the surface, for determining distance information on the respectively illuminated by the measuring light beam Site of the surface longitudinal scanning steps are performed, in which a match of along a transverse to the surface scan line variable scanning position is detected with the position of a light-reflecting spot on the surface of the object, which is characterized in that the measuring light beam by means of a measuring optics having a numerical aperture of not more than 0.3, in a focus area extending in the scanning direction about a central point is focused, and the scanning position is varied under control of the control and evaluation unit along the scanning line within a Feinabtastbereiches that at least partially with the focus area ( 27 ) matches.

The Task is further solved by a corresponding low-coherence interferometric.

advantageous Further developments of the invention are the subject of the dependent claims.

According to the invention measuring light by means of a measuring optics, which has a numerical aperture of not more than 0.3, preferably has not more than 0.2, extending in a along the Abtaststraaden Focus area focused.

By Use of a measuring optics With such a small nu meric aperture, the measuring light is less sharp, i. to a larger light spot, focused. This reduces the signal intensity and the lateral resolution. In the context of the invention it has been found that despite these apparent disadvantages excellent in the combined longitudinal and lateral ablation of curved surfaces Results when using a measuring optics with a small numerical Aperture and associated "worse" focus can be achieved.

• – Ein erster Anwendungsfall bezieht sich auf Oberflächen, die insgesamt eben sind, jedoch relativ grobe strukturelle Unebenheiten aufweisen, die mit vorbekannten Verfahren nur mit ständig variierender Fokussierung und deswegen relativ langsam abgetastet werden konnten. In solchen Fällen erlaubt die Erfindung die Abtastung der Oberfläche mit sehr viel höherer Geschwindigkeit. Sie macht sich dabei zu nutze, daß sich innerhalb des Fokusbereiches die Größe des von dem Meßlichtstrahl auf der Oberfläche erzeugten Lichtflecks in Abhängigkeit von der Longitudinalposition der Oberfläche nur wenig ändert und demzufolge die Intensität des reflektierten Lichtes weitgehend unabhängig von der Longitudinalposition der Oberfläche ist. Dadurch ist ohne Neueinstellung der Fokussierung eine Variation der Abtastposition über einen so großen räumlichen Bereich möglich, daß durch Verschieben der Abtastposition mit hoher Geschwindigkeit auch erhebliche Oberflächenrauhigkeiten, Stufen und Sprünge erfaßt werden können. Die Abtastposition läßt sich wesentlich schneller verschieben als der Fokusbereich, da dafür keine oder nur wesentlich geringere Massen mechanisch beschleunigt werden müssen.
• – Ähnliches gilt auch für gleichmäßig gekrümmte Oberflächen, beispielsweise die Innenwand eines Bohrlochs, sofern der Meßkopf bei der Änderung der Lateralabtastposition relativ zu der Oberfläche so bewegt wird, daß sich der Meßlichtweg um weniger als die Länge des Fokusbereiches ändert.
• – Wenn die zu untersuchende Oberfläche einen unregelmäßig gekrümmten Verlauf hat findet gemäß einer bevorzugten Ausführungsform der Erfindung bei Änderung der Lateralabtastposition zur Anpassung an die Form der Oberfläche eine Abtastungsgrobeinstellung (scanning coarse adjustment) statt, wobei der Feinabtastbereich und der Fokusbereich unter Kontrolle der Steuer- und Auswerteeinheit des Gerätes gleichgerichtet koaxial mit dem Meßlichtstrahl (d.h. auf der Abtastgeraden) verstellt werden. Sofern der unregelmäßig gekrümmte Verlauf der Oberfläche (beispielsweise als CAD/CAM-Daten) bekannt ist, kann eine Sollkurve vorgegeben werden, gemäß der der Fokusbereich in den unterschiedlichen Lateralabtastpositionen jeweils so eingestellt wird, daß die Oberfläche in dem Fokusbereich liegt. Für den Fall, daß die Oberflächenposition um mehr als einen vorbestimmten Grenzwert von der Sollkurve abweicht, ist der Steueralgorithmus bevorzugt so ausgebildet, daß bei der Abtastungsgrobeinstellung eine (von der vorbestimmten Sollkurve abweichende) Nachregelung der Position des Fokusbereiches stattfindet, wenn die Abweichung durch die Feinabtastung festgestellt wurde.
• – Die Erfindung eignet sich in besonderem Maße auch zur Untersuchung von Oberflächen, die unregelmäßig ge krümmt sind, ohne daß der Verlauf der Krümmung so gut vorbekannt ist, daß die vorstehend erläuterte Abtastungsgrobeinstellung mittels einer Sollkurve möglich wäre. In diesem Fall wird für die Abtastungsgrobeinstellung die bei mindestens einem vorausgehenden Longitudinal-Abtastschritt gewonnene Abstandsinformation zum Steuern der Verschiebung des Fokusbereiches benutzt. Mit anderen Worten findet bei der Abtastungsgrobeinstellung eine geregelte Einstellung des Fokusbereiches statt, bei der zur Einstellung der Position des Fokusbereiches als Istwert Informationen verwendet werden, die bei einem oder mehreren Longitudinal-Abtastschritten gewonnen wurden. Im Gegensatz zu vorbekannten Geräten ( US 6,330,063 ) wird bei der Erfindung der Fokus nicht ständig synchron mit der Abtastposition eingestellt, sondern eine Nachführung des Fokusbereiches erfolgt im Rahmen der Abtastungsgrobeinstellung nur bei Bedarf, und zwar auf der Grundlage der Abstandsinformation mindestens einer vorhergehenden, von der aktuellen lichtreflektierenden Stelle beabstandeten Stelle.

In the context of the invention, the optical properties of a lens with an unusually small numerical aperture, depending on the properties of the surface to be examined, are advantageously used in several respects:

• - A first use case refers to Surfaces that are generally flat, but have relatively rough structural unevenness that could be sampled with prior art methods only with constantly varying focus and therefore relatively slow. In such cases, the invention allows scanning of the surface at a much higher speed. It makes use of the fact that within the focus range the size of the light spot generated by the measuring light beam on the surface changes only slightly as a function of the longitudinal position of the surface and consequently the intensity of the reflected light is largely independent of the longitudinal position of the surface. As a result, without resetting the focus, a variation of the scanning position over such a large spatial area possible that significant surface roughness, steps and jumps can be detected by moving the scanning position at high speed. The scanning position can be moved much faster than the focus area, since no or only significantly lower masses need to be mechanically accelerated.
• - The same applies to uniformly curved surfaces, for example the inner wall of a borehole, provided that the measuring head is moved when changing the Lateralabtastposition relative to the surface so that the Meßlichtweg changes less than the length of the focus area.
• If the surface to be examined has an irregularly curved course, according to a preferred embodiment of the invention, when the lateral scanning position is changed to adapt to the shape of the surface, a scanning coarse adjustment takes place, the fine scanning area and the focusing area being under control of the control and scanning areas Evaluation unit of the device rectified coaxial with the measuring light beam (ie on the scanning line) can be adjusted. If the irregularly curved course of the surface is known (for example as CAD / CAM data), a setpoint curve can be specified according to which the focus area in the different lateral scanning positions is set such that the surface lies in the focus area. In the event that the surface position deviates from the setpoint curve by more than a predetermined limit, the control algorithm is preferably designed so that, in the sample coarse adjustment, a readjustment of the position of the focus area (deviating from the predetermined setpoint curve) takes place, if the deviation by the fine scan was determined.
• - The invention is particularly suitable for the investigation of surfaces that are irregularly curved ge, without the course of the curvature is so well known that the above-described Abtastgrogrobeinstellung would be possible by means of a setpoint curve. In this case, for the sample coarse adjustment, the distance information obtained in at least one preceding longitudinal scanning step is used to control the shift of the focus area. In other words, in the case of the coarse adjustment, a regulated adjustment of the focus area takes place, in which information is used as the actual value for setting the position of the focus area, which information was obtained during one or more longitudinal scanning steps. In contrast to previously known devices ( US 6,330,063 In the invention, the focus is not set constantly in synchronism with the scanning position, but tracking of the focus area is performed as needed within the scope of the sample coarse adjustment, based on the distance information of at least one preceding location spaced from the current light-reflecting location.

For the purposes of the present invention, the length L of the focus area can be calculated from the numerical aperture NA and the wavelength λ _{o of} the light used according to:

In this a is a scaling factor, the value of which is reasonable in the individual case Degree of blur near the limit of the focus area depends. Preferably, a ≤ 2, especially preferably a ≦ 1.5. The Feinabtastbereich is preferably completely within the so-defined th Focus area. But there are also possible applications in which the longitudinal scan based on interference signals resulting from reflections within the Focus range results, but the fine scan over the Borders of the focus area extends.

Preferably, measuring light from the near infrared spectral range with a central wavelength λ _o in the range of 800 to 1,300 nm is used. By a measuring optics with NA = 0.08, the measuring light is focused from this spectral range in a focus area with a length of about 150 microns. The diameter of the focus area is approximately 10 to 20 μm in the case of the values of λ _o and NA given as an example (depending on the specific wavelength λ _o and the specifically considered section of the focus area). In general, the focus area preferably has a length of about 100 to 300 μm and a width of 5 to 20 μm.

The variation of the scanning position in the context of fine scanning can be achieved by changing the length of the reference light path, in particular by adjusting the position of a reference reflector become. Preferably, the longitudinal scanning position is varied by means of a variable wavelength selection device in the detection light path according to WO 03/073041. Other methods for fast longitudinal scanning (with as little mechanical movement as possible) can be advantageously used within the scope of the invention. Examples are cited in WO 03/073041 as citations 4 to 6.

The measuring optics according to the invention with a numerical aperture of at most 0.3 is realized, in particular for the scanning of boreholes, preferably by means of a GRIN lens. This lens type is used in optics for various purposes. For example, in the DE 19819762 A1 a modulation interferometer using a gradient index lens (GRIN) lens.

The Invention allows an improved quality control in the industrial production of machine parts with curved, vor all concave surfaces, for example gears, injectors and boreholes. Especially in microsystems technology, there is a need for one fast, precise and cost-effective possibility for examining interiors of components, since even the smallest irregularities on the inner walls are disturbing. For example, they have a significant impact on that Flow behavior of a flowing through the interior Fluid.

Further Details and advantages of the invention will become apparent from an embodiment with reference to the attached Figures explained. The features shown therein can be used individually or in combination used to preferred embodiments of the invention create. Show it:

1 an embodiment of a Niederkohärenzinterferometrischen device;

2 an enlarged sectional view of the penetrating into a borehole probe of 1 represented device;

3 a schematic diagram of the measuring head of a low-coherence interferometric device in three phases A, B and C to explain the interaction of fine scanning and scanning coarse adjustment;

4 a highly abstracted schematic diagram of a scanning process in three phases A, B and C with different detection positions of the focus area;

5 an embodiment of a probe; and

6 the imaging conditions when using multiple measuring light sources.

That in the 1 and 2 shown embodiment of a low-coherence interferometric device consists of a relative to an object to be examined movable measuring head 1 and a stationary base unit 4 , The base unit 4 contains a low coherent, broadband measuring light source 30 whose light is through a lens 31 in an optical fiber 32 is coupled. The optical fiber 32 directs the light through an optical splitter 33 the optical fiber 5 to that to the measuring head 1 leads. The optical fibers 5 and 32 are preferably single-mode fibers.

In the measuring head 1 will that be out of the optical fiber 5 escaping light by means of the lens 10 collimated and a free beam splitter 13 fed. The free jet divider 13 divides the light into a measuring light path 23 and a reference light path 22 on.

On the reference light path 22 the reference light is through a lens 12 refocused and through a mirror 14 an elongated reference optics 15 fed. The reference optics 15 forms one of the lens 12 generated focus on a reference reflector 16 from. After reflection at the reference reflector 16 the reference light returns on the same way back to the free beam splitter 13 , The free jet divider 13 as well as the reference optics 15 and the reference reflector 16 are in a beam splitter unit 2 of the measuring head 1 attached.

On the measuring light path 23 the measuring light is from a lens 11 focused and a measuring optics 17 fed, the preferred as well as the reference optics 15 of the reference light path is designed as a rod optic with a GRIN lens. The measuring optics 17 is part of a very thin probe 3 with a diameter of about 500 to 800 microns, which can be inserted in holes with a diameter of less than 1 mm. On the from the beam splitter 13 opposite side of the measuring optics 17 is in the measuring light path 23 a deflecting element 18 arranged, which has a Meßlichtstrahl 23a on the surface to be examined 19 , In the illustrated embodiment, the inner surface of a borehole deflects. At a light-reflecting point 19a the surface 19 the sample to be examined, the measuring light is reflected and passes over the deflecting element 18 , the measuring optics 17 and the lens 11 back to the beam splitter again 13 where it is merged with the reference light that has passed the reference light path.

The intensity of the surface examined 19 reflected measuring light is depending on Be the surface is subject to great fluctuations. The associated problems in the evaluation are reduced by the fact that in the 1 shown low coherence interferometric device, the reference light path has a constant length. Therefore, the intensity fluctuations of the reference light, the intensity of which is usually much greater than that of the measuring light, minimal.

That in the beam splitter 13 Detection light formed from the measuring light and the reference light is transmitted via the lens 10 back into the optical fiber 5 coupled and the base unit 4 and thus the detector 35 fed. In the simplest case, the detector points 35 only a single detector element, which is preferably a PIN diode.

Different lengths of the reference light path and the Meßlichtweges cause that in the detection light certain wavelength ranges from the spectrum of the measuring light source 30 destroyed by destructive interference. In the base unit 4 is a variable wavelength selection device 34 arranged with a control and evaluation unit 6 connected is. It preferably passes light having wavelengths corresponding to a predetermined sequence of wavenumbers. This sequence is under the control of the control and evaluation unit 6 adjusted so that the scanning position to which the detector 35 detected low-coherence signal is varied. Further information can be found in WO 03/073041 A1 cited above, the content of which is incorporated by reference into the content of the present application. Is the scanning position with a light-reflecting point of the surface to be examined 19 match, so is the detector 35 an interference signal generated by the control and evaluation 6 is registered. The control and evaluation unit determines for this sampling position 6 the desired distance information.

A peculiarity of in 1 Niederkohärenzinterferometrischen device shown is that the Meßlichtstrahl 23 from the measuring optics 17 not in a sharp spot of light (of the order of the wavelength of light) on the surface to be examined 19 is focused, but in a perpendicular to the surface 19 extending focus area 27 , in which the width of the measuring light beam 23 only slightly changes. This is, as explained, by a measuring optics 17 which has a numerical aperture of not more than 0.3, preferably only 0.05 to 0.2. Within the focus area 27 the scanning position is under the control of the control and evaluation unit 6 along a scan line 9 within a fine scan range 29 which is part of the focus range and may for example have a length of 100 μm ( 2 ).

The deflection element 18 is, preferably together with the probe 3 that the measuring optics 17 contains, by means of the drive 20 rotatable about its longitudinal axis, so that an annular portion of the inner surface of a borehole can be examined. To examine the entire inner surface of the borehole becomes the measuring probe 3 pushed in the longitudinal direction in the hole. With gradual movement of the probe 3 Thus, the inner surfaces are scanned in successive annular areas. The measuring probe is preferred 3 steadily moved so that the successively scanned light-reflecting spots of the surface 19 adjoin each other helically.

The deflection element 18 is preferably designed as a prism, but may for example also be a mirror. Preferably, its deflection angle (angle α between the axis of the measuring optics 17 and the sampling line 9 ) 90 °, but this is not mandatory. Even if the measuring light beam 23 from the deflector 18 Boreholes can be examined by less or more than 90 °, provided that the arrangement is such that by rotation of the probe 3 the inner surfaces of a borehole can be scanned gapless. A deflection angle of 90 ° is advantageous, however, because with vertical reflection, a maximum proportion of the measuring light back into the probe 3 is reflected. The deflection element is preferred 18 interchangeable. It can be removable, for example, on the measuring optics 17 be attached or one of Meßoptik 17 and deflector 18 existing assembly may be interchangeable as a whole, so that the probe 3 each can be optimally adapted to the object to be examined (for example, a borehole or a gear).

In the embodiment shown, the entire measuring head 1 by means of a multi-axis actuator (not shown) three-dimensionally movable, wherein the movement under control of the control and evaluation unit 6 is precisely controllable. Inside the measuring head 1 is the distance of the measuring optics 17 from the beam splitter 13 by means of an actuator 21 adjustable. This is preferably achieved in that the entire beam splitter unit 2 relative to the probe 3 is displaceable.

By changing the distance of the measuring optics 17 from the beam splitter 13 can the position of the center F _{0 of} the focus area 27 be moved in the scanning direction so that the walls of the borehole within the focus area 27 (at the center of the hole 36 positioned probe 3 ) are located. In particular, the position of the focus area 27 be adapted to a diameter change of the borehole without a laterally formed on the surface to be examined Meßlichtfleck is moved.

In the described construction, the fine scanning area becomes 29 rectified and synchronously shifted coaxially with the focus area, wherein also the amount of displacement for both areas is the same, so the center Q _{0 of} the Feinabtastbereiches during the displacement has a constant distance to the center F _{0 of} the focus area. In general, it is advantageous if, in the case of scanning coarse adjustment, the shift of the focus area 27 and the shift of the fine scan area 29 rectified and synchronous takes place, with a shift by equal amounts is particularly advantageous.

Since the diameter of a borehole to be examined is generally known (at least to 100 μm), the distance between the measuring optics 17 and the beam splitter 13 be adjusted prior to insertion into the wellbore so that the surface in the focus area 27 lies. For example, the diameter of the borehole to be examined, for example 3 mm or 5 mm, can be fed to the control and evaluation unit via a keyboard (not shown) 6 be transmitted.

However, the position of the focus area is preferred 27 automatically, in the case shown by changing the distance between the probe 3 and the beam splitter unit 2 , discontinued. The control and evaluation unit uses this 6 the distance information obtained in the fine scan. The automatic positioning of the focus area 27 is based on an interaction of the fine scan and the sample coarse adjustment, which is based on 3 is explained in more detail.

3 shows as a highly schematized schematic diagram of the essential in this context functions of a device according to the invention in three movement phases A, B and C. The device largely corresponds to the basis of 1 and 2 explained embodiment, but wherein the Meßlichtstrahl 23a without deflecting straight on the surface to be scanned 19 is irradiated and the variation of the longitudinal scanning position by corresponding variation of the position of the reference reflector 16 is effected. The lengths of the scanning area and the focus area are denoted by q and f, respectively, and greatly exaggerated for better visibility. Although the measuring optics 17 For the sake of simplicity, it is shown as a simple lens, it is of course a preferably designed as a rod optical arrangement with a numerical aperture of less than 0.3.

The sub-figure A shows a state in which the center F _{0 of} the focus area 27 with the light-reflecting point 19a the surface 19 matches. The length of the reference light path 22 is set so that the center Q _{0 of} the Feinmeßbereiches 29 coincides with F ₀ . This means that the position of the reference mirror denoted Q ₀ ' 16 is set so that the optical path length of the reference light path 22 (between beam splitters 13 and reference reflector 16 ) and the optical path length of the measuring light path 23 (between reference reflector 13 and reflective site 19a ) coincides when the reference mirror is in the position indicated by Q ₀ '. Fine-tuning becomes the reference mirror 16 oscillated by ± q / 2. Accordingly, the longitudinal scanning position moves by ± q / 2 in the fine scanning area 29 ,

The sub-figure B shows a movement phase in which the surface 19 relative to the sub-figure A is shifted by Δs to the right. Since fine scanning is very fast, the new position of the surface is detected immediately. The corresponding reference signal is detected by the detector when the reference mirror 16 is in the position indicated by Q ₁ 'shifted by Δq position. The position of the focus area 27 is still the same as in subfigure A (.DELTA.f = 0).

The shift of the surface detected in the previous fine scan 19 is from the control and evaluation of the device to a control signal for the actuator 21 processed, the required Abtastgrobanpassung by appropriate displacement of the focus area 27 and the scanning range 29 (ie the centers F ₀ and Q _{0 of} these areas) causes. This will cause the beam splitter 13 and the reference reflector 16 (In the illustrated embodiment, the measuring optics 17 ) by an amount Δf = Δs shifted so that F ₀ and Q _{0 are} again in the longitudinal position of the light-reflecting point (part C). The center point of the scanning area is again in the original position (designated by Q ₀ 'in subfigure A) (Δq = 0).

For the practical success of the invention, it is important that the fine scanning takes place much faster than the scanning coarse adjustment. Quantitatively one can say that the average scanning speed of the fine scan v _q , which can be calculated from the length q of the scan area and the period T of the fine scan according to v _q = q / T, is at least ten times the maximum speed v _{f , max of} the movement of the center F _{0 of} the focus area in the sample coarse adjustment. Thereby, any change in the position of the light-reflecting spot detected in the fine scanning is immediately detected. This allows not only a fast detection of even fine surface structures, but also a virtually delay-free readjustment of the position of F ₀ and Q ₀ in the context of scanning coarse adjustment.

At the in 3 illustrated embodiment form the scanning coarse adjustment takes place in that within the measuring head 1 a corresponding shift (of the components 13 . 16 and 17 ) takes place. For some applications, especially for the scanning of relatively large objects with complicated surfaces, it is advantageous if instead the entire measuring head 1 precise controlled three-dimensional moves. Corresponding technologies are available, for example, from robotics.

4 illustrates a surface profile detection of a surface 19 that has a step. For scanning different light-reflecting areas 19a the surface 19 becomes the measuring light beam 23 moved laterally into the detection positions A, B and C shown. At the detection position A, the center F _{0 of} the focus area is located 27 , which is hereinafter assumed to coincide with the center Q _{0 of} the fine scanning area, below the surface 19 , From the control and evaluation unit 6 is therefore the distance between Q ₀ and the surface 19 registered with a negative sign. Subsequently, the Meßlichtstrahl by a lateral movement of the measuring head 1 along the surface 19 moved to the detection position B. Due to the in 2 shown stage in the profile of the surface 19 Q ₀ and F _{0 are} now above the surface 19 so that the distance between the center 28 of the focus area 27 and the light-reflecting location of the surface 19 is registered with a positive sign.

While for the detection position A, the distance between the center F _{0 of} the focus area 27 and the surface 19 is relatively small, in the detection position B, the distance of the center F ₀ from the surface 19 relatively large. In the subsequent lateral movement into the detection position C, therefore, the focus area 27 and the fine scanning area 29 from the control and evaluation unit closer to the surface 19 emotional. Such a displacement in the scanning direction requires a movement of the measuring head 1 or the beam splitter unit 2 relative to the probe 3 , This is due to the mass to be moved only much slower possible than varying the scanning position within the focus area. Therefore, movements of the measuring head 1 in the scanning direction usually slower than its uniform lateral movement along the surface 19 , Although of the control and evaluation unit already during the lateral movement of the measuring head of the detection position B to the detection position C (eg by pressing the in 1 shown actuator) a movement of the focus area 27 was started in the scanning direction, F ₀ in the detection position C has not yet its optimal position near the surface 19 reached. The setting process of the position of the focus area initiated during the transition from the detection position B into the detection position C. 27 is therefore also in the transition to (not shown) further detection positions continue until the distance between F ₀ and the surface 19 does not exceed a given threshold.

To as quickly as possible a surface profile of a surface 19 To be able to determine, so from the control and evaluation 6 in a first detection position when scanning a first light-reflecting point of the surface 19 gained distance information from the surface 19 used during a lateral movement to a second detection position, a movement of the focus area 27 to control in the scanning direction. In this way, the focus area must be 27 not set individually for each detection position. Instead, the focus area becomes 27 continuously shifted in the scanning direction using the distance information determined for the respective preceding detection position. As surface profiles on a length scale of some 10 microns with concave surfaces 19 Of components usually change continuously, can be described by the control of a movement of the focus area 27 the speed with which a concave surface 19 can be significantly increased.

A peculiarity of in 1 shown interferometric device is that in the measuring head 1 a CCD camera 42 is arranged. By means of this CCD camera 42 becomes an image of the surface to be examined 19 detected in the visible spectral range while the detector 35 is preferably sensitive to light in the near infrared spectral range.

The measuring head 1 includes a second light source 46 for generating visible light for the CCD camera 42 , This second light source 46 is a ring light, which is arranged around the Meßlichtweg around. From the second light source 46 emitted light passes through the measuring optics 17 to the object to be examined. The deflection element 18 is formed as a beam splitter cube, which at its 45 ° boundary layer, a reflection layer 37 which is reflective to light of the near infrared spectral range and transparent to light of the visible spectral range. Consequently, the in 2 with the arrow 25 symbolized light for the CCD camera 42 not deflected by it while light of the near infrared spectral range from the deflector 18 is deflected to the surface to be examined. In the embodiment shown, that of the CCD camera 42 detected light of the visible spectral range from the bottom of a borehole reflected.

The visible light reflected from the bottom of the well passes through the deflector 18 back to the measuring optics 17 and is about the lens 11 the beam splitter 13 fed. There it will be over a lens 40 and a mirror 41 on a CCD camera 42 directed. To improve the picture quality is in front of the CCD camera 42 an optical filter 44 arranged, the light from the near infrared spectral range stops. The CCD camera 42 is over a cable 45 connected to an image processing unit, which is part of the base part 4 arranged control and evaluation 6 is.

By means of the image processing unit are from the of the CCD camera 42 captured image of the object coordinates to control the measuring head 1 obtained, in particular the center of a borehole to be examined determined. Based on this data, the measuring probe 3 be led to this center. In this way, it is easier and faster possible, the probe 3 to introduce into a borehole, as the risk of damage to the probe 3 is avoided by a collision with the object.

Another advantage of the CCD camera 42 lies in the fact that the image captured by her can be used to a practically unavoidable eccentricity of the rotating probe 3 to capture and calibrate. Because of the CCD camera 42 detected light from the measuring optics 17 and visible light from the deflector 18 is influenced at most insignificantly, makes an eccentricity of the rotating probe 3 noticeable in that the visible light from the measuring optics 17 not in a circular area, but in a cyclically moving area on the surface to be examined 19 is shown. By means of the CCD camera 42 can in this way an eccentricity of the rotating probe 3 detected and in determining a location information about the surface to be examined 19 be taken into account.

5 shows a further embodiment of a measuring probe 3 , As explained above, the essential elements of the probe are 3 the rod optics 17 and the deflector 18 , At the in 5 The embodiment shown is the deflection element 18 designed as a beam splitter having an effective in the near infrared spectral range mirror 38 combined. This beam splitter has a reflection layer 37 which is semi-transmissive to light of the near infrared spectral range. In this way, that is from the measuring optics 17 emerging measuring light 23 in two opposite partial beams 23b and 23c divided, so that at the same time two opposite light-reflecting points 7 . 8th a borehole can be scanned.

By means of the deflector formed as a beam splitter 18 Two measuring light paths are created, which differ slightly. Namely, on a first measuring light path, the first partial beam of the measuring light is instantaneously from the reflection layer 37 the deflecting element 18 deflected by 90 ° and the surface to be examined 19 fed. On the second measuring light path, the second partial beam of the measuring light first passes through the reflection layer 37 through, then from the mirror 38 reflected and after re-entry into the beam splitter of the reflection layer 37 reflected and the second light-reflecting point 8th the surface to be examined 19 fed. From the surface 19 reflected measuring light passes through the two measuring light paths described back to the measuring optics 17 and from there to the detector 35 , The mirror 38 is preferably slightly curved, so that the two focus areas 27 each the same distance to the deflection 18 to have.

The by the arrangement of the beam splitter 37 and the mirror 38 conditional path length difference of the two Meßlichtwege is from the dimensions of the probe 3 known. The remaining path length difference of the two measuring light paths depends on how exactly the sensor 3 sits in the center of the hole. By means of the control and evaluation unit 6 can (for example, according to WO 03/073041) for each of the two Meßlichtwege and thus for the two opposite light-reflecting points 7 . 8th the surfaces to be examined 19 in each case a distance information can be obtained.

With the in 5 shown measuring probe 3 Consequently, not only can the speed at which a borehole be examined be increased, but it can also be independent of the described CCD camera 42 a mechanical eccentricity of the probe 3 determine and compensate. If, in fact, the beam splitter 37 is located in a deviating from the center of the wellbore position, the distance between the opposite light-reflecting points 7 . 8th the surface to be examined 19 to the beam splitter 37 differently. This difference is made over in the control and evaluation unit 6 obtained distance information is determined and can be used to compensate for the eccentricities. In particular, in this way the precision with which a diameter of the borehole can be determined can be increased.

In a suitable embodiment of the reflection layer 37 For example, it is possible to use two wavelengths simultaneously for low-coherence interferometric scanning of different surface points. This is the reflection layer 37 for light of a first wavelength range, for example around 830 nm, semi-transmissive and transparent for light of a second wavelength range, for example around 1300 nm. In the control and evaluation unit 6 These two wavelength ranges can be separated and used to generate and evaluate various low-coherence interferometric signals. While the measuring light in the region of the first wavelength range conveys information about walls of the borehole through the beam path described above, the ground of a borehole can be investigated with the measuring light of the second wavelength range. It is also possible in particular behind the first deflecting element 18 another deflector (not shown) to be arranged, with the light of the second wavelength range by an angle between 0 ° and almost 180 °, preferably 90 ° deflected and so can also be used to study the walls of the borehole. In this way, simultaneously adjacent light-reflecting points of the surface 19 to be examined.

To the speed with which a surface 19 can be studied, further increase, as in 6 shown - a plurality of spaced measuring light sources 30 be used. The measuring light sources 30 are arranged along a line, for example, by means of a parallel optical fiber array. Via a rotating additional optics 50 become measuring light sources 30 from the likewise rotating measuring optics 17 always transmitted with the correct angle to the inner surface of a borehole.

In the base part 4 is for each of the measuring light sources 30 a separate detector 35 or a detector element of a common detector 35 intended. In this way can be when using three Meßlichtquellen 30 a surface 19 examine three times as fast. Of course, the number of measuring light sources used 30 but not necessarily limited to three.

The additional optics 50 is designed as a prism with an odd number of reflections, such as a Dove prism or a Schmidt-Pechan prism. In the embodiment shown, the additional optics 50 executed as a frustoconical Dove prism, by means of a motor 51 around the same geometric axis as the probe 3 is rotatable. As can be seen from the beam path of the measuring light through the Dove prism 50 recognizes, is from the lens 11 incoming measuring light when entering the prism 50 its base area 52 fed and reflected there, leaving it to the side 53 exit. Rotates the dove prism 50 around the same axis as the measuring optics 17 the measuring probe 3 with half the rotational speed of the measuring optics 17 , so are the measurement light sources arranged along a line 30 always at the right angle to the surface to be examined 19 projected a borehole.

1

measuring head

2

Beam splitter

3

probe

4

base unit

5

optical fiber

6

Tax- and evaluation unit

7

reflective Job

8th

reflective Job

9

Abtastgerade

10

lens

11

lens

12

lens

13

beamsplitter

14

mirror

15

reference optics

16

reference reflector

17

measurement optics

18

deflector

19

surface

19a

reflective Job

20

drive

21

actuator

22

reference light

23

measuring light path

23a

measuring light beam

24

reference point

25

CCD light

27

focus area

28

center of the focus area

29

fine-scan

30

measuring light source

31

lens

32

optical fiber

33

splitter

34

Wavelength selection means

35

detector

36

well

37

reflective layer

38

mirror

40

lens

41

mirror

42

CCD camera

44

filter

45

electric wire

46

light source

50

additional optics

51

engine

52

footprint

53

page

Claims (32)

Low-coherence interferometric method for light-optical scanning of a surface ( 19 ) of an object, in particular a curved surface, by means of a measuring light beam directed onto the surface ( 23a ) and a short-coherence interferometer, which is a short-coherent measuring light source ( 30 ), a reference reflector ( 16 ), a measuring optics ( 17 ), a detector ( 35 ) and one with the detector ( 35 ) connected control and evaluation unit ( 6 ), in which by movement of the measuring light beam ( 23a ) are adjusted relative to the surface different Lateralabtastpositionen in which the Meßlichtstrahl ( 23a ) to different light-reflecting sites ( 19a ) impinges on the surface, for determining a distance information about the respectively illuminated by the Meßlichtstrahl body ( 19a ) surface longitudinal scanning steps are carried out, in which a match of a along a transverse to the surface scan line changing scanning position with the position of a light-reflecting point ( 19a ) on the surface ( 19 ) of the object is detected, wherein - that of the measuring light source ( 30 ) outgoing light by means of a beam splitter ( 13 ) on a Meßlichtweg ( 23 ) and a reference light path ( 21 ), - a first part of the light as a focused measuring light beam ( 23a ) is irradiated on the object and at the light-reflecting point ( 19a ), - a second part of the light as reference light on the reference reflector ( 16 ) and reflected there, and - the reflected measuring light and the reflected reference light are combined at a beam merger so that the resulting detection light when hitting the detector ( 35 ) generates an interference signal which contains information about the strength of the reflection of the measuring light as a function of the respectively set longitudinal scanning position, characterized in that the measuring light beam ( 23a ) by means of a measuring optics ( 17 ) having a numerical aperture of not more than 0.3, in a scanning direction about a central point ( 28 ) extending focus area ( 27 ) and the scanning position under control of the control and evaluation unit ( 6 ) along the scan line within a fine scan area (FIG. 29 ) that is at least partially aligned with the focus area ( 27 ) matches. Method according to Claim 1, characterized in that, when the lateral scanning position is changed in order to adapt to the shape of the surface, a scanning coarse adjustment takes place in which the fine scanning region ( 29 ) and the focus area ( 27 ) by means of the control and evaluation unit ( 6 ) are rectilinearly moved coaxially with the Meßlichtstrahl. Method according to Claim 2, characterized in that, in the case of sample coarse adjustment, the center (Q ₀ ) of the fine scanning region is at a constant distance from the center (F ₀ ) of the focal region (F ₀ ). 27 ), preferably with the center (F ₀ ) of the focus area ( 27 ) matches. Method according to one of Claims 2 or 3, characterized in that, in the case of the sample coarse adjustment, the position of the focus area ( 27 ) under control of the control and evaluation unit ( 6 ) is shifted using a predetermined target curve. Method according to Claim 4, characterized in that deviations from the predetermined setpoint curve are produced, if the longitudinal scanning results in the distance of the center (F ₀ ) of the focus area ( 27 ) from the surface ( 19 ) exceeds a predetermined limit. Method according to one of Claims 2 to 5, characterized in that, in the case of the sample coarse adjustment, the distance information obtained in at least one preceding longitudinal scanning step is used to control the shift of the focus range ( 27 ) is being used. Method according to one of the preceding claims, characterized in that the measuring light between the measuring optics ( 17 ) and the surface ( 19 ) a deflecting element ( 18 ), through which it is deflected, preferably by 90 °, in the direction of the surface. Method according to claim 7, characterized in that the deflecting element ( 18 ) is a prism. Method according to one of claims 7 or 8, characterized in that the deflection element ( 18 ) by means of a drive ( 20 ) is rotated to the measuring light beam ( 23a ) on an annular area of the surface ( 19 ) of the sample. Method according to one of claims 7 to 9, characterized in that the deflection element ( 18 ) divides the measuring light into two partial beams, which are deflected in opposite directions. Method according to one of claims 7 to 10, characterized in that the deflection element ( 18 ) is a beam splitter. Method according to one of the preceding claims, characterized in that the measuring light beam is divided into partial beams of different wavelength ranges, the partial beams are supplied to different light-reflecting points of the surface, the reflected partial beams are combined together with the reference light, and a low-coherence interference signal for each of the different wavelength ranges generated by the control and evaluation unit ( 6 ) is evaluated. Method according to Claim 12, characterized in that the two partial beams are each guided by a deflection element ( 18 ) towards the surface ( 19 ) are deflected, preferably deflected by 90 °. Method according to one of the preceding claims, characterized in that the detection light is detected by means of a variable wavelength selection device ( 34 ) is selected in dependence on its wavelength in such a way that to the detector ( 35 ) selectively reaches light having wavelengths corresponding to a predetermined sequence of wavenumbers. Apparatus for carrying out the method according to one of the preceding claims with a short-coherence interferometer, which has a short-coherent measuring light source ( 30 ), a reference reflector ( 16 ), a beam splitter ( 13 ), a measuring optics ( 17 ), a detector ( 35 ) and one with the detector ( 35 ) associated control and evaluation unit ( 6 ), and a control and evaluation unit ( 6 ) for controlling the relative movement of the measuring light beam relative to the surface and for controlling the longitudinal scanning required for setting different lateral scanning positions, characterized in that the measuring optics have a numerical aperture of not more than 0.3 so that the measuring light is in a scanning direction in the scanning direction a central point ( 28 ) extending focus area ( 27 ), and the control and evaluation unit ( 6 ) controls a fine scan in which the scan position along the scan line is varied within a fine scan area which is a partial area of the focus area. Apparatus according to claim 15, characterized in that the measuring optics ( 17 ) is a rod optics. Apparatus according to claim 16, characterized in that the measuring optics ( 17 ) includes a GRIN lens. Device according to one of claims 15 to 17, characterized in that the measuring optics ( 17 ) has a numerical aperture of at least 0.05 and at most 0.2. Apparatus according to one of Claims 15 to 18, characterized in that the distance of the measuring optics ( 17 ) from the beam splitter ( 13 ) is changeable. Apparatus according to claim 19, characterized in that the beam splitter ( 13 ) and the reference reflector ( 16 ) on a beam splitter unit ( 2 ), which are used to change the distance from the measuring optics ( 17 ) is displaceable relative to this. Device according to one of claims 15 to 20, characterized in that the measuring optics ( 17 ) is integrated into a thin measuring probe, preferably with a diameter of less than 3 mm, more preferably less than 1 mm. Device according to one of claims 15 to 21, characterized in that the beam splitter ( 13 ), the reference reflector ( 16 ) and the measuring optics ( 17 ) in a relative to the sample movable measuring head ( 1 ) are arranged. Device according to one of Claims 17 to 22, characterized in that the control and evaluation unit ( 6 ) controls a motor, by means of which the measuring head ( 1 ) is movable relative to the object. Apparatus according to any one of claims 15 to 23; characterized in that the measuring head ( 1 ) a CCD camera ( 42 ), which captures an image of the object, wherein the control and evaluation unit ( 6 ) by means of an image processing unit from the image coordinates for controlling the measuring head ( 1 ). Apparatus according to claim 24, characterized in that the signal from the CCD camera ( 42 ) captured image of the measuring optics ( 17 ) is displayed. Apparatus according to one of claims 24 or 25, characterized in that the detector ( 35 ) and the CCD camera ( 42 ) are sensitive in different spectral wavelength ranges. Apparatus according to claim 26, characterized in that the CCD camera ( 42 ) is sensitive to light from the visible spectral range. Device according to one of Claims 15 to 27, characterized in that the detector ( 35 ) is sensitive to light from the infrared, in particular the near infrared spectral range. Device according to one of Claims 24 to 28, characterized in that a second light source ( 46 ) for generating the from the CCD camera ( 42 ) detected light is provided. Apparatus according to claim 29, characterized in that the second light source ( 46 ) is a ring light in the measuring head ( 1 ) is arranged around the Meßlichtweg around. Apparatus according to any of claims 15 to 30; characterized in that it comprises a plurality of short-coherent measuring light sources ( 30 ), which emit light for generating an interference signal, wherein an additional optics ( 50 ) these measuring light sources ( 30 ) via the measuring optics ( 17 ) along a line on the surface to be examined ( 19 ) maps. Apparatus according to claim 31, characterized in that the additional optics ( 50 ) comprises a prism having an odd number of reflections, preferably a dove prism or a Schmidt-Pechan prism, which is rotatable at a rotational speed which is half as great as the rotational speed of the measuring optics ( 17 ).