EP1728045A1

EP1728045A1 - Low-coherence interferometric method and appliance for scanning surfaces in a light-optical manner

Info

Publication number: EP1728045A1
Application number: EP05715081A
Authority: EP
Inventors: Alexander KNÜTTEL
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-03-13
Filing date: 2005-03-10
Publication date: 2006-12-06
Also published as: WO2005088241A8; WO2005088241A1; DE102004012426A1

Abstract

The invention relates to a low-coherence interferometric appliance for scanning a surface (19) of an object in a light-optical manner, especially a concave surface (19), for example a borehole, using a short-coherence interferometer comprising a short-coherence measuring light source (30), a beam splitter (13) for splitting the light emitted by said source into a measuring light beam (23) and a reference light beam (22), a reference reflector (16), a measuring optical element (17), and a detector (35), to which the reflected measuring light beam (23) and the reference light beam (22) are supplied in order to detect a low-coherence interference signal. The measuring light beam (23) is focussed in a focal region (27) extending in a scanning direction, by a measuring optical element (17) having a maximum digital aperture of 0.3, a control and evaluation unit (6) varying a scanning position by a reference point inside said focal region, and registering the correspondence between the scanning position and the surface (19), in order to obtain information about the distance between the reference point and the surface (19) of the object. Location information about a light-reflecting position of the surface (19) is determined from the distance information and the position of the reference point.

Description

Low-coherence interferometric method and device for light-optical scanning of surfaces

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

A surface is usually scanned using sequential methods, in which distance information about its distance from the device (usually from its measuring head) is obtained for a specific point on the surface and then the device by means of a lateral displacement relative to that Surface is positioned so that the distance information can be obtained for another location on the surface. In other words, distance scans and lateral displacements take place alternately in order to gradually obtain the desired detailed three-dimensional information about the course of the surface.

The invention is particularly directed to low-coherence interferometric methods and devices for surface scanning. The distance is scanned using a measuring light beam directed by the measuring head of the device onto the surface to be examined. The direction of the light beam defines a scanning line that runs transversely (generally perpendicular) to the surface, which in practice almost always runs straight, although in principle a distance scanning on a curved scanning line is also possible. In the following, reference is made to a scanning line without restricting generality. Their direction is also referred to as the "z direction". Since this scan in the longitudinal direction of the

Light beam occurs, the term longitudinal scanning is used below. The term Low Coherence Distance Scan (LCDS) is used in the English-language literature for such methods.

Longitudinal scanning with the LCDS method is based on the fact that light from a low-coherent (spectrally broadband emitting) light source is divided into two light paths, namely a measuring light path that leads to the surface to be scanned and a reference light path is divided, and the two partial light paths before striking a detector are brought together in such a way that they interfere with each other. For this purpose, the device contains an interferometer arrangement, which usually comprises a beam splitter, a reference reflector and the detector in addition to the low-coherent light source. The light paths between these elements form interferometer arms. The light from the light source reaches the beam splitter through a light source arm and is split there. A first light component is radiated as a measuring light via an object arm in the scanning direction onto the surface to be examined, while a second light component as reference light reaches the reference reflector via a reflector arm. Both light components are reflected (the measuring light on the surface to be examined, the Reference light at the reference reflector) and returned to the beam splitter in the same light path (object arm or reference arm). There they are combined and fed to a detector via a detection arm of the interferometer in such a way that the light resulting from the combination ("detection light") generates an interference signal when it hits the detector, which provides information about the strength of the reflection of the measurement light as a function of the contains the respectively set longitudinal scanning position.

The scanning position along the scanning line is usually varied by changing the relationship between the lengths of the reference light path and the measurement light path. This changes the position on the scanning line for which the prerequisite for the interference of the measurement light and the reference light (namely that the optical path lengths of the two light paths differ from one another by a maximum of the coherence length of the light source) is fulfilled. The current scanning position is the position on the scanning line for which the optical length of the measuring light path coincides with the optical length of the reference light path (from beam splitting to beam combining) ("coherence condition"). As a rule, the reference light path is shortened or lengthened by moving the reference mirror in the direction of the reference light beam.

When scanning a surface longitudinally, the interference signal reaches a maximum if the scanning position coincides with the position of the surface. The desired distance information for the location of the surface to which the measuring light beam is directed is thereby determined. This is followed by a lateral displacement in order to move the area to another location Perform distance scanning. In order to accelerate the method, it is of course possible to work with a plurality of measurement light beams which are incident on the surface in parallel and thereby simultaneously carry out the Longitudinalina.1 scan for several points on the surface in a longitudinal scanning step. Here too, a method is required for scanning a larger surface, in which a large number of longitudinal scanning steps and lateral displacements take place alternately.

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 takes place without any translationally moving parts and is therefore faster than moving a reference mirror spatially. The device makes it possible, for example, to continuously monitor a film moving past a measuring head to determine whether a desired layer thickness is being maintained. WO 03/073041 and the documents cited therein contain further explanations about LCDS methods, to which reference is made here.

In order to be able to carry out precise measurements with a good signal yield with a low-coherence interferometric device, the measuring light must be focused on the area of the sample surface to be examined. When scanning flat surfaces, for example when a film or a rotating disk is passed past a measuring head, the focus can be maintained unchanged during the scanning of the entire surface. When scanning curved surfaces, however, there is a need to focus according to the Track contours of the surface to be scanned and. to change.

In US Pat. No. 6,144,449 it is proposed to ensure the necessary matching of the focusing with the respective scanning position by means of focus correction means, by means of which the position of the focus is brought into agreement with the respective scanning position at any time during the scanning process. However, the proposed focus correction means are complex and / or require a large amount of space. They are therefore not suitable for many applications or only to a limited extent.

Known low-coherence interferometric methods and devices allow the scanning of surfaces with high precision. However, they are for scanning surfaces with strong unevenness, i.e. Surfaces curved at least in sections, largely unsuitable. Only the scanning of flat surfaces, such as a rotating disk (US Pat. No. 5,473,431) or the above-mentioned film that is guided past a measuring head, is possible at high speed using known low-coherence interferometric devices.

The object of the invention is to show a way in which a low-coherence interferometric device can be used to scan any arbitrary, in particular (at least sectionally) curved surface of an object more quickly.

This object is achieved by a low-coherence interferometric method for the light-optical scanning of a surface of an object, in particular a curved surface, by means of a measuring light beam directed at the surface and a short coherence-in ferometers, which is a short-coherent measuring light source, a reference reflector, a measuring head, a detector and one connected to the detector. Control and evaluation unit, in which different lateral scanning positions are set by moving the measuring light beam relative to the surface, in which the measuring light beam strikes different light-reflecting points on the surface, for determining distance information about the point of the surface longitudinally illuminated by the measuring light beam - Scanning steps are carried out in which a correspondence of a scanning position which is variable along a scanning line running transversely to the surface with the position of a light-reflecting point on the surface of the object is detected, which is characterized in that the measuring light beam by means of measuring optics which have a numerical aperture of not more than 0.3, is focused into a focus region which extends in the scanning direction around a central point, and the scanning position under control of the control and evaluation unit along the r scanning lens within a fine scanning range is varied _y that at least partially matches the focus range.

The task is also solved by a corresponding low-coherence interferometric.

Advantageous developments of the invention are the subject of the dependent claims.

According to the invention, the measuring light is focused by means of measuring optics which have a numerical aperture of not more than 0.3, preferably not more than 0.2, in a focus area extending along the scanning line. By using measuring optics with such a small measurement aperture, the measuring light is focused less sharply, ie to a larger light spot. This reduces the signal intensity and the lateral resolution. Within the scope of the invention it was found that, despite these apparent disadvantages in the combined longitudinal and lateral imaging of curved surfaces, excellent results are achieved when using measuring optics with a small numerical aperture and associated "poorer" focusing.

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 ways: a first application relates to surfaces that are flat overall but have relatively rough structural bumps which could only be scanned with previously known methods with constantly varying focusing and therefore relatively slowly. In such cases, the invention allows the surface to be scanned at a much higher speed. It makes use of the fact that within the focus area 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. This makes it possible to vary the scanning position over such a large spatial area without having to readjust the focusing, that considerable surface roughness, steps and jumps can also be detected by moving the scanning position at high speed. The scanning position can be essentially move faster than the focus area, since no or only significantly smaller masses have to be accelerated mechanically.

The same also applies to uniformly curved surfaces, for example the inner wall of a borehole, provided the measuring head is moved relative to the surface when the lateral scanning position changes so that the measuring light path changes by 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 focus area being controlled by the control and evaluation unit of the device can be adjusted coaxially with the measuring light beam (ie on the scanning line). If the irregularly curved course of the surface (for example, as CAD / CAM data) is known, a target 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 target curve by more than a predetermined limit value, the control algorithm is preferably designed such that the position of the focus area (which deviates from the predetermined target curve) is readjusted when the scanning is roughly adjusted if the deviation occurs due to the fine scanning has been determined.

The invention is particularly suitable for examining surfaces that are irregularly shaped. are curved, without the course of the curvature being so well known that the above-described rough scanning adjustment would be possible by means of a nominal curve. In this case, the distance information obtained in at least one preceding longitudinal scanning step is used to control the displacement of the focus area for the rough scanning setting. In other words, a rough adjustment of the focus area takes place in the coarse scanning setting, in which information that is obtained in one or more longitudinal scanning steps is used as the actual value for setting the position of the focus area. In contrast to previously known devices (US Pat. No. 6,330,063), the focus is not always set synchronously with the scanning position during the invention, but the focus area is only adjusted as required as part of the rough scanning setting, based on the distance information, at least one previous the point spaced from the current light reflecting point.

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 _{0 of} the light used in accordance with: L = a * °

Here a is a scaling factor, the value of which depends on the degree of blurring in the individual case that is acceptable in the vicinity of the limit of the focus range. Preferably a <2, particularly preferably a <1.5. The fine scanning range is preferably completely within the focus area. However, applications are also possible in which the longitudinal scanning is based on interference signals which result from reflections within the focus range, but the fine scanning extends beyond the limits of the focus range.

Measuring light from the near infrared spectral range with a central wavelength λ ₀ in the range from 800 to 1,300 nm is preferably used. Measuring optics with NA = 0.08 focus the measuring light from this spectral range into a focus range with a length of over 150 μm. The diameter of the focus area is approximately 10 to for the values of λ ₀ and NA given as an example (depending on the specific wavelength λ ₀ and the section of the focus area considered in concrete terms)

20 μm. In general, the focus area preferably has a length of approximately 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. The longitudinal scanning position is preferably varied in the detection light path according to WO 03/073041 by means of a variable wavelength selection device. Other methods for fast longitudinal scanning (with as little mechanical movement as possible) can also be used advantageously within the scope of the invention. Examples are cited as citations 4 to 6 in WO 03/073041.

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 type of lens is used in optics for used different purposes. For example, DE 19819762 AI describes a modulation interferometer in which a GRIN lens (gradient index lens) is used.

The invention enables improved quality control in the industrial production of machine parts with curved, especially concave surfaces, for example gear wheels, injection nozzles and boreholes. In microsystem technology in particular, there is a need for a quick, precise and inexpensive way of examining the interior of components, since even the smallest irregularities on their inner walls are disruptive. For example, they have a significant influence on the flow behavior of a fluid flowing through the interior.

Further details and advantages of the invention are explained using an exemplary embodiment with reference to the accompanying figures. The special features illustrated therein can be used individually or in combination in order to create preferred embodiments of the invention. 1 shows an embodiment of a low-coherence interferometric device; FIG. 2 shows an enlarged detail of the measuring probe penetrating into a borehole of the device shown in FIG. 1; 3 shows 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 rough scanning adjustment; 4 shows a highly abstracted schematic diagram of a scanning process in three phases A, B and C with different detection positions of the focus area; Fig. 5 shows an embodiment of a sensor; and FIG. 6 shows the imaging relationships when using a plurality of measurement light sources.

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

In the measuring head 1, the light emerging from the optical fiber 5 is collimated by means of the lens 10 and fed to a free beam splitter 13. The free beam splitter 13 divides the light into a measuring light path 23 and a reference light path 22.

The reference light is refocused on the reference light path 22 by a lens 12 and fed to an elongated reference optics 15 by a mirror 14. The reference optics 15 images a focus generated by the lens 12 onto a reference reflector 16. After reflection at the reference reflector 16, the reference light returns to the free beam splitter 13 in the same way. The free beam splitter 13 and the reference optics 15 and the reference reflector 16 are fastened in a beam splitter unit 2 of the measuring head 1.

The measuring light is focused on the measuring light path 23 by a lens 11 and fed to a measuring lens 17, which, like the reference lens 15 of the reference light path, is preferably designed as a rod lens with a GRIN lens. The measuring optics 17 is part of a very thin measuring probe 3 with a diameter of approximately 500 to 800 μm, which can be inserted into boreholes with a diameter of less than 1 mm. On the side of the measuring optics 17 facing away from the beam controller 13, a deflection element 18 is arranged in the measuring light path 23, which deflects a measuring light beam 23a onto the surface 19 to be examined, in the exemplary embodiment shown the inner surface of a borehole. At a light-reflecting point 19a of the surface 19 of the sample to be examined, the measurement light is reflected and returns via the deflection element 18, the measurement optics 17 and the lens 11 back to the beam splitter 13, where it is combined with the reference light which has passed through the reference light path becomes.

The intensity of the measuring light reflected by the examined surface 19 is subject to large fluctuations depending on the nature of the surface. The problems associated with the evaluation are reduced in that the reference light path has a constant length in the low-coherence interferometric device shown in FIG. 1. The fluctuations in intensity of the reference light, the intensity of which is generally significantly greater than that of the measuring light, are therefore minimal.

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

Different lengths of the reference light path and the measurement light path lead to certain wavelength ranges in the detection light being extinguished by destructive interference from the spectrum of the measurement light source 30. A variable wavelength selection device 34 is arranged in the base unit 4 and is connected to a control and evaluation unit 6. It preferably allows light with wavelengths that correspond to a predetermined sequence of wave numbers to pass through. This sequence is adjusted under the control of the control and evaluation unit 6 so that the scanning position to which the low-coherence signal detected by the detector 35 relates is varied. Further information on this can be found in WO 03/073041 AI cited above, the content of which is made the content of the present application by reference. If the scanning position coincides with a light-reflecting point on the surface 19 to be examined, then an interference signal is generated by the detector 35 and is registered by the control and evaluation unit 6. For this

The scanning and evaluation unit 6 determines the desired distance information.

A special feature of the low-coherence interferometric device shown in FIG. 1 is that the measuring light beam 23 is not focused by the measuring optics 17 into a sharp light spot (in the order of magnitude of the light wavelength) on the surface 19 to be examined, but rather into a surface perpendicular to it Surface 19 extending focus area 27, in which the width of the Measuring light beam 23 changes only slightly. As explained, this is achieved by measuring optics 17 which have 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 varied under control of the control and evaluation unit 6 along a scanning line 9 within a fine scanning area 29, which is part of the focus area and can have a length of 100 μm, for example (FIG. 2).

The deflection element 18, preferably together with the measuring probe 3, which contains the measuring optics 17, can be rotated about its longitudinal axis by means of the drive 20, so that an annular region of the inner surface of a borehole can be examined. To examine the entire inner surface of the borehole, the measuring probe 3 is pushed into the borehole in the longitudinal direction. With gradual movement of the measuring probe 3, the inner surfaces are scanned in successive annular areas. The measuring probe 3 is preferably moved continuously so that the light-reflecting points of the surface 19 which are scanned one after the other adjoin one another in a helical shape.

The deflection element 18 is preferably designed as a prism, but can also be a mirror, for example. Its deflection angle (angle α between the axis of the measuring optics 17 and the scanning line 9) is preferably 90 °, but this is not mandatory. Even if the measuring light beam 23 is deflected by the deflecting element 18 by less or more than 90 °, boreholes can be examined, provided the arrangement is such that the inner surfaces of a borehole can be scanned without gaps by rotation of the measuring probe 3. A deflection angle of 90 ° is advantageous, however, because in the case of vertical reflection, a maximum proportion of the measuring light is returned to the measuring probe 3 is reflected. The deflection element 18 is preferably interchangeable. It can for example be removably secured to the measuring optical system 17 or consisting of ^'measuring optics 17 and deflector 18 assembly as a whole can be interchangeable, so that the measuring probe 3 optimal (for example, a wellbore or a gear) may be respectively adapted to the examined object.

In the exemplary embodiment shown, the entire measuring head 1 can be moved three-dimensionally by means of a multi-axis actuator (not shown), the movement being precisely controllable under the control of the control and evaluation unit 6. The distance between the measuring optics 17 and the beam splitter 13 can be adjusted within the measuring head 1 by means of an actuator 21. This is preferably achieved in that the entire beam splitter unit 2 is displaceable relative to the measuring probe 3.

By changing the distance of the measuring optics 17 from the beam splitter 13, the position of the center F _{0 of} the

Focus area 27 are shifted in the scanning direction so that the walls of the borehole are within focus area 27 (with probe 3 positioned centrally in borehole 36). In particular, the position of the focus area 27 can be adapted to a change in the diameter of the borehole without a measuring light spot formed on the surface to be examined being laterally displaced.

In the construction described, the fine-scan area 29 is shifted in the same direction and synchronously coaxially with the focus area, the amount of the shift being the same for both areas, i.e. the center Qo of the fine-scan area during the shift a constant distance from the center F _{0 of} the focus area Has. In general, it is advantageous if the shifting of the focus area 27 and the shifting of the fine scanning area 29 take place in the same direction and synchronously during the coarse scanning adjustment, a shift by equal amounts being particularly advantageous.

Since the diameter of a borehole to be examined is generally known (at least to an accuracy of 100 μm), the distance between the measuring optics 17 and the beam splitter 13 can be adjusted before being introduced into the borehole so that the surface lies in the focus region 27. For example, the diameter of the borehole to be investigated, e.g. 3 mm or 5 mm, are transmitted to the control and evaluation unit 6.

However, the position of the focus area 27 is preferably set automatically, in the illustrated case by changing the distance between the measuring probe 3 and the beam splitter unit 2. For this purpose, the control and evaluation unit 6 uses the distance information obtained during the fine scanning. The automatic positioning of the focus area 27 is based on an interaction of the fine scanning and the rough scanning setting, which is explained in more detail with reference to FIG. 3.

FIG. 3 shows a highly schematic basic sketch of the essential functions of a device according to the invention in three movement phases A, B and C. The device largely corresponds to the embodiment explained with reference to FIGS. 1 and 2, but with the measuring light beam 23a straight without a deflection element the surface 19 to be scanned is blasted and the variation of the longitudinal scanning position by corresponding ones Varying the position of the reference reflector 16 is effected. The lengths of the scanning area and the focus area are q and. f denotes and strongly represented ^¬ for better kennbarkeit He exaggerated. Although the measurement optics 17 is shown as a simple lens for the sake of simplicity, it is of course a question of an optical arrangement preferably designed as a rod optics with a numerical aperture of less than 0.3.

The partial figure A shows a state in which the center F _{0 of} the focus area 27 coincides with the light-reflecting point 19 a of the surface 19. The length of the reference light path 22 is set so that the center Qo of the fine measuring range 29 corresponds to F ₀ . This means that the position of the reference mirror 16, designated Q ₀ ', is set such that the optical path length of the reference light path 22 (between beam splitter 13 and reference reflector 16) and the optical path length of the measuring light path 23 (between reference reflector) 13 and the reflecting point 19a) when the reference mirror is in the position designated Q ₀ '. During the fine scanning, the reference mirror 16 is moved in an oscillating manner by ± q / 2. Accordingly, the longitudinal scanning position moves by ± q / 2 in the fine scanning area 29.

The partial figure B shows a movement phase in which the surface 19 is shifted to the right by Δs compared to the partial figure A. Since the 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 labeled Q _x 'shifted by Δq. The position of the focus area 27 is still the same as in partial figure A (Δf = 0). The displacement of the surface 19 realized in the context of the foregoing fine scan is processed by the control and evaluation unit of the device into a control signal for the actuator 21 that is, the required sample coarse adjustment by corresponding displacement of the focus ^¬ region 27 and the scanning region 29 (which Center points F ₀ and Q _{0 of} these areas). As a result, the beam splitter 13 and the reference reflector 16 (in the embodiment shown also the measuring optics 17) are shifted by an amount Δf = Δs such that F ₀ and Q _{0 are} again in the longitudinal position of the light-reflecting point (sub-figure C). The center of the scanning area is again in the original position (labeled Q ₀ 'in partial figure A) (Δq = 0).

It is important for the practical success of the invention that the fine scanning takes place much faster than the coarse scanning. It can be said quantitatively that the average scanning speed of the fine scanning v _g , which can be calculated from the length q of the scanning range and the period T of the fine scanning according to v _q = q / T, is at least ten times as large as the maximum speed v _f , _{max of} the movement of the center F _{0 of} the focus area in the coarse scanning setting. As a result, any change in the position of the light-reflecting point detected during the fine scanning is immediately detected. This not only enables fast detection of even fine surface structures, but also a practically instantaneous readjustment of the position of F ₀ and Q ₀ within the scope of the coarse scanning setting.

In the embodiment shown in FIG. 3, the rough adjustment of the scanning takes place in that within the Measuring head 1 a corresponding shift (the components 13, 16 and 17) takes place. For some applications, in particular for the scanning of relatively large objects with complicated surfaces, it is advantageous if instead the entire measuring head 1 is moved three-dimensionally in a precisely controlled manner. Appropriate technologies are available, for example, from robot technology.

Figure 4 illustrates surface profile detection of a surface 19 having a step. To scan different light-reflecting points 19a of the surface 19, the measuring light beam 23 is moved laterally into the detection positions A, B and C shown. The center F _{0 of} the is located at the detection position A.

Focus area 27, which in the following is assumed to coincide with the center Qo of the fine scanning area, below the surface 19. The control and evaluation unit 6 therefore registers the distance between Q ₀ and the surface 19 with a negative sign. The measuring light beam is then moved into the detection position B by a lateral movement of the measuring head 1 along the surface 19. Due to the step shown in FIG. 2 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 region 27 and the light-reflecting point on the surface 19 is positive Sign is registered.

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 is relatively large. During the subsequent lateral movement into the Detection position C, the focus area 27 and the fine scanning area 29 are therefore moved closer to the surface 19 by the control and evaluation unit. Such a displacement in the scanning direction requires a movement of the measuring head 1 or the beam splitter unit 2 relative to the measuring probe 3. Because of the mass to be moved, this is only possible much more slowly than varying the scanning position within the focus range. Therefore, movements of the measuring head 1 in the scanning direction are generally slower than its uniform lateral movement along the surface 19. Although the control and evaluation unit already during the lateral movement of the measuring head from the detection position B to the detection position C (e.g. by actuating the in FIG 1 actuator) a movement of the focus area 27 in the scanning direction was started, F ₀ in the detection position C has not yet reached its optimal position near the surface 19. The adjustment process of the position of the focus area 27 which is initiated during the transition from the detection position B to the detection position C is therefore also continued during the transition into further detection positions (not shown) until the distance between F ₀ and the surface 19 no longer exceeds a predetermined threshold value ,

In order to be able to determine a surface profile of a surface 19 as quickly as possible, the control and evaluation unit 6 uses the distance information from the surface 19 obtained in a first detection position when scanning a first light-reflecting point on the surface 19, already during a lateral movement to one to control a movement of the focus region 27 in the scanning direction in the second detection position. In this way, the focus area 27 does not have to be set individually for each detection position. Instead of which is shifted the focus area 27 continuously with Be ^¬ use of determined for each previous detection position distance information in the scanning direction. Since surface profiles mostly change continuously on a length scale of a few 10 micrometers for concave surfaces 19 of components, the described control of a movement of the focus area 27 allows the speed at which a concave surface 19 can be examined to be significantly increased.

A special feature of the interf erometric device shown in FIG. 1 is that a CCD camera 42 is arranged in the measuring head 1. Using this CC camera 42, an image of the surface e 19 to be examined is recorded in the visible spectral range, while the detector 35 is preferably sensitive to light in the near infrared spectral range.

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

The visible light reflected from the bottom of the borehole passes through the deflection element 18 back into the measuring optics 17 and is fed to the beam splitter 13 via the lens 11. There it is directed onto a CCD camera 42 via a lens 40 and a mirror 41. To improve the image quality, an optical filter 44 is arranged in front of the CCD camera 42, which prevents light from the near infrared spectral range. The CCD camera 42 is connected via a cable 45 to an image processing unit which is part of the control and evaluation unit 6 arranged in the base part 4.

By means of the image processing unit, coordinates for controlling the measuring head 1 are obtained from the image of the object captured by the CCD camera 42, in particular the center of a borehole to be examined is determined. On the basis of this data, the measuring probe 3 can be guided to this center. In this way it is easier and faster possible to insert the measuring probe 3 into a borehole, since the risk of damage to the measuring probe 3 due to a collision with the object is avoided.

Another advantage of the CCD camera 42 lies in the fact that the image captured by it can be used to capture and calibrate an eccentricity of the rotating measuring probe 3 that is hardly avoidable in practice. Since the light detected by the CCD camera 42 is imaged by the measuring optics 17 and visible light is at most insignificantly influenced by the deflecting element 18, an eccentricity of the rotating measuring probe 3 arises therein It is noticeable that the visible light from the measuring optics 17 is not imaged in a circular area, but in a cyclically moving area on the surface 19 to be examined. In this way, the eccentricity of the rotating measuring probe 3 can be detected by means of the CCD camera 42 and taken into account when determining location information about the surface 19 to be examined.

FIG. 5 shows a further exemplary embodiment of a measuring probe 3. As was explained in the foregoing, the essential elements of the measuring probe 3 are the rod optics 17 and the deflecting element 18. In the exemplary embodiment shown in FIG. 5, the deflecting element 18 is designed as a beam splitter that works with a mirror 38 effective in the near infrared spectral range is combined. This beam splitter has a reflection layer 37 which is semi-transparent to light in the near infrared spectral range. In this way, the measuring light 23 emerging from the measuring optics 17 is divided into two opposing partial beams 23b and 23c, so that two opposite light-reflecting points 7, 8 of a borehole can be scanned at the same time.

By means of the deflection element 18 designed as a beam splitter, two measuring light paths are created which differ slightly. On a first measurement light path, the first partial beam of the measurement light is deflected immediately by the reflection layer 37 of the deflection element 18 by 90 ° and supplied to the surface 19 to be examined. On the second measuring light path, the second partial beam of the measuring light first passes through the reflection layer 37, is then reflected by the mirror 38 and, after re-entering the beam splitter, is reflected by the reflection layer 37 and the second light-reflecting point 8 of the surface 19 to be examined. Measuring light reflected from the surface 19 returns via the two measuring light paths described 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 are each at the same distance from the deflection element 18.

The path length difference of the two measuring light paths caused by the arrangement of the beam divider 37 and the mirror 38 is known from the dimensions of the measuring probe 3. The remaining path length difference of the two measuring light paths depends on how exactly the sensor 3 sits in the center of the borehole. The control and evaluation unit 6 can (for example according to the

WO 03/073041), distance information can be obtained for each of the two measurement light paths and thus for the two opposite right-reflecting points 7, 8 of the surfaces 19 to be examined.

With the measuring probe 3 shown in FIG. 5, not only can the speed at which a borehole be examined be increased, but also a mechanical eccentricity of the measuring probe 3 can be determined and compensated independently of the CCD camera 42 described namely, if the beam splitter 37 is in a position deviating from the center point of the borehole, the distance between the light-reflecting points 7, 8 opposite the surface 19 to be examined and the beam splitter 37 differs. This difference is determined by the control and The distance information obtained is evaluated by the evaluation unit 6 and can be used to compensate for the eccentricities, in particular the precision with which a diameter of the borehole can be determined can be increased in this way. With a suitable configuration of the reflection layer 37, it is possible to use two wavelengths simultaneously for low-coherence interferometric scanning of different surface points. For this purpose, the reflection layer 37 is semitransparent for light of a first wavelength range, for example around 830 nm, and permeable for light of a second wavelength range, for example around 1,300 nm. These two wavelength ranges can be separated in the control and evaluation unit 6 and different low-coherence interferometric signals can be generated and evaluated therefrom. 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 bottom of a borehole can be examined with the measuring light of the second wavelength region. It is in particular also possible to arrange a further deflection element (not shown) behind the first deflection element 18, with which the light of the second wavelength range is deflected by an angle between 0 ° and almost 180 °, preferably 90 °, and thus also for examining the walls of the borehole can be used. In this way, adjacent light-reflecting points on the surface 19 can be examined at the same time.

In order to further increase the speed at which a surface 19 can be examined, a plurality of measuring light sources 30 spaced apart from one another can be used, as shown in FIG. The measurement light sources 30 are arranged along a line, for example by means of a parallel optical fiber array. Via a rotating additional optical system 50, measuring light sources 30 are always transmitted from the likewise rotating optical system 17 at the correct angle to the inner surface of a borehole. A separate detector 35 or a detector element of a common detector 35 is provided in the base part 4 for each of the measurement light sources 30. In this way, when using three measuring light sources 30, a surface 19 can be examined three times as quickly. Of course, the number of measurement light sources 30 used is not necessarily limited to three.

The additional optics 50 is designed as a prism with an odd number of reflections, for example as a Dove prism or a Schmidt-Pechan prism. In the exemplary embodiment shown, the additional optics 50 is designed as a truncated cone-shaped Dove prism, which can be rotated about the same geometric axis as the measuring probe 3 by means of a motor 51. As can be seen from the beam path of the measuring light through the Dove prism 50 shown, measuring light coming from the lens 11 is fed into the prism 50 when it enters the prism 50 and is reflected there so that it emerges to the side 53. If the Dove prism 50 rotates about the same axis as the measuring optics 17 of the measuring probe 3 at half the rotational speed of the measuring optics 17, the measuring light sources 30 arranged along a line are always projected at the correct angle onto the surface 19 of a borehole to be examined.

LIST OF REFERENCE NUMBERS

1 measuring head

2 beam splitter unit

3 measuring probe 4. Base unit

5 optical fiber

6 control and evaluation unit

7 light reflecting point

8 light reflecting point 9 scanning line

10 lens

11 lens

12 lens

13 beam splitters 14 mirrors

15 reference optics

16 reference reflector

17 measuring optics

18 deflector 19 surface

19a light reflecting point

20 drive

21 actuator

22 Reference light path 23 Measuring light path

23a measuring light beam

24 reference point

25 CCD light

27 Focus area 28 Center of the focus area Fine scanning range measuring light source lens optical fiber splitter wavelength selection device detector borehole reflection layer mirror lens mirror CCD camera filter cable light source additional optics motor base surface side

Claims

claims

1. 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 (23a) directed at the surface and a short-coherence interferometer, which is a short-coherent measuring light source (30), a reference reflector (16), measuring optics (17), a detector (35) and a control and evaluation unit (6) connected to the detector (35), in which, by moving the measuring light beam (23a) relative to the surface, different lateral scanning positions can be set, in which the measuring light beam (23a) impinges on different light-reflecting points (19a) of the surface, for determining distance information about the point (19a) of the surface illuminated by the measuring light beam, in which longitudinal scanning steps are carried out, in which a correspondence along one one running across the surface n scanning line variable scanning position with the position of a light-reflecting point (19a) on the surface (19) of the object is detected, wherein the light emanating from the measuring light source (30) is divided by means of a beam splitter (13) into a measuring light path (23) and a reference light path (21), - a first part of the light is radiated onto the object as a focused measuring light beam (23a) and on the light-reflecting one Point (19a) is reflected, a second part of the light is radiated as reference light onto the reference reflector (16) and reflected there, and the reflected measurement light and the reflected reference light are brought together at a beam junction in such a way that the resulting detection light strikes when it strikes 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 measuring optics (17) which have a numerical aperture of not more than 0.3, in the scanning direction by ei NEN central point (28) extending focus area (27) is focused, and the scanning position is controlled under control of the control and evaluation unit (6) along the scanning line within a fine scanning area (29) which at least partially matches the focus area (27).

2. The method according to claim 1, characterized in that when the lateral scanning position is changed to adapt to the shape of the surface, a rough scanning setting takes place, in which the fine scanning area (29) and the focus area (27) by means of the control and evaluation unit (6) rectified coaxial with the measuring light beam.

3. The method according to claim 2, characterized in that in the coarse scanning setting the center (Qo) of the fine scanning area has a constant distance from the center (F ₀ ) of the focus area (27), preferably with the center (F ₀ ) of the focus area (27 ) matches.

4. The method according to any one of claims 2 or 3, characterized in that the position of the focus area (27) is shifted under control of the control and evaluation unit (6) using a predetermined target curve in the coarse scanning.

5. The method according to claim 4, characterized in that there is a deviation from the predetermined target curve if the longitudinal scanning shows that the distance of the center (F ₀ ) of the focus area (27) from the surface (19) has a predetermined limit value exceeds.

6. The method according to any one of claims 2 to 5, characterized in that in the coarse scanning setting the distance information obtained in at least one preceding longitudinal scanning step is used to control the displacement of the focus area (27).

Method according to one of the preceding claims, characterized in that the measuring light between the measuring optics (17) and the surface (19) passes through a deflecting element (18) by 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.

9. The method according to any one of claims 7 or 8, characterized in that the deflecting element (18) is rotated by means of a drive (20) in order to direct the measuring light beam (23a) onto an annular region of the surface (19) of the sample ,

10. The method according to any one of claims 7 to 9, characterized in that the deflecting element (18) divides the measuring light into two partial beams which are deflected in opposite directions.

11. The method according to any one of claims 7 to 10, characterized in that the deflecting element (18) is a beam splitter.

12. The method according to any 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 on the surface, the reflected partial beams are brought together with the reference light, and a low coherence for each of the different wavelength ranges Interference signal generated and evaluated by the control and evaluation unit (6).

13. The method according to claim 12, characterized in that the two partial beams are each deflected by a deflecting element (18) in the direction of the surface (19), preferably deflected by 90 °.

14. The method according to any one of the preceding claims, characterized in that the detection light is selected by means of a variable wavelength selection device (34) in dependence on its wavelength such that the detector (35) selectively preferably receives light with wavelengths that have a predetermined Correspond to sequence of wave numbers.

15. Device for performing the method according to one of the preceding claims with a short-coherence interferometer, the short-coherent measuring light source (30), a reference reflector (16), a beam splitter (13), measuring optics (17), a detector (35 ) and a control and evaluation unit (6) connected to the detector (35), and a control and evaluation unit (6) for controlling the relative movement of the measuring light beam relative to the surface required for setting different lateral scanning positions and for controlling the longitudinal scanning, thereby characterized in that the measuring optics have a numerical aperture of not more than 0.3 so that the measuring light is focused in a focus area (27) extending in the scanning direction around a central point (28), 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.

16. Apparatus according to claim 15, characterized in that the measuring optics (17) is a rod optics.

17. Apparatus according to claim 16, characterized in that the measuring optics (17) includes a GRIN lens.

18. 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.

19. Device according to one of claims 15 to 18, characterized in that the distance of the measuring optics (17) from the beam splitter (13) is variable for coarse scanning.

20. Apparatus according to claim 19, characterized in that the beam splitter (13) and the reference reflector (16) are attached to a beam splitter unit (2) which is displaceable relative to the measuring optics (17) for changing the distance.

21. Device according to one of claims 15 to 20, characterized in that the measuring optics (17) in a thin measuring probe, preferably with a diameter of less than 3 mm, particularly preferably less than 1 mm, is integrated.

22. 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) are arranged in a measuring head (1) movable relative to the sample.

23. 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.

24. Device according to one of claims 15 to 23, characterized in that the measuring head (1) contains a CCD camera (42) which detects an image of the object, the control and evaluation unit (6) by means of an image processing unit from the Coordinates for controlling the measuring head (1) are determined.

25. Apparatus according to claim 24, characterized in that the image captured by the CCD camera (42) is imaged by the measuring optics (17).

26. Device 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.

27. Apparatus according to claim 26, characterized in that the CCD camera (42) is sensitive to light from the visible spectral range.

28. 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.

29. Device according to one of claims 24 to 28, characterized in that a second light source (46) is provided for generating the light detected by the CCD camera (42).

30. Apparatus according to claim 29, characterized in that the second light source (46) is a ring light which is arranged in the measuring head (1) around the measuring light path.

31. Device according to one of claims 15 to 30, characterized in that it has a plurality of short-coherent measuring light sources (30) which emit light for generating an interference signal, with an additional optical system (50) these measuring light sources (30) via the measuring optical system (17 ) along a line on the surface to be examined (19).

32. Apparatus according to claim 31, characterized in that the additional optics (50) contains a prism with an odd number of reflections, preferably a Dove prism or a Schmidt-Pechan prism, which is rotatable at a rotational speed that is half as much large as the speed of rotation of the measuring optics (17).