DE10115524A1 - Interferometric measuring device - Google Patents

Abstract

The invention relates to an interferometric measuring device (1) for measuring the shape of a surface of an object (O) with a radiation source (LQ) emitting short-coherent radiation, a beam splitter (ST) for forming an object beam guided to the object (O) via an object light path (OS) and a reference beam (RS) guided via a reference light path to a reflecting reference plane (RSP) and with an image recorder (BA), which records the radiation reflected by the object (O) and the reference plane (RSP) and brought to interference and an evaluation device for determining the surface shape. Good adaptability and manageability even at measuring points that are difficult to access is made possible by the fact that an optics (SO) that is rigid with respect to the object (O) is arranged in the object light path and that the rigid optics (SO) are optics that are movable in the direction of their optical axis (BO). follows.

Description

State of the art

The invention relates to an interferometric measuring device for Shape measurement of a surface of an object with a short-coherent Radiation emitting radiation source, a beam splitter to form an over an object light path to the object guided object beam and one via directed a reference light path to a reflective reference plane Reference beam and with an image sensor, which of the object and the Radiation reflected and brought to interference on the reference plane increases and an evaluation device for determining the surface shape leads.  

An interferometric measuring device of this type is in DE 41 08 944 A1 specified. In this known interferometric measuring device based on the measuring principle of so-called white light interferometry or short coherence based on interferometry, a radiation source emits short-coherent radiation, via a beam splitter into an object beam illuminating a measurement object and a reflective reference plane in the form of a reference mirror illuminating reference number is divided. To the object surface in To scan the depth direction, the reference mirror is Move the control element in the direction of the optical axis of the reference light path. If the object light path and the reference light path match, the result is in the area of the coherence length a maximum of the interference contrast, which by means of a photoelectric image sensor and a downstream evaluation device recognized and for determining the contour of the object surface on the The basis of the known deflection position of the reference mirror is evaluated becomes.

Other such interferometric measuring devices or interferometric Measurement methods based on white light interferometry are described in P. de Groot, L. Deck, "Surface profiling by analysis of white-light interferograms in the spatial frequency domain "J. Mod. Opt., Vol. 42, No. 2, 389-401, 1995 and T. Maack, G. Notni, W. Schreiber, W.-D. Prenzel, "Endoscopic 3-D shape measuring system", in Yearbook for Optics and Precision Mechanics, Ed. W.-D. Prenzel, publishing house Schiele and Schoen, Berlin, 231-240, 1998.

With the interferometric measuring devices or measuring methods mentioned There is a difficulty in making measurements on different, in particular  hard-to-reach places such as B. in deep cavities or narrow channels with sufficient lateral resolution. In the (not pre-published lichten) German patent application No. 199 48 818 is to fix this Problem suggested, in the arm of the object light path at least a twos to generate a map, which also means a greater lateral resolution a narrow cavity or channel is reached. On the other hand, there is from an enlargement of the numerical aperture a smaller depth of field and also a scan of a surface area, the normal (view direction) obliquely to the axis of the imaging device of the object light path oriented leads to problems with the scanning in the depth direction.

The problem of keeping the depth of field in the depth scan can be avoided that not the reference plane or this forming reference mirror is moved, but the reference light path fixed held and the object light path is changed instead. This can again in two ways, namely by the fact that the Object itself is moved in the depth direction, or on the other hand by the fact that the interferometric part of the measuring device is moved relative to the object. Though are such changes in the object light path as a measure for depth scanning device in white light interferometry, for example, from the above Journal articles known per se, but they are technical difficult to achieve, especially in the manufacture of parts.

The invention has for its object an interferometric measurement provide device of the type mentioned, the simple Adaptability to different measurement problems allowed, the achievement  the best possible measurement result with the simplest possible structure and the simplest possible measurement is favored.

This object is achieved with the features of claim 1. After that is provided that in the object light path a reference to the object (during the Measurement) rigid optics is arranged and that the rigid optics are an in Moving optics follow the direction of their optical axis (during the measurement).

With the rigid optics and the movable optics connected downstream diverse possibilities, in a simple way different surfaces to be measured even in hard-to-reach places. For example, with a deflecting element the possibility of an oblique to the direction of movement of the depth scan (surface scan) directed position in To sense depth direction. With other images that deform the wavefront elements such as refractive, diffractive or reflective elements (e.g. Lenses, concave mirrors, gratings etc.) or a combination of such optical Elements result in further adaptability to a respective measurement sample lem without having to extensively modify the overall structure of the measuring device have to.

It is envisaged that the rigid optic will be made entirely or partially as an endoscope is formed, so there is the possibility, even with a measurement in narrow Cavities to achieve a relatively large lateral resolution.

With the measure that the rigid optics form part of an intermediate image is optics, the effort, the measuring device to different  Adapt measurement tasks, still significantly favored. Here it is particularly favorable if the intermediate image is in the intermediate image generating optics located in the object light path.

To achieve a robust against lateral relative movement of the object Measurement is advantageously provided that the rigid optics follow the object Infinitely depicted.

Different constructions consist of the fact that the moving optics are completely outside half, partly inside and outside, or completely inside the object light way is.

With the measures that the moving optics completely or partially from opti elements that are movable in the optical axis can be e.g. B. just build a zoom lens.

The measures that an image of the ref border plane in the depth of field that images the object on the image sensor end optics (imaging optics). It is advantageous here that the image of the Reference plane lies in the image plane of the imaging optics, and furthermore that the image of the reference plane moves synchronously when the moving optics move moved with the image plane of the imaging optics.

An advantageous embodiment of the invention also consists in that the rigid optic is designed as a rigid intermediate imaging device with  the at least one intermediate image of the object surface that is rigid with respect to the object preferably in the object light path - is generated, and that as a moving optic the following lens optics in the beam path behind the rigid intermediate image Direction of its optical axis movable to scan the normal to it Axis-aligned rigid intermediate image in the depth direction and imaging the same directly or via one or more intermediate images on the Image sensor is formed. By generating the z. B. in the object light path lying rigid intermediate image of the object surface with the rigid intermediate Image imaging device in the object light path is on the one hand also in narrow Channels or holes the object surface to be measured with a relatively large lateral resolution can be recorded and with the image sensor and the reshaped evaluated evaluation device with respect to the depth structure. The The rigid intermediate image can be scanned with relatively simple measures Lich, since only a few optical components of the Ob have to be moved around, the depth of the rigid intermediate image always in the depth of field of the movable lens Optics remains, since the object plane of the moving lens optics as it were through the rigid intermediate image is moved and in this way the interference maxima in the area of greatest warp fe can be evaluated. In addition, the rigid intermediate image is always normal directed to the direction of movement of the lens optics or alignable, as well an oblique arrangement with respect to the axis of the object light path Object surface slightly normal to the axis of the moving lens optics is reproducible. This makes it easy to measure surface areas  sen, the normals directed obliquely to the direction of movement of the lens optics is.

The image quality and accuracy of the evaluation is thereby started tends to make the intermediate imaging device one for all in the intermediate image depicted object points have the same magnification. For example The structure can be such that the intermediate imaging device as telecentric imaging device is formed in a 4f arrangement.

To achieve accurate measurement results, it is also advantageous that in the Reference light path to compensate one of the optics in the object light path at least partially corresponding (or identical) optics is present.

One for the flat evaluation of the surface area under consideration Partial training of the measuring device is that the image sensor has flat image recording elements (pixels) and that for each Pixel the position of the lens optics is detected, at which the highest Interference contrast occurs.

The depth scan of an obliquely arranged object surface becomes one simple way that allows one of the normal of the object surface deviating viewing direction of the intermediate imaging device Imaging unit is provided for generating the intermediate image.

A simple, easy-to-use structure of the measuring device is thereby favorable that a movable unit besides the movable lens optics,  a lighting unit with the light source and the beam splitter or except the moving object optics only includes the beam splitter.

A design of the measuring device that is favorable for handling and construction tion further consists in that the rigid intermediate imaging device as Endoscope is formed.

With the measures that illuminate the object and the reference plane a fiber optic is provided, there is the advantage that reflections on the Lenses of the imaging device can be reduced.

Different training options for the light paths result from that the object light path and the reference light path as well as further light paths as Lenses have achromatic lenses, grin (gradient index) lenses or rod lenses.

The shape measurement is favored by the fact that in the imaging optics there is an optical element inside or outside the object light path through which the image is in a favorable position for evaluation is rotatable.

He can thereby access a measuring point within an object be lightened that in the object light path as a rigid optic or part of the rigid optics a folding endoscope is arranged in at least two Folded positions with an angle between a folded and a unfolded state is adjustable.  

It is advantageously provided that the folding endoscope two with one Has articulated tubes in which optical components ten of the folding endoscope including a deflecting element are.

The folding positions can be easily adjusted by the fact that in the area of the Articulated a spring mechanism cooperating with both tubes is not.

The invention is described below with reference to exemplary embodiments took explained in more detail on the drawings. Show it:

FIGS. 1A and 1B is a schematic representation of an object rigid deflection unit in two different positions Tiefenabtast a first embodiment of an interferometric measuring device with a respect to

Figs. 2A and 2B is an illustration of another embodiment of the interferometric metric measuring device, wherein the deflector element is replaced by forming from the wavefront-deforming elements, in two different scanning positions,

Fig. 3 device, another embodiment of the interferometric measuring, with a different arrangement of the optically rigid elements in the object,

FIGS. 4A and 4B, another embodiment of the interferometric measuring device, in which the size of a moving unit is reduced Refe rence of the light path,

Fig. 5A and 5B show a further embodiment in which both movable Op tik be moved as well as the reference mirror,

Fig. 6 is a schematic illustration of another embodiment of an interferometric measuring device,

Fig. 7 is an illustration of area of the interferometric measuring apparatus of Fig. 6 with an arrangement for measuring an oblique upper object,

Fig. 8 shows a further example of the interferometric measuring apparatus in which the number of moving elements relative to FIGS. 6 and 7, Ringert ver,

Fig. 9 apparatus, a further embodiment of the interferometric measurement in which the number of moving elements is further Ringert ver,

Fig. 10 processing a further embodiment of the interferometric Messvorrich, is pre see a Lichtleitphaseranordnung in the lighting,

Fig. 1l another embodiment of the interferometric Messvorrich processing with a means for rotating the image,

Fig. 12 is an embodiment of the interferometric measuring apparatus with a folding endoscope during insertion operation,

Fig. 13, the measuring apparatus of FIG. 12 in the measuring position and

FIG. 14a) to c) several views of the folding endoscope.

In the case of an interferometric measuring device 1 shown in FIGS . 1A and 1B, short-coherent radiation from a radiation source or light source LQ (for example a light-emitting diode or superluminescent diode), the coherence length of which is typically of the order of magnitude of approximately 10 μm (for example third up to 100 µm), is guided via a lens L4 and further optical elements onto a beam splitter ST and is split into an object beam OS directed to a surface of an object O and a reference beam RS directed to a reference mirror RSP.

In the object light path or object arm, a rigid optic SO in the form of a reflecting deflection unit AE is arranged in front of the object O in such a way that the inclined object O is always scanned in the depth direction perpendicular to its surface, as shown in FIGS. 1A and 1B, in which two different scanning depths are shown, which are caused by the deflection of a moving unit BEW by means of a motion generator BE attached to it, for example a piezo element. A virtual reference plane VR is therefore in the normal direction at different depths with respect to the object O. The rigid optics SO, which in the present case is formed by the deflection unit AE, are rigid with respect to the object O. It is followed by a movable optics BO in the beam path is arranged in the moving unit BEW in the beam path between the beam splitter ST and the image sensor A.

If the movement of the moving unit BEW the object light path and the reference light path match, results in Range of coherence length a maximum of the interference contrast, which by means of of the photoelectric image sensor BA and a downstream evaluation device recognized and for determining the contour of the object surface on the Basis of the known deflection position is evaluated.

In the further exemplary embodiment shown in FIGS . 2A and 2B, in which two different deflection positions of the moving unit BEW are shown by means of the motion generator BE, the rigid optics SO is configured differently from the previous exemplary embodiment, namely by imaging, deforming the wavefront Elements in the form of lenses L2, L3. With the rigid optics SO, a rigid intermediate image is generated in an intermediate image plane ZE, which is scanned in the depth direction by means of the moving optics located in the moving unit BEW. With this measure, there is a simple possibility of adapting a scanning unit that is constant per se to different measuring situations, with z. B. measurement in narrow cavities is achieved with a relatively large lateral resolution. There is also no problem with the depth of field, since the object surface is always optimally aligned with the imaging movable optics BO. The object O is imaged on the image sensor BA via the imaging optics, which contains the rigid optics SO and the movable optics BO. With a suitable arrangement of the components in the moving unit BEW, it can be sufficient that the image of the reference plane VR is moved with the image plane of the imaging optics.

In a further exemplary embodiment shown in FIG. 3, the rigid optics SO comprise an imaging element which changes the wavefront in the form of the lens L3 and a deflection unit in the form of a refractive element, so that in turn an object surface lying obliquely with respect to the depth scanning direction is always measured in the correct position . Another, the wavefront-changing element in the form of the lens L2 is here within the moving unit BEW, so that there is also a simple possibility of adaptation to a scanning device, but on the other hand the depth measurement range is limited to the depth of field, since the overall image is flat does not migrate with the depth scan. The rigid optics SO represent the object infinitely.

In the embodiment shown in FIGS. 4A and 4B, likewise in two different scanning positions, the moving unit BEW is designed to be very small, a larger diameter of imaging lenses in the reference light path being used in connection with the beam distribution on the beam splitter ST.

In a further exemplary embodiment shown in FIGS . 5A and 5B, a moving unit BEW is provided in the reference light path and in the object light path, which are deflected synchronously by means of an associated movement generator BE.

Further preferred exemplary embodiments are shown in FIGS. 6 to 10. According to the exemplary embodiment according to FIG. 6, an intermediate imaging device SO (hereinafter also referred to as a bayonet lens) with intermediate imaging lenses L2, L3, with which a rigid lens is arranged, is arranged in the object light path or object arm as the rigid optics SO Intermediate image SZB of the object surface is generated. The length of the reference light path corresponds to the object light path, so that a virtual reference VR lies in the area of the object surface. An image of the virtual reference VR is generated as an image of the reference plane BR in the area of the rigid intermediate image SZB with the bayonet optics SO. The bayonet optics SO are endoscopic for a favorable construction and handling and are attached, for example, to a housing containing the rest of the optical system. Another possibility is that the bayonet optics SO are mechanically separated from the housing and coupled to the object O in a fixed manner.

The reference light path or reference arm contains the optical elements of the Object arms essentially corresponding optical elements as compensated Optics KSO, so that disruptive optical properties of the optics in object light be compensated away.

The rigid intermediate image SZB created by the bayonet optics SO is at depths direction, d. H. parallel to its normal by means of parallel to the normal direction, d. H. along its optical axis moving optics BO in depths direction scanned (depth scan). The image of the reference plane BR is in the archipelago depth range of the movable lens, preferably in the object plane of the movable lens or the movable lens optics. The depths is done by moving the movable lens BO relative to the rigid Zwi The image of the SZB is moved, ensuring that the image of the Re ferenzebene BR moved synchronously with the movable lens optics BO.

As a result, when the movable lens BO is scanned, the image of the Re ferenzee plane BR moved by the rigid intermediate image SZB.

The rigid intermediate image SZB is created directly by the movable lens optics BO or via at least one intermediate image on a variety of ne image recorders BA lying one next to the other, e.g. B. a CCD camera is shown and in a subsequent evaluation device device for determining the surface shape z. B. by detecting the maxima  of the interference contrast evaluated, the respective position of the movable lens BO is used.

High interference contrast occurs in the image of the object on the image sensor BA then when there is a path difference in the object arm and the reference arm is smaller than the coherence length. To obtain a 3D height profile various methods known per se (cf. the pressure mentioned at the beginning fonts) can be applied.

The structure shown in FIG. 6 and also the following figure exemplarily includes a Michelson interferometer. With the rigid bayonet optics SO, several intermediate images can also be created. For the scanning, the area shown in broken lines is moved, which, for. B. can lie within a housing on which is mounted in a bayonet look SO. Alternatively, the bayonet optics SO can also be separated from the housing and fixed to the object.

In the construction shown in FIG. 7, the surface of the object O to be measured is arranged obliquely with respect to the optical axis of the bayonet optics SO, and a deflection element AE or another imaging unit is positioned in front of the object, via which a movable element can be moved with respect to the optical axis Whether jective BO's normally oriented rigid intermediate image SZB is generated. By scanning the rigid intermediate image SZB, the scanning of the oblique object surface can be carried out with simple measures, since the viewing direction of the optical axis of the movable lens BO on the intermediate image has 0 °. The scanning axis now only has to be directed parallel to the axis of the movable lens BO. The viewing direction of the bayonet optics SO is therefore independent of the scanning axis of the depth scanning.

In the construction of the interferometric measuring device 1 shown in FIG. 8, the number of moving components which carry out the depth scanning is substantially reduced, as the number of elements shown in the dashed area shows, which essentially comprises the reference arm, the beam splitter ST and includes the movable lens BO.

Another embodiment of the interferometric measuring device 1 with scanning of the rigid intermediate image SZB is shown in FIG. 9. Here, in addition to the movable lens BO, the beam splitter ST and the lighting unit with the light source LQ are moved. A slight displacement of the reference beam transverse to the optical axis of the reference arm has practically no effect on the measurement result due to the relatively small scanning movement distance.

In the exemplary embodiment shown in FIG. 10, the object O is alternatively illuminated via a fiber optic with light guides LL, which at least partially runs within the bayonet optics SO. This fiber-optic lighting has the advantage that reflections on the lenses of the bayonet optics SO are reduced. In order to compare the optical path lengths and the dispersion in the object arm and the reference arm, the fiber lengths and geometries in the two interferometer arms should be chosen to match as closely as possible.

For the lenses contained in the interferometric measuring device different forms of training can be chosen, e.g. B. achromatic, grin (Gra serves index) lenses or rod lenses.

In the exemplary embodiment shown in FIG. 11, an optical element DE for rotating the image is contained in the imaging optics. Is the rigid optics SO rotated with respect to the object, for. B. for measuring different segments of a radially symmetrical object (such as a valve seat), the image of the object O also rotates on the image sensor BA. However, it is advantageous to have a fixed image of the object on the image sensor BA. This can be achieved by providing an optical element (e.g. reversion prism, Dove prism, etc.) in the imaging optics, preferably outside the object light path, by means of which the rotation of the image can be compensated for again.

In Figs. 12 to 14 a further embodiment of the interferometric rule measuring device is shown, wherein the rigid optics is designed as a folding SO endoscope KL.

The folding endoscope KL consists, for example, of two tubes T1, T2, each having tube axes TA1, TA2 and connected to one another by a joint G in order to form a pivot axis KA of one tube T1 relative to the other tube T2. For example, the two tubes T1, T2 can assume two different folding positions, which differ by a folding angle a, as can be seen from FIGS . 14b) and c). In the present case, the two tubes T1, T2 are mechanically manufactured in such a way that the two tube axes TA1, TA2 form an angle of 0 ° in the folded state, while in the expanded state they are oriented to one another by the predetermined folding angle α. Between the tubes T1, T2, which contain the optical components OKL of the folding endoscope, there is a spring mechanism with a spring F in the area of the joint G. If the folding endoscope KL is in the folded state, the spring F is excited while she is relaxed when unfolded. In the present case, the endoscope optics is designed for the unfolded state. The endoscope optics contains at least one optical steering element, e.g. B. a mirror KSP, which is arranged in the region of the joint G. Prisms or gratings through which the optical axis is deflected in accordance with the tube axes TA1, TA2 are also conceivable as deflection elements. As shown in FIG. 12, when the object O is introduced, the folding endoscope KL is in the folded state. This can be achieved with the spring under tension by using the endoscope's own guide. The object itself can also serve as a guide, e.g. B. a guide bore in a valve seat measurement, as shown in FIGS. 12 and 13. If the folding endoscope KL is completely inserted into the object O or the component, the joint G should be exposed so that the folding endoscope KL can assume the unfolded state. The folding endoscope KL is manufactured in such a way that exactly the measuring point MST is observed in the unfolded state.

The object surface to be observed is by the folding endoscope KL in when unfolded, illuminated with a flat wave and directly or over an intermediate image is imaged on the image sensor BA (eg CCD camera. Im Reference light path becomes the reference beam RS from the reference mirror RSP right inflected. To compensate for the endoscope optics, the ref  renzlichtweg or reference arm a similar or ent to the endoscope optics speaking optics can be used. The data is evaluated as in Zu described in connection with the preceding embodiments.

The interferometer can also do differently than a Michelson interferometer can be realized (e.g. as common path, Mach-Zehnder, etc.).

Claims (24)

1. Interferometric measuring device ( 1 ) for measuring the shape of a surface of an object (O) with a short-coherent radiation emitting radiation source (LQ), a beam splitter (ST) for forming an object beam (OS) guided via an object light path to the object (O) ) and a reference beam (RS) guided via a reference light path to a reflecting reference plane (RSP) and with an image sensor (BA) which records the radiation thrown back by the object (O) and the reference plane (RSP) and brought to interference and an evaluation device for determining the surface shape, characterized in that
that an optics (SO) which is rigid with respect to the object (O) is arranged in the object light path and
that the rigid optics (SO) are followed by a moving optics (BO) in the direction of their optical axis. 2. Measuring device according to claim 1, characterized, that the rigid optics (SO) on the wavefront deforming elements has.   3. Measuring device according to claim 1 or 2, characterized, that the rigid optics (SO) are wholly or partially designed as an endoscope is. 4. Measuring device according to one of the preceding claims, characterized, that the rigid optic is part of an optic that creates an intermediate image. 5. Measuring device according to one of the preceding claims, characterized, that the rigid optics (SO) represent the object after infinity. 6. Measuring device according to one of the preceding claims, characterized, that the moving optics (BO) are completely outside, partly inside and is outside or entirely within the object light path. 7. Measuring device according to one of the preceding claims, characterized, that the moving optics (BO) wholly or partly from optical ele elements exist that are movably mounted in the optical axis. 8. Measuring device according to one of the preceding claims, characterized,  that an image of the reference plane (VR) in the depth of field of the abbil optics. 9. Measuring device according to claim 8, characterized, that the image of the reference plane (VR) in the image plane of the imaging Optics lies. 10. Measuring device according to claim 8 or 9, characterized, that the image of the reference plane (VR) moves with the moving Optics (BO) moved synchronously with the image plane of the imaging optics. 11. Measuring device according to one of the preceding claims, characterized in that
that the rigid optics (SO) is designed as a rigid intermediate imaging device (L2, L3) with which at least one intermediate image (SZB) of the object surface that is rigid with respect to the object (O) is generated, and
that as moving optics (BO), a lens optics following in the beam path behind the rigid intermediate image (SZB) is movable in the direction of its optical axis (ROA) for scanning the rigid intermediate image (SZB) oriented normal to this axis in the depth direction (Z) and Imaging the same is formed directly or via one or more intermediate images on the image sensor (BA). 12. Measuring device according to claim 11, characterized, that the intermediate imaging device (L2, L3) one for all in the inter object image (SZB) depicted object points on the same scale owns. 13. Measuring device according to claim 12, characterized, that the intermediate imaging device (L2, L3) as telecentric Ab education device is formed in 4f arrangement. 14. Measuring device according to one of the preceding claims, characterized, that in the reference light path for compensation one of the optics in the object appropriate optical path (KSO) is at least partially present. 15. Measuring device according to one of the preceding claims, characterized in that
that the image sensor (BA) has surface-mounted image recording elements (pixels) and
that the position of the lens optics (BO) at which the highest interference contrast occurs is detected for each pixel. 16. Measuring device according to one of the preceding claims, characterized,  that with a deviating from the normal of the object surface Direction of view of the intermediate imaging device (L2, L3) a deflection unit (AE) for generating the intermediate image (SZB) is provided. 17. Measuring device according to one of the preceding claims, characterized, that a movable unit besides the movable lens optics (BO), a lighting unit with the light source (L2) and the beam splitter (ST) or apart from the movable lens optics (BO) only the beam splitter (ST) includes. 18. Measuring device according to one of claims 1 l to 17, characterized, that the rigid intermediate imaging device (L2, L3) as an endoscope is trained. 19. Measuring device according to one of the preceding claims, characterized, that to illuminate the object (O) and the reference plane (RSP) one Fiber optics (LL) is provided. 20. Measuring device according to one of the preceding claims, characterized,  that the object light path and the reference light path as well as other light paths as lenses (L1, L2, L3, L4, KSO) single lenses, grin (gradient Index) lenses, rod lenses, diffractive elements, prisms or their com have combinations. 21. Measuring device according to one of the preceding claims, characterized, that in the imaging optics inside or outside the object Lichtweges is an optical element (DE) through which the Image can be rotated into a position that is favorable for the evaluation. 22. Measuring device according to one of the preceding claims, characterized, that in the object light path as rigid optics (SO) or part of the rigid Optics (SO) a folding endoscope (KL) is arranged in at least two folding positions with an angle (α) between one folded and an unfolded state is adjustable. 23. Measuring device according to claim 22, characterized, that the folding endoscope (KL) two with a joint (G) with each other has connected tubes (T1, T2) in which optical components (OKL) of the folding endoscope (KL) including a deflecting element (KSP) are accommodated.   24. Measuring device according to claim 23, characterized, that in the area of the joint (G) one with both tubes (T1, T2) men acting spring mechanism (F) is arranged.