Abstract:
An interferometric system for measuring surfaces of a measured object using an optical system. The optical system has a beam splitter, which directs measuring beams in a first beam path and measuring beams in a second beam path onto the surfaces of the measured object with the aid of two mirrors. The beam paths which are formed by the light beams which are reflected on the surfaces at least partially overlap in an area having identical beam direction. In this manner, measured surfaces of the measured object are at least partially imaged on an identically irradiated surface of a detector, such as an image recorder.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an optical system for illuminating at least one surface of a measured object and an interferometric system for measuring surfaces of a measured object. 
       BACKGROUND INFORMATION 
       [0002]    An interferometric measuring device for measuring an object, in particular for measuring the thickness of the object, is discussed in German patent application DE-OS 10 2004 045 806 A1. Described is a construction of a Michelson interferometer having a light source, a beam splitter for forming reference beams and object beams, a reference mirror in the reference light path, and an image recorder. A special objective is situated in the object light path. It divides the object beams into two partial object beams. 
         [0003]    Both partial object beams are each directed onto a deflection mirror. The partial object beams which are deflected on the mirrors are used for the simultaneous, perpendicular illumination of two object surfaces, which are parallel to one another. The partial object beams which are reflected on the object surfaces are superimposed with the reference beams and are incident on the image recorder of the measuring device. Interference phenomena occur in the case of the superposition when the reference beams and the reflected partial object beams have the same optical path length. The image recorder registers the interference patterns which are formed. This optical path length at which interferences occur is ascertained for each of the reflected partial object beams of an object surface. This is performed by displacing the reference mirror in the reference light path until reference patterns are formed. On the basis of the ascertained optical path lengths for both partial object beams, the distance of both object surfaces to one another, and thus the thickness of the object at the location of the measured object surfaces, are inferred. 
         [0004]    The division of the object beams in the special objective into two partial object beams is performed in the form of an aperture division. Two partial cross sections of the object beams, which are contiguous per se, are each separated into two partial object beams having a separate beam path. The partial object beams which are reflected on the object surfaces are joined together again in the same direction at the position of the performed aperture division. 
         [0005]    The object beams which are joined together have non-overlapping partial cross sections, which are contiguous per se, and through which exclusively reflected partial object beams of an object surface pass. This has the result that only a specific partial surface of an image recorder, which is typically square, is exclusively exposed by the particular reflected partial object beams of an object surface. Accordingly, the measured object surfaces are imaged separately from one another on the particular partial areas of the image recorder. A unique assignment of the measuring signals on a partial surface of the image recorder to the measured object surfaces is thus made possible. 
       SUMMARY OF THE INVENTION 
       [0006]    The exemplary embodiments and/or exemplary methods of the present invention are based on the object of proposing an optical system for illuminating at least one surface of a measured object, in which a beam path of light beams which are reflected on a first object surface at least partially overlaps with a beam path of light beams which are reflected on a second object surface, all light beams being oriented in an identical direction in the area of the overlap. 
         [0007]    Furthermore, it is the object to situate an optical system in an interferometric measuring system, measured object surfaces being imaged at least partially on an identically irradiated surface of a detector, such as an image recorder. 
         [0008]    These objects are achieved according to the exemplary embodiments and/or exemplary methods of the present invention by an optical system for illuminating at least one object surface and by an interferometric system for measuring object surfaces according to the characterizing features of the independent claims. 
         [0009]    The optical system according to the present invention for illuminating at least one object surface provides at least one first optical element, which implements at least one first beam path and at least one second beam path if light beams of a radiation-generating light source are introduced in it. The light beams in the first beam path are directed, which may be perpendicularly, onto a first object surface via a second optical element, which is downstream in the light path from the first optical element. Furthermore, light beams in the second beam path are directed, which may be perpendicularly, onto a second object surface via a third optical element, which is downstream in the light path from the first optical element. 
         [0010]    The light beams in the first beam path advantageously originate from the largest possible area of the cross section of the light beams which are introduced onto the first optical element, which may be from the complete cross section. Furthermore, the light beams in the second light beam path also originate from the largest possible area of the cross section of the light beams which are introduced onto the first optical element, which may also be from the complete cross section. 
         [0011]    In addition, further optical elements may be situated for guiding the introduced light beams, the light beams in the at least one first beam path, and/or the light beams in the at least one second beam path. In this case, this means guiding the light beams to an object surface and/or guiding the reflected light beams away from the particular object surface. 
         [0012]    The light beams, which are directed at least approximately perpendicularly onto the object surfaces and then reflected, then pass through the first and/or second beam paths previously formed to an object surface back in the opposite direction. 
         [0013]    The reflected light beams in the first beam path and the reflected light beams in the second beam path are advantageously oriented parallel to one another at least on one longitudinal section of their light path in each case. 
         [0014]    A particularly great advantage results if the first and the second beam paths of the reflected light beams have an at least partial spatial overlap. It is additionally advantageous if the reflected light beams from the first and second beam path are oriented parallel to one another within this overlap. This means that both the reflected light beams from the first beam path and also the reflected light beams from the second beam path pass through the spatial overlap of the beam paths. The overlap of the beam paths of the reflected light beams may be initiated by the system of the at least one first optical element. A complete overlap of the beam paths of the reflected light beams is particularly advantageous. The cross section of the overlap advantageously corresponds to the total cross section of the light beams introduced into the optical system. 
         [0015]    The optical system according to the exemplary embodiments and/or exemplary methods of the present invention may be situated in a measuring beam path of an interferometer measuring head. The measuring beams are thus introduced into the optical system according to the present invention and used for illuminating the object surfaces. In this manner, an interferometric system according to the present invention for measuring object surfaces is thus advantageously obtained. 
         [0016]    The interferometric system according to the exemplary embodiments and/or exemplary methods of the present invention has the advantage with respect to the related art that ideally a large measuring field may be implemented as the illuminated object surface, in particular when the entire cross section of the measuring beams introduced within the optical system according to the present invention is used in each case to form the at least one first beam path and/or the at least one second beam path. 
         [0017]    It is very particularly advantageous that a large detector surface, which may be the entire detector surface, may be used to image the particular measured object surface on a detector. This is made possible in that the beam paths of the light beams which are reflected on the object surfaces at least partially, which may be completely, overlap and the reflected light beams may be incident on the detector surface centered in the middle. This means that at least partially identical areas of the detector surface may be exposed during the measurement of a first object surface and a second object surface. Thus, during the measurement of circular object surfaces, a surface of the detector, which is four times as large in comparison to the related art, may advantageously be used for the imaging. 
         [0018]    The lateral resolution thus also advantageously doubles during the measurement of an object surface using an interferometric system according to the present invention. This means that the measurement of the same object surfaces is performed substantially more precisely than is the case using a measuring device known heretofore. It is also possible to use a smaller detector for an at least equal measuring precision and/or an object surface to be measured which is at least equally large. Vice versa, it is also possible to maintain the detector size, and instead to reduce the cross section of the measuring beams which are introduced into the optical system according to the present invention. The object surfaces to be measured are thus advantageously illuminated using a greater light intensity. Therefore, even poorly reflecting object surfaces may be imaged on the detector. 
       SUMMARY OF THE INVENTION 
       [0019]    Advantageous refinements and improvements of these systems specified herein are possible through the measures listed and further described herein. 
         [0020]    Thus, in a refinement of the optical system according to the present invention for illuminating at least one surface of a measured object, the optical elements may be situated in such a way that the light beams which are directed onto the first object surface are directed parallel or antiparallel to the light beams which are directed onto the second object surface. Object surfaces which are situated parallel to one another may accordingly be illuminated. The distance of both object surfaces to one another may be ideally measured within an interferometric system according to the present invention. The thickness of the object at the position of the measured object surfaces may therefore be determined in the case of parallel surfaces directed in opposite directions. The parallelism of the object surfaces to one another may also be checked. 
         [0021]    In a refinement of the optical system according to the present invention, it is particularly advantageous if at least one optical element is a beam splitter. The guidance of the light beams which are incident on the beam splitter may advantageously be influenced in such a way that the light beams are split and therefore light beams are formed at least in a first beam path and light beams are formed at least in a second beam path. A beam splitter is to be understood in this case as an optical element, in which light beams in the at least one first formed beam path and in the at least one second formed beam path originate from at least one identical cross-sectional area of the light beams which are introduced into the beam splitter, which may be from the entire cross section. A very simple beam splitter of this type is a partially mirror-coated glass pane, which is situated at an angle of 45° to the introduced light beams, for example. If a part of the incident light beams is reflected on the object surface of the pane at an angle of 90°, a further part penetrates the pane. In its widespread form, such a beam splitter includes two prisms, which are cemented together at their base (e.g., using an optical adhesive such as the UV adhesive Norland Optical Adhesive 63). 
         [0022]    In a specific embodiment, the first optical element for forming at least one first beam path and at least one second beam path is a beam splitter. The at least one first object surface and the at least one second object surface are advantageously illuminated simultaneously. 
         [0023]    Furthermore, in one embodiment variant, a beam splitter is used in which the light beams in the formed first beam path may be oriented perpendicularly to the light beams which are incident on the beam splitter. Furthermore, a beam splitter is proposed in which the light beams in the formed second beam path are directed rectified and without deflection onto the light beams incident on the beam splitter. Such a proposed optical system may be positioned within an interferometric system according to the present invention in such a way that the optical axis of the formed second beam path is on the optical axis of the measuring beams which are formed in the interferometric system. 
         [0024]    In general, it is advantageous for an optical system according to the present invention if at least one optical element is a mirror or a prism, in order to deflect light beams in a desired beam direction by reflection, for example. Light deflections as a result of light refraction within the prism may be avoided. Otherwise, spectral splitting of the light additionally occurs. In the case of an interferometric system according to the present invention having an optical system used in this way, deviating measurement results may occur. 
         [0025]    A further advantageous specific embodiment of the optical system according to the present invention proposes that a mirror and/or a prism be used as the second and/or third optical element, which is downstream from the first optical element. The use of prisms may be desired, in particular if such a proposed optical system is used within an interferometric system according to the present invention. Interferometric systems typically have a fixed—so-called—operating length. This operating length is the maximum covered optical path of measuring beams which are reflected on an object surface, in which focused imaging of the object surface may still be achieved on the detector via the objective. Prisms advantageously lengthen this operating distance by one-third of the glass distance traveled. In addition, the danger of air flows, which typically negatively influence the measurement result, occurring in the beam path is reduced. 
         [0026]    It is generally advantageous for an optical system according to the present invention if at least one optical element for refracting the light beams is a lens or a lens system. The cross section of the light beams may advantageously be reduced in the direction of the light path. The light intensity in the adjoining beam path may thus be enlarged. It is also possible to enlarge the cross section of the light beams. Overall, for example, a larger object surface may thus be illuminated. 
         [0027]    Optical elements which may deflect light beams by reflection in more than one beam direction are generally advantageous for an optical system according to the present invention. For example, this is possible by rotatably arranging the optical element. A rotatably arranged first optical element, such as a mirror or a prism, is advantageous in particular. It is thus possible to illuminate exclusively one object surface in a corresponding rotational position. All light beams which are incident on the rotatable first optical element may be directed exclusively on the object surface via the second optical element, for example. In the same manner, the second object surface may be exclusively illuminated by a changed rotational position. 
         [0028]    Such a proposed optical system within an interferometric system according to the present invention has the advantage that only one object surface may always be imaged on the detector for an object measurement. A clear assignment of the measuring signals of the detector to the measured object surface is thus provided. Because all measuring beams which are introduced into the optical system may thus be used for illuminating the object surface, an advantageously large light yield results for the measuring procedure. 
         [0029]    Optical elements, which may be folded out of the at least one first beam path or the at least one second beam path formed by the first optical element, are generally advantageous for an optical system according to the present invention. This is advantageously performed in such a way that the elements no longer participate in guiding the light beams. Mirrors and/or prisms may particularly be used. It is thus possible to exclusively illuminate one object surface at a time in such a system of the optical elements. Thus, for example, light beams in a first beam path may be directed onto a first surface. The illumination of the second object surface is suspended simultaneously, in that the second optical element is folded out of the second beam path, for example. The light beams in the second light beam path are thus no longer directed onto the second object surface. 
         [0030]    Such a proposed optical system according to the present invention within an interferometric system according to the present invention has the advantage that only one object surface may always be imaged on the detector for an object measurement. A unique assignment of the measuring signals of the detector to the measured object surface is thus provided. 
         [0031]    Exemplary embodiments of the present invention are shown in the drawings and are described in greater detail in the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  schematically shows a first specific embodiment of an interferometric measuring system according to the present invention having an optical system for illuminating at least one surface of a measured object, in longitudinal section. 
           [0033]      FIG. 2   a  schematically shows an embodiment variant of an optical system having an aperture in a first position within an interferometric system, in longitudinal section. 
           [0034]      FIG. 2   b  shows the embodiment variant from  FIG. 2   a  having a second position of the aperture, in longitudinal section. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    A first specific embodiment of an interferometric system  300  according to the present invention for measuring object surfaces is shown in  FIG. 1 . An interferometer measuring head is identified by reference numeral  100 . Interferometer measuring head  100  is constructed in the form of a planar measuring interferometer. The construction contains a radiation-generating light source  10 , which emits short coherent light beams, such as white light, into an illumination optics  20 . An illumination beam path  30  is thus formed. A first roof prism  40 , which is situated in illumination beam path  30 , causes a deflection of the light beams in such a way that the light beams are directed perpendicularly onto a first beam splitter  50 . First beam splitter  50  causes a division of the light beams into one part of reference beams within a reference beam path  70  and one part of measuring beams within a measuring beam path  205 . 
         [0036]    The reference beams are oriented in the same direction and the measuring beams are oriented at a right angle to the light beams of illumination beam path  30 , which are incident on beam splitter  50 . The reference beams are deflected via a second roof prism  60 , which is situated in reference beam path  70 , in such a way that they are directed perpendicularly onto a reference element  75 , such as a reference mirror. The reference beams which are reflected on reference element  75  thus pass back through reference beam path  70  again in the opposite direction via roof prism  60  up to beam splitter  50 . Through a further perpendicular deflection by beam splitter  50 , the reference beams are finally directed perpendicularly through an objective  80 , which is situated in the light path, onto a detector  90 , such as a photosensitive element of a camera. The beam direction of the reference beams, which are incident on detector  90 , is opposite to that of the measuring beams, which are formed by first beam splitter  50 . 
         [0037]    An optical system  200  according to the exemplary embodiments and/or exemplary methods of the present invention is situated in measuring beam path  205  of interferometer measuring head  100 . The measuring beams are directed onto a second beam splitter  250 . The optical axis of measuring beam path  205  is advantageously congruent with the optical axis of second beam splitter  250 . An offset of the optical axes is permissible, in particular up to an offset dimension at which it is ensured that the measuring beams are incident on second beam splitter  250  using the entire cross section of measuring beam path  205 . Second beam splitter  250  corresponds to a first optical element within optical system  200  for forming a first beam path  210  and a second beam path  220 . Light beams in first beam path  210  are oriented perpendicularly to the measuring beams, while in contrast the light beams in second beam path  220  run as a linear extension to the measuring beams, which are incident on second beam splitter  250 . 
         [0038]    The light beams both in first beam path  210  and also in second beam path  220  may advantageously originate from the entire cross section of measuring beam path  205 . Accordingly, the light beams within first and second beam paths  210 ,  220  are also measuring beams, each having a light intensity which is less due to the division. In addition, the cross section of first and second beam paths  210 ,  220  advantageously corresponds to the cross section of measuring beam path  205 . 
         [0039]    A first mirror  260  is situated in first beam path  210  in such a way that measuring beams are also directed perpendicularly by reflection onto a first object surface  281  of a measured object  280 . A second mirror  270  is also situated in second beam path  220  in such a way that measuring beams are directed onto a second object surface  282  of measured object  280 . Measured object  280  is situated having second object surface  282  resting on a radiation-transparent support  240 . Accordingly, proposed interferometric system  300  is positioned in space in such a way that first object surface  281  is illuminated from above in vertical spatial direction y and second object surface  282  is illuminated from below through radiation-transparent support  240  in vertical spatial direction y. 
         [0040]    The measuring beams which are reflected on first and second object surfaces  281 ,  282  pass back through the first and second beam paths  210 ,  220  in the opposite direction via first and second mirrors  260 ,  270  up to second beam splitter  250 . The reflected measuring beams from second beam path  220  are introduced without deflection axially parallel into objective  80 , such as a telecentric objective, and directed perpendicularly onto detector  90 . The measuring beams in first beam path  210 , in contrast, are deflected by second beam splitter  250  perpendicularly to their beam direction up to this point and are introduced axially parallel into objective  80  and also directed perpendicularly onto detector  90 . 
         [0041]    The beam path of the measuring beams which are reflected on first object surface  281  and the beam path of the measuring beams which are reflected on second object surface  282  are congruent within imaging beam path  230  of objective  80 . The reflected measuring beams which are incident on detector  90  interfere with the reference beams, which are also contained in imaging beam path  230  and are incident on detector  90 . Measured object  280  is situated within optical system  200  in such a way that the measuring beams which are incident on first object surface  281  and then reflected and the measuring beams which are incident on second object surface  282  and also reflected pass through different optical paths. The total path difference of both optical path lengths may be greater than the coherence length of light source  10 . This prevents the measuring beams which are reflected from first object surface  281  and second object surface  282  from interfering with one another. An interference with the reference beams occurs when the optical path length of the measuring beams corresponds to the optical path length of the reference beams. 
         [0042]    The optical path length of the reference beams may be increased or decreased by displaceably arranging reference element  75  on the optical path axis of the reference beams. A change and adaptation of the optical path length for the measuring beams of first object surface  281  or second object surface  282  may be achieved by a relative displacement of interferometer measuring head  100  and optical system  200  in the direction of optical axis M. An alternative embodiment variant provides that first mirror  260 , second mirror  270 , and/or second beam splitter  250  are each displaceably arranged relative to the measured object in the direction of their optical axis A, B, C, optionally in addition to a possible displacement of interferometer measuring head  100 . 
         [0043]    The distance of first object surface  281  to second object surface  282  corresponds to the object thickness. Before the object thickness of a measured object  280  is ascertained by a measuring procedure, interferometric system  300  is calibrated in a first step using a gauge block of known thickness. The gauge block is positioned on support  240  in place of measured object  280 . Interferometer measuring head  100  is displaced into the position in which the optical path from beam-splitting surface  250   a  of second beam splitter  250  up to first object surface  281  corresponds to the optical path from beam splitting surface  250   a  of second beam splitter  250  up to reference element  75 . In this position, the measuring beams which are reflected on first object surface  281  interfere with the reference beams, so that a corresponding first interference pattern is visible on detector  90 . The gauge block is positioned in such a way that the optical path from beam-splitting surface  250   a  of second beam splitter  250  to second object surface  282  is slightly longer or shorter than the optical path length of the reference beams. The measuring beams which are reflected on second object surface  282  therefore do not interfere with the reference beams. 
         [0044]    A second interference pattern is accordingly also not visible on detector  90 . The second reference pattern only becomes visible on detector  90  due to the displacement of reference element  75 , for example, in the direction of optical axis S of reference beam path  70 . The first reference pattern is simultaneously no longer visible on detector  90 . The displacement of reference element  70 , for example, between the first interference pattern, which is implemented by first object surface  281 , and the second reference pattern, which is implemented by second object surface  282 , is measured using a high-precision position encoder, for example. The displacement path of reference element  75  which is thus ascertained is stored as a calibration constant. Subsequently, measured object  280  is measured in the same manner and the distance of reference element  75  between the first implemented interference pattern and the second implemented interference pattern is ascertained. The object thickness is calculated from the known thickness of the gauge block, the previously ascertained calibration constant, and the distance of reference element  75  which is ascertained for the measured object. 
         [0045]    A further specific embodiment of an interferometric system according to the exemplary embodiments and/or exemplary methods of the present invention provides at least one light source  10   a , which emits long coherent light, such as laser light. The remaining construction of an interferometer measuring head  100  remains unchanged. 
         [0046]    If a light source  10   a  having long coherent light beams is used, simultaneous exposure of detector  90  by measuring beams, which are reflected on first object surface  281  and second object surface  282 , is ideally to be suppressed. Otherwise, interference may also occur between the measuring beams, which are reflected on first object surface  281  and second object surface  282 , in spite of different optical path lengths. Interference patterns, which could be assigned uniquely to first or second object surface  281 ,  282  by the displacement of reference element  75  in the direction of optical axis S, for example, are therefore not implemented. 
         [0047]    In a first embodiment variant of an interferometric system using long coherent light, a light source  10   a  is provided in such a way that interferometer measuring head  100   a  may be used as a multiple wavelength interferometer. For example, two laser beams may be used for this purpose, whose wavelengths have a very small difference in comparison to one another. In a similar way, such a light source  10   a  may be used that interferometer measuring head  100   a  is implemented as an interferometer having a displaceable wavelength. For this purpose, for example, a laser source may be provided as light source  10   a , which may be tuned to various laser frequencies. In this embodiment variant of an interferometric system, an embodiment of an optical system  200   a  according to  FIG. 2   a  may be situated in measuring beam path  205 . The construction of optical system  200   a  essentially corresponds to the above-described construction of optical system  200  in  FIG. 1 . In contrast thereto, a movable aperture  225  is provided in first and second beam paths  210 ,  220 . During the measurement of first object surface  281 , aperture  225  in second beam path  220  may be positioned in such a way that no measuring beams are incident on second object surface  282 . First object surface  281  may therefore be exclusively imaged on detector  90  by interference with the reference beams. 
         [0048]    A similar method is used in the measurement of second object surface  282 . According to  FIG. 2   b , aperture  225  may be situated in first beam path  281  in such a way that no measuring beams are incident on first object surface  281 . Second object surface  282  may therefore be exclusively imaged on detector  90  by interference with the reference beams. In this manner, the planarity, the parallelism of both object surfaces  281 ,  282  to one another, and the object thickness may be measured. To ascertain the object thickness, during the measurement of first object surface  281 , the phase difference of the two different components of laser beams in the measuring beams, having minimally different wavelengths, are ascertained on detector  90 . The number of the periods within the optical path which result in such a phase shift may thus be determined. The optical path length which is covered by the measuring beams which are reflected on first object surface  281  is accordingly also known. In the same manner, the optical path length which is covered by the measuring beams which are reflected on second object surface  282  may also be ascertained. The object thickness may thus be inferred in the simplest manner. 
         [0049]    The interferometric system may further be varied in that light source  10   a  emits long coherent light having a first light wavelength or long coherent light having a second light wavelength by being turned on and off electrically. For example, two alternately activated laser units are conceivable for the provision of light beams having different light wavelengths. A construction, which essentially corresponds to the already described construction of optical system  200  in  FIG. 1 , may be used for optical system  200 . 
         [0050]    In contrast thereto, a first color filter for filtering the measuring beams having the first light wavelength is situated within first beam path  210 . In a similar manner, a second color filter for filtering the measuring beams having the second light wavelength is positioned in second beam path  220 . As a function of whether the light having the first light wavelength or the light having the second light wavelength is turned on for the illumination, either first object surface  281  or second object surface  282  may thus be imaged on detector  90 . Alternatively, instead of the color filter and second beam splitter  250 , a dichroic (color-separating) beam splitter  250  may be used as the first optical element. 
         [0051]    An additional embodiment variant of the interferometric system provides a light source  10   a , which emits long coherent light having a first polarization direction or long coherent light having a second polarization direction, in each case by being turned on and off electrically. For example, two alternately activated laser units are conceivable, which may have having laser beams which are polarized perpendicularly to one another. 
         [0052]    A construction may particularly be used for optical system  200  which essentially corresponds to the above-described construction of optical system  200  in  FIG. 1 . In contrast thereto, second beam splitter  250  which is used is a polarized beam splitter. Either first object surface  281  or second object surface  282  may be imaged on detector  90  as a function of whether the light having the first polarization direction or the light having the second polarization direction is turned on for the illumination. 
         [0053]    In general, the above-described embodiment variants of an interferometric system for measuring object surfaces  281 ,  282  using long coherent light beams are also possible using short coherent light beams. The signal quality is thus positively influenced. 
         [0054]    This is also true for an alternative specific embodiment of an interferometric system according to the present invention for measuring object surfaces using short coherent or long coherent light. 
         [0055]    The construction essentially corresponds to the construction of interferometric system  300  in  FIG. 1 . In contrast thereto, light source  10  may emit short coherent light or also long coherent light. In contrast to optical system  200 , instead of second beam splitter  250 , a further mirror is advantageously used, which may be folded out of measuring beam path  205 . In a first position, the mirror is situated in such a way that all measuring beams from interferometer measuring head  100  may be directed on first mirror  260  and thus illuminate first object surface  281 . In this case, first object surface  281  is exclusively imaged on detector  90 . In a further position, the mirror is folded completely out of measuring beam path  205 . In this case, all measuring beams from interferometer measuring head  100  may be directed onto second mirror  270  and accordingly may exclusively illuminate second object surface  282 . Accordingly, in this beam guidance of the measuring beams, second object surface  282  may be exclusively imaged on detector  90 .