Abstract:
According to an embodiment, a measuring device for measuring a laser beam comprises a magnification lens system with a total of two lenses which are arranged in series in the beam path of the laser beam and whose foci are coinciding, as well as a camera which is arranged behind the two lenses in the focus of the last lens and includes an electronic image sensor which generates an electronic image of the magnified laser beam. The lenses together with the camera are adjustable along the beam path relative to a reference point of the measuring device, for the purpose of locating the beam waist of the laser beam and of determining a diameter profile of the laser beam. The measuring device further comprises an adapter enclosing the beam path for coupling the measuring device to a laser system which provides the laser beam. The adapter forms an abutment surface or the laser system, which is axially directed with respect to a beam axis of the laser beam, and permits the measuring device to be coupled in situ at the installation site of the laser system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a United States national phase application of co-pending international patent application number PCT/EP2008/007575, filed Sep. 12, 2008, which claims the benefit of EP Application No. 07 017 968.4 filed Sep. 13, 2007, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a measuring device for measuring a focused laser beam. The laser beam to be measured is employed, in particular, in the ophtalmic laser surgery, such as in a photorefractive treatment of the cornea or the lens. Of course, this is merely an example; principally, the invention is not subject to any limitations with respect to the application purpose of the laser beam to be measured. A precise knowledge of certain beam parameters is generally necessary for many laser applications, with only two out of numerous other examples besides the laser surgery of the eye, the photolithography in the semi-conductor technology as well as the laser scanning or laser writing of optical data carriers being mentioned herein. 
         [0003]    Beam parameters of interest of a laser beam often include the beam diameter, in particular in the area of the beam waist, as well as the beam divergence, but other parameters, too, such as the diffraction index (usually expressed as M 2 ) or the intensity profile transverse to the beam direction. 
         [0004]    For the measurement of a laser beam so-called knife edge or slit methods are known, in which a blade or a sufficiently narrow slit is moved transversely through the beam and transmitted power is measured depending on the blade or slit position, respectively, by means of a detector. By the differentiation of the transmission curve which has been obtained in this manner, the intensity profile may subsequently be determined, by means of which in turn the beam diameter may be determined. 
         [0005]    As an alternative to the above transmission measuring methods which are based on “scanning” of the beam, it is known, e. g. from U.S. Pat. No. 4,917,489, to magnify the laser beam by means of a single magnification lens and to direct the magnified laser beam following a suitable intensity attenuation to an image pickup device (image sensor). It is then possible to calculate the beam diameter at the object-side lens focus under consideration of the magnification factor of the lens from the generated image of the laser beam. 
         [0006]    In contrast, an inventive measuring device for the measurement of a focused laser beam comprises a magnification lens system with at least two lenses disposed in series in the beam path of the laser beam, wherein each pair of successive lenses of the magnification lens system has coinciding focal points, an electronic image sensor disposed behind the magnification lens system at a focal point thereof, for capturing an image of the magnified laser beam, an adapter which encloses the beam path, for coupling the measuring device to a laser system which provides the laser beam, with the adapter forming an abutment surface for the laser system, which abutment surface is axially directed with respect to a beam axis of the laser beam, as well as longitudinal adjustment means which permit a simultaneous adjustment of the lenses of the magnification lens system and the image sensor along the beam path with respect to a reference point of the measuring device provided on the adapter. 
       SUMMARY 
       [0007]    Preferred embodiments of the inventive measuring device are indicated in the dependent claims. 
         [0008]    In the reference literature on the optics of Gaussian beams (see also e. g. Kühlke, Dietrich: “Optik, Grundlagen and Anwendungen”, Harri Deutsch Verlag, 1st edition, 1998), a dependency of the magnification factor of a single magnification lens of the beam diameter of the laser beam to be measured is taught. When taking the quotient of the waist diameter of the magnified beam and the waist diameter of the non-magnified beam as the magnification factor, it follows in particular that the magnification factor of a single magnification lens for smaller waist diameters may be significantly higher than that for larger waist diameters. The assumption of a nominal, constant magnification scale may thus lead to significant measurement errors when using a single magnification lens for the measurement of foci with different waist diameters. 
         [0009]    The inventors realised that a multiple lens system is able to eliminate the dependency of the magnification scale from the beam diameter, if each pair of successive lenses has coinciding foci. In this case, an invariably constant magnification scale may be assumed, regardless of how large the waist diameter is of the beam to be measured. This permits a much more precise determination of the actual waist diameter of the laser beam to be measured. 
         [0010]    The longitudinal adjustment means of the inventive measuring device permit a defined adjustment of the lenses and the image sensor as a whole along the beam path relative to a reference point which belongs to the measuring device. Belonging to the measuring device means that the reference point is located at or within the measuring device and is thus stationary relative to the measuring device as a whole. The adjustability of the ensemble consisting of the lenses and the image sensor along the beam path enables to precisely locate the beam waist of the laser beam to be measured and consequently the precise adjustment of the measuring device, so that the focus of the laser beam coincides with the focus of the first lens of the magnification lens system. This situation will be present when the beam cross-section which is captured by the image sensor has its smallest value. With the inventive mutual arrangement of the lenses and the image sensor, the beam waist will be imaged on the sensor surface of the image sensor when it is located in the focus of the first lens. This position may easily be found by a longitudinal adjustment of the ensemble consisting of the lenses and the image sensor relative to the beam focus. 
         [0011]    The longitudinal adjustability of the lenses and the image sensor is alternatively or additionally useful for the determination of the beam diameter at various positions along the laser beam, in other words, for the determination of a diameter profile of the laser beam. Such a diameter profile allows to gain information e. g. with respect to the divergence of the beam and the diffraction index. Needless to say that a suitably programmed electronic evaluation unit will be provided for the determination of these and other beam parameters, which evaluates and interprets the electric image signals which have been supplied by the image sensor and buffered in a storage, if required. 
         [0012]    The adjustable range which is provided by the longitudinal adjustment means for the system consisting of the lenses and the image sensor preferably extends across at least three times the Rayleigh length of the laser beam. 
         [0013]    If desired, the longitudinal adjustment means may additionally provide for a longitudinal adjustment of the lenses relative to each other or/and relative to the image sensor, e. g. for calibration purposes or for the compensation of component and assembly tolerances. 
         [0014]    According to an embodiment which provides for a particularly short design of the measuring device, the magnification lens system comprises a total of two lenses. For an adequate magnification scale, a relatively small focal length of the first lens is required, so that the first lens will have to be made relatively small. Larger lenses which can be handled and installed more conveniently are possible if the magnification lens system comprises a total of four lenses. The magnification power may then be divided into tow stages—consisting of one pair of lenses each—so that with the same total magnification the magnification of each pair of lenses may be smaller than that of the single pair of lenses in the embodiment with a total of two lenses. 
         [0015]    All lenses of the magnification lens system may be designed as convergent lenses each. Alternatively, it is possible that the magnification lens system comprises at least one divergent lens, with at least the last lens being designed as a convergent lens. Due to its negative focal length, the use of a divergent lens permits a shortening of the structural space compared to a convergent lens. This may be utilised in particular with embodiments comprising more than two lenses, where a greater focal length of the first lens may be chosen so that the benefit in structural space due to the use of a divergent lens is greater than with solutions having a total of two lenses, where the first lens has a comparatively short focal length. In each case, the last lens is to be designed as a convergent lens, because the image sensor requires a real image of the laser beam. 
         [0016]    In order to keep the wave front deformation by the magnification lens system small it was found to be of advantage when at least one lens of the magnification lens system has one planar lens face and has its curved lens face directed toward the beam portion having smaller divergence. The magnification lens system therefore preferably includes merely plano-convex or piano-concave lenses, but no lenses which are curved on both sides, although this is of course not mandatory. 
         [0017]    Component and assembly tolerances may cause that at least some of the optical components of the measuring device are not exactly oriented with respect to the laser beam in the direction transverse to the beam path. The measuring device therefore preferably comprises transverse adjustment means which permit an adjustment at least of the first lens of the magnification lens system transversely, in particular, vertically, to the beam path relative to the above mentioned reference point of the measuring device. It was found that a misalignment of the first lens of the magnification lens system transverse to the beam path may have a particularly strong impact on the location of the image position on the image sensor, the wave front deformations, and the image distortions, but in any case stronger than corresponding misalignments of the other lenses. For this reason, at least the first lens should be transversely adjustable. According to a variant, it alone may be transversely adjustable, i. e. independent of the other lenses and independent of the image sensor. According to another variant, the first lens together with at least a further lens of the magnification lens system, in particular together with all lenses of the magnification lens system and preferably also together with the image sensor may be transversely adjustable as a unit. 
         [0018]    The transverse adjustment means suitably permit an independent adjustment of at least the first lens of the magnification lens system in two mutually orthogonal transverse directions. 
         [0019]    A preferred configuration of the measuring device provides for at least some of the lenses, in particular all lenses of the magnification lens system in the beam path, to be installed in series in a lens barrel which is adjustable by means of the longitudinal adjustment means in the direction of the barrel axis relative to the reference point. The lens barrel may preferably be displacably guided in the axial direction, but secured against rotation, in a guide receiving opening of a guide body which is designed in particular tubular, with the longitudinal adjustment means being effective between the lens barrel and the guide body. 
         [0020]    One end of the lens barrel may axially protrude from the guide body and is adapted in the region of its protruding end for mechanical coupling to a camera accommodating the image sensor. 
         [0021]    For the compensation of any play between the lens barrel and the guide body it is recommended that elastic bias means are effective between the lens barrel and the guide body, which axially bias the lens barrel relative to the guide body. 
         [0022]    In a preferred embodiment of the measuring device the guide body carries the adapter. The adapter is preferably a component which is separate from the guide body and is replaceably connected to the guide body. This permits a modular construction of the measuring device for which a plurality of selectively usable, different adapters for coupling to different laser systems may then be made available. 
         [0023]    The discussed transverse adjustment means may be effective between the adapter and the guide body. 
         [0024]    The adapter suitably forms an abutment surface for the laser system, which is axially directed with respect to a beam axis of the laser beam. This abutment surface may then serve as a reference point for the longitudinal and, if required, for the transverse adjustment of the optical components of the measuring device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention will be explained in more detail in the following with reference to the accompanying drawings in which: 
           [0026]      FIG. 1  is a schematic illustration of the relative arrangement of a pair of lenses and an image sensor for capturing an enlarged image of a laser beam. 
           [0027]      FIG. 2  is a schematic illustration of the relative arrangement of two pairs of lenses and an image sensor for capturing an enlarged image of a laser beam. 
           [0028]      FIG. 3  is a longitudinal cross-section through an embodiment of an inventive measuring device. 
           [0029]      FIG. 4  shows an enlarged section of the measuring device of  FIG. 3 , and 
           [0030]      FIG. 5  another enlarged section of the measuring device of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIGS. 1 and 2  show two different exemplary configurations of a magnification lens system to be used in an inventive measuring device. In the variant of  FIG. 1  the magnification lens system comprises a total to two optically thin lenses  10 ,  12  both of which are adapted as plano-convex convergent lenses. The laser beam to be measured is initially incident on the lens  10  and subsequently on the lens  12 , before its magnified image is captured by an electronic image sensor  14 . The image sensor may, for example, be a CCD or a CMOS image sensor. The optical axis of the magnification lens system is indicated at  16 . In order to prevent optical imaging errors it is desirable that the beam axis of the laser beams coincides with the optical axis  16 . Due to the fact that a misalignment of the optical components of the measuring devices transversely to the beam direction of the laser beam caused by component or assembly tolerances can often not be avoided completely, suitable transverse adjustment means are provided in preferred embodiments of the inventive measuring device, by means of which the lenses of the magnification lens system and the image sensor as well, if required, can be adjusted transversely to the beam direction in such a manner that the optical axis  16  coincides essentially exactly with the beam axis. 
         [0032]    The lens  10  of  FIG. 1  has a focal length f 1 ; its two foci (object side and image side) are identified by  18  and  20 , respectively. 
         [0033]    The lens  12  has a focal length f 2 . It is arranged in such a manner that its object side focus coincides with the image side focus  20  of the first lens  10 . The lens  12  has a focus  22  on the image side. The sensor plane of the image sensor  14  coincides with said focus  22 . 
         [0034]    If with this mutual arrangement of the lenses  10 ,  12  and of the image sensor  14 , the beam waist of the laser beam to be measured lies in the focus  18 , it is imaged on the image sensor  14  at a magnification scale which is independent of the waist diameter. The image sensor  14  is connected with electronic evaluation means (not shown in detail) which receive the image signals of the sensor  14 , convert them into a digital form, if required, and determine the magnified as well as the actual (non-magnified) waist diameter therefrom. For an attenuation of the intensity of the laser beam one or several neutral density filters (grey filters) may be arranged in its beam path. In the exemplary configuration of  FIG. 1 , in particular, such a grey filter may be provided between the lens  12  and the image sensor  14 . The optical thickness of the grey filter will then lead to a minor displacement of the focus  22 , which has to be taken into account when positioning the image sensor  14 . 
         [0035]    It can be seen that the curved sides of the lenses  10 ,  12  are facing the beam portion having smaller divergence. The magnification of the waist diameter, which is effected by the first lens  10 , is accompanied by a parallelisation of the beam, which causes a smaller divergence of the laser beam at the image side of the lens  10  than at the object side. That which applies for the lens  12  is, however, that the laser beam has a higher divergence at its image side. This results in the arrangement of the lenses shown in  FIG. 1 , with their curved sides facing each other. The measure to have the planar lens sides face the beam portions of higher divergence and to have the curved lens sides face the beam portions of smaller divergence enables to keep the wave front deformations caused by the lenses  10 ,  12  small and to thereby reduce the image distortions to a minimum. 
         [0036]    If, in a numerical example, the magnification scale is assumed to be approx. 20 (i. e. the waist diameter at the image sensor  14  is to be 20 times larger than the non-magnified waist diameter), then a focal length of e. g. approx. 3 mm for the lens  10  and a focal length of approx. 60 mm for the lens  12  may be selected. These focal lengths enable to achieve a comparatively overall short size of the measuring device. The above numerical specifications are, of course, merely exemplary; in particular other values for the magnification scale may be worth striving for, which may require other focal lengths of the lenses  10 ,  12 . 
         [0037]    In the variant of  FIG. 2  the magnification lens system comprises a total of four—again optically thin—lenses which are now identified by  24 ,  26 ,  28 ,  30 . The image sensor is still identified by  14  and the optical axis by  16 . The second lens  26 , when viewed in the direction of the beam propagation, and the fourth lens  30  each are designed as plano-convex convergent lenses, while the first lens  24  and the third lens  28  each are designed as plano-concave divergent lenses. The pair of lenses  24 ,  26  forms a first magnification stage of the magnification lens system, while the second pair of lenses  28 ,  30  forms a second magnification stage. The magnification factors of the two magnification stages may be identical or different; the overall magnification scale results from the product of the magnification factors of the two magnification stages. 
         [0038]    In the illustrated exemplary case, the two convergent lenses  26 ,  30  have the same focal length; the two foci of the convergent lens  26  are identified by  32 ,  34 , those of the convergent lens  30  by  36 ,  38 . Due to their negative focal length, the divergent lenses  24 ,  28  each are arranged behind the focus of the following convergent lens, but again in such a manner that their (virtual) foci coincide with the focus of the respective successive convergent lens. This means that the focus  32  is at the same time a (virtual) focus of the lens  24  and the foci  34 ,  36  also correspond to one (virtual) focus each of the divergent lens  28 . 
         [0039]    The use of divergent lenses in lieu of convergent lenses permits a shortening of the size of the measuring device, with this effect being more pronounced with the four-lens configuration of  FIG. 2  than with the two-lens configuration of  FIG. 1 . 
         [0040]    To present another numerical example: If the magnification scale of the magnification lens system is again assumed to be approx. 20, then the convergent lenses  26 ,  30  each may have a focal length of approx. e. g. 42 mm, while a focal length of approx. −9 mm each may be selected for the divergent lenses. Again, these are, of course, merely exemplary specifications which may be changed at any time, in particular if another magnification scale of the magnification lens system is desired. 
         [0041]    For an explanation of a concrete construction example, reference is now made to  FIGS. 3 to 5 . The measuring device illustrated therein is generally identified by  40 . It is a hand-portable device which may be coupled to existing laser systems, without the necessity of transporting the laser system to a separate measuring site. Thus, the measuring device  40  permits an in situ measurement immediately at the installation site of the laser system. 
         [0042]    The measuring device  40  is provided with an adapter  42  with a sleeve-like adapter main portion  44  which encloses the optical axis  16 . In the region of the left-hand axial end in  FIG. 3 , the adapter  42  is formed for coupling to a laser system which provides the laser beam. The coupling means provided for this purpose comprise an axially directed annular abutment surface  46  which serves for an axial abutment and thus for the axial positioning of the measuring device  40  relative to the laser system. The mentioned coupling means further comprise an annular clamping portion  48 , which is preferably integrally formed with the adapter main portion  44 , for the radial clamping of an edge of an objective or another portion of the laser system. The clamping portion  48  is provided with a plurality of threaded holes  50  which are distributed in the circumferential direction for the insertion of radial clamping screws (not shown). 
         [0043]    In the region of its other axial end, the adapter main portion  44  is removably connected with an adjustment block  52  which forms a receiving opening  54  for a tubular guide body  56 . The adjustment block  52  is provided with threaded holes  58  for the insertion of fastening screws which are not shown in detail, by means of which the adapter  42  may be secured to the adjustment block  52 . The guide body  56  is held axially stationary in the receiving opening  54  of the adjustment block  52 , may, however, be adjusted relative to the adjustment block in a radial plane by means of adjustment screws (not shown in detail) of the adjustment block  52 . For this purpose, the adjustment block  52  may comprise an inner sleeve which clamps the guide body  56  radially in a manner not shown in detail, and is held in a housing of the adjustment block  52  in a transversely adjustable manner. Suitable adjustment blocks are commercially available. For example, reference is made to an adjustment device with the product name “XY-Justieraufnahmeplatte OH40” available from OWIS GmbH, which may be used as the adjustment block  52 , if required, slightly modified. The adjustment block  52  permits an independent transverse adjustment of the guide body  56  in two mutually orthogonal radial directions. Thus, the guide body  56  may be adjusted in a plane normal to the axis in any direction. 
         [0044]    A fixed axial location of the guide body  56  with respect to the adjustment block  52  in the illustrated embodiment is ensured by an axial shoulder  60  at the outer circumference of the guide body  56  as well as by a screw-on ring  62  which is screwed on the guide body  56 . The adjustment block  52  is held axially between the axial shoulder  60  and the screw-on ring  62 . 
         [0045]    The guide body  56  forms an axial guide receiving opening  66  for a lens barrel  68  which together with the lenses of a magnification lens system may be preassembled as a construction unit and as such a preassembled construction unit inserted into the guide body  56 . In the present exemplary case, the magnification lens system corresponds to the variant of  FIG. 1 , so that the only two existing lenses of the measuring device in  FIGS. 3 to 5  are also identified by  10  and  12 . It is understood, however, that another lens configuration, such as the one of  FIG. 2 , may equally be installed into the lens barrel  68 . 
         [0046]    The lens barrel  68  is guided axially movably but secured against rotation in the guide receiving opening  66  of the guide body  56 . For the anti-rotation of the two bodies against one another, the lens barrel  68  comprises an axial, elongated recess  70  which is formed into its outer peripheral surface, into which an anti-rotation screw (not shown in detail) engages which may be inserted into a corresponding threaded hole  72  of the guide body  56 . 
         [0047]    It can be seen that the lens barrel  68  and the guide body  56  together extend into the sleeve-shaped main portion  44  of the adapter  42 , in particular to the same depth, and that the lens barrel  68  in the area of its other axial end protrudes from the guide body  56 . In the region of the protruding end, the lens barrel  68  is coupled to a digital camera  74  wherein an image sensor (not shown in detail in  FIGS. 3 to 5 ) is installed, such as the image sensor  14  of  FIGS. 1 and 2 . An axial abutment surface  76  at the free axial end of the lens barrel  68  permits a defined axial positioning of the camera  74  relative to the lens barrel  68  and consequently relative to the lenses  10 ,  12  which are installed therein. The coupling of the camera  74  with the lens barrel  68  may, e. g., be a screw coupling or a radial clamping coupling. The configuration of the measuring device  40  with a camera  74  which is removably coupled to the lens barrel  68  makes it possible to revert to commercial camera models. 
         [0048]    It has already been mentioned that the lens barrel  68  together with the components inserted therein may be preassembled to a construction unit. Besides the two lenses  10 ,  12 , these components comprise a front ring  78  which is pushed into the lens barrel  68  as the first component in the preassembly and provides a flat contact surface for the planar side of the lens  12 . The front ring  78  contacts an axial shoulder  80  which is formed at the transition between a portion with a larger diameter of the inner body opening and a portion with a smaller diameter. Following the lens  12 , a spacer tube  82  is pushed into the lens barrel  68 . The spacer tube  82  is followed by a lens assembly  84  including the lens  10 , which again is a preassembled construction unit. The lens assembly  84  comprises an approximately cup-shaped lens mount  86  which comprises a central through hole for the laser beam in its bottom. The lens  10  is located above this hole and with its planar lens side bears against the cup bottom of the lens mount  86 . A centering disk  88  provides for the radial centering of the lens  10  in the lens mount  86 . The lens mount  86  has a thread at the inside of its cup wall, into which a lens clamping screw  90  as well as a threaded disk  92  are inserted. In order to not hurt the lens  10 , the lens clamping screw  90  is not tightened against the lens  10 , but locked by means of the threaded disk  92 . 
         [0049]    The lens assembly  84  which has been preassembled in this manner is pushed into the lens barrel  68 , until the lens mount  86  abuts the spacer tube  82 . In the area of the adapter side end, the lens barrel  68  is provided with a female thread into which two further threaded disks  94 ,  96  can be engaged which serve to secure the remaining components in the lens barrel  68 . The two threaded disks  94 ,  96  are again preferably locked against one another in order to prevent the transmission of axial clamping forces via the lens mount  86  and the spacer body  82  to the lens  12 . 
         [0050]    It has already been explained earlier that the lens barrel  68  is guided axially movable relative to the guide body  56  in the guide receiving opening  66 . Longitudinal adjustment means are effective between the lens barrel  68  and the guide body  56 , which enable a defined longitudinal adjustment of the lens barrel  68  relative to the guide body  56 . These longitudinal adjustment means comprise an adjusting member  98  which is in threaded engagement with the guide body  56  and simultaneously in an axial supporting engagement with the lens barrel  68 . In the illustrated exemplary case, the adjusting member is a threaded sleeve which is seated on the guide body  56  and radially extends behind a supporting collar  100  formed in the area of the camera-near end of the lens barrel  68 . The threaded sleeve  98  is provided with a female thread by means of which it is screwed onto a complementary male thread of the guide body  56 . An annular space  102  is formed between the threaded sleeve  98  and the lens barrel  68 , in which a bias spring  104  adapted as a helical pressure spring is received. The bias spring  104  bears with one end axially against the supporting collar  100  of the lens barrel  68  and with the other end axially against the guide body  56  and exerts a bias force which urges the two bodies  56 ,  68  apart. This axial bias force is directed from the lens barrel  68  via its supporting collar  100  to an annular web  106  which is formed at the camera-near end of the threaded sleeve  98  and protrudes radially inwards. In order to reduce the friction between the guide body  68  and the threaded sleeve  98 , a sliding ring  108  made of a material with advantageous sliding properties is put onto the lens barrel  68  between the supporting collar  100  and the annular web  106 . The bias spring  104  is sufficiently strong to prevent any unintentional play-induced movement between the lens barrel  68  and the guide body  56  under normal handling conditions of the measuring device  40 . 
         [0051]    The threaded sleeve  98  enables a defined longitudinal adjustment of the system including the lenses  10 ,  12  and the camera  74  relative to the guide body  56  and thus relative abutment surface  46  of the adapter  42 , which serves as a reference point. Due to the comparatively large thread diameter (large when compared, for example, with a micrometer screw), a very sensitive longitudinal adjustment of the lens barrel  68  may be obtained. In this manner, an adjustment accuracy of only a few micrometers can be achieved in the axial direction. Under the assumption of a diameter of approx. 34 mm of the threaded sleeve  98 , such a high and precise adjustment accuracy can be achieved with a thread pitch of approx. 0.5 mm (under the additional assumption that a human hand is capable of minimally rotating the threaded sleeve  98  by 0.5 to 1 mm). The maximum adjustment travel of the lens barrel  68  in the axial direction may, for example, is in the range of a few millimetres, which is sufficent for carrying out M 2  measurements with the usual beam diameters, which require a longitudinal adjustability of the lens barrel  68  by three time the Rayleigh length. 
         [0052]    For the attenuation of the intensity of the laser beam, a grey filter  110  is additionally installed in the lens barrel  68 , which is pushed from the camera-near end of the lens barrel  68  into it and secured therein by means of a clamping sleeve  112 , with an elastomer ring  114  sandwiched in between. In this manner, the grey filter  110  may be exchanged so that grey filters with different densities may be used for different beam intensities.