Abstract:
A method and an apparatus for making visible a mark on a spectacle lens are disclosed. An illumination light beam is directed to the spectacle lens. The illumination light beam runs through the spectacle lens and, after having run through the spectacle lens, is reflected on a reflector configured as a retroreflector, then runs again through the spectacle lens, and is finally fed to a camera as an observation light beam. The reflector is moved. Further, a measurement light beam is directed to said spectacle lens and fed to a sensor for measuring a physical property of the spectacle lens. The measurement light beam is generated by a first light source and the illumination light beam is generated by a second light source. The first and the second light sources are physically distinct units.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application is a continuation-in-part of international patent application PCT/EP2004/007425 filed on Jul. 7, 2004 and published in German language, which international patent application claims priority from German patent application DE 103 33 426.2 filed on Jul. 17, 2003. 

   FIELD OF THE INVENTION 
   The present invention is related to the field of manufacturing spectacle lenses. 
   More specifically, the invention is related to a method for making visible a mark on a spectacle lens, wherein an illumination light beam is directed to the spectacle lens, which runs through the spectacle lens and, after having run through the spectacle lens, is reflected on a reflector configured as a retroreflector, then runs again through the spectacle lens, and is finally fed to a camera as an observation light beam. 
   Further, the invention, further, is related to a method for measurement of a physical property of a spectacle lens provided with a mark, wherein a measurement light beam is directed to the spectacle lens and fed to a sensor, wherein, further, for making visible the mark, an illumination light beam is directed to the spectacle lens, which runs through the spectacle lens and, after having run through the spectacle lens, is reflected on a reflector, then runs again through the spectacle lens, and is finally fed to a camera as an observation light beam. 
   Moreover, the invention is related to an apparatus for making visible a mark on a spectacle lens comprising: 
   an illumination light source arranged on a first side of the spectacle lens for generating an illumination light beam, for identifying the mark; 
   a reflector arranged on the side of the spectacle lens opposite the first side, and being configured as a retroreflector; 
   a camera for receiving an observation light beam coming from the spectacle lens; 
   first optical means for guiding the illumination light beam through the spectacle lens; and 
   second optical means for guiding the observation light beam reflected by the reflector through the spectacle lens to the camera. 
   Still further, the invention is related to an apparatus for measuring a physical property of a spectacle lens provided with a mark, comprising: 
   an illumination light source arranged on a first side of the spectacle lens for generating an illumination light beam, for identifying the mark; 
   a reflector arranged on the side of the spectacle lens opposite the first side; 
   a camera for receiving an observation light beam coming from the spectacle lens; 
   first optical means for guiding the illumination light beam through the spectacle lens; and 
   second optical means for guiding the observation light beam reflected by the reflector through the spectacle lens to the camera, 
   a measurement light source for generating a measurement light beam for a measurement of a physical property of the spectacle lens; 
   a sensor; and 
   third optical means for guiding the measurement light beam from the measurement light source to the spectacle lens, and from the spectacle lens to the sensor. 
   Finally, the invention is related to an apparatus for making visible a spectacle lens provided with a mark. 
   BACKGROUND OF THE INVENTION 
   Spectacle lenses, in particular so-called progressive spectacle lenses, are provided with marks, the position of which being detected during the manufacture of the spectacle lens, and being processed, in order to chuck the spectacle lens in a correct position, to work it, to stamp it, and, finally, to bring it into the spectacle frame of the end user. Marks are applied to spectacle lenses in a durable manner, i.e. by diamond scratching methods, or during the forming when plastic material spectacle lenses are molded, or by laser marking. Moreover, the term “mark” as used in the context of the present invention, also comprises other irregularities of the spectacle lens, for example streaks within the glass or the plastic material. 
   If, in the context of the present application, “spectacle lenses” are referred to, it is to be understood that this term also comprises contact lenses or other comparable optical elements. 
   In order to avoid that the user of the spectacle is irritated by the marks when the spectacle is used, the marks are configured such that they are visible only under very special light circumstances. The detection of the position of a mark on a spectacle lens during the production process is, therefore, quite difficult. An additional problem is that the spectacle lenses in a production process have very different optical effects due to the specific requirements of the later user of the spectacle. Therefore, within a production line spectacle lenses of highly different optical effects are following one after the other so that these optical effects have to be taken into account in a fast sequence during the subsequent working of individual spectacle lenses. 
   The recognition of a mark on a spectacle lens is particularly difficult, when the spectacle lens is provided with a phototropic coating. Such spectacle lenses are commonly referred to in the art as HIP (High Index Phototropic) lenses. Phototropic coatings of the type of interest in the present context are relatively thick. The coating thickness is about 30 μm, whereas other lens coatings (antireflex coatings etc.) have a thickness of only about 2-3 μm. The marks are applied to the lens body prior to the coating. Therefore, they are covered by the coating that is applied later. Whereas this does not present a real problem for conventional thin coatings for what concerns the recognition of marks, a problem arises when the mark is covered by a thick phototropic coating. The marks become optically smeared thereby, i.e. the edges of the mark become blurred. In contrast to the situation with thin coatings only a low frequency phenomenon appears. 
   For a control of progressive lenses both in the far and in the near design reference point, it is necessary to measure the effect of the progressive lenses at predetermined coordinate points on the spectacle lens depending on the applied marks. For a manual or for an automatic measurement these marks must, therefore, be made visible. According to prior art methods and apparatuses, this is done by means of rhombic gratings or strip patterns which are imaged blurred, wherein the edge transitions bright/dark make the mark visible. 
   A disadvantage of this prior art approach, in particular during the automatic recognition of a mark, is that the grating is imaged differently, depending on the optical effect of the particular spectacle lens under investigation, namely as a function of the particular dioptric effect of the spectacle lens. It is, therefore, necessary to make considerable efforts for the recognition of marks with respect to the algorithms used. Prior art methods, therefore, have not yet matured into a complete, safe and automatic recognition. Under actual practice it is, therefore, necessary that even with automatic control devices a person of particular trained skill has to manually interfere into the production process for correcting wrong recognitions. 
   But also in a situation where the marks are recognized within a production process by means of a manual examination action, the situation is quite similar. In that case different illuminations are used depending on the particularly used marking process in order to make the marks visible. According to prior art apparatuses this is affected by manually interchanging illumination units. However, also with these manual methods the marks are quite obscure and difficult to recognize so that errors are possible during the positioning and the orienting of the particular spectacle lens. This holds true in particular with regard to the amount of time being available for recognizing the mark. For these reasons, it is necessary, in particular for plastic material spectacle lenses, to additionally mark the spectacle lenses prior to the recognition of the marks as such, namely by means of a felt pen or the like (so-called “pointing”) which results in additional work and time consumption. 
   Corresponding considerations also apply for other areas within the processing of such spectacle lenses, namely for stamping automats, which, according to the actual state of the art likewise require the assistance of an operating person. The person observes the spectacle lenses on a screen in order to manually correct within the system positions of the marks that have not been properly recognized automatically. This is, for example, affected by means of roller sphere input. This disadvantage likewise results in a reduction in productivity of a video-assisted, manually operated stamping machine. 
   Document U.S. Pat. No. 3,892,494 discloses a method and an apparatus for locating optical microeffects on optical components, for example lenses. A laser beam is directed on the component under investigation via a beam splitter, namely a semitransparent mirror. The laser beam runs through the component and impinges on the other side thereof on a retroreflector, for example a retroreflecting foil from which it is again reflected back through the component and then runs back on the same ray path until it is deflected by the beam splitter directed to a camera. 
   A disadvantage of this prior approach is that it may result in problems for spectacle lenses of highly different curvature. Due to the highly different curvature the observation ray path must be long and stopped down in order to achieve a sufficient depth of field. On the other hand, the structures of the retroreflector shall not be imaged sharply because for avoiding wrong interpretations it is desired to have a relatively homogeneous background. As a consequence, the retroreflector in these applications must be located very far behind the plane of the spectacle lens to be measured and, further, it should be configured very large, because strongly negative spectacle lenses would image the retroreflector on a very small scale so that the entire lens could no more be seen over the retroreflector. 
   In addition, it is important in the context of the present invention that not only the recognition of marks or of other irregularities on spectacle lenses is concerned, but moreover the integration of this recognition process into a measuring instrument or into a working process. In such instances, however, a sensor is arranged behind a spectacle lens, i.e. on the same side as the retroreflector of the prior art apparatus, for measuring physical properties of the spectacle lens. For design reasons it is, therefore, impossible to locate the retroreflector far behind the plane of the spectacle lens. 
   Document U.S. Pat. No. 4,310,242 discloses an apparatus for measuring the optical qualities of windshields in situ. For that purpose an optical apparatus is also used here having a light source, a beam splitter, a retroreflector positioned behind the windshield to be measured, as well as a camera. A fine pattern is projected through the beam splitter on a retroreflecting screen, such that a real image of this pattern is generated on the retroreflecting screen being deformed by the windshield positioned within the ray path. Via the beam splitter the camera also looks in the direction of projection through the windshield under investigation on the retroreflective screen. Inhomogeneities, tension double refractions, streaks and the like are clearly made visible in such a way. 
   Document DE 43 43 345 A1 disclosed methods and apparatuses for measuring the reflective and the transmissive, respectively, optical properties of a sample. A measuring radiation is directed on a sample and is reflected by the sample so that it impinges on a retroreflector which, again, sends the measuring radiation back via the object to the light source where a decoupling a detector takes place. 
   Another similar approach is also described in document EP 0 169 444 A2. 
   In a prior art vertex refractometer “Focovision SG1” a light beam is emitted from a light source through a green filter and is directed on a spectacle lens to be examined via a beam splitter. The light beam runs through the spectacle lens and impinges on a sensor head located behind the rear side of the spectacle lens. In such a way physical properties of the spectacle lens may be measured. Moreover, on the rear side there is a plane in which exchangeable illumination accessories may be located. These illumination accessories illuminate the spectacle lens from the rear such that the marks become visible. A corresponding observation light beam runs from the spectacle lens to the beam splitter, is there reflected and then fed to a camera via other optical means. In a first illumination accessory a sharply limited bright light bundle is directed on the spectacle lens under a flat angle. Marks that have been applied by scratching will then appear bright due to the irregular shape of the scratch in front of a dark background. A second illumination accessory, in contrast, is provided for spectacle lenses in which the marks have not been applied by scratching but by forming or by laser beams instead. This second illumination accessory therefore, comprises a bright line grating being illuminated from below, as well as a plurality of auxiliary lenses arranged one besides the other by means of which these bright gratings may be imaged to infinity. 
   This prior art apparatus, therefore, is relatively difficult to operate. Further, the point in which the measuring beam emitted by the light source impinges on the spectacle lens coincides with the point in which the observation light beam exits from the spectacle lens. This may result in errors during the processing. 
   DE 197 40 391 discloses still another observation apparatus for masked markings, i.e. marks. In this apparatus, a lens, being provided with a masked marking, is illuminated with an illumination light. The masked marking is then observed as a shadow of the lens generated by the observation light. 
   In this apparatus we have the disadvantage that the marking will be shifted depending on the type of the lens and its local prismatic effect or is reduced in size or amplified, respectively, by the positive or by the negative effect of the lens. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object underlying the invention to provide methods and apparatuses of the type specified at the outset such that the afore-mentioned disadvantages are avoided. In particular, it shall become possible to treat spectacle lenses during a production process in that on the one hand side the marks applied thereto are recognized in their correct position, on the other hand side also a measurement of the spectacle lens within the same process and within the same apparatus becomes possible. All this shall be accomplished with apparatus and method means being as simple as possible. 
   It is another object of the invention to provide an apparatus which allows the recognition of marks also below relatively thick coatings, in particular phototropic coatings, and to handle such spectacle lenses in an industrial manufacturing process. 
   According to the method specified first at the outset, this object is achieved by the present invention in that the reflector is moved. 
   According to the method specified second at the outset, this object is achieved by the present invention in that the measurement light beam is generated by a first light source and the illumination light beam is generated by a second light source, the light sources being physically distinct units. 
   According to the apparatus specified first at the outset, this object is achieved by the present invention in that the reflector is connected to a drive motor for moving the reflector. 
   According to the apparatus specified second at the outset, this object is, finally, achieved by the present invention in that the measurement light source and the illumination light source as well as the first optical means and the third optical means are each physically distinct units. 
   According to the apparatus mentioned third at the outset, this object is achieved by the present invention in that the apparatus comprises a support with the spectacle lens resting thereon, the spectacle lens being provided with a phototropic coating, the mark being applied to the spectacle lens below the phototropic coating, an illumination light source for generating an illumination light beam, the illumination light source being configured as a point light source, a camera for receiving an observation light beam, first optical means for guiding the illumination light beam from the illumination light source to the spectacle lens, and through the spectacle lens and the mark, and second optical means for guiding the illumination light beam after having run through the spectacle lens as an observation light beam to the camera. 
   The object underlying the invention is thus entirely solved. 
   If, namely, the reflector being configured as a retroreflector, is moved, a more homogeneous background is generated, as compared to the prior art. The marks distinguish from this more homogeneous background significantly clearer, and, therefore, with more contrast. The spectacle lenses to be examined appear constantly bright during the measurement. At the edges of the marks, however, such a strong scattering takes place, that the scattered light does no more fulfill the condition of the retroreflection, with the consequence that the marks appear dark in front of a bright background. By moving the retroreflector, the structure thereof becomes blurred and waves, inhomogeneities, soiling, etc. of the retroreflector become no more apparent. 
   The measure to use physically distinct components for the ray path of the measurement light and of the observation light and, at least partially, to also use distinct ray paths, has the effect that the measurement of physical properties of the spectacle lens and the recognition of marks on the spectacle lens may be clearly separated one from the other methodwise. 
   The use of a point light source (so-called “artificial star”) has the advantage that low frequency phenomena become apparent. The recognized image of the mark, therefore, becomes sharper at its edges. 
   In a preferred embodiment of the invention the inventive method with the moved retroreflector may be combined with a vertex refractometer so that the measurement may easily be effected at those points on the spectacle lens, the position of which relative to the mark being defined on the spectacle lens. For that purpose, the inventive method is combined with the vertex refractometer such that an image of the mark must assume a certain position on a camera image during the making of the measurement. For that purpose one uses an illumination ray path and a measurement ray path which may be optically decoupled as described in detail, although they partially use identical paths. 
   If in the context of the present application the term “retroreflector” is used, this is to be understood to mean an area which reflects incoming light over a large range of incident angles essentially into the same direction in which it has come in. In practice, one uses plane or curved surfaces being provided with a retroreflective surface, for example with glass pearls or on which many small triple mirrors or mirrored triple prisms are applied in a regular arrangement. Such surfaces are generally known from back reflectors on vehicles, traffic signs, light barriers, etc. For the inventive method it is advantageous when the individual retroreflective structures on the retroreflector are essentially smaller than 1 mm. 
   In preferred embodiments of the inventive method, the reflector is moved essentially periodically, in particular in rotation. As an alternative, a movement in a parallel rotational translation may also be considered, in which the retroreflector is, for example, displaced within a plane and its center is moved on a circular path without rotating the retroreflector as such. Besides that any linear movement of the retroreflector is possible. 
   All these movements are essentially effected transversely with regard to the direction of propagation of the illumination light beam. 
   When the retroreflector is moved periodically, another preferred embodiment of the invention consists in adapting the frequency of the periodic movement of the reflector to a synchronizing signal of the camera. In particular, it is preferred when the frequency is revolution-synchronized, in particular phase-synchronized with the synchronizing signal. 
   This measure has the advantage that the electronic processing of the video signals derived from the observation light beam becomes particularly simple. 
   In another group of embodiments a good effect is achieved when the reflector is moved at a distance of between 1 cm and 30 cm from the spectacle lens. 
   This measure has the advantage that a particularly homogeneous background of the moved retroreflector is achieved. 
   A particularly good effect is, further, achieved when a video signal generated from the observation light beam within the camera is emphasized in contrast over neighborhoods being each finite, by means of at least one folding generation. Preferably, when doing so, high spatial frequencies are emphasized more than low spatial frequencies, for example due to a differentiating effect. 
   The video signal generated from the observation light beam within the camera may be used for various purposes according to the invention. On the one hand it may be used for identifying the mark by means of pattern recognition. On the other hand it may be used for identifying a position of the spectacle lens on a support, and finally for identifying a dioptric effect of the spectacle lens. 
   According to the invention, various ray paths may be combined entirely of over certain lengths, in order to be able to execute the method within a space being as small as possible. 
   According to a first variant, the illumination light beam is coupled into the ray path of the observation light beam. According to a second variant, the measurement light beam is coupled into the ray path of the observation light beam. In a third variant, finally, the measurement light beam is coupled into the ray path of the illumination light beam. 
   According to embodiments of the inventive apparatus, this is preferably affected by corresponding beam splitters or by other appropriate optical means, for example through-bored mirrors. 
   In connection with the beam splitters used, it is preferred to provide a light trap for a portion of the illumination light beam running through the beam splitter. 
   If a beam splitter is provided for coupling the illumination light beam into the ray path of the observation light beam, this is preferably affected such that an entrance pupil of the camera and an exit pupil of the illumination light source are arranged at conjugate positions with regard to the beam splitter. 
   According to the invention, it is, further, preferred when the illumination light beam is blanked out at least in the point of impingement of the measurement light beam on the spectacle lens. 
   This measure has the advantage that interactions are avoided that might occur because the illumination light beam impinges on the sensor that should only receive the measurement light beam. 
   In so far it is particularly preferred when the illumination light beam is generated as a light beam an annular-shaped cross section. 
   Under apparatus aspects this is affected in that either the illumination light source generates an illumination light beam having an annular-shaped cross section or in that the illumination light source comprises a slide having an opaque spot in the ray path of the illumination light beam. 
   In further preferred embodiments of the invention the illumination light beam and the measurement light beam are generated with a different wave length. 
   This measure, too, has the advantage that both light beams may be exactly separated one from the other with regard to their electronic processing. 
   Preferably, the illumination light beam is generated as a red light and the measurement light beam is generated as a green light. 
   Moreover, it is preferably possible to guide the measurement light beam impinging on the sensor through a filter acting as a stop filter for the light wave length of the illumination light beam. 
   This measure too, helps to separate the two ray paths one from the other. 
   Moreover, the retroreflector may be configured plane or domed. 
   In a preferred embodiment of the invention the point light source has an aperture ratio of less than 1/200, preferably of less than 1/500 relative to the mark. 
   This measure has the advantage that an optimum compromise between an ideal point light source and a practical point light source can be found. When a point light source with an aperture ratio of 1/500 is used, a high power light emitting diode with a selection aperture stop may be used having a diameter of 1 mm and a distance of 500 mm to the spectacle lens surface provided with the mark. 
   Moreover, it is preferred when the point light source has a wavelength in the transition between visible and invisible light, preferably of less than 400 nm or of more than 750 nm. 
   This measure has the advantage that stray light and ambient light may well be suppressed by receiver-sided filters. 
   Further advantages will become apparent from the description and the enclosed drawing. 
   It goes without saying that the features mentioned before and those that will be explained hereinafter, may not only be used in the particularly given combination, but also in other combinations, or alone, without leaving the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are shown in the drawing and will be explained in further detail throughout the subsequent description. 
       FIG. 1  shows a highly schematic side elevational view of a first embodiment of an inventive apparatus; 
       FIG. 2  shows a likewise schematic representation of a second embodiment of an inventive apparatus, namely a vertex refractometer, wherein additional optical means are provided for measuring a spectacle lens; 
       FIGS. 3-5  show three distinct embodiments of illumination light sources as may be used in the embodiments according to  FIGS. 1 and 2 ; 
       FIG. 6  on an enlarged scale shows a top plan view on a retroreflector as may be used in the apparatus of  FIG. 1 , however, with a drive for a parallel rotational transformation; and 
       FIG. 7  a schematic side elevational view for explaining another possibility for coupling illumination light. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1  reference numeral  10  as a whole designates a first embodiment of an apparatus according to the present invention. Apparatus  10  comprises a support  12  having a central opening  14 . A spectacle lens  16  is disposed on support  12  over opening  14 . 
   As one can see from the enlarged section in  FIG. 1 , spectacle lens  16  is provided with a mark indicated at  18 . Mark  18  is applied into the surface of spectacle lens  16 . In the embodiment shown, spectacle lens  16 , further, is provided with a thick coating, in particular with a phototropic coating, covering mark  18 . The thickness of the coating is about 30 μm. The invention, however, is not limited to spectacle lenses having such thick coatings. 
   An illumination light source  20  is directed preferably transversely with regard to the optical axis of spectacle lens  16  which, in the embodiment shown, coincides with the optical axis  21  of a camera  36 . In particular if spectacle lens  16  is provided with thick coating  19 , illumination light source may preferably be a point light source. It has an aperture ratio of less than 1/200, preferably of less than 1/500 relative to the upper surface of spectacle lens  16 . If illumination light source  20  is configured by a high power light emitting diode (for example Type SFH4230 of the Osram company) with a selection aperture stop in front thereof, the diameter of the aperture stop may be 1 mm and the distance from the aperture stop to the surface of the spectacle lens  16  provided with mark  18  may be 16,500 mm if the aperture ratio is 1/500. 
   Illumination light source  20  is preferably operated at a light wave length being outside or at the transition between visible and invisible light, i.e. for example below 400 nm or above 750 nm. 
   Illumination light source  20  emits an illumination light beam  22 . Illumination light beam  22  impinges on a beam splitter  24 , for example a semi-permeable mirror, and is reflected therefrom in the direction of optical axis  21  of camera  36 . That portion of illumination light beam  22  emanating from illumination light source  20  and running through beam splitter  24  is absorbed by a light trap  26  located behind beam splitter  24 . 
   Illumination light beam  22  runs through spectacle lens  16  wherefrom it impinges on a retroreflector  30 , which, in the illustration of  FIG. 1 , is positioned below spectacle lens  16 . Retroreflector  30  is provided with a retroreflecting coating  32 . It may be configured as a conventional retroreflecting foil or as a retroreflector having regularly arranged triple mirrors or mirrored triple prisms. 
   Illumination light beam  22  reflected from retroreflector  30  now again runs through spectacle lens  16 , in the opposite direction, and is then fed as an observation light beam  34  to camera  36 , for example a CCD-camera. Camera  36  is focused on spectacle lens  16  and generates an image of spectacle lens  16  in which mark  18  is visible in front of the background of retroreflector  30 . 
   In the embodiment shown, a drive shaft is connected to a drive motor  38  via a highly schematically indicated actuating connection  37 . Drive motor  38  rotates retroreflector  30  about a vertical axis being preferably flush with axis  21  of camera  36  as well as with the axis of illumination light beam  22 . In  FIG. 1  this is indicated by an arrow  39 . 
   In the embodiment shown in  FIG. 1  the retroreflecting coating  32  is made continuous in the area of vertical rotational axis of retroreflector  30 . Due to that a pattern may remain recognizable in the area of the rotational axis and within a small circular neighborhood thereabout and under unfavorable circumstances which, however, disturbs only little in practice. It should be emphasized already at this instance that retroreflector  30  for that reason may be configured annular-shaped, as is the case, for example, in the embodiment of  FIG. 2  discussed below. 
   Instead of rotating retroreflector  30  as a whole about a vertical axis, one may also let it oscillate linearly and transversely with regard to axis  21 . If in that case, retroreflectors are used having a continuous pattern of the retroreflecting elements, care must be taken that an appropriate direction of the linear oscillating movement of the pattern is set. 
   Finally, in still another variant it is also possible to move retroreflector  30  in a parallel rotational translation, similar to the oscillating movement of a grinding plate of a hand-held oscillating grinding machine. A corresponding drive mechanism for that purpose is shown in  FIG. 6  and will be discussed in further detail below. 
   Seen as a whole it is important for the movement of retroreflector  30  that the regular structure of retroreflector  30  and, as the case may be, a dirt or soiling sticking thereto, become blurred with the movement. Is has already been mentioned that the main component of movement of retroreflector  30  should extend essentially transversely to optical axis  21  of camera  36 . 
   For the retroreflecting coating  32  of retroreflector  30  one preferably uses a continuous pattern of individual elements, for example regularly arranged triple prisms or triple mirrors. In that case it makes sense to couple the movement of retroreflector  30  with vertical synchronizing pulses of camera  36 . 
   For that purpose a circuitry as shown in  FIG. 1  may be used. The circuitry consists of an electronic control unit  40  being connected to camera  36  via a first electrical line  41  and being connected to drive motor  38  via a second electrical line  42 . Electronic control unit  40 , in turn, delivers control commands to drive motor  38  via a third electrical line  43 . 
   The vertical synchronizing pulses of camera  36  are transmitted to electronic control unit  40  via first line  41 . Motor  38  delivers encoder pulses via second electrical line  42 , the encoder pulses being compared with the vertical synchronizing pulsed within electronic control unit  40 . From that comparison a control signal for current or for the voltage of drive motor  38  is derived and transmitted via third electrical line  43 . The control may effect a revolution synchronization, i.e. an adaptation of revolutions of drive motor  38  to the frequency of the vertical synchronizing pulses. However, it is particularly preferred if, moreover, a rigid phase coupling is effected such that a predetermined constant phase relation between the periodic movement of drive motor  38  (for example of its rotational movement) and the vertical synchronizing pulses of camera  36  is ensured. 
   By utilizing a moved retroreflector  30 , the background before which spectacle lens  16  is imaged homogeneously within camera  36 . Therefore, one avoids a disadvantage of prior art apparatuses, in which in addition to the mark to be recognized there is still another blurred structure superimposed which may even have the same order of magnitude as the mark to be recognized. If, in contrast, retroreflector  30  is moved as described above, one may subtract a homogeneous background image during the image processing. In prior art apparatuses this is impossible because the inhomogeneous background pattern would be imaged in different sizes depending on the differing curvature of the spectacle lenses. 
   It is of particular advantage during the observation of the contrasted image to let the video signal of camera  36  additionally run via a contrasting apparatus. The contrasting apparatus, for example, executes a local folding operation on the grey values by utilizing a core function having a differentiating character a plurality of directions. For each pixel P[ij] one computes the sum 
   
     
       
         
           
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   One may, for example, continuously compute the contrasting function on a universal computer and display the image (being shortened at the rims of the image area by one core length each). It is even simply possible to implement such a contrasting function in hardware, without using a computer, and to display the result in real time on a video screen. For that purpose a video digitizer is required and as many delay lines as the folding core has lines (in the above notation n). Further, a so-called convolver chip is required, as is, for example, commercially available under the denomination PDSP 16488 from the Plessey company. 
   In the embodiment of  FIG. 2 , reference numeral  44  designates an apparatus, namely a vertex refractometer having a light tight housing  46 . 
   In  FIG. 2  only one mark  52  is shown. In reality, however, two marks are provided being located at a predetermined distance above and below, respectively, the plane of the drawing. The position of these two marks define the location of a spectacle lens at which a physical property, for example the optical power of spectacle lens  50 , shall be measured. Spectacle lens  50  corresponds to spectacle lens  16  of  FIG. 1 , i.e. it may be likewise provided with a thick, phototropic coating (not shown in  FIG. 2 ). 
   A measurement light beam  64  is directed on this location. For that purpose spectacle lens  50  is laid down on a tubular support  54  by an operator and is there manually adjusted relative to the marks  52 . Insofar, the apparatus may be a conventional vertex refractometer. On this vertex refractometer spectacle lens  50  is positioned during the measurement such that marks  52  being displayed in a contrasted fashion, come to lie at a predetermined position within the camera image. Marks  52 , therefore, are not at the location of measurement as such at a predetermined geometric relation thereto. 
   Only on its right-hand side in  FIG. 2  housing  46  has an opening  48  being accessible from the exterior. Within opening  48  there is a support for spectacle frame  50  being provided with mark  52 . 
   Below spectacle lens  50  there is an annular-shaped retroreflector  56  which is only shown merely schematically. Retroreflector  56  is adapted to be rotated by means of an appropriate drive (not shown) as indicated with an arrow  58 . Insofar the same applies as was already explained above with regard to the embodiment of  FIG. 6 . 
   In housing  46  there is an upper chamber  60  being equipped with a measurement light source  62  at the left-hand end thereof. Measurement light source  62  emits a measurement light beam  64 . Measurement light beam  64  first runs through a first color filter  66  and then through an aperture stop  68  before it is deflected downwardly at a prism  70  or a corresponding mirror. Measurement light beam  64  then runs through a prisms compensator  72  as well as through a hole in a through-bored mirror  73  and then impinges on the surface of spectacle lens  50 . It then runs through spectacle lens  50  as well as through support  54  and, in another preferred embodiment of the invention, runs through a second color filter  74  before it impinges on a sensor  76 . 
   Within a central chamber  77  of housing  46  there is an illumination light source  78 , three embodiments of which being discussed below in connection with the  FIGS. 3 through 5 . 
   Illumination light source  78  may likewise be a point light source, in particular when spectacle lens  16  is covered by a thick phototropic layer, as discussed above. 
   Illumination light source  78  emits an illumination light beam  79 . Illumination light beam  79  first impinges on a deviation mirror  80  and then on a semi-permeable mirror acting as a beam splitter  81 . That portion of illumination light beam  79  running through beam splitter  81  is absorbed within a light trap  75  being positioned behind beam splitter  81 . Illumination light beam  79  is essentially deviated to the right-hand side by beam splitter  81  for then impinging on a prism  82  or a mirror, deviating illumination light beam  79  downwardly. After still another deviation by a deviation mirror  83  illumination light beam  79  runs through a window  84  of opening  48  and impinges on through-bored mirror  73  which deviates illumination light beam  79  again upwardly where it impinges on spectacle lens  50  and illuminates mark  52 . 
   Illumination light beam  79  being reflected from spectacle lens  50  and from mark  52 , respectively, now configures an observation light beam  85  which, first, runs upwardly and then via through-bored mirror  73 , deviation mirror  83 , prism  82  and beam splitter  81  on another prism  86  or a corresponding mirror which deviates observation light beam  85  downwardly where it is fed to a CCD-camera  88  via a lens  87 . Prism  86 , lens  87  and CCD-camera  88 , positioned within a left chamber  89  of housing  46 . 
   The entrance pupil of lens  87  and the exit pupil of illumination light source  78  are located at conjugate positions with regard to beam splitter  81 . 
   The apparatus  44  of  FIG. 2  operates as follows: 
   Within a measurement branch, measurement light beam  64  is emitted in the fashion described above from measurement light source  62  to spectacle lens  50 , runs there through and impinges on sensor  76 . In such a way physical properties of spectacle lens  50  may be measured. For a better distinction from illumination light beam  79  and from observation light beam  85 , respectively, measurement light beam  64  is emitted in another light wavelength, for example as green light. For that purpose, first color filter  66  is configured as a green filter. Second color filter  74  in front of sensor  76 , in contrast, has the function of a band stop filter which does not let other light wavelengths pass, in particular those of illumination light beam  78 . In such a way it is avoided that other light except from measurement light beam  64  impinges on sensor  76 . 
   Concurrently, illumination light beam  79  from illumination light source  78  is directed on spectacle lens  50  in the manner described above, for illuminating mark  52 . The reflected image of mark  52  reaches CCD-camera  88  as the observation light beam  85  and, from thereon, is processed as a video signal. 
   While all that happens, retroreflector  56  below spectacle lens  50  is moved (arrow  58 ), namely in the already described manner, i.e. about support  54 , either rotating or in a parallel rotational translation. 
   The frequency of the afore-mentioned periodic movement of retroreflector  56  is selected such that it is adapted to the reading frequency of camera  88 . 
   Also insofar, it goes without saying that a further contrasting may be effected by a folding operation of the type discussed above. 
   As an alternative or in addition to sensor  76  a measurement of a physical parameter of spectacle lens  50  may be made via the evaluation of the video signal of CCD-camera  88 . 
   In general, however, it is desired to decouple the measuring branch on the one hand and the illumination/observation branch on the other hand. 
   For that purpose, illumination light source  78  is preferably configured as shown in three embodiments in  FIGS. 3 through 5 . 
   It is a common feature of all three embodiments of  FIGS. 3 through 5  that light is fed thereto via a light guide  90 . Of course, this does not exclude that the light may also be generated within illumination light source  78  itself, for example by means of a laser, a laser diode, a light emitting diode or the like. 
   In the embodiment of  FIG. 3  light guide  90  within illumination light source  78   a  emits illumination light beam  79   a  which, at this instance, is a diverging light beam. By means of a downstream collimating optical unit  91  illumination light beam  79   a  is made parallel and then impinges on a transparent slide  92  which is provided with a central black spot  93  in the area of the optical axis. By means of an imaging optical unit  94  illumination light beam  79   a  is now directed on deviation mirror  80  ( FIG. 2 ). 
   By imaging the central black spot  93  on the surface of spectacle lens  50  it is ensured that no illumination light will enter into the opening of tubular support  54  used for the optical measurement. This is exactly the area in which measurement light beam  64  impinges on spectacle lens  50 . By doing so one prevents that illumination light impinges on sensor  76  via spectacle lens  50 . 
   As has already been mentioned, second color filter  74  may be arranged in front of sensor  76 , wherein second color filter  74  acts as a band stop filter for the light wavelength of the illumination light. If, for example, the measurement light is green light, the illumination light may preferably be red light. 
   In the second embodiment of  FIG. 4  illumination light source  78   b  has likewise a collimating optical unit  95  behind the exit of light guide  90  acting on diverging illumination light beam  79   b . In this instance a color filter  96  is arranged in front of collimating optical unit  95  which, in the manner described before, may be a red filter. 
   In the third embodiment of  FIG. 5 , finally, a annular-shaped outlet  97  is provided within illumination light source  78   c  at the free end of light guide  90 , for generating an illumination light beam  79   c  having an annular-shaped cross section. Illumination light beam  79   c  is directed on deviation mirror  80  via an imaging optical unit  98 . 
   In this embodiment, too, very much like in the embodiment of  FIG. 3 , a central, stopped-down area is generated, in which no illumination light impinges on the surface of spectacle lens  50 , and in which the measurement light may be fed to sensor  76  via spectacle lens  50 . 
     FIG. 6  illustrates the generation of a parallel rotational translation, for example of retroreflector  30 . For that purpose there are, for example, three cams  100   a ,  100   b  and  100   c  located below retroreflector  30 . Cams  100   a ,  100   b  and  100   c  are each journalled stationary for rotation about a first axis  101 , as indicated by an arrow  102 . 
   At equal distances from axis  101 , cams  100   a ,  100   b  and  100   c  are journalled with a second axis  103  at retroreflector  30 , wherein axes  101  and  103  extend parallel. If cams  100   a ,  100   b  and  100   c  are driven at equal phase (arrow  106 ), for example by means of a circumferential drive belt  104  and a common drive shaft  105  or a common friction wheel, retroreflector  30  is caused to make a tumbling movement, as indicated at  30 ′ and  30 ″. 
   According to this tumbling movement the center  107  of retroreflector  30  runs on a circular path  108 , the radius of which being equal to the excentricity, i.e. equal to the distance between the axes  101  and  103 . Accordingly, any point on the surface of retroreflector  30  likewise moves along such a circular path. 
   For compensating unbalanced masses, cams  100   a ,  100   b  and  100   c  may be provided with balancing masses. The amount of excentricity depends on the periodicity of the movement of retroreflector  30  in the first place. 
   It goes without saying that the drive illustrated in  FIG. 6  is only understood to be an example and that, for example, another number of cams may be used. It is likewise possible to use other types of drive, as are, for example, conventional oscillating grinding machines. 
     FIG. 7 , finally, shows still another variant in which no beam splitter ( 24  in  FIG. 1 and 81  in  FIG. 2 , respectively) is used. 
   For that purpose the illumination source is centrally arranged in front of the camera lens. In the embodiment shown the plane parallel glass plate  111  is arranged in front of camera lens  110  in a direction transverse with regard to axis  109 . Glass plate  111  carries the end of an optical fiber  112  at its center. Optical fiber  112  emits the illumination light beam  113 . In order to avoid that light is directly reflected into the camera, a small area  114  on camera lens  110  is blackened.