Abstract:
Disclosed is an apparatus for determining surface properties, comprising at least a first radiation device which emits radiation onto a surface to be analysed, at least a first radiation detector device which receives at least part of the radiation emitted by the at least one radiation device and then scattered or reflected by the surface and outputs at least a first measurement signal which is characteristic of the reflected or scattered radiation, and at least a second radiation detector device which receives at least part of the radiation emitted by the at least one radiation device and then scattered or reflected by a surface and outputs at least a second measurement signal which is characteristic of the reflected or scattered radiation. According to the disclosure, the first radiation detector device is offset by a first predefined angle β 1  with respect to the direction of the radiation reflected by the surface, and the further radiation detector device is offset by further predefined angle γ 1  with respect to the direction of the radiation reflected by the surface, and the ratio between the value of the further predefined angle γ 1  and the value of the first predefined angle β 1  is at least 1.5:1.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method and an apparatus for determining surface properties such as, in particular but not exclusively, the colour, orange peel or similar properties of a surface to be analysed. 
   The invention will be described with reference to surfaces of motor vehicles. However, it is pointed out that the invention can also be used on other surfaces, such as for example the coatings of furniture, of floor coverings and the like. 
   BACKGROUND OF THE INVENTION 
   The optical impression of objects or of the surfaces thereof, particularly surfaces on motor vehicles, is greatly determined by the surface properties thereof. Since the human eye is suitable only to a limited extent for the objective determination of surface properties, there is a need for aids and apparatuses for the qualitative and quantitative determination of surface properties. 
   Surface properties such as, for example, gloss, orange peel, colour, macrostructure or microstructure, image sharpness, haze, surface structure and/or surface topography and the like are determined. 
   Furthermore, coatings which contain so-called effect pigments are enjoying great popularity in recent times. 
   The prior art discloses apparatuses in which a radiation device emits radiation onto the surface to be analysed and the radiation reflected and/or scattered by this surface is received by a detector and evaluated. These apparatuses operate satisfactorily and allow objective classification of the surface in question. 
   However, particularly when using the abovementioned effect pigments, the problem arises that said effect pigments may change their optical impression as a result of different physical causes. For example, it is possible that an incorrect application of paint to the surface may lead to the situation where the layer thicknesses are different at different points. The concentration of colour pigments at different areas of the surface is also accordingly different. As a result of such changes, the colour impression of such a surface will also change. 
   In addition, however, incorrect orientations of the abovementioned effect pigments may lead to colour changes of the surface. Ideally, these effect pigments are oriented uniformly in a given direction and in particular in the plane of the surface. However, incorrect orientations of the individual effect pigments or of groups of effect pigments may occur, i.e. individual or several pigments are rotated with respect to other pigments. In such areas, therefore, the light will also be reflected differently than in the case of uniformly oriented effect pigments. 
   However, the observer is unable to determine the physical causes of colour changes and is therefore unable to take any suitable counter-measures. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is therefore to provide an apparatus and a method which allows the user to ascertain the physical causes of colour changes. 
   According to the invention, this is achieved by an apparatus according to claim  1 . Advantageous embodiments and further developments form the subject matter of the dependent claims. 
   The apparatus according to the invention for determining surface properties comprises at least a first radiation device which emits radiation onto a surface to be analysed. Also provided is at least a first radiation detector device which receives at least part of the radiation emitted by the at least one radiation device and then scattered and/or reflected by the surface and outputs at least a first measurement signal which is characteristic of the reflected and/or scattered radiation. Also provided is at least one further radiation detector device which receives at least part of the radiation emitted by the at least one radiation device and then scattered and/or reflected by the surface and outputs at least a further measurement signal which is characteristic of the reflected and/or scattered radiation. 
   According to the invention, the first radiation detector device is offset by a first predefined angle with respect to the direction of the radiation reflected by the surface, the further radiation detector device is offset by a further predefined angle with respect to the direction of the radiation reflected by the surface, and the ratio between the value of the further predefined angle and the value of the first predefined angle is at least 1.5:1. 
   Due to this arrangement of the two radiation detector devices, one radiation detector device is arranged closer to the direction of the reflected light than the other. This radiation detector device will thus receive a greater proportion of the reflected light than the other radiation detector device. If concentration fluctuations of the colour pigments then occur in the case of layer thickness changes, this will have essentially the same effect both on the scattered light and on the reflected light. It is thus possible for example that the intensities both of the reflected light and of the scattered light decrease. 
   By contrast, a non-uniform distribution of effect pigments means that less light is reflected in a given direction, but the proportion of scattered light increases. In this case, the intensity measured by the detector unit arranged close to the reflection angle would be reduced, and the intensity measured by the radiation detector device further away from the reflection angle would increase. It is thus possible to ascertain, from the respective changes in the measured radiation intensities, the physical causes underlying colour changes. 
   By using the present invention, it is possible to check the parameters of painting devices for example. Components of the paint can also be checked to ensure they have a constant quality. 
   Preferably, the ratio between the value of the further predefined angle and the value of the first predefined angle is at least 2, preferably at least 2.5, more preferably at least 3.0 and particularly preferably is approximately 4.0. This ensures that one of the two radiation detector devices is arranged much closer to the reflection angle than the other, so that the more remote radiation detector device no longer receives essentially any part of the reflected light. In this way, the differentiation according to physical causes can be achieved in a particularly efficient manner. 
   Preferably, the apparatus can be moved with respect to the surface so that different areas of the surface can be analysed, and furthermore a processor device is provided which at least partially compares first and further measurement signals recorded at a first area of the surface with first and further measurement signals recorded at a second area of the surface, and based on this comparison outputs at least one characteristic value regarding the state of the surface. It can thus be deduced for example, from the comparison of two such measurement signals, whether an intensity at the two radiation detectors increases or decreases in an identical manner or whether these signals behave oppositely. It can thus be deduced from this comparison whether the colour changes are based on concentration changes of the colour pigments or on a change in orientation of the effect pigments. 
   Preferably, a relationship between measurement signals is recorded and this relationship is particularly preferably a difference or a ratio. The cause of colour changes can thus be determined from corresponding differences or ratios. 
   In a further preferred embodiment, the first predefined angle has a value between 5° and 35°, preferably between 10° and 30°, more preferably between 15° and 25° and particularly preferably of approximately 15° with respect to the direction of the reflected light. 
   In a further preferred embodiment, the further predefined angle has a value between 30° and 100°, preferably between 40° and 80°, more preferably between 50° and 70° and particularly preferably of 60° with respect to the direction of the reflected radiation. 
   In a further preferred embodiment, the radiation device emits the radiation at a predefined emission angle α 1  onto the surface, and this angle is between 5° and 45°, preferably between 10° and 35°, more preferably between 10° and 25° and particularly preferably in the region of 15° with respect to a direction perpendicular to the surface. 
   When selecting this angle, care must be taken in particular to ensure that it is not selected to be too large, since if the angle is too large there is a very large offset of the reflected radiation due to unevenesses. In addition, the colour measurement is made more difficult if the angles are too large. On the other hand this angle should also not be too small, in order to ensure that the angle between the emitted and the reflected radiation is large enough to be able to arrange radiation detector devices therebetween. 
   In a further preferred embodiment, an absorption device is provided which absorbs the radiation reflected by the surface or a large part of said radiation. This absorption device is thus preferably arranged at the reflection angle with respect to the emitted radiation. Preferably, the absorption device is arranged in such a way that it also covers predefined angle ranges of for example 2° to 10° degrees around the reflection angle. The absorption device may in this case be designed as a radiation-absorbing tube or the like. By providing this absorption device, undesired effects due to scattered light are reduced. 
   In a further preferred embodiment, a second radiation detector device is provided which receives at least part of the radiation emitted by the at least one radiation device and then scattered and/or reflected by the surface and outputs at least a second measurement signal which is characteristic of the reflected and/or scattered radiation. In this case, this second radiation detector device is preferably arranged at a second angle with respect to the radiation reflected by the surface, and this second angle is essentially equal and opposite to the first predefined angle. 
   Radiation is understood to mean any type of radiation, such as for example infrared light, ultraviolet light, infrared light, light in the visible wavelength range, X-ray radiation and the like. The radiation used is preferably light in the visible range and particularly preferably a standardised white light. 
   A property characteristic of the radiation is understood to mean, in particular but not exclusively, the intensity thereof, the spectral range or wavelength thereof, the polarisation thereof or a combination of these properties. A characteristic measurement signal is thus understood to mean a measurement signal which is characteristic of at least one of these properties. 
   According to the invention or in this embodiment, the two radiation detector devices are thus arranged essentially the same distance away from the direction of the reflected radiation. Preferably, both the radiation device and the radiation detector devices are arranged in the same plane, so that the observed beam paths also run essentially in one plane which is particularly preferably perpendicular to the surface to be analysed. 
   In the case of an exactly perpendicular orientation of the apparatus with respect to the surface, it is to be expected that the two radiation detector devices receive essentially the same radiation intensity since they are located essentially the same distance away from the expected maximum of the radiation at the reflection angle. The intensity distribution reaches a maximum at the respective reflection angle and then decreases for example in a Lorenz or Gaussian form towards the flanks. If tilting of the apparatus with respect to the surface occurs in the plane of the beam paths, for example due to curvatures, the angle of the reflected light will also change. It is thus possible for example that the reflected light is reflected by the surface in a direction which is closer to the first radiation detector device and further away from the second radiation detector device. In this case, the first radiation detector device will detect a higher intensity than the second radiation detector device. The tilt angle of the apparatus with respect to the surface can be deduced from a relationship between the measured intensities, and a measurement result can be corrected on the basis of this deduction. 
   If, on the other hand, the state of the surface changes, for example the layer thickness thereof, then possibly the intensity of the reflected light will change but not the direction thereof. Here, the intensity detected by the two radiation detector device will thus change in the same way. The observer can deduce therefrom that there is no tilting of the apparatus with respect to the surface. 
   An essentially equal but opposite angle is understood to mean that the two angles of the radiation detector devices have essentially the same value with respect to the direction of the reflected light, with tolerances of up to 3° also being possible. In a further embodiment, it would also be possible to arrange the radiation detector devices at different angles with respect to the direction of the reflected light, and to take this into account accordingly when evaluating the intensity ratios. 
   Here, too, a processor device is preferably provided which, from a relationship between the first measurement signal and the second measurement signal, takes account of the orientation of the apparatus with respect to the surface. For example, a straight orientation of the apparatus can be deduced from identical intensities or an intensity difference of 0. This intensity difference of 0 occurs in particular when the two radiation detector devices are arranged at equal and opposite angles with respect to the direction of the reflected light. If the angles do not exactly correspond to one another, this must accordingly be taken into account by the processor device. In this case, the intensity distribution of the reflected light will particularly preferably also be taken into account. 
   By providing further radiation detector devices for example outside the abovementioned plane of the beam paths, corresponding measurements also for tilting of the apparatus perpendicular to the abovementioned plane are possible, and it is thus possible overall to record the precise tilting of the apparatus in three-dimensional space with respect to the surface. 
   However, it is pointed out that the first radiation detector device and the second radiation detector devices, which are arranged at an essentially equal and opposite angle with respect to the direction of the reflected light, can also be used independently of the further radiation detector device or without the latter and thus are also claimed independently thereof. 
   In a further preferred embodiment, an intensity measuring device is provided which measures the intensity of the radiation emitted by the radiation device before said radiation impinges on the surface. If for example a white LED is used as the radiation device or radiation source, the intensity thereof is not exactly constant over time. For instance, changes in the intensity may be caused for example due to heating of the radiation source. Since the intensities determined in each case by the radiation detector device are also relevant for determining surface properties, the intensity measuring devices serves to carry out a calibration of the respectively determined intensities. This is particularly beneficial in the pulsed mode. 
   Preferably, at least a first radiation device has at least one radiation source selected from a group of radiation sources including thermal radiation sources, such as in particular but not exclusively incandescent lamps, halogen lamps, coherent and non-coherent semiconductor radiation sources, such as in particular but not exclusively LEDs, gas discharge radiation sources, lasers, combinations thereof and the like. The radiation source used is particularly preferably an LED which emits white light. 
   In a further preferred embodiment, a layer thickness measuring device is provided. The respective layer thickness can thus be determined for example inductively. In a further preferred embodiment, the apparatus comprises a movement device for moving the apparatus with respect to the surface along a predefined, preferably essentially rectilinear movement direction. This movement device may comprise wheels for example, wherein particularly preferably at least one wheel is coupled to a distance measuring device. It is thus possible to record a profile of a surface to be analysed. In addition, it is also possible that information about the location of the apparatus with respect to the surface originates from a robot which carries the apparatus. 
   In a further preferred embodiment, at least one radiation detector device allows locally resolved reception of the radiation impinging thereon. In this case, the radiation detector device may have a CCD chip or the like. In this way, it is possible not just to determine intensities of the impinging radiation but also to display a differentiated image of the surface. 
   The present invention also relates to a method for determining surface properties, wherein in a first method step radiation is emitted onto a surface to be analysed. In a further method step, at least part of the radiation emitted onto the surface and reflected by the latter is detected by means of a first radiation detector device and a first signal characteristic of this reflected radiation is output. Furthermore, at least part of the radiation emitted onto the surface and reflected by the latter is detected by means of a second radiation detector device and a second signal characteristic of this reflected radiation is output. In a further method step, the first signal is compared with the second signal and, based on this comparison, a geometric position of the radiation device with respect to the surface is taken into account. More specifically, preferably a tilting or the angle of inclination of the apparatus with respect to the surface in the plane of the radiation devices is determined by comparing the first and second signal. 
   Preferably, at least one signal is corrected by taking account of the tilting of the radiation device or of the apparatus as a whole with respect to the surface. This may be for example a signal which was determined by the radiation detector device for the surface, and the correction accordingly takes place by taking account of any tilting. 
   Preferably, the first radiation detector device is offset by a first predefined angle with respect to the direction of the radiation reflected by the surface and the second radiation detector device is offset by a second predefined angle with respect to the direction of the radiation reflected by the surface, wherein the angles are essentially equal and opposite to one another with respect to the direction of the radiation reflected by the surface. 
   Preferably, an apparatus of the type described above is used to carry out the method according to the invention. 
   The present invention also relates to a method for determining surface properties, wherein in a first method step radiation is emitted onto a surface to be analysed, and in a further method step at least part of the radiation emitted onto the surface and reflected by the latter is detected by means of a first radiation detector device and a first signal characteristic of this reflected radiation is output. Furthermore, at least part of the radiation emitted onto the surface and reflected by the latter is detected by means of a further radiation detector device and a further signal characteristic of this reflected radiation is output. Finally, the first signal is compared with the further signal and a value characteristic of this comparison is output. Preferably, an apparatus of the type described above is used to carry out the method according to the invention. 
   As an alternative or in addition, a certain type of colour change or the cause thereof is identified by comparing the first signal and the second signal or the first signal and a further signal which originates from a further radiation detector device. Colour changes are generally caused by those states of the surface which differ from an ideal surface, i.e. a surface which in particular has no concentration fluctuations of the colour pigments due to changes in layer thickness and no incorrect orientations of effect pigments (e.g. aluminium flakes). In particular, by means of the method according to the invention, changes in the colour pigment concentration can be distinguished from those effects caused by an incorrect orientation of effect pigments. More specifically, based on the value characteristic of the comparison, it is possible to distinguish between signal changes caused by a change in layer thickness and signal changes caused by an incorrect arrangement of pigments. 
   Preferably, the first signal and the further signal are output in each case in two different areas of the surface and at lest one surface property is determined from a comparison of the first and further signals. By comparing the first and further signals, it is possible to check whether the two signals change in one specific direction or in opposite directions. As described above, this can in turn be used to deduce the physical causes of colour changes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and embodiments will emerge from the appended drawings, in which: 
       FIG. 1  shows a diagram to illustrate a first problem on which the invention is based; 
       FIGS. 2   a - 2   c  show schematic diagrams to illustrate a second problem on which the invention is based; 
       FIG. 3  shows an optical block of an apparatus according to the invention; 
       FIG. 4  shows an apparatus according to the invention; 
       FIG. 5   a  shows a first diagram to illustrate measurement results; 
       FIG. 5   b  shows a second diagram to illustrate measurement results; and 
       FIG. 5   c  shows a third diagram to illustrate measurement results. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an intensity distribution of radiation emitted onto a surface and reflected by the latter. Here, the dashed line bearing the marking 0° denotes the reflection angle. For the sake of simplification, the respective angles and the lines thereof have been plotted on a straight line. 
   If the apparatus is not tilted with respect to the surface, the intensity curve  31  is to be expected. In this case, the two radiation detector devices  5  and  15 , which are arranged at −15° and 15°, in each case receive the same intensity I 0 . However, if the apparatus is tilted with respect to the surface, the intensity distribution or the signal is shifted, as shown for example by the dotted line  33 . In this case, the radiation detector device arranged at +15° will receive a higher intensity I 2  and the radiation detector device arranged at −15° will receive a corresponding lower intensity I 1 . The tilting can be taken into account from the ratio between the intensity values I 1  and I 2 . Particularly preferably, the two radiation detector devices are arranged in such a way that intensity I 0  is located in that region of the intensity distribution in which an approximately linear intensity profile with respect to the angle is to be expected. In the case of tilting, the differences between I 1  and I 0  and between I 2  and I 0  are then approximately the same. 
     FIGS. 2   a  to  2   c  relate to a further problem on which the invention is based. Here, reference  8  denotes the surface or a corresponding coating. Within this coating, in some types of coating there is a large number of so-called pigments or flakes  36   a ,  36   b . In the case of a proper coating, the flakes are oriented essentially horizontally, like the flake  36   a  for example. In the event of defects, however, oblique positions of the flakes may arise, as shown for example in the case of the flake  36   b.    
   On the other hand, however, thickness fluctuations of the respective coating may arise, and these thickness fluctuations also have an effect on the optical impression of the surface and also on the measurement result. For example, in the region in which the coating is too thin and has a thickness d 2 , it is possible that a basecoat shows through and falsifies the overall impression. With the measurement method known from the prior art, it is not possible to determine from a recorded image or from a measurement that has been carried out whether fluctuations stem from a concentration fluctuation or from an incorrect orientation of the flakes. 
   Three different measurement situations are shown in  FIGS. 2   a  to  2   c . In said figures, reference  3  denotes a radiation device and references  15  and  7  denote radiation detector devices. The radiation device emits radiation onto the surface at an emission angle α 1  of −15° with respect to the median perpendicular M. This radiation is reflected at a reflection angle of 15° with respect to the median perpendicular M (arrow P 3 ). The radiation detector device  15  receives a certain part of this reflected radiation, but the further radiation detector device  7  on the other hand receives only scattered radiation. 
     FIG. 2   a  shows a starting situation in which a measurement is carried out at the location of a correctly oriented flake  36   a , wherein the layer thickness here has the value d 1 .  FIG. 2   b  shows a situation in which, although the flake is correctly oriented, nevertheless the layer thickness has a reduced value d 2 . As mentioned above, changes in intensity may occur in this region on account of the reduced layer thickness or the colour pigment concentration. 
   However, this change in intensity will have the same effect on the radiation detector device  15 , which is arranged at −15° with respect to the direction of the reflected radiation, and on the radiation detector device  7 , which is arranged at −45° with respect to the direction of the reflected radiation. Layer thickness changes, which can also be referred to as clouds, therefore act on the respective signals in the same way or in the same direction. 
     FIG. 2   c  shows a situation in which the emitted light impinges on the flake  36   b , which is rotated in terms of its orientation. However, the illustrated flake  36   b  also represents a large number of flakes. In this case, the rotation means that the reflected signal no longer impinges at 15°. This is illustrated by the arrow P 3  shown in dashed line. In this case, therefore, the radiation detector device  15  will receive a higher proportion of the reflected light. By contrast, the intensity of the scattered light, which impinges on the radiation detector device  7 , will not change significantly or will change in some other way. In this case, therefore, the intensity of the radiation which impinges on the second radiation detector device  15  and the radiation which impinges on the further detector device  7  thus change differently or in opposite directions. The cause of intensity changes can thus be determined by comparing the received intensities. With a large number of incorrectly oriented flakes, generally the intensity of the reflected light will decrease and the intensity of the scattered light will increase. 
   In other words, although such so-called orientation clouds (i.e. areas containing incorrectly oriented flakes) change the properties of colour changes, they nevertheless in each case have a different effect on the respectively measured signals, depending on the arrangement of the radiation detector device in the circumferential direction. In this case, too, the percentage deviation of the signals received in each case by the two radiation detector devices  7  and  15  is summed or compared, wherein this summing or comparison may take place over different ranges of length distances. In this way, the physical cause of colour changes can be ascertained and the types of colour changes can be distinguished from one another. 
   With reference to  FIG. 1 , it can be seen that thickness changes here too change the sensor signals at −15° and +15° in the same direction, and for example the total signal is increased. Here, too, the percentage deviation between the signals at +15° and −15° are summed. However, as already mentioned, tilting of the surface gives rise to opposite changes in the signals of the radiation detector devices. In this case, however, the sum of the two signals (which are received at −15° and +15° with respect to the direction of the reflected light) is essentially independent of the tilting of the apparatus with respect to the surface since, as mentioned, the arrangement of the radiation detector device is selected in such a way that the measurement is carried out in each case in the essentially linear region of the flanks of the signals  31  and  33 . 
     FIG. 3  shows an optical block  10  of an apparatus according to the invention for analysing surface properties. Here, reference  3  denotes a radiation detector device which emits radiation onto a surface  8  to be analysed. During operation, the apparatus is moved along the arrow P with respect to the surface in order to optically scan the surface in this way. 
   In principle, it would also be possible to arrange a plurality of radiation devices parallel to one another, for example in a direction perpendicular to the plane of the figure. In addition, optical elements such as cylindrical lenses may also be provided, which cause radiation to be emitted onto the surface along a line perpendicular to the plane of the figure. In this way, it is possible to optically measure simultaneously not just linear elements but rather two-dimensional elements. 
   Reference  13  denotes a light source which may for example be a white LED. The radiation emitted by the LED passes through a diaphragm  14  and a lens  12  onto the surface  8 . Here, the radiation device is arranged at an emission angle α 1  of 15° with respect to the median perpendicular M. The reflected light is thus also reflected at an angle of 15° by the surface  8 . Reference  11  denotes an absorption device which essentially absorbs the reflected radiation. This may be for example a tube or the like. The absorption device is preferably closed in order to prevent the entry of external light into the apparatus. 
   Reference  5  denotes a first radiation detector device which is arranged at an angle β 1  of +15° with respect to the direction of the reflected light. In the present embodiment, the first radiation detector device  5  is thus arranged at an angle of 30° with respect to the median perpendicular M. A second radiation detector device  15  is arranged at an angle β 2  of −15° with respect to the direction of the reflected radiation and in this case is located vertically above the surface  8 . These two radiation detector devices  5 ,  15  have photocells, and preferably CCD chips, which also allow locally resolved analysis of the radiation. In addition, the two radiation detector devices have lenses  17  and also diaphragms (not shown in detail). 
   Reference  16  denotes an intensity measuring device which is provided next to the radiation device  3 . By means of this intensity measuring device, part of the radiation is coupled out of the radiation device and, as mentioned above, is used to calibrate and stabilise the measurement results. This intensity measuring device  16  may likewise have optical elements (not shown) such as diaphragms, filters, photodiodes and the like. 
   Reference  7  denotes a further radiation detector device. This further radiation detector device  7  is arranged at an angle of γ 1 =−45° with respect to the direction of the reflected radiation and therefore receives light scattered by the surface. This radiation detector device preferably also allows locally resolved reception of the radiation impinging thereon. As explained above, the physical cause of intensity fluctuations can be deduced from a comparison of the measurement signals received by one of the radiation detector devices  5  or  15  with the signal received by the radiation detector device  7 . 
   However, besides the radiation detector devices shown, further radiation detector devices may also be provided, for example at large angles with respect to the median perpendicular, such as 70° or 80° for example. In addition, it would also be possible to provide further radiation devices and also filter elements which separate light of different spectral components from one another. 
     FIG. 4  shows an apparatus  1  according to the invention with the optical unit  10  shown in  FIG. 3 . In addition, the apparatus has a housing  20  in which the aforementioned optical unit  10  is installed. Reference  21  denotes an opening in the lower housing region, through which the radiation passes from the radiation device  3  onto the surface  8 . Reference  25  denotes a wheel of the apparatus for moving the apparatus with respect to the surface. Preferably, this wheel or else a different wheel is coupled to a distance measuring device in order to determine a distance traveled by the apparatus  1  with respect to the surface. Reference  23  denotes a display device such as a screen. Reference  6  denotes a processor device which compares the individual measurement signals with one another. 
   By the cooperation between the distance measuring device and the individual radiation detector devices, a profile of the determined data can be recorded over the surface and a distance/time profile can be determined. 
     FIGS. 5   a ,  5   b  and  5   c  show spectra recorded by the apparatus according to the invention. Here, reference  41  denotes a spectrum which was recorded from a reference surface, and reference  42  denotes a spectrum which was recorded with a further surface to be analysed. The standard deviation σ of the determined intensities is plotted on the Y coordinate in each case. For each surface, a total of four measured values were recorded, with different spatial regions (hereinafter also referred to as spatial wavelengths) in millimetres being plotted on the coordinate. 
   More specifically, this means that averaging was carried out over distances between 6 and 13 millimetres in the first region, averaging was carried out in a distance range between 11 and 23 millimetres in the second region, averaging was carried out over distances in a range from 19 to 42 millimetres in the third region, and averaging was carried out in a distance range between 33 and 70 millimetres in the fourth region. These different distances represent the distance of an observer from the respective surface, for example a motor vehicle paintwork. The third region with distances from 19 to 42 millimetres thus corresponds to a viewing distance of between 2 and 3 metres. 
     FIG. 5   a  shows the light intensity changes which were recorded by the radiation detector device arranged at 45° Here, a surface was selected which had thickness changes in its base coating.  FIG. 5   c  shows a corresponding diagram for the radiation received at 15°, i.e. for example by the radiation detector device  5 . As can be seen by comparing  FIGS. 5   a  and  5   c , due to the change in layer thickness the signals of both radiation detector devices change in the same way here, i.e. they both have a maximum in the region of 19 to 42 millimetres. 
     FIG. 5   b  shows a comparison of the intensities recorded by the two radiation detector devices at 15° and 45°. Here too, like in the two other graphs, the signals of the two radiation detector devices arranged at +15° and −15° with respect to the direction of the reflected light were averaged or summed. It can be seen from  FIG. 5   b  that the difference or ratio of the two intensities has a relatively small standard deviation which, as mentioned above, makes it possible to deduce that the surface has a uniform orientation of the flakes but is subject to certain thickness fluctuations. 
   In the case of effects brought about by a non-uniform orientation of the flakes, the standard deviation would accordingly assume higher values. 
   Preferably, therefore, an apparatus is provided which comprises both a first and a second and also a further radiation detector device. In this way, different causes of measurement deviations can be determined in a particularly simple manner, be these caused by tilting of the apparatus with respect to the surface, by a changing orientation of the flakes or by a change in layer thickness. 
   All the features disclosed in the application documents are claimed as essential to the invention in so far as they are novel individually or in combination with respect to the prior art. 
   
     
       
             
           
             
             
           
         
             
                 
             
             
               List of references 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                1 
               apparatus 
             
             
                3 
               radiation device 
             
             
                5 
               first radiation detector device 
             
             
                6 
               processor device 
             
             
                7 
               further radiation detector device 
             
             
                8 
               surface 
             
             
               10 
               optical block 
             
             
               11 
               absorption device 
             
             
               12 
               lens 
             
             
               13 
               light source 
             
             
               14 
               diaphragm 
             
             
               15 
               second radiation detector device 
             
             
               16 
               intensity measuring device 
             
             
               17 
               lens 
             
             
               20 
               housing 
             
             
               21 
               opening in the lower housing region 
             
             
               23 
               display device 
             
             
               25 
               wheel of the apparatus 
             
             
               31, 33 
               signals 
             
             
               36a, 36b 
               flake (pigment) 
             
             
               41, 42 
               spectrum 
             
             
               I 0 , I 1 , I 2   
               intensity 
             
             
               P, P3 
               arrow 
             
             
               d 1 , d 2   
               layer thicknesses 
             
             
               α 1   
               emission angle 
             
             
               β1, β2, γ1 
               angles of the radiation detector devices 
             
             
               M 
               median perpendicular