Patent Publication Number: US-2016245760-A1

Title: Device and Method for Measuring Sheets, More Particularly Windshields of Vehicles

Description:
BACKGROUND 
     The invention relates to a device and a method for measuring panes, more particularly windshields of vehicles. The device comprises a light source and a light sensor, which are arranged in such a way that a light beam emanating from the light source passes through the pane and is incident on the light sensor. 
     If a light beam is incident on a pane under an angle of incidence which includes an angle unequal to 0° with the normal of the pane, there may be internal reflection within the pane, by means of which the light beam is split into a primary beam and a secondary beam. An observer peering onto the light source through the pane sees a double image of the light source. A double image arises, in particular, if the pane is wedge-shaped in the relevant region, i.e. if the two outer faces are not parallel to one another, or if the pane is curved at said location. 
     By way of example, in the case of windshields of vehicles, such double images are perceived as bothersome if the light of an approaching vehicle is visible in duplicate form in darkness. It is known to measure windshields in respect of the generation of double images. 
     The double image angle, i.e. the angle included between the primary beam and the secondary beam, is of particular interest. To this end, a light beam is guided through the pane onto a light sensor and the size of the distance between the primary beam and the secondary beam on the light sensor is established. 
     In these measurements, the problem arises that it is not entirely straightforward to measure the primary beam and the secondary beam on a light sensor since the primary beam is regularly many times brighter than the secondary beam. 
     SUMMARY 
     The invention is based on the object of providing a device by means of which the double images generated by a pane can be measured more easily. The object is achieved by the features of claim  1 . Advantageous embodiments are specified in the dependent claims. 
     According to the invention, the light sensor has a dynamic range of more than 8 bit in the case of a linear resolution. 
     At first, a few terms are explained. Dynamic range denotes the quotient of the highest brightness and the lowest brightness which can be detected by the light sensor. If the light sensor detects the brightness in digital form with a resolution of 8 bit, 256 brightness levels are available. If the light sensor has a linear resolution, there is a proportional relationship between the brightness and the brightness levels. Thus, the brightness difference between two adjacent brightness levels has substantially the same size at a low brightness and at a high brightness. 
     A dynamic range of 256 is theoretically available in the case of 8 bit with a linear resolution. In practice, the dynamic range is substantially lower because it is not possible to differentiate low brightness from noise. In actual fact, a dynamic range of the order of 20 is provided in the case of 8 bit with a linear resolution. 
     If a non-polarized light beam is incident on a pane under an acute angle, the light beam is split into a primary beam and a secondary beam. If, by way of example, the assumption is made that the light beam is incident on the pane at an angle of 60° and the glass of the pane has a refractive index of the order of 1.5, the primary beam is brighter than the secondary beam by a factor of approximately 70. If the primary beam and the secondary beam are incident on a light sensor with 8 bit linear resolution, the light sensor is unable to reliably detect both beams. 
     A reliable detection of both the primary beam and the secondary beam by way of a light sensor is possible using the dynamic range that is increased in a manner according to the invention. In particular, this opens up the possibility of automating the measurement of the pane. In the previous manual measurements, it was possible to bypass the problem by changing the sensitivity of the light sensor during the measurement. Initially, the sensitivity could be set to be so high that the secondary beam was uniquely identifiable but the light sensor was completely overdriven by the primary beam. Subsequently, the sensitivity was reduced in such a way that the secondary beam disappeared and the primary beam had a sensible resolution. Such a procedure is not acceptable for an automatic measurement. 
     In an advantageous embodiment, the dynamic range corresponds to at least 12 bit in the case of a linear resolution. A light sensor with a nonlinear resolution can contribute to increasing the dynamic range. Preferably, the nonlinear resolution is selected in such a way that the brightness distance between two adjacent brightness levels increases with increasing brightness. In a preferred embodiment, the light sensor has a logarithmic resolution. The fact that a light sensor with a logarithmic resolution is generally less suitable for distinguishing between closely adjacent brightness levels is not a relevant disadvantage within the scope of the invention because only two light beams, the brightness levels of which differ significantly, are to be detected. In the case of a logarithmic resolution, a dynamic range which is readily sufficient to detect the primary beam and the secondary beam in parallel can be obtained with 8 bit. 
     The light sensor preferably has a sensor face covered by a multiplicity of pixels. The resolution according to the invention is preferably provided for the individual pixels. 
     In an advantageous embodiment, the light beam has a linear polarization, wherein the polarization direction includes an angle of between 50° and 130° with the plane of incidence. The plane of incidence is spanned by the axis of the light beam incident on the pane and the normal of the pane at the location at which the light beam is incident on the pane. 
     A light beam can be described as a superposition of a multiplicity of electromagnetic waves, wherein each individual wave has a linear polarization direction that is directed perpendicular to the direction of propagation of the light. The light beam formed by the superposition of the individual waves has a linear polarization if the individual waves of the relevant polarization direction are present in the light beam with a higher intensity than other polarization directions. It would be ideal for the invention if the light beam were to be composed exclusively from individual waves of the relevant linear polarization direction. In practice, this will usually not be realizable, and it will be necessary to make do with the relevant polarization direction being present with a significantly higher intensity than other polarization directions. 
     When measuring the pane, the linear polarization of the light beam can be aligned in a targeted manner relative to the plane of incidence of the light beam. The plane of incidence is spanned by the axis of the light beam and the normal of the pane at the location at which the light beam is incident on the pane. The normal of the pane denotes the axis that is at right angles to an imaginary tangential plane placed onto the pane at the location at which the light beam is incident. The light source should be arranged in such a way that the light beam does not coincide with the normal of the pane. The pane is transparent such that the light beam can pass therethrough. The pane preferably consists of a material, the refractive index of which is greater than the refractive index of air. The pane is not a component of the device according to the invention. 
     The brightness of the secondary beam increases by the targeted alignment of the polarization direction and it becomes easier to measure the primary beam and the secondary beam. 
     The difference in the brightness between the primary beam and the secondary beam is caused by the fact that the primary beam crosses the pane directly while the secondary beam experiences two additional reflections in the interior of the pane. The magnitude of the portion of the reflected light compared to the portion of the transmitted light depends, inter alia, on the polarization direction of the light. In accordance with the invention, the polarization direction of the light is selected in such a way that an increased portion of the light is reflected in the interior of the pane, i.e. contributes to the brightness of the secondary beam. The greatest brightness of the secondary beam is achieved when the polarization direction of the light beam includes an angle of 90° with the plane of incidence. Then, the brightness of the secondary beam is higher by a factor of approximately 2 than in the case of a non-polarized light beam. A relevant increase in the brightness sets in an angular range between 50° and 130°. Preferably, the angle lies between 70° and 110°, more preferably between 80° and 100°. 
     After the emergence from the pane, the primary beam and the secondary beam are spatially separated from one another in such a way that they can be evaluated separately from one another by means of the light sensor. Depending on the wedge angle and the curvature of the pane, the primary beam and the secondary beam include an angle therebetween, as a consequence of which the distance between the two beams increases with the distance from the pane. It would be possible to establish the position of the primary beam and the position of the secondary beam in succession by way of a light sensor. However, the light sensor preferably is dimensioned and arranged in such a way that both the primary beam and the secondary beam are incident on the light sensor. Then, the two beams can be measured at the same time. 
     The light sensor can have an evaluation unit which automatically establishes the position of the primary beam and of the secondary beam on the light sensor. Such an evaluation unit renders it possible to automate the measurement of the pane overall. It is possible to calculate specific properties of the pane in an automatic manner, for example whether the pane meets certain standards. Appropriate information can be output on a display of the evaluation unit. 
     For the measurement, it is advantageous to use a concentrated light beam, the extent of which across the direction of propagation is small. If the light beam is collimated, the measurement result is independent of the distance between the light source and the pane. By way of example, a collimated light beam can be obtained by virtue of arranging a suitable collimation lens between the light source and the pane. In a preferred embodiment, a laser is used as a light source, said laser emitting a collimated light beam per se. 
     The linear polarization can be obtained by the light beam by virtue of said light beam passing through a suitable polarization filter between the light source and the pane. The polarization filter is transmissive to light with the relevant polarization direction, while other polarization directions are damped or preferably completely suppressed. Additionally or alternatively, use can be made of a light source; by way of example, the use of a He-Ne laser with a suitable linear polarization comes into question. 
     The alignment of the plane of incidence can depend on the position at which the light beam is incident on the pane. In order to be able to adapt the direction of polarization to different planes of incidence, it is advantageous if the polarization filter and/or the light source is/are designed in such a way that the linear polarization direction is adjustable. Preferably, the relevant element is mounted in a manner rotatable about the axis of the light beam. 
     If the primary beam and the secondary beam include an angle therebetween, the distance between the two beams is dependent on the distance at which the pane is measured. Consequently, an exact adjustment of the distance between the pane and the light sensor is generally required in order to be able to draw conclusions about the properties of the pane from the positions of the primary beam and the secondary beam on the light sensor. 
     In an advantageous embodiment, a converging lens through which the primary beam and the secondary beam pass is arranged between the pane and the light sensor. If the light sensor is arranged in the focal plane of the converging lens, the position of primary beam and secondary beam on the light sensor is independent of the distance between the pane and the converging lens. The device can be configured in such a way that the light sensor and the converging lens are components of an analysis instrument, in which the light sensor and the converging lens are held at a fixed distance from one another. Measuring the pane is made easier in this way because the light sensor has the appropriate distance from the converging lens and the distance between the converging lens and the pane does not influence the measurement. Consequently, the relevant adjustment is dispensed with. 
     It is not necessary for the converging lens according to the invention to be an individual lens element. Rather, the same effect can be achieved if the converging lens is a lens system made of a plurality of individual lens elements and the light sensor is arranged in the focal plane of the lens system. The diameter of the converging lens is preferably greater than 30 mm and can, for example, lie between 40 mm and 60 mm. With these dimensions, the converging lens is regularly suitable for capturing both the primary beam and the secondary beam. 
     The invention moreover relates to a method for measuring panes. In the method, a light beam is guided through a pane onto a light sensor. According to the invention, use is made of a light sensor which has a dynamic range of more than 8 bit in the case of a linear resolution. The method can be developed with further features which are described in the context of the device according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in an exemplary manner below on the basis of an advantageous embodiment, with reference being made to the attached drawings. In detail: 
         FIG. 1  shows a schematic illustration of a device according to the invention; 
         FIG. 2  shows a magnified section from  FIG. 1  in the case of a pane with a wedge angle; 
         FIG. 3  shows a magnified section from  FIG. 1  in the case of a pane with a curve; 
         FIG. 4  shows a magnified sectional illustration along the line A-A in  FIG. 1 ; and 
         FIG. 5  shows a block diagram of an evaluation unit according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A device according to the invention in  FIG. 1  comprises a light source  14  in the form of a He—Ne laser. The light source  14  emits a collimated light beam  15  in the direction of a windshield  16  of a motor vehicle to be measured. The light beam  15  is incident on the pane  16  at an acute angle. When passing through the pane  16 , the light beam is split into a primary beam  17  and a secondary beam  18  which, when leaving the pane  16 , include a double image angle δ therebetween. 
     The primary beam  17  and the secondary beam  18  are captured by an analysis instrument  19 . The analysis instrument  19  comprises a tube-shaped housing, at the front end of which a converging lens  20  is arranged. The converging lens  20  forms an objective of the analysis instrument  19 , through which the primary beam  17  and secondary beam  18  enter into the housing. Arranged at the other end of the housing is a light sensor  21 , on which the primary beam  17  and the secondary beam  18  are incident. By way of example, the light sensor  21  can be a CCD camera. The distance between the converging lens  20  and the light sensor  21  corresponds to the focal length of the converging lens  20 ; i.e., the light sensor  21  is arranged in the focal plane of the converging lens  20 . By way of example, the converging lens  20  can have a diameter of 50 mm and a focal length of 300 mm. 
     The primary beam  17  and the secondary beam  18  are incident on the light sensor  21  with a distance d therebetween. Since the light sensor  21  is arranged in the focal plane of the converging lens  20 , the distance d is not dependent on the distance between the converging lens  20  and the pane  16 . It is therefore not necessary to bring the analysis instrument  19  to an exactly defined distance from the pane  16 . The double image angle δ can be established from the distance d according to the following formula: 
     
       
         
           
             δ 
             = 
             
               
                 arctan 
                  
                 
                   d 
                   f 
                 
               
               ≈ 
               
                 d 
                 f 
               
             
           
         
       
     
     Here, f denotes the focal length of the converging lens  20 . For small angles (less than 0.1 radians), the double image angle δ emerges as approximately the quotient of d and f. From the double image angle δ, it is possible to draw conclusions about the properties of the pane  16 , for example about geometric properties in the region in which the light beam  15  passed through the pane  16 . 
     In accordance with  FIG. 2 , the splitting of the light beam  15  into the primary beam  17  and the secondary beam  18  emerges, for example, during the passage of the light beam  15  through a pane  16  which has a wedge angle, i.e. in which the two outer faces are not parallel to one another. In accordance with  FIG. 3 , a corresponding split into the primary beam  17  and secondary beam  18  emerges when the light beam  15  passes through a curved pane  16 . By way of example, it is possible to draw conclusions about the wedge angle or the radius of curvature of the pane  16  from the double image angle δ. Moreover, by way of a comparison with corresponding thresholds, it is possible to determine whether the double image angle δ itself meets the specifications. 
     The light beam  15  coming from the light source  14  spans the plane of incidence with the normal  22  of the pane. The normal  22  of the pane is perpendicular to the pane  16  at the location at which the light beam  15  is incident on the pane  16 . In the case of a curved pane  16 , the normal  22  of the pane is perpendicular to the tangential plane  23  which is placed against the pane  16  at the relevant location, see  FIG. 3 . 
     The light beam  15  generated by the light source  14  is collimated and has a linear polarization. The polarization direction  24 , which is indicated by two arrows in  FIG. 4 , is aligned perpendicular to the plane of incidence  15 ,  22 . Compared to a non-polarized light beam, the brightness of the secondary beam  18  is increased by approximately a factor of  2  as a result of the selection of the polarization direction. 
     The light sensor  21  is a matrix sensor which has a matrix made of light-sensitive photodiodes. In each photodiode, the incidence of a light beam releases a number of charge carriers, said number being proportional to the brightness. A brightness level is established on the basis of the number of charge carriers and an assignment between the photodiode and the brightness level is undertaken. In the case of a conventional linear assignment, the number of charge carriers increases linearly from brightness level to brightness level, as a consequence of which the dynamic range of the light sensor  21  is restricted. 
     An increased dynamic range is desired for the device according to the invention, which is why the light sensor  21  has a logarithmic resolution. The number of released charge carriers therefore increases exponentially from brightness level to brightness level. As a result, the light sensor  21  has an increased dynamic range and it is possible to establish both the primary beam  17  and the secondary beam  18  sufficiently accurately with the light sensor  21 , even if the primary beam  17  is, for example, brighter than the secondary beam  18  by a factor of  30 . 
     In accordance with  FIG. 5 , the digital values are guided from the light sensor  21  to an evaluation unit  25  and stored in a memory  26  there. A computational module  27  establishes the distance d with which the primary beam  17  and the secondary beam  18  are incident on the light sensor  21  from the values stored in the memory  26 . On the basis of the known focal length f of the converging lens  20 , the double image angle δ which the primary beam  17  and the secondary beam  18  include when emerging from the pane  16  can be established in a further computational step. A setpoint value for the double image angle δ is stored in a second memory  28 . The computational module  27  compares the established value with the value from the memory  28  and outputs information on a display  29  as to whether the pane  16  meets the specifications.