Patent Publication Number: US-2021173076-A1

Title: Measurement setup, reference reflector as well as method for measuring attenuation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 16/008,979, filed on Jun. 14, 2018, the disclosure of which is incorporated herein in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     Embodiments of the present disclosure relate generally to a measurement setup for measuring attenuation through an irregular surface of a device under test. Further, embodiments of the present disclosure relate generally to a reference reflector for use in a measurement setup for measuring attenuation through an irregular surface. Embodiments of the present disclosure also relate generally to a method for measuring attenuation of an irregular surface of a device under test. 
     BACKGROUND 
     Nowadays, radar sensors are used in vehicles for observing the environment of the respective vehicle, for instance the distance to a preceding vehicle or rather the distance to a vehicle behind is observed by the radar sensors. A driver of the vehicle may be warned if a distance observed becomes too small. Typically, the radar sensors are mounted behind the front car bumper as well as the rear car bumper. Since the car bumpers are painted with a metallic paint, undesirable attenuations of the radar signals occur which are caused by the paint. 
     Generally, the occurring attenuation is highly dependent on the paint color, its density, its metallic content, its thickness, its mixture and so on. Thus, it is hard to control or rather to predict the resulting attenuation value of a car bumper in industrial production since each individual paint has to be examined individually. 
     The measurement of the attenuation is further complicated as the car bumpers typically have an irregular surface. 
     So far, point-like measurements are done in industrial production wherein a few singular points are investigated. The complete field of view of the radar sensors, also called radar field of view, cannot be covered by such a measurement type in a practical time. Hence, it is not possible to get a reliable overview of the attenuation of the whole car bumper. 
     For scientific or rather academic applications, a radio frequency sensor module can be used that is remotely placed behind the respective car bumper wherein a direct transmission measurement is performed, for instance the scattering parameter S 21  obtained by a vector network analyzer is taken into account. However, these measurements are impractical in industrial production due to several facts, for instance high tolerances and the radio frequency cables that have to be extended into the inside body of the bumper. 
     Accordingly, there is a need for a possibility to measure attenuation through an irregular surface of a device under test such as a car bumper in a practical and cost-efficient manner so that it can be applied in industrial production of the respective device under test. 
     SUMMARY 
     Embodiments of the present disclosure provide a measurement setup for measuring attenuation through an irregular surface of a device under test. In an embodiment, the measurement set up comprises: 
     a positioning system; 
     a reference reflector having a collection of diffuse scattering members; 
     a three dimensional imaging system having a field of view, the three dimensional imaging system being configured to use electromagnetic signals in the frequency range of a radar system; 
     the measurement setup having a reference state and a measurement state; 
     the positioning system, in the reference state, being configured to position the reference reflector without the device under test in the field of view of the imaging system; 
     the positioning system, in the measurement state, being further configured to position the reference reflector and the device under test in the field of view of the imaging system, the device under test being located between the imaging system and the reference reflector; 
     the imaging system, in the reference state, being configured to take a reference image of the reference reflector; 
     the imaging system, in the measurement state, being configured to take a measurement image of the reference reflector while the device under test is arranged between the imaging system and the reference reflector; 
     the imaging system being further configured to compare the images taken in the reference state and the measurement state to determine the attenuation of the device under test. 
     Accordingly, the attenuation of the device under test having an irregular surface such as a car bumper can be measured easily and in a cost-efficient manner by the measurement setup since reflection imaging against a reference target is used for generating a reference image. The reference image, namely the image taken in the reference state, is later used for comparison purposes in order to determine the attenuation of the device under test with its irregular surface. Put it another way, the attenuation of the device under test is obtained by measuring the collective relative change in the reflectivity of the diffuse scattering members of the reference reflector used in both measurements, namely the reference measurement as well as the measurement of the device under test. Thus, the reflection of the device under test itself, namely its irregular shape, is removed as this reflection occurs at a wrong reflection plane, namely a reflection plane being distanced to the reference reflector. In some embodiments, the imaging system relies on the reflection of the reference reflector for determining the attenuation of the device under test. 
     The three dimensional imaging system uses electromagnetic signals in the frequency range of radar systems, for instance an ultimate radar system. The applied frequency range corresponds to the one of the radar sensors used by the device under test, for instance the car bumper. In an embodiment, the electromagnetic signals may have a frequency of between, for example, 75 and 82 GHz, for instance a frequency of 77 GHz or 79 GHz, so that the wavelength of the electromagnetic signals corresponds substantially to 4 mm Thus, the three dimensional imaging system may be a highly resolved millimeter wave panel (mmWave panel) so as to image through the device under test, namely the reference reflector placed behind the device under test. 
     Generally, the three dimensional imaging system has a defined radiation pattern, namely radiation beam(s), so that the electromagnetic signals impinge on the device under test in a defined manner. 
     As the measurement setup relies on the reflection of the reference reflector, it is understood that at least a part of the signals emitted by the three dimensional imaging system goes through the device under test, for example in a parallel or rather straight manner Hence, the transmission thickness, namely the thickness of the material of the device under test to be traversed by the electromagnetic signals, is constant. 
     In the measurement state, the reference reflector may be located inside the device under test, namely the car bumper. Hence, the reference reflector may be embedded in the device under test. Alternatively, the reference reflector is positioned (closely) behind the device under test, for example directly behind. In some embodiments, the reference reflector is positioned to correspond to the intended position of the at least one radar sensor being used by the device under test. 
     The positioning system ensures that the reference reflector is positioned in the correct position. For this purpose, the positioning system may position the reference reflector in the reference state and in the measurement state so that the same location of the reference reflector is ensured for the reference reflector during both measurements, namely the reference measurement and the measurement of the device under test. 
     The collection of the diffuse scattering members may correspond to a matrix so that the individual scattering members are located in a matrix-like arrangement. The respective distances between neighbored scattering members may be equal. Thus, the individual scattering members can be distributed over a surface of the reference reflector in a homogeneous manner. 
     The diffuse scattering members ensure that the electromagnetic signals radiated by the three dimensional imaging system are scattered in all directions. 
     According to an aspect, the imaging system being configured to locate the reference reflector and its scattering members by using geometric matching. This can be ensured due to the reference image taken in the reference state since only the reference reflector is located in the field of view of the three dimensional imaging system. Thus, the respective locations of the individual scattering members can be located by the imaging system. The imaging system can locate and extract the collective reflectivity of the reference reflector at the individual scattering members. Put it another way, the three dimensional imaging system, in the measurement state, already knows the locations of the diffuse scattering members or at least expects the individual scattering members at certain locations (due to the same position of the reference reflector and the geometric matching). Accordingly, the referenced image taken provides some kind of a template with regard to the locations of the individual scattering members. By using this template in the measurement state, the imaging system is enabled to identify the scattering members in the image taken in the measurement state more easily. 
     For instance, locations within a range of 3 mm deviation of the template, namely 3 mm to the left and/or 3 mm to the right of the expected locations, may be taken into account. Hence, minor deviations do not impair the measurement result. 
     Another aspect provides that the imaging system being configured to compare the image strength at the locations of at least two scattering members of the reflector while comparing the images taken in the reference state and the measurement state. The attenuation of the device under test can be retrieved easily by taken the image strength into account of both images taken so that the attenuation merely corresponds to the difference in reflectivity strength of both images. Since the reflectivity strength or rather the image strength is taken into account at the location of scattering members, it becomes obvious that the irregular surface of the device under test is not taken into account. In some embodiments, reflections at the irregular surface are assigned to a different reflection plane compared to the one of the reference reflector. In other words, the reflection of the device under test itself, namely its irregular surface, is removed so that these signals do not contribute to the determination of the attenuation of the device under test. 
     According to an aspect, the imaging system is further configured to disregard the reflection originating from the irregular surface. The imaging system only relies on reflections of the reference reflector for determining the attenuation of the device under test. Since the irregular surface is assigned to a different reflection plane, these reflections do not contribute. In some embodiments, disturbing contributions of the irregular surface are disregarded since the imaging system relates to the reflection plane assigned to the reference reflector. 
     Moreover, the imaging system may be configured to take only the scattering members into account that contribute to the intersection between the irregular surface of the device under test and an intended field of view of at least one radar sensor of the device under test. The intended field of view of the at least one radar sensor, also called radar field of view, corresponds to a specific area on the irregular surface of the device under test. As this area is of interest for the attenuation determination, only these scattering members are taken into account that have a contribution to this specific area. The other areas of the irregular surface which do not intersect with the radar field of view are less interesting so that the scattering members assigned to these areas are neglected. Put it another way, some scattering members are excluded if they are located outside of the intended radar field of view of the device under test on the irregular surface of the device under test. Thus, only specific scattering members are selected that are relevant for the intended radar field of view with respect to the irregular surface of the device under test. 
     Moreover, the reference reflector may be positioned by the positioning system such that the scattering members face the imaging system. Hence, the electromagnetic signals transmitted by the imaging system impinge on the scattering members of the reference reflector (after traversing the device under test in the measurement state) so that the electromagnetic signals are reflected by the scattering members. In some embodiments, the scattering members reflect the electromagnetic signals towards the imaging system. 
     The imaging system is generally configured to receive the reflected electromagnetic signals and to process these signals appropriately. 
     The field of view of the imaging system may be aligned with a radar field of view provided by at least one radar sensor of the device under test. The electromagnetic signals emitted by the imaging system may cross the irregular surface of the device under test in the same area as the radar signals of the intended radar field of view do. Thus, the whole intersection surface, namely the surface area of the irregular surface that is intersected by the radar field of view, is radiated by the three dimensional imaging system. In addition, neighbored areas on the irregular surface are not irradiated by the imaging system. 
     Moreover, the positioning system may be at least one of a fixed positioning system or a movable positioning system. The fixed positioning system ensures that at least the reference reflector is held at the same position during both measurement. The moveable positioning system is able to irradiate the device under test under different positions with regard to the imaging system. However, the moveable positioning system also ensures that the reference reflector is held in the same position during the reference measurement as well as the measurement of the device under test as it the positioning system is controlled to take the same position. 
     For instance, the positioning system comprises a robot. The robot can be controlled appropriately so that the desired positions for the reference reflector as well as the device under test can be obtained. 
     Another aspect provides that the positioning system comprises at least one gripper being configured to position at least one of the reference reflector or the device under test. The gripper can be used for easily holding the at least one respective component, namely the reference reflector and/or the device under test. 
     In some embodiments, the gripper is capable of holding the irregular surface of the device under test in the desired position, namely in front of the reference reflector. 
     For instance, the at least one gripper has a vacuum sucker. The device under test with its irregular surface can be held easily by the gripper. Further, the component may be gripped at different locations easily without any wear. 
     The positioning system may be free of highly reflective components in the area used for measuring the attenuation. The measurements are not disturbed by reflections of the positioning system. 
     For instance, at least the gripper is free of highly reflective components since the gripper is located close to the reference reflector and/or the device under test during the respective measurement. 
     The reference reflector may comprise a base to which the several individual scattering members are attached, the several individual scattering members each having a curved tip for reflection, the size of the several individual scattering members being comparable to the wavelength of a signal used for attenuation measurement, the several individual scattering members being arranged with regard to the base such that front and back reflections occur which are separable by the imaging system. Thus, the distance between the tips of the individual scattering members and the base to which the several individual scattering members are attached by their opposite ends is high enough so that the imaging system can separate the reflections that occur at the base and the tips of the several individual scattering members. Thus, two reflection planes are provided by the reference reflector. 
     In an embodiment, the base comprises an electromagnetic absorber. The electromagnetic absorber ensures that background reflection is reduced, namely the back reflection. The absorber may be provided by an electromagnetic absorbing layer. The layer may be painted, sprayed and/or coated on the base. 
     The curved tip may be ellipsoidal or hemispherical, for example. 
     The scattering members may be formed by sticks having a diameter corresponding to the wavelength of the signal used for measurement. Thus, the individual scattering members are shaped substantially cylindrical with a certain diameter. 
     Further, embodiments of the present disclosure provide a reference reflector for use in a measurement setup for measuring attenuation through an irregular surface, the reference reflector comprising a base and several individual scattering members attached to the base, the several individual scattering members each having a curved tip for reflection, the size of the several individual scattering members being comparable to the wavelength of a signal used for attenuation measurement, the several individual scattering members being arranged with regard to the base such that front and back reflections occur which are separable by an imaging system. The front reflection may occur by the tips of the several individual scattering members whereas the back reflection is assigned to the reflection at the base. Thus, the several individual scattering members are long enough so that the imaging system can differentiate between both reflection planes. 
     The base may comprise an electromagnetic absorber. The electromagnetic absorber ensures that background reflection is reduced, namely the back reflection. The absorber may be provided by an electromagnetic absorbing layer. The layer can be provided by a paint, a spray and/or coating. 
     Further, embodiments of the present disclosure provide a method for measuring attenuation of an irregular surface of a device under test, with the following steps: 
     positioning a reference reflector having a collection of diffuse scattering members in front of a three dimensional imaging system that uses electromagnetic signals in the frequency range of a radar system; 
     taking a reference image of the reference reflector by using the imaging system; 
     positioning the device under test and the reference reflector in front of the imaging system so that the device under test is arranged between the reference reflector and the imaging system; 
     taking a measurement image of the reference reflector while the device under test is arranged between the imaging system and the reference reflector by using the imaging system; and 
     comparing the images taken to determine the attenuation of the device under test. 
     The characteristics and advantages mentioned above with regard to the measurement setup also apply for the method mentioned above in a similar manner. 
     In some embodiments, the collective reflectivity at the scattering members is located and extracted by the imaging system after the reference image was taken. In a similar manner, the collective reflectivity at the scattering members is located and extracted by the imaging system after the measurement image was taken. Hence, the respective collective reflectivity can be compared. 
     When comparing both images, the attenuation can be determined easily while referring to the collective reflectivity at the scattering members, for example the difference. 
     According to an aspect, the image strength at the locations of at least two scattering members of the reference reflector are compared while comparing the images taken. The attenuation of the device under test is determined by comparing the image strength of the reflected signal at locations assigned to the reference reflector. Hence, the reference plane assigned to the reference reflector is taken into account. In other words, the reflection of the irregular surface of the device under test is removed as it is assigned to a different reflection plane. 
     According to another aspect, a reflection originating from the irregular surface is disregarded. Thus, only the reflections occurring from scattering members of the reference reflector, for example the ones assigned to the radar field of view, are taken into account for measurement purposes. In some embodiments, the reflections originating from the irregular surface are assigned to a different reflection plane compared to the one of the reference reflector. 
     Moreover, only the scattering members may be taken into account that contribute to the intersection between the irregular surface of the device under test and an intended field of view of at least one radar sensor of the device under test. Thus, a certain area of the irregular surface of the device under test is measured with regard to attenuation since this area will be irradiated by the intended radar field of view of the at least one radar sensor of the device under test. In some embodiments, only this area of the irregular surface is of interest with regard to the transmittance of the device under test. 
     The attenuation of the device under test can be determined by averaging attenuation values due to the attenuation values determined for the respective scattering members taken into account. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of the claimed subject matter will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a representative measurement setup in a reference state according to an aspect of the present disclosure; 
         FIG. 2  shows the measurement setup of  FIG. 1  in a measurement state; 
         FIG. 3  shows the measurement setup of  FIG. 2  in another perspective view without the positioning system; 
         FIG. 4  shows a detail of  FIG. 3 ; 
         FIG. 5  shows a perspective view of a representative reference reflector according to an aspect of the present disclosure; 
         FIG. 6  shows a top view of the reference reflector of  FIG. 5 ; 
         FIG. 7  shows a side view of the reference reflector of  FIGS. 5 and 6 ; and 
         FIG. 8  shows a flow-chart illustrating a representative method for measuring attenuation according to an aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings, where like numerals reference like elements, is intended as a description of various embodiments of the disclosed subject matter and is not intended to represent the only embodiments. Each embodiment described in this disclosure is provided merely as an example or illustration and should not be construed as preferred or advantageous over other embodiments. The illustrative examples provided herein are not intended to be exhaustive or to limit the claimed subject matter to the precise forms disclosed. 
     In  FIG. 1 , a measurement setup  10  for measuring attenuation through an irregular surface of a device under test is shown in a reference state. The measurement setup  10  comprises a three dimensional system  12  that has several antennas for transmitting and/or receiving electromagnetic signals as will be described later. 
     The three dimensional imaging system  12  uses electromagnetic signals in the frequency range of a radar system such as an ultimate radar system. Thus, the electromagnetic signals may have a frequency between 75 and 82 GHz, for example 77 GHz and 79 GHz, so that the wavelength of the electromagnetic signals corresponds to 4 mm Therefore, the three dimensional imaging system  12  is also called mmWave imaging system. 
     The measurement setup  10  further comprises a positioning system  14  that has a robot  16  and at least one gripper  18  for gripping components and holding the respective component in a desired position. The at least one gripper  18  may be located at an end of a robotic arm of the robot  16 . In the shown embodiment, the gripper  18  has a vacuum sucker  20  so that the respective component is held by the gripper  18  via a vacuum applied. 
     In  FIG. 1 , the measurement setup  10  is shown in its reference state wherein the measurement setup  10  is used to take a reference image of a reference reflector  22  that is held by the at least one gripper  18  in a reference position. The reference reflector  22  is shown in more detail in  FIGS. 5 to 7 . The reference reflector  22  comprises a base  24  being provided by a plate from which diffuse scattering members  26  extend in a substantially perpendicular direction. 
     As shown in  FIGS. 5-7 , the diffuse scattering members  26  are arranged in a matrix on the base  24  wherein the scattering members  26  each have a curved tip  28 , for instance a hemisphere or an ellipsoidal tip, so that electromagnetic signals impinging on the scattering members  26  are scattered in a diffuse manner. The size of the several individual scattering members  26  is substantially comparable to the wavelength of the signals emitted by the imaging system  12 . In some embodiments, the scattering members  26  are shaped cylindrically wherein their diameter corresponds to the wavelength of the electromagnetic signals emitted by the imaging system  12 , namely about 4 mm. 
     Furthermore, the several individual scattering members  26  have a length that ensures that two different reflection planes are provided by the reference reflector  22  which can be differentiated by the imaging system  12 . The reflection planes provided correspond to the reflection at the tips  28 , also called front reflection, as well as reflections at the base  24 , also called back reflection. Hence, front and back reflections occur when the electromagnetic signal impinges on the reference reflector  22 . In some embodiments, the scattering members  26  correspond to sticks. 
     As shown in  FIG. 6 , the individual scattering members  26  are spaced apart from each other by substantially the same distance so that a homogenous distribution of scattering members  26  is obtained. 
     In  FIG. 1 , the field of view  30  of the imaging system  12  is illustrated appropriately which irradiates the reference reflector  22  via a certain area as will be described later with respect to  FIG. 2 . 
     For measuring the attenuation of a device under test, the reference reflector  22  is positioned in front of the imaging system  12  via the positioning system  14  in a first step S 1  (see  FIG. 8 .) The reference reflector  22  is positioned such that the scattering members  26  face the imaging system  12  so that the electromagnetic signals emitted by the imaging system  12  impinge on the scattering members  26  which scatter the signals in a diffuse manner. 
     In a second step S 2 , the imaging system  12  takes a reference image of the reflectance of the reference reflector  22 . In some embodiments, the image strength at the different locations assigned to the scattering members  26  is evaluated by the imaging system  12 . Hence, the collective reflectivity at the individual scattering members  26  is located and extracted so that a template with regard to the locations of the individual scattering members  26  is obtained. 
     In a next step S 3  illustrated in  FIG. 2 , a device under test  32  is held by the positioning system  14  wherein the device under test  32  is positioned between the imaging system  12  and the reference reflector  22 . 
     Then, the device under test  32  as well as the reference reflector  22  positioned behind the device under test  32  are irradiated by the imaging system  12  (step S 4 ). Thus, the electromagnetic signals emitted by the imaging system  12  go through the device under test  32  and impinge on the reference reflector  22  located being the device under test  32 . The electromagnetic signals are attenuated by the device under test  32  while they go through (transverse) the device under test  32 . Again, the imaging system  12  locates and extracts the collective reflectivity at the individual scattering members  26  so that a measurement image of the reference reflector  22  is taken while the device under test  32  is disposed between the imaging system  12  and the reference reflector  22 . 
     After the imaging system  12  has taken the reference image (S 2 ) as well as the measurement image (S 4 ), the imaging system  12  compares the images taken so as to determine the attenuation of the device under test  32  (step S 5 ). 
     Therefore, the imaging system  12  relies on a geometric matching with regard to the reference reflector  22  which can be done since the imaging system  12  has taken the reference image in advance so that the different scattering members  26 . 
     The positioning system  14  generally ensures that the reference reflector  22  is positioned in the same position for both the reference measurement and the measurement of the device under test  32 . Hence, a relative reflection imaging against the reference reflector  22  is used for determining the attenuation of the device under test  32 . In other words, the geometric matching is done by the imaging system  12  so as to locate the reference reflector  22 , for example its scattering members  26 , in the measurement state compared to the reference state. Therefore, the three dimensional imaging system  12 , in the measurement state, already knows the locations of the diffuse scattering members  26  due to the geometric matching or at least expects the individual scattering members  26  at certain locations 
     As already discussed, the device under test  32  may be a car bumper comprising a radar sensor with an intended radar field of view  34  as illustrated in  FIG. 2 . The field of view  30  of the imaging system  12  corresponds to the intended radar field of view  34  as will become obvious by taking  FIGS. 3 and 4  into account. In other words, the field of view  30  of the imaging system  12  is aligned with the radar field of view  34 . 
     Thus, the attenuation of the irregular surface of the device under test  32  can be determined for the specific area that interacts with the at least one radar sensor of the device under test  32  in real application. The attenuation is measured for the intersection of the radar field of view  34  and the irregular surface of the device under test  32  since the field of view  30  of the imaging system  12  covers the same area at the irregular surface of the device under test  32 . 
     The imaging system  12  is further configured to take only the scattering members  26  into account that contribute to the intersection between the irregular surface of the device under test  32  and the radar field of view  34  of the at least one radar sensor of the device under test  32  or rather the field of view  30  of the imaging system  12  covering the same area on the irregular surface. Thus, the contributions of the scattering members  26  are only used for determining the attenuation of the device under test  32  that may have an influence on the signal attenuation in real operation. 
     As already discussed above, the imaging system  12  relies on the reflections that occur on the reference reflector  22  for determining the attenuation of the device under test  32 . The scattering members  26  of the reference reflector  22  itself are long enough so that two different reflection planes occur, namely the one assigned to the tips  28  of the diffuse scattering members  26  and the one assigned to the base  24 . In some embodiments, the base  24  may be covered by an electromagnetic absorber  36  so that background reflection is reduced, namely the back reflection at the base  24 . The electromagnetic absorber  36  may be provided by a layer that can be established by a spray, a coating and/or a paint. Therefore, the imaging system  12  substantially receives only the reflections that occur at the tips  28  of the scattering members  26 . 
     The imaging system  12  is also configured to disregard reflections from the irregular surface as these reflections are also assigned to a different reflection plane with respect to the reflection plane of the reference reflector  22 , for example the reflection plane of the tips  28 . 
     For improving the measurements, the positioning system  14  is free of highly reflective components in the area used for measuring the attenuation of the device under test  32 . This can already be ensured by providing the gripper  18  without any highly reflective components since the gripper  18  is assigned to the measurement area where the reference reflector  22  and/or the device under test  32  are/is held by the gripper  18  during the respective measurement. 
     Generally, the positioning system  14  may be a fixed positioning system so that the location of the reference reflector  22  is always the same. Alternatively, the positioning system  14  may be a moveable positioning system wherein the positioning system  14  is controlled appropriately so that it can be ensured that the reference reflector  22  is positioned or rather moved in the same position for measurement. The movable positioning system  14  generally ensures that differently sized devices under test  32  can be tested easily by the measurement setup  10 . 
     In general, the imaging system  12  is able to take only a certain reflection plane into account for determining the attenuation. The reflection plane used is assigned to the tips  28  of the diffuse scattering members  26 . 
     In some embodiments, reflections being too close, namely those of the irregular surface of the device under test  32 , are not taken into consideration for determining the attenuation of the device under test  32 . In a similar manner, reflections at the base  24  of the reference reflector  22  are disregarded by the imaging system  12 . Those reflections are already suppressed by the electromagnetic absorber  36 . 
     Hence, each diffuse scattering member  26  scatters the respective electromagnetic signals towards the imaging system  12  that in turn can neglect certain scattering members  26 , for example their reflected signals, so that only those reflections are taken into account that correspond to the intersection of the radar field of view  34  and the irregular surface of the device under test  32 . 
     Using the reference reflector  22  ensures that the attenuation of the device under test  32  having an irregular surface can be done easily and in a cost-efficient manner. Accordingly, the respective measurements can be done at industrial sites so that the radar systems of a vehicle can be calibrated more easily. 
     The present application may reference quantities and numbers. Unless specifically stated, such quantities and numbers are not to be considered restrictive, but exemplary of the possible quantities or numbers associated with the present application. Also in this regard, the present application may use the term “plurality” to reference a quantity or number. In this regard, the term “plurality” is meant to be any number that is more than one, for example, two, three, four, five, etc. The terms “about,” “approximately,” “near,” etc., mean plus or minus 5% of the stated value. For the purposes of the present disclosure, the phrase “at least one of A and B” is equivalent to “A and/or B” or vice versa, namely “A” alone, “B” alone or “A and B.”. Similarly, the phrase “at least one of A, B, and C,” for example, means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B, and C), including all further possible permutations when greater than three elements are listed. 
     The principles, representative embodiments, and modes of operation of the present disclosure have been described in the foregoing description. However, aspects of the present disclosure which are intended to be protected are not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. It will be appreciated that variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present disclosure. Accordingly, it is expressly intended that all such variations, changes, and equivalents fall within the spirit and scope of the present disclosure, as claimed.