Abstract:
A device and a method are described for measuring a camera having an image sensor, in particular a fixed-focus camera, the device including: a camera support for accommodating at least one camera to be tested in at least one camera position, a collimator device for emitting collimator light of a test pattern having different object distances, a mirror system for deflecting the collimator light to the camera position, the mirror system having at least one first mirror device which is pivotable by a mirror adjustment device into multiple pivot positions, and a second mirror device having multiple second mirror units, the second mirror units reflecting the light emitted by the first mirror device in its different pivot positions to the camera position for imaging the test pattern in different image regions of an image sensor.

Description:
BACKGROUND INFORMATION 
       [0001]    Cameras which are used in driver assistance systems of vehicles are generally manufactured in a fixed-focus design, i.e., having fixed focusing. These cameras must be checked for a correct alignment of the image plane of the objective with the surface of the image sensor. For this purpose, a test pattern is generally recorded at various positions in the image field, which is projected by a motor-tunable collimator from various virtual object distances. The distance range may respectively be varied in multiple steps between infinite and 2 m in order to ascertain the object distance at which the test object is focused. An MTF (modulation transfer function) is advantageously calculated as a measure of the contrast for each position and for each object distance from the definable pulse response in the recorded image of the test pattern. The tuning of the object distance is also referred to as a through-focus scan. 
         [0002]    The cameras are generally to be focused in such a way that the depth of field range reliably covers the relevant distance range for the driver assistance function. Generally, objects at a large distance and those at a shorter distance are to be imaged having sufficiently good contrast. This property is to be achieved at the different positions of the image field, i.e., including image regions which are farther apart, such as the corners of the imager and of the image provided by it. 
         [0003]    The cameras are generally tested by rotating the camera and the collimator relative to each other in order to be able to record the test pattern in various image field positions. Thus, a support accommodating the camera or the collimator is rotatable or pivotable. Correspondingly, many runs of the tunable collimator are required at the different pivot positions. 
         [0004]    If the camera to be tested or the collimator is rotated or tilted, an exact centering on the center of rotation of the alignment unit is required in each case. Generally, in such measurements, the test pattern is repositioned to another image field position as an external loop of the measuring procedure, and the through-focus scan is carried out as an inner loop, since the alignment of the test pattern is more time-consuming. 
         [0005]    Measurements of this kind are therefore generally complex and require exact adjustments of the camera and/or the collimator in different pivot positions. 
         [0006]    German Published Patent Appln. No. 10 2004 010 958 describes a device for manufacturing a camera, in which a first calibration field is accommodated in a support device and a second calibration field is provided for larger virtual object distances separately from the support device, the second calibration field being able to be detected by the camera via multiple mirror units accommodated in the support device. The position of the image sensor of the camera may be finely positioned using a hexapod robot. It is thus possible to implement larger regions of virtual object distances using the multiple calibration fields and the deflection device from the mirror units. 
       SUMMARY 
       [0007]    The device according to the present invention for measuring a camera has multiple advantages relative to the related art. A mirror system having a pivotable first mirror device is thus provided in order to check various image regions or regions of the imager of the camera successively. By setting different pivot positions, a test pattern which is output by a collimator device may be directed toward the various image regions or regions of the imager chip without requiring an adjustment of the camera. An adjustable first mirror device directs the light output by the light-emitting device at different tilt angles to second mirror units of a second mirror device, which deflect this light to the camera or to the entrance pupil of the camera. The camera position is precisely defined by the camera support. 
         [0008]    With regard to the optical light course, the camera position is understood to be in particular the position of an entrance pupil of the objective of the camera to be accommodated. 
         [0009]    The camera may thus be accommodated in a fixed, non-adjustable camera accommodation. The camera accommodation may in particular be situated securely or rigidly with respect to a collimator accommodation and a second mirror device. 
         [0010]    Thus, only the setting of different tilt angles of the first mirror device is required for measuring the imager or the camera, without tilting or pivoting the camera and/or the collimator as a whole. The first mirror device may in particular be pivoted about two offset axes, for example, pivot axes which are orthogonal to each other, in order to thus image the test patterns successively on the two-dimensional surface of the imager chip. The second mirror units are advantageously planar. The pivotable or tiltable first mirror device is also advantageously planar, so that the focusing takes place solely via the collimator device. 
         [0011]    According to one particularly advantageous design, the second mirror units of the second mirror device are situated in such a way that the total length of the optical light path from the collimator device to the camera position or the entrance pupil of the camera is completely or at least essentially the same at the different pivot positions. For example, differences may be permitted within a tolerance value. The total length thus consists in particular of the light paths from the collimator device to the pivotable first mirror device, from there to the second mirror device, and from there to the camera. Such a design may in particular be carried out via a hemispherical and/or a rotational ellipsoid arrangement of the second mirror units. For this purpose, the second mirror units may be securely accommodated in a second mirror support, for example, embedded or rigidly secured. 
         [0012]    A second mirror unit is advantageously provided for each pivot position, i.e., in particular different value pairs of two tilt angles, so that an exact matching of the optical path lengths may be carried out. 
         [0013]    One particular advantage is that the mass to be pivoted may be kept small, since it is necessary to adjust only the first mirror device, which, for example, may be one single planar first mirror which is set at different tilt angles via a mirror adjustment device. The setting of different object distances or focuses may be carried out in a manner known per se using a focusable collimator device, which, however, may not be additionally pivoted. 
         [0014]    Thus, the various cameras may be accommodated successively in the camera support and measured by setting different focuses and different tilt angles. These settings may be run through via an inner and an outer loop. 
         [0015]    One additional advantage of the present invention is that, in contrast to conventional systems, the loop structure of the measuring procedure may be changed, so that the collimator runs through only one single through-focus scan in an outer loop, and the adjustable first mirror unit quickly sets the respective image positions in succession in an inner loop. 
         [0016]    One additional advantage is that when adjusting the first mirror unit, which may be designed having a small mass and is quickly and precisely adjustable, the measurement may be carried out in a shorter testing period than when pivoting large masses such as the camera or the collimator device. 
         [0017]    In addition to the measurement of identical cameras, cameras having a different aperture angle may also be measured. For this purpose, the second mirror device is advantageously designed having different mirror sets for the cameras having different aperture angles, i.e., generally having mirror units which are farther apart for cameras having a larger aperture angle. Such a second mirror device may thus be used for different cameras without having to replace, modify, or adjust it, with only a software adjustment having to be carried out for the control signals for controlling the mirror adjustment device and/or the adjustment device of the collimator device. 
         [0018]    In addition to measuring monocular cameras, the measurement of stereo camera systems is also possible. Both individual cameras may be successively measured or checked on the one hand by setting different pivot positions, so that here as well, only the adjustment of the control signals must be carried out. The second mirror device advantageously has different mirror sets for both axes of the stereo camera system, which lie on hemispheres or paraboloids of revolution which are offset from each other. Thus, for such a design as well, only one shared second mirror device having fixed mirror sets is required, without having to change or replace the second mirror device. 
         [0019]    Therefore, one advantage is also that by using a second mirror device having multiple mirror sets for cameras having a different aperture angle and/or stereo cameras, more complex measurements are possible, in which only one configuration adjustment of the software is required for controlling the mirror adjustment device and/or the focus of the collimator device. 
         [0020]    The adjustable first mirror unit may in particular have a first mirror which is tiltable about two axes. This may, for example, be implemented using a motorized two-axis unit. Alternatively, however, a combined system made up of two mirrors which are pivotable at various tilt angles, for example, galvanometer mirrors, may be implemented, which are already known per se, for example, in laser processing. 
         [0021]    Furthermore, according to the present invention, defined climatic conditions may also be set. Multiple measurements may thus be set successively in different climatic conditions, i.e., the temperature or humidity for the camera to be tested. Since the camera may be accommodated in the camera support securely and without setting different pivot angles, problems of insulating a pivotable camera accommodation are eliminated. The securely accommodated camera may thus be subjected to different conditions without great effort. 
         [0022]    An additional advantage is that only three axes, which may be set precisely by an evaluation and control device, must be motorized, namely, one axis for the through-focus scan of the light-emitting device or collimator device, and two pivot axes of the first mirror device. The calibration of the infinite position of the collimator, which is relevant to the measurement accuracy, is required only for one single collimator. Deviations for the various measuring field positions may be avoided. 
         [0023]    The use of one single collimator having deflection of the test pattern onto the various measurement positions furthermore also simplifies the adjustment of the spectral design of the light source, which is indeed required, for example, for the measurement of night vision systems or image evaluation systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a side view of a measuring device according to one specific embodiment. 
           [0025]      FIG. 2  shows the principle of the fixed second mirror device in a front view. 
           [0026]      FIG. 3  shows a front view of a fixed second mirror device according to another specific embodiment for the successive measurement of two different cameras. 
           [0027]      FIG. 4  shows a front view of a fixed second mirror device according to another specific embodiment for measuring a stereo camera device having two individual cameras. 
           [0028]      FIG. 5  shows a flow chart of a measurement method according to one specific embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    According to  FIG. 1 , a measuring device  1  has a focusable collimator  2  acting as a collimator device, a mirror adjustment device  3  having a pivotably accommodated first mirror  4 , a fixed second mirror device  5 , and a camera support  6  having a camera  8  to be tested which is accommodated in a camera position  7 . 
         [0030]    Collimator  2  has, in a manner known per se: 
         [0031]    a housing  2   a,  an optics device  9 , a light source, for example, in the form of an LED unit  10  in the rear area of housing  2   a,  and a test pattern  12  which is adjustable along optical axis A by an adjustment device  15  in housing  2   a.    
         [0032]    Optics device  9  is preferably an eyepiece (collective lens) having a fixed position in housing  2   a.  Test pattern  12  may, for example, be an etched plate or glass panel, for example, as apparent in the illustration in  FIG. 3 , a cross-shaped test pattern  12 , which is illuminated from behind by LED unit  10  and thus appears toward the front, i.e., toward eyepiece  9 , as a light-emitting object, which is subsequently to be imaged over the entire optical arrangement by camera  8 . The various longitudinal positions of test pattern  12  in collimator  2  are thus used to represent different object distances which are to be imaged by camera  8  to be tested. 
         [0033]    Collimator  2  or its collimator housing  2   a  is securely accommodated in a collimator accommodation  11  of measuring device  1 . Adjustment device  15  for setting the focus of collimator  2  is thus adjustable with respect to collimator accommodation  11 . Light  14  emitted by collimator  2  runs along optical axis A to pivotable first mirror  4 . First mirror  4  is pivotable via mirror adjustment device  3  about two orthogonal pivot axes C and D, neither of which runs parallel to optical axis A. The respective pivot positions of first mirror  4  are not orthogonal to optical axis A, so that incident light  14  is not reflected directly to collimator  2 , but rather to second mirror device  5 . Pivot axes C and D advantageously run parallel to first mirror  4 . Instead of one single mirror which is pivotable about two pivot axes C and D, a mirror device having two first mirrors which are pivotable about each pivot axis may also generally be provided. 
         [0034]    First mirror  4  is preferably planar. In the illustration in  FIG. 1 , pivot axis C is perpendicular to the image plane, and the other pivot axis D is in the image plane. 
         [0035]    First mirror  4  reflects incident light  14  corresponding to its pivot positions in different directions as light paths  16 - 1 ,  16 - 2 , i.e.,  16 - i , where i=1, 2, 3, . . . ; light paths  16 - i  and  16 -(i+1) are shown in  FIG. 1  by way of example. Each light path  16 - i  is directed to a second mirror unit  18 - i  of second mirror device  5 , where i=1, 2, . . . . Individual second mirror units  18 - 1 ,  18 - 2 , . . . are advantageously in turn planar and accommodated on a concave-, spherical- or hemispherical-, or ellipsoid-shaped second mirror support  19 . 
         [0036]    Planar designs of first mirror  4  and second mirror units  18 - i  are advantageous, so that the focusing is determined solely via the collimator  2 , and precise manufacturing of mirrors  4  and mirror units  18 -i is cost-effective. However, non-planar designs of first mirror  4  and/or second mirror units  18 - i  are also generally possible and must then correspondingly be taken into account when ascertaining the respective object distance of camera  8 . 
         [0037]    Individual second mirror units  18 - i  reflect each of incident light paths  16 - i  to entrance pupil  8   a  of the objective of camera  8 , whose image sensor (imager chip)  21  thus provides an image which (essentially) corresponds to the front view of second mirror device  5  shown in  FIG. 2 . 
         [0038]    Images B 12  of the test pattern at positions of second mirror units  18 - i  are correspondingly illustrated in  FIG. 2  and the additional specific embodiments in  FIGS. 3 and 4 . For the sake of clarity,  FIGS. 2 through 4  thus depict the superimpositions of the front view of the second mirror device and the image recorded by image sensor  21  of the camera or generated image signal S 1 . 
         [0039]    Image signal S 1  is output to a control and evaluation device  30 , which in turn outputs control signals S 2  to mirror adjustment device  3  and adjustment device  15 . Control and evaluation device  30  is schematically depicted here and may correspondingly also be formed from multiple units for control and evaluation. 
         [0040]    Depending on the setting of pivot angles (tilt angles) a about the C-axis and B about the D-axis of mirror  4 , image B 12  of test pattern  12  is thus shown in different horizontal positions (x-axis) and vertical positions (y-axis) of an image signal Si which is output by camera  8 . A matrix arrangement of images B 12  of test pattern  12  thus results at the different angle values of α and β. 
         [0041]    Different image field positions in image sensor  21  of camera  8  are thus set via pivot angles α and β of mirror adjustment device  3 , so that the position and quality may be checked. 
         [0042]    Individual second mirror units  18 - i  are advantageously situated in such a way that the entire optical distance from first mirror  4  in its different angle positions via second mirror units  18 - i  to entrance pupil  8   a  of camera  8  is the same for all i. This is achieved via the hemispherical or ellipsoid arrangement of second mirror units  18 - i  on second mirror support  19 , since the sum of the distances from first mirror  4  (or the point of intersection of pivot axes C and D in mirror  4 ) via each of second mirror units  18 - i  to objective  8   a  is constant in such an arrangement. 
         [0043]    Collimator accommodation  11 , second mirror support  19 , and camera support  6  are thus situated in a fixed position relative to each other. They are advantageously accommodated in a frame of measuring device  1 . Camera support  6  allows a defined camera position  7  of camera  8  in the optical arrangement. 
         [0044]    More complex mirror designs, for example, having more than one fixed mirror device  5  or having two mirrors which are tiltable about various pivot axes, are also generally possible. However, the depicted design is advantageous with respect to the formation of equal optical path lengths via the hemispherical or ellipsoid arrangement of second mirror units  18 - i.    
         [0045]    For a complete measurement of the imaging properties of optical camera  8 , its image signals S 1  are recorded at various virtual object distances at various positions on image sensor (imager)  21 . The various virtual object distances are set by adjustment device  15  of collimator  2  and correspond to real object distances, for example, between 2 m and infinity. 
         [0046]    An inner loop and an outer loop are advantageously run through for detecting all values. Preferably, angle settings α and β corresponding to the x and y directions in image sensor  21  are run through in the inner loop, and the through-focus scan is carried out via adjustment of collimator  2  in the outer loop. A focus setting of collimator  2  is thus respectively set by adjustment device  15 , and all values of α and β are subsequently run through for this focus setting, an image signal S 1  respectively being recorded. Then, the next setting of adjustment device  15  is subsequently set, in which all angle values α and β are successively set and image signals S 1  are recorded, and so forth. 
         [0047]    The measurements using measuring device  1  may also be carried out under different climatic conditions. Thus, respective measurements using all focus settings and angle settings α and β may be carried out for different temperature values and/or humidity values. Tilt designs of mirror  4  for such different temperature conditions and/or climatic conditions are thus technically easily implementable, since second mirror  4  has only a small mass. 
         [0048]    Cameras  8  having different aperture angles γ may also be used with measuring device  1 . The same measuring device  1  may advantageously be implemented for cameras  8  having different aperture angles γ solely as a configuration adjustment of the evaluation software for evaluating or processing the image signals S 1 , without modification or hardware-based adaptation. 
         [0049]      FIG. 3  shows a front view of a second mirror device  5  having two mirror sets, i.e., a mirror set  18  made up of mirror units  18 - i , where i=1 to 9, for a camera  8  having an additional aperture angle γ, and an additional mirror set  20  made up of mirror units  20 - i , where i=1 to 9, for an additional camera  8  having a smaller aperture angle γ. Optical axis E of camera  8  runs symmetrically through both mirror sets  18  and  20 , which are thus designed to be symmetrical to each other or are designed for enlarged or reduced images. 
         [0050]    In  FIG. 3 , the outer box of second mirror device  5  thus corresponds to the image sensor  21  of camera  8  having a larger aperture angle; correspondingly, the inner box surrounding mirror units  20 - i  corresponds to image sensor  21  of camera  8  having a smaller aperture angle for the same optical axis E. 
         [0051]      FIG. 4  shows a design of second mirror device  5  for measuring stereo camera devices having two cameras, i.e., having two optical axes E 1  and E 2  which are offset from each other. For this purpose, a shared first adjustable mirror  4  and a shared second mirror device  5 , which has a left mirror set  18  for the left camera of the stereo camera device having left second mirror units  18 -i, where i=1 to 9, and a right mirror set  28  for the right camera having right second mirror units  28 - i , where i=1 to 9, may in turn be used. Left mirror units  18 - i  are thus situated on a first hemisphere or a first paraboloid of revolution having an optical axis E 1 , and right mirror units  28 - i  of right camera  8  are situated on a second hemisphere or a second ellipsoid of revolution having an optical axis E 2  which is offset for this purpose, all mirror units  18 - i  and  28 - i  being rigidly accommodated in second mirror device  5 . 
         [0052]    The measurement method according to the present invention is shown on a flow chart in  FIG. 5  by way of example. It starts at step St 0 , and then an outer loop St 1  and an inner loop St 2  are run through. Outer loop St 1  is used to set different through-focus values. For this purpose, values of a parameter from k=1 to k=n are run through by adjustment device  15  of collimator  2 . Inner loop St 2  is used to set different tilt angles α, β. For this purpose, a parameter n runs from n=1 to n=j. 
         [0053]    The depiction of the loops in  FIG. 5  having an initial fixing of parameters k and n to 1 and subsequent increments is only exemplary. 
         [0054]    In step St 3 , images are respectively recorded by camera  8 , and image signals S 1  are generated which are subsequently evaluated in step St 4 . The method is ended in step St 5  for the specific embodiments in  FIG. 2 . For measuring cameras  8  having different aperture angles γ, different cameras  8  are consecutively inserted into camera support  6 , and the measurement method according to  FIG. 5  is carried out. For measuring a stereo camera, after the start in St 0 , inner and outer loops St 1  and St 2  may also each be run through for both individual cameras  8  with measurements in St 3 , followed by a combined evaluation in step St 4 .