Abstract:
A vehicle camera system includes a camera that includes an image sensor, a filter mask, and a control and evaluation device, to which the image sensor outputs an image signal with frames that correspond to different exposure times. The image sensor includes an arrangement of sensor pixels outputting pixel signals, and the filter mask includes an arrangement of filter pixels situated in front of respective ones of the sensor pixels, where different filter pixels have different transmission behavior. The control and evaluation device compares to each other pixel signals (a) contained in the frames of different exposure times, and (b) output by sensor pixels which record light filtered differently by the filter pixels.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a camera system, in particular for a vehicle, and to a method for ascertaining pieces of image information of a detection area. 
       BACKGROUND 
       [0002]    In general, image sensors of camera systems are read out by a control device as a function of an ambient brightness using different exposure times to avoid underexposure or overexposure. 
         [0003]    During the day, the exposure times are therefore in general very short. However, it may thus occur that only briefly illuminating light sources, in particular pulsed light sources, are no longer continuously detected, in particular when the read-out time is within or below the order of magnitude of the pulse duration of the light source. For example, pulse frequencies of light emitting diodes (LEDs) in camera systems in the automotive field are less than 0.1 ms during the day at approximately 90 Hz to 100 Hz read-out times of the image sensors. 
         [0004]    To prevent an LED detected by the camera system from flickering, DE 10 2010 024 415 A1 provides for the image sensor to constantly or continuously detect incident light using a maximal read-out time. A pause time, during which the image sensor does not convert the light incident on it into electrical signals or is not sensitive to light, is selected to be lower than a pulse duration of light pulses to be detected, so that the sensor does not “overlook” a light pulse. An attenuation filter or a diaphragm is provided to ensure that the sensor is not overexposed or that the sensor does not go into saturation, the attenuation filter or the diaphragm having to be adapted according to the lighting conditions (during the day or at night) since at night less light strikes the sensor than during the day. 
         [0005]    The disadvantage of this approach is that additional means are required in the camera system to vary the diaphragm or the attenuation filter in each case as a function of the lighting conditions. 
         [0006]    DE 10 2005 033 863 A1 shows an image recording system in which, in addition to a camera, a radiation sensor is provided, which ascertains an average brightness level of an image scene and compares it to the brightness of an image recorded by the camera. The radiation sensor also senses pulsed light sources by using a longer read-out time. If a discrepancy arises between the two brightness levels, a warning signal, which indicates to the driver that a fault exists in the display representation, is output; and the exposure phase of the camera and the activation phase of the pulsed light source are synchronized, or areas of the images represented on the display are replaced by areas with accordingly longer exposure. 
         [0007]    The disadvantage of this approach is that a further radiation source, which must be synchronized with the camera, is required, whereby the manufacturing complexity and costs increase. 
         [0008]    DE 698 08 024 T2 describes a further method and an image recording system for generating a video signal having an expanded dynamic range. Such systems are also known as high dynamic range (HDR) images, in which consecutive images are generated using different parameter settings, in particular using different exposure times, so that subsequently an optimized image may be created from image sections having different brightness levels, for example by taking over dark sections from the longer exposed image and lighter sections from the shorter exposed image. In this way, the consecutive frames of the output image signal may directly be set differently, so that a sequence of different frames is generated. For this purpose, different exposure times are fixedly set and the images are subsequently evaluated. 
       SUMMARY 
       [0009]    According to example embodiments of the present invention, a filter mask that includes different filter pixels is provided in front of an image sensor, which includes a pixel arrangement made up of multiple sensor pixels. The filter pixels exhibit different transmission behaviors, for example, different characteristics of a degree of not allowing incident light to partially pass, i.e., to absorb or to reflect it. The different transmission behavior preferably denotes a different attenuation of the intensity which is allowed to pass. 
         [0010]    The filter mask includes first and second filter pixels of different transmission behaviors, in particular different attenuation, which are situated in a regular or irregular arrangement or raster in front of the sensor pixels. For example, the first filter pixels provide no attenuating or a moderately attenuating effect, and the second filter pixels provide a stronger attenuating effect. 
         [0011]    This may take place, on the one hand, by forming second filter pixels as a gray filter, which attenuates light incident in a relevant wavelength range in each case on a pro-rata basis or proportionally, for example in each case one quarter or half of the incident intensity. As an alternative, the attenuation can also be achieved by a polarization filter, for example, which thus attenuates half the intensity of incident unpolarized light, for example. However, wavelength-selective attenuations are also generally possible, for example with the aid of a color filter, which thus allows only light of a relevant wavelength range to pass and attenuates other wavelength ranges. 
         [0012]    The pixel arrangement of the—preferably identically designed—sensor pixels on the image sensor can be divided into individual sensor areas, in which sensor pixels provided with upstream first filter pixels, which hereafter are referred to as first sensor pixels, and sensor pixels provided with upstream second, more strongly attenuating, filter pixels, which hereafter are referred to as second sensor pixels, are situated adjacent or abutting each other, for example three first sensor pixels and one second sensor pixel. At the same incident light intensity, the signal strengths of the first pixel signals output by the first sensor pixels are thus greater than the signal strengths of the second pixel signals output by the second sensor pixels. 
         [0013]    Moreover, the image sensor is read out using different read-out times or exposure times. Advantageously, a multimodal camera control unit is designed with two or more sub-sequences, which are assigned different exposure times. In the case of a two-modal camera control unit, a first sub-sequence with individual first frames, which are each assigned to a shorter first exposure time, and a second sub-sequence made up of second frames, which are each assigned to a greater second exposure time, are thus generated. These sub-sequences advantageously alternate, as is known per se from multimodal camera control units, which are used in particular also for high dynamic range (HDR) camera systems. 
         [0014]    In the evaluation, the less strongly attenuated first pixel signals having a shorter exposure time are compared to more strongly attenuated second pixel signals having a longer exposure time. The comparison is carried out in particular between first and second pixel signals of a sensor area, e.g., of preferably mutually abutting sensor pixels. For this purpose, for example, initially mean values may be formed from the first and second pixel signals of a sensor area, which are then used for the comparison. 
         [0015]    The comparison takes the different attenuation factors and different exposure times into consideration. This can take place, for example, with the aid of a correction using a fixed correction factor. In the simple case of an attenuation by the second filter pixels by the factor 0.5 compared to the first filter pixels, and a ratio of the exposure times with the factor 2, approximately identical signal strengths, i.e., charge amounts generated by the exposure in the sensor pixels, should thus be ascertained—not considering background noise or sensor noise and without the case of oversaturation. 
         [0016]    The present invention is thus based on the idea to compare pixel signals from adjacent or mutually abutting sensor pixels, having different attenuation and different exposure times, to each other to ascertain short-term intensity changes, in particular intensity increases, which are detected during the longer exposure time, but not during the shorter exposure time. In this way, it is possible to detect in particular light pulses of pulsed light sources, such as LEDs, which, due to the short read-out times of the first exposure time, are not detected, if necessary, during the measurements using a longer exposure time. Such intensity contributions can thus initially be ascertained by the comparison. Subsequently, advantageously only one image signal from the pixel signals having a shorter exposure time are used for further image processing, if the comparison shows or suggests that no relevant intensity increases are present. 
         [0017]    Initially, the comparison is preferably used to detect possible errors in an image signal having a short exposure, contrary to HDR, for example, where pieces of information are deliberately taken from the multiple modes or exposure times to form the image signal. Contrary to HDR, in particular a comparison of pixel signals of a pixel area that are attenuated to different degrees is also combined with the comparison of the different exposure times. 
         [0018]    The filter mask can be regular or irregular. It can regularly cover, e.g., every fourth pixel. An irregularity can be provided to avoid systematic errors due to light points moving vertically in the detected image, for example (in the case of vertical pitch of the host vehicle). 
         [0019]    According to the present invention, several advantages are achieved. It is possible to detect short-term light signals, in particular light pulses, such as of LEDs, even if their pulse duration is within or below the order of magnitude of the shorter exposure time. Nonetheless, a high signal quality and thus image quality can be achieved by using for this purpose pixel signals that are less strongly attenuated, or in particular not attenuated, and have short exposure times. As a result of the additional measurement or the second mode having longer exposure times, a comparison can be carried out, which is used to avoid errors or discrepancies due to non-detection of pulsed light sources. 
         [0020]    Since the second sensor pixels are thus essentially used for comparison, the number of the second sensor pixels in the pixel area can be selected to be smaller and thus, if necessary, have no relevant impairing effect on the resolution of the non-attenuated or less strongly attenuated first sensor pixels. 
         [0021]    The filter mask can in particular also be used for supplementary functions of the camera system. For example, the second filter pixels having a stronger attenuation can be used as a polarization filter, which is used in relevant functions on a supplementary basis. Such a function can be a night vision device, for example, in which the polarized IR radiation ,which is reflected by detected objects, is output, the second filter pixels being adjusted to this polarization and allowing light reflected in such a way to pass in a preferred manner or without relevant attenuation. Such a camera system can thus utilize the second filter pixels for the comparison for ascertaining short-term intensity increases during daytime operation, and it can utilize the second filter pixels as a polarization filter for the selective detection of reflected radiation during nighttime operation with active illumination of the surroundings. 
         [0022]    In this way, a multifunctional camera system can be created, which allows the detection of short-term light pulses without, or without relevant additional, hardware complexity. 
         [0023]    Accordingly, a method according to the present invention for ascertaining pieces of image information is provided. Moreover, a vehicle including such a camera system is provided according to the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  shows a vehicle including a camera system according to an example embodiment of the present invention. 
           [0025]      FIG. 2  shows top views of a filter mask and the image sensor of  FIG. 1 , according to an example embodiment of the present invention. 
           [0026]      FIG. 3  shows representations of the chronological progression of the radiation intensity impinging on different sensor pixels, according to an example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]      FIG. 1  schematically shows a windshield  2  and a vehicle interior  3  of a vehicle  1 . A camera system  24  includes a control and evaluation device  12  and includes a camera  4  attached in vehicle interior  3 . Camera  4  includes a lens  5 , an image sensor  7 , and a filter mask  6  upstream of the image sensor  7 , with respect to a direction of light sensed by camera  4 . In an example embodiment, filter mask  6  is advantageously situated or designed directly on image sensor  7 . Camera  4  records light  8  of a detection area  9  of vehicle surroundings  10  through windshield  2  and converts this light  8  or the pieces of image information of light  8  into image signals S 1 , which it outputs to the control and evaluation device  12 . Image signals S 1  represent—in any suitably appropriate conventional manner of representation—a sequence of frames (images) A 1 , B 1 , A 2 , B 2 , A 3 , B 3 , . . . , control and evaluation device  12  activating and reading out image sensor  7  in a multimodal camera control unit, using two different read-out times (exposure times) τ1 and τ2, where τ2&gt;τ1. The sequence of frames can thus be divided into two sub-sequences A 1 , where i= 1 ,  2 ,  3 , . . . (i.e., A 1 , A 2 , A 3 , . . . ), and Bi, where i = 1 ,  2 ,  3 , . . . (i.e., B 1 , B 2 , B 3 , . . . ), the frames of first sub-sequence Ai being read out using the shorter first read-out time τ1 and the frames of second sub-sequence Bi being read out using the longer second read-out time τ2. In this example embodiment, sub-sequences Ai and Bi alternate; however, embodiments in which, for example, only every third or fourth image is assigned to second sub-sequence Bi are also possible. 
         [0028]    Filter mask  6  includes first filter pixels F 1  without absorption or without relevant absorption of light  8 . First subareas F 1  are thus (at least largely) transparent. Filter mask  6  also includes second filter pixels F 2 , which cause a relevant absorption of light  8 . According to one preferred example embodiment, second filter pixels F 2  are gray pixels, or act as gray filters, and thus relatively uniformly absorb or reflect a portion of incident light  8  in a relevant, visible spectral range. According to the shown example embodiment, second filter pixels F 2 —shown hatched in FIG.  1 —are situated in a regular manner, approximately every fourth filter pixel being a second filter pixel F 2 . In alternative example embodiments, irregular arrangements and/or different ratios of first filter pixels F 1  to second filter pixels F 2  are provided. 
         [0029]    Filter mask  6 , with its filter pixels F 1  and F 2 , is situated in front of image sensor  7 , which includes any suitably appropriate conventional pixel array of sensor pixels, e.g., in a CMOS or CCD design. The sensor pixels behind first filter pixels F 1  are denoted as first sensor pixels P 1 , and the sensor pixels behind second filter pixels F 2  are denoted as second sensor pixels P 2 . In this way, second sensor pixels P 2  undergo a gray filtration, while first sensor pixels P 1  do not. First sensor pixels P 1  supply pixel signals PS 1 , and second sensor pixels P 2  supply pixel signals PS 2 . 
         [0030]    A pixel area  16  made up of four mutually abutting sensor pixels, which is illustrated in  FIG. 2  by a thicker line, contains three first sensor pixels P 1  and one second sensor pixel P 2 . The pixel array made up of sensor pixels P 1  and P 2  can thus be considered as a raster or an over-raster made up of multiple pixel areas  16 . 
         [0031]    Image signals  51  thus result as an alternating sequence of frames A 1 , B 1 , A 2 , B 2 , which each contain first pixel signals PS 1  and second pixel signals PS 2 , which are recorded and processed in control and evaluation device  12 . 
         [0032]    Control and evaluation device  12  compares pixel signals PS 1 , PS 2  of a shared pixel area  16  of the first frames of first sub-sequence Ai and of the second frames of second sub-sequence Bi to each other. For example, in an example embodiment, an average value of the three first pixel signals PS 1  of a respective pixel area  16  is initially determined, the average value being compared to second pixel signal PS 2  of the respective pixel area  16  in question, so that the intensity differences due to the lateral offset of sensor pixels P 1  and P 2  are slightly reduced; otherwise intensity differences due to the lateral offset may be neglected, as is also the case when using color filter masks. 
         [0033]    In the comparison, preferably directly consecutive frames Ai and Bi are compared to each other, so that, in a first approximation, a detection area  9 , which was not changed by a relative movement or by the travel of vehicle  1 , may be assumed. 
         [0034]    Thus, four different pixel signal values are obtained by filter mask  7  having two different filter pixels F 1 , F 2  and two modes having different exposure times τ1, τ2, namely:
       first pixel signals PS 1 (A) of first sub-sequence Ai, i.e., with a short first exposure time τ1;   first pixel signals PS 1 (B) of second sub-sequence Bi, i.e., with a long second exposure time τ2;   second pixel signals PS 2 (A) with a short first exposure time τ 1 ; and   second pixel signals PS 2 (B) with a long second exposure time τ 2 .       
 
         [0039]    The ratio of the attenuation of F 2  to F 1  is referred to as d, i.e., d&lt;1, and the ratio of τ2 to τ1 is referred to as w, i.e., w&gt;1. 
         [0040]    In a pixel area  16 , PS 1 (B) is thus the greatest value and PS 2 (A) is the lowest value in terms of the magnitude of the intensity or signal strength. During the day, P 1  may already go into saturation at a longer exposure time τ2, so that PS 1 (B) is no longer used. 
         [0041]    PS 2 (B) and PS 1 (A) should be neither in undersaturation nor in oversaturation, so that, in terms of the magnitude of the intensity or signal strength, with a chronologically unchanged intensity of light  8  (neglecting sensor noise and further influences): PS 2 (B)=d*w*PS 1 (A) and PS 1 (A)=PS 2 (B)/(d*w). 
         [0042]    However, differences may occur, in particular with pulsed light  8 . For example, in particular light  8  of LEDs is operated in pulse frequencies of approximately 90 Hz to 100 Hz, i.e., corresponding to frequencies in the range of 0.1 ms. First read-out time τ1 is in the range of 0.1 ms, for example, while longer second read-out time τ2 is placed in a time range which detects individual pulses with certainty, e.g., &gt;0.2 ms, for example, in the range of 0.5 ms to 1 ms. 
         [0043]      FIG. 3  schematically shows intensity I of light  8  as a function of time t, for each of a first sensor pixel P 1  and a second sensor pixel P 2 , i.e., signals PS 1  and PS 2 . Here, an example is shown in which a light pulse  20  falls into the time period τ2-τ1, and therefore is not detected by τ1, but is detected by τ2. This light pulse  20  is thus detectable in resulting pixel signal PS 2 (B), but not in resulting PS 1 (A). Light pulse  20  can also be detected in subsequent τ1, for example, and can thus occur in a portion PS 1 (A), but, precisely this is recognized as being problematic. 
         [0044]    Thus if [PS 1 (A)=PS 2 (B)/(d*w)] is also no longer met within tolerance limits f, which considers sensor noise, e.g., if PS 1 (A) is not within the range of [f*PS 2 (B)/(d*w); 1/f*PS 2 (B)/(d*w)], where f&lt;1, e.g., f=0.8, then a short-term intensity increase that is not detected in τ1 can be detected. In this frame, PS 1 (A) is thus recognized as erroneous. 
         [0045]    According to one preferred example embodiment, control and evaluation device  12  considers only signals PS 1 (A) and use these for image evaluation during daytime operation if no error is recognized, and second pixel signals PS 2 (B) are used only for the comparison to detect whether a light pulse  20  is present in PS 1 (A). It is recognized here that signals PS 1 (A), i.e., at a short exposure time τ1, are of a better quality and generally sufficient for image processing. If an erroneous PS 1 (A) is recognized, it is replaced by the corresponding PS 2 (B). 
         [0046]    Control and evaluation device  12  subsequently outputs corrected image signals S 2 , or further signals ascertained therefrom. 
         [0047]    Control and evaluation device  12  can also be formed by multiple individual devices, e.g., a control device for activating image sensor  7  or its sensor pixels P 1  and P 2 , and a further evaluation device. 
         [0048]    Second filter pixels F 2  can also attenuate with the aid of polarization instead of as gray filters, e.g., camera system  24  is equipped as a night vision system with an additional IR lamp  22  on a supplementary basis, which is activated by control signals S 3  from control and evaluation device  12  to emit polarized IR radiation  23  into detection area  9 , which, after a reflection from an object in detection area  9 , is in turn preferably allowed to pass by second filter pixel F 2 . 
         [0049]    The polarization plane of F 2  is adapted to the polarization of radiation  23 , so that camera  6  is provided in a multifunctional manner, on the one hand, for detecting pulsed LEDs in day-time operation, and, on the other hand, as a night vision system during night-time operation. Different exposure times are set in night-time operation. First pixel signals PS 1  without polarization filtration then contain pieces of image information of the ambient light, and second pixel signals PS 2  contain pieces of image information of the night vision function. 
         [0050]    Moreover, embodiments of second filter pixels F 2  with wavelength-selective filtration are also possible. In general, F 2  can thus also be used to form a color pattern on a supplementary basis. 
         [0051]    Moreover, more than just two different filter pixels F 1 , F 2  can be provided. Moreover, a larger number of exposure times, i.e., a multimodal camera control unit with more than two modes, can also be provided. 
         [0052]    The entire camera system  24  thus includes camera  4 , control and evaluation device  12 , and further devices, such as IR lamp  22 , for example.