Patent Publication Number: US-10321085-B2

Title: Pixel unit for an image sensor, image sensor, method for sensing a light signal, method of controlling a pixel unit, and method for generating an image using a pixel unit

Description:
CROSS REFERENCE 
     The present application claims the benefit under 35 U.S.C. § 119 of German Patent Application No. DE 102016212765.9 filed on Jul. 13, 2016, which is expressly incorporated herein by reference in its entirety. 
     BACKGROUND INFORMATION 
     The present invention relates to a device, a method, and a computer program. 
     Sampling performed by an image sensor typically requires an exposure time that is relatively short in comparison to a resulting image frame rate. The exposure time functions as a low-pass filter, which, in an idealized consideration, can be regarded as a square wave in the time domain. 
     SUMMARY 
     The present invention includes a pixel unit for an image sensor, an image sensor, a method for sensing a light signal, a method for controlling a pixel unit, a method for generating an image using a pixel unit; in addition, a device that employs at least one of these methods; and, finally, a respective computer program. Advantageous embodiments of the device are described herein. 
     A pixel unit for an image sensor is provided, the pixel unit having the following features: 
     a possibly complex photodiode for converting a light signal into a processing signal; 
     a multiplexer unit designed to use the processing signal to generate at least a first multiplexer signal and a second multiplexer signal that deviates therefrom; and 
     a storage device having at least a first storage unit for buffer storing a time-limited integral value of the first multiplexer signal and a second storage unit for buffer storing a time-limited integral value of the second multiplexer signal. 
     This allows time-overlapped sampling of the low-pass filtered multiplexer input signal. An image signal, which represents an image recorded by the pixel unit, may be reconstructed from the time-limited integral values of the two multiplexer signals stored in the storage devices. 
     Using the processing signal, a plurality of different signals may advantageously be synchronously generated, for example, at least two or at least three, and, in accordance with a specific embodiment, for each continuous instant, or at least for many discrete instants. 
     A pixel unit may be understood to be a unit for sensing a light signal. A photodiode may be understood to be a possibly complex, light-sensitive element for converting a photon stream into an electron stream. A light signal may be understood, for instance, to be a continuous-time electromagnetic signal that is not constant and has any (spatial and temporal) (modulation) rate of change. A processing signal may be understood to be an electrical output signal from the photodiode. A multiplexer unit may be understood to be a signal multiplexer. For example, the multiplexer unit may be designed as a continuous divider, as a discrete-time divider, thus as a time division multiplexer. A multiplexer signal may be understood to include signals generated by the multiplexer unit by amplifying the processing signal, for example. The first and the second storage units may each be capacitors, for example. In accordance with a specific embodiment, the deviation between the multiplexer signals results from a signal amplification, respectively attenuation (thus multiplication) performed during generation of at least one of the signals. 
     The approach presented here is based on the realization that, by using a hardware structure composed of a multiplexer, in combination with at least two storage units, it is possible to adapt a pixel design and a pixel readout structure of an image sensor in a way that allows a low-pass filter to temporally correctly sample a light signal. 
     In accordance with a specific embodiment, the multiplexer unit may be designed for amplifying the processing signal by a first amplification factor to generate the first multiplexer signal or, additionally or alternatively, for amplifying it by a second amplification factor to generate the second multiplexer signal. The first amplification factor and the second amplification factor may deviate from one another. This specific embodiment allows the storage units to be variably loaded as a function of the instant the first multiplexer signal and the second multiplexer signal are generated. 
     In accordance with a specific embodiment, the multiplexer unit may be designed for synchronously generating the first multiplexer signal and the second multiplexer signal. If further multiplexer signals are generated, then they may likewise be generated synchronously. The multiplexer unit in accordance with this specific embodiment may advantageously be in the form of a continuous divider. 
     In accordance with another specific embodiment, the multiplexer unit may be designed for generating the first multiplexer signal at a first instant and the second multiplexer signal at a second instant. This presents itself, in particular, when the processing signal is a discrete-time signal. However, this approach may also be used for a continuous-time processing signal. In accordance with this specific embodiment, the multiplexer unit may be in the form of a discrete-time divider. 
     In accordance with another specific embodiment, the photodiode, the multiplexer unit and the storage device may be serially interconnected. In this case, the first storage unit and the second storage unit may be parallel interconnected. This makes possible a simple and rugged interconnection within the pixel unit. 
     It is also advantageous when the pixel unit features a readout contact for synchronously and separately reading out the first storage unit and the second storage unit. A readout contact may be understood to be a contact element that is electroconductively connected to the first storage unit and the second storage unit. This permits a simple electrical contacting of the pixel unit. 
     In addition, the photodiode may be designed to feed the multiplexer unit a continuous electrical current as the processing signal. Using the processing signal, the multiplexer unit may be designed here to generate a first electrical current as the first multiplexer signal and a second electrical current, which deviates therefrom, as the second multiplexer signal. The multiplexer unit may be thereby realized as a continuous divider. 
     Alternatively, the complex photodiode may be designed for feeding a discrete-time signal featuring a time scheme as the processing signal, to the multiplexer unit. In accordance with a specific embodiment, the multiplexer unit is designed for using the time scheme to generate the first multiplexer signal and the second multiplexer signal. For example, a first pulse of the processing signal may be fed to the first storage unit, and a subsequent second pulse of the processing signal to the second storage unit. In such a case, the pulses may still each be appropriately amplified, for example, manipulated or multiplied by the multiplexer unit. 
     In accordance with another specific embodiment, the pixel unit may feature at least one additional storage unit, interposed between the photodiode and the multiplexer unit, for buffer storing the processing signal. The multiplexer unit may be designed here to generate the first multiplexer signal or, additionally or alternatively, the second multiplexer signal, using a processing signal buffer stored in the additional storage unit. The additional storage unit may be an intermediate circuit capacitor, for example. This makes it possible to ensure an efficient processing of even weak light signals. 
     In accordance with a specific embodiment, the additional storage unit constituting an additional capacitance is charged with a few electrons in the “pulsed mode” directly following the photodiode and then multiplexed in the individual storage modes that are associated with the buffer memories. This makes possible a low capacitance for a substantial amplification. 
     In the continuous mode, this capacitance (if present) is continuously discharged by the continuous operation of the multiplexer. Thus, if indicated, the capacitance also already functions as a low-pass filter, for example as an RC circuit. 
     In accordance with one specific embodiment, in the continuous mode without the capacitor, the multiplexer does not function with voltages, but with currents. 
     The pixel unit may be realized by a multilayer composite of a first semiconductor layer and a second semiconductor layer. The photodiode may be configured here in the first semiconductor layer. Depending on the specific embodiment, the multiplexer unit, or, additionally or alternatively, the storage device may be configured in the second semiconductor layer. The multilayer composite may be what is commonly known as a stacked die composite, for example. This specific embodiment allows the pixel unit to be miniaturized cost-effectively. 
     A semiconductor layer may be optionally in the form of a wafer or a silicon-based functional unit. 
     In accordance with another specific embodiment, the multiplexer unit may be designed to use the processing signal to generate synchronously or at least at another instant another multiplexer signal, that deviates at least from the first multiplexer signal or, additionally or alternatively, from the second multiplexer signal. Correspondingly, the storage device may have at least one further storage unit for buffer storing a time-limited integral value of the further multiplexer signal. This makes it possible to increase the number of buffer stored multiplexer signals. 
     The approach presented here also provides an image sensor having at least one pixel unit in accordance with one of the preceding specific embodiments. 
     Moreover, the approach described here provides a method for sensing a light signal, the method including the following steps: 
     converting the light signal into a processing signal; 
     using the processing signal to generate at least a first multiplexer signal and a second multiplexer signal that deviates therefrom; and 
     buffer storing a time-limited integral value of the first multiplexer signal in a first storage unit and a time-limited integral value of the second multiplexer signal in a second storage unit. 
     This method may be implemented, for example, using a pixel unit in accordance with one of the preceding specific embodiments. 
     In addition, the approach presented here also provides a method for controlling a pixel unit in accordance with one of the preceding specific embodiments, the method including the following steps: 
     inputting the processing signal; and 
     using the processing signal to output a control signal for controlling the multiplexer unit. 
     In accordance with a specific embodiment, an independent “timer” supplies the multiplexer with a control signal thereof, thus with a first multiplexer signal having any desired weighting function G 1 ( t ) and with a second multiplexer signal having any desired weighting function G 2 ( t ). The weighting signals may be globally generated. 
     The present invention also provides a method for generating an image using a pixel unit in accordance with one of the preceding specific embodiments, the method including the following steps: 
     inputting the first multiplexer signal and the second multiplexer signal via an interface to the storage device; and 
     reconstructing an image signal representing the image using the first multiplexer signal and the second multiplexer signal. 
     These methods may be implemented as software or hardware, for example, or as a software and hardware hybrid, in a control unit, for example. In particular, an implementation may be accomplished in the lower layer in the stacked die. 
     The reconstruction may advantageously take place in the sensor-free layer of the stacked die to avoid having to provide many memory stores in the overall system. 
     To this end, the device may have at least one processing unit for processing signals or data, at least one storage unit for storing signals or data, at least one interface to a sensor or an actuator for inputting sensor signals from the sensor or for outputting data signals or control signals to the actuator, and/or at least one communication interface for inputting or outputting data that are embedded in a communication protocol. The processing unit may be a signal processor, a microcontroller or the like, for example, it being possible for the storage unit to be a flash memory, an EPROM or a magnetic storage unit. The communication interface may be adapted for inputting or outputting data wirelessly and/or by wire; a communication interface, capable of inputting or outputting data by wire, then inputting these data, for example, electrically or optically from a corresponding data transmission line or outputting them into a corresponding data transmission line. 
     A device may be understood here to be an electrical device that processes sensor signals and outputs control and/or data signals as a function thereof. The device may have an interface implemented as hardware and/or software. When implemented as hardware, the interfaces may be the part of what are commonly known as system ASICs, for example, that include a wide variety of device functions. However, the interfaces may also be separate, integrated circuits or be at least partially composed of discrete components. When implemented as software, the interfaces may be software modules that are present on a microcontroller, for example, in addition to other software modules. 
     Also advantageous is a computer program product or computer program having program code, which may be stored on a machine-readable medium or storage medium, such as a semiconductor memory, a hard disk or an optical storage medium, and is used to perform, implement and/or control the steps of the method in accordance with one of the aforedescribed specific embodiments, particularly when the program product or program is executed on a computer or a device. 
     Exemplary embodiments of the present invention are illustrated in the figures and explained in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a light sensor of an image sensor in accordance with an exemplary embodiment. 
         FIG. 2  schematically shows an image sensor in accordance with an exemplary embodiment. 
         FIG. 3  schematically shows an image sensor in accordance with an exemplary embodiment. 
         FIG. 4  schematically shows a device in accordance with an exemplary embodiment. 
         FIG. 5  is a diagram for schematically illustrating three control signals for controlling a multiplexer unit in accordance with an exemplary embodiment. 
         FIG. 6  is a diagram for schematically illustrating a low-pass filter characteristic. 
         FIG. 7  is a diagram for schematically illustrating an exposure scheme for using appropriate overlapping low-pass filters for ideal temporal sampling. 
         FIG. 8  is a diagram for schematically illustrating an image sensor performing a typical temporal sampling. 
         FIG. 9  shows a flow chart of an exemplary embodiment of a method for sensing a light signal. 
         FIG. 10  is a flow chart of a method for controlling a pixel unit in accordance with an exemplary embodiment. 
         FIG. 11  is a flow chart of a method for generating an image using a pixel unit in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     In the following description of advantageous exemplary embodiments of the present invention, the same or similar reference numerals are used for the elements that are shown in the various figures and whose function is similar, there being no need to repeat the description of these elements. 
       FIG. 1  schematically shows a light sensor of an image sensor  100  in accordance with an exemplary embodiment. Shown is a basic design of a pixel structure that allows image sensor  100  to perform temporally correct sampling. Image sensor  100  includes a pixel unit  102  having a photodiode  104  for converting a light signal  106  into a processing signal  108 , a multiplexer unit  110 , as well as a storage device  112  composed of a first storage unit  114  and a second storage unit  116 . Storage device  112  is linked to a multiplexer output of multiplexer unit  110 . In accordance with this exemplary embodiment, photodiode  104 , multiplexer unit  110 , and storage device  112  are serially interconnected. Multiplexer unit  110  is designed for generating a first multiplexer signal  118  and at least a second multiplexer signal  120  from processing signal  108  synchronously or at different points in time. In the case of a synchronous generation, multiplexer unit  110  may be configured as a continuous multiplexer. In the case of a generation at different points in time, multiplexer unit  110  may be configured as a discrete-time multiplexer. Multiplexer unit  110  is designed for transmitting first multiplexer signal  118  via multiplexer output into first storage unit  114  that is configured for buffer storing a time-limited integral value of first multiplexer signal  118 . Accordingly, multiplexer unit  110  is designed for transmitting second multiplexer signal  120 , analogously to first multiplexer signal  118 , via the multiplexer output into second storage unit  116 , and for buffer storing a time-limited integral value there. 
     In accordance with  FIG. 1 , in addition to the two storage units  114 ,  116 , storage device  112  includes at least one optional further storage unit  122  for buffer storing at least one further multiplexer signal  124  that multiplexer unit  114  generates using processing signal  108 . The at least three storage units  114 ,  116 ,  122  are parallel interconnected, for example, and each electroconductively connected in a switchable manner to a readout contact  126  for synchronously outputting the three buffer stored multiplexer signals  118 ,  120 ,  124 . In accordance with the exemplary embodiment shown in  FIG. 1 , the three storage units  114 ,  116 ,  122  are in the form of three capacitors C 1 , C 2 , C 3 . 
       FIG. 2  schematically shows an image sensor  100  in accordance with an exemplary embodiment. Image sensor  100  shown in  FIG. 2  essentially corresponds to the image sensor previously described with reference to  FIG. 1 , with the distinction that an optional additional storage unit  200  is connected between photodiode  104  and multiplexer unit  110 , in this case an additional capacitor C S  as a buffer storage downstream of photodiode  104 . Additional storage unit  200  is configured for buffer storing processing signal  108 , multiplexer unit  110  receiving buffer stored processing signal  108  from additional storage unit  200  and processing in a manner previously described with reference to  FIG. 1 . 
       FIG. 3  schematically depicts an image sensor  100  in accordance with an exemplary embodiment, for instance an image sensor, as previously described with reference to  FIG. 2 . It shows the pixel structure being divided into photon-electron conversion and signal conditioning in stacked die technology. For this, pixel unit  102  is realized using a multilayer composite of a first semiconductor layer  300 , also referred to as signal generation layer, and a second semiconductor layer  302 , also referred to as signal post-processing layer. First semiconductor layer  300  includes photodiode  100 , as well as additional storage unit  200 . Second semiconductor layer  302  includes multiplexer unit  110 , storage device  112 , as well as a device  304  for generating an image signal  306  representing an image. Device  304 , which is connected, for example, via the readout contact to the three storage units  114 ,  116 ,  122 , is configured for reconstructing temporal image signal  306  using the three memory contents, also referred to as memory signals. 
     In such a case, it holds for all described exemplary embodiments that multiplexer signals  118 ,  120 ,  124  are not low-pass filtered. The low-pass filtering first occurs in storage devices  114 ,  116 ,  122  configured here as capacitors. Reconstruction is then carried out from the memory signals referred to here as capacitor signals. 
       FIG. 4  schematically depicts a device  304  in accordance with an exemplary embodiment, for instance a device, as previously described with reference to  FIG. 3 . Device  304  includes an input unit  410  for inputting three multiplexer signals  118 ,  120 ,  124 . A reconstruction unit  420  is designed for reconstructing image signal  306  from storage devices  114 ,  116 ,  122  using the three memory signals. 
     In accordance with this exemplary embodiment, input unit  410  is also designed for inputting processing signal  108  and for transmitting it to an output unit  430 . Output unit  430  is designed for outputting a control signal  432  in response to receiving processing signal  108 , in order to control the multiplexer unit. 
       FIG. 5  is a diagram for schematically illustrating three control signals  432 ,  500 ,  502  for controlling a multiplexer unit in accordance with an exemplary embodiment. The three control signals may be generated, for example, by a device previously described with reference to  FIGS. 3 and 4 . At every point in time, there is a signal weight for each memory node C of the storage device. 
       FIG. 6  is a diagram for schematically illustrating a low-pass filter characteristic  600 . Shown is the low-pass filter characteristic of a nominal sampling having an essentially triangular curve shape. 
       FIG. 7  is a diagram for schematically illustrating an exposure scheme for ideal temporal sampling by respective overlapping low-pass filters  700 ,  702 ,  704 ,  702 ,  704 ,  706 . 
       FIG. 8  is a diagram for schematically illustrating an image sensor performing a typical temporal sampling. Shown is a square wave signal  800  representing a low-pass filter in the time domain. 
       FIG. 9  shows a flow chart of an exemplary embodiment of a method  900  for sensing a light signal. Method  900  may be carried out, for example, using a pixel unit as previously described with reference to  FIG. 1 through 5 . In this case, the light signal is converted into the processing signal in a first step  910 . In a further step  920 , the first multiplexer signal and the second multiplexer signal deviating therefrom are generated using the processing signal. Finally, in a step  930 , a time-limited integral value of the first multiplexer signal is buffer stored in the first storage unit, and a time-limited integral value of the second multiplexer signal in the second storage unit. 
       FIG. 10  shows a flow diagram of a method  1000  for controlling a pixel unit in accordance with an exemplary embodiment, such as of a pixel unit, as previously described with reference to  FIG. 1 through 5 . 
     In this case, the processing signal is input in a first step  1010 . In a second step  1020 , the control signal for controlling the multiplexer unit of the pixel unit is output using the processing signal. 
       FIG. 11  shows a flow chart of a method  1100  for generating an image using a pixel unit in accordance with an exemplary embodiment, for instance a pixel unit, as previously described with reference to  FIG. 1 through 5 . Here, in a first step  1110 , the first multiplexer signal and the second multiplexer signal are input via an interface to the storage device, for instance via the readout contact. In a second step  1120 , the image signal representing the image is reconstructed using the time-limited integral values of the two multiplexer signals stored in the storage devices. 
     The approach presented here is described again in different words in the following with reference to  FIG. 1 through 11 . 
     Present-day image sensors typically function in accordance with the following principle. An optical system generates a light image on a planar sensor, also referred to as an image sensor or imager. The image sensor samples this image spatially and temporally. The temporal sampling is defined by the image frame rate, in some designs also by the individual light sensor sampling rate. 
     The customary camera components thereby fulfill the following functions. The optical system functions as a spatial low-pass filter. Today, alight sensor is composed of several million individual light sensors, each light sensor constituting a spatial sampling point. The exposure time functions as a temporal low-pass filter. 
     For an aliasing-free sampling, the low-pass filter should be adapted to the sampling frequency in a way that allows the sampling frequency to be at least greater than twice the cut-off frequency of the low-pass filter in accordance with the Nyquist-Shannon sampling theorem. It subsequently holds that the signal should always propagate through the low-pass filter prior to the sampling. 
     If this does not occur, the sampled signal contains aliasing artefacts. The spatial aliasing effects are usually discernible as a color fringe at high contrast edges, by disappearing and reappearing on point-like objects, and as what is generally referred to as a Moiré effect on objects having a uniform, periodic pattern. 
     In the temporal signal, aliasing effects appear as rolling shutter effects, where a moving, straight object is imaged as a curved object, as wagon wheel effects, where a wheel appears to rotate backwards starting after a certain speed, and as problems when capturing pulsed light sources, such as LED traffic signs or the taillights of vehicles. The human eye also perceives such effects, showing that temporal aliasing effects cannot be completely prevented in this case either. 
     For applications in the machine vision sector, the optical flow is an important auxiliary quantity. The optical flow quality may be appreciably enhanced by a temporally correctly sampled signal. 
     The method of functioning of present-day image sensors is based on a relatively short exposure time in comparison to the resulting image frame rate. The exposure time acts as a low-pass filter, which, in an idealized consideration, may be regarded as a square wave in the time domain. 
     In the frequency domain, the square wave low-pass filter features width t exp  as the Si function: 
     
       
         
           
             
               
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and, at the same time, represents a normal approximation of the cut-off frequency of the low-pass filter.
 
     If customary exposure times of t exp ≈500 μs having customary image refresh rates or sampling frequencies of t frame ≈1/30≈33 ms are compared, then the sampling frequency f sample  and the cutoff frequency of low-pass filter f Nyq . are obtained: 
     
       
         
           
             
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       FIG. 8  shows the currently customary sampling scheme. Thus, the Nyquist theorem is clearly violated. This is not a problem as long as no high frequencies occur, as in the natural world, such as when clouds slowly pass by or in the case of photography. 
     In the video-based driver assistance sector, rapid scene changes are particularly important, especially in standard application cases, such as, for example, when computing the optical flow on pedestrian extremities, or when detecting LED lights, such as brake lights or traffic signs. For that reason, the light signal should also be temporally correctly sampled. 
     For a temporally correct sampling, 
                 f     Nyq   .         f   sample       ≤     1   2           
should be fulfilled.
 
     From this, however, it is also derived that: 
     
       
         
           
             
               
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     This means that the exposure time for a correct sampling should last twice as long as a customary frame. In addition, an improved low-pass filter requires a temporally adapted signal modelling.  FIGS. 6 and 7  show the Nyquist-conform sampling scheme. 
     In the Nyquist-conform case, the actual frame rate is dependent on the width of the low-pass filter. In the case of customary image sensors, an integration time longer than 1 ms during the day is difficult due to boundary conditions, such as SNR, or a design suited for nighttime use is hardly possible. As a result, the frame rate also increases in the case of a temporally correct sampling. However, this may again be reduced to a lower frame rate by a downstream image processing, in order to obtain the data rate. Since the output signal does not contain any aliasing effects, at a reduced frame rate, the final signal also does not contain any aliasing in the case of correct downsampling. 
     Depending on the low-pass design, the individual frames should, therefore, be weighted and exposed in an overlapping manner. This problem is already known as standard sampling in the context of image sensors in spatial sampling. While, spatially, an unsharpness of the optical system ensures that the signal is low-pass filtered beyond the pixel dimension, a corresponding counterpart for the time dimension is missing in the methods known heretofore. 
     On the other hand, the approach described here provides a pixel design option for realizing a temporal low-pass filter for a temporally correct sampling. 
     In the pixel design sector, it should be ensured that the signal is subject to an analog low-pass filtering before it is temporally quantized. In this case, photodiode  104  is used as a photon-to-electron converter. The output signal of the photodiode, also referred to as processing signal  108 , should now be transmitted through an electrical-temporal low-pass filter. Appropriate implementation options are described in the following. 
     For each photodiode unit  104 , two or more storage units are required for the overlapping of the temporal sampling. The storage units are realized as capacitors, for instance. While the integration in the first frame is not yet concluded, the integration of the second frame begins already. When the first frame is concluded, the integration of the third frame begins. Since the first frame also requires a finite read-out time, at least three storage units, for example, are connected to multiplexer unit  110 . Generally, it holds that: if the low-pass filter is supposed to be wider than [have a wider bandwidth that of] the image frame rate, more storage units are needed in correspondence with the overlappings. 
     If the post-processing is complex enough, light signal  306  may be correctly reconstructed from the signals of the three storage units  114 ,  116 ,  122  at a desired instant and also output at the target frame rate subsequently to further low-pass filtering. 
     Moreover, it is possible here to compute and output the average variation in the signal between two points in time. This is also referred to as the time-gradient image. 
     From the individual exposures of the images, it is also possible to compute the extent of the time variance of the signal, also referred to as the feature mask. In this case, the reconstructed pixels are marked using a threshold value function. For example, if the signal of a sensor element varies within a sampling period by more than the threshold value, for example by more than 25 percent, then the image region in question is marked by a marker bit as fluctuating. This may be helpful for detecting artificial light sources. The threshold value may be adaptable. 
     Pixel unit  102  has a basic design having the following sequence: photodiode  104 , optional additional storage unit  200 , multiplexer unit  110 , and memory capacitors  114 ,  116 ,  122 . Functionally, multiplexer unit  110  ensures the temporal low-pass characteristics.  FIG. 5  illustrates possible control signals that act upon multiplexer unit  110  in order to fulfill this task. For each point in time, multiplexer unit  110  provides a signal weight that corresponds to the low-pass characteristic. 
     This weight is applied to photodiode signal  108  or to the signal of additional storage unit  200 . 
     Technically, multiplexer unit  110  is used, for example, as a continuous divider. To this end, processing signal  108  is realized as current and continuously fed to multiplexer unit  110 . Depending on the point in time, it generates currents of different intensities for the individual storage units. Thus, multiplexer unit  110  is a complex circuit, which applies temporally distinct amplification factors to the photo stream and thus loads the storage units with varying intensity in accordance with the current points in time. Thus, relative to the sampling instant thereof, the storage units receive a temporally low-pass filtered signal at the end. This is important for the overall functioning method. 
     The described continuous process may also be realized as a temporally discrete process. In this regard, photodiode  104  is read out repeatedly during the low-pass time span, for example several hundred times. The result signal in the form of buffer-stored processing signal  108  is now present as an accumulated number of electrons, either in the junction capacitance or in additional storage unit  200 . In multiplexer unit  110 , these electrons are fed to the storage units in accordance with the actual weight. 
     The just described signal divider may also be transformed into the time domain. To that end, photodiode  104  is again read out several hundred times in the low-pass time span. However, the duration of an individual read-out pulse is now adapted to conform to the actual weight of the low-pass filter. Thus, multiplexer unit  110  feeds the signal generated per pulse to only one of the three memory nodes  114 ,  116 ,  122 . This is useful when it is only possible to collect a few electrons in certain illumination situations. 
     In classic semiconductor processes, circuits are realized side by side in a planar 2D design. However, today&#39;s semiconductor processes have the capability of stacking and connecting a plurality of circuits in 3D arrays. This is also referred to as stacked die technology. 
     Complex hardware structures are needed for the case of temporally correct sampling, as mentioned above. Typically, a 2D design does not allow this to be realized cost-efficiently. However, this obstacle may be overcome using stacked die processes. 
     In this regard, the units for the photon-to-electron conversion are left in an upper layer  300 , while multiplexer unit  110  and units for implementing further post-processing steps, for instance, to form gradients or to reconstruct signals at the frame instant, are displaced to a lower layer  302 . Depending on the exemplary embodiment, a different fabrication technology may be used to also establish second layer  302  as first layer  300 , for instance in 14 or 7 nm, thereby allowing appreciably more complex standard signal conditioning steps.  FIG. 3  is a schematic view of this type of design of pixel unit  102 . 
     If an exemplary embodiment includes an “AND/OR” logic operation between a first feature and a second feature, then this is to be read as the exemplary embodiment in accordance with a specific embodiment having both the first feature, as well as the second feature and, in accordance with another specific embodiment, either only the first feature or only the second feature.