Abstract:
An apparatus for producing an image of an object which is illuminated by a pulse of artificial light and by ambient light. That apparatus has a two dimensional array of photodetector elements each of which includes a semiconductor device which responds to impingement of light by producing an electrical signal. A mechanism is provided to remove the effects of the ambient light from the electrical signal to produce a resultant signal at an output terminal that corresponds only to the magnitude of the artificial light striking the semiconductor device. This apparatus is immune from the semiconductor device becoming saturated by intense ambient illumination and can be configured to detect high speed objects.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention relates to apparatus for producing a two-dimensional image, and in particular to circuits for such apparatus which reduce effects produced by very intense ambient light.  
           [0005]    2. Description of the Related Art  
           [0006]    Electronic vision systems are utilized in a variety of applications to automatically inspect objects and control one or more functions in response to the analysis of the resultant image. Such systems have been proposed use in automobiles to produce an image of the front seat in the vehicle to determine whether a person is present, and the position and size of such a person. That information derived from an image can be employed to control the deployment of an air bag in the event of an accident. For example, if an infant or a small child is found on the seat, deployment of the air bag may be inhibited to prevent injury of the occupant. Whereas, when an adult person is identified on the seat, the air bags can be deployed normally during an accident. The rate at which an air bag deploys for an adult also can be regulated depending upon the size of that individual. Air bag deployment may also be inhibited when the seat is found to be unoccupied or when an object, such as a bag of groceries, is found to be present.  
           [0007]    The use of such imaging systems in an automobile and elsewhere is greatly affected by the level of light that illuminates the object being imaged. For example, very intense illumination produced by direct sunlight may cause saturation of the semiconductor optical detectors commonly used in imaging equipment. The detector saturation adversely affects image resolution and the ability to image fine details of the object being viewed. In the automotive application described previously, the fine details are required to analyze image depth, which is often needed in the control of air bags.  
           [0008]    Therefore, it is desirable to reduce the adverse effects produced by bright ambient illumination saturating the detectors of an imager array.  
         SUMMARY OF THE INVENTION  
         [0009]    An apparatus produces an image of an object which is illuminated by a pulse of artificial light. That apparatus comprises a two dimensional array of photodetector elements, each of which includes a semiconductor device that responds to impingement of light by producing an electrical signal indicating the magnitude of that light. The electrical signal having a first component representing the amount of the artificial light and a second component representing the amount of ambient light. A circuit is connected to the semiconductor device to remove the second component from the electrical signal to produce a resultant signal at an output terminal that corresponds the magnitude of the artificial light which strikes the semiconductor device.  
           [0010]    In one embodiment of the present apparatus, the mechanism that removes the second component from the electrical signal comprises a first storage capacitor and a second storage capacitor. A first transistor couples the semiconductor device to the first storage capacitor and a second transistor couples the semiconductor device to the second storage capacitor. A differential amplifier has one input connected to the first storage capacitor, another input connected to second storage capacitor, and the output terminal.  
           [0011]    In another embodiment, the circuit which removes the second component from the electrical signal comprises a capacitor and an amplifier connected in series between the semiconductor device and the output terminal.  
           [0012]    In both of those embodiments that apparatus preferably has a plurality of first conductors extending along one dimension of the two-dimensional array, and a plurality of second conductors extending along another dimension of the two-dimensional array. Each photodetector element further includes an output selection transistor coupling its output terminal to one of the plurality of second conductors in response to a control signal on one of the plurality of first conductors. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram illustrating an imaging system used in a motor vehicle;  
         [0014]    [0014]FIG. 2 is depicts the matrix of photodetector elements in an imager of the imaging system;  
         [0015]    [0015]FIG. 3 is a schematic diagram of the electrical circuit for one of the photodetector elements;  
         [0016]    [0016]FIG. 4 graphically depicts signal waveforms at different points of the electrical circuit; and  
         [0017]    [0017]FIG. 5 is a schematic diagram of an alternative embodiment of the electrical circuit a detector element. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    With initial reference to FIG. 1, an imaging system is employed to produce a two-dimensional image of the area of seat  12  within a motor vehicle. The imaging system  10  comprises a camera  14  which incorporates an imager  15  formed by a plurality of photodetector elements, each comprising a semiconductor device which produces an electrical current in response to the intensity of light striking the device.  
         [0019]    Referring to FIG. 2, in order to form an image, the photodetector elements  20  of the imager  15  are arranged in rows and columns of a two-dimensional array and are interconnected by conductors  22  and  24 , respectively for each row and column. The signal produced by an individual photodetector element can be selected by applying a control signal to the conductor  22  for the associated row and observing the signal that the detector applies to the conductor  24  for the associated column. This is a common technique for reading the image data from a two-dimensional array of photodetector elements.  
         [0020]    The operation of the camera  14  is governed by a control circuit  16  which also provides a signal to a strobe circuit  18  which activates a light source  19  that illuminates the seating area. As will be described, the illumination of the object by the light source  19  is synchronized with the image acquisition. The light source  19  may emit infrared light to which the image detector  15  is sensitive. In that way, operation of the imaging system  10  does interfere with the ability of a driver located on seat  12  to operate the motor vehicle.  
         [0021]    Each photodetector element  20  within the imager  15  comprises the circuit  30  shown in FIG. 3. That circuit  30  comprises a semiconductor photodiode  32  which produces an electrical signal that varies in response to the intensity of light striking the photodiode. The photodiode  32  is connected to the input of an amplifier  34  which increases the electrical signal. The input of the amplifier  34  also is connected to a source of negative voltage (Vdd) by a reset transistor  36  which when conductive clears accumulated charge on the photodetector element  20 .  
         [0022]    The output of the amplifier  34  is connected by a first transistor switch  38  to a first storage capacitor  40  connected between ground and the inverting input of a differential amplifier  42 . The output of the first amplifier  34  also is connected by a second transistor switch  44  to a second storage capacitor  46  that is connected between ground and the non-inverting input of the differential amplifier  42 . As will be described, the first storage capacitor  40  stores a charge which corresponds to the level of ambient light striking the photodiode  32  and the second storage capacitor  46  holds a charge corresponding to the output of the photodiode  32  when the light source  19  is strobed. The differential amplifier  42  produces an output signal corresponding to the difference between those charges.  
         [0023]    An output selection transistor  48  couples the output of differential amplifier  42  to the imager&#39;s column conductor  24  associated with the particular photodetector element  20 . The control electrode of the output selection transistor  48  is connected to the associated row conductor  22  of the imager  15 . When the output selection transistor  48  is rendered conductive by an active signal on the row conductor  22 , the output signal for the detector circuit  30 , produced by the differential amplifier  42 , is applied to the associated column conductor  24 . As is conventional practice, the image produced by the imager is read-out by sequentially activating each row conductor  22 , and selectively reading the signals on each of the column conductors  24 . All the photodetector elements  20  are connected to a common reset bus, not shown, to reset the elements between image acquisition periods. As will be described, each photodetector element  20  also is connected to a Strobe Sample bus and an Ambient Sample bus, which carry signals that control image acquisition.  
         [0024]    With reference to FIGS. 3 and 4, all the detector circuits  30  in the imager  15  operate simultaneously during the image acquisition phase. In that phase, a first low level pulse  50  is applied to the reset bus. This low level turns off the reset transistor  36  allowing any output produced by the photodiode  32  to be picked up by the amplifier  34 , thus producing a voltage level at node  35  which corresponds to the intensity of the light striking the photodetector element. During the first pulse  50  on the reset bus, a high logic level pulse  52  occurs on the ambient sample signal bus that causes the first transistor switch  38  to become conductive. This applies the voltage level at node  35  across the first storage capacitor  40  which charges to a corresponding level. The ambient sample pulse  52  occurs a given amount of time after the commencement of the first low level reset bus pulse  50  in order to allow the signal produced by the photodiode  32  to propagate through the amplifier  34  and produce a valid output at node  35  before sampling by the first storage capacitor  40 . Following termination of the ambient sample pulse  52 , the reset bus signal again goes high at region  54  which resets the detector circuit  30 . Thus the charge stored on the first storage capacitor  40  corresponds to the level of ambient light striking the photodiode  32 .  
         [0025]    Thereafter, a second low level pulse  56  is applied to the reset bus, and simultaneously, the strobe circuit  18  is triggered which activates the light source  19  producing a pulse of light  58  that illuminates the seat  12 . During this light pulse and the second reset bus pulse  56 , a brief high level pulse  59  is applied to the strobe sample bus which turns on the second transistor switch  44 . This applies the amplified output of the photodiode  32  across the second storage capacitor  46 , thereby producing a charge on that capacitor that corresponds to the level of light striking the photodiode  32 . During this second reset bus pulse  56 , the first transistor switch  38  is non-conductive so that the output of the photodiode at node  35  does not affect the previously stored charge.  
         [0026]    At this time, the charge of the first capacitor  40  corresponds to the amount of ambient light striking the photodiode  32  and the second storage capacitor  46  holds a charge corresponding to the level of light received by the detector during the strobe pulse  58 . The differential amplifier  42  produces an output signal that represents that difference between the voltage across the first storage capacitor  40  and the voltage across the second storage capacitor  46 . That output signal corresponds to the strobe light from the object being imaged which is received by the photodiode  32 . In other words, the ambient light level has been subtracted from the amount of light received during excitation of strobe light source  19  to produce the output signal. Therefore, the effects due to very bright ambient light conditions which saturate the photodiode  32  are removed when producing an image of the seat  12 .  
         [0027]    The operation of the imager can be enhanced so that the amount of charge transferred to the first storage capacitor  40  is varied by altering the timing between the falling edge of the first low level reset pulse  50  and the ambient sample pulse  52 . Thus, if the ambient light intensity is sufficiently great to cause saturate the signal at node  35 , then a shorter time between these pulses causes less charge to be transferred and produces a reduced output signal. Conversely, when the ambient light intensity is relatively low, a greater output signal can be produced by extending the time between the falling edge of the first low level reset pulse  50  and the ambient sample pulse  52 . A similar adjustment of the time between the falling edge of the second low level reset pulse  56  and the strobe sample pulse  59  also can be performed.  
         [0028]    [0028]FIG. 5 illustrates an alternative embodiment of the present invention. This version takes advantage of the high frequency content of the output signal from the photodetector that results from the relatively short strobe light pulse. In this circuit  60 , the photodiode  62  is connected between ground and node  64 . The circuit is reset by a reset transistor  66  that between acquisition of images connects node  64  to a source Vdd of negative voltage. The node  64  is coupled by a storage capacitor  70  to the input of an amplifier  68 . The output of the amplifier  68  is connected by a detector selection transistor  72  to the column conductor  24  associated with this photodetector element in the imager, when the detector selection transistor is rendered conductive by an active signal level on the associated row conductor  22 .  
         [0029]    The signal produced by the photodiode  62  has a relatively low frequency component due to the steady state character of the ambient light, such as sunlight or interior vehicle lighting, and that component is blocked by the capacitor  70 . Other low frequency components resulting from dark current Sand certain types of noise also are blocked. However, the photodiode  62  output signal has a relatively high frequency component due to the short strobe pulsation of light source  19 . This high frequency component passes through the capacitor  70  to the amplifier  68 . Thus, when the detector circuit  60  is selected for read-out by an active signal on the row conductor  22 , the signal applied to the associated column conductor  24  corresponds only to the response of the photodiode  62  to the strobe light and not to the ambient light.  
         [0030]    It should be noted that this photodetector element design also is useful to detect objects moving at high speed. The difference output without strobe illumination results in a black (except for noise) output when the camera views objects that are not in motion. However, when objects are moving fast enough (defined by the optics chosen) to be imaged on different photodetector elements  20  between the ambient and strobe sample pulses, the moving object will be shown twice on the image frame: a negative image first, then a positive image. The dual storage elements allow the capture of very high speed moving objects such as bullets while suppressing background clutter. The dual image has the added feature of providing two dimensional trajectory information.