Abstract:
A 3D camera for determining distances to regions in a scene comprising: a photosurface having a plurality of pixels each of which comprises a circuit having a light sensitive element that provides a current responsive to light incident thereon, wherein the circuit comprises, at least one amplifier inside the pixel, having an input and an output; at least one feedback capacitor separate from the light sensitive element and connected between the input and output of each of the at least one amplifier; at least one controllable connection through which current flows from the light sensitive element into the input of the at least one amplifier; a light source; and, a controller that, controls the light source to illuminate the scene with light, opens and closes the at least one controllable connection to gate or modulate current from the light sensitive element of a pixel in the photosurface responsive to the time dependence of the gating or modulation of the light, controls the at least one controllable connection to provide a current for correcting biases caused by at least one of background light or dark current, and determines a distance to a region imaged on the pixel responsive to an amount of charge integrated on the feedback capacitor responsive to the gated or modulated current and the corrected biases.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. application Ser. No. 09/806,252 which is a U.S. National Phase filing of PCT application PCT/IL98/00476, the disclosures of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to cameras that provide measurements of distances to objects and parts of objects that they image and in particular to integrating functions of such cameras on a single chip.  
       BACKGROUND OF THE INVENTION  
       [0003]     Three dimensional optical imaging systems, hereinafter referred to as “3D cameras”, that are capable of providing distance measurements to objects and points on objects that they image, are used for many different applications. Among these applications are profile inspection of manufactured goods, CAD verification, robot vision, geographic surveying and imaging objects selectively as a function of distance.  
         [0004]     Some 3D cameras provide simultaneous measurements to substantially all points of objects in a scene they image. Generally, these 3D cameras comprise a light source, such as a laser, which is pulsed or shuttered so that it provides pulses of light for illuminating a scene being imaged and a gated imaging system for imaging light from the light pulses that is reflected from objects in the scene. The gated imaging system comprises a camera having a photosensitive surface, hereinafter referred to as a “photosurface”, such as a CCD camera, and a gating means for gating the camera open and closed, such as an optical shutter or a gated image intensifier. The reflected light is registered on pixels of the photosurface of the camera only if it reaches the camera when the camera is gated open.  
         [0005]     To image a scene and determine distances from the camera to objects in the scene, the scene is generally illuminated with a train of light pulses radiated from the light source. For each radiated light pulse in the train, following an accurately determined delay from the time that the light pulse is radiated, the camera is gated open for a period of time hereinafter referred to as a “gate”. Light from the light pulse that is reflected from an object in the scene is imaged on the photosurface of the camera if it reaches the camera during the gate. Since the time elapsed between radiating a light pulse and the gate that follows it is known, the time it took imaged light to travel from the light source to the reflecting object in the scene and back to the camera is known. The time elapsed is used to determine the distance to the object.  
         [0006]     In some of these 3D cameras, only the timing between light pulses and gates is used to determine the distance from the 3D camera to a point in the scene imaged on a pixel of the photosurface of the 3D camera. In others, the amount of light registered by the pixel during the time that the camera is gated open is also used to determine the distance. The accuracy of measurements made with these 3D cameras is a function of the rise and fall times and jitter of the light pulses and their flatness, how fast the gating means can gate the camera open and closed.  
         [0007]     A 3D camera using a pulsed source of illumination and a gated imaging system is described in “Design and Development of a Multi-detecting two Dimensional Ranging Sensor”, Measurement Science and Technology  6  (September 1995), pages 1301-1308, by S. Christie, et al, and in “Range-gated Imaging for Near Field Target Identification”, Yates et al, SPIE Vol. 2869, p 374-385 which are herein incorporated by reference.  
         [0008]     Another 3D camera is described in U.S. Pat. No. 5,081,530 to Medina, which is incorporated herein by reference. A 3D camera described in this patent registers energy in a pulse of light reflected from a target that reaches the camera&#39;s imaging system during each gate of a pair of gates. Distance to a target is determined from the ratio of the difference between the amounts of energy registered during each of the two gates to the sum of the amounts of energy registered during each of the two gates.  
         [0009]     A variation of a gated 3D camera is described in U.S. Pat. No. 4,935,616 to Scott, which is incorporated herein by reference. In this patent, a 3D camera is described in which a light source and imaging system, instead of being fully gated, are “modulated”. In a preferred embodiment of the invention, the light source comprises a CW laser. The intensity of light radiated by the laser is modulated so that the intensity has an harmonically varying component. The sensitivity of the camera&#39;s imaging system to light is also harmonically modulated. When a target that is illuminated by the modulated laser light reflects some of the incident laser light, the reflected light has the same modulation as the laser light. However, modulation of the reflected light that reaches the imaging system from the target has a phase difference with respect to the modulation of the imaging system that depends upon the distance of the target from the camera. The intensity that the camera registers for the reflected light is a function of this phase difference. The registered intensity is used to determine the phase difference and thereby the distance of the target from the camera.  
         [0010]     Other “gated” 3D cameras and examples of their uses are found in PCT Publications WO97/01111, WO97/01112, and WO97/01113 which are incorporated herein by reference.  
         [0011]     An optical shutter suitable for use in 3D cameras is described in PCT patent application PCT/IL98/00060, by some of the same applicants as the applicants of the present application, the disclosure of which is incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0012]     Some aspects of preferred embodiments of the present invention relate to providing an improved 3D camera wherein gating or modulating apparatus for the 3D camera is incorporated on a photosurface of the camera on which light detectors of the camera are also situated.  
         [0013]     In accordance with one aspect of some preferred embodiments of the present invention, each pixel in the photosurface includes its own pixel circuit for gating the pixel on or off or for modulating the sensitivity of the pixel to incident light.  
         [0014]     In some preferred embodiments of the present invention the same pixel circuit functions to gate the pixel on or off and to modulate the sensitivity of the pixel to incident light.  
         [0015]     In some preferred embodiments of the present invention each pixel is gated on or off or modulated independently of other pixels. In other preferred embodiments of the present invention pixels on the photosurface are grouped into different pixel groups. The pixels belonging to a same pixel group are gated on or off or modulated substantially simultaneously. Pixel groups are gated on and off or modulated in different combinations and time sequences.  
         [0016]     In some preferred embodiments of the present invention, pixels in different pixel groups are located in different regions of the photosurface. In some preferred embodiments of the present invention, the different regions are different parallel bands of pixels of the photosurface. In some preferred embodiments of the present invention, the different regions are substantially equal area regions of the photosurface.  
         [0017]     Some aspects of preferred embodiments of the present invention relate to providing a photosurface that images a scene and provides measurements of distances to objects in the scene in a single frame.  
         [0018]     Some aspects of preferred embodiments of the present invention relate to providing a photosurface that comprises pixels having outputs that are automatically corrected for biases and noise resulting from background light and dark current from a light sensitive element of the pixel.  
         [0019]     In accordance with another aspect of some preferred embodiments of the present invention, a photosurface is provided comprising pixels, wherein each pixel includes a photodiode or other, preferably linear, light sensitive current source such as a photoresistor, or photogate, a charge accumulator, hereinafter referred to as, but not necessarily limited to an “integrator” and a variable connection. The photodiode is connected to the integration circuit via the variable connection. Preferably, the integrator comprises an amplifier, preferably, an operational amplifier with capacitive feedback.  
         [0020]     In some preferred embodiments of the present invention the variable connection is a switch controllable to be either open or closed. When the photodiode is exposed to light to which it is sensitive and the pixel control switch is closed, a current flows into the integrator from the photodiode that is substantially proportional to the intensity of light incident on the photodiode. A charge, hereinafter referred to as a “photocharge”, is accumulated by an integrator. The amount of photocharge accumulated is proportional to the integral over time of the intensity of light to which the photodiode is exposed during the time that the pixel control switch is closed. The integrated photocharge is used as a measure of the pixel response to the light to which it is exposed. The switch is said to gate the pixel on when the switch is closed and to gate the pixel off when the switch is open. The switch is hereinafter referred to as a “gate switch”.  
         [0021]     In some preferred embodiments of the present invention the variable connection operates to modulate the sensitivity of the pixel to incident light. In these preferred embodiments, the modulator is controllable, using methods known in the art, so that the magnitude of the resistance between the photodiode and the integrator across the modulator can be set to values within some range of values. When light is incident on the photodiode, the magnitude of photocurrent flowing between the photodiode and the storage capacitor is a function not only of the intensity of the incident light but also of the value of the modulator resistance. By controlling the value of the modulator resistance the amount of photocharge integrated by the integrator in a given period of time for a given intensity of incident light, and thereby the sensitivity of the pixel to incident light, is controlled or modulated. When operating in a modulating mode the variable connection is referred to as a “modulator”.  
         [0022]     In some preferred embodiments of the present invention the modulator modulates pixels so that pixel sensitivities vary harmonically. In some preferred embodiments of the present invention all pixels in a photosurface are modulated harmonically with a same frequency of modulation. In other preferred embodiments of the present invention different groups of pixels in a photosurface are modulated harmonically with different frequencies.  
         [0023]     In some preferred embodiments of the present invention a variable connection is controllable to function only as a gate switch. In other preferred embodiments of the present invention it is controllable to function only as a modulator. In still other preferred embodiments of the present invention, it may be controllable to function as either a gate switch or a modulator. The pixel control switch is appropriately connected using methods known in the art, via a control line, to a controller that transmits control signals to operate the pixel control switch as a gating switch or as a modulator.  
         [0024]     Gate switches and modulators of pixels in a photosurface can be controlled, in accordance with preferred embodiments of the present invention, to gate or modulate different combinations of pixels and to gate pixels and groups of pixels with different timing sequences. Similarly, pixel storage capacitors can be addressed and “read” in different combinations and in different timing sequences.  
         [0025]     Preferably, the pixels are packed on the photosensitive surface with a pitch less than 50 microns. More preferably the pixels are packed with a pitch less than 30 microns. Preferably, the photosurface is produced using CMOS technology and the pixel control switch is a FET or MOSFET. Using CMOS technology, light sensitive photosurfaces comprising arrays of pixels suitable for visual imaging can be produced, wherein each pixel of the photosurface contains a light sensitive component such as a photo-diode and electronic switching, control and logic elements. For example, U.S. Pat. No. 5,345,266 describes a pixel comprising a photodiode and a transistor. Peter Denyer in a talk given at the 1996 SSCTC Workshop On CMOS Imaging Technology, Feb. 7, 1996, described a pixel comprising electronic elements that is on the order of 12 microns on a side and in which the photodiode occupies 60% the pixel area.  
         [0026]     There is thus provided, in accordance with a preferred embodiment of the invention, a photosurface comprising a plurality of light sensitive pixels, wherein each pixel of the plurality of pixels comprises an electronic circuit, each of the circuits comprising:  
         [0027]     a single light sensitive element that provides a current responsive to light incident thereon;  
         [0028]     at least one charge accumulator separate from the light sensitive element; and  
         [0029]     at least one variable connection through which current flows from the light sensitive element into the integrator.  
         [0030]     Preferably, the charge is accumulated on a capacitor. Preferably, the at least one charge accumulator comprises at least one amplifier, having an input and an output, the at least one capacitor being connected as a feedback capacitor of the amplifier, and wherein the at least one variable connection connects the light sensitive element to the input of the amplifier. Preferably, the amplifier is an operational amplifier.  
         [0031]     Preferably the photosurface comprises at least one data bus and wherein the circuit comprises at least one address switch, which connects a data bus to an output of one of the at least one amplifiers, either directly or via another switch.  
         [0032]     Preferably, the at least one variable connection comprises at least one gate switch. Preferably, the at least one capacitor comprises a single capacitor and the at least one gate switch comprises a single gate switch.  
         [0033]     In preferred embodiments of the invention, such photosurfaces are used in 3D cameras. Preferably such cameras comprise a controller that gates each pixel in the photo surface on and off by controlling the gate switch associated with the capacitor to be closed or open. Preferably, the camera comprises a light source that radiates a plurality of light pulses, having a pulse width, that illuminate objects in the scene, wherein the controller gates pixels in the photosurface on or off at times coordinated with times at which light pulses of the plurality of light pulses are radiated.  
         [0034]     In a preferred embodiment of the invention, the at least one capacitor comprises first and second capacitors connected as feedback capacitors respectively to first and second amplifiers to form first and second integrators; and the at least one gate switch comprises first and second gate switches, the first gate switch connecting the light sensitive element to the input of the first amplifier and the second gate switch connecting the light sensitive element to the input of the second amplifier. Preferably, the at least one address switch comprises first and second address switches, the first address switch connecting the output of the first amplifier to the data bus and the second address switch connecting the output of the second differential amplifier to the data bus.  
         [0035]     Alternatively, the photosurface comprises a differential amplifier having positive and negative inputs and an output, wherein the output of the first differential amplifier is connected to the positive input of the differential amplifier, the output of the second differential amplifier is connected to the negative input of the differential amplifier and wherein the output of the differential amplifier is connected by the at least one address switch to the data bus.  
         [0036]     In preferred embodiments of the invention, these photosurfaces are used in 3D cameras.  
         [0037]     Preferably, the 3D camera comprises a controller that gates pixels in the photo surface on and off by controlling at least one of the first and second gate switches of the circuits of the pixels to be closed or open. Preferably, the 3D camera comprises a light source that radiates a plurality of light pulses that illuminate objects in the scene, the light pulses having a pulse width, wherein the controller gates pixels in the photosurface on or off at times responsive to times at which light pulses of the plurality of light pulses are radiated.  
         [0038]     In a preferred embodiment of the invention, the controller is operative to:  
         [0039]     gate pixels on for a first gate period after a first time lapse following each radiated light pulse of a first plurality of radiated light pulses such that current from the light sensitive element is integrated by the first integrator; and  
         [0040]     gate pixels on for a second gate period after a second time lapse following each radiated light pulse of a second plurality of radiated light pulses such that current from the light sensitive element is integrated by the second integrator.  
         [0041]     Preferably, the mid points of first and second gate periods are delayed with respect to the radiated light pulses that they respectively follow by the same amount of time. Preferably, the duration of the first gate period is substantially equal to the pulse width of the radiated light pulses. Preferably, the duration of the second gate is greater than or equal to three times the pulse width.  
         [0042]     Alternatively, in a preferred embodiment of the invention the controller is operative to:  
         [0043]     gate pixels on for a first gate period after a first time lapse following each radiated light pulse of the plurality of radiated light pulses such that current from the light sensitive element is integrated by the first integrator; and  
         [0044]     gate pixels on for a second gate period after a second time lapse following each radiated light pulse of the plurality of the plurality of radiated light pulses such that current from the light sensitive element is integrated by the second integrator.  
         [0045]     Preferably, the first time lapse is such that light reflected from the object reaches the light sensitive element during the first gate period, such that current therefrom responsive to background light, light reflected from the radiated light pulse by objects in the scene plus dark current is integrated on the first integrator.  
         [0046]     Preferably, the second time lapse is such that light reflected from the object does not reach the light sensitive element during the second gate period, such that current therefrom responsive to background light plus dark current is integrated on the second integrator.  
         [0047]     In a preferred embodiment of the invention, the at least one capacitor is connected to the at least one amplifier by a plurality of switches such that:  
         [0048]     for a first combination of open and closed switches a first terminal of the at least one capacitor is connected to the input of the amplifier and a second terminal of the at least one capacitor is connected to the output of the amplifier; and  
         [0049]     for a second combination of open and closed switches the first terminal of the at least one capacitor is connected to the output of the amplifier and the second terminal of the at least one capacitor is connected to the input of the amplifier.  
         [0050]     In preferred embodiments of the invention, the above photosurfaces are used in 3D cameras.  
         [0051]     In a preferred embodiment of the invention, the 3D camera comprises a controller that gates pixels in the photo surface on and off by controlling the at least one gate switch in the circuits of the pixels to be closed or open. Preferably, the 3D camera comprises a light source that radiates a plurality of light pulses having a pulse width that illuminate objects in the scene and wherein the controller gates pixels in the photosurface on or off at times responsive to times at which light pulses of the plurality of light pulses are radiated.  
         [0052]     Preferably, the controller gates pixels on for a first and second gate periods following each light pulse in the plurality of light pulses and wherein during the first gate period current in the light sensitive element is responsive to background light and light of the radiated light pulse reflected from the objects in the scene plus dark current is integrated on the capacitor and increases voltage across the capacitor and wherein during the second gate current responsive to background light plus dark current is integrated on the capacitor and decreases voltage across the capacitor.  
         [0053]     Preferably, the duration of the first gate and the duration of the second gate are controlled to be equal to a high degree of accuracy.  
         [0054]     Preferably, the duration of the first and second gates is substantially equal to the pulse width of the radiated light pulses.  
         [0055]     Preferably, the pixel circuit of the photosurface comprises a reset switch connected to the light sensitive element and wherein when the reset switch is closed, voltage across the light sensitive element is set to a predetermined magnitude. Preferably, the controller controls the reset switch and wherein before the controller gates a pixel on the controller closes and opens the reset switch of the pixel at least once.  
         [0056]     In a preferred embodiment of the invention, the at least one variable connection comprises at least one modulator. Preferably, the at least one modulator comprises one modulator and wherein the at least one capacitor comprises one capacitor. Preferably, the at least one modulator is controllable to modulate the current from the light sensitive element harmonically. Alternatively, the at least one modulator is controllable to modulate the current from the light sensitive element pseudo randomly.  
         [0057]     In preferred embodiments of the invention, these photosurface are used in a 3D camera.  
         [0058]     preferably, the 3D camera comprises a controller that controls modulators in the pixels of the photosurface to modulate currents from the light sensitive elements of the pixels. preferably, the modulators modulate the currents harmonically. In one preferred embodiment of the invention, different pixels of the photosurface are modulated at different frequencies of modulation.  
         [0059]     In a preferred embodiment of the invention, the 3D camera comprises a light source that radiates a beam of light having an harmonically modulated intensity and wherein the controller controls the modulators to modulate the currents harmonically so that the beam of light and the currents are modulated harmonically at the same frequency of modulation and in phase.  
         [0060]     In a preferred embodiment of the invention, the controller controls each pixel of the pixels in the photosurface independently of other pixels in the photosurface. In an alternative preferred embodiment of the invention, pixels in the photosurface are grouped into different pixel groups and pixels in a same pixel group are controlled by the controller simultaneously and wherein each pixel group is controlled independently of other pixel groups.  
         [0061]     There is further provided, in accordance with a preferred embodiment of the invention, a method of removing the effects of background and dark current from a signal generated from a gated reflection of a pulsed source of light reflected from an object, the method comprising;  
         [0062]     generating a value based on gating a reflection of a pulsed source of light reflected from an object;  
         [0063]     generating a second value based on gating when no reflected light is present; and  
         [0064]     subtracting the values to form a corrected values.  
         [0065]     Preferably, gating of the reflection of the pulsed source is so timed and of such a duration that only a portion of the light from the source reflected from the object is utilized in generating the value.  
         [0066]     Alternatively or additionally, gating of the reflection of the pulsed source is so timed and of such a duration that all of the light from the source reflected from the object is utilized in generating the value, such that the value is a normalizing value.  
         [0067]     There is further provided, in accordance with a preferred embodiment of the invention, a method of removing the effects of background and dark current from a signal generated from a gated reflection of a pulsed source of light reflected from an object and normalizing the signal, the method comprising;  
         [0068]     providing a value in accordance with preferred method described above;  
         [0069]     providing a normalizing value generated in accordance with the above method; and 
        normalizing the value utilizing the normalizing value.        
 
         [0071]     The invention will be more clearly understood by reference to the following description of preferred embodiments thereof read in conjunction with the figures attached hereto. In the figures identical structures, elements or parts which appear in more than one figure are labeled with the same numeral in all the figures in which they appear. The figures are listed below and: 
     
    
     BRIEF DESCRIPTION OF FIGURES  
       [0072]      FIG. 1A  shows a schematic of a photosurface and a circuit diagram of pixels in the photosurface, in accordance with a preferred embodiment of the present invention;  
         [0073]      FIG. 1B  shows a schematic of a photosurface divided into band shaped pixel groups, in accordance with a preferred embodiment of the present invention;  
         [0074]      FIG. 1C  shows a schematic of a photosurface divided into square shaped pixel groups, in accordance with a preferred embodiment of the present invention;  
         [0075]      FIG. 1D  shows a schematic of a photosurface divided into pixel groups that are used to simultaneously provide an image of a scene and distance measurements to points in the scene, in accordance with a preferred embodiment of the present invention;  
         [0076]      FIG. 2  shows a schematic of a photosurface and a circuit diagram of pixels in the photosurface, in accordance with another preferred embodiment of the present invention, in which the pixel circuit comprises a modulator;  
         [0077]      FIG. 3  shows a schematic of a photosurface and a circuit diagram of pixels in the photosurface, in accordance with a preferred embodiment of the present invention, in which pixel outputs are automatically corrected for biases due to background light and dark current;  
         [0078]      FIG. 4  shows a schematic of another photosurface and a circuit diagram of pixels in the photosurface, in accordance with a preferred embodiment of the present invention, in which pixel outputs are automatically correctable for biases due to background light and dark current;  
         [0079]      FIG. 5  shows a time drawing of light and gating pulses illustrating a method of removing background and dark current effects for producing normalized light values; and  
         [0080]      FIG. 6  shows a schematic of a photosurface being used to determine distances to objects in a scene, in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0081]      FIG. 1A  shows a schematic of a photosurface  20  for use in a 3D camera in accordance with a preferred embodiment of the present invention. Elements in  FIG. 1A , and in similar subsequent figures, are not shown to scale and their relative sizes have been determined for ease and clarity of presentation. Only those parts of photosurface  20  that are relevant to the discussion are shown in  FIG. 1 .  
         [0082]     Photosurface  20  comprises a plurality of pixels  22 , wherein each pixel comprises a pixel circuit  24 , in accordance with a preferred embodiment of the present invention, shown schematically and in greatly exaggerated scale in inset  26 . Pixel circuit  24  comprises a back biased photodiode  28 , connected at a node  30  to a variable connection that operates as a gate switch  36 . Gate switch  36  connects photodiode  28  to an integrator  32  shown inside a dotted rectangle  34 . Integrator  32  comprises a storage capacitor  38  and an amplifier  40  having a positive input  42 , a negative input  44  and an output  46 . Voltage on output  46  is substantially proportional to charge on capacitor  38 . Gate switch  36  connects photodiode  28  to negative input  44  when gate switch  36  is closed and disconnects photodiode  28  from negative input  44  when gate switch  32  is open. Positive input  42  is preferably grounded.  
         [0083]     A reset switch  48  selectively connects node  30  to ground. When reset switch  48  is closed any intrinsic capacitance of photodiode  28  is charged to Vdd. When both reset switch  48  and gate switch  36  are closed, any accumulated charge on storage capacitor  38  is discharged and voltage on output  46  is set to zero.  
         [0084]     Output  46  of amplifier  40  is connected via an address switch  50  to a readout bus  52 . Address switch  50 , reset switch  48  and gate switch  36  are controlled to be closed and open by control signals from a controller to which they are connected by control lines (not shown) using methods known in the art. Preferably, the controller and photosensitive surface  20  are integrated on a same substrate (not shown).  
         [0085]     In a typical operating cycle of a pixel  22 , in accordance with a preferred embodiment of the present invention, when photosurface  20  is used to determine distances to objects in a scene, the pixel is first reset. This is accomplished by closing reset switch  48  and gate switch  36  to discharge storage capacitor  38 . Gate switch  36  is then opened. The scene is then illuminated with a train of light pulses, preferably radiated from a pulsed or shuttered laser. Light reflected from each of the radiated light pulses by objects in the scene is collected by collecting optics (not shown) and focused onto photosurface  20 . Preferably, an appropriate filter (not shown) that transmits substantially only light having a wavelength radiated by the laser, shields pixels  22  in photosurface  20 .  
         [0086]     At accurately determined times following the time that each light pulse is radiated, reset switch  48  is opened and gate switch  36  is closed. Gate switch  36  remains closed for an accurately determined period of time and is then opened. Pixel  22  is thereby sensitive to light from the laser during a gate that has an accurately determined gate width (the period of time during which gate switch  36  is closed) and an accurately determined start time following the time that each light pulse is radiated.  
         [0087]     If photodiode  28  is exposed to light from a radiated light pulse that is reflected from a region of an object in the scene, and/or background light during the gate, a current, hereinafter referred to as a “photocurrent”, flows from photodiode  28  into storage capacitor  38 . Storage capacitor  38  integrates the photocurrent and a “photocharge” is accumulated on storage capacitor  38 . The photocurrent is proportional to the intensity of the light incident on photodiode  28  from the region of the object and from background light. The amount of photocharge accumulated on storage capacitor  38  is equal to the time integral of the photocurrent during the gate.  
         [0088]     By opening and closing reset switch  48  each time before closing gate switch  36  it is assured that every time photodiode  28  is connected to input  44  node  30  has been set to ground and the voltage across the intrinsic capacitance of photodiode  28  is set to Vdd. As a result any change in voltage across the intrinsic capacitance of photodiode  28  that occurs during periods of time between gates does not affect the amount of charge integrated by storage capacitor  38  during a gate. Such changes might result from dark currents or light incident on photodiode during periods between gates.  
         [0089]     The total amount of photocharge accumulated by storage capacitor  38  for the train of radiated light pulses is the sum of the photocharges accumulated during all of the gates that follow radiated light pulses in the train of light pulses.  
         [0090]     Following the last gate, the amount of photocharge on storage capacitor  38  is determined by closing address switch  50 . When this occurs output  46  of amplifier  40  is connected to readout bus  52  and a charge is deposited on readout bus  52  that is proportional to the photocharge accumulated on storage capacitor  38 . The charge deposited on readout bus  52  is sensed and registered using methods known in the art.  
         [0091]     The registered photocharge from pixel  22  is preferably normalized to the reflectivity of the region of the object imaged on pixel  22  using methods described in PCT Publications WO97/01111, WO97/01112, and WO97/01113 referenced above. Corrected and normalized registered photocharges from a plurality of pixels  22  are then processed to provide distances to objects in the scene and images of the objects as described in the referenced PCT publications.  
         [0092]     Following the readout of the photocharge, reset switch  48  and gate switch  36  are closed so as to discharge any accumulated photocharge on storage capacitor  38  and reset the voltage on output  46  to zero. A next operating cycle can now be initiated.  
         [0093]     Gate switches  36  and reset switches  48  of pixels  22  on photosurface  20  can be controlled, in accordance with preferred embodiments of the present invention, to gate pixels  22  on and off in different combinations and with different timing sequences. In some preferred embodiments of the present invention gate switch  36  and reset switch  48  of each pixel  22  is controlled independently of gate and reset switches  36  and  48  of other pixels  22 . Different combinations of pixels  22  are gated on and off in different timing sequences by controlling individual pixels  22 . In other preferred embodiments of the present invention pixels  22  are grouped into different “pixel groups”. Gate switch control lines to pixels  22  in a same pixel group are appropriately connected together so that pixels  22  belonging to the pixel group are gated on and off together and different combinations of pixel groups are gated on and off in different timing sequences.  
         [0094]     In some preferred embodiments of the present invention different pixel groups define different regions of photosurface  20 . For example,  FIG. 1B  shows pixel groups that divide photosurface  20  into parallel strips  60 . All pixels  22  in a strip  60  belong to the same pixel group and are gated simultaneously.  FIG. 1C  shows pixel groups that divide photosurface  20  into square substantially equal area regions  62 . Applications of different configurations of pixel groups are described in PCT publication WO 97/01111, referenced above.  
         [0095]     A photosurface, in accordance with a preferred embodiment of the present invention, may also be divided into pixel groups that are usable to simultaneously provide images of objects in a scene and distance measurements to the objects.  
         [0096]     Assume that the scene is illuminated with a train of light pulses and that light reflected from each of the radiated light pulses by objects in the scene is collected by collecting optics and focused onto the photosurface. The photosurface is divided into pixel groups, in accordance with a preferred embodiment of the present invention, in which each pixel group comprises two contiguous pixel sub-groups, a first pixel subgroup and a second pixel sub-group. Each pixel subgroup includes at least one pixel. Preferably, the area of the second pixel subgroup surrounds the area of the first pixel subgroup. Preferably, the pixel groups of the photosurface have small areas.  
         [0097]     The first pixel subgroups of the photosurface are used to perform distance measurements to the objects in the scene. The second pixel subgroups of the photosurface are used to provide images of the objects.  
         [0098]     In accordance with a preferred embodiment of the present invention the first pixel subgroup of each pixel group is gated on with a relatively short gate at a predetermined time following each light pulse in the train of light pulses (i.e. the pixels of the sub-group are simultaneously gated with a relatively short gate). Preferably, the gate width of the short gate is equal to the pulse width of the light pulses in the train of light pulses. The amount of light registered by a pixel in a first pixel subgroup is a function of the distance from the pixel of a region of an object in the scene that is imaged on the pixel and the intensity of light incident on the pixel from the region. The distance to the region is determined from the amount of light registered on the pixel normalized to the intensity of light incident on the pixel from the region.  
         [0099]     The second pixel subgroup of each pixel group is gated on with a relatively long gate at a predetermined time following each light pulse in the train of light pulses. Preferably, the gate width of the long gates is at least three times the pulse width of the light pulses. (In the case where the pulse width and the short gate width are not equal, preferably, the long gate width is equal to at least two pulse widths plus a short gate width). Preferably, the mid points of the long and short gates are substantially coincident. The amount of light collected on a pixel of a second subgroup is a function of the intensity of light incident on the pixel from a region of an object in the scene that is imaged on the pixel.  
         [0100]     A region imaged by the first subgroup of a pixel group is contiguous with a region imaged by the second subgroup of the pixel group. The intensity of light registered by pixels in the second subgroup of pixels is used to estimate the intensity of light that is incident on pixels in the first subgroup. Estimates of intensity for pixels in the first pixel subgroup are made from intensities registered on pixels in the second pixel subgroup by appropriate averaging and interpolation techniques known in the art. The estimated intensity of incident light on pixels in the first subgroup is used to normalize the amount of light registered on pixels in the first subgroup in order to determine distances to the objects.  
         [0101]      FIG. 1D  shows photosurface  20  divided into pixel groups  63  that are usable to simultaneously provide an image of an object illuminated by an appropriate train of light pulses and distance measurements to the object, in accordance with a preferred embodiment of the present invention. Each pixel group  63  preferably comprises nine pixels  22 . The nine pixels  22  are preferably grouped into a first pixel subgroup  65  comprising a single pixel  22  and a second pixel subgroup  67  comprising eight pixels  22 . Inset  70  of  FIG. 1D  shows a pixel group  63  in which pixels  22  belonging to second subgroup  67  are textured and the single pixel  22  belonging to first pixel subgroup  65  is shown untextured. First pixel subgroup  65  is used for distance measurements to objects in a scene and second pixel subgroup  67  is used for imaging objects in a scene.  
         [0102]      FIG. 2  shows a schematic of a photosurface  80  comprising pixels  82  in accordance with another preferred embodiment of the present invention. Each pixel  82  comprises a pixel circuit  84  shown in inset  86 . Pixel circuit  84  is identical to pixel circuit  24  shown in FIG. IA except that in pixel circuit  84  a modulator  88  replaces gate switch  36  in pixel circuit  24 . Modulator  88 , unlike gate switch  36 , modulates current flowing from photodiode  28  into storage capacitor  38  rather than either just preventing photocurrent from flowing or enabling photocurrent to flow into storage capacitor  38 . Modulator  88  is preferably a FET and is shown in  FIG. 2  by the graphical symbol for a FET. Modulator  88  is connected by a control line  90  to a controller (not shown) that controls the value of the resistance of modulator  88  between photodiode  28  and input  44  of amplifier  40 . The resistance of modulator  88  modulates the magnitude of photocurrent that flows through photodiode  28  into storage capacitor  38 .  
         [0103]     When pixels  82  in photosurface  80  are modulated harmonically, in accordance with a preferred embodiment of the present invention, photosurface  80  is useable to determine distances to objects using an harmonically modulated light source according to methods described in U.S. Pat. No. 4,935,616 cited above.  
         [0104]     For example, assume that a target (not shown) located at a distance D from photosurface  80  is illuminated with laser light that is modulated so that the intensity of the laser light may be written as I=I o (1+sin(ωt)). Assume that pixels  82  of photosurface  80  are harmonically modulated, in accordance with a preferred embodiment of the present invention, so that the sensitivity of pixels  82  can be represented by S=So(0.5)(1+sin(ωt). Assume further that light reflected by the target is properly collected and focused onto photosurface  80  for a period of time equal to NT where N is an integer and T=2π/ω, is the period of modulation of the laser light and the pixel sensitivities. Then the amounts of photocharge accumulated on a capacitors  30  of pixels  82 , onto which an image of the target is focused, will be proportional to RI o S o (NT)(0.5+0.25cos Θ) where R is a proportionality constant and Θ=2Dω/c where c is the speed of light. The amplitude, RI O S O (NT), can be determined, in accordance with a preferred embodiment of the present invention, by imaging the target with modulated laser light for a known period of time, which period of time is preferably equal to NT, without modulating the sensitivity of pixels  82 .  
         [0105]     In the above example pixels  82  of photosurface  80  are modulated harmonically. In some preferred embodiments of the present invention pixels are modulated non-harmonically. For example, pixels may be modulated pseudo-randomly.  
         [0106]      FIG. 3  shows another photosurface  100  comprising pixels  102  for use in a 3D camera in accordance with a preferred embodiment of the present invention, to determine distances to objects in a scene. Like photosurface  20  shown in FIG. IA, photosurface  100  is preferably used with a pulsed or shuttered laser and is preferably shielded by an appropriate optical filter that transmits substantially only light having a wavelength equal to that of light radiated by the laser.  
         [0107]     However, unlike pixels  22  in photosurface  20 , the outputs of pixels  102  in photosurface  100  are automatically corrected for biases caused by background light to which they are exposed and from dark currents. Background light is any light incident on pixels  102  that is not from light radiated to illuminate objects in the scene. Such background light may originate from sources of light (natural as well as man made) other than the laser that radiate light having the same wavelengths as light radiated by the laser. Background light might also arise because the optical filter that shields photosurface  100  might not be perfectly opaque to light having wavelengths not radiated by the laser.  
         [0108]     Pixels  102  comprise a pixel circuit  104  shown in greatly exaggerated scale in inset  105 . Pixel circuit  104  comprises a photodiode  28  connected to a node  30  and preferably back biased with a voltage Vdd, and first and second integrators  110  and  112  respectively shown inside dotted circles  114  and  116 . First and second integrators  110  and  112  are preferably identical and similar in structure and operation to integrator  32  shown in FIG. IA as part of pixel circuit  24 . A first gate switch  120  is used to connect and disconnect photodiode  28  to and from first integrator  110  and a second gate switch  122  is used to connect and disconnect photodiode  28  to and from second integrator  112 .  
         [0109]     First integrator  110  comprises a first storage capacitor  130  and first amplifier  132 , which amplifier  132  has positive and negative inputs  134  and  136  and an output  138 . Second integrator  112  has a second storage capacitor  140  and an amplifier  142  having positive and negative inputs  144  and  146  and an output  148 . Preferably, integrators  110  and  112  are identical. Output  138  of first integrator  132  is connected to a positive input  150  of an amplifier  152  and output  148  of second amplifier  142  is connected to a negative input  154  of amplifier  152 . Amplifier  152  has an output  156 . Voltage on output  156  is proportional to the difference of voltages on outputs  138  and  148 . This voltage is proportional to the charge generated by reflection from the object of light from the illumination source.  
         [0110]     When first gate switch  120  is closed and second gate switch  122  is open, photocurrent from photodiode  28  is integrated by first storage capacitor  130 . Similarly, when first gate switch  120  is open and second gate switch  122  is closed, photocurrent from photodiode  28  is integrated by second storage capacitor  140 . Node  30  is connected to a reset switch  48 . When reset switch  48  is closed the intrinsic capacitance of photodiode  28  is charged to voltage Vdd. When reset switch  48  and gate switch  120  are closed storage capacitor  130  is discharged. Similarly, storage capacitor  140  is discharged when reset switch  48  and gate switch  122  are closed.  
         [0111]     Output  156  of differential amplifier  152  is connected via an address switch  50  to a readout bus  52 . When address switch  50  is closed, voltage on output  156 , which is proportional to the difference between the amounts of photocharge on first and second storage capacitors  130  and  140  respectively, is sensed via on readout bus  52 . The sensed voltage is a measure of the intensity of the response of a pixel  102  to light from an object imaged on the pixel  102 .  
         [0112]     A controller (not shown) controls each of the switches in circuit  100  via appropriate control lines (not shown) that connect the controller to the switches.  
         [0113]     When photosurface  100  is used to determine distances to objects in a scene, a train of light pulses radiated from the laser illuminates the scene. Following each light pulse in the train of radiated light pulses, each pixel  102  in photosurface  100  that is used to determine distances to the objects is gated on twice.  
         [0114]     The first time a pixel  102  is gated on, for a “first gate”, photodiode  102  is connected to first integrator  110  and disconnected from second integrator  112  and photocurrent is integrated on first storage capacitor  130 . The second time pixel  102  is gated on, for a “second gate”, photodiode  102  is connected to second storage capacitor  140  and disconnected from first capacitor  110  so that photocurrent is integrated on second storage capacitor  140 . The gate widths of the first and second gates are controlled to be equal to a high degree of accuracy. Each time before photodiode  28  is connected to one or the other of integrators  110  and  112 , reset switch  48  is closed so as to charge the intrinsic capacitance of photodiode  28  to Vdd and set the voltage of node  30  to ground. As explained in the discussion of FIG. IA this prevents any changes in voltage across the intrinsic capacitance of photodiode  28  that occur between gates from affecting the amounts of charge accumulated on storage capacitors  130  and  140 .  
         [0115]     The first gate is timed with respect to the radiated light pulse so that pixel  28  accumulates photocharge on first storage capacitor  130  generated by light incident on photodiode  28  that is reflected from the radiated light pulse by an object in the scene. During the first gate, storage capacitor  130  also accumulates photocharge from background light and charge generated by dark current in photodiode  28 . The voltage on output  138  of first amplifier  132  is therefore proportional to dark current, photocurrent generated by background light and light reflected by an object in the scene that is integrated during the first gate.  
         [0116]     The second gate is timed to follow the first gate after a sufficiently long delay so that light from the radiated light pulse reflected by objects in the scene is no longer incident on pixel  102 . During the second gate therefore, pixel  102  accumulates photocharge on second storage capacitor  140  generated only by background light and dark current. The voltage on output  148  of second amplifier  142  is therefore proportional to dark current and photocurrent generated by background light that is integrated during the second gate.  
         [0117]     Since the voltage on output  156  of amplifier  152  is proportional to the difference between the voltages on output  138  of first amplifier  132  and output  148  of second amplifier  142 , the output of pixel  102  is proportional to photocharge generated only by light that is from the radiated light pulse that is reflected by an object in the scene. Biases in the response of pixel  102  to light resulting from background light and from dark current are substantially removed.  
         [0118]     In a variation of pixel circuit  104  amplifier  152  is omitted and each of first and second integrators  110  and  112  respectively is connected to data bus  52  by its own address switch. In this variation of pixel circuit  104 , following the last radiated light pulse in the train of light pulses, the voltage on output  138  and  148  of each pixel  102  is separately read out and corrections for the effects of background light and dark current on the output of each pixel  102  is preferably performed digitally.  
         [0119]      FIG. 4  schematically shows another photosurface, photosurface  170 , comprising pixels  172  wherein each pixel  172  comprises a pixel circuit  174  shown in inset  176  that automatically corrects the output of the pixel for biases causes by background light and dark current in accordance with a preferred embodiment of the present invention. This circuit operates with one capacitor and one amplifier and removes the effects of background light and dark current by switching the direction in which current flows into the capacitor.  
         [0120]     Pixel circuit  174  comprises an amplifier  178  and five gate switches, gate switches  180 ,  181 ,  182 ,  183  and  184 , which control the operating cycle of pixel circuit  174  and route photocurrent from a photodiode  28  (back biased by voltage Vdd) to a storage capacitor  186 . Amplifier  178  has positive and negative inputs  190  and  192  and an output  194 . Output  194  can be connected to a readout bus  52  by an address switch  50 . Storage capacitor  186  is connected between two nodes,  196  and  198 . A reset switch  48  connected to a node  30  is used to ground node  30  and reset the voltage across the intrinsic capacitance of photodiode  28  to Vdd.  
         [0121]     Photosurface  170  is useable to measure distances to a target illuminated by a train of light pulses, in accordance with a preferred embodiment of the present invention. Following each light pulse in the train of light pulses, pixels  172  are gated on twice. Each pixel  172  is gated on for a first gate following the light pulse to receive reflected light from the target and subsequently gated on for a second gate to receive background light and measure dark current. The second gate is delayed with respect to the first gate so that during the second gate no reflected light from the target is incident on pixel  172 . The gate widths of the two gates are carefully controlled to be equal to a high degree of accuracy. Preferably the gate widths of the two gates are substantially equal to the pulse widths of the light pulses that illuminate the target.  
         [0122]     In a typical operating cycle of a pixel  172 , capacitor  30  is reset before the first pulse of a train of light pulses illuminating a target by closing gate switches  181  and  182  or gate switches  183  and  184 . Thereafter, following each light pulse, reset switch  48  is closed while gate switch  180  is open in order to reset the voltage across the intrinsic capacitance of photodiode  28  to Vdd. Pixel  172  is then gated on for a first gate following (after an appropriate time delay) the light pulse by opening gate switch  48  and closing gate switches  180 ,  181  and  183 . Node  196  is connected thereby to output  194  of amplifier  178  and node  198  is connected to negative input  192  of amplifier  178 . During the first gate, photocurrent generated by light reflected by the target and background light, plus dark current, flow into storage capacitor  186  and increase the potential difference across storage capacitor  186 . At the end of the first gate, gate switches  180 ,  181  and  183  are opened and subsequently reset switch  48  is closed to again reset the voltage across the intrinsic capacitance of photodiode  28  to Vdd.  
         [0123]     To begin the second gate, reset switch  48  is opened and gate switches  180 ,  182  and  184  are closed (gate switches  181  and  183  are open). Nodes  196  and  198 , which during the first gate were connected to output  194  and input  192  respectively, now have their connections reversed. Node  196  is connected to input  192  and node  198  is connected to output  194 . As a result, current from photodiode  28  that flows into storage capacitor  186  during the second gate reduces the voltage across storage capacitor  186 . This current is the sum of dark current and photocurrent generated by background light. Therefore at the end of the second gate the contribution to the potential difference across capacitor  186  that existed at the end of the first gate due to dark current and photocurrent generated by background light is subtracted from the voltage across storage capacitor  186 . At the end of the second gate, the potential difference across capacitor  186  and the charge accumulated on the capacitor is due only to light reflected by the target from the light pulse.  
         [0124]     Voltage on output  194  of amplifier  178  is therefore proportional only to the amount of photocharge generated by light from the train of light pulses that is reflected by the target. The effects of background light and dark current have been effectively eliminated from the output of pixels  172  in photosurface  170 . To read the output of pixel  172  following the last pulse of the train of light pulses, gate switch  50  is closed to connect output  194  to readout bus  52 .  
         [0125]     In order to determine distances to the target the output of each pixel  172  used to measure distance to the target must be normalized to the intensity of the reflected light incident on the pixel from the region of the target that is imaged on the pixel. This is preferably done by grouping pixels  172  in photosurface  170  into pixel groups and using some of the pixel groups to acquire distance data from the target and using other pixel groups to acquire imaging data (intensity data) as described in the discussion of FIG. ID. Alternatively photosurface  170  may be exposed twice to the target, once to acquire a frame of distance data from the target and a second time to acquire a frame of imaging data from the target. As described above both the distance data and the imaging data are automatically corrected for the effects of background light and dark current. Outputs of pixels  172  that are used to acquire image data from the target are used to normalize outputs of pixels  172  that are used to acquire distance data from the target.  
         [0126]      FIG. 5  shows a generalized system for producing normalized, background and dark-current corrected signals, in accordance with a preferred embodiment of the invention.  FIG. 5  is a time drawing in which the timing of two pulses and four gates are shown. A background and dark current corrected signal is derived by accumulating charge from a light sensitive device during a first gating period  302 . This includes charge generated by source light reflected from the object  300  during (part of) the period as well as charge generated by background light and dark current  301 . During a second gating period  304 , preferably having the same extent as gate  302 , charge which is accumulated is caused only by background and leakage current. The difference between the two shaded areas corresponds to the net charge from the source light reflected from the object. This difference is, however, not yet normalized.  
         [0127]     In order to normalize, the total amount of light from source during the entire period of its illumination by the source is accumulated, as in the prior art, during a third gating period  306 , which is made long enough to include all of the reflected light  300 . As with respect to period  302 , the light during this period includes background, source reflection and dark current. During a fourth gate  308 , preferably having the same width as gate  306 , charge is accumulated which has as its source only background and dark current. When this charge is subtracted from the charge accumulated during period  306 , a true normalizing value (net of background and dark current) is determined. This “net” normalizing signal is used to normalize the net source light reflection charge, as determined from the accumulations during gates  302  and  304 .  
         [0128]      FIG. 5  shows gates  302 / 304  and  306 / 308  acquired in pairs on successive pulses. For this case, the charges may be accumulated utilizing for example a circuit such as that shown in  FIG. 3  or  4 . However, as described above, they may be acquired for the same pulse utilizing different, adjacent pixels or during different frames, in which case the circuit of FIG. IA may be used. However, it should be understood that the methodology described with respect to  FIG. 5  has more general applicability than to the photosurfaces described above and can be utilized in a wider range of pulsed detection systems for the elimination of background and dark current and for normalization.  
         [0129]      FIG. 6  schematically shows a photosurface  200  having pixels  202  comprised in a 3D camera that is being used to determine distances to an object  204 , in accordance with a preferred embodiment of the present invention. Only the parts of the 3D camera that are relevant to the discussion are shown. Elements shown in  FIG. 6  are not to scale and their relative dimensions have been chosen to facilitate ease and clarity of exposition.  
         [0130]     The 3D camera comprises a light source, preferably a laser  206 , that illuminates objects being imaged with a train of light pulses or a light beam having a modulated intensity. A lens  208  collects light from objects imaged by the 3D camera and focuses the collected light on pixels  202  of photosurface  200 . 3D camera comprises a controller  210  that synchronizes gating or modulating pixels  202  with light pulses or with the intensity modulation of light radiated by laser  206 , respectively.  
         [0131]     In the case that laser  206  radiates light pulses, pixels  202  are “gated” pixels that comprise pixel circuits, in accordance with a preferred embodiment of the present invention, of the types shown in  FIGS. 1A, 3 , or  4 . Pixels  202  are gated in response to the times at which light pulses are radiated by laser  206 , in accordance with a preferred embodiment of the present invention, as described above.  
         [0132]     In the case where laser  206  radiates an intensity modulated light beam, pixels  202 , are “modulated” pixels that comprise, in accordance with a preferred embodiment of the present invention, pixel circuits of the type shown in  FIG. 2 . Pixels  202  are modulated in response to the time dependence of the intensity modulation, in accordance with a preferred embodiment of the present invention, as described above.  
         [0133]     In  FIG. 6  laser  206  is shown illuminating object  204  with a plurality of light pulses represented by wavy arrows  212 . Regions of object  204  reflect light from radiated light pulses  212  in reflected light pulses that are represented by wavy arrows  214 . Controller  210  gates pixels  202  on and off with respect to the times that light pulses  212  are radiated, pulse widths of light pulses  212 , and a range of distances to object  204  that it is desired to measure.  
         [0134]     As discussed explicitly for photosurface  24  shown in  FIG. 1A , pixels in the other photosurfaces in accordance with preferred embodiments of the present invention that are described above may be gated (or modulated as the case might be) in different combinations and with different timing sequences. Furthermore, pixels may be controlled individually or in groups.  
         [0135]     It should also be recognized that different pixels or pixel groups in photosurfaces, in accordance with preferred embodiments of the present invention, may be made sensitive to different wavelengths of light. For example, in some preferred embodiments of the present invention, pixels in a photosurface are grouped into groups of three contiguous pixels in which each pixel is sensitive to a different one of the primary additive colors R, G, B.  
         [0136]     Furthermore, whereas preferred embodiments of the present invention are shown comprising a photodiode as an element that generates current in a pixel circuit in response to incident light, other light sensitive current generators, such as photoresistors or photogates may be used instead of the photodiodes shown.  
         [0137]     The present invention has been described using non-limiting detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. Variations of embodiments described will occur to persons of the art. The scope of the invention is limited only by the following claims. In the claims, when the words “comprise”, “include” or “have” or their conjugations are used they mean “including but not limited to.”