Abstract:
An optical imaging system comprising: a taking lens system that collects light from a scene being imaged with the optical imaging system; a 3D camera comprising at least one photosurface that receives light from the taking lens system simultaneously from all points in the scene and provides data for generating a depth map of the scene responsive to the light; and an imaging camera comprising at least one photosurface that receives light from the taking lens system and provides a picture of the scene responsive to the light.

Description:
RELATED APPLICATIONS 
     The present application is a U.S. national application of PCT/IL99/00490, filed Sep. 8, 1999. 
     FIELD OF THE INVENTION 
     The invention relates to cameras that provide an image of a scene and measurements of distances to regions in the scene. 
     BACKGROUND OF THE INVENTION 
     3D cameras that provide distance measurements to objects and points on objects that they image are well known in the art. Gated 3D cameras comprise a photosensitive surface, such as a CCD or CMOS camera, hereinafter referred to as a “photosurface”, and a gating means for gating the camera open and closed, such as an electro-optical shutter or a gated image intensifier. 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 an appropriate light source. Generally, the radiated light pulses are infrared (IR) light pulses. 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. 
     In some of these 3D cameras, only the timing between light pulses and gates is used to determine distance from the 3D camera to a region 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. In 3D cameras in which the amount of light is used to determine distances to the imaged region, the amount of light registered on a pixel is sometimes corrected for reflectivity of the imaged region, dark current and background light. 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, and how fast the gating means can gate the camera open and closed. 
     Gated 3D cameras that determine distances to objects in a scene that they image responsive to amounts of light registered on pixels of photosurfaces comprised in the 3D cameras are described in PCT Publications WO 97/01111, WO 97/01112, and WO 97/01113, the disclosures of which are incorporated herein by reference. 
     A gated 3D camera as shown in WO 97/01111 comprises first and second homologous photosurfaces and a light source that illuminates a scene being imaged with the camera with a train of, preferably IR, light pulses. The first photosurface, hereinafter referred to as a “distance photosurface”, is gated on with a short gate following the time that each light pulse in the pulse train is radiated. The portion of light from each light pulse in the pulse train that is reflected by a region of the scene and enters the 3D camera, which is registered on a pixel of the distance photosurface, is a function of the distance of the region from the pixel. The second photosurface, hereinafter referred to as a “normalization photosurface”, is preferably not gated. The portion of light from each light pulse in the pulse train that is reflected by a region of the scene and enters the 3D camera, which is registered on a pixel of the normalization photosurface, is independent of the distance of the region from the pixel. The amount of light registered on the pixel is a measure of the total amount of light reaching the camera from the imaged region. An amount of reflected light registered on a pixel of the distance photosurface from all the light pulses in the pulse train is normalized to an amount of reflected light from all the light pulses registered on a corresponding pixel in the normalization photosurface. Normalized amounts of light are used to determine distances to regions in the scene. 
     U.S. Pat. No. 5,434,612 to Nettleton, the disclosure of which is incorporated herein by reference, describes a gated 3D camera comprising first, second and third photosurfaces. A scene imaged with this camera is not illuminated with a train of light pulses but with a single light pulse from a laser and the three photosurface are gated with respect to the time that the light pulse is radiated. The first photosurface is a distance photosurface. It is gated with a short gate so that a portion of the light pulse reflected by a region of the scene that is collected by the camera and registered on a pixel of the photosurface is a function of the distance of the region from the pixel. The second photosurface is a normalization photosurface. It is gated with a long gate so that the amount of reflected laser light registered on a pixel of the photosurface from an imaged region is a measure of the total amount of light reaching the camera from the imaged region. The third photosurface is used to measure background light by measuring the amount of light reaching the camera in a band of wavelengths near to wavelengths of light radiated by the laser. A filter that transmits light in the band of wavelengths close to the wavelengths of the laser light but blocks light having a wavelength the same as a wavelength of light radiated by the laser shields the third photosurface. The third photosurface is gated simultaneously with the normalization photosurface by a long gate having a same gate width as the gat that gates the second photosurface. A photosurface used to measure background light is hereinafter referred to as a “background photosurface”. 
     Amounts of light registered on the background photosurface are used to correct the amounts of light registered on pixels of the distance and normalization photosurfaces for background light. Background corrected amounts of light registered by pixels on the normalization photosurface are used to normalize background corrected amounts of light registered by pixels on the distance photosurface. Distances to regions in the scene are determined from the background corrected normalized amounts of light registered by pixels on the distance photosurface. 
     Generally photosurfaces used in 3D cameras are gated by an external fast shutter. Certain types of CCD cameras allow for gating image acquisition on and off during a frame by turning the photosurfaces on and off. However, turn-on and turn-off times of these photosurfaces are generally much too long to enable gating the photosurfaces for the purposes of accurate distance measurements by turning them on and off. Typically turn-on and turn-off times for CCD photosurfaces are on the order of microseconds while gating for accurate distance measurements requires turn-on and turn-off times on the order of nano-seconds or less. 
     An electro-optical shutter suitable for use in 3D cameras, such as those described in the cited patent and patent applications is described in PCT Publications WO 99/40478, the disclosure of which is incorporated herein by reference. 
     Generally, a 3D camera is used in conjunction with an imaging camera, such as a video camera, that provides an image, hereinafter referred to as a “picture”, of a scene being imaged with the 3D camera responsive to visible light from the scene. The 3D camera provides a “depth map” of the scene while the imaging camera provides a picture of the scene. Distances provided by the depth map are associated with visible features in the picture. In some applications distances associated with a picture of a scene are used to “window” the scene and remove unwanted features and/or objects in the scene, such as for example a background, from the picture. Such applications are described in PCT publication in WO 97/01111 cited above. 
     PCT patent application PCT/IL98/00476, entitled “Distance Measurement with a Camera”, by some of the same inventors as the inventors of the present invention, the disclosure of which is incorporated herein by reference, describes a photosurface comprising pixels each of which has its own circuit that is controllable to gate the pixel on or off. A single photosurface of this type is useable to simultaneously provide the functions of a distance, background, and normalization photosurface of a 3D camera as well as an imaging camera. However, as the number of functions that the photosurface performs increases, the resolution of the photosurface decreases. 
     It is advantageous to have a simple robust optical system comprising a 3D camera and an imaging camera that is easily adjustable to simultaneously optimize quantities of light available from a scene imaged by the system that reach the cameras. 
     SUMMARY OF THE INVENTION 
     An aspect of some preferred embodiments of the present invention relates to providing an improved optical system, hereinafter referred to as a “3D imager”, comprising a 3 D camera and an imaging camera, for acquiring depth maps and picture of a scene. 
     An aspect of some preferred embodiments of the present invention relates to providing a 3D imager that is relatively easily adjusted so that its 3D and imaging cameras may simultaneously receive optimum amounts of light available from a scene being imaged with the 3D imager. 
     An aspect of some preferred embodiments of the present invention relates to providing a 3D imager comprising an improved gating system for gating photosurfaces comprised in its 3D camera. 
     A 3D imager, in accordance with some preferred embodiments of the present invention, comprises a single taking lens boresighted with a gated 3D camera and an imaging camera. Preferably, the imaging camera is a color camera. Preferably the 3D camera comprises three photosensitive surfaces, a distance photosurface, a normalization photosurface and a background photosurface. Preferably, the 3D imager is used with a pulsed IR light source that illuminates a scene being imaged with the 3D imager with a train of preferably IR, light pulses. Light from the light pulses reflected by objects in the scene is used by the 3D camera to provide a depth map of the scene. Visual light from the scene is used by the imaging camera to provide a picture of the scene. 
     While there is usually more than ample amount of visible light available to form a quality picture of the scene, the quantity of light available for the purpose of providing a depth map of the scene is usually small. As a result, the 3D imager usually requires that the taking lens be set to a much higher f-number to produce a quality picture of the scene than an f-number required to provide accurate distance measurements to the scene. 
     To provide proper control of the amounts of light reaching the 3D camera and the imaging camera, the 3D imager comprises a system that controls the amount of light reaching the imaging camera from the taking lens independently of the amount of light reaching the 3D camera. 
     Preferably, the 3D camera comprises at least two irises. A first iris of the at least two irises controls the amount of visible light collected by the taking lens that reaches the imaging camera. A second iris controls either the amount of IR light collected by the taking lens that reaches the 3D camera or the total amount of IR and visible light collected by the taking lens that enters the 3D imager. As a result, the 3D imager can be adjusted to control independently amounts of light reaching the imaging camera and the 3D camera from a scene being imaged with the 3D imager. Therefore, subject to a level of illumination of the scene, a 3D imager, in accordance with a preferred embodiment of the present invention, is adjustable to simultaneously optimize the amounts of light from the scene that reach its 3D and imaging cameras. 
     In accordance with some preferred embodiments of the present invention all photosurfaces comprised in the 3D camera are gated with a same single fast shutter. This substantially simplifies the construction and control of the 3D imager. Preferably the 3D camera comprises three photosurfaces, a distance photosurface, a background photosurface and a normalized photosurface. The three photosurfaces are independently controllable to the turned on and off. Preferably, all light transmitted from the taking lens to the 3D camera passes through the single fast shutter. After passing through the shutter, portions of the light arc directed to each of the three photosurfaces, for example by a prism. Preferably, the prism is a totally internal reflection (TIR) prism. At any given time, to determine which of the photosurfaces is gated by the fast shutter, all photosurfaces except a photosurface that is to be gated by the fast shutter are shut off. 
     It is to be noted that whereas the photosurfaces are controllable to be turned on or off, the speed with which a photosurface can be switched between on and off states is generally not fast enough to gate the photosurfaces for the purposes of accurate distance measurements. Therefore, for 3D cameras, gating photosurfaces by turning them on and off is generally not practical. 
     In some preferred embodiments of the present invention, the 3D camera and associated optical components are housed as a single unit, hereinafter referred to as a “3D module”. The 3D module comprises portals for optically coupling a taking lens and imaging camera to the 3D module using methods and techniques known in the art. In some preferred embodiments of the present invention, the 3D camera, imaging camera and associated electrical and optical components are integrated together as a single unit to which a taking lens is optically coupled. 
     There is therefore provided, in accordance with a preferred embodiment of the present invention an optical imaging system comprising: a taking lens system that collects light from a scene being imaged with the optical imaging system; a 3D camera comprising at least one photosurface that receives light from the taking lens system simultaneously from all points in the scene and provides data for generating a depth map of the scene responsive to the light; and an imaging camera comprising at least one photosurface that receives light from the taking lens system and provides a picture of the scene responsive to the light. 
     Preferably, the 3D camera and the imaging camera are boresighted with the taking lens system. Additionally or alternatively, the at least one photosurface of the 3D camera and the at least one photosurface of the imaging camera are preferably homologous. 
     In some preferred embodiments of the present invention, an optical imaging system comprises a light controller adjustable to control the amount of light from the taking lens system that reaches the imaging camera without affecting the amount of light from the taking lens system that reaches the 3D camera. Preferably, the light controller comprises an iris. Alternatively, the light controller preferably comprises a neutral density filter. 
     In some preferred embodiments of the present invention, an optical imaging system comprises a light controller adjustable to control the amount of light collected by the taking lens system that enters the imaging system. Preferably, the light controller comprises an iris. 
     In some preferred embodiments of the present invention, an optical imaging system comprises a light controller adjustable to control the amount of light from the taking lens system that reaches the 3D camera without affecting the amount of light from the taking lens that reaches the imaging camera. Preferably, the light controller comprises an iris. 
     The 3D camera in an optical imaging system in accordance with some preferred embodiment of the present invention is a gated 3D camera. Preferably, the optical imaging system comprises a pulsed light source that radiates a train of light pulses to illuminate a scene being imaged with the optical imaging system. Preferably, the pulsed light source radiates IR light. 
     According to some preferred embodiments of the present invention, the 3D camera comprises at least 2 photosurfaces. Preferably, the 3D camera comprises a light guide that receives light from the taking lens system and directs portions of the light that is receives to each of the at least two photosurfaces. Preferably, the 3D camera comprises a single shutter, which when gated open enables light from the taking lens system to reach the light guide. 
     The optical imaging system preferably comprises a controller that gates the single shutter open and closed. Preferably, the controller controls each of the photosurfaces to be activated and deactivated and wherein when a photosurface is activated, it is sensitive to light incident thereon. Preferably, the controller gates on the single shutter it activates one and only one of the at least two photosurfaces. 
     Preferably, the at least two photosurfaces comprises three photosurfaces. Preferably, following a time that at least one light pulse is radiated, the controller gates on the single shutter for a first gate and turns on a first photosurface and the first gate is timed so that light reflected from the at least one light pulse by a region in the scene is registered by the first photosurface. 
     Following a time that at least one light pulse in the train of light pulses is radiated, the controller preferably gates on the single shutter for a second gate and activates a second one of the photosurfaces and the second gate is timed so that during the second gate no light from the at least one light pulse reflected by the region is registered by the second photosurface. 
     Preferably, following a time that at least one light pulse in the train of light pulses is radiated the controller gates on the single shutter for a third gate and activates a third one of the photosurfaces and the controller controls the gate width and timing of the third gate so that during the third gate substantially all light from the at least one pulse that is reflected by the region, which is collected by the taking lens system, is registered by the third photosurface. 
     In some preferred embodiments of the present invention the light guide of the 3D camera is a three-way prism. 
     Some optical imaging systems in accordance with preferred embodiments of the present invention comprise a beam splitter that receives light from the taking lens system and directs a portion of the received light towards the 3D camera and a portion of the received light to the imaging camera. 
     In some preferred embodiments of the present invention the light guide of the 3D camera is a four-way prism that receives light from the taking lens system and directs a portion of the received light to the imaging camera. 
     In some preferred embodiments of the present invention the imaging camera comprises a color camera. 
     In some optical imaging systems, in accordance with preferred embodiments of the present invention, the imaging camera is a color camera comprising separate R, G and B photosurfaces and the imaging system comprises a four way prism that receives light from the taking lens system and directs a portion of the received light to each of the R, G and B photosurfaces and to the single shutter of the 3D camera. 
     There is further provided in accordance with a preferred embodiment of the present invention a gated 3D camera comprising: a taking lens system that collects light from a scene imaged with the 3D camera; at least 2 photosurfaces; a light guide that receives light from the taking lens and directs portions of the light that it receives to each of the at least two photosurfaces; and a single shutter, which when gated enables light from the taking lens system to reach the light guide. 
     Preferably, the controller controls each of the photosurfaces to be activated and deactivated and wherein when a photosurface is activated, it is sensitive to light incident thereon. Preferably, each time that the controller gates on the single shutter it activates one and only one of the at least two photosurfaces. Preferably, the at least two photosurfaces comprises three photosurfaces. 
     The 3D camera preferably comprises a light source that radiates a train of light pulses to illuminate a scene being imaged with the 3D camera. Preferably, following a time that at least one light pulse is radiated, the controller gates on the single shutter for a first gate and turns on a first photosurface and the first gate is timed so that light reflected from the at least one light pulse by a region in the scene is registered by the first photosurface. 
     Preferably, following a time that at least one light pulse in the train of light pulses is radiated, the controller gates on the single shutter for a second gate and activates a second one of the photosurfaces and the second gate is timed so that during the second gate no light from the at least one light pulse reflected by the region is registered by the second photosurface. 
     Preferably, following a time that at least one light pulse in the train of light pulses is radiated, the controller gates on the single shutter for a third gate and activates a third one of the photosurfaces and the controller controls the gate width and timing of the third gate so that during the third gate substantially all light from the at least one pulse that is reflected by the region, which is collected by the taking lens system, is registered by the third photosurface. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       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 figures are labeled with the same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
         FIG. 1  schematically shows a 3D imager comprising a 3D module, in accordance with a preferred embodiment of the present invention; 
         FIG. 2A  schematically shows a 3D imager comprising a 3D module, in accordance with a preferred embodiment of the present invention; 
         FIG. 2B  shows a timing diagram for shuttering photosurfaces comprised in the 3D imager shown in  FIG. 2A , in accordance with a preferred embodiment of the present invention; 
         FIG. 3  schematically shows a 3D imager comprising a 3D module, in accordance with a preferred embodiment of the present invention; and 
         FIG. 4  schematically shows a 3D imager comprising a 3D module and color imager integrated in a single unit, in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  schematically shows a 3D imager  20  comprising a 3D module  22 , in accordance with a preferred embodiment of the present invention, shown inside dashed boundary  24 . 3D module  22  comprises a beam splitter  26 , two refocusing lens system  28  and  30 , referred to as “refocusers”  28  and  30 , and a 3D camera  32  having components shown inside a dashed boundary  34 . 3D module  22  is coupled to a taking lens system  35  and an imaging camera  36 , which is shown by way of example as a video camera. Preferably, video camera  36  is a color video camera. Taking lens  35  and video camera  36  may be any suitable taking lens and video camera, for example a CCD or CMOS camera, readily available on the commercial market. A pulsed light source  38  radiates pulse trains of, preferably IR light pulses, to illuminate scenes being imaged with 3D imager  20 . 3D imager  20  is shown, by way of example, imaging a scene  40  having two elephants  42  and  44 . 
     Visual light and IR light from IR source  38  that is reflected from scene  40  and collected by taking lens  35  is represented by large arrows  46 . Collected light  46  is transmitted by taking lens  35  to beam splitter  26 . Visual light, represented by arrows  48 , in collected light  46 , is transmitted by beam splitter  26  and enters refocuser  28 . Refocuser  28  generally comprises a field lens  50 , an iris  52  and a relay lens  54 . Refocuser  28  is boresighted with taking lens  35  and preferably positioned so that an image of scene  40  formed by taking lens  35  from visual light  48  is located substantially at the location of field lens  50 . Light from field lens  50  passes through iris  52  and continues towards relay lens  54 , which transmits the light to video camera  36 . Video camera  36  is boresighted with refocuser  28  and taking lens  35 . The amount of light received by video  36  from refocuser  28  is controlled by iris  52 . Received visual light  48  is imaged by video  36  to form a picture of scene  40 . 
     IR light, represented by dashed arrows  60 , in collected light  46  that is incident on beam splitter  26  is reflected by beam splitter  26  to refocuser  30 , which is boresighted with taking lens  35 . Refocuser  30  is generally similar to refocuser  28  and preferably comprises a field lens  62 , an iris  64  and a relay lens  66 . Refocuser  30  is preferably positioned so that an image of scene  40  formed in IR light  60  by taking lens  35  is located at the location of field lens  62 . Refocuser  30  transmits IR light that it receives towards 3D camera  32  which is boresighted with refocuser  30  and taking lens  35 . The amount of IR light  60  transmitted by refocuser  30  to 3D camera  32  is controlled by iris  64 . 
     Irises  52  and  64  are controllable independently of each other and therefore enable the amounts of IR and visual light reaching 3D camera  32  and video camera  36  respectively to be controlled independently of each other, in accordance with a preferred embodiment of the present invention. 3D imager  20  therefore can easily be adjusted to simultaneously optimize the amounts of IR light reaching 3D camera  32  and visual light reaching video camera  36  from taking lens  35  to provide simultaneous quality depth maps and pictures of scene  40 . 
     It is to be noted that commercially available taking lenses are generally supplied with an iris and taking lens  35  is shown with an iris  67 . In operation of 3D imager  20 , iris  67  is preferably permanently set to maximum open and the amounts of light reaching 3D camera  32  and video camera  36  are controlled by irises  64  and  32  respectively. However, in some preferred embodiments of the present invention iris  67  is used in place of one of irises  52  or  62 . For example, generally a greater fraction of IR light collected by taking lens  35  is needed to produce a quality depth map of a scene than the fraction that is needed of visible light collected by taking lens  35  to provide a quality picture of the scene. As a result, iris  67  may conveniently be used in place of iris  64  to modulate IR light reaching 3D camera  32 . 
     3D camera  32  preferably comprises a three-way prism  68  and preferably three photosurfaces  71 ,  72  and  73 . Three-way prism  68  receives IR light  60  transmitted from refocuser  30  and directs portions of the received IR light to each of photosurfaces  71 ,  72  and  73 . Photosurfaces  71 ,  72  and  73  are respectively shuttered by fast shutters  81 ,  82  and  83 . Fast shutters  81 ,  82  and  83 , and fast shutters in other preferred embodiments of the present invention, are preferably either gated image intensifiers or fast solid state shutters of a type described in above cited PCT patent application PCT/IL98/00060. In some preferred embodiments of the present invention in which 3D photosurfaces are shuttered individually by their own fast shutters, as are photosurfaces  71 ,  72  and  73 , the shutters are comprised in the photosurfaces. Photosurfaces that comprise shuttering apparatus are described in PCT patent application PCT/IL98/00476 cited above. A controller (not shown) controls shutters  81 ,  82  and  83 . 
     Photosurfaces  71 ,  72  and  73  function respectively as a distance photosurface, a background photosurface and a normalization photosurface and are labeled accordingly “D”, “N” and “B” in  FIG. 1 . The choice of which photosurface  71 ,  72  or  73  is a distance, background or normalization photosurface is arbitrary and the choices shown in  FIG. 1  are choices of convenience. The controller controls shutters  81 ,  82  and  83  to gate photosurfaces  71 ,  72  and  73  following each IR pulse radiated by IR source  38  with a sequence of gates similar to prior art gate sequences used in gated 3D cameras. Prior art gating sequences are described in above cited references PCT Publications WO 97/01111, WO 970112, WO 97/01113, U.S. Pat. No. 5,434,612 and PCT Application PCT/IL98/00476. 
     Whereas, in the preceding paragraph it is implied that photosurfaces  71 ,  72  and  73  are similar or substantially identical, and this is generally the case, in some preferred embodiments of the present invention, different ones of the photosurfaces in 3D camera  32  are different. For example, a photosurface used to measure background light or normalization light may have a lower resolution than a resolution of a photosurface used as a distance photosurface and this can be advantageous. 
     Shutter  81  is controlled to respectively gate distance photosurface  71  with a relatively short gate and normalization photosurface  73  with a relatively long gate. Preferably the short gate has a gate width that is equal to the pulse width of IR pulses radiated by pulsed IR source  38 . Preferably the time centers of the short gates and long gates coincide. The controller preferably controls shutter  83  to gate background photosurface  72  with a short gate. The short gate of background photosurface  72  is timed to occur when no IR light from pulsed IR source  38  that is reflected from scene  40  is incident on taking lens  35 . Preferably the short gates of distance photosurface  71  and background photosurface  72  are equal. During the short gate of background photosurface  72 , substantially only background light and dark current effects are registered by background photosurface  72 . 
     The amounts of light registered on pixels of distance photosurface  71 , background photosurface  72  and normalization photosurface  73  are sensed and transmitted to a processor (not shown) using methods known in the art and processed as in prior art to provide a depth map of scene  40 . 
     Photosurfaces comprised in 3D cameras and imaging cameras that are used in 3D imagers, in accordance with preferred embodiments of the present invention, are preferably homologous. Two photosurfaces are said to be homologous if there is a one to one mapping of regions of one of the photosurfaces onto the other and the positions of any two regions that map onto each other are similar in their respective photosurfaces. 
       FIG. 2  shows another 3D imager  100  imaging scene  40 . 3D imager  100  comprises a 3D module  102  optically coupled to taking lens  35  and video camera  36 . 
     3D module  102 , having components shown inside dashed boundary  104 , is similar to 3D module  22 . 3D module  102  comprises beam splitter  26  and refocuser  28  that transmits visual light  48  from taking lens  35  to video camera  36 , which generates a picture of scene  40  responsive to the visual light that it receives. 3D module  102  comprises a 3D camera  106  having components shown inside dashed boundary  108 . 
     However, unlike 3D module  22 , 3D module  22  does not comprise a refocuser  30  that irises and transmits IR light from beam splitter  26  to 3D camera  106 . Instead, IR light  60  from beam splitter  26  passes through a fast shutter  110  and a relay lens  112  that directs the IR light to 3D camera  106 . Preferably, an iris  111  controls the amount of light from taking lens  35  transmitted to 3D camera  106 . 3D camera  106  is similar to 3D camera  32  shown in  FIG. 1  and comprises a three-way prism  68  that directs portions of IR light  60  incident on three-way prism  68  to a distance photosurface  121 , a background photosurface  122  and a normalization photosurface  123 . However, unlike photosurfaces  71 ,  72  and  73  in 3D camera  32 , Photosurfaces  121 ,  122  and  123  are not individually gated by their own fast shutters as are photosurfaces  71 ,  72  and  73  in 3D camera  32 . Fast gating for all photosurfaces  121 ,  122  and  123  is done by fast shutter  110 , which is common to all photosurfaces  121 ,  122  and  123 . Photosurfaces  121 ,  122  and  123  are turned on and off to determine which photosurface registers light during a gate of fast shutter  110 . Preferably, only one photosurface is turned on during a gate of fast shutter  110 . 
     Fast shutter  110  is controlled by a controller (not shown) to be gated open with a sequence of short and long gates in synchrony with IR pulses radiated by IR source  38 . Preferably, two sequential short gates having a same gate width follow every other IR light pulse in a train of light pulses radiated by IR light source  38  to illuminate scene  40 . Preferably, a long gate follows every IR pulse that is not followed by the two sequential short gates. 
     During the first short gate, IR light reflected from scene  40  is collected by taking lens  35  and transmitted to 3D camera  106  and only distance photosurface  121  is turned on. Only distance photosurface  121  registers amounts of light incident on 3D camera  106  during the first short gate. The amount of reflected IR light registered on distance photosurface  121  from a region of scene  40 , compared to the total amount of reflected IR light reaching taking lens  35  from the region is useable to determine the distance of the region from 3D camera  106 . 
     The timing of the first short gate with respect to an IR light pulse from which reflected light is registered by distance photosurface  121  and the gate width of the short gate, determine a center for a range of distances for which 3D camera  106  can provide distance measurements to regions in scene  40 . The width of the range is determined by the pulse width of the IR pulses and the gate width of the short gate. 
     The second short gate is timed to occur when no IR light reflected from scene  40  reaches taking lens  35 . During the short gate only background light is collected by taking lens  35  and transmitted to 3D camera  106 . Only background photosurface  122  is turned on and registers light incident on 3D camera  106 . Background photosurface  106  exclusively acquires background light information from scene  40 . 
     The time centers of the long gates and the first short gates are preferably delayed by the same time with respect to their respective IR light pulses. During the long gates, only normalization photosurface  123  is turned on. Normalization photosurface  123  registers amounts of IR light that are responsive to total amounts of reflected IR light reaching taking lens  35  from regions of scene  40 . 
     It should be noted that, while photosurfaces  121 ,  122  and  123  are turned on and off to gate 3D camera  106 , gating of 3D camera  106  for accurate distance measurements cannot be accomplished without fast gate  110  and only by turning on and off photosurfaces  121 ,  122  and  123 . Accurate distance measurement require that photosurfaces  121 ,  122  and  123  be gated on and off in times on the order of nanoseconds or less. Photosurfaces such as CCD photosurfaces can generally be turned on and off in times on the order of microseconds. 
     However, whereas turning photosurfaces on and off can not generally be used for fast shuttering of photosurfaces, it can be used for irising photosurfaces. For example, a photosurface in some video cameras can be turned off for a fraction of a frame time to control an amount of light that the photosurface registers. In some preferred embodiments of the present invention irising of an imaging camera is accomplished by controlling the length of time that a photosurface comprised in the imaging camera is on during a frame time. 
     As in prior art, amounts of light registered on pixels of distance photosurface  121  are corrected for background and normalized using amounts of light registered on corresponding pixels of on background photosurface  122  and normalization photosurface  123 . The corrected and normalized amounts of light are used to determine distances to reflecting regions of scene  40 , in this case, by way of example distances to elephants  42  and  44 . 
       FIG. 2B  shows, as function of time, a graph  130  of gates of fast shutter  110  and associated periods of time, hereinafter referred to as “on times” during which photosurfaces  121 ,  122  and  123  are turned on, in accordance with a preferred embodiment of the present invention. The gates and on times are shown by way of example synchronized with four IR pulses, represented by rectangles  131 ,  132 ,  133  and  134  on a time line  136 , of a pulse train of IR pulses radiated by IR source  38  shown in  FIG. 2A  to illuminate scene  40 . The choice of four IR pulses in the train of pulses is by way of example only and is a choice of convenience. 
     The gates of fast gate  110  are represented by dashed rectangles  141 ,  142  and  143 . Gates  141  are short “distance gates” and gates  142  are short “background gates”. A distance gate  141  and a background gate  142  follow every other IR pulse (the odd numbered IR pulse in  FIG. 2B ) in the pulse train radiated by IR source  38 . Gates  143  arc long “normalization” gates that preferably follow every IR pulse (the even numbered IR pulses in  FIG. 2B ) that is not followed by a distance gate  141  and a background gate  142 . 
     The on time of distance, background and normalization photosurfaces  121 ,  122  and  123  are shown as trapezoids  150  on timelines  151 ,  152  and  153  respectively. Distance photosurface  121  is on only during short distance gates  141 . Background photosurface  122  is on only during short background gates  142  and normalization photosurface  123  is on only during long normalization gates  143 . The sloped side edges of the trapezoids indicate the relatively long periods of time required to turn on and off a photosurface compared to the short turn-on and turn-off times required to gate photosurfaces quickly enough to provide accurate distance measurements. 
     An intensity of light received by taking lens  35  from elephant  42  as a function of time is represented by the height of an “intensity” line  160  above a base line  162 . Rectangular “peaks”  164  in the height of intensity line  160  occur when IR light reflected from an IR pulse by elephant  42  reaches taking lens  35 . The height of intensity line  160  outside of peaks  164  represents background light received from elephant  42 . Amounts of light received from elephant  42  that are registered by photosurfaces  121 ,  122  and  123  during gates  141 ,  142  and  143  are represented by shaded areas  171 ,  172  and  173  respectively. 
     Similarly, an intensity of light received by taking lens  35  from second elephant  44  as a function of time is represented by the height of an intensity line  180 , which has IR “reflection peaks”  184 , above a base line  182 . Shaded areas  181 ,  182 , and  183  represent amounts of light from second elephant  44  registered by photosurfaces  121 ,  122  and  123  during gates  141 ,  142  and  143  respectively. 
     For elephant  42 , background corrected light registered by distance photosurface  121  is represented by an area equal to the sum of the areas  171  minus the sum of areas  172 . Background corrected light registered by normalization photosurface  123  is represented by an area equal to the sum of areas  173  minus the product of the ratio of the gate width of long gates  143  to the gate width of short gates  142  times the sum of areas  173 . For elephant  44 , background corrected light registered by distance photosurface  121  is represented by an area equal to the sum of areas  181  minus the sum of areas  182 . Background corrected light registered by normalization photosurface  123  for elephant  44  is represented by an area equal to the sum of areas  183  minus the product of the ratio of the gate width of long gates  143  to the gate width of short gates  142  times the sum of areas  183 . 
     Elephant  44  is further away from 3D imager  100  than elephant  42 . As a result, the amount of background corrected light registered by distance photosurface  121  normalized to background collected light registered by normalization photosurface  123  is less for elephant  44  than for elephant  42 . 
       FIG. 3  shows another 3D imager  200  imaging scene  40 , in accordance with a preferred embodiment of the present invention. 
     Imager  200  comprises a 3D module  202  having components shown inside dashed border  204 , which is coupled to taking lens  35  and video camera  36 . 3D module  202  comprises a 3D camera  206  having components shown inside dashed border  208 . 3D camera  206  comprises a four-way prism  210  that directs portions of IR light  60  in light  46  that is collected by taking lens  35  from scene  40  to distance, background and normalization photosurfaces  71 ,  72  and  73  respectively. Each of photosurfaces  71 ,  72  and  73  is gated by its own fast shutter  81 ,  82  and  83  respectively. Photosurfaces  71 ,  72  and  73  are gated similarly to the manner in which photosurfaces  71 ,  72  and  73  comprised in 3D camera  206 , shown in  FIG. 1 , are gated. Amounts of IR light  60  collected by taking lens  35  that reaches 3D camera  206  is controlled by iris  67  comprised in taking lens  35 . 
     Four-way prism  210  also directs visual light  48  in light  46  collected by taking lens  35  to a refocuser  28  that preferably comprises a field lens  50  an iris  52  and a relay lens  54 . Refocuser  28  transmits visual light  48  that it receives from four-way prism  210  to video camera  36 . 
       FIG. 4  schematically shows another 3D imager  220  imaging scene  40  in accordance with a preferred embodiment of the present invention. 3D imager  220  comprises a unit  222 , hereinafter referred to as a “combination unit”, having components located within a border  224 . Combination unit  222  comprises a 3D camera  226  having components shown inside a dashed boundary  228  and three color cameras, a Red (R) camera  230  a Green (G) camera  232  and a Blue (B) camera  234 . Combination unit  222  is coupled to a taking lens  35 , which is boresighted with 3D camera  226  and color cameras  230 ,  232  and  234 . 
     Combination unit  222  comprises a four-way prism  236  that directs visual light  48  collected by taking lens  35  to each of color cameras  230 ,  232  and  234  and IR light  60  collected by taking lens  35  to a fast shutter  110  that shutters 3D camera  226 . Preferably, combination unit  222  comprises an iris  111  that controls the amount of light  60  transmitted from taking lens  35  to 35D camera  226 . IR light  60  that fast shutter  110  transmits is incident on a relay lens  238  that relays the light to a three-way prism  68 , which directs portions of the IR light  60  that it receives to a distance photosurface  121 , a background photosurface  122  and a normalization photosurface  123 . 3D camera  226  is substantially the same as 3D camera  106  shown in  FIG. 2A  and is gated by controlling fast shutter  110  and photo surfaces  121 ,  122 , and  123  in the same manner in which 3D camera  106  is gated. 
     Whereas combination unit  22  is shown comprising a 3D camera in which all the photosurfaces are shuttered by single shutter, in some preferred embodiments of the present invention, combination unit  222  comprises a 3D camera similar to 3D camera  32  shown in  FIG. 1  in which each photosurface of the 3D camera is shuttered by its own shutter. 
     Combination unit  222  preferably comprises an adjustable neutral density filter  240  located between taking lens  35  and four-way prism  236 . Neutral density filter  240  is chosen so that it does not substantially attenuate IR light. Neutral density filter  240  is used to control the amount of visible light  48  collected by taking lens  35  that reaches color cameras  230 ,  232  and  234 . The amount of IR light  60  reaching 3D camera  226  is controlled by iris  67  comprised in taking lens  35 . 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb. 
     The present invention has been described using detailed descriptions of preferred embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described preferred embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons of the art. The scope of the invention is limited only by the following claims.