Abstract:
An electronic imaging device ( 27 ) includes an optical objective ( 28 ) for collecting optical radiation from an object, the objective having an optical axis, and an image sensor ( 24 ), including a matrix of optical detectors arranged in a plane that is substantially non-perpendicular to the optical axis, the image sensor having a lateral dimension in the plane. A turning mirror ( 38 ) has an optical surface that is positioned so as to reflect the radiation collected by the objective in order to form a focused image in the plane of the image sensor, while a maximum distance from the optical surface to the plane of the image sensor is substantially less than the lateral dimension of the image sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/381,478, filed May 16, 2002, whose disclosure is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to electronic imaging systems, and particularly to miniature camera heads and associated illumination devices, especially for use in endoscopy.  
       BACKGROUND OF THE INVENTION  
       [0003]     Miniature, remote-head cameras are commonly used in endoscopy and other areas of minimally-invasive surgery. A solid-state imaging sensor is fixed in the distal end of an endoscope, along with suitable imaging optics and an illumination source, in order to capture images within body cavities and passageways. In general it is desirable to reduce the endoscope diameter and at the same time to improve the image quality obtained from the distal-end camera head. These two objectives are often mutually contradictory, since increasing the resolution of the sensor generally requires increasing its size, which leads to increasing the diameter of the endoscope.  
         [0004]     A wide variety of distal-end camera heads have been described in the patent literature, based mainly on integration of the sensor, typically a CCD-based sensor, with suitable miniature optics. Some exemplary camera head designs are described in U.S. Pat. Nos. 4,604,992, 4,491,865, 4,746,203, 4,720,178, 5,166,787, 4,803,562, and 5,594,497. Some systems and methods for reducing the overall dimensions of the distal end of an endoscope containing an image sensor are described in U.S. Pat. Nos. 5,929,901, 5,986,693, 6,043,839, 5,376,960, and 4,819,065, and in U.S. Patent Application Publication No. 2001/0031912 A1. One technique that has been suggested for reducing endoscope diameter is to orient the image sensor in a plane that is parallel to the axis of the imaging optics, rather than perpendicular to the plane as in conventional optical designs. Implementations of this technique are described in U.S. Pat. Nos. 4,692,608, 4,646,721 and 4,986,642 and in the above-mentioned U.S. Patent Application Publication 2001/0031912 A1. The disclosures of all the above publications are incorporated herein by reference.  
         [0005]     Although most endoscopes provide the user with a single, two-dimensional image, endoscopes with three-dimensional imaging capability are also known in the art. For example, endoscopes that generate stereoscopic images by using two different optical paths are described in U.S. Pat. Nos. 5,944,655, 5,222,477, 4,651,201, 5,191,203, 5,122,650, 5,471,237, 5,673,147, 6,139,490, and 5,603,687, whose disclosures are likewise incorporated herein by reference.  
         [0006]     Endoscopes typically use an external illumination source to provide radiation to the distal end of the endoscope via fiber optics. On the other hand, some endoscopes employ illumination devices integrated within the endoscope itself, either at the distal end or at the proximal end of the endoscope. For example, the use of Light Emitting Diodes (LEDs) for this purpose is described in U.S. Pat. Nos. 6,318,887, 6,331,156, 6,260,994, 6,371,907, and 6,340,868, whose disclosures are incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0007]     In embodiments of the present invention, a miniature camera head assembly comprises an objective for collecting optical radiation from an object, and an image sensor, which is oriented in a plane that is substantially non-perpendicular to the optical axis of the objective. Typically, the sensor plane is parallel to the optical axis. A turning mirror, typically a prism, directs the radiation collected by the objective to form a focused image on the image sensor.  
         [0008]     The camera head assembly is constructed and configured so as to reduce the radial dimensions of the assembly (measured in a plane perpendicular to the optical axis) to a substantially smaller size than has been achieved in comparable assemblies known in the art. Typically, the assembly is capable of fitting inside a tube, such as the insertion tube of an endoscope, whose diameter is smaller than the diagonal dimension of the image sensor. The reduction of diameter is achieved, inter alia, by a novel optical design, which allows the height of the turning mirror above the image sensor to be reduced in comparison to designs known in the art in which the sensor is oriented parallel to the optical axis. Additionally or alternatively, novel methods for mounting the image sensor chip within the camera head are used to reduce the diameter still further.  
         [0009]     Camera head assemblies in accordance with the present invention are thus useful particularly in producing endoscopes of small diameter, relative to endoscopes of comparable resolution that are known in the art. Embodiments of the present invention may additionally be used in other imaging applications in which size and weight are at a premium, such as in military and surveillance cameras and industrial cameras for diagnostics of small cavities.  
         [0010]     There is therefore provided, in accordance with an embodiment of the present invention, an electronic imaging device, including: 
        an optical objective for collecting optical radiation from an object, the objective having an optical axis;     an image sensor, including a matrix of optical detectors arranged in a plane that is substantially non-perpendicular to the optical axis, the image sensor having a lateral dimension in the plane; and     a turning mirror, having an optical surface that is positioned so as to reflect the radiation collected by the objective in order to form a focused image in the plane of the image sensor, while a maximum distance from the optical surface to the plane of the image sensor is substantially less than the lateral dimension of the image sensor.        
 
         [0014]     Typically, the maximum distance from the optical surface to the plane of the image sensor is less than approximately 75% of the lateral dimension of the image sensor, and the plane of the image sensor is substantially parallel to the optical axis.  
         [0015]     In some embodiments, the turning mirror includes a prism, haying an exit face adjacent to the image sensor and an entrance face adjacent to the objective, and the optical surface includes a reflective face of the prism oriented at a diagonal between the entrance and exit faces. In one embodiment, a surface of the prism opposite the exit face is flattened so as to reduce a height of the entrance face of the prism so that the height is substantially less than the lateral dimension of the image sensor, and the flattened surface of the prism has edges that are phased so as to fit the prism within a tube in which the device is contained. Additionally or alternatively, the entrance face of the prism is shaped so as to define an indentation, in which the objective is positioned.  
         [0016]     Typically, the image sensor includes a semiconductor chip on which the matrix of optical detectors is formed, wherein the chip is thinned following fabrication of the optical detectors on the chip.  
         [0017]     There is also provided, in accordance with an embodiment of the present invention, an electronic imaging device, including: 
        an optical objective for collecting optical radiation from an object, the objective having an optical axis;     an image sensor oriented in a plane that is substantially non-perpendicular to the optical axis, and including: 
            a semiconductor chip including a monolithic array of optical detectors and having a predetermined chip area; and     a chip package on which the chip is mounted, the package having a total area no greater than about 200% of the chip area; and    
            a turning mirror, having an optical surface that is positioned so as to direct the radiation collected by the objective to form a focused image in the plane of the image sensor.        
 
         [0023]     Preferably, the chip package has a total area no greater than about 150% of the chip area, and more preferably no greater than about 120% of the chip area.  
         [0024]     In some embodiments, the device includes an electronic circuit board, on which the image sensor is mounted, wherein the chip package includes a ball grid array (BGA) for contacting the circuit board. In one embodiment, the circuit board is formed so as to define an opening therethrough, and the image sensor is mounted adjacent to the opening, so that the chip package is located on a first side of the circuit board, while the turning mirror is located on a second side of the circuit board, opposite the first side, so as to direct the radiation through the opening onto the image sensor.  
         [0025]     There is additionally provided, in accordance with an embodiment of the present invention, an endoscope, including: 
        an insertion tube of predetermined diameter, the tube having a longitudinal axis and a distal end;     an image sensor fixed within the insertion tube, the image sensor including a matrix of optical detectors arranged in a plane that is substantially non-perpendicular to the longitudinal axis, the image sensor having a diagonal dimension in the plane that is substantially greater than the diameter of the insertion tube; and     imaging optics fixed adjacent to the distal end of the tube for focusing optical radiation from an object onto the image sensor so as to form an image of the object on the image sensor.        
 
         [0029]     In one embodiment, the endoscope includes one or more light emitting diodes (LEDs) mounted at the distal end of the insertion tube so as to illuminate the object. The endoscope may also include an electronic circuit board, which includes a first mounting area on which the image sensor is mounted, and a second mounting area on which the one or more LEDs are mounted, wherein the second mounting area is angled relative to the first mounting area.  
         [0030]     Alternatively, the endoscope includes a light source located proximally to the distal end of the insertion tube, and a light guide, which passes through the insertion tube so as to emit light from the distal end of the tube to illuminate the object.  
         [0031]     There is further provided, in accordance with an embodiment of the present invention, an endoscope, including: 
        an insertion tube having a longitudinal axis and a distal end; and     an electronic imaging device, mounted within the distal end of the insertion tube, and including:     an optical objective for collecting optical radiation from an object, the objective having an optical axis, which is substantially parallel to the longitudinal axis of the insertion tube;     an image sensor, including a matrix of optical detectors arranged in a plane that is substantially non-perpendicular to the optical axis, the image sensor having a lateral dimension in the plane; and     a turning mirror, having an optical surface that is positioned so as to reflect the radiation collected by the objective in order to form a focused image in the plane of the image sensor, while a maximum distance from the optical surface to the plane of the image sensor is substantially less than the lateral dimension of the image sensor.        
 
         [0037]     Typically, the turning mirror includes a prism, having an exit face adjacent to the image sensor and an entrance face adjacent to the objective, and the optical surface includes a reflective face of the prism oriented at a diagonal between the entrance and exit faces, wherein a surface of the prism opposite the exit face is flattened and phased so as to fit within the insertion tube.  
         [0038]     There is moreover provided, in accordance with an embodiment of the present invention, an endoscope, including: 
        an insertion tube having a longitudinal axis and a distal end; and     an electronic imaging device, mounted within the distal end of the insertion tube, and including: 
            an optical objective for collecting optical radiation from an object, the objective having an optical axis, which is substantially parallel to the longitudinal axis of the insertion tube;     an image sensor oriented in a plane that is substantially non-perpendicular to the optical axis, and including:     a semiconductor chip including a monolithic array of optical detectors and having a predetermined chip area; and     a chip package on which the chip is mounted, the package having a total area no greater than about 200% of the chip area; and     a turning mirror, having an optical surface that is positioned so as to direct the radiation collected by the objective to form a focused image in the plane of the image sensor.    
               
 
         [0046]     There is furthermore provided, in accordance with an embodiment of the present invention, an electronic imaging device, including: 
        first and second optical objectives for collecting optical radiation from an object, the objectives having respective first and second optical axes, which are mutually substantially parallel;     first and second image sensors, including respective matrices of optical detectors, which are arranged back-to-back in respective first and second planes that are substantially non-perpendicular to the optical axes, the image sensors having a lateral dimension in the respective planes; and     first and second turning mirrors, having respective first and second optical surfaces that are positioned so as to reflect the radiation collected by the first and second objectives, respectively, so as to form respective first and second images in the first and second planes of the image sensors, while a maximum distance from the first optical surface to the first plane and from the second optical surface to the second plane is substantially less than the lateral dimension of the image sensors.        
 
         [0050]     Typically, the first and second image sensors are adapted to generate respective first and second electrical signals responsively to the optical radiation that is incident thereon, and the device includes an image processor, which is coupled to receive the first and second electrical signals and to process the signals so as to produce a stereoscopic image of the object.  
         [0051]     In a disclosed embodiment, the device includes a circuit board, having first and second sides, wherein the first and second image sensors are mounted respectively on the first and second sides of the circuit board, and the first and second planes are substantially parallel to the optical axis.  
         [0052]     There is also provided, in accordance with an embodiment of the present invention, an endoscope, including: 
        an insertion tube having a longitudinal axis and a distal end; and     an electronic imaging device, mounted within the distal end of the insertion tube, and including: 
            first and second optical objectives for collecting optical radiation from an object, the objectives having respective first and second optical axes, which are mutually substantially parallel;     first and second image sensors, including respective matrices of optical detectors, which are arranged back-to-back in respective first and second planes that are substantially non-perpendicular to the optical axes, the image sensors having a lateral dimension in the respective planes; and     first and second turning mirrors, having respective first and second optical surfaces that are positioned so as to reflect the radiation collected by the first and second objectives, respectively, so as to form respective first and second images in the first and second planes of the image sensors, while a maximum distance from the first optical surface to the first plane and from the second optical surface to the second plane is substantially less than the lateral dimension of the image sensors.    
               
 
         [0058]     There is further provided, in accordance with an embodiment of the present invention, imaging apparatus, including: 
        a camera head including an image sensor, which is adapted to capture an electronic image of an object; and     a light source, which includes: 
            an array of light emitting diode (LEDs), which are adapted to generate optical radiation;     an array of optical fibers, having respective proximal and distal ends, wherein the proximal ends are respectively coupled to the LEDs so that each of the fibers receives the radiation emitted by a respective one of the LEDs, and the distal ends are arranged to convey the radiation to a vicinity of the camera head so as to illuminate the object; and     a controller, which is adapted to drive the LEDs to emit the optical radiation at different, respective intensities so as to adjust illumination of the object.    
               
 
         [0064]     Typically, the controller is adapted to drive the LEDs responsively to the image of the object, so as to adjust for uneven brightness in the image.  
         [0065]     There is moreover provided, in accordance with an embodiment of the present invention, a method for electronic imaging, including: 
        aligning an optical objective to collect optical radiation from an object along an optical axis;     arranging an image sensor, including a matrix of optical detectors, in a plane that is substantially non-perpendicular to the optical axis, the image sensor having a lateral dimension in the plane; and     positioning an optical surface of a turning mirror so as to reflect the radiation collected by the objective in order to form a focused image in the plane of the image sensor, such that a maximum distance from the optical surface to the plane of the image sensor is substantially less than the lateral dimension of the image sensor.        
 
         [0069]     There is furthermore provided, in accordance with an embodiment of the present invention, a method for electronic imaging, including: 
        aligning an optical objective for collecting optical radiation from an object along an optical axis;     orienting an image sensor in a plane that is substantially non-perpendicular to the optical axis, the image sensor including a semiconductor chip, which includes a monolithic array of optical detectors and having a predetermined chip area and is mounted on a chip package having a total area no greater than about 200% of the chip area; and     positioning an optical surface of a turning mirror so as to direct the radiation collected by the objective to form a focused image in the plane of the image sensor.        
 
         [0073]     There is moreover provided, in accordance with an embodiment of the present invention, a method for endoscopic imaging, including: 
        providing an insertion tube of predetermined diameter, the tube having a longitudinal axis and a distal end;     fixing an image sensor within the insertion tube, the image sensor including a matrix of optical detectors arranged in a plane that is substantially non-perpendicular to the longitudinal axis, the image sensor having a diagonal dimension in the plane that is substantially greater than the diameter of the insertion tube; and     aligning imaging optics adjacent to the distal end of the tube, so as to focus optical radiation from an object onto the image sensor in order to form an image of the object on the image sensor.        
 
         [0077]     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0078]      FIG. 1  is a block diagram that schematically illustrates an endoscopic imaging system, in accordance with an embodiment of the present invention;  
         [0079]      FIGS. 2 and 3  are schematic, sectional diagrams of camera head assemblies, in accordance with embodiments of the present invention;  
         [0080]      FIG. 4  is a schematic end view of the camera head assembly of  FIG. 2 ;  
         [0081]      FIG. 5  is schematic optical ray diagram of a prism used in a camera head assembly, in accordance with an embodiment of the present invention;  
         [0082]      FIG. 6  is a schematic optical ray diagram showing elements of a camera head assembly, in accordance with an embodiment of the present invention;  
         [0083]      FIGS. 7A and 7B  are schematic side and end views, respectively, of a prism for use in a camera head assembly, in accordance with an embodiment of the present invention;  
         [0084]      FIG. 8  is a schematic end view of a prism for use in a camera head assembly, in accordance with an alternative embodiment of the present invention;  
         [0085]      FIG. 9  is a schematic optical ray diagram showing elements of a camera head assembly, in accordance with another embodiment of the present invention;  
         [0086]      FIG. 10  is a schematic top view of a sensor assembly used in a camera head, in accordance with an embodiment of the present invention;  
         [0087]      FIG. 11  is a schematic sectional view of a sensor assembly used in a camera head, in accordance with an embodiment of the present invention;  
         [0088]      FIG. 12  is a schematic sectional view of a sensor assembly used in a camera head, in accordance with another embodiment of the present invention;  
         [0089]      FIG. 13  is a schematic top view of a illumination assembly for use in an endoscopy system, in accordance with an embodiment of the present invention; and  
         [0090]      FIGS. 14 and 15  are a schematic sectional side view and an end view, respectively, of a stereoscopic camera head assembly, in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0091]     Reference is now made to  FIG. 1 , which is a block diagram that schematically illustrates an endoscopic imaging system  20 , in accordance with an embodiment of the present invention. System  20  comprises an endoscope  21 , which is connected by a cable  12  to a processing unit  16 . The endoscope comprises an insertion tube  23 , containing a miniature camera head at its distal end  25 , as shown and described hereinbelow. Typically, the endoscope also contains an internal light source, for illuminating the area adjacent to the distal end of the endoscope, which is imaged by the camera head. Alternatively or additionally, an external light source  14  may be used to provide illumination via a fiberoptic bundle  15  to a light guide within endoscope  21 . The external light source may alternatively be coupled optically to the distal end of the endoscope via one or more liquid-filled light guides. Light source  14  typically comprises one or more solid-state emitters, such as LEDs, as described below. Alternatively, the light source may comprise a gas discharge lamp, as is known in the art.  
         [0092]     A processing unit  16  receives signals from the miniature camera head via cable  12 , and processes the signals to generate video images on a display  18 . Processing unit  16  may be either a stand-alone unit with image processing capabilities and control circuitry, or a personal computer (PC) with suitable front-end circuits and software. Alternatively, the functions of the processing unit may be performed by electronics within endoscope  21 . The electronics, as well as a light source for providing illumination at distal end  25 , may be contained within a handle (not shown) that is used in manipulating the endoscope. In some embodiments, as illustrated below in  FIGS. 15 and 16 , endoscope  20  provides stereoscopic image information, and the processing unit may perform three-dimensional image reconstruction and display. Processing unit  16  may further function as a controller for light source  14 , as is further described below.  
         [0093]      FIG. 2  is a schematic, sectional illustration showing a miniature camera head assembly  27  within insertion tube  23 , in accordance with an embodiment of the present invention. One or more light sources  30  illuminate the region immediately distal to endoscope  21 . Typically, light sources  30  comprise white-light LEDs, but alternatively, miniature light source of different types may be used, including LEDs of other colors or infrared LEDs or, in some applications, a miniature incandescent source, as is known in the art.  
         [0094]     An optical objective  28 , mounted at distal end  25 , collects and focuses light from objects illuminated by light source  30 . A turning mirror, typically comprising a right angle prism  38 , reflects the light collected by objective  28  to focus on the focal plane of an image sensor  24 . Sensor  24  typically comprises a two-dimensional matrix of detector elements, based on CMOS, CCD or other solid-state imaging technology, as is known in the art. For example, sensor  24  may comprise a MI0133 CMOS imaging array, produced by Micron Technology Inc., of Boise, Id., comprising 377×312 detector elements, giving an imaging area of about 2×1.8 mm, out of overall chip dimensions of 3×3.7 mm (with a diagonal dimension of about 4.8 mm). Typically, prism  38  is arranged to turn the optical axis of the focused rays by 90°, so that the focal plane of the sensor is substantially parallel to the optical axis of objective  28 . Alternatively, the turning mirror and image sensor may be arranged so that the sensor is oriented at a different angle, non-perpendicular to the optical axis of the objective.  
         [0095]     Sensor  24  is mounted on a circuit substrate, such as a printed circuit board  40 , by balls  29 , which are arranged in a ball grid array (BGA), as is known in the art. This method of packaging and mounting sensor  24  enables the sensor to be contained in a chip-scale package, which is not much wider than the sensor chip itself. This and other methods of chip-scale packaging and mounting of the sensor chip are described in detail hereinbelow. A cable  22  passing through endoscope  21  connects assembly  27  to processing unit  16 . One or more controller and communication interface chips  26  on board  40  serve to pass electrical signals from image sensor  24  to processing unit  16  and to receive control inputs from the processing unit. Cable  22  is typically mechanically secured to board  40  by a cable clamp  34 . A perpendicular extension  41  of board  40  may be provided for mounting light sources  30 . Alternatively, board  40  may comprise a flexible end portion, which is bent to mount light sources  30 . Further alternatively, any other suitable means known in the art may be used to mount the light sources at distal end  25 . A working channel  42 , which runs substantially the entire length of endoscope  21 , is located beneath board  40 .  
         [0096]      FIG. 3  is a schematic, sectional illustration of a miniature camera head assembly  37 , in accordance with another embodiment of the present invention. Assembly  37  is similar to assembly  27 , shown in  FIG. 2 , except that in the assembly  37 , a remote light source is used to provide illumination to distal end  25  via a fiberoptic light guide  44  running the length of endoscope  21 . The remote light source may comprise external light source  14  or an internal light source (not shown) located proximally within endoscope  21 . An illumination lens  31  directs the light from light guide  44  onto objects distal to the endoscope, in place of light source  30 . Light guide  44  typically comprises multiple optical fibers, which may branch at distal end  25  in order to output light through multiple illumination lenses. As a further alternative, one or more distal-end light sources  30 , as shown in  FIG. 2 , may be used in combination with one or more lenses  31 , fed by light guide  44 .  
         [0097]      FIG. 4  is a schematic end view of insertion tube  23 , illustrating the elements of assembly  27  ( FIG. 2 ) at distal end  25 . Four light sources  30  are shown in the present embodiment, but more or fewer illumination sources may be employed as necessary to provide adequate illumination. Assembly  37  may be similarly configured, using illumination lenses  31  in place of light sources  30 . It will be observed that the minimum achievable diameter of insertion tube  23  is determined approximately by the lateral (width) dimensions of sensor  24  and board  40  and by the height of prism  38  above the plane of sensor  24 . Techniques and optical designs for reducing the height of prism  38  are described hereinbelow. Preferably, the chip on which sensor  24  is fabricated is thinned, and the chip package (if used) and board  40  are designed so that the board is not much wider than the chip itself. In this manner, the diameter of tube  23  may be made smaller than the diagonal dimension of sensor  24 .  
         [0098]     Minimizing the overall radial dimensions of assemblies  27  and  37  is a major consideration in their design, so that insertion tube  23  may itself be made narrower and pass more easily through narrow body passages. As noted above, typical lateral dimensions for image sensor  24  are 3×3.7 mm. The sensor chip as fabricated is typically about 0.7 mm thick. Board  40  has a typical thickness of 0.3 mm. Tube  23  has a wall thickness of about 0.15 mm. In the view shown in  FIG. 4 , it can be seen that the diameter of tube  23  is limited in the horizontal direction by the width of board  40  plus twice the wall thickness of the insertion tube. Taking the board width to be 3 mm, the minimal diameter in the horizontal direction is given by: 
 
diameter≧3+2*0.15=3.3 mm  (1) 
 
         [0099]     In the vertical direction, on the other hand, the minimal diameter of tube  23  is limited by the thickness of board  40 , plus twice the thickness of sensor  24  and twice the prism height (assuming board  40  to be roughly centered within tube  23 ), plus twice the thickness of the insertion tube. In conventional optical designs, the height of the prism (or other turning mirror) can be no less than the lateral dimension of the sensor array, i.e., 2 mm in the present example. The limit of the diameter in the vertical direction is then: 
 
diameter≧0.3+2*(0.7+2+0.15)=6.0 mm  (2) 
 
 In embodiments of the present invention, however, the prism height is reduced, as shown in  FIG. 4 , so that the limiting diameter of insertion tube  23  in the vertical direction is substantially reduced as well. The sensor chip may be thinned, as well, below the 0.7 mm standard thickness. As a result of these steps, the minimal diameter of tube  23  is reduced, preferably to less than the diagonal dimension of sensor  24  (4.8 mm in the present example). 
 
         [0100]     Reference is now made to  FIG. 5 , which is a simplified ray diagram of prism  38 , showing how a minimal height H of the prism is determined, in accordance with an embodiment of the present invention. A central ray  54  and extreme rays  53  and  55  emanate from an aperture stop  50  of optical assembly  27  and are reflected at a reflecting surface  52  of prism  38  to focus on sensor  24 . Ray  54  is incident at the center of sensor  24 , while rays  53  and  55  correspond to the edges of the image. Let D be the distance from stop  50  to the point at which ray  54  reflects from surface  52 , and let A be the horizontal distance between the points at which rays  54  and  55  impinge on sensor  24 . (In other words, A should be roughly half the width of the active area of sensor  24 , so that A=Y½ mm=1 mm in the present example.) The relationship between the height H of prism  38  and the distances A and D can be expressed as: 
 
 H= 2 A *( A+D )/(2 A+D )  (3) 
 
         [0101]     Thus, reducing D for a given value of A allows the height H of prism  38  to be reduced. Let C be defined as the horizontal distance between the front surface of prism  38  and the point at which ray  54  reflects from surface  52 , while B is defined as the horizontal distance between the points at which rays  54  and  53  impinge on sensor  24 . Assuming the distances C and B to be roughly equal, as shown in  FIG. 5 , and B to be roughly equal to A, the minimal distance, D min , is: 
 
 D   min   =B=C   (4) 
 
 which gives: 
 
 H   min =1.33*A  (5) 
 
 Inserting the value H=1.33 mm into equation (2) gives 4.66 mm as the limiting diameter of tube  23 . It will be observed that this limit is less than the diagonal dimension of sensor  24 , which is about 4.8 mm, as noted above. 
 
         [0102]     In practical optical designs, it may be difficult to reduce D to the D min  value given by equation (4), because this constraint would appear to require that aperture stop  50  be located at the entrance face of prism  38 . Even at a larger value of D, however, it is still possible, based on the present invention, to reduce H substantially below the nominal height of H≈2A that is typical of turning mirrors known in the art (i.e., equal height and base dimensions of the prism, with the base dimension roughly equal to the lateral dimension of the image sensor). For example, for D=2A, a prism of height H=1.5*A may be used, so that the height of the prism is approximately 75% or less of the lateral dimension of sensor  24 . As shown below in  FIG. 9 , it is also possible to design the turning prism so that C&lt;B, whereby H can be reduced still further, down to a limit of H min ≈A.  
         [0103]      FIG. 6  is a ray diagram showing an exemplary optical design of assembly  27 , in accordance with an embodiment of the present invention. Light ray groups  72  and  74  define the edges of an image collected by objective  28  and reflected by surface  52  onto a focal plane  70  of sensor  24 . Prism  38  is designed so that ray group  72  exits from the prism at its lower left corner, while ray group  74  exits from the prism at its lower right corner. The prism is truncated in a plane  80  at a height that is slightly greater than 1.33*A. Because of the short distance D between aperture stop  50  and surface  52 , the upper portion of the surface (above plane  80 ) is not required and can be eliminated in the manner shown in this figure.  
         [0104]     Objective  28  in this embodiment comprises a protective window  73 , followed by two lenses  75  and  77 , with air gaps between them. Prism  38  and both lenses are made from PMMA. Aperture stop  50  is located at the first surface of lens  77 . The front focal length of objective  28  is 10 mm in water. Table I below lists the optical parameters of this design:  
                     TABLE I                       EXEMPLARY OPTICAL DESIGN PARAMETERS OF  FIG. 6                                  Protective window 73 made from BK7 with thickness 0.1 mm.       Air gap from protective window to first surface of lens 75: 0.05 mm.       Radius of curvature of first surface of lens 75: −.7982 (concave surface).       Thickness of lens 75: 0.3 mm.       Radius of curvature of second surface of lens 75: 1.593 (concave surface).       Air distance between second surface of lens 75 and aperture       stop 50: 0.157 mm.       Aperture stop diameter: 0.29 mm.       Air distance between aperture stop 50 and first surface of lens 77: 0.       Radius of curvature of first surface of lens 77: 1.756 (convex surface).       Thickness of lens 77: 0.4 mm.       Radius of curvature of second surface of lens 77: −0.547 (convex surface)       Air distance between second surface of lens 77 and prism 38: 0.1 mm.       45° prism, base 2 mm × 2 mm.       Distance between exit surface of prism and sensor 24: 0.15 mm.                  
 
         [0105]     Reference is now made to  FIGS. 7A and 7B , which are schematic side and end views, respectively, of a prism  39  with further reduced dimensions in accordance with an embodiment of the present invention. Side phases  76  are cut on either side of plane  80 , so as to allow the prism to be more easily integrated into a round tube, as shown in  FIG. 4 . The side phases do not affect the optical performance of prism  39 , since they involve removal of material only from areas of surface  52  that are not used in reflecting the image rays from objective  28  to sensor  24 .  
         [0106]      FIG. 8  is a schematic end view of another prism  81 , having rounded phases  78 , in accordance with an alternative embodiment of the present invention. Alternatively or additionally, any number of straight side phases may be employed to yield a prism with a shape ranging from that shown in  FIGS. 7A and 7B  to that shown in  FIG. 8 .  
         [0107]      FIG. 9  is a ray diagram showing an exemplary optical design of another camera head assembly  87 , in accordance with a further embodiment of the present invention. In this embodiment, a prism  83  and an accompanying objective  85  are designed so that the height of the prism is reduced even more than in the preceding embodiment. For this purpose, objective  85  (and hence aperture stop  50 ) is placed closer to surface  52 , by means of an indentation formed in the entrance surface of prism  83 . Equivalently, a front extension may added to the prism. In either case, the lower right portion of the prism serves to prevent deformation of ray group  74 .  
         [0108]     Objective  85  in this embodiment comprises three lenses  90 ,  92  and  94 , of which lenses  92  and  94  are doublets, with air gaps between the lenses. Aperture stop  50  is located between lenses  92  and  94 . The front focal length of objective  28  is 30 mm in water. Table II below lists the optical parameters of this design:  
                     TABLE II                       EXEMPLARY OPTICAL DESIGN PARAMETERS OF  FIG. 9                                  Lens 90 made from SK16 with thickness 0.1 mm.       First radius of curvature of lens 90 is infinity (flat), second       is 0.927 (concave).       Air gap from lens 90 to first surface of lens 92: 0.05 mm.       Lens 92 made from SK16/SFL6.       Radius of curvature of first surface of lens 92: 0.461 (convex surface).       Radius of curvature of second surface of lens 92: 0.4135       (concave to first element and convex to cemented element).       Radius of curvature of third surface of lens 92: 1.111 (concave surface).       Thicknesses of lens 92: 0.1/0.1 mm (overall doublet thickness 0.2 mm).       Air distance between third surface of lens 92 and aperture       stop 50: 0.05 mm.       Air distance between aperture stop 50 and first surface       of lens 94: 0.05 mm.       Lens 94 made from SFL6/SK16.       Radius of curvature of first surface of lens 94: 2.376 (convex surface).       Radius of curvature of second surface of lens 94: 7.4557       (concave to first element and convex to cemented element).       Radius of curvature of third surface of lens 94: 19.037 (concave surface).       Thicknesses of lens 94: 0.1/0.1 mm (overall doublet thickness 0.2 mm).       Air distance between third surface of lens 94 and prism 83: 0.05 mm.       45° prism, 1.7 mm × 1.7 mm base, made from SFL6.       Distance between exit surface of prism and focal plane 70: 0.05 mm.                  
 
         [0109]     Reference is now made to  FIG. 10 , which is a schematic top view of the sensor assembly portion of camera head assembly  27 , which was shown in sectional view in  FIG. 2 . In this embodiment, image sensor  24  is contained in a ShellOP package, which is produced by ShellCase Ltd., of Jerusalem, Israel, or in another, similar type of package. The input/output (I/O) pads of the image sensor chip are extended to the bottom of the die by extension wires (not shown), where they are connected to board  40  by balls  29 , as noted above. This technology is one implementation of Ball Grid Array (BGA) packaging technology, as is known in the art. Before packaging the sensor chip in the ShellOP package, the silicon die is thinned, so that the total thickness of the chip plus package is typically about 0.7 mm. The limiting diameter of insertion tube  23 , as given by equation (2), is maintained accordingly.  
         [0110]     Note also that the total area of sensor  24  plus its package, as measured in the plane of  FIG. 10 , is only slightly greater than the area of the sensor chip itself. Therefore, the limiting diameter of the insertion tube in the horizontal direction, as given by equation (1), is not substantially increased by the chip package. Preferably, in this and the other embodiments described below, sensor  24  is contained in a chip-scale package that is, in terms of total area in the plane of the sensor, no more than 200% of the area of the sensor chip itself. More preferably, the total area of the chip-scale package is not more than 150% of the area of the sensor chip, and most preferably, not more than about 120%. It will be understood that these dimensions are approximate, and may be varied by ±25% while still realizing this aspect of the present invention.  
         [0111]     Similarly, board  40  is preferably only minimally wider than the package of sensor  24  (wherein the width dimension in this case is taken in the vertical direction in  FIG. 10 , or equivalently, the horizontal direction in  FIG. 4 ). In order to minimize the required width of the board, sensor  24  may be produced with I/O pads on only two sides of the chip, typically on the left and right sides in the view shown in  FIG. 10 .  
         [0112]      FIG. 11  is a schematic sectional view of a sensor assembly  100  used in a camera head, in accordance with another embodiment of the present invention. In this embodiment, image sensor  24  is contained in another type of BGA package, such as a flip-chip package, as is known in the art, or a package of the type produced by ShellCase. The sensor is mounted below a “window”  104  in board  40 , and the sensor I/O pads (not shown) are connected to PCB  40  by balls  102 . This method of BGA packaging of image sensor  24  similarly includes a step of thinning the silicon die, to yield a typical overall chip package thickness of 0.4 mm. This mounting scheme, with sensor  24  mounted below board and prism  38  protruding above it, is useful in further reducing the diameter limit imposed by equation (2).  
         [0113]      FIG. 12  is a schematic sectional view of a sensor assembly  105  used in a camera head, in accordance with still another embodiment of the present invention. In this embodiment, image sensor  24  is fitted into a recess in board  40 , which is made to fit the sensor chip. The sensor is electrically coupled to board  40  by bonding wires  106 , as is known in the art. As in the preceding embodiments, the sensor chip is preferably thinned before assembly into board  40 . The chip may be fitted into the recess in board  40  as a bare chip without further packaging. Alternatively, the bare chip may be mounted directly on the surface of board  40 , without a special recess.  
         [0114]      FIG. 13  is a schematic top view of an array  114  of LEDs  107 , each coupled to a respective light guide  112 , in accordance with an embodiment of the present invention. The light guides are joined into bundle  15 , which feeds light to endoscope  21 . LEDs  107  may be individually controlled, in order to compensate for uneven illumination intensity within the field of view of sensor  24  at the distal end of endoscope  21 , as described, for example, in the above-mentioned U.S. Patent Application Publication 2001/0031912 A1. LEDs  107  may all emit the same color light, such as white light, or alternatively, different LEDs may be configured to emit different colors, include infrared light.  
         [0115]      FIGS. 14 and 15  schematically illustrate a miniature camera head assembly  115  for stereoscopic imaging, in accordance with an embodiment of the present invention.  FIG. 14  is a sectional side view of assembly  115 , while  FIG. 15  is an end view, seen from the right side of  FIG. 14 . The design of assembly  115  is based on the principles of assembly  27 , as described above in detail, and is thus suitable for use within the distal end of an endoscope. Two image sensors  128  and  130  in assembly  115  are mounted back-to-back, on opposite sides of board  40 . BGA mounting may be used for this purpose, for example, as described above. Each sensor has a respective turning prism  118 ,  120  and an objective  122 ,  124 . Thus, image sensors  128  and  130  capture images of objects in the field of view of objectives  122  and  124 , along optical axes that are parallel but mutually displaced. Processor  16  ( FIG. 1 ) receives the signals generated by the image sensors, and processes the signals to produce pseudo-three-dimensional pictures on display  18 . Based on the design principles described above, including minimizing the heights of prisms  118  and  120  and reducing the width and thickness of sensors  128  and  130 , the overall radial diameter of assembly  115  can be made substantially smaller than that of miniature stereoscopic camera heads that are known in the art for producing images of comparable resolution.  
         [0116]     Although the embodiments described above are directed particularly to endoscopic imaging, the principles of the present invention may similarly be applied in other areas of electronic imaging in which size and weight are at a premium, such as in military and surveillance cameras and industrial cameras for diagnostics of small cavities. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.