Patent Publication Number: US-2023161147-A1

Title: Optical systems for multi-sensor endoscopes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application a continuation application of U.S. patent application Ser. No. 13/882,004, entitled “Optical Systems for Multi-Sensor Endoscopes” and filed on May 23, 2013, which is a national stage entry of PCT Application Number PCT/IL2011/000832, of the same title and filed on Oct. 27, 2011, which relies on U.S. Provisional Patent Application No. 61/407,495, of the same title and filed on Oct. 28, 2010, for priority, all of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a wide FOV objective lens system for an endoscope. 
     BACKGROUND 
     Endoscopes have attained great acceptance within the medical community, since they provide a means for performing procedures with minimal patient trauma, while enabling the physician to view the internal anatomy of the patient. Over the years, numerous endoscopes have been developed and categorized according to specific applications, such as cystoscopy, colonoscopy, laparoscopy, upper GI endoscopy and others. Endoscopes may be inserted into the body&#39;s natural orifices or through an incision in the skin. 
     An endoscope is usually an elongated tubular shaft, rigid or flexible, having a video camera or a fiber optic lens assembly at its distal end. The shaft is connected to a handle, which sometimes includes an ocular for direct viewing. Viewing is also usually possible via an external screen. Various surgical tools may be inserted through a working channel in the endoscope for performing different surgical procedures. 
     There are known various endoscopes employing in their front insertion part, optical heads for viewing the interior of a body cavity or lumen such as the lower digestive track. Such optical head normally includes at least an illumination means for illuminating the object, an objective lens system and a sensor array. 
     U.S. Pat. No. 6,956,703 discloses an objective lens for endoscopes comprises a front lens unit component and a rear lens unit component, between which a aperture stop is located, wherein the front lens unit component comprises, in order from the object side, a first lens having a negative refractive power, and a second lens having a positive refractive power which directs a surface of the small radius of curvature toward the object side; wherein the rear lens unit component comprises a third lens having a positive refractive power which directs a surface of the small radius of curvature toward the image side, a fourth lens having a positive refractive power, and a fifth lens having a negative refractive power; and wherein the fourth lens and the fifth lens are cemented. The following condition is satisfied: 2.0&lt;|f3/f|&lt;3.0 where f is the composite focal length of the total system and f 3 is the focal length of the third lens. Still, the complexity of the objects that are inspected by the endoscope (for example, the asymmetric colon environment), requires high quality images capturing a wide Field of View (FOV), which cannot be accomplished using only one detector. 
     More efforts have been undertaken to improve the optical design of these systems and to create a wide FOV, as seen for example, in U.S. Pat. No. 5,870,234 entitled “Compact wide angle lens”, as well as U.S. Pat. No. 6,476,851 entitled “Electronic endoscope”. Although these patents bring the advantage of a wide FOV they mainly provide a front view. Another disadvantage is a significant distortion in the periphery looking at the borders of the wide view image. 
     These disadvantages may be partially solved by using a multi image lens for example as shown in US patent application number 2005/0168616 entitled “Methods and apparatus for capturing images with a multi-image lens” or by using other Omni-directional optical solutions, as disclosed, for example, in U.S. Pat. No. 7,362,516 entitled “Optical lens providing Omni-directional coverage and illumination”. These technologies may support a wide FOV with relatively low distortion in the periphery of the image however they suffer from a major disadvantage of low optical resolution on side views. Another disadvantage of these technologies is the complexity and space consuming design which typically eliminates the possibility to combine other crucial features like jet, working channels and illuminating sources to the endoscope. 
     There is still a need in the art for endoscopes, such as colonoscopies, that provide a wide FOV, a wide range of Depth of Field/Depth of Focus (DOF) and acceptable resolution within the required dimensions of the device used of a medical application. 
     SUMMARY 
     The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. 
     It is an object of the current invention to provide optical system(s) for front looking and side looking cameras to be housed in the same head (tip) of an endoscope. The cameras together with their respective optic systems are adapted to provide a high quality image capturing a wide FOV of the complex environment examined by the use of the endoscope. According to some embodiments, there is provided an endoscope comprising at least a front looking camera and a side looking camera being essentially perpendicular to one another. According to some embodiments any of the cameras may include a small-sized image sensors such as CCD or CMOS sensors (hereinafter referred to as CCD but can also mean CMOS or any other sensor). In order to keep the outer diameter of the front part of the endoscope as small as possible, the optical systems used in the plurality of cameras need to be compact. Specifically, the optical track of the side looking camera needs to be short. In the case where two side looking cameras are positioned along the same axis, preferably essentially, perpendicular to the long axis of the endoscope, the minimal diameter of the endoscope&#39;s head is limited to at least twice the total length of the cameras (which typically includes the optical track of the camera, the sensor, and any electronic circuitry and wiring which may be located behind the sensor). Shortening the total length should not affect the FOV or cause distortion. Both optical characteristics should be maintained together with minimal total length. 
     Additionally, working channel(s) and fluid channel(s) need to traverse the endoscope&#39;s head. Thus, the diameter of the cameras and its optical systems needs to be small to allow for space occupied by the channels. Since different sensors may be used per field of view it opens the opportunity for additional working channels space giving big advantage to this application. 
     In order to effectively work within the confined space in a body cavity, the cameras may be equipped with wide-angle lens, capable of imaging close objects and a wide range of working distances with preservation of image quality. 
     Optionally, several optical modules (cameras) in one endoscope head, with similar or different designs may be used, optionally each tuned to its desired Depth of Field (DOF). 
     According to some embodiments, there is provided herein an optical system for a tip section of a multi-sensor endoscope, the system comprising: a front-pointing camera sensor; a front objective lens system; a side-pointing camera sensor; and a side objective lens system, wherein at least one of the front and side objective lens systems comprises a front sub-system and a rear sub-system separated by a stop diaphragm, wherein the front sub-system comprises, in order from the object side, a first front negative lens and a second front positive lens, the rear sub-system comprises, in order from the object side, a first rear positive lens, an achromatic sub-assembly comprising a second rear positive lens and a third rear negative lens, wherein the following condition is satisfied: 
     f (first rear positive lens) ≤1.8f, where f is the composite focal length of the total lens system and f (first rear positive lens)  is the focal length of the first rear positive lens. 
     The front sub-system may further include an additional front positive lens (such as a meniscus lens) disposed between the first front negative lens and the second front positive lens (as seen, for example, in  FIG.  4   c   ). 
     The rear sub-system may further include a rear protective glass situated between the third rear negative lens and the front-pointing and/or side-pointing camera sensor, wherein the rear protective glass is adapted to protect a detector array of the front-pointing and/or side-pointing camera sensor. 
     According to some embodiments, the front-pointing camera sensor and the front objective lens system may be adapted to provide a Depth of Focus (DOF) of between 4 and 110 mm. Optical system having a Depth of Focus (DOF) of between 4 and 110 mm may mean that the optical system is adapted to image objects at an object distance of 4-110 mm. The front-pointing camera sensor and the front objective lens system may be adapted to provide a Depth of Focus (DOF) of between 3.5 and 50 mm. The front-pointing camera sensor and the front objective lens system may be adapted to provide an effective spatial resolution of at least 60 lines per mm at Depth of Focus (DOF) of between 5 and 50 mm. The front-pointing camera sensor and the front objective lens system may be adapted to provide an effective angular resolution of about 2′ per degree or less at Depth of Focus (DOF) of between 5 and 50 mm. The front-pointing camera sensor and the front objective lens system may be adapted to provide a Field of View (FOV) of at least 150 degrees. The front-pointing camera sensor and the front objective lens system may be adapted to provide a Field of View (FOV) of at least 170 degrees. 
     According to some embodiments, the front-pointing camera sensor and the front objective lens system have a total optical length of 5 mm or less. 
     According to some embodiments, the side-pointing camera sensor and the side objective lens system may be adapted to provide a Depth of Focus (DOF) of between 3.5 and 50 mm. The side-pointing camera sensor and the side objective lens system may be adapted to provide an effective spatial resolution of at least 60 lines per mm at Depth of Focus (DOF) of between 5 and 50 mm. The side-pointing camera sensor and the side objective lens system may be adapted to provide a Depth of Focus (DOF) of between 3 and 30 mm. The side-pointing camera sensor and the side objective lens system may be adapted to provide an effective angular resolution of about 2′ per degree or less at Depth of Focus (DOF) of between 4.5 and 25 mm. The side-pointing camera sensor and the side objective lens system may be adapted to provide a Field of View (FOV) of at least 150 degrees. The side-pointing camera sensor and the side objective lens system may be adapted to provide a Field of View (FOV) of at least 170 degrees. 
     According to some embodiments, the side-pointing camera sensor and the side objective lens system may have a total optical length of 5 mm or less (for example, 4 mm or less, 3 mm or less). 
     According to some embodiments, the diameter of the first front negative lens may be 2.5 mm or less (without the barrel or lens holder). 
     According to some embodiments, there is provided an objective lens system for at least one of a front-pointing camera sensor and a side-pointing camera sensor of a multi-sensor endoscope, the objective lens system comprising: a front sub-system and a rear sub-system separated by a stop diaphragm, wherein the front sub-system comprises a first front negative lens and a second front positive lens, and the rear sub-system comprises a first rear positive lens, an achromatic sub-assembly comprising a second rear positive lens and a third rear negative lens, wherein the following condition is satisfied: 
     f (first rear positive lens) ≤1.8f, where f is the composite focal length of the total lens system and f(first rear positive lens) is the focal length of the first rear positive lens. 
     According to some embodiments, there is provided a tip section of a multi-sensor endoscope comprising an optical system comprising: a front-pointing camera sensor; a front objective lens system; a side-pointing camera sensor; and a side objective lens system, wherein at least one of the front and side objective lens systems comprises a front sub-system and a rear sub-system separated by a stop diaphragm, the front sub-system comprises a first front negative lens and a second front positive lens, the rear sub-system comprises a first rear positive lens, an achromatic sub-assembly comprising a second rear positive lens and a third rear negative lens, wherein the following conditions are satisfied: 
     f (first rear positive lens) ≤1.8f, where f is the composite focal length of the total lens system and f (first rear positive lens)  is the focal length of the first rear positive lens. 
     More details and features of the current invention and its embodiments may be found in the description and the attached drawings. 
     Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some embodiments of the invention are herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
       In order to understand the invention and to see how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
         FIG.  1   a    schematically depicts an external isometric view of an endoscope having multiple fields of view according to an exemplary embodiment of the current invention. 
         FIG.  1   b    schematically depicts a front view of an endoscope having multiple fields of view according to an exemplary embodiment of the current invention. 
         FIG.  1   c    schematically depicts a side view of endoscope having multiple fields of view according to an exemplary embodiment of the current invention. 
         FIG.  2   a    schematically depicts a cross section of an endoscope having multiple fields of view, for use within bodily cavity according to an exemplary embodiment of the current invention. 
         FIG.  2   b    schematically depicts a cross section of an endoscope front head having multiple fields of view showing some details of the head according to an exemplary embodiment of the current invention. 
         FIG.  2   c    schematically depicts a cutout isometric view of an endoscope having multiple fields of view according to another exemplary embodiment of the current invention. 
         FIG.  2   d    schematically depicts another cutout isometric view of an endoscope having multiple fields of view according to an exemplary embodiment of the current invention. 
         FIG.  3    schematically depicts a cross section of a lens assembly of a camera head, according to an exemplary embodiment of the current invention. 
         FIG.  4   a    schematically illustrates example of light propagation within an objective lens systems according to an exemplary embodiment of the current invention. 
         FIG.  4   b    schematically illustrates another example of light propagation within an objective lens system according to an exemplary embodiment of the current invention. 
         FIG.  4   c    schematically illustrates another example of light propagation within an objective lens system according to an exemplary embodiment of the current invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not necessarily limited in its application to the details set forth in the following description or exemplified by the Examples. The invention is capable of other embodiments or of being practiced or carried out in various ways. 
     The terms “comprises”, “comprising”, “includes”, “including”, and “having” together with their conjugates mean “including but not limited to”. 
     The term “consisting of” has the same meaning as “including and limited to”. 
     The term “consisting essentially of” means that the composition, method or structure may include additional ingredients, steps and/or parts, but only if the additional ingredients, steps and/or parts do not materially alter the basic and novel characteristics of the claimed composition, method or structure. 
     As used herein, the singular form “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof. 
     Throughout this application, various embodiments of this invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination or as suitable in any other described embodiment of the invention. Certain features described in the context of various embodiments are not to be considered essential features of those embodiments, unless the embodiment is inoperative without those elements. 
     In discussion of the various figures described herein below, like numbers refer to like parts. In some cases, pluralities of similar or identical elements are marked with same numbers followed by letters, in some cases; same number without the letter refers to any of these elements. The drawings are generally not to scale. For clarity, non-essential elements were omitted from some of the drawing. 
     The optical setup for endoscopes typically used in the prior art requires a relatively large overall optical length (total optical track) of the entire optical system, which is disadvantageous for endoscopes, in particular those used as colonoscopes and gastroscopes, particularly if used in endoscopes having side-viewing camera or cameras, such as endoscopes according to embodiments of the present invention. 
     In addition, in sensors (such as CCD sensors) used in endoscopes of the prior art, the pixels are partially covered by a photo-shielding film, so that the light energy is concentrated in the center of the pixel, where there is a “window” in the photo-shielding film. This improves the signal-to-noise ratio and increases the light utilization efficiency. However, this also causes the sensor to be sensitive to incident angles between the light rays which have passed the micro-lenses of the sensor and the optical axis of the system. Thus, light rays having relatively small incident angles may reach the pixel, while light rays having relatively large incident angles (between the light rays which have passed the micro-lenses of the sensor and the optical axis of the system) may not reach the “window” and thus the pixel, leading to significant energy losses. The losses are maximized at the edges of the field of view, i.e. for light rays having incident angles close to that of the chief ray. 
     There is thus provided herein, according to some embodiments, a lens system (assembly) configured for use in an endoscope, such as colonoscope, particularly for use in a multi-sensor endoscope/colonoscope. The lens system, (optionally together with the sensor) according to some embodiments of the invention, has a short total optical length (track), for example, 5 mm or less. The lens system, according to some embodiments of the invention, is configured to provide a large incident angle, for example, a chief incident angle (for example the incident angles forming by rays R 6  in  FIGS.  4   a - 4   c   ) larger than 20°, larger than 25°, larger than 30° or between about 20-40°. The lens system, according to some embodiments of the invention provides minimal distortion (for example, less than 80%). 
     According to some embodiments, the sensor which is used together with the lens system, is configured to have a window in the photo-shielding film configured to allow rays having large incident angle (for example, a chief incident angle larger than 20°, larger than 25°, larger than 30° or between about 20-40°) to reach the pixel and thus improve the distortion. According to some embodiments, the width of the window (or any other dimensional parameter) may be about 30-60% of the width of the corresponding pixel. According to some embodiments, the micro-lenses of the sensor may be configured to provide substantially aplanatic conditions. In other words, the sensor may be configured to provide an image substantially free of aberrations. 
       FIG.  1   a    schematically depicts an external isometric view of an endoscope (for example, a colonoscope)  200  having multiple fields of view according to an exemplary embodiment of the current invention. 
     According to an exemplary embodiment of the current invention, head  230  of endoscope  200  comprises at least a forwards looking camera (such as a TV camera) and at least one side looking camera (such as a TV camera). 
       FIG.  1   a    shows front camera element  236  of forwards looking camera  116  (seen in  FIG.  2   c   ) on the front face  320  of head  230 . The term “camera element” may generally refer to a camera and the optical system/assembly related to the camera. Optical axis of forwards looking camera  116  (seen for example in  FIG.  2   a   ) is substantially directed along the long dimension of the endoscope. However, since forwards looking camera  116  is typically a wide angle camera, its Field of View (FOV) may include viewing directions at large angles to its optical axis. Additionally, optical windows  242   a  and  242   b  of discrete light sources such as Light Emitting Diodes (LEDs)  240   a  and  240   b  are also seen on front face  320  of head  230 . It should be noted that number of LEDs used for illumination of the FOV may vary. Distal opening  340  of working channel  262  (seen for example in  FIG.  2   d   ) may preferably be located on front face  320  of head  230 , such that a surgical tool inserted through working channel  262  and deployed beyond front face  320  may be viewed by forwards looking camera  116 . 
     Distal opening  344  of a fluid channel may preferably also be located on front face  320  of head  230 . The fluid channel leading to distal opening  344  may be used as a jet channel for cleaning the colon. 
     Liquid injector  346  having a nozzle  348  aimed at front camera element  236  is used for injecting fluid to wash contaminants such as blood, feces and other debris from front camera element  236  of forwards looking camera. Optionally the same injector is used for cleaning both front camera element  236  and one or both optical windows  242   a  and  242   b . Injector  346  may receive fluid (for example, water and/or gas) from the fluid channel or may be fed by a dedicated cleaning fluid channel. 
     Visible on the side wall  362  of head  230  is the front camera element  256  of side looking camera  220  (two such cameras are seen in  FIG.  2   a   ) and optical window  252  of a discrete light sources such as LED  250 . It is noted that the number of the discrete light sources may vary. Optical axis of side looking camera  220  may be substantially directed perpendicular to the long dimension of the endoscope. However, since side looking camera  220  is typically a wide angle camera, its field of view may include viewing directions at large angles to its optical axis. 
     Liquid injector  366  having a nozzle  368  aimed at front looking camera element  256  is used for injecting fluid to wash contaminants such as blood, feces and other debris from front camera element  256  of side looking camera. Optionally the same injector is used for cleaning both front camera element  256  and optical windows  252 . Preferably, injectors  346  and  366  are fed from same channel. An optional groove  370  helps directing the cleaning fluid from nozzle  368  towards front camera element  256 . Groove  370  may be beneficial when side wall  362  is near or pressed against the rectal wall. Optionally, injector  366  may be at least partially recessed in groove  370 , thus reducing the maximum diameter of head  230  and reduce the risk of injury to the rectal wall due to friction with injector  366 . 
     In the depicted embodiment, flexible shaft  260  is constructed of a plurality of links  382  connected to each other by pivots  384 . Links  382  allows pushing, pulling and rotating the endoscope while pivots  384  provide limited flexibility. The shaft is preferably covered with an elastic sheath (removed for clarity in this figure). The lumen in links  382  holds the working channel  262 . Not seen in this figure are the fluid channel connected to opening  344 , optional cleaning fluid channel and electrical cables supplying power to the LEDs and cameras and transmitting video signals from the camera. Generally, the shaft may also comprise mechanical actuators (not seen), for example cables attached to the links for directing and aiming the head during use. 
     It should be noted that while only one side looking camera is seen in  FIG.  1   a   , optionally, according to some embodiments, two or more side looking cameras may be located within head  230 . When two side looking cameras are used, the side looking cameras are preferably installed such that their field of views are substantially opposing. According to some embodiments, Different configurations and number of side looking cameras are possible and covered within the general scope of the current invention. 
       FIG.  1   b    schematically depicts a front view of head  230  of endoscope  200  having multiple fields of view according to an exemplary embodiment of the current invention. 
     According to an exemplary embodiment of the current invention, head  230  of endoscope  200  comprises at least a forwards looking camera and at least one side looking camera.  FIG.  2   b    shows a front camera element  236  of forwards looking camera  116  on the front face  320  of head  230 . Additionally, optical windows  242   a  and  242   b  of LEDs  240   a  and  240   b  are also seen on front face  320  of head  230 . Distal opening  340  of working channel and distal opening  344  of a fluid channel are preferably also located on front face  320  of head  230 . Liquid injector  346  having a nozzle  348  is also visible in this view. 
     Additionally, Liquid injectors  366   a  and  366   b  aimed at side looking camera element  256   a  and  256   b  respectively are used for injecting fluid to wash contaminants such as blood, feces and other debris from front camera element  256  of side looking cameras. 
       FIG.  1   c    schematically depicts a side view of endoscope  200  having multiple fields of view according to an exemplary embodiment of the current invention. 
       FIG.  1   c    shows front camera element  256  of side looking camera  220 , groove  370  and optical window  252  on the side wall  362  of head  230 . Liquid injectors  346  and  366  are also visible in this view. 
       FIG.  2   a    schematically depicts a cutout isometric view of an endoscope  400  having multiple fields of view according to another exemplary embodiment of the current invention. 
     According to an exemplary embodiment of the current invention, head  230  of endoscope  200  comprises at least a forwards looking camera  116  and two side looking cameras  220   a  and  220   b.    
     Optical windows  242   a  and  242   b  of LEDs used for forward illumination are also seen on front face of head  230 . 
     Distal opening  340  of working channel is preferably located on front face of head  230  such that a surgical tool inserted through the working channel  262  and deployed beyond front face may be viewed by forwards looking camera  116 . 
     Distal opening  344  of a fluid channel is preferably also located on front face of head  230 . The fluid channel leading to distal opening  344  may be used as a jet channel for cleaning the colon. 
     Liquid injector  346  having a nozzle aimed at front camera element of camera  116  is used for injecting fluid to wash contaminants such as blood, feces and other debris from front camera element of forwards looking camera  116 . Optionally the same injector is used for cleaning the front camera element and one or both optical windows  242   a  and  242   b . Injector  346  may receive fluid from the fluid channel or may be fed by a dedicated cleaning fluid channel. 
     Visible on right hand side of head  230  is the front camera element  256   b  of side looking camera  220   b  and optical window  252   b  of side illuminating LED. 
     Liquid injector  366   b  having a nozzle aimed at front camera element  256   b  is used for injecting fluid to wash contaminants such as blood, feces and other debris from front camera element  256   b  of side looking camera  220   b . Optionally the same injector is used for cleaning both front camera element  256   b  and optical windows  252   b . An optional groove  370   b  helps directing the cleaning jet from injector  366   b  towards front camera element  256   b.    
     Although not seen in this figure, it is understood that equivalent elements  366   a ,  370   a ,  256   a  and  252   a  are present on the left hand side of head  230 . 
     Preferably, all the injectors  346  and  366  are fed from same channel. 
     In the depicted embodiment, flexible shaft  260  is constructed of a plurality of links  382  (only one is marked for clarity). Electrical cable  396  within shaft  260  is seen connected to cameras  116 ,  220   a  and  220   b . The same or separate electrical cable is used to power the LEDs. 
       FIG.  2   b    schematically depicts a cross section of an endoscope  200  having multiple fields of view showing some details of the head  230  according to an exemplary embodiment of the current invention. 
     According to the current invention, head  230  of endoscope  200  comprises at least a forwards looking camera  116  and two side looking cameras  220   a  and  220   b . Each of cameras  116  and  220  is provided with an optical imaging system such as lens assemblies (systems)  132  and  232  respectively and solid state detector arrays  134  and  234  respectively. Front camera elements  236  and  256  of cameras  116  and  220  respectively may be a flat protective window, but optionally an optical element used as part of the imaging systems such as solid state detector arrays  134  and  234  respectively. Optionally, cameras  116  and  220  are similar or identical, however different camera designs may be used, for example, field of views  118  and  218  may be different. Additionally or alternatively, other camera parameters such as: resolution, light sensitivity, pixel size and pixel number, focal length, focal distance and depth of field may be selected to be same or different. 
     Light is provided by Light Emitting Diodes (LED) that illuminates the field of views. According to some embodiments, white light LEDs may be used. According to other embodiments, other colors of LEDs or any combination of LEDs may be used (for example, red, green, blue, infrared, ultraviolet). 
     In the depicted embodiment, field of view  118  of forwards looking camera  116  is illuminated by two LEDs  240   a  and  240   b  located within the endoscope head  230  and protected by optical window  242   a  and  242   b  respectively. 
     Similarly, in the depicted embodiment, field of views of side looking camera  220  is illuminated by a single LED  250  located within the endoscope head  230  and protected by optical window  252 . It should be noted that number of LED light sources and their position in respect to the cameras may vary within the scope of the current invention. For example few LEDs may be positioned behind the same protective window, a camera and an LED or plurality of LED may be located behind the same protective window, etc. 
     Head  230  of endoscope  200  is located at the distal end of a flexible shaft  260 . Similarly to shafts of the art, shaft  260  comprises a working channel  262  for insertion of surgical tools. Additionally, shaft  260  may comprises channels for irrigation, insufflation, suction and supplying liquid for washing the colon wall. 
       FIG.  2   c    schematically depicts a cross section cutout of an endoscope  200  showing some details of the head  230  according to an exemplary embodiment of the current invention. For simplicity, details of one of the two side looking cameras are marked in the figure. 
     According to the current invention, head  230  of endoscope  200  comprises at least one side looking camera  220 . Each of cameras  220  is provided with an optical imaging system such as lens assemblies  232  and solid state detector arrays  234 . Front camera element  256  of camera  220  may be a flat protective window or an optical element used as part of the imaging system  232 . 
       FIG.  2   d    schematically depicts a cross section of an endoscope  200  having multiple fields of view showing some details of the head  230  according to an exemplary embodiment of the current invention. 
     According to some embodiments of the current invention, the interior of the head  230  comprises forward looking and side looking cameras  116  and  220 , respectively. Cameras  116  and/or  220  comprise lens assemblies  132  and  232  (not shown), respectively, having a plurality of lenses  430  to  434  and protective glass  436  (not shown) and a solid state detector arrays  134  and  234  (not shown) connected to a printed circuit board  135  and  235  (not shown) respectively. It is noted that cameras  116  and  220  or any element related to them (such as lens assemblies  132  and  232 , lenses  430  to  434  and protective glass  436 , solid state detector arrays  134  and  234  and/or printed circuit board  135  and  235 ) may be the same or different. In other words the front looking camera and the side looking camera(s) may be the same or different in any one or any combinations of their components or other element related to them (such as optical elements). 
       FIG.  3    schematically depicts a cross section of cameras  116  or  220 , showing some details of lens assemblies  132  and  232  according to an exemplary embodiment of the current invention. It should be noted that according to some embodiments of the invention, cameras  116  and  220  may be similar or different. Optionally, the focusing distance of camera  116  is slightly different than that of camera  220 . Differences in focusing distances may be achieved, for example, by (slightly) changing the distance between the lenses that comprise the lens assemblies  132  and/or  232 , or between the lens assembly and the detector array. 
     Air gap “S” between lenses  431  and  432  acts as a stop. Air gap S may affect the focal range (the distance between the closest object and farther objects that can be imaged without excessive blurring caused by being out of optimal focusing of the lens system). 
     According to an exemplary embodiment of the current invention, cameras  116  and  220  comprise lens assemblies  132  and  232  respectively. The lens assemblies comprise a set of lenses  430  to  434  and protective glass  436 . 
     Lenses  430  to  434  are situated within a (optionally metallic) barrel  410  and connecter thereto (for example, glued in barrel  410 ). Any one of lens assemblies  132  and/or  232  may also include an adapter  411 , optionally, as shown in  FIG.  3   , positioned within barrel  410 . Adapter  411  is configured to adjust the location of one or more of the lenses and adjust the distance between lenses. Adapter  411  may also be configured to function as a stop (in this case, between lenses  432  and  433 . Protective glass  436  is situated in proximity to the solid state detector arrays  134  or  234  and is optionally attached thereto. 
     Focal distance (the distance to the object to be optimally focused by the lens system) may be changed by changing the distance between lenses  434  and protective glass  436 . As lens  434  is fixed to the barrel, and protective glass  436  is fixed to lens holder  136  ( 236 ), this distance may be varied by changing the relative positioning of lens holder  136  ( 236 ) with respect to barrel  410 . The space between the lenses  434  and protective glass  436  may be an empty space or may be filled with glass or other transparent material, or a tubular spacer may be inserted to guarantee the correct distance between these lenses. Optionally, optical filters may be placed within the space. Cameras  116  and  220  further comprise solid state detector arrays  134  and  234  respectively. Solid state detector arrays  134  and  234  may each be connected to printed circuit boards. An electrical cabling may connect the printed boards to a central control system unit of the endoscope. 
     Solid state detector arrays  134  and  234  are attached to lens holders  136  and  236  respectively. Lens holder  136  or  236  are attached to lens assemblies  132  or  232  respectively by attaching detector array cover to barrel  410 . 
     In some applications, protective glass  436  may be a flat-flat optical element, acting primarily as a protection of the detector array (such as detector arrays  134  and  234 ), and may optionally be supplied with the array. However, optical properties of protective glass  436  need to be accounted for in the optical design. 
     In order to assemble lens assemblies  132  or  232 , lens  430  may first be inserted from left, then  431 , and  432  from right. Lenses  433  and  434  which may be glued together (or separated for example by air) are then inserted from right. The complete set is now assembled in a barrel. The assembled detector (such as detector arrays  134  and  234 ), protective glass  436  and cover  136 ( 236 ) are then added. 
       FIGS.  4   a ,  4   b  and  4   c    illustrate three examples for the lens assemblies such as lens assemblies  132  and  232  according to the present invention, having objective lens systems  510 ,  520  and  530  respectively. The sensor used in the lens assemblies  132  and  232 , according to this exemplary embodiment, may be a Charge Coupled Device sensor (CCD) having an array of micro-lenses but other sensors, such as CMOS, may also be used. 
     In an exemplary embodiment of the invention, a color CCD camera having resolution of approximately 800×600 pixels were used with total active area of approximately 3.3×2.95 mm. The optical resolution of the lens, according to exemplary embodiments of the current invention, was designed to match the resolution of the sensor. The objective lens system  510  ( 520 / 530 ) are preferably corrected for chromatic; spherical and astigmatism aberrations. In an exemplary embodiment of the invention, objective lens system  510  ( 520 / 530 ) is approximately 4.60 mm (4.62) total length, measured from front face of front lens to the front surface of the sensor. In an exemplary embodiment of the invention, objective lens systems  510  and  520  are wide angle objectives having approximately 170 degrees acceptance angle. In an exemplary embodiment of the invention, objective lens system  510  ( 520 / 530 ) has a short focal distance of measured from the front surface of the front lens to the imaged object. In an exemplary embodiment of the invention objective lens system  510  ( 520 / 530 ) has Depth of Focus (DOF) allowing to effectively image objects between 4-110 mm (or between, 3.5-50 mm). In an exemplary embodiment of the invention, objective lens system  510 ,  520  and  530  has maximum diameter of about 2.5 mm, defined by the diameter of the front lens, and is housed in a barrel having maximum outer diameter of approximately 3.6 mm. It should be noted that other design parameters may be selected within the general scope of the current invention. 
     The objective lens system  510  ( 520 / 530 ) has an optical axis “0” depicted by the dashed line. The lens system comprises a front sub-system  510   a  ( 520   a / 530   a ) and a rear sub-system  510   b  ( 520   b / 530   b ). 
     Front sub-system  510   a  ( 520   a ) ( FIGS.  4   a    ( 4   b )) comprises a front lens  430  ( 430 ′) located closest to the object to be viewed, having a negative power and lens  431 ( 431 ′) having a positive power. 
     Front lens  430  ( 430 ′) is oriented with its concave surface facing toward the optical image formed and away from the object to be viewed and optionally having a diameter substantially greater than the largest dimension of the rear sub-system  510   b  in the direction perpendicular to the optical axis. Lens  431 ( 431 ′) has a positive power. 
     Rear sub-system  510   b  ( 520   b ) comprises lenses  432 ,  433 ;  434 ; and protective glass  436  (lenses  432 ;  433 ;  434 ′; and  436 ′), wherein  432  ( 432 ′), has a positive power,  433  ( 433 ′) has a positive power,  434  ( 434 ′) has a negative power, and  436  ( 436 ′) has essentially no optic power. It is noted that protective glass  436  ( 436 ′) may be a part of the sensor or a part of the rear sub-system  510   b  ( 520   b ). Lenses  433  and  434  ( 433 ′ and  434 ′) of the rear sub-system  510   b  ( 520   b ) compose an achromatic sub-assembly (a compound achromatic sub-assembly as seen in  FIG.  4   a   , where lenses  433  and  434  are cemented or non-compound achromatic sub-assembly as seen in  FIG.  4   b   , where lens  433 ′ and lens  434 ′ are separated). Lens  433  ( 433 ′) may be biconvex with radius of curvature of its front surface being smaller than radius of curvature of its rear surface, as indicated in Tables T 1 , T 2  below. 
     Lens  432  of the objective lens systems  510  may have a focal length f 432  satisfying the following condition: f 432 ≤1.8f, where f is the composite focal length of the total system. Particularly, for the data indicated in Table T 1  f 432 =2.05 and f=1.234 mm, the condition: f 432 ≤1.8f is satisfied. 
     Lens  432 ′ of the objective lens systems  520  may have a focal length f 432 ′ satisfying the following condition: f 432 ≤1.8f. 
     Particularly, for the data indicated in Table T 2  f 432 =2.05 and f=1.15 mm, the condition: f 432 ≤1.8f is satisfied. 
     The lenses may be coated with an anti-reflection coating (AR coating) for further improving the efficiency of the lens assemblies  132  ( 232 ). 
     An effective aperture stop S 1  (S 2 ) is formed between lenses  431  and  432  ( 431 ′ and  432 ′). Effective aperture stop S 1  (S 2 ) may separate between front sub-system  510   a  ( 520   a ) and rear sub-system  510   b  ( 520   b ). 
     Front sub-system  530   a  ( FIG.  4   c   ) comprises a front lens  430 ″ located closest to the object to be viewed, having a negative power and lens  431 ″, having a positive power. Front sub-system  530   a  ( FIG.  4   c   ) further comprises an additional front positive lens (such as the meniscus lens  429 ) disposed between the first front negative lens  430 ″ and the second front positive lens  431 ″. 
     Front lens  430 ″ is oriented with its concave surface facing toward the optical image formed and away from the object to be viewed and optionally having a diameter substantially greater than the largest dimension of the rear sub-system  530   b  in the direction perpendicular to the optical axis. 
     Rear sub-system  530   b  comprises lenses  432 ″,  433 ″,  434 ″; and protective glass  436 ″, wherein  432 ″, has a positive power.  433 ″ has a positive power,  434 ″ has a negative power, and  436 ″ has essentially no optic power. It is noted that protective glass  436 ″ may be a part of the sensor or a part of the rear sub-system  530   b . Lenses  433 ″ and  434 ″ compose an achromatic sub-assembly of the rear sub-system  530   b  and may or may not be cemented to each other. Lens  433 ″ may be biconvex with radius of curvature of its front surface being smaller than radius of curvature of its rear surface, as indicated in Table T 3  below. 
     Lens  432 ″ of the objective lens systems  530  may have a focal length f 432  satisfying the following condition: f 432″ ≤1.8f, where f is the composite focal length of the total system. Particularly, for the data indicated in Table T 3  f 432″ =2.26 and f=1.06 mm, the condition: f 432″ ≤1.8f is satisfied. 
     The lenses may be coated with an anti-reflection coating (AR coating) for further improving the efficiency of the lens assemblies  132  ( 232 ). 
     An effective aperture stop S 3  is formed between lenses  431 ″ and  432 ″. Effective aperture stop S 3  may separate between front sub-system  530   a  and rear sub-system  530   b.    
     Tables T 1  T 2  and T 3  summarize the parameters of lenses in the objective lens systems  510 ,  520  and  530 , respectively, according to some embodiments of the current invention: 
     
       
         
           
               
             
               
                 TABLE T 1   
               
             
            
               
                   
               
               
                 (FOV = 164°, DOF = 3-110 mm. f = 1.234 mm, total optical track 4.09 mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Semi-Diamete   
                 Semi-Diamet   
                   
               
               
                 Len   
                 Type 
                 R 1   
                 R 2   
                 Th 
                 D 
                 Glass 
                 d 1 /2 
                 d 2 /2 
                 f mm   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 430   
                 Negative 
                 15 
                 0.7 
                 0.2 
                 0.18 
                 N-LASF3 
                 1.2 
                 0.64 
                 −0.837 
               
               
                 431   
                 Plano-convex 
                 0.9 
                 Infinit   
                 0.56 
                 0.27 
                 N-LASF3 
                 0.8 
                 0.8 
                 1.02 
               
               
                 S 1   
                 Stop 
                   
                   
                   
                 0.05 
                   
                 0.104 
               
               
                 432   
                 Plano-convex 
                 Infinit   
                 −1.0 
                 0.75 
                 0.09 
                 FK5 
                 0.8 
                 0.8 
                 2.05 
               
               
                 433   
                 Biconvex 
                 1.93 
                 −4.2 
                 0.75 
                 0.005 
                 N-LAK2   
                 1.1 
                 1.1 
                 2.13 
               
               
                 434   
                 Biconcave 
                 −4.2 
                 4.44 
                 0.3 
                 0.65 
                 N-SF66 
                 1.1 
                 1.2 
                 −2.3 
               
               
                 436   
                 Protection Gla   
                 Infinit   
                 Infinit   
                 0.3 
                 0 
                 N-BK7 
                 1.5 
                 1.5 
                 Infinity 
               
               
                   
               
               
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE T 2   
               
             
            
               
                   
               
               
                 (FOV = 164°, DOF = 3-110 mm, f = 1.15 mm, total optical track 4.09 mm) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                   
                   
                 Semi-Diamet   
                 Semi-Diamet   
                   
               
               
                 Len   
                 Type 
                 R 1   
                 R 2   
                 Th 
                 D 
                 Glass 
                 d 1 /2 
                 d 2 /2 
                 f mm   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 430   
                 Negative 
                 6 
                 0.7 
                 0.2 
                 0.3 
                 N-LASF3 
                 1.2 
                 0.66 
                 −0.91 
               
               
                 431   
                 Plano-convex 
                 1.26 
                 Infinit   
                 0.5   
                 0.2   
                 N-LASF3 
                 0.8 
                 0.8 
                 1.43 
               
               
                 S 1   
                 Stop 
                   
                   
                   
                 0.0   
                   
                 0.105 
               
               
                 432   
                 Plano-convex 
                 Infinit   
                 −1.0 
                 0.6   
                 0.1   
                 FK5 
                 0.8 
                 0.8 
                 2.05 
               
               
                 433   
                 Biconvex 
                 1.67 
                 −1.65 
                 0.7   
                 0.3   
                 FK5 
                 0.95 
                 0.95 
                 1.83 
               
               
                 434   
                 Meniscus 
                 −1.33 
                 −12.0 
                 0.3   
                 0.4   
                 N-SF66 
                 1.0 
                 1.2 
                 −1.65 
               
               
                 436   
                 Protection Glas   
                 Infinit   
                 Infinit   
                 0.3   
                 0 
                 N-BK7 
                 1.5 
                 1.5 
                 Infini   
               
               
                   
               
               
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     Table 3, shows an example of a six-component system also comprising an additional positive lens  429  (for example, as indicated in Table 3, a meniscus lens). 
                     TABLE T 3                    (FOV = 164°, DOF = 3-110 mm, f = 1.06 mm, total optical track 4.69 mm)                                                                                 Semi-Diamet     Semi-Diamet             Lens   Type   R 1     R 2     Th   D   Glass   d 1 /2   d 2 /2   f mm                                                               430″   Negative   4.3   0.75   0.2   0.22   N-LASF     1.3   0.72   −1.0         429    Meniscus   0.95   0.9   0.44   0.18   N-SF66   0.8   0.65   5.75       431″   Plano-convex   2.0   Infinit     0.75   0.02   N-LASF     0.8   0.8   2.26       S 3     Stop               0.02       0.116       432″   Plano-convex   Infinit     −1.0   0.78   0   N-PSK5   0.8   0.8   1.69       433″   Biconvex   2.52   −2.0   0.50   0.15     YGH52   0.8   0.8   1.49       434″   Biconcav   −1.44   11.0   0.25   0.91   PBH56   0.8   0.9   −1.5         436″   Protection Glas     Infinit     Infinit     0.3   0   N-BK7   1.5   1.5   Infini                     indicates data missing or illegible when filed            
R 1 —radius of curvature of the lens front surface (front surface is the surface facing the direction of the object);
 
R 2 —radius of curvature of the lens rear surface (facing away from the object);
 
Th—thickness of the lens—from center of front surface to center of rear surface;
 
Glass—lens glass type;
 
d 1 —radius of the front optical surface of the lens;
 
d 2 —radius of the rear optical surface of the lens;
 
D—distance between components such as lenses, measured front center of rear surface of the component, such as lens to the front surface of the next optical element (in the case of a stop, S, the distance is measured front center of rear surface of a component on the front side of the stop, to the front surface of the next component),
 
As commonly used, radius of curvature equal to infinity is interpreted as planar. All lenses are optionally spherical.
 
       FIGS.  4   a ,  4   b  and  4   c    also show the propagation of five incident rays of light R 1  to R 6  through the objective lens system  510 ,  520  and  530  respectively, from the front lens  430  ( FIG.  4   a   ),  430 ′ ( FIG.  4   b   ) or  430 ″ ( FIG.  4   c   ) till the creating of an image of the object at an image plane. 
     Rays R 1  to R 6 , enter the lens assembly at angles α 1  (alpha 1) to α 6  (alpha 6), respectively, for example, essentially equal to the following angles: α 1 =0°, α 2 =45°, α 3 =60°, α 4 =75° and α 5 =84°. The corresponding incident angles (the angles between the light rays which have passed the micro-lenses of the sensor and the optical axis of the system) are □ 1  (beta 1)-□ 6  (beta 6). According to some embodiments, the chief incident angle (for example the incident angles forming by rays R 6  in  FIGS.  4   a - 4   c   ) is larger than 20°, larger than 25°, larger than 30° or between about 20-40°. The lens system, according to some embodiments of the invention provides minimal distortion (for example, less than 80%). 
     The optical system assembly  132  ( 232 ) may be assembled by a method comprising the step of: 
     Optionally, cementing the rear doublet of lenses  433 - 434  ( 433 ′- 434 ′); 
     and: 
     Assembling in the barrel the front lenses  430  ( 430 ′); 
     Assembling lens  431  ( 431 ′) in the barrel; 
     Assembling lens  432  ( 432 ′) in the barrel; 
     Assembling in the barrel, the rear doublet  433 - 434  ( 433 ′- 434 ′); and 
     Note that front lens  430  ( 430 ′) may be assembled last. 
     Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.