Abstract:
An endoscopic apparatus and method allow for robust autofocusing operation regardless of contrast characteristics of the environment. The endoscopic apparatus and method can focus on a field stop edge and store default information of the apparatus focused on the edge. Later, if the endoscope fails to focus on an internal target object during the course of normal operation, the endoscope recalls the default information from memory to focus the lens on the edge of the field stop. Thus, internal structural objects in front of the endoscopic apparatus are brought into reasonable focus for visualization.

Description:
BACKGROUND OF THE INVENTION 
     Autofocusing cameras typically use edge and contrast detection methods to autofocus on objects. Among others, Ide (U.S. Pat. No. 6,157,783), Nishida (U.S. Pat. No. 5,710,662), Koyanage et al. (U.S. Pat. No. 5,352,882, and Kaneda (U.S. Pat. No. 5,027,147), all incorporated by reference, discuss the edge and contrast detection methods. If the contrast detection methods of autofocusing cameras are applied to endoscopic systems, such systems may have difficulties focusing on an object at a target site under harsh conditions. For example, a fogged body cavity environment, fumes from electrosurgery or laser surgery or a low contrast content of the field of view in general can prevent appropriate autofocusing on the internal target object. The situation is aggravated by relatively large depths of focus of endoscopes, especially of small diameter scopes. Thus, generally, an operator of an endoscopic system focus on the internal target object manually. 
     SUMMARY OF THE INVENTION 
     An apparatus and method according to the present invention allows for robust autofocusing operation regardless of contrast characteristics of the environment. 
     One aspect of the invention is a method for maintaining a focused image viewed by an autofocusing endoscope having an optical system including determining an transition between an imaging area and a non-imaging area, focusing the optical system on the transition, and storing in a memory a position of the optical system focused on the edge. 
     Embodiments of this aspect of the invention may include one or more of the following features. The method includes selecting at least a portion of the imaging area for focusing, determining a contrast value of at least the portion of the imaging area, comparing the contrast value of at least the portion of the imaging area with a preset value, moving the optical system to the position of the optical system stored in the memory to focus on the edge if the contrast value is below the preset value, and focusing the optical system on at least the portion of the imaging area if the contrast value is above the preset value. The contrast within the focusing zone can be checked at a predetermined frequency. 
     In another aspect of the invention, an article including a machine-readable medium that stores machine-executable instructions causes an endoscopic apparatus to determine an transition between an imaging area and a non-imaging area, focus the optical system on the transition, and store in a memory a position of the optical system focused on the edge. 
     The article can further cause the endoscopic apparatus to select at least a portion of the imaging area for focusing, determine a contrast value of at least the portion of the imaging area, compare the contrast value of at least the portion of the imaging area with a preset value, move the optical system to the position of the optical system stored in the memory to focus on the edge if the contrast value is below the preset value, and focus the optical system on at least the portion of the imaging area if the contrast value is above the preset value. The article can further cause the endoscopic apparatus to check the contrast within the focusing zone at a predetermined frequency. 
     In still another aspect of the invention, an endoscopic apparatus includes an optical system, an autofocusing module for focusing the optical system on a selected focusing zone, and a memory component for storing a position of the optical system focused on the edge. The selected focusing zone includes a zone around an edge between an imaging area and a non-imaging area. The autofocusing module includes an image size detector for finding the edge between the imaging area and the non-imaging area, a zone selector for selecting the focusing zone including the zone around the edge, and an edge detector for focusing the optical system on the edge. 
     The endoscopic apparatus can further include a driver for moving the optical system and a focus controller for controlling the driver. The focus controller can interact with the autofocusing module to focus the optical system on the edge. 
     Also, the endoscopic apparatus can include a photosensitive device for converting an image transmitted from the optical system into video signals. The video signals are transmitted to the autofocusing module for signal processing. 
     Among other advantages, the endoscopic apparatus and method of the present invention allow the optical system to focus on a field stop edge—the edge of a field of view—and store default information of the apparatus focused on the edge. Later, if the apparatus fails to focus on an internal object during the course of normal operation, the apparatus recalls the default information from memory to focus the lens on the field stop edge. In the present invention, manual focusing is obviated. Thus, objects in front of the endoscopic apparatus are brought into reasonable focus for visualization automatically. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates one embodiment of an endoscopic apparatus in accordance with the present invention. 
     FIG. 2 illustrates an optical system of an endoscopic apparatus in accordance with the present invention. 
     FIG. 3 illustrates an enlarged section of the optical system of FIG.  2 . 
     FIG. 4 shows a control unit of an endoscopic apparatus in accordance with the present invention. 
     FIG. 5 illustrates an autofocus module according to an embodiment of the present invention. 
     FIG. 6 shows steps for storing autofocus information of a field stop edge according to an embodiment of the present invention. 
     FIG. 7 shows steps for autofocusing on a field stop edge according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an endoscopic system  5  according to an embodiment of the present invention. An optical end  6  of the endoscopic system  5  is inserted into the body, and video signals from the optical end  6  is transmitted to a control unit  7 , which processes the signals to show internal structures on a video monitor  8 . A light source  9  is provided to the endoscopic system  5 . 
     Referring to FIG. 2, the optical end  6  leads to an optical system  10 , having a field stop  11 , an ocular lens  12 , a coupler lens  13 , and a photosensitive imaging device  14 , such as a CCD or CMOS imager. An object under examination is visualized through the field of view of the field stop  11 . 
     Referring to FIG. 3, the ocular lens  12  of the optical system  10  forms a virtual image of the object in the field of the view defined by the field stop  11 ; and the coupler lens  13  projects the virtual image formed by the lens  12  to the photosensitive imaging device  14 . The coupler lens  13  is shown schematically as one single lens for illustration purposes only. The lens  13  may actually be a complex lens assembly of fixed lenses and movable lenses for zooming and focusing on a target. In a variation of the system, the ocular lens  12  may be omitted and the virtual image may be projected directly on the imaging device  14  by the coupler lens  13 . 
     A projected image on the imaging device  14  is called a real image. The projected real image includes an effective imaging area  15  (the bold line) and a non-imaging black area  16  (the dashed line). The video monitor  8  of FIG. 1 illustrates the imaging area  15  and the black area  16 . 
     Referring to FIG. 4, the control unit  7  includes an autofocus module  20 , a focus control microcontroller  30 , and a memory  32 . The focus control microcontroller  30  is coupled to the autofocus module  20 , and a memory  32  is connected to the microcontroller  30 . A focus motor drive  34  is coupled to the microcontroller  30  for driving a focusing lens motor  36 , which moves a lens for focusing. The lens moved for focusing may be the coupler lens  13  or the ocular lens  12  or both. 
     When the endoscopic system  5  is initialized optionally by the user or automatically during the initial system boot-up, a video output signal  18  from the imaging device  14  is fed to the autofocus module  20 . The video signal  18  is processed by the autofocus module  20  to find the edge of the field stop  11  (the function of the autofocus module  20  is discussed in greater detail later). Then, to focus on the edge of the field stop  11 , the focus control microcontroller  30  interacts dynamically with the autofocus module  20  and activates the focus motor drive  32 , which drives the focusing lens motor  36  to focus the coupler lens  13  or the ocular lens  12  or both on the edge of the field stop  11 . The lens position focused on the edge of the field stop  11  (field stop focus position) is stored in the memory  32 . Thus, whenever an object projected to the effective imaging area  15  cannot be focused, a default setting of the field stop focus position stored in the memory  32  during system initialization is recalled to provide a reasonable visualization of the object. 
     Referring to FIG. 5, to find the edge of the field stop  11  in the video signal and to focus on the edge, the autofocus module  20  has at least the following circuit components: an image size detector  22 , an image zone selector  24 , and an edge detector  26 . The components of the autofocus module  20  operate as follows. 
     Referring to FIG. 6, after the endoscopic system  5  is initialized, a video signal  18  is sent from the imaging device  14  to the autofocus module  20  for image processing (Step  100 ). The image size detector  24  finds an edge of the field stop within the video signal image (Step  102 ). The image size detector  24  does this by, for example, digitally filtering the high frequencies of the video signal  18  to extract out the edge component. 
     Then, the image zone selector  24  selects an image zone around the field stop edge (Step  104 ). The microcontroller  30  then controls the focus motor drive  34  to send signals to the focusing lens motor  36  to focus the coupler lens  13  on the field stop edge in the image zone (Step  106 ). 
     The contrast of the field stop edge is generally high because the edge represents a transition from the imaging area  15  to the black area  16 . Step  106  is repeated until the edge detector  26  detects the highest and the sharpest contrast, that is, the highest focalization, of the field stop edge (Step  108 ). A degree of focalization is related to a high frequency content of a signal. Therefore, the edge detector  26  detects a degree of focalization by, for example, evaluating the high frequency content of the signal. 
     The autofocus module  20  then sends a signal to the microcontroller  30  to stop the motor  36  (Step  110 ). Then, the information of the field stop focus position is stored in the memory  32  (Step  112 ). 
     After the field stop focus position is stored in the memory  32 , the steps as shown in FIG. 7 are executed. An imaging zone, generally set at the center of the imaging area  15 , is selected by the image zone selector  24  for autofocusing (Step  200 ). The edge detector  26  then evaluates the contrast and compares it with a preset minimum value (Step  202 ). If the contrast is determined to be low relative to a preset value (step  204 ), then the autofocusing of the image in the imaging zone is stopped (Step  206 ), and the default field stop focus position is recalled from the memory  32  (Step  208 ). If the contrast is determined to be above a preset value (Step  204 ), then the autofocusing is enabled (Step  210 ). From either Step  208  or  210 , the contrast can be rechecked periodically, for example, every 2 seconds, by going back to Step  202  to constantly monitor and update the visual field (Step  212 ). 
     The foregoing technique can be implemented on a machine executable program. The program can be stored on a storage medium such as random access memory (RAM) and read by a programmable machine incorporated in the endoscopic system  5 . 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.