Abstract:
An autofocus method for an intra-oral camera modulates the focus of a liquid lens in a cycle that has at least first, second, and third focus positions and obtains an image at each focus position, measuring focus of the obtained image. The position of the liquid lens is adjusted according to the measured focus. Steps of modulating the focus of the liquid lens in the cycle with at least first, second, and third focus positions, and obtaining the image at each focus position and measuring focus of the obtained image are repeated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and is a US national phase filing of PCT application No. PCT/CN2012/072302 filed Mar. 14, 2012 that is entitled “AUTOFOCUS METHOD USING LIQUID LENS” in the names of Zaiguang Yu, Guijian Wang and Zhaohua Liu; which itself claims benefit of Provisional application U.S. Ser. No. 61/454,651, provisionally filed on Mar. 21, 2011 that is entitled “AN AUTO-FOCUS METHOD USING LIQUID LENS” in the names of Zaiguang Yu, Guijian Wang and Zhaohua Liu; the disclosures of both priority applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to apparatus for dental imaging. More specifically, the invention relates to autofocus methods for an intra-oral camera having a liquid lens. 
     BACKGROUND OF THE INVENTION 
     While there have been improvements in detection, treatment and prevention techniques, dental caries remains a prevalent condition affecting people of all age groups. If not properly and promptly treated, caries could lead to permanent tooth damage and even to loss of teeth. Thus dental imaging based on an intra-oral camera is of great interest. 
     There exist known intra-oral cameras, such as those available from ACTEON Inc. of Mount Laurel, N.J., USA. Generally, intra-oral cameras are operated over a large working distance range that typically varies between about 1 mm to about 50 mm. They must also have a sizable depth of field (DOF), which is different at different working distances. Thus, focus adjustment is necessary to provide good image quality. However, for most of the known intra-oral cameras including the one disclosed in U.S. Pat. No. 6,019,721 (Holmes et al.), focus adjustment is performed manually by operator adjustment to the distance between a lens and an imaging sensor. Conventional intra-oral cameras must be separately adjusted for each image. This makes these conventional cameras poorly suited for obtaining images in the dental office and practitioners can find cameras without an autofocus capability more difficult to use. 
     System optics for intra-oral cameras must provide a large depth of field (DOF) and as wide a field of view (FOV) as is possible. Some of the existing intraoral camera use a small NA (numerical aperture) that can provide sufficient DOF (depth of field) to help reduce the requirements for focus adjustment. However, an optical system having a small NA has limitations and is poorly suited for providing the high resolution needed for dental examination. The small NA can be unable to provide sufficient luminous flux for intra-oral use. 
     A number of solutions that have been proposed for intraoral imaging use a liquid lens as part of the imaging optics. The liquid lens can adjust more readily to intraoral requirements and is advantaged with respect to FOV and DOF. Autofocus, however, remains a problem. Accordingly, there is a need to provide an intraoral camera having an auto focus capability that does not require additional components and that operates quickly enough for use in the dental office. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an intra-oral camera having a liquid lens and capable of providing autofocus. Embodiments of the present invention are advantaged for providing a camera that is compact, provides suitable imaging, and does not require focus by the operator or practitioner. 
     These objects are given only by way of illustrative example, and such objects may be exemplary of one or more embodiments of the invention. Other desirable objectives and advantages inherently achieved by the disclosed invention may occur or become apparent to those skilled in the art. The invention is defined by the appended claims. 
     According to one aspect of the present invention, there is provided an autofocus method for an intra-oral camera comprising:
         modulating the focus of a liquid lens in a cycle that has at least first, second, and third focus positions;   obtaining an image at each focus position and measuring focus of the obtained image;   adjusting lens position according to the measured focus; and   repeating steps of modulating the focus of the liquid lens in the cycle with at least first, second, and third focus positions, and obtaining the image at each focus position and measuring focus of the obtained image.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the embodiments of the invention, as illustrated in the accompanying drawings. The elements of the drawings are not necessarily to scale relative to each other. 
         FIG. 1  shows a schematic view of components of an intra-oral camera of the present invention. 
         FIG. 2A  is a side view schematic diagram that shows a liquid lens in a zero voltage state. 
         FIG. 2B  is a side view schematic diagram that shows the two-electrode liquid lens with a non-zero applied voltage. 
         FIG. 2C  is a side view schematic diagram that shows working principles of the two-electrode liquid lens. 
         FIG. 3  is a logic flow diagram that shows an autofocus sequence. 
         FIG. 4A  is a timing diagram that shows a portion of the autofocus detection sequence for a rolling shutter imager. 
         FIG. 4B  is a timing diagram that shows a portion of the autofocus detection sequence for a global shutter imager. 
         FIG. 5A  shows autofocus test results when the liquid lens position is in focus. 
         FIG. 5B  shows autofocus test results when the liquid lens position is out of focus in one direction. 
         FIG. 5C  shows autofocus test results when the liquid lens position is out of focus in the opposite direction from that of  FIG. 5B . 
         FIG. 5D  shows autofocus test results when the liquid lens position is out of focus, with its re-focus direction dependent on the current lens position. 
         FIG. 5E  shows autofocus test results when the liquid lens position is out of focus, with its re-focus direction dependent on the current lens position and on sensed focus values. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following is a detailed description of the preferred embodiments of the invention, reference being made to the drawings in which the same reference numerals identify the same elements of structure in each of the several figures. 
     Where they are used, the terms “first”, “second”, “third”, and so on, do not necessarily denote any ordinal or priority relation, but may be used for more clearly distinguishing one element or time interval from another. 
       FIG. 1  shows components of an intra-oral camera  10  of the present invention according to one embodiment. Intra-oral camera  10  comprises an illumination system  11 , an imaging system  12 , and an imaging sensor  16 . Imaging system  12  includes a liquid lens  36  having multiple electrodes, liquid lens driver element  38 , and a microprocessor  34 . Intra-oral camera  10  is intended for imaging a target tooth or other structure that is within the mouth of a patient, and to do this expediently and accurately. 
     Imaging system  12  provides a large depth of field (DOF) and forms an image of the target onto sensor  16 . Liquid lens  36  in cooperation with an imaging lens  22  allows focus of imaging system  12  without the need for adjustment by an operator. The liquid lens that is used provides an adjustable lens element disposed at a position along the optical path, optical axis O, and actuable to change refraction with respect to each of two orthogonal axes in response to received adjustment signals from driver element  38 . The position of the liquid lens along the optical axis O is also adjustable, with its linear position along the optical axis O also controlled by driver element  38 . The use of this type of adjustable lens differentiates intra-oral camera  10  of the present invention from conventional intra-oral cameras and from many other types of conventional cameras that are intended for other uses. 
     In order to better understand how embodiments of the present invention are advantaged over camera embodiments using conventional liquid lenses, it is useful to review how the liquid lens operates. Referring to  FIGS. 2A-2C , a conventional liquid lens  36   a  generally includes two kinds of liquids of equal density. The liquids are sandwiched between two transparent windows  107  in a conical vessel. In one embodiment, one liquid is conductive water  103 , while the other is oil  101  for providing a measure of stability for the optical axis  105 . Liquid lens  36   a  further includes electrodes  109  and  113  insulated from oil  101  but in electrical contact with water  103 ; and variable voltage can be selectively applied to the electrodes as an adjustment signal. An insulator  111  is deposited between electrodes  109  and  113  to separate them. The interface  115  between oil  101  and water  103  changes its shape depending on the voltage applied across the conical structure. As shown in  FIG. 2A , when zero volts are applied, interface  115  is slightly curved and the surface of oil  101  becomes slightly concave. When the voltage is increased to about 40 volts, the surface of oil  101  becomes highly convex, as shown in  FIG. 2B . In this way, liquid lens  36   a  can attain the desired refraction power by means of changing the voltage applied on the electrodes. 
       FIG. 2C  summarizes the working principle of liquid lens  36   a  having two electrodes  109  and  113 . Liquid lens  36   a  works based on the electro-wetting phenomenon described below: a drop of water  103  is deposited on a substrate made of metal, covered by a thin insulating layer. The voltage applied to the substrate generates an electrostatic pressure to force the liquid to change its shape so as to modify the contact angle at the liquid interface. Two iso-density liquids are employed in the liquid lens: one is an insulator such as oil  101  while the other is a conductor such as water  103 . The variation of voltage leads to a change of curvature of the liquid-liquid interface  115 , which in turn leads to a change of optical power or refraction of the lens. Generally, the two liquid components of the liquid lens are immiscible and have different optical indices. The relative shape of the interface between liquids determines the refractive properties of the lens. The relative indices of refraction of the two liquids must differ from each other by some amount in order to provide adjustable refraction. 
       FIG. 3  is a logic flow diagram that shows steps in an autofocus sequence. The autofocus method has two states: a convergence testing state  200  and a refocusing state  300 . In a startup step  180 , the intraoral camera  10  is energized and begins to capture image frames. In a vibration cycling step  210 , the liquid lens is vibrated or modulated, rapidly changing the lens shape so that the lens has three focal positions. An image is captured for each vibration position of the liquid lens. A focus detection step  220  then executes, in which the focus of each captured image is measured and the relative focus at each vibration position is analyzed, as described in more detail later. If convergence testing shows that focus is acceptable, autofocus terminates at a termination step  230 . If convergence testing shows that focus needs adjustment, refocusing state  300  is executed. In refocusing state  300  a hill-climbing method is used to adjust lens position as part of a lens scanning step  310 , to detect the peak value, and to determine its corresponding focus position. A focus assessment step  320  determines whether to repeat lens scanning step  310  or to return to convergence testing state  200 . Each of the steps in convergence testing state  200  and refocusing state  300  can be repeated any number of times, as needed. 
     To detect focus state and decide focus direction, the liquid lens is rapidly modulated to provide slight vibration, with the cycle timing demonstrated in  FIGS. 4A and 4B . In  FIG. 4A , the timing sequence is shown for a sensor that has a rolling shutter, such as a conventional CMOS (Complementary Metal-Oxide Semiconductor) sensor. A frame sequence  140  shows the timing of image capture frames for the sensor in rolling shutter mode. An imaging sequence  150  shows the three positions of the liquid lens used for obtaining an image. The three positions that form a single cycle are center, near, and far offset positions, each with a different focus. A suitable value is chosen for the amplitude of the lens focus change to each of the three near, center, and far positions. As shown in the timing diagram, the change in liquid lens position is executed between image capture frames. Each image capture sequence in the cycle spans two image capture frames for the rolling shutter device. The full cycle thus spans six image capture frames. 
     In  FIG. 4B , the timing sequence is shown for a sensor that has a global shutter, such as a conventional CCD (Charge-Coupled Device) sensor. A frame sequence  140  shows the timing of image capture frames for the sensor in global shutter mode. An imaging sequence  152  shows the cycle of three positions of the liquid lens used for obtaining an image with global shutter timing. Again, positions in each cycle are center, near, and far offset positions, each with a different focus. As shown in the timing diagram, the change in liquid lens position is executed between image capture frames. Each image capture sequence in the cycle spans a single image capture frame for the global shutter device. The full cycle thus spans three image capture frames. 
     The diagram of  FIG. 5A  shows autofocus test results from convergence testing state  200  of  FIG. 3  when the liquid lens position is in focus. Focus is represented on the vertical axis. Focus in the captured image can be measured in a number of ways, such as by analyzing contrast and high frequency components extracted from the image. Graphs  240   a ,  240   b , and  240   c  are shown. When in focus, the center value is not less than either of the near and far offset values and preferably exceeds the near or far offset values. 
       FIG. 5B  shows graphs  242   a  and  242   b  that show autofocus test results when the liquid lens position is out of focus in a near-scanning direction. The near offset value exceeds both center and far offset values. 
       FIG. 5C  shows graphs  244   a  and  244   b  that show autofocus test results when the liquid lens position is out of focus in the opposite direction from that of  FIG. 5B . Here, the liquid lens position is out of focus in a far-scanning direction. The far offset value exceeds both center and near offset values. 
       FIG. 5D  shows a graph  246  with autofocus test results when the liquid lens position is out of focus, with its re-focus direction dependent on the current lens position. 
       FIG. 5E  shows a graph  248  with autofocus test results when the liquid lens position is out of focus, with its re-focus direction dependent on the current lens position and on sensed focus values. 
     The autofocus method of the present invention provides a straightforward method for automatic measurement of focus and adjustment of liquid lens state and position. External devices or measurements are not needed to achieve the needed focus position for intraoral imaging. 
     Illumination system  11  ( FIG. 1 ) is configured to direct light from a light source in order to illuminate the tooth or other target for improved imaging at imaging sensor  16 . The light source can be one or more light emitting diodes (LEDs) or any other known light source. Illumination system  11  can be integrated into the intra-oral camera  10  package or can be provided from a separate device. An optical fiber or other light guide could be provided for directing illumination toward target  1  from an external light source. 
     Imaging sensor  16  records the image of the target tooth at a fixed position. Imaging sensor  16  can be a complementary metal-oxide-semiconductor (CMOS) device, charge coupled device (CCD), or any other known sensor array type. 
     Though intra-oral camera  10  of the present invention is designed for imaging an intra-oral target, this device may be used in other suitable applications, particularly where the camera width requirement is fairly constrained, such as for endoscope applications.