Autofocus method using liquid lens

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.

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.

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; andrepeating 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.

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. 1shows components of an intra-oral camera10of the present invention according to one embodiment. Intra-oral camera10comprises an illumination system11, an imaging system12, and an imaging sensor16. Imaging system12includes a liquid lens36having multiple electrodes, liquid lens driver element38, and a microprocessor34. Intra-oral camera10is 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 system12provides a large depth of field (DOF) and forms an image of the target onto sensor16. Liquid lens36in cooperation with an imaging lens22allows focus of imaging system12without 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 element38. 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 element38. The use of this type of adjustable lens differentiates intra-oral camera10of 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 toFIGS. 2A-2C, a conventional liquid lens36agenerally includes two kinds of liquids of equal density. The liquids are sandwiched between two transparent windows107in a conical vessel. In one embodiment, one liquid is conductive water103, while the other is oil101for providing a measure of stability for the optical axis105. Liquid lens36afurther includes electrodes109and113insulated from oil101but in electrical contact with water103; and variable voltage can be selectively applied to the electrodes as an adjustment signal. An insulator111is deposited between electrodes109and113to separate them. The interface115between oil101and water103changes its shape depending on the voltage applied across the conical structure. As shown inFIG. 2A, when zero volts are applied, interface115is slightly curved and the surface of oil101becomes slightly concave. When the voltage is increased to about 40 volts, the surface of oil101becomes highly convex, as shown inFIG. 2B. In this way, liquid lens36acan attain the desired refraction power by means of changing the voltage applied on the electrodes.

FIG. 2Csummarizes the working principle of liquid lens36ahaving two electrodes109and113. Liquid lens36aworks based on the electro-wetting phenomenon described below: a drop of water103is 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 oil101while the other is a conductor such as water103. The variation of voltage leads to a change of curvature of the liquid-liquid interface115, 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. 3is a logic flow diagram that shows steps in an autofocus sequence. The autofocus method has two states: a convergence testing state200and a refocusing state300. In a startup step180, the intraoral camera10is energized and begins to capture image frames. In a vibration cycling step210, 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 step220then 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 step230. If convergence testing shows that focus needs adjustment, refocusing state300is executed. In refocusing state300a hill-climbing method is used to adjust lens position as part of a lens scanning step310, to detect the peak value, and to determine its corresponding focus position. A focus assessment step320determines whether to repeat lens scanning step310or to return to convergence testing state200. Each of the steps in convergence testing state200and refocusing state300can 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 inFIGS. 4A and 4B. InFIG. 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 sequence140shows the timing of image capture frames for the sensor in rolling shutter mode. An imaging sequence150shows 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.

InFIG. 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 sequence140shows the timing of image capture frames for the sensor in global shutter mode. An imaging sequence152shows 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 ofFIG. 5Ashows autofocus test results from convergence testing state200ofFIG. 3when 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. Graphs240a,240b, and240care 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. 5Bshows graphs242aand242bthat 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. 5Cshows graphs244aand244bthat show autofocus test results when the liquid lens position is out of focus in the opposite direction from that ofFIG. 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. 5Dshows a graph246with autofocus test results when the liquid lens position is out of focus, with its re-focus direction dependent on the current lens position.

FIG. 5Eshows a graph248with 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 system11(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 sensor16. The light source can be one or more light emitting diodes (LEDs) or any other known light source. Illumination system11can be integrated into the intra-oral camera10package or can be provided from a separate device. An optical fiber or other light guide could be provided for directing illumination toward target1from an external light source.

Imaging sensor16records the image of the target tooth at a fixed position. Imaging sensor16can be a complementary metal-oxide-semiconductor (CMOS) device, charge coupled device (CCD), or any other known sensor array type.

Though intra-oral camera10of 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.