Abstract:
A method and apparatus for focusing an image on a pixel array. The method includes the steps of continuously changing the distance between a lens and a pixel array between a first distance and a second distance and obtaining an image projected onto the pixel array through the distance is changing. The apparatus includes a lens and an electromechanical structure to continuously change the distance between the lens and the pixel array between the first distance and the second distance.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention generally relate to imaging devices, and specifically to imaging devices employing an adjustable focus assembly. 
       BACKGROUND OF THE INVENTION 
       [0002]    To focus an image on an imaging array in a conventional digital camera a lens adjustment assembly is used to mechanically move one or more lenses in a linear direction relative to an image array from a known starting position, such as “infinity”, to a focused position, at which the image is focused on the image array. The lens is then held at the focused position, while the image is acquired. Many lens adjustment assemblies move the lens using an electromechanical actuator, which may include technology such as voice-coils, electrically active polymers, or piezoelectric actuators. This movement may be made against resistance from a bias source, for example, a spring, which serves to return the lens to the lens starting position when the actuator is turned off. 
         [0003]    Most cameras, including digital cameras, have an automatic focus feature (referred to herein as “auto focus”) by which objects viewed through the camera can be focused on automatically. Auto focus systems are generally categorized as either active or passive systems. Active systems actually determine the distance between the camera and the subject of the scene, e.g., by measuring the total travel time of ultrasonic waves or infrared light emitted from the camera. Based on the total travel time, the distance between the camera and the subject of the scene may be calculated and the lens adjustment assembly moves the lens from the starting position to a focused position correlated to the calculated distance of the camera to the object. 
         [0004]    Passive auto focus systems, on the other hand, rely on the light that is naturally reflected by the subject in the scene. One example of a passive auto focus system is a system that uses contrast analysis of a captured image to determine the best focal position for the camera lens. In a contrast analysis auto focus system, adjacent areas of a scene are compared with each other to measure differences in intensity among the adjacent areas. An out-of-focus scene will include adjacent areas that have similar intensities, while a focused scene will likely show a significant contrast between areas in which the subject of the scene is located and other areas of the scene (e.g., background objects). During focusing, the lens adjustment assembly moves the lens from the starting position to a number of intermediate positions until the focused position is finally determined (that is, when the lens position results in an image having the maximum intensity difference between adjacent areas). 
         [0005]    In either passive or active auto focus systems, energy is used by the electromagnetic actuator to move and maintain the lens at various positions against the resistance of the spring.  FIG. 1  shows the relative amount of power needed for the distance that the lens is to be displaced in a conventional lens adjustment assembly. It can be seen from  FIG. 1  that the power needed to displace the lens increases as the amount of displacement increases. In some conventional lens adjustment assemblies, the electromagnetic actuators may require a large amount of power, which, for imaging devices operating on a limited power supply such as batteries, will drain the batteries and diminish the usefulness of the imaging device. 
         [0006]    Accordingly, there is a desire and need for an imaging device with an auto focus capability that mitigates against these shortcomings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a graph of the relative amount of power needed for lens displacement in a conventional lens adjustment assembly. 
           [0008]      FIG. 2  is a cross-sectional view of an imaging device and a lens adjustment assembly according to an embodiment described herein. 
           [0009]      FIG. 3  shows a graph of the displacement of a lens over time according to an embodiment described herein. 
           [0010]      FIGS. 4A-4C  are diagrams of lenses positioned according to a method of chromatic aberration correction according to an embodiment described herein. 
           [0011]      FIG. 5  is a cross-sectional view of an imaging device and a lens adjustment assembly according to another embodiment described herein. 
           [0012]      FIG. 6  is a cross-sectional view of an imaging device and a lens adjustment assembly according to another embodiment described herein. 
           [0013]      FIG. 7  is a block diagram of an imaging device according to embodiments described herein. 
           [0014]      FIG. 8  is a block diagram of a processor system that includes an imaging device according to embodiments described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    In the following detailed description, reference is made to various specific embodiments that may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed, and that structural and electrical changes may be made. 
         [0016]    In various embodiments described herein, an imaging device includes a lens adjustment assembly in which the lens is mounted to a mechanism that moves in a resonant manner throughout its range of motion. The timing of the resonant motion of the lens is synchronized to the imaging device so that an image may be obtained at the optimum focus position of the lens. The resonant motion of the lens is made possible by an elastic component that returns energy on every cycle and therefore may be achieved with very low power. 
         [0017]      FIG. 2  is a cross-sectional view of an imaging device  100  having a lens adjustment assembly  105  according to an embodiment. Imaging device  100  includes a substrate  102  supporting an imaging device circuit  104  having a pixel array. In the illustrated embodiment, the imaging device circuit  104  is shown as fabricated on top of substrate  102 , but for this and other embodiments, the imaging device circuit  104 , including the pixel array, may also be fabricated directly within a semiconductor substrate  102  along with other circuitry, if desired. The substrate  102  includes interconnects  116 , which connect the imaging device circuit  104  to exterior circuits, and solder balls  118  through which the imaging device circuit  104  may be mounted to and electronically connected with other devices. Imaging device  100  also includes a lens adjustment assembly  105  that includes a support structure  106 , a flexible membrane  108 , a magnetic ring  110 , actuation coils  114 , and a lens  112 . 
         [0018]    In the lens focusing process, the actuation coils  114  act as a driving unit that is activated to provide excitation pulses that exert a force on the magnetic ring  110  and begin resonant motion of the lens  112 . The flexible membrane  108  is an elastic component that allows the lens  112  and magnetic ring  110  to move up and down in the directions denoted by arrow A to an upper displacement position  112 A and a lower displacement position  112 B. In one embodiment, the range of motion of the lens  112  between the upper displacement position  112 A and the lower displacement position  112 B may be about 0.5 mm. 
         [0019]      FIG. 3  shows a graph of the displacement of the lens  112  over time in conjunction with the excitation pulses. At time zero, the actuation coils  114  apply a first excitation pulse P 1  to the magnetic ring  110  to begin resonant movement of the lens  112 . Subsequent excitation pulses P 2 , P 3 , P 4  are applied to the magnetic ring  110  at the beginning of each resonant cycle. As shown in  FIG. 3 , the lens  112  has achieved a steady state resonant movement by the third excitation pulse. In other embodiments, however, a steady state resonant movement of the lens  112  may be established by fewer or more excitation pulses. 
         [0020]    The frequency and amplitude of the resonant movement of the lens  112  are dependent on the mass of the moving elements, i.e., the lens  112  and the magnetic ring  110 , and the spring constant of the flexible membrane  108 . More or less displacement of the lens  112  may be achieved by using larger or smaller excitation pulses, respectively. The amplitude should be large enough to cover the desired focal length range of the lens  112 . 
         [0021]    The flexible membrane  108  is elastic so that the energy from the resonant motion of the lens  112  is conserved as the lens  112  moves up and down during each cycle. After steady state resonant motion is achieved, the excitation pulses from the actuation coils  112  are only needed to overcome motion loss due to friction, which may be very small. Due to the conservation of energy, very little power is needed for the excitation pulses to maintain the resonant motion of the lens  112  when compared to conventional methods of moving a lens from a start position to a focus position and holding it there while an image is acquired. In one embodiment, a the excitation pulses from the actuation coils  112  may be generated using a voltage between about zero to 10 V and a power of about 100 μW to maintain the resonant motion of the lens  112 . 
         [0022]    Once the lens  112  has achieved a steady state resonant motion, the timing of the resonant motion of the lens  112  is synchronized to the operation of the imaging device circuit  104  by a timing signal sent to the device&#39;s pixel array so that an image may be obtained at a desired lens position, such as the focus position, e.g., the lens position at which an image is most focused.  FIG. 3  shows a repeating timing signal (S 1 , S 2  . . . ) that is generated by the lens adjustment assembly  105  and is sent to the imaging device circuit  104  when the lens  112  is at the zero displacement point. Using the timing signal (S 1 , S 2  . . . ) the imaging device circuit  104  may interpolate the position of the lens  112  at any given time. In other embodiments, a timing signal may be sent to the imaging device circuit  104  when the lens  112  is at another point in the cycle, such as the upper displacement position  112 A, the lower displacement position  112 B, or other positions. 
         [0023]    In one embodiment, a global shutter may be used so that the entire image is obtained by the pixel array of imaging device circuit  104  in a single instant at a particular lens  112  position. In another embodiment, an image may be constructed from multiple images obtained during multiple cycles to create a “hyper-focused” image. The frequency of the resonant motion of the lens  112  may be sufficiently high so that an image is obtained over many, even hundreds, of cycles. 
         [0024]    In another embodiment, an imaging device using a system other than a global shutter, such as a rolling shutter, may read out a portion of the imaging device circuit  104  pixel array during each pass through the focus position over multiple cycles. For example, in an embodiment using a rolling shutter system, signals from one or more rows of pixels may be obtained and read out from the imaging device circuit  104  pixel array at a particular focus position during each cycle. 
         [0025]    In another embodiment, selected regions of an image may be designated to be captured either in focus or out of focus. For example, in a “portrait mode” in which it is desired to focus only on a single subject, the portion of the image that contains the subject may be read out by the imaging device circuit  104  at a lens  112  position at which the subject is focused and the portion of the image that does not contain the subject, (i.e., the background), may be read out by the imaging device circuit  104  at a lens  112  position at which the background is not in focus. In another embodiment, multiple subjects in an image may be read out in focus while the remainder of the image may be read out unfocused. In one embodiment, the amplitude of the resonant motion of the lens  112  may be reduced in portrait mode. 
         [0026]    The imaging device  100  may be used with active or passive auto focus techniques. In an active auto focus technique, the distance between the imaging device  100  and a subject may be measured and correlated to the desired focus position of the lens  112  using a stored look-up table. When the lens  112  is at the desired focus position during its resonant motion, the pixel array obtains an image. As described above, the imaging device circuit  104  determines the position of the lens  112  by interpolating the position of the lens using the timing signals (S 1 , S 2  . . . ). In a passive auto focus technique, the imaging device circuit  104  may obtain a number of scene images at different positions as the lens  112  travels through the resonant motion cycle. Adjacent areas of the scenes for each of the images may then be compared with each other to measure differences in intensity and determine which focus position provides the optimum focus for the scene. 
         [0027]    In another embodiment, the imaging device  100  may be used to compensate for chromatic aberrations due to refractive differences in red, green, and blue light. As shown in  FIGS. 4A-4C , red  450 , green  452 , and blue light  454  each have different focal points in a silicon substrate due to their different wavelengths. To account for this difference, the different wavelengths of light may be captured by the imaging device circuit  104  at different lens  112  positions during the resonant motion cycle. 
         [0028]    As shown in  FIG. 4A , red light  450  is optimally focused at a lens  112  position relatively closest to the imaging device circuit  104 . The red light  450  from an image may be obtained by the imaging device circuit  104  at this position by only activating the pixels in the imaging device circuit  104  that have a red color filter. As shown in  FIG. 4B , green light  452  is optimally focused at a lens  112  position between that of red  450  and blue light  454 . Green light may be read out by the imaging device circuit  104  at this position by activating only the pixels having a green color filter. As shown in  FIG. 4C , blue light  454  is optimally focused at a lens  112  position relatively farthest from the imaging device circuit  104 . Blue light may be captured by the imaging device circuit  104  at this position by activating only the pixels having a blue color filter. 
         [0029]    The red, green, and blue images may be obtained during the same or different cycles. The red, green, and blue images may then be combined to form one image having all three colors, in which each color has been optimally focused. 
         [0030]      FIG. 5  is a cross-sectional view of an imaging device  500  having a lens adjustment assembly  505  according to an another embodiment. Imaging device  500  includes a substrate  502  supporting an imaging device circuit  504 , which includes a pixel array. The substrate  502  includes interconnects  516  and solder balls  518  as described above with regard to the embodiment shown in  FIG. 2 . Imaging device  500  also includes a lens adjustment assembly  505  that includes a support structure  506 , a magnetic ring  510 , ring supporter  522 , voice coils  514 , spring coils  520 , and a lens  512 . 
         [0031]    The voice coils  514  are a driving unit that provides excitation pulses that exert a force on the magnetic ring  510  and begin a resonant motion of the lens  512 . The spring coils  520  are elastic components that allow the lens  512  and magnetic ring  510  to move up and down in the directions denoted by arrow A to an upper displacement position  512 A and a lower displacement position  512 B. In one embodiment, the range of motion of the lens  512  may be about 0.5 mm. 
         [0032]    The displacement of the lens  512  over time in conjunction with the excitation pulses in imaging device  500  is also depicted by  FIG. 3 . In imaging device  500 , the voice coils  514  apply excitation pulses to the magnetic ring  510  to begin resonant movement of the lens  512 . The spring coils  520  conserve the energy of the resonant motion of the lens  512  as the lens  512  moves up and down during each cycle. Therefore, the excitation pulses from the voice coils  112  are only needed to overcome motion loss due to friction. 
         [0033]    The frequency and amplitude of the resonant movement of the lens  512  are dependent on the mass of the moving elements, i.e., the lens  512 , the ring supporter  522 , and the magnetic ring  510 , and the spring constant of the spring coils  520 . A greater or smaller cycle amplitude may be achieved by using larger or smaller excitation pulses. 
         [0034]    As described above with regard to imaging device  100 , the timing of the resonant motion of the lens  512  is synchronized to the operation of the imaging device circuit by a timing signal sent to the pixel array so that an image may be obtained at a desired lens position. The imaging device  500  is suitable for auto focus and other applications described above with regard to imaging device  100 . 
         [0035]      FIG. 6  is a cross-sectional view of an imaging device  600  having a lens adjustment assembly  605  according to an another embodiment. Imaging device  600  includes a substrate  602  supporting an imaging device circuit  604 . The substrate  602  includes interconnects  616  and solder balls  618  as described above with regard to imaging device  100 . Imaging device  600  also includes a lens adjustment assembly  605  that includes a support structure  606 , an electrically active polymer (EAP) actuator  610 , and an elastomeric lens  612 . 
         [0036]    An electrically active polymer is an electrostatically driven polymer, such as an ionomer, that expand in volume in response to an applied voltage and decreases in volume when the voltage is removed. The electrically active polymer actuator  610  is a driving unit that includes an electrically active polymer that may be activated by an applied voltage. The electrically active polymer actuator  610  may be arranged in a ring surrounding the elastomeric lens and may be configured so that the electrically active polymer actuator  610  moves at least in the direction of arrow A when activated and in the direction of arrow B when deactivated. 
         [0037]    The electrically active polymer actuator  610  provides excitation pulses that exert a force on the elastomeric lens  612  to begin resonant motion in the elastomeric lens  612 . The elastomeric lens  612  itself is an elastic component and is arranged so that the movement of the electrically active polymer actuator  610  may cause resonant motion causing the elastomeric lens  612  to deform from a first shape having a lower displacement position  612 A to a second shape having an upper displacement position  612 B. The deformation of the elastomeric lens  612  causes the focus position of the lens  612  to move as the shape of the elastomeric lens  612  changes. 
         [0038]    The displacement of the elastomeric lens  612  over time in conjunction with the excitation pulses in imaging device  600 , is also depicted by  FIG. 3 . In imaging device  600 , the electrically active polymer actuator  612  applies the excitation pulses to the elastomeric lens  612  to begin resonant movement of the elastomeric lens  612 . The elastomeric lens  612  conserves the energy of the resonant motion of the elastomeric lens  612  as the elastomeric lens  612  deforms from a first shape to a second shape during each cycle. 
         [0039]    The frequency and amplitude of the resonant movement of the elastomeric lens  612  are dependent on the elasticity and shape of the elastomeric lens  612 . A greater or smaller cycle amplitude may be achieved by using larger or smaller excitation pulses. 
         [0040]    As described above with regard to imaging device  100 , the timing of the resonant deformation of the elastomeric lens  612  is synchronized to the operation of the imaging device circuit by a timing signal sent to the pixel array so that an image may be obtained at a desired lens position. The imaging device  600  is suitable for auto focus and other applications described above with regard to imaging device  100 . 
         [0041]      FIG. 7  shows a block diagram of an imaging device  900 , e.g. a CMOS imaging device, which includes a lens adjustment assembly according to embodiments described herein. A timing and control circuit  932  provides timing and control signals for enabling the reading out of signals from pixels of the pixel array  930 . In one embodiment, the timing and control circuit  932  may include an auto focus circuit, which receives auto information including a lens position where the best focus is obtained and the timing signals (S 1 , S 2  . . . ) which are used to determine the actual lens position at a given time, and may provide the resonant timing pulses (P 1 , P 2  . . . ) to the lens adjustment assembly to coordinate the timing of the pixel array with the motion of the lens. Alternatively, the circuit receiving the auto focus information and supplying the resonant timing pulses (P 1 , P 2  . . . ) may be a circuit separate and apart from the timing and control circuit  932 . The pixel array  930  has dimensions of M rows by N columns of pixels, with the size of the pixel array  930  depending on a particular application. 
         [0042]    Signals from the imaging device  900  may be read out a row at a time using a column parallel readout architecture if a rolling or global shutter technique is used. The timing and control circuit  932  selects a particular row of pixels in the pixel array  930  by controlling the operation of a row addressing circuit  934  and row drivers  940 . Signals stored in the selected row of pixels are provided to a readout circuit  942 . The signals read from each of the columns of the array sequentially or in parallel using a column addressing circuit  944 . For image acquisition, pixel array  930  can be operated in either a rolling shutter or global shutter mode. 
         [0043]    In either a rolling shutter or global shutter system, pixel signals corresponding to a pixel reset signal Vrst and an image pixel signal Vsig are provided as outputs of the readout circuit  942 , and are typically subtracted in a differential amplifier  960  and the result digitized by an analog to digital converter  964  to provide a digital pixel signal. The digital pixel signals represent an image captured by pixel array  930  and are processed in an image processing circuit  968  to provide an output image. 
         [0044]      FIG. 8  shows an imaging system  1000 , for example, a camera system, that includes an imaging device  900  constructed and operated in accordance with the various embodiment described above, such as imaging devices  100 ,  500 , and  600 . The imaging system  1000  is shown in  FIG. 8  as a camera system. Without being limiting, such a system  1000  could include a computer system, scanner, machine vision, vehicle navigation, video phone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system, or other imaging system. 
         [0045]    Camera system  1000 , for example a digital still or video camera system, generally comprises a central processing unit (CPU)  1002 , such as a control circuit or microprocessor for conducting camera functions, that communicates with one or more input/output (I/O) devices  1006  over a bus  1004 . Imaging device  900  also communicates with the CPU  1002  over the bus  1004 . The processor system  1000  also includes random access memory (RAM)  1010 , and can include removable memory  1015 , such as flash memory, which also communicates with the CPU  1002  over the bus  1004 . The imaging device  900  may be combined with the CPU processor with or without memory storage on a single integrated circuit or on a different chip than the CPU processor. In a camera system, a lens adjustment assembly  1020  according to embodiments described herein, such as lens adjustment assemblies  105 ,  505 , and  605 , is used to focus image light onto the pixel array  930  of the imaging device  900  and an image is captured when a shutter release button  1022  is pressed. 
         [0046]    Although the embodiments described above include methods and apparatuses for moving a lens in a resonant motion, it should be appreciated that in other embodiments, the lens may be stationary and the imaging device circuit  104  pixel array may instead be moved in a resonant motion. 
         [0047]    The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modification and substitutions to specific structures and methods can be made and features of the various disclosed embodiments may be combined without departing from the spirit and scope of the invention, which is defined by the appended claims.