Abstract:
A method for controlling a lighting source in a capsule camera improves image quality by avoiding over-exposure or under-exposure in all regions of an image, while concurrently reducing significantly power dissipation in the capsule camera. A capsule camera using the method includes: (1) one or more sensor arrays each having one or more pixels in one or more designated regions in the field of view of the capsule camera; (2) lighting elements each providing illumination to one or more of the designated regions; and (3) a control unit that (a) extracts a parameter value from the pixels of each region; (b) evaluates the parameter value at each region; and (c) adjusts the lighting elements providing illumination to each region according to the evaluation. The parameter value may be an average value of the pixels. The purpose of the adjustment is to bring the parameter value for the region to within a predetermined range. In one embodiment, the control unit adjusts an amount of light provided by each lighting element, which may be given by integrating a light intensity of the lighting element over time. In one implementation, the light intensity in each lighting element is substantially constant and the control unit adjusts an exposure time for each lighting element. The lighting element may be, for example, a light emitting diode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to swallowable capsule cameras for imaging of the gastro-intestinal (GI) tract. In particular, the present invention relates to the control of the light sources in the camera. 
         [0003]    2. Discussion of the Related Art 
         [0004]    Devices for imaging body cavities or passages in vivo are known in the art and include endoscopes and autonomous encapsulated cameras. Endoscopes are flexible or rigid tubes that are passed into the body through an orifice or surgical opening, typically into the esophagus via the mouth or into the colon via the rectum. An image is taken at the distal end using a lens and transmitted optically to the proximal end located outside the body, either by a lens-relay system or by a coherent fiber-optic bundle. Alternatively, an instrument may record an image electronically at the distal end (e.g., using a CCD or CMOS array) and transfers the image data electrically to the proximal end through a cable. Endoscopes allow a physician control over the field of view and are well-accepted diagnostic tools. However, they have a number of limitations, present risks to the patient, and are invasive and uncomfortable for the patient. The cost of these procedures restricts their application as routine health-screening tools. 
         [0005]    Because of the difficulty traversing a convoluted passage, endoscopes cannot reach the majority of the small intestine and special techniques and precautions—that increase cost—are required to reach the entirety of the colon. Endoscopic risks include the possible perforation of the bodily organs traversed and complications arising from anesthesia. Moreover, a trade-off must be made between patient pain during the procedure and the health risks and post-procedural down-time associated with anesthesia. Therefore, endoscopy is necessarily an in-patient service that involves a significant amount of time from clinicians and thus is a costly procedure. 
         [0006]    An alternative in vivo image sensing technique is capsule endoscopy. In capsule endoscopy, a camera is housed in a swallowable capsule, along with a radio transmitter for transmitting data (which consists primarily of images recorded by the camera) to a base-station receiver or transceiver in a data recorder located outside the body. The capsule may also include a radio receiver for receiving instructions or other data from a base-station transmitter. Instead transmitting in a radio frequency, lower frequency electromagnetic signals may be used. Power may be supplied inductively from an external inductor to an internal inductor within the capsule or from a battery within the capsule. 
         [0007]    An early example of a camera in a swallowable capsule is described in the U.S. Pat. No. 5,604,531, issued to the Ministry of Defense, State of Israel. A number of patents assigned to Given Imaging describe more details of such a system, using a transmitter to send the camera images to an external receiver. Examples are disclosed in U.S. Pat. Nos. 6,709,387 and 6,428,469. There are also a number of patents to the Olympus Corporation describing a similar technology. For example, U.S. Pat. No. 4,278,077 shows a capsule with a camera for the stomach, which includes film in the camera. U.S. Pat. No. 6,800,060 shows a capsule which stores image data in an atomic resolution storage (ARS) device. 
         [0008]    An advantage of an autonomous encapsulated camera with an internal battery is that the measurements may be made with the patient ambulatory, out of the hospital, and with only moderate restrictions of activity. The base station includes an antenna array surrounding the bodily region of interest and this array can be temporarily affixed to the skin or incorporated into a wearable vest. A data recorder is attached to a belt and includes a battery power supply and a data storage medium for saving recorded images and other data for subsequent uploading onto a diagnostic computer system. 
         [0009]    A typical procedure consists of an in-patient visit in the morning during which clinicians attach the base station apparatus to the patient and the patient swallows the capsule. The system records images beginning just prior to swallowing and records images of the GI tract until its battery completely discharges. Peristalsis propels the capsule through the GI tract. The rate of passage depends on the degree of motility. Usually, the small intestine is traversed in 4 to 8 hours. After a prescribed period, the patient returns the data recorder to the clinician who then uploads the data onto a computer for subsequent viewing and analysis. The capsule is passed in time through the rectum and need not be retrieved. 
         [0010]    The capsule camera allows the GI tract from the esophagus down to the end of the small intestine to be imaged in its entirety, although it is not optimized to detect anomalies in the stomach. Color photographic images are captured so that anomalies need only have small visually recognizable characteristics, not topography, to be detected. The procedure is pain-free and requires no anesthesia. Risks associated with the capsule passing through the body are minimal; certainly, the risk of perforation is much reduced relative to traditional endoscopy. The cost of the procedure is less than that of traditional endoscopy because of the decreased requirements in clinician time, clinical facilities and anesthesia. 
         [0011]    As the capsule camera becomes a viable technology for inspecting gastrointestinal tract, various methods for storing its image data have emerged. For example, U.S. Pat. No. 4,278,077 discloses a capsule camera that stores image data in chemical films. U.S. Pat. No. 5,604,531 discloses a capsule camera that transmits image data by wireless to an antenna array attached to the body or provided inside a vest worn by the patient. U.S. Pat. No. 6,800,060 discloses a capsule camera that stores image data in an expensive atomic resolution storage (ARS) device. The stored image data may then be downloaded to a workstation, which is normally a personal computer for analysis and processing. The results may then be reviewed by a physician using a friendly user interface. However, these methods all require a physical media conversion during the data transfer process. For example, image data on chemical film are required to be converted to a physical digital medium readable by the personal computer. The wireless transmission by electromagnetic signals requires extensive processing by an antenna and radio frequency electronic circuits to produce an image that can be stored on a computer. Further, both the read and write operations in an ARS device rely on charged particle beams. 
         [0012]    A capsule camera using a semiconductor memory device, whether volatile or nonvolatile, is capable of a direct interface with a CMOS or CCD image sensor, where the image is captured, and a personal computer, where the image may be analyzed. The high density and low manufacturing cost achieved in recent years made the semiconductor memory the most promising technology for image storage in a capsule camera. According to Moore&#39;s law, which is still believed valid, the density of integrated circuits doubles every 24 months. Meanwhile, CMOS or CCD sensor resolution continues to improve, doubling every few years. Recent advancement in electronics also facilitate development in capsule camera technology. For example, (a) size and power reductions in light emitting diodes (LEDs) promotes the use of LEDs as a lighting source for a capsule camera; (b) new CMOS image sensors also reduce power and component count; (c) the continued miniaturization of integrated circuit allows integrating many functions on a single silicon substrate (i.e., system-on-a-chip or “SOC), resulting in size and power reductions. 
       SUMMARY 
       [0013]    A method for controlling a lighting source in a capsule camera improves image quality by avoiding over-exposure or under-exposure in all regions of an image, while concurrently reducing significantly power dissipation in the capsule camera. 
         [0014]    According to one embodiment of the present invention, a capsule camera having adjustable illumination control includes: (1) one or more sensor arrays each having one or more pixels in one or more designated regions in a field of view of the capsule camera; (2) lighting elements each providing illumination to one or more of the designated regions; and (3) a control unit that (a) extracts a parameter value from the pixels of each region; (b) evaluates the parameter value at each region; and (c) adjusts the lighting elements providing illumination to each region according to the evaluation. The parameter value may be an average value of the pixels. The purpose of the adjustment is to bring the parameter value for the region to within a predetermined range. In one embodiment, the control unit adjusts an amount of light provided by each lighting element, which may be given by integrating a light intensity of the lighting element over time. In one implementation, the light intensity in each lighting element is substantially constant and the control unit adjusts an “on” time for each lighting element. The lighting element may be, for example, a light emitting diode. 
         [0015]    Each designated region may be illuminated by multiple lighting elements. In one implementation, each lighting element illuminates a designated region driven by a common current mirror circuit. The current in each lighting element may be reflected from the common current mirror circuit by a transistor of a predetermined conductivity type. 
         [0016]    According to one embodiment of the present invention, the capsule camera includes a motion detection circuit which compares the extracted parameter values in two exposures to detect motion of the capsule camera. The exposures may be two successive exposures of the capsule camera. In this embodiment, the control unit operates in an active mode and a monitor mode. The control unit enters the monitor mode when no motion of the capsule camera is detected in successive exposures in the active mode, and enters the active mode when motion is detected in the monitor mode. Exposures in the active mode are provided within a first range of light amounts and exposures in the monitor mode are provided in a second range of light amounts, the light amounts within the first range being substantially greater than the light amounts in the second range. The first range is provided to yield images with sufficient detail for a human reviewer to perform a diagnosis. 
         [0017]    According to one embodiment of the present invention, in the first exposure of the monitor mode, the parameter value extracted from the last exposure in the active mode is scaled based on the first and second ranges of light amounts. The scaled parameter value is then used by the motion detection circuit in the comparison. In the monitor mode, the motion detection circuit compares the extracted parameter value for each frame against the parameter value extracted from the last exposure in the active mode. The criteria for motion detection in the active mode and in the monitor mode may be different. The criterion for motion detection for successive frames with the same exposure may also be different from the criterion for motion detection for successive frames with different amounts of exposure. Upon returning to the active mode from the monitor mode, the lighting elements are returned to settings used for taking the last frame in a previous active mode operation. One or more of the lighting elements are not activated in the monitor mode to achieve power saving goals. 
         [0018]    According to one embodiment of the present invention, the capsule camera includes component cameras each facing a different direction, so that the fields of view of the component cameras together provide a panoramic field of view (e.g., 360-degree). 
         [0019]    The present invention is better understood upon consideration of the detailed description below in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1A  shows capsule camera  100  inside a gastrointestinal tract  10  taking measurements with its light sources  20 A and  20 B. 
           [0021]      FIG. 1B  shows capsule camera  100  in narrow portion  11  of GI tract  10 . 
           [0022]      FIG. 2  illustrates exemplary control scheme  200  for driving the LED&#39;s of capsule camera  100  (e.g., LEDs  20 A and  20 B of  FIGS. 1A and 1B ), in accordance with one embodiment of the present invention. 
           [0023]      FIG. 3  shows amounts of light  301 ,  302  and  303  provided to the LEDs of capsule  100  for taking consecutive frames  311 ,  312  and  313  under the control scheme of  FIG. 2 . 
           [0024]      FIG. 4  shows control scheme  400  which includes motion detection function  401 , in accordance with one embodiment of the present invention. 
           [0025]      FIG. 5  illustrates amounts of light  501 ,  502  and  503  provided by the LEDs of capsule camera  100  for taking frames  510 ,  511  and  512 , under the control scheme of  FIG. 4 . 
           [0026]      FIG. 6  shows lights amounts  601 ,  602  and  603  provided by the LEDs of capsule camera  100  under control scheme  400  of  FIG. 4 ; light amounts  601  and  602  being provided in the monitor mode and light amount  603  being provided after returning to the active mode. 
           [0027]      FIG. 7  is a flow chart illustrating the operations of control unit  201  both in the monitor mode and the active mode, in accordance with one embodiment of the present invention. 
           [0028]      FIG. 8  is a cross section of housing  107  of capsule camera  100 , showing cameras  801 - 804  each facing outward in a different direction, thereby compositing a panoramic view. 
           [0029]      FIG. 9  shows lighting control scheme  900 , having N separate regions  901 - 1  to  901 -N illuminated by LEDs  903 - 1  to  903 -M, according to one embodiment of the present invention. 
           [0030]      FIGS. 10A and 10B  show respectively designs  1001  and  1005  each providing different constant driving currents for LEDs  903 -I,  903 -(I+1), . . . ,  903 -(I+J) to illuminate specified region K, in accordance with one embodiment of the present invention. 
       
    
    
       [0031]    To facilitate cross-references, like elements in the figures are assigned like reference numerals. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]      FIG. 1A  shows capsule camera  100  inside a gastrointestinal tract  10  taking measurements with its light emitting diode (LED) light sources  20 A and  20 B. As shown in  FIG. 1A , capsule camera  100  includes housing  107 , LEDs  20 A and  20 B, baffler/reflector  108 , and image sensor  21 . Image sensor  21  may include more than one sensor array. The image on image sensor  21  is processed by image processor  102  using digital signal processing techniques. Selected images are compressed in image compression module  103  and stored into memory  104 . Power supply  105  provides power for capsule camera  100 &#39;s operations. After capsule camera  100  is recovered, output port  106  allows a user to upload to a workstation the stored images and other data. In another embodiment, instead of storing the images on-board, a transmitter is provided in capsule camera  100  which transmits the image data to a receiver outside of the patient&#39;s body, where the images may be processed or archived for later review. The present invention is applicable to both systems that store the image data on-board and systems that transmit the image data. 
         [0033]    The amount of light provided for each exposure is given by the sum of the light provided by all the LEDs. The light provided by each LED is the product of the LED “on” time and its intensity. By adjusting the amounts of light provided in LEDs  20 A and  20 B, it is desired that image of point B (which is far away), resulting from the light reflected from point B onto image sensor  21 , is not under-exposed, and the image of point A (which is much closer), from light reflected from point A, is not over-exposed. GI tract  10  is not a uniform pipe, but includes both wide and narrow portions.  FIG. 1B  shows, for example, capsule camera  100  in narrow portion  11  of GI tract  10 . In portion  11 , LED light sources  20 A and  20 B are kept at suitable lighting levels, so that the image of illuminated objects at image sensor  21  is not over-exposed, and to save battery power. 
         [0034]    It is significant that, for each capsule endoscopic procedure, tens to hundreds of thousands of images are taken and stored, thus requiring a large amount of physician time to read and archive the images. During the procedure, which normally takes from a few hours to more than 10 hours, a capsule camera continues to take pictures of the GI tract at the rate of one to a few frames per second, as it travels by peristalsis activities of the GI tract. The total number of images taken ranges from tens of thousands to hundreds of thousands. Even if software is provided in the workstation to accelerate the physician&#39;s or technician&#39;s viewing process, tens of minutes of physician or technician time is still required to examine these images. The requirements for archiving, retrieval and transferring large amount of data is also severe. Moreover, for each image, power is also required for lighting, image processing and storing. As capsule camera  100  does not move or moves very little relative to GI tract  10  much of the time, camera  100  records an image only when a significant movement is detected relative to the last image. 
         [0035]    Capsule camera  100 , however, is still required to take images for use in motion detection. There is a range of exposures that result in clear pictures suitable for review by human eyes. At a lower exposure (i.e., under an under-exposure condition), the human eyes are not efficient. However, so long as the lower exposure is above the system noise level, image processing techniques may be applied to the image data to differentiate features in the image, even if the image is too dark for human eyes. Such an image may still be used to detect motion. Therefore, lighting may be reduced for motion detection images, which are not used for a physician&#39;s later analysis. Further, for such purpose, only some of the LED&#39;s of the lighting system need to be on, and only images of a portion of the entire field of view need be acquired for the capsule camera&#39;s motion detection circuits. The capsule camera returns to normal lighting conditions when it determines that significant movement has occurred. 
         [0036]    Images taken for the purpose of motion detection need only involve a subset of the pixels on image sensor  21 , and the exposure provided for each pixel may be much lower than that required for an image intended for physician review. Thus, for motion detection images, the power output of LED light sources  20 A and  20 B may be a fraction of that required for an image suitable for human review. Moreover, the subset of pixels on image sensor  21  used for motion detection may be selected from specified areas of the pixel array or arrays, where one of the lighting sources  20 A and  20 B—the one having a higher percentage of light reaching the region in the field of view corresponding to the subset of pixels—needs to be activated. Because only a smaller number of pixels of the whole image are involved in acquisition and processing motion detection images, power is reduced. The motion detection images are discarded, so that a physician&#39;s time spent in reviewing and archiving redundant images is reduced. 
         [0037]    Autonomous capsule camera  100  operates from a power source. Normally, the power source may be a battery system, or a changing magnetic field imposed from outside which is used by the power supply circuits in the capsule to generate power. Because it is desirable that the procedure is performed as an out-patient procedure with the patient ambulatory, the battery approach is preferred. Normally, the size of capsule camera  100  is limited by the physical size of the battery. A higher detection rate requires a higher resolution and a higher frame rate, which in turn demand a larger battery capacity with larger dimensions. Power saving achieved when the capsule is quiescent (i.e., not moving) enables using a smaller capsule camera, thus making it easier to swallow. Such a capsule camera is desirable for use especially by a child, an elderly person, or a very sick patient. Such a smaller capsule camera also enhances the detection rate. Thus power conservation is practiced whenever possible without compromising performance. 
         [0038]    In  FIG. 1A , the lighting near point A is primarily provided by LED  20 A, while around points B and C, the light is primarily provided by LED  20 B. To avoid an over-exposure at the image of point A, LED  20 A should be adjusted to a lower light level to avoid an overly bright and saturated image. Conversely, around points B and C, LED  20 B must provide a stronger lighting. Relative to point A, a proper exposure of points B and C require a higher intensity or a longer exposure time, or both. By the same principles, in  FIG. 1B , where the space is much smaller, the light intensities of both  20 A and  20 B should be adjusted to be much lower. 
         [0039]    Inside the GI tract, capsule camera  100  may move forward (and at times backward), rotate or move in other ways. The movements are generally slow, however. Therefore, if the images are taken at a fast enough rate, capsule camera  100 &#39;s position relative to the GI tract change little from one image to the next. Lighting may therefore be adjusted for all regions of interest by controlling the driving parameters to all LED&#39;s accordingly in a continuous fashion. 
         [0040]      FIG. 2  illustrates exemplary control scheme  200  for driving the LED&#39;s of capsule camera  100  (e.g., LEDs  20 A and  20 B of  FIGS. 1A and 1B ), in accordance with one embodiment of the present invention. After the exposure of a previous frame, control unit  201  (e.g., image processor  102 ) analyzes a selected region covered by LED  20 A, using pixel values at pixel subset  202  of the image for that region. If the image is over- or under-exposed, an adjustment to achieve the appropriate lighting is determined for the next image. (The pixel value is expected to be in linear proportion to the exposure). The same procedure is performed for another selected region, which is covered by LED  20 B using pixel values at pixel subset  203  of the image. The parameters for a proper exposure are then stored in medium  204  (e.g., flip-flops, registers or another temporary storage medium). In one implementation, one exposure parameter is the “on” duration for an LED (e.g., LEDs  20 A or  20 B) that is driven by a constant current source (current source  205  or  206 ). Such an arrange results in a simple driving circuit design. Other schemes—such as, for example, increasing the light intensity of an LED by providing a higher current—are also possible. 
         [0041]      FIG. 3  illustrates consecutive frames  311 ,  312  and  313  taken under the control scheme of  FIG. 2 .  FIG. 3  shows both quantities expressed in pixel values (referring to the axis on the right) and quantities expressed in light intensity (referring to the axis on the left). The pixel value for this purpose may be selected from those that can be taken from the image with a reasonable amount of image processing. One example is the average value of a subset of pixels in a region covered by a light source. Alternatively, the pixel value may correspond to the highest occurrence, or another parameter that represents the total brightness. In this detailed description, the average pixel value is used merely for illustrative purposes. Other parameters may be used within the scope of the present invention. On the right axis, AH and AL indicate, respectively, the upper bound and the lower bound of the range of pixel values suitable for human review and analysis. ML represents the brightness below which the noise in the system interferes with the capsule camera&#39;s ability to handle the image processing necessary for the light control. MH is an upper bound set for the motion detection exposures, selected to most effectively save power. 
         [0042]      FIG. 3  shows amounts of light (i.e., exposures)  301 ,  302  and  303  provided by the LEDs of capsule camera  100  for frames  311 ,  312  and  313 , respectively. In  FIG. 3 , the luminances or intensities of LEDs  20 A and  20 B are kept constant, so that exposures  301 ,  302  and  303  can be controlled by the durations at LED  20 A (TL 1 A, TL 2 A and TL 3 A) and the durations at LED  20 B (TL 1 B, TL 2 B and TL 3 B), when LEDs  20 A and  20 B are turned on, respectively. After frame  311  is taken, control unit  201  obtains the average pixel values  304  and  305  from the image at the pixel subsets covered by LEDs  20 A and  20 B, respectively. As shown in  FIG. 3 , average pixel value  304  for the region covered by LED B is below AL. Thus, control unit  201  calculates the proper amount of light (i.e. exposure time in this case, as the LEDs have constant luminance) for the next frame, assuming the same scene. For the same scene, the average pixel value depends linearly on the amount of light. As average pixel value  305  for region A is within the range between AH and AL, no adjustment to the exposure time is required for LED  20 A. As average pixel values  306  and  307  for frame  312  are within the range between AH and AL, no adjustment to exposure times for LEDs  20 A and  20 B are provided to take frame  313 . 
         [0043]      FIG. 4  shows control scheme  400  which includes motion detection function  401 , in accordance with one embodiment of the present invention. Motion detection function  401  achieves power savings in the operations of capsule camera  100  by storing or transmitting an image only when the image shows a significant movement relative to a previous image. Motion detection function  401  may be implemented in a variety of ways. Some methods for motion detection are disclosed, for example, in co-pending U.S. patent application, entitled “IN VIVO AUTONOMOUS CAMERA WITH ON-BOARD DATA STORAGE OR DIGITAL WIRELESS TRANSMISSION IN REGULATORY APPROVED BAND,” Ser. No. 11/533,304, filed on Sep. 19, 2006. The copending application is hereby incorporated by reference in its entirety. Motion detection detects whether or not a significant enough movement has occurred within the field of view of interest. When there is no movement, the next frame would be taken in the monitor mode, in which the amount of light for the exposures is reduced to a level such that the pixel values in the image for regions A and B are between MH and ML. This amount of light is selected to be low but sufficient to allow control unit  201  to reliably determine if a significant movement has taken place. In the monitor mode, if any movement is detected, control unit  201  returns to an active mode to capture an image that is within the range for human review. In some embodiments, in the monitor mode, not all the LED&#39;s are turned on, as is the case in the active mode. In practice, a single LED provides sufficient light to detect motion. 
         [0044]    It is possible that, at the first frame taken after entering into the monitor mode, capsule camera  100  actually moves. The pixel value in a region covered by one turned-on LED is linearly dependent on the amount of light provided by the LED. Therefore, in some embodiments, the average pixel values in the same region covered by the turned-on LED may still be compared to detect motion between the last frame taken before entering the monitor mode and the first frame after entering the monitor mode. This is achieved by scaling the average pixel value in the last frame of the active mode according to the amounts of light provided by the LED in the two frames.  FIG. 5  shows amounts of light  501 ,  502  and  503  provided by the LEDs of capsule camera  100  for taking frames  510 ,  511  and  512 , under control scheme  400  of  FIG. 4 . As shown in  FIG. 5 , the same amounts of light  501  and  502  are provided by LEDs  20 A and  20 B for taking frames  510  and  511 . Control unit  201  detects that no motion occurred between frames  510  and  511 , and thus enters into monitor mode after frame  511 . An amount of light  503 , which is provided only by LED  20 A, is used for illuminating the field of view in the monitor mode. LED  20 B is turned off to save power. To detect if motion occurred between frames  511  and  512 , the average pixel value  507  for region A of frame  511  is scaled by the ratio TL 3 A:TL 2 A, which is the ratio of exposure times for taking frames  511  and  512 . The scaled average pixel value is then compared to measured average pixel value  508  of frame  512 . If motion is detected, control unit  201  returns to active mode after just one frame in monitor mode. Because a slight error may exist in the driving circuit (i.e., the actual deliveries of the two different light amounts may not be exactly according to the predetermined ratio), in some embodiments, the threshold selected for motion detection between two frames in different modes may be different from the threshold selected for motion detection in the active mode, when the amount of light does not change between two frames. Similarly, different thresholds of motion detection may be used between two frames of the same light amount and between two frames of different light amounts, even in the same mode. 
         [0045]    In one embodiment, the motion detection function uses the average pixel value of the last frame before entering the monitor mode to compare with the average pixel value of a current frame. In this method, one or more frames may have been taken in the monitor mode that the motion detection function cannot detect a significant movement from frame to frame. However, when the average pixel value of the current frame is compared to the last frame before entering the monitor mode, the accumulated difference in the average pixel value may be sufficient to reach the threshold of motion detection. At that point, capsule camera  100  returns to active mode to capture an image that a physician can review. 
         [0046]      FIG. 6  shows lights amounts  601 ,  602  and  603  provided by the LEDs of capsule camera  100  under control scheme  400  of  FIG. 4 ; light amounts  601  and  602  being provided in the monitor mode and light amount  603  being provided after returning to the active mode. In  FIG. 6 , motion is detected between frames  611  and  612 . Therefore, control unit  201  returns to the active mode and takes frames using both LEDs  20 A and  20 B using, for each LED, the same light amount used for taking the last frame prior to entering the monitor mode. In the active mode, control unit  201  stays in the active mode as long as movement is detected between two consecutive frames. It is possible that the first frame taken in the active mode shows no movement from the last frame taken in the monitor mode. In that situation, control unit  201  returns to the monitor mode after only one frame in the active mode. Alternatively, for the first frame in the active mode following the monitor mode, the light amount provided by LED  20 A is determined from scaling the light amount used for the last frame in the monitor mode, and the light provided by LED  20 B is the light amount provided by LED  20 B for the last frame the previous time control unit  201  was in the active mode. 
         [0047]    In one embodiment, the image is low-pass filtered before performing motion detection in the monitor mode, so as to reduce the noise. A lower noise level allows a further reduction of the MH and ML levels. In another implementation, adjacent pixels are resampled to subdue noise to lower the MH and ML levels. For example, a 64×64 sub-region may be resampled by combining (e.g., summing) 4 adjacent pixels to achieve a 32×32 sub region, which may then be used for motion detection. 
         [0048]      FIG. 7  is a flow chart illustrating the operations of control unit  201  in both the monitor mode and the active mode, in accordance with one embodiment of the present invention. In the embodiment of  FIG. 7 , LEDs  20 A and  20 B and two regions illuminated respectively by these LEDs are used to control the operations. For illustrative purpose only, LEDs  20 A and  20 B are modeled as constant current sources, so that each LED&#39;s turned-on duration (i.e., exposure time) determines the amount of light provided by the LED in each frame. In another embodiment, in which the current may be varied, the amount of light provided is the variable light intensity integrated over the exposure period. Of course in all the embodiments, the exposure time precedes the image data acquisition, processing, transmission and storage function. As shown in  FIG. 7 , operations  703 - 707  are carried out in active mode  750  and operations  712 - 716  are carried out in monitor mode  752 . 
         [0049]    In active mode  750 , when a frame is taken at step  703 , the response or pixel values lum-A and lum-B derived from regions A and B, respectively, are examined at step  704  to determine if they are each within the active mode luminance range (e.g., between AH and AL of  FIG. 6 ). If the pixel values are within the active mode luminance range, the motion detection function examines if motion has occurred between the present frame and a previous frame (step  705 ). If one or more of the pixel values are not within the active mode luminance range, the appropriate lighting control value or values (e.g., the LED “on” time or times) are adjusted, where necessary, to bring the pixel values back to within range (step  707 ). If motion is not detected (step  706 ), the lighting control values for LEDs  20 A and  20 B are stored, and control unit  201  exits active mode  750 . Otherwise, control unit  201  returns to step  703  to wait for the next frame to be taken. 
         [0050]    In one embodiment, when motion is detected, the mage is stored or transmitted even when the pixel values are not within the desired range (e.g., between AH and AL as shown in  FIG. 6 ). In another embodiment, when motion is detected, only image regions where average pixel values are in the desired range (i.e., between AH and AL) are stored or transmitted. Image regions where the pixel values are out of the desired range are discarded. In still another embodiment, the range used at step  704  to determine if the lighting should be adjusted may be different from the range used to determine if an image is to be stored or transmitted. 
         [0051]    Upon leaving active mode  750 , the pixel values for regions A and B and the light amounts provided are examined (step  708 ) to determine which of regions A and B has a larger pixel value to light amount ratio. The LED which results in the lesser response is turned off (step  709  or step  710 ) before entering into monitor mode  752 . At step  711 , the appropriate lighting control value in monitor mode  752  for the other LED (i.e., the LED which provide the greater pixel value to light amount ratio from regions A and B) is set. 
         [0052]    In monitor mode  752 , when a frame is taken in step  712 , the response or pixel value lum-A or lum-B derived from the active one of regions A and B is examined at step  713  to determine if it is within the monitor mode luminance range (e.g., between M 1  and ML of  FIG. 6 ). If the pixel value is within the active mode luminance range, the motion detection function examines if motion has occurred between the present frame and a previous frame (step  714 ). If motion is detected (step  715 ), the stored lighting control values for LED  20 A and  20 B from active mode  750  are restored, and control unit  201  exits monitor mode  752 . Otherwise, control unit  201  returns to step  712  to wait for the next frame to be taken. 
         [0053]    More than two LEDs are expected to be used in a practical implementation of capsule camera  100 . There may also be more than one image sensor array.  FIG. 8  is a cross section of housing  107  of capsule camera  100 , showing cameras  801 - 804  each facing outward in a different direction, thereby compositing a panoramic view. As cameras  801 - 804  each have a field of field that is more than 90 degrees wide, and so long as the fields of views of adjacent cameras overlap inside capsule housing  107 , a 360-degree total field is provided perpendicular to the longitudinal direction in which capsule camera  100  travels. The current state-of-the-art is capable of providing lens and sensor arrays that are each in the order of 1 mm in each dimension. Using these components, capsule camera  100  may be implemented with housing  107 , which may have a 1-cm diameter. 
         [0054]      FIG. 9  shows lighting control scheme  900 , having N separate regions  901 - 1  to  901 -N illuminated by LEDs  903 - 1  to  903 -M, according to one embodiment of the present invention. The images of regions  901 - 1  to  901 -N may situate in different sensor arrays, with each region receiving light from one or more LED&#39;s. When entering into the monitor mode, some of the LED&#39;s may be turned off to save power. The motion detection circuits  904  may compare, using successive images, subsets of pixel values in one or more regions. 
         [0055]    The power requirement for an LED constant current driver includes a constant current source in a current mirror circuit. Resistors in the constant current source are normally selected to have very high values so as to reduce the operating current of the constant current source. Since a conventional semiconductor process does not reliably provide high-value resistors with good precision, such resistors are typically implemented outside the integrated circuit by discrete components. In a capsule camera, where space is limited, these resistors and their interconnections with the integrated circuit may cause space and manufacturing difficulties.  FIGS. 10A and 10B  show respectively designs  1001  and  1005  each providing different constant driving currents for LEDs  903 -I,  903 -(I+1), . . . ,  903 -(I+J) to illuminate specified region K, in accordance with one embodiment of the present invention. In each of designs  1001  and  1005 , the current in the current mirror circuit (i.e., current mirror circuit  1003  or current mirror circuit  1004 ) is reflected in ( 1 #) (J+1) currents to drive LEDs  903 -I,  903 -(I+1), . . . ,  903 -(I+J). Design  1001  provides the currents through PMOS transistors  1007 -I,  1007 -(I+1), . . . ,  1007 -(I+J). Similarly, design  1002  provides the currents through NMOS transistors  1008 -I,  1008 -(I+1), . . . ,  1008 -(I+J). In these designs, one current mirror circuit is provided for each given region to drive multiple LEDs, so as to save space and improve manufacturing yield. 
         [0056]    The above detailed description is provided to illustrate the specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims: