# Patent

## Title
Methods to compensate manufacturing variations and design imperfections in a capsule camera

## Abstract

A method characterizes manufacturing imperfections in a camera and variations in its operating environment to allow images captured by the camera to be compensated for these defects. The method includes: (a) illuminating a field of view of the optical elements under a controlled condition; (b) exposing multiple images onto the image sensor under the controlled condition; (c) extracting from the multiple images parameter values for pixels of the image sensor; and (d) compensating images taken subsequently in the camera using the parameter values. The controlled condition includes an external light source for illumination, and the image sensor is sensitive to color components.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for compensating for variations in a manufacturing process. In particular, the present invention relates to applying such methods to optical instruments.
2. Discussion of the Related Art
The capsule camera has proved to be an effective minimally-invasive way for examining the gastrointestinal (GI) tract. One example of a swallowable capsule camera 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 in greater detail a capsule camera system which uses a transmitter to send images captured by the camera to an external receiver. Other capsule camera examples include U.S. Pat. Nos. 6,709,387 and 6,428,469. There are also other patents to the Olympus Corporation regarding capsule cameras. For example, U.S. Pat. No. 4,278,077 discloses a film camera in a capsule designed for the stomach. U.S. Pat. No. 6,939,292 discloses a capsule camera with a buffering memory and a transmitter. U.S. Pat. No. 6,800,060 discloses a capsule camera which stores image data in an atomic resolution storage (ARS) device.
Various advancements in electronics may now be incorporated into the capsule camera. For example, an LED provides a compact and low-power lighting source. CMOS image sensors reduce the component count and power. The further miniaturizing of integrated circuit geometry allows SOC (system-on-a-chip) techniques to be used, which reduce the size and power of the capsule camera. However, size reduction pushes the limits of manufacturing tolerances. For example, manufacturing processes for semiconductor image sensors inherently include not only lot-to-lot, wafer-to-wafer, and die-to-die variations, but also pixel-to-pixel variations within a die. Even the analog circuits sensing the charge stored in a cell to provide digital image data vary from location to location within a die. In addition, LEDs are known for variations in their light spectrum and intensity. Also, because the center of a lens passes light better than at the edge, a higher intensity results in the center of the image than at the edges. There are also variations from lens to lens, even if the lenses are fabricated from the same mold.
Another example results from the fact that illumination sources in a capsule camera illuminate objects only a few centimeters away, the objects receive non-uniform light intensities. Moreover, the transparent window provided in the capsule housing for the LED light to pass through is not perfectly transparent. Thus, when the light sources in the capsule (e.g., LEDs) are turned on, the transparent window reflects some of the light within the camera's field of view back to the light sources and the image sensor.
As the capsule camera is intended to allow a physician to examine the inside of a patient's GI tract, its accuracy and image quality are of paramount concern.
SUMMARY
According to one embodiment of the present invention, a method is provided for characterizing manufacturing variations and design imperfections in a capsule camera and imperfections in its operating environment to allow images captured by the capsule camera to be compensated for these defects.
According to one embodiment of the present invention, a method for characterizing a camera includes: (a) illuminating a field of view of the optical elements under a controlled condition; (b) exposing multiple images onto the image sensor under the controlled condition; (c) extracting parameter values of a model for the image provided on the image sensor from the multiple images; and (d) compensating images taken subsequently in the camera using the parameter values. The objects in the field of view may have a predetermined color, contrast or pattern. In one instance, the controlled condition includes an external light source for illuminating the field of view, and the image sensor is sensitive to a plurality of color components. The field of view may be illuminated at a predetermined light intensity for each of the color components. The camera may include an archival memory for storing the images taken. Alternatively, the camera may include a transmitter for sending images taken to an external depository. The camera may also be associated with an identification, and the transmitter sends the images together with the identification to identify the images sent.
According to one embodiment of the present invention, the model of the image may include a multiplicative factor for compensating pixel-to-pixel variations, an average pixel value across a plurality of pixels, or a leakage current in a pixel of the image sensor. The leakage current is integrated over a time of exposure.
According to one embodiment of the present invention, the camera includes an illumination system within the camera. In one characterization, the images are taken in a light absorption environment. The model of the image includes a correction for light reflected from the camera itself; the correction may be a function of a current in a light emitting diode and time of exposure.
According to one embodiment of the present invention, a camera includes an optical element, an image sensor and an archival memory for storing characterization data based on a model of the image on the image sensor.
According to one embodiment of the present invention, a method for characterizing a camera includes (a) placing the camera inside a light absorption environment, and taking a first number of exposures, and (b) placing the camera within a uniform lighting environment, and taking a second number of exposures. In one instance, the exposures are taken using an illumination system within the camera for different exposure times. In one application, a model is provided for the light of the illumination system reflected from the camera's housing. In one instance, the exposures are taken using external lighting conditions of predetermined characteristics.
The present invention is better understood upon consideration of the detailed description and the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an example of a capsule camera in the GI tract.
FIG. 2 illustrates a method for characterizing pixel-to-pixel variations and design imperfection in a capsule camera, in accordance with one embodiment of the present invention.
FIG. 3 illustrates a method for characterizing pixel-to-pixel variations and design imperfection in a capsule camera under its own illumination, in accordance with one embodiment of the present invention.
FIG. 4 shows a method for using a capsule camera using the characterization procedures described above, in accordance with one embodiment of the present invention.
FIG. 5 illustrates an alternative embodiment of the present invention described in conjunction with FIG. 4.
FIG. 6 illustrates an alternative embodiment of the present invention described in conjunction with FIG. 4.
FIG. 7 illustrates the alignment between an image sensor array and the footprint of an image produced by an optical system, such as the lens system of a capsule camera.
FIG. 8A shows a Lambertian curve of a radiation pattern of an LED (i.e., angle versus illuminance) and FIG. 8B the luminance of LED versus angle.
FIG. 9 shows a grid-distortion plot of a lens.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an exemplary capsule camera in the GI tract. As shown in FIG. 1, swallowable capsule camera system 01 is seen inside body lumen 00, which may be, for example, the colon, the small intestines, the esophagus, or the stomach. Capsule camera system 01 is entirely autonomous while inside the body, with all of its elements encapsulated in a capsule housing 10 that provides a moisture barrier, protecting the internal components from bodily fluids. Capsule housing 10 includes at least a transparent window, so as to allow light from the light-emitting diodes (LEDs) of illuminating system 12 to pass through the wall of capsule housing 10 to illuminate the walls of lumen, and to allow the scattered light from the lumen 00 walls to be collected and imaged within the capsule camera. Capsule housing 10 also protects lumen 00 from direct contact with the foreign material inside capsule housing 10. Capsule housing 10 is provided a shape that enables it to be swallowed easily and later to pass through the GI tract efficiently. Generally, capsule housing 10 is sterile, made of a non-toxic material, and is sufficiently smooth to minimize the chance of lodging within the lumen.
As shown in FIG. 1, capsule camera system 01 includes illuminating system 12 and a camera that includes optical system 14 and image sensor 16. An image captured by image sensor 16 may be processed by image processor 18, which performs various image processing functions, such as determining whether or not the capsule is moving relative to the portion of the GI tract within the optical view of the camera. Image processor 18 may be implemented by a digital signal processor (DSP) or a central processing unit (CPU). Image processor 18 may have one or more partial frame buffers. A semiconductor non-volatile archival memory 20 may be provided to allow the images to be retrieved throughout output port 28 at a docking station or workstation outside the body, after the capsule is recovered. The image in archival memory 20 may be stored in a compressed form, using data compression module 22. (Data compression module 22 may be implemented in hardware, or in software running on image processor 18). Capsule camera system 01 is powered by battery power supply 24. Capsule camera system 01 may be propelled through the GI tract by peristalsis.
Illuminating system 12 may be implemented by LEDs. In FIG. 1, the LEDs are located adjacent the camera's aperture, although other configurations are possible. The light source may also be provided, for example, behind the aperture. Other light sources, such as laser diodes, may also be used. Alternatively, white light sources or a combination of two or more narrow-wavelength-band sources may also be used. In one embodiment, white LEDs may be formed by various components, such as a blue LED or a violet LED, along with phosphorescent materials that are excited by the LEDs' light to emit light at longer wavelengths. The portion of capsule housing 10 that allows light to pass through may be made from bio-compatible glass or polymer.
Optical system 14, which may include multiple refractive, diffractive, or reflective lens elements, provides an image of lumen 00 within its field of view on image sensor 16. Image sensor 16 may include charged-coupled devices (CCD) or complementary metal-oxide-semiconductor (CMOS) type sensor devices that convert the received light intensities into corresponding electrical signals. Image sensor 16 may have a monochromatic response or include a color filter array to allow a color image to be captured (e.g. using the RGB or CYM color space representations). The analog signals from image sensor 16 are preferably converted into digital form to allow use of digital processing techniques. Such a conversion may be accomplished using an analog-to-digital (A/D) converter inside image sensor 16 (as in the current case), or at another location inside capsule housing 10. The A/D unit may be provided between image sensor 16 and the rest of the system. LEDs in illuminating system 12 are synchronized with the operations of image sensor 16. One function of control module 26 (not shown) is to control the LEDs during image capture operation.
Image processor 18 may select an image to retain when the image shows enough motion relative to a previous image in order to conserve the limited storage space available. The captured images may be stored in on-board archival memory system 20. Output port 28 shown in FIG. 1 is not operational in vivo, but uploads data to a workstation after the capsule camera is recovered from a journey through the body.
When the LEDs are turned on, their light passes through the transparent window in capsule housing 10 to illuminate objects in the GI tract. The reflected light from these objects passes through the transparent window and reaches lens 14, which focuses the image on imaging sensor 16. Some of the light emitted from the LEDs is reflected back from from the surface of the transparent window, as shown in long-short dash line 103, through lens 14 onto image sensor 16. Because every pixel in a cell array in image sensor 16 is in some sense unique, image sensor 16 may be characterized under a uniform lighting condition provided externally—with the LEDs in capsule camera system 01 turned off—to determine pixel-to-pixel variations in image sensor 16. In fact, the different readouts of these pixels represent both the pixels' variations and the lens' contribution to non-uniform pixel values.
FIG. 2 illustrates a method for characterizing pixel-to-pixel variations and design imperfection in a capsule camera, in accordance with one embodiment of the present invention. As shown in FIG. 2, a capsule camera including at least a lens system, an image sensor and a transmitter or an archival memory is prepared (step 201). Next, at step 202, objects illuminated by light of a known color, intensity, contrast or pattern is placed in the field of view of the lens system and exposures are made. These images are then used to characterize or calibrate the capsule camera system. The images may be stored in the archival memory or transmitted. At a subsequent time (step 203), these images are used to calibrate the images captured from a patient for clinical diagnosis purposes.
One method for compensating variations in pixel responsivity and the field dependence on the camera's response (i.e., the non-uniformity in sensor illumination even with a uniform object illumination) is to present the camera with a field of uniform color and brightness during calibration. A test image is then taken under this illumination condition and used to obtain an average value AR for the red component of all pixels. If a particular pixel's red component signal is PRi, the red component of this pixel may be compensated during normal operation by multiplying the red component of the measured pixel value by a factor AR/PRi. Other color components may be compensated in a like manner. (This method may also be applied likewise to color components in other color space domains).
Characterization of the capsule camera system under its own illumination (e.g., illumination system 12) may be carried out. FIG. 3 illustrates a method for characterizing pixel-to-pixel variations and design imperfection in a capsule camera under its own illumination, in accordance with one embodiment of the present invention. The “white balance” of the combined illumination and imaging system can be corrected. As shown in FIG. 3, a capsule camera including at least a lens system, an image sensor, an illumination system within the capsule camera's housing, and a transmitter or an archival memory is prepared (step 301). Next, at step 302, objects of known color, contrast, and pattern are illuminated by the illumination system in the field of view of the lens system and exposures are made. These images are then used to characterize or calibrate the capsule camera system. The images may be stored in the archival memory or transmitted. At a subsequent time (step 303), these images are used to calibrate the images captured from a patient for clinical diagnosis purposes.
During calibration, images may be taken of various colored fields. The average red, green, and blue pixel responses for each test field can be compared to an ideal responses and correction factors determined. For example, if a measured average value for the red component is AR and the expected average pixel value is ARE then the factor ARE/AR may be used to correct color fidelity—by multiplying each pixel in a measured image by this factor. Similar factors may be derived for the green and blue components. Different test color fields may yield different calibration factors for the three primary colors. A weighted average of calibration factors over the test conditions can be determined and stored for later image correction. The compensation factors can be stored in capsule the camera system 01, in the data recorder, or in the workstation software.
Within image sensor 16, each pixel value is also affected by effects not relating to light input. Such effects may be seen in an output value of a “dark” pixel. A dark pixel may be a pixel not within the field of view of the optical system 14 and which is typically covered by an opaque material. An output value of such a pixel is not attributed to light input, but effects such as thermally-induced charge leakage. An operational pixel may be compensated by adding as an offset the thermally-induced leakage current measured at a dark pixel (or the average thermally-induced leakage current of a group of dark pixels). The thermally-induced leakage current is a function of both temperature and time. The temperature of a capsule camera in operation is expected to be relatively fixed at the human body temperature, plus a few degrees determined by both the power consumption of the capsule camera and the thermal conductance. As a first-order estimate, the thermally-induced leakage current is proportional to the time between a pre-charge of the pixel and the readout.
The pixel-to-pixel variations, including the effects of lens system 14, may be compensated according to the following equations (1) for the R, G and B color components, respectively:

R=FR×PR+ClR×tl+CR 

G=FG×PG+ClG×tl+CG 

B=FB×PB+ClB×tl+CB  (1)

where, PR, PG, and PB, are the measured component values for the three color components read from a pixel in image sensor 16, FR, FG, and FB are multiplication factors provided for correcting pixel-to-pixel variations and for adjusting the average pixel value, and CR, CG and CB are offsets—which may be positive or negative—for each pixel. Parameters Clr, Clg and Clb are provided to compensate for the thermally-induced leakage current. The contribution of the leakage current to each color component is proportional to time t1, which is the time duration between cell pre-charge and cell readout. In order to solve for FR, ClR, and CR, three test images must be taken that include two different illumination intensities and two different values of t1.

The data for each color component is read out. As a first-order estimate (e.g., without compensating for thermally-induced leakage current, discussed below), two parameters Fi and Ci may be stored or transmitted for each color component. Raw color image data may also be stored or transmitted to reduce the space requirement or transmission power. These raw color image data may correspond to a small number of images of red, blue, and green objects illuminated at different intensities. Alternatively, images of a number of objects each with a uniform known color may be used to derive these coefficients. For example, lighting conditions in which the same RGB ratio is used, but different intensities are provided may be used to solve for the Cr, Cg and Cb parameters.
Parameters Clr, Clg and Clb are provided to compensate for the thermally-induced leakage current. The contribution of the leakage current to each color component is proportional to time t1, which is the time duration between cell pre-charge and cell readout. This leakage current may be calculated either in the capsule camera or outside the capsule camera. For calculation outside the capsule camera, the timing parameters for each image are stored in the flash memory or transmitted by wireless. The memory space needed to store these timing parameters is miniscule relative to the size of the image. In the above discussion, although an RGB space is used in this example for illustrative purpose, other color spaces may also be used.
As discussed above, to make compensation available during operations, the coefficients may be calculated for each pixel at the time of manufacture and stored. Alternatively, the images data taken under known uniform illumination conditions may be stored for subsequent calculations of the coefficients. As the number of images taken during operations in a capsule camera is in the order of tens of thousands, the data storage requirement for a few or even a few tens of test images for characterization is minimal, and may be stored in the on-board archival memory (e.g., memory 20). These test data may also be transmitted by wireless to an external recorder, which may be used later in a physician's examination of the images. Each capsule camera may be provided an identification number (ID). Based on the ID, the test data may be provided on and downloaded from a website at the time the physician examines the captured image data.
The computational requirement (e.g., in terms of circuit area and power) for extracting the coefficients based on the simple relationships discussed above is relatively low and, either the test image data (e.g. values for each color component) or the coefficients, which may be calculated by on-board circuits, may be easily stored or transmitted. To illustrate the power and circuit requirements, consider an order-of-magnitude estimate for a JPEG circuit required to process thirty 300 k resolution images. (There are many selections of JPEG circuits available from various popular foundries' libraries, each consisting of no more than about 50 k gates.) For a 0.09 μm process, the average intrinsic gate capacitance is estimated to be:

4CiWL≅(5 ff/μm2)(0.5 μm)(0.09 μm+0.2 μm)≅3 ff

(The factor of 4 assumes that an equivalent gate has 4 transistors.) The 0.2 μm term represents fringing capacitance. For a one-clock-per-code-word JPEG standard widely adopted today, about 30 MHz is needed to run a thirty frames per second real time speed. Therefore, assuming a wiring capacitance of twice the intrinsic capacitance and an activation rate of 1⁄3 of all circuits, the power required is estimated to be:

0.5 CV2*30 MHz 0.5*(2+1)*3 ff*50 k*(1/3)*(1V)2*30 MHz=2.25 mW

The energy for processing 30 frames is 2.25 mW×1 sec=2.25 mJ, which is a miniscule value within the capability of the batteries of the capsule camera. By comparison, the equations for calculating the compensation coefficients above are much simpler than the computational requirements for JPEG processing, which includes space-frequency domain discrete cosine transform (DCT) operations and entropy encoding, among other steps specified in ISO 10918-2 JPEG coding standard. Further, the number of test images required for characterization is normally substantially less than 30 images. Therefore, extracting the coefficients in the capsule camera itself may also be easily achieved in practice.

Thus, the cost of correcting the captured images using the methods discussed herein, and other methods are not cumbersome operations. In addition, both the compression ratio and the quality of the compressed image are more favorable in a corrected image than an uncorrected image because the pixel-to-pixel variations in the uncorrected image are presented as noise in the image data. Without an elimination procedure, such noise is blended in with the useful information and, consequently, increases the number of the bits in a compressed image. The short range differences (e.g., between adjacent pixels) increase the frequency-domain data, thus impacting both the compressed file size and the image quality. During compression, the processing of the noise in the image data further increases power consumption. For example, in the case of MPEG processing, a previous reference frame has to be decompressed in order to code differences between the current frame and a reference frame. The long range differences (i.e., between pixels a substantial away from each other within the sensor array) affect motion estimation in MPEG-like compression. Motion estimation in such a compression algorithm is affected by both short range differences and long range differences, because the compression is carried out by space- and frequency-domain transformations after motion estimation.
Light from the illumination sources are, to some extent, reflected by the inner and outer surfaces of capsule housing 10 and other objects within capsule housing 10 (e.g., particulate contamination). Such reflections may be both specular and scattered. Some reflected light may pass into the camera's input pupil within the camera's field of view to collect on the image sensor. Light may reflect multiple times from multiple objects before reaching image sensor 16.
The spurious signals resulting from reflections of the LED light are independent of the scene photographed. To characterize the reflected illuminating light picked up by image sensor 16 from the surface of the transparent window in capsule housing 10, the capsule camera may be placed completely in a light-absorbing environment (i.e., no light reflects from the environment). Under this arrangement, the light sensed at image sensor 16 consists only of the light emitted from the LEDs that is reflected from capsule housing 10 or from other objects within the capsule. For simplicity, the following discussion assumes LED light sources but other light sources could be used instead, such as those based on incandescence, fluorescence, field-ionization, phosphorescence, or any other light emission process.
The measured red, green, and blue pixel signals may be expressed as,




  
    
      
        
          
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where I1, . . . , In are the currents in LED1 to LEDn, respectively, and times tLED1, . . . , tLEDn are the durations of the corresponding LED currents. The subtractive terms such as ClR and CR are offsets determined using the previously described procedures. Since the LED emitted optical power is roughly linear with current, the reflected light sensed at the image sensor is expected to depend linearly on the currents flowing in individual LEDs integrated over time. For simplicity, equations (2) assume the current Ii for LEDi is constant but the current terms in equations (2) may also each be expressed as an integral of the instantaneous current over time, rather than multiplication of current and time.

Measurements are made with n different combinations of LED currents to allow solving the n independent linear equations for each color. Ideally, each LED is turned on by itself in turn. The n equations for red may be solved using n images, and the values of leakage current factor, ClR, and offset CR obtained from the image tests of equations (1). Alternatively, two additional images may be taken and, using equations (2), the leakage current and offset factors may be obtained. In that case, the number of total images needed is n+2.)
The reflections from the outer surface of capsule housing 10 depend on the medium in which the capsule camera is immersed. Often, in the GI tract, the capsule is immersed in a watery liquid. In some cases, the capsule will be only partially covered in liquid. One set of calibration data of LED reflections may be taken with the capsule in a black container filled with water. Another set of calibration data may be taken without water. When the GI image data are corrected, the calibration data set which provides optimal correction can be selected. A different set of calibration data may be selected to correct a different image region. The suitable correction for an image region may be indicated by various criteria such as:

    
    
        1. Minimize high frequency components of spatial frequency spectrum.
        2. Minimize sharp edges in the image, using image processing algorithms for detecting edges.
        3. Maximize change between successive images. Usually, the medium touching the capsule within an image region is the same from one frame to the next, so that the suitable correction in each pixel for LED reflections off the capsule housing 10's outer surface is invariant from one frame to the next. If the correction is correctly identified (i.e., using the proper calibration data set), the change in image is maximized.
        4. Yield negative corrected pixel signals. A negative value indicates that the wrong calibration data set is applied.
    
    


The uniqueness of each LED and the non-uniform light that is cast on objects by the LEDs—due to both their respective positions and the finite distances between the objects and the light sources—may also be compensated. Even without knowing the distances inside the small intestines and inside the large intestine with balloon inflated, a correction may still be made since the light sources' locations and strengths are known and an average human being's intestines have certain characteristic range in shape and size.
Tests or characterizations designed to extract the compensation parameters may be carried out during the manufacturing process (e.g. during the assembly process), storing the resulting images and the associated information or derived coefficients inside the capsule camera or externally.
Alternatively, the characterization or test may be performed by a hospital technician or by a physician, who runs an automatic test program with the capsule camera placed into a container providing the controlled characterization environment. The test images with associated information or extracted coefficients may be stored in an on-board memory or, if the capsule camera has a wireless transmitter and an antenna, the coefficients database or the images with associated information may be transmitted outside. These data may be retrieved at a later time at a workstation for display and archiving. The characterization data may be archived with a patient's health record or with the raw (i.e., uncorrected) image data. Alternatively, the corrected image and the characterization data are stored, with a description of how the compensation procedures are carried out. The physicians may have the freedom to undo the compensation using the characterization data.
FIG. 4 shows a method for using a capsule camera using the characterization procedures described above, in accordance with one embodiment of the present invention. As shown in FIG. 4, a capsule camera including at least a lens system, an image sensor, an illumination system with two LEDs inside the capsule housing, an archival memory and an output port is prepared (step 401). Next, at step 402, the capsule camera is placed in a light absorbing environment without external light sources. Two images are then taken using illuminations by the illumination system at different relative illuminations1. The images are then stored in the archival memory. At step 403, the capsule camera is placed in an environment of uniform lighting condition using an external lighting source. Three images of different uniform lighting conditions and pixel precharge to readout times are captured. The images are then stored in the archival memory. At step 404, the capsule camera is swallowed by a patient to take images of the patient's GI tract, using the capsule camera's illumination system as light source. These images are captured into the archival memory system. At step 405, after the capsule camera is expelled from the patient's body, the capsule camera is recovered, and the captured image and the images taken at steps 402 and 403 (if stored) are retrieved through the output port of the capsule camera. The images taken at steps 402 and 403 are used to extract the parameter values in accordance with equations (2) and (1), respectively, for characterization or calibration of the capsule camera system. The captured images are then compensated using the extracted parameter values. The term “relative illumination” refers to the product of exposure time and light intensity.
Alternatively, the test or characterization data may be transmitted through an output port (e.g., output port 28) or transmitted by wireless means to outside the capsule camera for external storage. FIG. 5 illustrates an alternative embodiment of the present invention described in conjunction with FIG. 4. In the method illustrated in FIG. 5, instead of an archival memory, the capsule camera is provided with a non-volatile memory buffer and a transmitter. The images taken in the capsule camera, including the images taken at steps 502 and 503, are first stored in non-volatile memory buffer and transmitted to an external receiver, where the images are stored for use in the subsequent steps. In one embodiment, the stored correcting image data or the coefficients are transmitted to the receiver wirelessly just before the capsule takes measurements of the patient.
The capsule camera may be marked with an ID and the externally stored test data or the parameters extracted from the test data, may be made available on websites, by e-mail, in a flash memory, on a VCD or another means. In one embodiment, after the capsule camera is recovered from traversing the GI tract, the measurements data may be downloaded to a workstation, where the test data, or the parameters extracted from the test data, may be retrieved to compensate the captured image data. Alternatively, an ID or ID information may be embedded in the test or characterization image or coefficient data. A technician at a workstation may use the embedded ID to retrieve the test or characterization data or coefficients over the internet, or from another storage media, to perform compensation.
FIG. 6 illustrates an alternative embodiment of the present invention described in conjunction with FIG. 4. In the method illustrated in FIG. 6, instead of an archival memory, the capsule camera is provided with device ID and a transmitter and antenna. The images taken in the capsule camera, including the images taken at steps 602 and 603, are transmitted to an external receiver together with the device ID. The images are stored at an external depository, which may be made available for retrieval over the internet. These images retrieved from the depository are used in the subsequent steps.
FIG. 7 illustrates the alignment between an image sensor array (e.g., sensor array 16) and the footprint of an image produced by an optical system, such as lens 14 of capsule camera 01 of FIG. 1. Ideally, as shown in FIG. 7, if all the components are defect-free and are perfectly aligned, the optical center of a captured image coincides perfectly with the center of the sensor array. In practice, however, the center of the captured image and the center of the sensor array are not perfectly aligned. In addition, imperfections typically exist in the components of the optical system. Normally, the captured image is designed to be slightly smaller than the size of the sensor array to ensure that the entire image is captured within the sensor array. Using the test images for characterization discussed above, a misalignment of the captured image and the center of the sensory array may be derived. For example, under uniform lighting, the average value of a color component in each column of the sensor array indicates where the edges of the captured image are (e.g., scanning from left to right, the column on the right at which the intensity transitions from dark to light, and the column on the left where the image transitions from light to dark). The same procedure along the rows indicates where the edges are at the top and at the bottom of the image.
As another example, when a test image consists of a repetitive pattern, a DCT—such as that used in JPEG or MPEG processing—may be used to calculate the frequency-domain image information. The image footprint is located at where the DCT values increase abruptly, corresponding to the positions at which the pattern becomes clear and discernable. An offset between the optical center and the center of the sensor array (e.g., the displacement expressed in x- and y-coordinates) may be stored for later reference in registers or in the non-volatile memory (e.g., archival memory 20). By cropping the operational boundaries of the sensor array to the actual image footprint, or a “meaningful” portion of the image footprint (i.e., the area where the actual field of view is expected), power is not wasted on the areas in the sensor array outside of the actual useful field of view. In addition, the space required for storing the image, or the bandwidth required for transmitting the image, is also reduced. The workstation utilization rate is enhanced and archiving management is also made easier.
In another example, the edge information may be used to provide the alignment information. Edge enhancement is used in many of today's commercial cameras or image post processing software. One way to extract an edge is to find the difference in luminance between pixels in adjacent columns. The difference provides edge information in one direction. The same procedure maybe used on corresponding pixels in adjacent rows to get edge information in the direction to the columns. The edges obtained for the column and row directions may be used together.
Illustrated also in FIG. 7 is the fact that, due to individual variations in the lens, the magnification factor for each lens is not the same across capsule cameras, thereby resulting in different footprints from camera to camera. An alternative representation of the image footprint may be, for example, two vertices of the rectangular image footprint.
As discussed above, image compensation may be carried out within the capsule camera (e.g., image processor 18, or by controlling the sensor analog gain and providing an offset to each color components of every pixel.) The storage space required for compensation or characterization parameter values may be provided in a separate non-volatile flash memory, in a memory provided in an application-specific integrated circuit (ASIC) or in the image sensor. Alternatively, the compensation or characterization parameter values may also be stored in the same flash non-volatile storage for captured image data (e.g., archival memory 20).
FIG. 8A shows a Lambertian curve of a radiation pattern of an LED (a polar plot showing illuminance as a function angle relative to an axis of the LED) and FIG. 8B the luminance as a function of the angle (linear scale). These figures show the imperfections of an actual LED design.
FIG. 9 shows a grid-distortion plot of a lens. As shown in FIG. 9, the distortion in the middle is minimal (i.e., the actual grid points match well with the reference grid) but the distortion becomes pronounced away from the center.
There is a fundamental difference between the design issues posed by FIGS. 8A and 8B and FIG. 9, as large manufacturing variations exist among the LED products. Alternatively, in lens manufacturing, as measured by grid distortion, variations are small and deterministic. The problem shown in FIG. 9 may be compensated in the most part, even after data acquisition by just inverse mapping of the acquired image on the work station or another computer before displaying. The imperfections in FIGS. 8A and 8B are caused by both design imperfections and manufacturing variations. Such imperfection or variations may be efficiently handled using the methods in FIGS. 2-6 above.
In one embodiment, the grid distortion could be compensated using the methods in FIGS. 2-6 by imaging a known pattern, for example a grid, and detecting an edge (as previously described) to calculate the degree of distortion of the image on the sensor surface caused by optical imperfections (design imperfection as well as manufacturing variations) before displaying. Other problems (e.g., lateral color) may be solved similarly. If deterministic, the image may be compensated on a work station before the image is displayed, and if the defects are caused by both by design imperfection and manufacturing variations, only the methods in FIGS. 2-6 may be used realistically to compensate to get a higher fidelity of image and higher detection rate in case of medical device application.
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 accompanying claims.

## Claims

1. A method for characterizing a camera having an image sensor comprising a plurality of pixels, the method comprising:
illuminating a field of view of the camera under a controlled condition;
exposing one or more test images of the illuminated field of view onto the image sensor of the camera under the controlled condition;
extracting from the one or more test images parameter values for the pixels of the image sensor, taking into account pixel-to-pixel variations in the image sensor;
storing the extracted parameter values in an archival memory of the camera;
retrieving the parameter values from the archival memory; and
compensating an image captured in the image sensor at a subsequent time using the retrieved parameter values, taking into account a difference between an operating condition at the subsequent time and the controlled condition.

2. A method as in claim 1, further comprises providing objects in the field of view of a predetermined color, contrast or pattern.

3. A method as in claim 1, further comprises preparing the controlled condition by providing an external light source for illuminating the field of view.

4. A method as in claim 1, wherein the image sensor is sensitive to a plurality of color components.

5. A method as in claim 1, wherein the field of view is illuminated at a predetermined light intensity.

6. A method as in claim 5, wherein objects providing predetermined patterns in the multiple test images are provided in the field of view.

7. A method as in claim 1, wherein the camera further comprises a transparent window through which the exposures are taken and a light source behind the transparent window, the transparent window being covered with liquid.

8. A method as in claim 1, wherein the archival memory stores the images taken.

9. A method as in claim 1, wherein the archival memory comprises flash memory.

10. A method as in claim 1, wherein the parameter values include a factor for compensating pixel-to-pixel variations.

11. A method as in claim 1, wherein the parameter values compensate pixel value across a plurality of neighboring pixels.

12. A method as in claim 1, wherein the parameter values are based on expected pixel values under the control condition.

13. A method as in claim 1, wherein the parameter values include a factor for compensating a leakage current in the corresponding pixel of the image sensor.

14. A method as in claim 13, wherein the leakage current is integrated over a time of exposure.

15. A method as in claim 1, wherein the camera further comprises a transmitter for sending images taken, parameter values or other data to an external depository.

16. A method as in claim 15, wherein the camera is associated with an identifier associated with the image sensor, and wherein the images, parameter values or other data are sent together with the identifier to identify the images sent.

17. A method as in claim 1, further comprises preparing the controlled condition by providing an illumination system within the camera.

18. A method as in claim 17, wherein the preparing the controlled condition further comprises providing the camera in a light absorption environment.

19. A method as in claim 17, wherein the parameter values include a correction for light reflected from a transparent window of the camera itself.

20. A method as in claim 19, wherein the correction is a function of a current in a light emitting diode.

21. A method as in claim 19, wherein the correction is further a function of a time of exposure.

22. A method as in claim 1, wherein the operating condition relates to an elapsed time between pre-charge and readout in the pixel.

23. A method as in claim 1, wherein the operating condition relates to temperature.

24. The method of claim 1, wherein the camera further comprises one or more optical elements, and wherein the parameter values take into account pixel-to-pixel variations due to the optical elements.

25. A camera, comprising:
an image sensor having a plurality of pixels; and
means for preserving in an archival memory a record of characterization data created for the pixels of the image sensor, taking into account pixel-to-pixel variations in the image sensor, the characterization data (i) being derived from one or more test images projected onto the image sensor under a controlled condition, and (ii) being retrieved for compensating an image captured in the image sensor at a subsequent time, wherein the image compensation takes into account a difference between an operating condition at the subsequent time and the controlled condition.

26. A camera as in claim 25, wherein the means for preserving a record comprises a non-volatile storage device.

27. A camera as in claim 25, wherein the means for preserving a record comprises a transmitter for transmitting the record to an external recording device.

28. A camera as in claim 27, wherein the image sensor is associated with an identifier and wherein the record is subsequently retrieved from the external recording device using the identifier.

29. A camera as in claim 25, wherein the characterization data include a factor for compensating the pixel-to-pixel variations.

30. A camera as in claim 29, further comprising compressing an image, after compensating the pixel-to-pixel variations in the image.

31. A camera as in claim 25, wherein the characterization data compensate a pixel value across a plurality of neighboring pixels.

32. A camera as in claim 25, wherein the characterization data are based on expected pixel values obtained under the control condition.

33. A camera as in claim 25, wherein the characterization data compensate a leakage current in a pixel of the image sensor.

34. A camera as in claim 32, wherein the leakage current is integrated over a time of exposure.

35. A camera as in claim 25, wherein the characterization data is obtained by taking exposure based on an external light source of predetermined characteristics.

36. A camera as in claim 25, further comprising an illumination system behind a transparent window through which the test images are taken.

37. A camera as in claim 36, wherein the characterization data comprises image data taken in a light absorption environment.

38. A camera as in claim 37, wherein the characterization data correct for light reflected from the transparent window.

39. A camera as in claim 38, wherein the correction is a function of a current in a light emitting diode.

40. A camera as in claim 38, wherein the correction is further a function of a time of exposure.

41. A camera as in claim 25, wherein the camera is housed in a swallowable capsule.

42. A camera as in claim 41, wherein some of the multiple images are taken in a liquid environment.

43. A camera as in claim 25, wherein the operating condition relates to an elapsed time between pre-charge and readout in the pixel.

44. A camera as in claim 25, wherein the operating condition relates to temperature.

45. The camera of claim 25, wherein the camera further comprises an optical element, and wherein the characterization data take into account pixel-to-pixel variations due to the optical elements.

46. A method for characterizing a camera having a housing that includes a transparent window and an image sensor that includes a plurality of pixels, comprising:
placing the camera inside a light absorbing environment, and taking a first set of one or more test images under illumination by a first light source behind the transparent window within the housing, the light absorbing environment created to provide a controlled condition such that the test images primarily capture light reflected from the transparent window and objects within the housing;
processing the one or more test images to extract parameter values for the pixels of the image sensor, the parameter values being related to variations in the pixels of the image sensor; and
storing the extracted parameter values in an archival memory.

47. A method as in claim 46, wherein the light source is based on one of: a light emitting diode source, an incandescence source, a fluorescence source, a field-ionization source, and a phosphorescence source.

48. A method as in claim 47, wherein the test images are taken using different relative illuminations.

49. A method as in claim 46, wherein the test images are taken using different exposure times.

50. A method as in claim 46, wherein the camera further comprises a second light source, each light source emitting light at an intensity that depends on its current, the method further comprises taking the first set of test images at a first combination of time integrated source currents and taking a second set of one or more test images using the second light source at a second combination of time-integrated source currents.

51. A method as in claim 50, wherein some of the test images are taken in a liquid environment.

52. A method as in claim 50, further comprising compensating images subsequently captured in the image sensor based on the parameter values.

53. A method as in claim 52, wherein the parameter values are derived from the test images and the additional test images as sensed in the image sensor.

54. A method as in claim 53, wherein the parameter values include a factor for compensating the pixel-to-pixel variations.

55. A method as in claim 53, wherein the parameter values compensate a pixel value across a plurality of neighboring pixels.

56. A method as in claim 50, wherein the first and second sets of test images are stored in an archival memory in the camera.

57. A method as in claim 56, wherein the archival memory comprises a non-volatile memory.

58. A method as in claim 50, wherein the first and second sets of test images are transmitted to an external receiver for storage.

59. A method as in claim 50, wherein the parameter values are transmitted to an external receiver for storage.

60. A method as in claim 50, wherein the image sensor is associated with an identification, and wherein the first and second sets of test images are transmitted with the identification to allow subsequent retrieval for use in the compensating.

61. A method as in claim 50, wherein the image sensor is associated with an identification, and wherein the extracted parameter values are transmitted with the identification to allow subsequent retrieval.

62. The method of claim 46, wherein the parameter values are adjusted for each pixel of the image sensor based on an elapsed time between pre-charge and readout of the pixel.