Abstract:
A system for identifying test tube types and properties in a sample handling machine using visual information automatically obtained by an optical imager and then processed using vision processing methods. The system includes an optical imager positioned to capture images containing one or more test tubes in a rack and a microcontroller programmed to extract predetermined regions of interest and interpret the optical information in the image to decipher the dimension of the test tubes, determine the presence or absence of caps on the test tubes, decode any encoded data, and interpret custom symbologies. The system may then determine the nature of the test tubes or other containers presented before the image and provide that information to the sample handling machine to assist with processing of samples.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to machine vision systems and, more specifically, to a system and method for determining the nature of a test tube and associated cap using optical imaging techniques. 
         [0003]    2. Background of the Art 
         [0004]    Machine vision plays an important role in automated and robotic systems, such as assembly line manufacturing, quality control inspection, and sample processing. Conventional systems are generally comprised of an optical imager, such as a charged coupled device (CCD) or similar device using digital imaging technology, that is positioned to capture images of objects that pass in front of it. In low-light or enclosed applications, machine vision systems may include an illumination source, such as a bank of light emitting diodes (LEDs), positioned proximately to the imager. The images are subsequently processed to decode information contained in the resulting two-dimensional image, such as 1D linear codes, 2D stacked/matrix codes, OCR fonts, and postal codes. The image captured by the machine vision system may also be subjected to more advanced processing, such as shape recognition or detection algorithms, that provide information about the object of interest in the image. However, the characteristics of digital images taken by machine vision systems, such as the contrast of the image, often limit the processing techniques that may be employed and adversely affects the accuracy of the results obtained from the processing of the image contents. 
         [0005]    In sample handling systems, such as blood analyzers and the like, samples are moved to and from diagnostic modules for automatic testing and retesting using a loading rack that holds a plurality of carriers, such as test tubes filled with samples. Proper identification of the samples, decoding of information encoded into labels on the test tube, recognition of the test tube type, and even determining whether the tube contains a cap may be critical for timely and accurate processing of samples. 
       SUMMARY OF THE INVENTION 
       [0006]    It is a principal object and advantage of the present invention to provide a system and method for identifying the contents of image captured by a machine vision system. 
         [0007]    It is an additional object and advantage of the present invention to provide a system and method for identifying the type of test tube in a rack handling system. 
         [0008]    It is a further object and advantage of the present invention to provide a system and method for identifying whether a test tube in a rack handling system is associated with a cap. 
         [0009]    Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter. 
         [0010]    In accordance with the foregoing objects and advantages, the present invention comprises a system for identifying a test tube or other object that is presented to an optical imager as the test tube or object moves along an assembly line or process. In a preferred embodiment, the optical imaging platform is programmed to perform decoding of information contained within the image, such as any barcodes or recognizable symbology, as well as for more advanced image processing, such as pattern matching and shape detection, that allows for an accurate and efficient identification of the nature of the test tube or object, as well as any data or information encoded on barcodes or other indicia that are placed on the test tube. More particularly, the present invention comprises an optical imager for capturing images of at least one test tube positioned in a sample handling rack and a microcontroller associated with the optical imager for interpreting information contained in images captured by the imager. The microcontroller is preferably programmed to extract barcode information from captured images, extract information encoded into predetermined geometric symbologies in the images, and interpret visual information in regions of interest to determine whether a test tube is present and to identify the geometry of the test tube. In addition, the microcontroller is programmed to interpret visual information to determine whether a cap is present on the test tube, and then determine what type of test tube has been captured in the image. The information may then be provided to the main line processing of the sample handling machine to assist with identification and processing of samples. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic of a system for determining the nature of a test tube and associated cap in a sample handling device according to the present invention. 
           [0013]      FIG. 2  is a schematic of an imager according to the present invention. 
           [0014]      FIG. 3  is a schematic of a test tube and rack of a sample handling device according to the present invention. 
           [0015]      FIG. 4  is a high level flowchart of a machine vision process according to the present invention. 
           [0016]      FIG. 5  is a flowchart of a calibration process according to the present invention 
           [0017]      FIG. 6  is an image of a test tube according to the present invention. 
           [0018]      FIG. 7  is a processed image of a test tube according to the present invention. 
           [0019]      FIG. 8  is a further processed image of a test tube according to the present invention. 
           [0020]      FIG. 9  is another processed image of a test tube according to the present invention. 
           [0021]      FIG. 10  is an image of a test tube region of interest according to the present invention. 
           [0022]      FIG. 11  is a processed image of a test tube region of interest according to the present invention. 
           [0023]      FIG. 12  is a further processed image of a test tube region of interest according to the present invention. 
           [0024]      FIG. 13  is an image of a test tube cap region according to the present invention. 
           [0025]      FIG. 14  is a processed image of a test tube cap region of interest according to the present invention. 
           [0026]      FIG. 15  is an image of an indicia region of interest according to the present invention. 
           [0027]      FIG. 16  is a processed image of an indicia region of interest according to the present invention. 
           [0028]      FIG. 17  is a further processed image of an indicia region of interest according to the present invention. 
           [0029]      FIG. 18  is a yet further processed image of an indicia region of interest according to the present invention. 
           [0030]      FIG. 19  is another processed image of an indicia region of interest according to the present invention. 
           [0031]      FIG. 20  is a vertical histogram of the image of  FIG. 19 . 
           [0032]      FIG. 21  is an image of a test tube region of interest according to the present invention. 
           [0033]      FIG. 22  is a histogram of a test tube region of interest in the absence of a cap. 
           [0034]      FIG. 23  is a histogram of a test tube region of interest in the presence of a cap. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Referring now to the drawings, wherein like numerals refer to like parts throughout, there is seen in  FIG. 1 , a machine vision system  10  according to the present invention. System  10  comprises an optical imager  12  positioned on one side of a target, such as such as one or more test tubes  14 . Preferably, a retro-reflective background  16  is positioned behind test tube  14  in alignment with imager  12  to reflect light onto the rear of tube  14 . 
         [0036]    Imager  12  preferably comprises a complementary metal oxide semiconductor (CMOS) image sensor and is capable of reading and interpreting two-dimensional images, such as 1D linear codes, 2D stacked/matrix codes, OCR fonts, RSS (Reduced Space Symbology) codes, and postal codes, as well as provides image capturing for use in a wide range of applications, such as image and shape recognition, signature capture, image capture, and optical character recognition (OCR). As seen in  FIG. 2 , imager  12  may include an on-board illumination source  18  comprising one or more light emitting diodes (LEDs) of various wavelengths, to provide illumination of tube  14 . For example, imager  12  may include red LEDs for general illumination and green LEDs for targeting. Illumination source  18  may be separately attached to imager  12  and positioned proximately thereto. 
         [0037]    Imager  12  also includes a microcontroller  30  for managing imaging and illumination operations, performing processing of captured images, and communicating with a host  32 , such as a host computer or a rack handling system, through a host interface  34 . Host  32  preferably controls imaging of objects  14  based on host commands received from to host interface  34 . Similarly, microcontroller  30  is capable of providing data to host device  32  via interface  34 . 
         [0038]    Host interface  34  may comprise a conventional RS232 transceiver and associated  12  pin FFC jack. Alternatively, interface  34  may comprise other conventional buses, such as USB, IEEE, 1394, IrDA, PCMCIA, or Ethernet (TCP/IP). Interface  34  may also comprise a wireless transceiver for wireless communication to a host computer and is programmed with the applicable protocols for interfacing with a host computer, such as Bluetooth® or 802.11 protocols. Microcontroller  30  is electrically connected to an imaging engine  36  for driving the optical imaging of a target object and receiving image data. Microcontroller  30  is also connected to an illumination engine  38  used for controlling timing and illumination source  18 . Optionally, imaging engine  36  and illumination engine  38  may be provided in a single unit interconnected to microcontroller  30 . 
         [0039]    Imager  12  may comprise an IT4X10/80 SR/SF or IT5X10/80 series imager available from Hand Held Products, Inc. of Skaneateles Falls, N.Y. that is capable of scanning and decoding most standard barcodes including linear, stacked linear, matrix, OCR, and postal codes. The IT5X10/80 series imager is a CMOS-based decoded output engines that can read 2D codes, and has image capture capabilities. Imager  12  obtains an optical image of the field of view and, using preprogrammed algorithms, deciphers the context of the image to determine the presence of any decodable barcodes, linear codes, matrix codes, and the like. As will be explained hereinafter, imager  12  may further be programmed to perform other image processing algorithms, such as shape recognition, culling, match filtering, statistical analysis, and other high-level processing techniques, in addition to barcode detection. Microcontroller  30  may comprise a MC9328MXL VH15 microprocessor, available from Freescale Semiconductor, Inc. of Chandler, Ariz. that is programmed prior to implementation in imager  12 , or programmed anytime thereafter, such as by using interface  34  to upgrade the firmware used to control microcontroller  30 . 
         [0040]    Reflective background  16  comprises a thin film or sheet having reflective properties that is aligned to reflect all or a portion of light emitting from illumination source  18  back to imager  12 . Reflective background  16  preferably includes retroreflective characteristics. Positioning of reflective material  16  saturates the background, thus improving the contrast of the image taken by imager  12 , allowing for the use of image processing techniques without the need for additional illumination sources or sophisticated illumination control circuitry. Preferably, reflective background  16  comprises seven millimeter retro-reflective sheeting. Sheeting generally comprises a layer of glossy mylar bonded to a liner by an adhesive, such as a layer of permanent acrylic. The layer of mylar and the layer of adhesive are preferably one millimeter thick each and the liner may comprise 90# polyethylene coated paper, resulting in a reflective sheeting of approximately seven millimeters in thickness. Acceptable reflective sheeting comprises the Series 680 Reflective Sheeting available from 3M of St. Paul, Minn. 
         [0041]    Referring to  FIG. 3 , test tubes  14  may comprise any variety of shapes and sizes positioned in a rack  20 . In some cases, tubes  14  may be positioned on top of an insert  22  positioned in rack  20 , which are typically used to elevate tubes  14  for sample handling and processing. Test tubes  14  may include a cap  24  or similar device to contain fluids within tube  14 . Tube  14  may further include a barcode  26  or comparable symbology placed on an intermediate portion of test tube  14 . Insert  22  may include a label containing an indicia  28  to help with identification of tube  14 , as will be explained in further detail herein. Indicia  28  preferably comprises a custom or predetermined symbology printed onto a retro-reflective label that is then adhered to insert  22  or even tube  14 . 
         [0042]    As seen in  FIG. 4 , microcontroller  30  may be programmed to execute an advanced machine vision process  40 . Machine vision process  40  is preferably into two parallel sub-systems; a barcode decoding sub-process  42  and a tube identification sub-process  44 . This separation of execution of barcode decoding sub-process  42  and tube identification sub-process  44  allows for flexibility. For example, barcode decoding sub-process  42  and tube identification sub-process  44  could be performed on two different processors to maximize processing speed. 
         [0043]    Prior to performing machine vision process  40 , microcontroller  30  must be configured using a calibration process  50  to ensure proper alignment of system  10  and to provide location information need for subsequent vision processing operations. Calibration process  50  is also implemented to provide the foundation for test tube identification according to the present invention and is necessary for identification of tube  14  and barcode  26  decoding, as well as for ensuring that rack  20  is properly aligned with respect to imager  12 . For example, proper tube identification and barcode decoding rely on positioning of tube  14  near the center of a retro-reflective region created in a captured image as a result of the use of retro-reflective background  16 , and ensuring that the retro-reflective region is rectangular, thereby verifying that the angle of imager  12  relative to tube  14  is correct. Calibration process  50  is thus responsible for analyzing a sample image and deducing the rectangle of the retro-reflective region and the offset of the center of the tube insert from the center the retro-reflective region. 
         [0044]    Before commencing with calibration process  50 , rack  20  must be positioned in front of imager  12  so that it can capture an image including the center of tube  14 , as tube  14  preferably includes indicia  28  located in a central position so that calibration process  50  can easily identify the center of tube  14  by locating the center of indicia  28 . 
         [0045]    Referring to  FIG. 5 , the first step in calibration process  50  is to acquire an image  52  that contains rack  20  and test tube  14  therein, including indicia  28  on tube  14 , as illustrated in  FIG. 6 . The next step in process  50  is to dynamically adjust the exposure time  54  so that a retro-reflective region of interest  56  that surrounds the upper portion of tube  14  and a label region of interest  58  that contains indicia  28  are the largest and brightest regions within their respective portions of the captured image. As seen in  FIG. 7 , adaptive thresholding may used to segment the image into retro-reflective region of interest  56  and label region of interest  58 . 
         [0046]    Process  50  continues with the step of identifying the location  60  of retro-reflective region of interest  56  and label region of interest  58  with the image. By searching the thresholded image of  FIG. 7 , the bounding rectangles of retro-reflective region of interest  56  and label region of interest  58  can be deducted using either an image segmentation technique or a lateral histogram technique. 
         [0047]    As unexpected specular reflection or misplaced test tubes  14  may inhibit these techniques from working properly, the predetermined rectangular shape and size of retro-reflective region of interest  56  and label region of interest  58  must be confirmed to determine the location of these regions. Confirmation begins by searching for four vertex points, identified as P 1 , P 2 , P 3  and P 4  in  FIG. 8 , that define rectangles that enclose retro-reflective region of interest  56  and label region of interest  58 . The rectangle formed by the four vertex points is then checked by tracing and checking for any significant white lines connecting P 1  and P 2 , connecting P 1  and P 4 , and connecting P 3  and P 4 . The significant white lines must be of sufficient length and devoid of any significant interruptions (or number of interruptions) to qualify. As seen in  FIG. 8 , the four points in the left side of the thresholded image define retro-reflective region of interest  56 , while the average of the detected four points in the right side of the thresholded image provide the center of label region of interest  58 . 
         [0048]    The final step of calibration process  50  is to confirm proper alignment  62  using the location information provided by the processing explained above. The parameters of retro-reflective region of interest  56  (i.e., location, height and width), and the center of label region of interest  58  provide the requisite information to allow for proper horizontal and vertical alignment of imager  12 . The offset dimension of retro-reflective region of interest  56  relative to label region of interest  58  will enable rack  20  to be moved to the correct horizontal position. In addition, the information may also be used to check if the parameters are in the tolerance range within which system  10  can perform normally. Once system  10  is properly calibrated using calibration process  50 , the final locations of retro-reflective region of interest  56  and label region of interest  58  may be stored in memory in imager  12  for sequent vision processing functions so that retro-reflective region of interest  56  and label region of interest  58  may be extracted from a captured image and further processed. More specifically, the retro-reflective region of interest  56  is used for detecting the type of tube  14 , and label region of interest  58  is used by system  10  for decoding indicia  28 . 
         [0049]    Because of the retro reflective material, the backgrounds of both retro-reflective region of interest  56  and label region of interest  58  are quiet bright, i.e., near to saturation, which creates maximal contrast with the rest of the image. Due to such contrast, simple thresholding may be used to segment out the retro-reflective region of interest  56  and label region of interest  58 . However, aging and drifting of the LEDs, damage to the reflective-material, and exterior lighting condition pose the potential threat to degrade this contrast. To correct for these variations, the exposure time of imager  12  may be adjusted by system  10  to ensure that the right background and contrast are used. Setting the exposure time is very important, because if it is set too high, the front tube object will be washed out. If the exposure is set too low, there will be a lot of noise appearing in retro-reflective region of interest  56 . One solution is to adjust the exposure time dynamically to ensure that the right background and contrast are used. For example, whenever system  10  fails to identify tube  14  after calibration, system  10  may sample a test region  64  within retro-reflective region of interest  56  to see if it is uniform and bright enough, as seen in  FIG. 9 . If retro-reflective region of interest  56  is saturated, the exposure time may be decreased incrementally until tube  14  can be identified. Otherwise, the exposure time may be incremented until tube  14  is identified. The adjusted exposure time may then be saved and used in the tube identification process, as described below, until a failure is encountered. In the event of a failure, the operational exposure time may be dynamically adjusted until determining the proper exposure and saved for future use. 
         [0050]    Once system  10  is calibrated and the operational exposure time is selected, as described above, machine vision process  40  may be executed. Referring to  FIG. 4 , the first step in process  40  is to acquire an image  70 . Using the calibrated information, process  40  continues with the extraction  72  of retro-reflective region of interest  56  and extraction  74  of label region of interest  58 . In addition, a barcode region of interest is extracted  78  from the image. Process  40  then continues with barcode decoding sub-process  42  and/or tube identification sub-process  44 . 
         [0051]    Barcode decoding sub-process  42  involves the application of conventional or commercially available barcode decoding algorithms  80  to barcode region of interest, such as those provided with the IT5X10/80 series imager identified above. Because any barcode  26  is positioned inside a predetermined region located in a predetermined relationship to retro-reflective region of interest  56  and label region of interest  58 , system  10  may identify barcode region of interest using the calibration location information and provide such information to the onboard barcode decoder of imager  12  to expedite the barcode location process. 
         [0052]    Tube identification sub-process  44  generally involves examining retro-reflective region of interest  56  to locate the top part of any tube  14  positioned therein to check whether a tube is present  82 , determine the geometry  84  of the top of tube  14 , and detect  86  any cap  20  positioned on tube  14 . Tube identification sub-process  44  also searches for and applying the appropriate decoding algorithms  88  to decodes indicia  28  located in label region of interest  58 . The extracted tube features, the binary decision of cap presence from retro-reflective region of interest  56 , and decoded indicia  28  are then used as inputs to a rule-based decision system to determine the tube type  90 . The determination of step  90 , the determination that no tube is present at step  82 , and/or the barcode information extracted at step  80  may then be reported  92  to host  32 . 
         [0053]    With respect to step  84 , once the system is calibrated and imaging exposure time is correctly set, the geometrical features of tube  14  can be easily recognized inside retro-reflective region of interest  56 . An example of retro-reflective region of interest  56  is seen in  FIG. 10 . Edge detection is first performed on the retro reflective region of interest  56  to extract the boundary of the tube. An image pixel f (x,y) is declared on the edge if it meets any of the following condition, 
         [0000]        abs (ƒ( x,y )−ƒ( x+ 1 ,y ))&gt; s   e    
         [0000]        abs (ƒ( x,y )−ƒ( x+ 2 ,y ))&gt; s   e    
         [0000]        abs (ƒ( x,y )−ƒ( x,y+ 1))&gt; s   e    
         [0000]        abs (ƒ( x,y )−ƒ( x,y+ 2))&gt; s   e   (1) 
         [0054]    where f (x,y) stands for the image intensity at the x th  row and the y th  column, and 
         [0055]    abs(.) denotes the absolute function. 
         [0056]    The threshold s e  defines what the level of image intensity transition can be regarded as edge. Instead of using the original sub-image of  FIG. 10 , s e  is determined adaptively from the statistics of the processed image that is transformed from the original image using the robust image normalization. The robust image normalization is used to account for the potential illumination changes. The detected edge may be seen in  FIG. 11 , where the edge pixels and background pixels are represented by the black pixels and the white pixels, respectively. The edge detection is quiet simple and efficient since it does not require computing the high order statistics of the image. More sophisticated edge detection algorithms may be used for this purpose, but they generally do not improve accuracy significantly while consuming considerable time. Moreover, the above-described edge detection method can guarantee the detected edges are thick enough, which makes the later shape recognition more reliable. The edges may display some imperfect calibration and rack alignment, leading to some unwanted connected objects linking to the tube boundary that may confuse system  10 . Therefore, system  10  preferably should isolate the connected tube boundary so that the geometrical features may be analyzed more correctly. Most of retro-reflective region of interest  56  is cropped (as shown by the dashed rectangle  94  in  FIG. 11 ), so that on the inside the cropped version of the retro-reflective region  56  the objects on the top border and the bottom border will be disconnected from the tube boundary and can be removed using binary object segmentation techniques. 
         [0057]    As with the calibration process, a simple algorithm may be used to extract the shape features of the tube. The operation of this algorithm is illustrated in  FIG. 12 , where system  10  detects the salient point P 1 , which is on the top-left corner of the tube edge. Starting from P 1 , system  10  then traces to the right until a few pixels from the right border of retro-reflective region of interest  56 , and then checks if there is a connected edge (dashed line). The edge must be long enough and have interruptions less than a predefined number of pixels. In addition, the line connecting the start point P 1  and the end point P 2  must be parallel to the horizontal axis within a few degrees of tolerance. Using the same logic, the bottom salient point P 4  may be detected on the left bottom corner of the tube edge, and a trace and check for a horizontal or near-horizontal edge (dashed line) to P 3  (circle). 
         [0058]    There is seen in  FIGS. 13-14 , an example of the operation of this process on a test tube having a cap, where the points P 2  and P 3  are not linearly positioned with respect to the sides of tube  14  as in  FIG. 12 , but instead are positioned proximately to two concave bays at points P 2  and P 3  that are created by the presence of cap  24 . This visual information may also be used by system  10  to determine the presence of the tube. 
         [0059]    Locating the four salient points and evaluating their relative locations help system  10  determine the presence of tube  14  and the shape characters of tube  14 . For example, the presence of all four salient points is the evidence that system  10  uses to determine if there is a tube  14  in rack  20 . Second, if a tube  14  is found, the system  10  may perform a tube shape check on these four points (for example, if the column difference between the point P 1  and P 2  is found above the predefined threshold, system  10  can declare that either the tube is tilt or the cap is loose). Finally, system  10  uses these four points to deduct the tube geometry. For example, the height of the tube, in pixels, is determined as the average of the columns of P 1  and P 2 . The height can be converted to inches or millimeters based on the projection matrix of imager  12 , as trained in the calibration process. 
         [0060]    Tube identification sub-process  44  also searches for and decodes indicia  28  located in label region of interest  58 . Indicia  28  is preferably a customized label that helps system  10  identify the type of the tube. For example, indicia  28  may comprise a label placed on an insert that is used to position tube  14  higher in rack  20 , thereby bringing tube  14  into retro-reflective region of interest  56 . As described above, calibration process  40  provides the center of label region of interest  58 . Tube identification sub-process  44  can then extract  84  label region of interest  58 , or at least a portion of the image captured at step  70  that is large enough to contain indicia  28 , as seen in  FIG. 15 . Because indicia  28  preferably comprises retro-reflective material, indicia  28  will be a large, white object in comparison to a dark background. As with the calibration process, the step of decoding  84  first uses adaptive thresholding to segment out indicia  28 , as seen in  FIG. 16 . Tube identification sub-process  44  may then apply a four-point method such as that described above to locate and extract label region of interest  58 , as seen in  FIG. 17 . If no white object is found, or the found object is too small, tube identification sub-process  44  may determine that there are no indicia  28  and assign the corresponding flag to the tube identification decision-making process. If a likely indicia  28  is found, label region of interest  58  defined by the four points seen in  FIG. 17  is extracted and the image morphology (binary erosion and dilation) is used to remove any small or noise-like objects to prune label region of interest  58 . The de-noised label region of interest  58  is seen in  FIG. 18 . 
         [0061]    Decoding of indicia  28  may then commence using a lateral histogram technique to locate any predetermined elements of indicia  28 , as illustrated by rectangle  96  in  FIG. 19  and resulting the vertical histogram seen in  FIG. 20 . For example, a custom symbology may include a number of icons comprised of various arrangements of geometric elements, such as horizontal and vertical bars. The identified geometric elements may then be verified against the specifications or standard used to define the symbols comprising acceptable indicia  28 . It should be recognized by those of skill in the art that any conventional symbols may be used, or custom symbols developed specifically for the particular application of system  10 . 
         [0062]    Once tube  14  is detected and the tube height is determined, tube identification sub-process  44  may then perform cap detection  86  by analyzing the statistical characteristics of the image intensity in a cap sample region  98  at the very top region of tube  14 , as seen in  FIG. 21 . As tubes  14  are generally transparent glass and caps are opaque plastic, transparent glass and opaque cap reflect light in different ways, thereby causing the image intensity inside cap sample region  98  for plain tubes  14  to be in one region of the histogram, as seen in  FIG. 22 , while the image intensity inside cap sample region  98  when a cap is present will takes up the other region of the histogram, as seen in  FIG. 23 . Therefore, by polling out the image intensity in cap sample region  98 , system  10  may determine the presence or absence of a cap. 
         [0063]    Aging and drifting of the LED lights and the translucent liquid residue that might adhere to the interior of the top of the tubes complicates the detection process by dragging the intensity of the cap sample region of the bare tube to the middle region of the histogram and may result in a failure to detect. One possible remedy for this problem is to increase the exposure time to such a level that the majority of the image intensity in the top region of the bare tube is saturated. On the contrary, no matter how high the exposure time is, the majority of the image intensity in the top region of the bare tube cannot be saturated. Accordingly, correct cap detection  86  may be performed by thresholding the saturation ratio (i.e., the number of saturated pixel to the total image pixels in the cap sample region). If the exposure time is increased enough to make the reliable cap detection, however, the front tube will be washed out and we cannot extract any shape information. 
         [0064]    To address the conflicting requirements of the tube shape extraction and cap detection, system  10  is preferably configured to acquire a second, high exposure image of rack  20  at step  70 . The first image acquired in step  70  using the operational exposure time is used to detect tube  14 , decode any barcode  26 , and decode indicia  28 , while the second image of step  70  is used for cap detection  86 . Based on the deducted shape information, e.g., the tube height, cap sample region  98  may be located and extracted from the high exposure image, and then processed to detect the presence or absence of cap  24 . 
         [0065]    Generally, items such as test tubes  14  are geometrically so simple that it is not possible to extract enough features from a captured image to distinguish different tube types. While a few primitive geometrical features, e.g., tube height, tube width, etc., may be extracted, these parameters do not provide enough information to distinguish the tube type because of the use of inserts below the test tubes and the presence or absence of tube caps. In order to properly classify the type of tube  14  in an application having multiple types, system  10  should preferably be able to discern the type of insert  22  used to raise tube  14  into position. Directly identifying the type insert  22  may be difficult because inserts  22  are often hidden in rack  20 , and therefore hard to illuminate, and because barcodes  26  posted on tubes  14  and the liquid inside tubes  14  may obscure the transition between tube  14  and insert  22 . Because the number of inserts  22  are generally limited, and the height of each insert  22  is known, a different indicia  28  may be assigned to each insert  22  and placed thereon so that system  10  can easily identify the insert type by decoding indicia  28 . In the case of different style tubes  14  that are associated with the same insert, and thus the same indicia  28 , the geometric information may be additionally considered to discriminate between the tube types. 
         [0066]    It should be recognized that a simple rule-based decision algorithm may be implemented in microcontroller  30  to use the determined tube dimensions, cap presence or absence, and indicia to specifically identify one or more potential tube types in a particular system  10 . For example, a database including a list of all possible tube types along with their respective parameters would allow for retrieval of a particular tube type based on the identification of one or more of the parameters that may be identified by system  10  as described above.