Abstract:
A portable imaging-based measurement device is developed to perform 2D projection based measurements on an object that is difficult or dangerous to access. This device is equipped with self calibration capability and built-in operating procedures to ensure proper imaging based measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/247,010 filed Sep. 30, 2009, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY FUNDED RESEARCH 
     This invention was made with United States government support under Award No. DE-SC-0003254 awarded by the U.S. Department of Energy (DOE), Office of Science (SBIR), entitled “Recovery Act-Imaging-based Optical Caliper for Objects in Hot Manufacturing Processes”. The United States government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to imaging based dimensional measurement, and specifically, to a portable imaging-based device that can be used for the measurement of critical dimensions of an object with built-in self calibration. 
     BACKGROUND OF THE INVENTION 
     Imaging based measurement is widely in use. One can find imaging-based measurement systems based on triangulation, interferometry or 2D projection. However, these devices are subject to good alignment, critical calibration procedures and typically sensitive to lighting conditions. This limits the ability to use these imaging instruments on a portable basis. 
     It is preferable to have a non-contact dimension measurement device that is inexpensive and portable for certain applications. For instance, in a steel rolling mill, the roll closure (the gap between two rolls) is a critical parameter relating to the quality of the finished products. While fixed location dimensional measurement instruments are commonly available to the steel mills, they are too expensive to be installed after every rolling stand. Therefore, these instruments are typically installed at the finishing end, after the last rolling stand. Nevertheless, there is a need to verify the dimensions after every stand on a periodic basis, such as after a new machine setup or after a predetermined time period (e.g., every 30 minutes). The difficulty involved in taking such a measurement is that the object to be measured is not only very hot, typically ranging from 700° C. to 1,250° C., but is also moving. It is not practically possible to stop the object for measurement because a constant no-moving contact can damage the rolls. There are two practices now for taking a measurement for the hot object while moving. The first practice is to use a hand-held caliper for those objects moving at a relatively slow speed, such as after a roughing mill. In this practice, an operator must bring a caliper into contact with the hot moving object to obtain the measurement. As may be appreciated, this practice is somewhat dangerous and potentially inaccurate. The other practice, for objects moving at a faster speed, such as after an intermediate mill, involves using a piece of a specially treated wood. The wood piece is held against the hot, moving object, thereby allowing the hot object to burn a portion of the wood piece into the shape of the object. The worker then measures the burned portion to obtain the dimension of the object. A portable non-contact dimension measurement device could find itself valuable in this application. 
     Further, it is known to use a hand-held optical instrument for measuring a distance to an object (e.g., such as a range finder); however, such a device does not help when the dimension of the object is desired. In addition, a LIDAR (light detection and ranging) system is known in the art, which is capable of scanning with a laser beam for topographical data, but such a system is expensive and not portable. 
     One challenge in providing a portable two-dimensional (2D) projection based measurement device is that the projection size in an image may be greatly affected by the setup and/or configuration, or known as the spatial relationship between the imaging sensor, typically a camera, and the object to be measured. Portability in such a measurement device thus prevents knowing in advance the object-to-measurement device spatial relationship. 
     Another challenge in providing a portable measurement device is the unknown nature of the lighting conditions and setup the may exist at a time and place when a measurement is desired to be taken. In particular, those of ordinary skill in the art know that in general, the quality of an image may greatly affect the information that can be extracted from the image, which for present purposes would greatly affect measurement results. For example, it is undesirable to have any portion of the object to be measured so bright that it will saturate (i.e., image brightness over the limit) the imaging system. Such saturation effect will cause the boundary detection algorithms, used to define the boundaries of an object to derive the dimensions, to fail. Furthermore, such saturation may cause a spill over effect, with excessive photons flooding adjacent image pixels (i.e., in typically used imaging technologies). This will result in a swollen object in the image, thereby causing faulty measurements. It is also undesirable to have the object too dark, in that the imaging sensors will behave non-linearly, due to the so-called dark current effect. Dark current effect is caused by long wavelength photons penetrating into the substrate of the imaging sensor. When the brightness is low (e.g., less than 15% of the full scale, for instance), the dark current effect may distort the image, particularly the object boundaries. In contrast to a portable device, in a fixed installation, a typical imaging system may experience the benefit of a fixed and well-controlled lighting environment to ensure proper exposure and working range. However, as suggested, this outcome is almost impossible for a portable device because the lighting conditions and setup cannot be known in advance. Even today&#39;s digital cameras, equipped with automatic electronic shutter, aperture and flash lights, cannot fully address some extreme cases, such as imaging a steel bar at 1,000° C. 
     There is therefore a need and value to design a measurement device, capable of portable use and capable of measuring object dimensions based on 2D projection, that eliminates or minimizes one or more of the problems or shortcoming described above. 
     SUMMARY OF THE INVENTION 
     The present invention provides a measurement apparatus for determining a dimension of an object. The apparatus includes an imaging system, a parameter identifier and a computing unit. The imaging system is configured to acquire an image of the object. In one embodiment, the image may be the two-dimensional (2D) projection of the 3D object as captured on an imaging sensor. The parameter identifier is configured to determine at least one calibration parameter, preferably during or near in time to the actual image capture (i.e., self-calibration at measurement time). In an embodiment, the at least one calibration parameter may comprise a working distance from the imaging system to the object. The parameter identifier does not need to have information in advance of the image capture of the spatial relationship to the object, which makes embodiments of the apparatus suitable for portable use. The computing unit is configured to determine the dimension of the object from the image based on the at least one calibration parameter. 
     In an embodiment, the computing unit is further configured to determine a dimensional model. The dimensional model reflects the measurement-time calibration that enables the use of the apparatus for dimensional measurements. The apparatus is calibrated such that the actual dimensions of a physical object can be mapped onto the image grid, which is expressed in pixels. With this model, the computing unit analyzes the image so as to determine (i.e., “count”), for example, the number of pixels in the image between boundary lines of an object. The boundary lines (edges) of the object in the image may be identified (i.e., by the computing unit) through execution of image processing techniques, known as edge detection algorithms. The computing unit uses the determined object dimension (in the pixel space), in conjunction with at least the dimensional model, to arrive at the actual dimension of the object in physical space. 
     In one embodiment, the model includes (1) a relatively accurate estimation of the working distance—a system calibration parameter determined at measurement time (moment of imaging) and corresponding to the distance from the imaging system (i.e., lens) to the object being imaged, and (2) a lens focus length—typically known in advance and corresponding to the distance from the lens optical center to an imaging sensor part of the imaging system. 
     In alternate embodiments, the apparatus includes an ability to generate display readouts to guide a user in making adjustments to certain settings (e.g., focus, exposure, etc.) so as to ensure acceptable quality image (e.g., in gray scale, no saturation). 
     In still further embodiments, the apparatus may further include the ability to generate display readouts that allow users to identify and select the shape and/or configuration of the objects to be measured, such as a straight bar, a rectangular box, a circle, and so forth. Therefore, acquiring additional spatial relationships such as the existence of “parallel boundary lines”, “perpendicular boundary lines” and the like. The information is then used by the apparatus to overcome the difficulty of alignment with a handheld device. 
     Embodiments of the inventive apparatus, in view of the built-in functionality to address both unpredictable spatial relationships as well as unpredictable lighting setup and conditions, are suitable for portable use and may be used to obtain dimensional measurements for difficult to access objects, such as those at an elevated temperature or those in a narrow space. 
     The foregoing and other aspects, features, details, utilities, and advantages of the present disclosure will be apparent from reading the following description and claims, and from reviewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described by way of example, with reference to the accompanying drawings: 
         FIG. 1  is a schematic and block diagram view of an apparatus for determining a dimension of an object (measurement apparatus), in one embodiment. 
         FIG. 2  is a flowchart showing a method of determining a dimension of an object in physical space. 
         FIG. 3  is a diagrammatic and block diagram view of a measurement apparatus, in a further embodiment. 
         FIGS. 4A-4B  are diagrammatic views of an image of a 3D object, showing boundary lines (edges) of the object, in relation to a two-dimensional (2D) coordinate system onto which the object is projected. 
         FIG. 5  is a diagrammatic view of an image of a 3D object, showing boundary lines (edges) of the object, in relation to a two-dimensional (2D) coordinate system onto which the object is projected and being corrected for alignment. 
         FIGS. 6A-6B  are isometric views of the front and back, respectively, of a measurement apparatus, in a still further embodiment. 
         FIG. 7  is a schematic view of a measurement apparatus and an object showing a first source of geometric error. 
         FIG. 8  is a schematic view of a measurement apparatus and an object showing a second source of geometric error. 
         FIG. 9  is a flowchart showing a method for adjusting an object dimension to correct for geometric errors, as may be incorporated in a still further measurement apparatus embodiment. 
         FIG. 10  is diagrammatic and block diagram view of distributed network that includes a still yet further embodiment a measurement apparatus in communication with a data center computer. 
         FIG. 11  are simplified cross-sectional views of various object geometries (shapes). 
         FIG. 12  is a flowchart diagram showing a method of guiding a user in making adjustments to certain settings, as may be incorporated in a still further measurement apparatus embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIG. 1  is a diagrammatic and block diagram view of an apparatus  8  for determining a dimension of an object (hereinafter sometimes referred to as a “measurement apparatus”). As shown, apparatus  8  includes a computing unit  10  configured to output measurement data  12  (e.g., an object dimension in physical space) using at least a dimensional model  11 , a source of power  14 , an imaging system  26  (including at least an imaging device  20  and a lens  24 ) configured to acquire an image of a target object  60 , and in-situ parameter identifier  30  configured to determine at least one system calibration parameter  31 . 
     Computing unit  10  may include conventional processing apparatus known in the art, capable of executing pre-programmed instructions stored in an associated memory, all performed in accordance with the functionality described in this document. It is contemplated that the methods described herein, at least in one embodiment, will be programmed with the resulting software being stored in an associated memory and where so described, may also constitute the means for performing such methods. Implementation of the invention, in software, in view of the foregoing enabling description, would require no more than routine application of programming skills by one of ordinary skill in the art. Such a computing unit may further be of the type having both ROM, RAM, a combination of non-volatile and volatile (modifiable) memory so that the software can be stored (embodied) and yet allow storage and processing of dynamically produced data and/or signals. One of ordinary skill in the art will appreciate that computing unit  10  may additionally and/or alternately include or be coupled with supporting hardware of various kinds, particularly in light of its image processing functionality described below. 
     Dimensional model  11  is configured to allow computing unit  10  to convert an object&#39;s dimension (in pixel space) to an object&#39;s dimension (in physical space of the object). Model  11  may be implemented as a data structure embodied in a memory associated with computing unit  10 . Details of an exemplary embodiment of model  11  will be set forth below. 
     Measurement data  12  comprises dimensional information concerning object  60  including, but not limited to, the dimension of object  60  in physical space (e.g., for a longitudinally-extending bar that is round/circular in cross-section, the dimension may be the bar&#39;s diameter). It should be understood, however, that other information concerning the dimension measurement event may be stored and output by apparatus  8  (e.g., see  FIG. 10  and related description). 
     Power source  14  is configured to provide operating power for the other portions of apparatus  8 . In an embodiment, power source  14  may comprise conventional batteries (e.g., replaceable and/or re-chargeable) suitable for adapting apparatus  8  for portable use. 
     Imaging system  26  is configured to acquire an image of object  60  (i.e., the object whose dimension is to be determined). The image so obtained may be organized in a two-dimensional array (X, Y), for capturing the two-dimensional (2D) projection of a three-dimensional (3D) object  60  thereon. Imaging system  26  may include a plurality of sub-components, including imaging device  20  and lens  24 . The lens  24  may have a predetermined focal length (D f ) that is selected to provide a corresponding, desired field of view. It should be understood that various constituent components of imaging system  26  have been omitted for clarity. Imaging system  26  may comprise conventional components. 
     In-situ parameter identifier  30  is configured to acquire system calibration parameters, shown in block form as calibration parameters  31 , during or at least near in time to the capture of an image of object  60  whose dimension is to be measured. 
     For a measurement of the object&#39;s dimension, imaging system  20  is used to capture the image of target object  60  through a principle of line of sight  40 . While a single apparatus  8  is primarily useful for determining a dimension of object  60 , if multiple apparatuses are synchronized to take measurements from different perspectives of the object, the resulting plurality of measurements can be used to determine features of the object, for example, to determine geometric measurements such as eccentricity and ovality. 
     It should be understood, however, that computing unit  10  cannot determine the size or dimension of object  60  with the object&#39;s image alone. Certain additional parameters are required. As described briefly above in the Summary, apparatus  8  must be calibrated at the same time or sufficiently near the image capture time to account for and prevent adverse affects of movement of the apparatus, changes in lighting, etc. Computing unit  10  can then determine from the object image an object dimension in pixel space which is converted into an object dimension in physical space. To achieve this end goal, the image pixel size must be therefore be determined (“calibrated”) at the same time (or near so) as the image is taken. To facilitate this calibration, parameter identifier  30  is used to detect necessary parameters, such as the working distance (best shown in  FIG. 3 ) between imaging lens  24  and object  60  by way of a parameter detecting means  50 . The parameter detecting means  50  may comprise conventional components known in the art, for example, may be structural lighting, such as laser spot, ultrasonic wave, or other known technologies, and in accordance with known approaches, such as optical triangulation, interferometry, and/or sonic/ultrasonic response. Generally, the information obtained by the imaging system  26  (e.g. image) and parameter identifier  30  (e.g., system calibration parameters  31 ) are processed by computing unit  10  to determine measurement data  12  (e.g., the object&#39;s dimension). 
     The object  60  whose dimension in physical space is to be measured may take a wide variety of forms. For example, in one embodiment, object  60  may comprise a relatively hot (700° C. to 1,250° C.), moving rod of metal (e.g., steel), such as may be found in a steel mill or the like. Such an object  60  may take wide variety of radial cross-sectional geometries (shapes), such as illustrated in  FIG. 11 . As shown in  FIG. 11 , such shapes may be round (circular)  176 , square  178 , rectangle  180 , diamond  182 , oval  184  or hexagon  186 , to list a few. 
     In the view of the possible variation in geometry, shape, etc. of object  60 , a further system calibration parameter  31 , in an embodiment, may be the degree of parallelism and/or perpendicularity of edges of object  60  in the 2D projection plane. In an embodiment, to take a measurement of an object properly would require that the dimensions of an object to be measured are in the same plane that is perpendicular to the camera (lens) principal axis line. For hand-held, portable measuring devices, this requirement is a practical impossibility (i.e., no perfect alignment). To measure an object such as the diameter of a straight round bar, for instance, apparatus  8  may be configured to assume or otherwise force the object&#39;s edges to be parallel (e.g., the 2D projection of 3D the object, as seen in the acquired image, is expected to be a rectangle). These assumptions may be embedded into the software of computing unit  10  for a “diameter measurement”, in the case of a round object. 
     As an example,  FIG. 4A  represents a typical picture of a 2D projection of a straight round bar. Those experienced in the art shall know that the bar boundaries  34  and  36  would not be straight and parallel in the original image due to movements of the object  60  or the portable apparatus  8 , as well as the non-perfect alignment. To overcome this issue, this invention adopts the approach of applying the advance knowledge of the 2D projection shape of the object to be measured for correcting misalignment. That is, with the knowledge of the 2D projection shape being a “rectangle”, the software in the computing unit  10  would be able to “force the boundaries  34  and  36  to be parallel”, as illustrated in  FIG. 4B . Specifically, the following steps can be followed by the computing unit  10  (this example assumes the expected shape to be a rectangle): 
     The first step (a): A best fit straight line  234  can be generated as y=a 34 x+b 34 , in which a 34  and b 34  are the mathematical slope and offset of the line equation, for the boundary  34 . Similarly, a best fit line  236  y=a 36 x+b 36  can be generated for the boundary  36 . 
     The second step (b): Check if the two best fitted lines  234  and  236  are parallel (whether a 34 =a 36 ). If they are parallel, it is not necessary to force for parallelism. 
     The third step (c): If lines  234  and  236  are not parallel, there would be an intersection point  330 , which can be mathematically solved for. This intersection point  330  would be the theoretical disappearing point of the two parallel lines due to a perspective view. 
     The fourth step (d): With this intersection point  330 , we can define a centerline  200  of the object  60  by equally splitting the angle  210  between lines  234  and  236  (each of the resulting two equal angles being denoted β). The two boundary edges  34  and  36 , should be parallel to this centerline  200  if the measurement plane is perpendicular to the camera principal axis  2 . 
     The fifth step (e): The next step is to define the points of invariant. As the diameter measurement depends on the working distance, we will fix the points on the boundaries  34  and  36  associated with the spot  220  where the working distance is measured on the object  60 . This is possible, particularly with the embodiment of a laser spot (see below description in connection with the embodiment of  FIG. 3 ). A line  230  that passes through the spot  220  and is perpendicular to the centerline  200  can be generated and intersect with both lines  234  and  236 , resulting in two stationary points  334  and  336 . 
     The sixth step (f): With now the constraints of having the centerline  200  and the points  334  and  336  being stationary (invariants during the image transformation mapping), we can perform the image mapping to convert the original image as in  FIG. 4A  into the alignment-corrected image shown in  FIG. 4B . 
     Those skilled in the art shall know that there exists image mapping algorithms to transform an image with two unparallel lines into an image with two parallel lines. It is also possible to further transform (e.g., by rotation) the corrected image in  FIG. 4B  into one shown in  FIG. 5 , in which now the centerline  200  is parallel to the Y axis, for the ease of computing the distance between edges  34  and  36 . A user of apparatus  8  need not even be aware of this working assumption. 
     For certain other shapes, apparatus  8  may be configured to “assume” a few known features of the 2D projection of the object, such parallelism (or perpendicularity) of certain edges (with respect to other edges), or of prismatic shapes. Based on these programmed assumptions, computing unit  10  is configured to process all the acquired data, through the dimensional model such as converting an oval into a circle, a trapezoid into a rectangle, to arrive at the object&#39;s dimension(s) (in physical space). 
     In alternate embodiments, computing unit  10  may be configured with a user interface that will display multiple shape/geometry choices to a user and provide a mechanism for the user to select the desired working assumptions (e.g., which edge will be perpendicular or parallel to another edge, etc.) on the 2D projection. For example, this interface may be akin to allowing the user to select the shape of the object (e.g., round, square, etc.—see  FIG. 11 ). This user interface may be particularly useful when apparatus  8  is adapted for measuring the dimension(s) of complex geometry objects (e.g., a box). 
     In yet another embodiment, computing unit  10  may be configured with a user interface that will display the image with detected “edges”, and allow users to select the edges and assign relations (e.g., parallelism, perpendicularity, roundness, angle, etc.). Taking  FIG. 4A  as an example, the image frame  28  would be displayed to the user after the snapshot with the boundaries  34  and  36  detected. The user can then select boundaries  34  and  36 , and assign the “parallelism requirement” between the two boundaries for the computing unit  10  to further process the image. 
     With continued reference to  FIG. 1 , although parameter identifier  30  is shown as a separate block from computing unit  10 , it should be understood that this separation is for description purposes only, and that certain computational steps performed by computing unit  10  and parameter identifier  30  may in fact be performed on the same processing unit. Parameter identifier  30 , in whole or in part, may be but need not be a separate component. 
       FIG. 2  is a flowchart showing a method of determining a dimension in physical space of a target object. The method begins in a “Start” step, which marks the start of a measurement cycle, typically initiated by the user by depressing a key on apparatus  8 . 
     In steps  64  and  68 , computing unit  10 , upon commencement of a measurement cycle, will initiate requests to imaging device  20  and parameter identifier  30  for needed data. As described generally above, imaging system  20  will acquire an image (including the 2D projection of the object) and parameter identifier  30  will determine one or more system calibration parameters  31 , for example, a working distance as defined above. The method then proceeds. 
     In step  66 , computing unit  10  may pre-process the acquired image of the object, for example, through image de-noising or the like. Thereafter, computing unit  10  will determine the object&#39;s dimension in pixel space. The pixel space is the coordinate system space of the image obtained by imaging device  20 . Computing unit  10  may employ conventional algorithms, such as edge detection algorithms, in order to determine object boundaries and thus the object dimension(s) in pixel space. In other words, the distance(s) between selected edges of object  60  in the image can be determined by computing unit  10 , thereby establishing that dimension in pixel space. Those of ordinary skill in the art will appreciate that a variety of different image processing approaches may be employed to identify the object boundaries and then to determine the pixel distance therebetween. The foregoing may constitute a means for determining an object dimension from the image in the pixel space. 
     In step  70 , computing unit  10  will use system calibration parameters  31  obtained from parameter identifier  30  to establish dimensional model  11 , which enables computing unit  10  to convert the object dimension(s) in the pixel space into the object dimension(s) in the physical space of the object. An exemplary embodiment of model  11  will be described below. The foregoing may constitute a means for establishing a dimensional model in accordance with the determined system calibration parameters. 
     In step  72 , computing unit  10  applies dimensional model  11  to convert the pixel space dimension(s) into physical space dimension(s). The foregoing may constitute a means for converting the object dimension in the pixel space into the object dimension in the physical space in accordance with the dimensional model. 
     In step  74 , computing unit  10  displays the determined object dimension (e.g., on a display screen associated with apparatus  8 ), may store the determined object dimension for later retrieval, and/or transmit the determined object dimension to another computing unit (e.g., see  FIG. 10  and related description of a networked data center computer). 
       FIG. 3  is a diagrammatic and block diagram of an alternate measurement apparatus embodiment  8   a . One of ordinary skill in the art will recognize that the following description of apparatus  8   a  is exemplary only and not limiting in nature—there are many ways to implement embodiments of the present invention. 
     In the illustrated embodiment, parameter identifier  30  includes a laser spot source  32 . Laser spot source  32  is placed at a location and an attitude that is fixed with respect to the imaging device  20 . As shown in  FIG. 3 , laser spot source  32  is offset from imaging device  20  by a known distance L. Also, laser spot source  32  is configured to project its laser spot  52  at a known angle α with respect to a direction parallel to the normal line of the imaging device  20 . In an embodiment, the laser spot source  32  is configured to project its laser beam in the field of view of the imaging system. The parameter detecting means  50 , in the illustrated embodiment, is the laser spot  52 . 
     As the laser spot  52  impinges the outer surface of object  60 , a representation of the laser spot in the image, designated by reference numeral  54 , is formed on an imaging chip  22  in the imaging device  20  by lens  24 . Computing unit  10  is configured, through image processing, to distinguish the image of laser spot  54  from other imaging features, for example, by the combination of intensity, pattern, position and others (i.e., in other words, to recognize the representation of the laser spot  54  in the acquired image). Computing unit  10  will then determine attributes of laser spot  54 , perhaps the most significant of which is the position of laser spot  54  in the pixel space. However, computing unit  10  is configured to analyze certain factors associated with the laser spot  54  in order to accurately calculate the spot position in the pixel space. These other factors may include but are not limited to the observed spot shape and intensity distribution, among other factors. By analyzing the attributes of the image of laser spot  54  (e.g., the position of laser spot  54  in pixel space), along with the known location and attitude parameters (such as L and α) of laser spot source  32  and the focal length D f  of lens  24 , computing unit  10  can determine, in real-time, system calibration parameters  31 , such as the working distance D w . In an embodiment, the parameter identifier  30  is configured to determine a system calibration parameter  31  (e.g., working distance) during the image acquisition. 
     For instance, computing unit  10  may be configured to calculate distance  56 , denoted as d l , between the image of laser spot  54  and principal line  2  of imaging chip  22 . Those skilled in the art shall know that d l  can be a physical quantity (i.e., defined in the physical space of the chip) if the pixel size on imaging chip  22  is known in advance. Computing unit  10  is thus configured to derive the working distance D w  as follows:
 
 D   w   =L /( d   l   /D   f +tan α),  (1)
 
     in which L, D f  and α are predetermined data associated with the laser spot source and imaging system known to the computing unit and d l  can be obtained by computing unit  10  via image analysis. 
     Once computing unit  10  determines the working distance D w , computing unit  10  can further derive other measurement parameters such as the image pixel sizes of the object image  62  and thus the mapped dimensions of the object  60  in physical space. In  FIG. 3 , distance  56  on imaging chip  22  corresponds to a physical distance  58  on the object. 
       FIG. 4A  is a representation of an image of an object, showing boundary lines (edges) of the object, in relation to a two-dimensional (2D) coordinate system. In  FIG. 4A , an image  28  contains a representation of object  60 , with a pair of edges  34 ,  36  extending in a generally vertical direction in the Figure but not corrected for parallelism. The image  28  would be corrected by way of the assumption of parallelism such that edges  34  and  36  are now parallel with each other and with respect to a centerline but where the centerline remains at an angle with respect to the Y-axis, as in  FIG. 4B . Finally, the image  28  may be still further transformed such that edges  34  and  36  (and centerline) are now substantially parallel to the Y axis, as in  FIG. 5 . 
     With respect to  FIG. 5 , principal axis  2  extends into the paper. Furthermore, image  28  is shown in relation to a 2D coordinate system (X, Y), designated coordinate system  38 . In the illustration, for a given position off the principal line  2  of a feature (e.g., a first edge) on object  60 , there would be a mapped position on imaging chip  22 . Through image analysis, computing unit  10  can determine a corresponding physical position relative to the principal line  2  on imaging chip  22  and then calculate the physical position off the principal line  2  on object  60  by an amplification factor D w /D f . 
     Accordingly, in an embodiment, dimensional model  11  may include (i) a factor to convert pixels (e.g., counted between edges) from the image to a physical dimension on imaging chip  22 ; and (ii) an amplification factor defined by D w /D f  to convert the physical dimension of the object&#39;s projection on the imaging chip  22  to a physical dimension of or on the object itself. 
     For example illustrated in  FIG. 5 , computing unit  10  may be configured to determine the edge to edge distance  76  of object  60 . This is shown as the X-axis distance (dimension) between edge boundaries  34 ,  36 . Computing unit  10  will determine a first physical position off the principal line  2  for the first edge  36  (hereafter D 36 ) and a second physical position for the second edge  34  (hereafter D 34 ). Then, computing unit  10  may compute the distance (i.e., dimension) between the two edges  34 ,  36  through simple subtraction D 36 −D 34 . 
     In summary, parameter identifier  30  in this embodiment comprises laser spot source  32  and imaging device  20 , and parameter detecting means  50  is laser spot  52 . Those skilled in the art shall know that the laser spot can be replaced by other structured light and may be in different shapes, colors and radiating sources, such as a laser line or a projected line image, among other implementations. Other than the implementation as in  FIG. 3 , those skilled in the art shall also know that the parameter identifier  30  can also comprise other technologies such as a laser interferometer, a sonic or ultrasonic echo detector, and/or other means. 
       FIGS. 6A-6B  are isometric front and back views, respectively, of a still further measurement apparatus embodiment  8   b , reflecting measurement apparatus  8   a  of  FIG. 3 . Apparatus  8   b  thus also includes a computing unit  110 , an imaging device  120  and a laser source  132  that are integrated therein. Laser source  132  is fixed with respect to imaging device  120 . Imaging device  120  is comprised of a camera and a lens. The lens could have manual or automatic iris and focus controls. Apparatus  8   b  as shown includes a grasping handle  118  and strip mounting ears  116 , to facilitate portability and hand-holding operations. Computing unit  110  also has a screen  112  and a control button  114 . It can be operated from hard buttons such as the control button  114  or preferably through a touch screen, to minimize the number of hard buttons. Apparatus  8   b  may be operated using battery power. Apparatus  8   b  further includes a charger jack  102  provided in the body/shell for recharging the re-chargeable batteries included as the power source. Apparatus  8   b  may also include a standard camera mounting hole (¼″-20—not shown) such that apparatus  8   b  may be used on standard tripods. 
       FIG. 7  is a schematic view of a measurement apparatus and an object showing the possible occurrence of a measurement error. In particular, there is a not insignificant source of error from the geometric variation of the object to be measured. As illustrated in  FIG. 7 , an apparatus  100  is used to measure the two-dimensional (2D) projected diameter of object  60 . Without loss of generality, the instant example shows that object  60  is round/circular in radial cross-section, although it should be understood this is exemplary only and not limiting in nature (the object may take the form of many other shapes—see  FIG. 11  and related description). 
     The diameter of object  60  to be measured in this case is 2r, in which r is the radius of the round bar (object  60 ). Nevertheless, the working distance measured by the integrated sensor in apparatus  100  would be distance  44 , hereby designated as w. This working distance  44  (“w”) is different that the working distance (“W”) from the lens of apparatus  100  to the measuring plane  42  of the object  60 .  FIG. 7  also shows line of sight  40  for reference. This difference in the actual working distance versus the apparent working distance (W-w), unless corrected or otherwise accounted for, can cause an error in the object dimension calculation, which is, as described above, based on the working distance measured to be w. 
       FIG. 8  is a schematic view of apparatus  100  and object  60  showing a second source of error. While the first source of error above involved an error in the apparent working distance, another source of error may arise due to perspective error. The diameter  46  that is actually measured (now with perspective error), hereby designated as d, is due to the fact that the line of sight is different from that needed to view the actual diameter  48 , hereby designed as D. Nevertheless, those skilled in the art would appreciate that actual diameter  48  (“D”) can be derived from the measured diameter d based on trigonometry, taking note that angle  144  is the same as angle  142 , which for reference purposes will be designated as angle θ. Based on the foregoing, actual diameter  48  (“D”) may be determined as a function of the measured diameter d, or
 
 D =ƒ( d )= d /sin θ.  (2)
 
       FIG. 9  is a flowchart showing a method for adjusting a system calibration parameter, namely the working distance, so as to arrive at an object dimension this is compensated to correct for the above-noted geometric errors. To overcome the foregoing described errors, computing unit  10 , in an embodiment, is configured to perform the following method: 
     1. (Step  150 ): Determining the working distance w i . This step may be performed as described above. 
     2. (Step  152 ): Estimate measured bar diameter d i  using w i . This step may be performed as described above. This step may constitute a means for determining a measured object dimension using the current value of the calibration parameter. 
     3. (Step  154 ): Estimate actual bar diameter D i  by ƒ(d i ). This step may be performed by evaluating the function ƒ(d i ). This step may constitute a means for computing a corrected object dimension as a function of the current measured object dimension wherein the function ƒ(d i ) is defined at least in part on the cross-sectional geometry of the object to be measured. 
     4. (Step  156 ): if |D i −D i−1 |&lt;threshold, stop (Step  158 ). This step may constitute a means for determining a difference between the current corrected object dimension and the most recently computed corrected object dimension computed in the previous step (step  154 ). The threshold is selected based on a desired level of accuracy. 
     5. (Step  160 ): r i =D i /2. 
     6. (Step  160 ): w i+1 =w o +r i . This step (and the previous step r i =D i /2) may constitute a means for updating the current value of the calibration parameter (e.g., working distance) when the difference in step  156  is not less than the threshold. 
     7. (Step  160 ): Advance index and repeat steps 2 to 6. 
     Note that the steps (2) through (6) above are intended to continue to search for an update to add to the originally-calculated working distance parameter until the updated working distance parameter (initial value plus the updated, incremental value) results in a suitably accurate object dimension. The final (compensated) value of the actual bar diameter D i  (object dimension) when the method stops may be used as the outputted object dimension. Alternatively, the final (compensated) object dimension may be calculated based on the updated working distance (i.e., the current value of the term w i+1 ) when the method stops. Either of these approaches may constitute a means for calculating a final, measured object dimension based. 
     Further, note that the term w o  above represents the initial value of the working distance as calculated based on methodology outlined above in connection with  FIGS. 1-5 , and stays the same as the method loops (i.e., only the term r i  updates as the method loops as the actual bar diameter D gets closer and closer to an accurate value). The foregoing steps may constitute a means for adjusting one or more system calibration parameters to correct for geometric errors. 
     Those of ordinary skill in the art will appreciate that the function ƒ(d) described above is dependent on the cross-section geometry of object  60 , although the 2D projection of object  60  (i.e., as seen in the acquired image  28 ) is a rectangle. In one embodiment, apparatus  100  is configured to default to a round/circular shape (i.e., as taken in radial cross-section) when performing the above-described adjustment to correct or compensate for geometrical errors. In an alternate embodiment, however, apparatus  100  is configured with a user interface configured to allow a user to select an object cross-section shape that the computing unit will use to select the corresponding geometry dependent function ƒ(d) in refining the measurement results. Exemplary shapes documented in  FIG. 11  round, square, rectangle, diamond, etc. would also serve this purpose. 
     It bears emphasizing that the shape selection just described above in connection with the correction of geometric errors (i.e., function ƒ(d)) is different from the shape selection described in connection with correction for misalignment. In other words, the inventive measurement apparatus may be configured with a user interface for two different shape selections by a user. The first shape selection is to select the expected 2D projection shape for the purpose of determining parallelism and/or perpendicularity (see  FIGS. 4A ,  4 B and  5 ) for correcting misalignment (i.e., to correct for when the measuring plane is non-perpendicular to the camera principal axis). With the expected 2D shape known, the computing unit can analyze the image (as illustrated in  FIGS. 4A-4B ) and can further be mapped into a plane that is perpendicular to the camera principal axis. The second shape selection is to select the cross-sectional shape while measuring a side view 2D projection of the object for adjusting and correcting for perspective error. The selected cross-sectional geometry may make very little difference to the geometry as seen in the 2D projection image of the object. For example, the 2D projection will in very many cases be seen as a rectangle, even though the cross-sectional shape varies (e.g., a straight bar of different cross sectional shapes). 
       FIG. 10  is diagrammatic and block diagram view of distributed network that includes a further measurement apparatus embodiment  8   c , shown in electronic communication with a data center computer.  FIG. 10  shows that apparatus  8   c  may locally store various information  162  relating to the measurement event, for example, the measurement data  164   1  (e.g., object dimension(s)), time/date  164   2  when the measurement was taken, as well as other information  164   n . In this embodiment, apparatus  8   c  includes a communication module  166  configured to transmit all or only a portion of such information  162  (e.g., measurement data  164   1 ) to a data center computer  168  via its communication module  170 . The communication module  170  may accommodate various wired connections known in the art, such as USB, RS232, TCP/IP (i.e., over Ethernet), to name a few but may also provide for wireless connections such as WiFi, Bluetooth, etc. (i.e., communication module  166  would be a wireless transmitting/receiving unit configured to operate in accordance with a predetermined wireless communication protocol—for example, one of a WiFi-compliant and Bluetooth compliant communication module). 
     The user of a measurement apparatus is likely to record the measurement result in a log or an equivalent as to when, where, what and how the measurement was taken. The communications function described above facilitates (and eases) the burden of recording measurement information from the user&#39;s point of view. In a further embodiment, the apparatus is configured for automatic logging of the measurement results. In a still further embodiment, data center  168  may have a communication line  172  coupling data center computer  168  to a local area network and/or wide area network (e.g., a steel mill network) for wider distribution and use of the object measurement data. 
       FIG. 12  is a flowchart diagram showing a method of guiding a user in making setting adjustments, as may be incorporated into a still yet further measurement apparatus embodiment. As described in the Background, those of ordinary skill in the art know that imaging sensors will not react to light linearly if the image is too dark (dark noise) or too bright (saturation). Therefore, it is necessary for the computing unit (i.e., computing units in the any of the embodiments, e.g. computing unit  110  in  FIGS. 6A-6B ) to provide a user interface to guide the user in making adjustments to pertinent exposure settings so that the image of interest is in the linear range of the imaging sensor. The guidance could be a brightness index of the image of interest (displayed to the user). It is also possible for the apparatus to automatically adjust the image brightness to a preset proper level through automatic gain adjustment or iris adjustment, or both. Likewise, those skilled in the art will appreciate the value of correct focus, and understand that correct focus can be facilitated through a focus index in the image (and displayed to the user) to guide the user or automatic focus adjustment. 
     In one embodiment, the measurement apparatus may include multiple readouts displayed on the screen (e.g., screen  112 — FIG. 5B ), which are refreshed continuously by the computing unit as the user adjusts the respective settings for focus, exposure, and the like. The readouts that are displayed would provide guidance as to when each setting has been adjusted into an acceptable range (e.g., too low, too high, acceptable range). 
     In an alternate embodiment, the measurement apparatus may include a composite readout displayed on the screen (e.g., screen  112 — FIG. 5B ), which is refreshed continuously as the user adjusts a single setting. The single readout that is displayed would provide guidance is the form of an “acceptable” or “not acceptable” adjustment. The user will only have to adjust a single setting until the output indicator is brought into an acceptable zone. In a still further embodiment, the measurement apparatus displays or otherwise outputs a “condition affirmation” when the setting(s) have been adjusted so as to result in an image meeting predetermined quality standards. As a further alternative, the use of the measurement apparatus itself may be disabled until the setting(s) have been adjusted into an acceptable zone. Disabling the measurement apparatus prevents measurement of dimension data that may be of questionable accuracy. 
       FIG. 12  is an exemplary method of implementing a user guidance interface feature. The method begins in step  188 . 
     In step  188 , the computing unit assesses at least one setting for focus, exposure or the like, as described above. 
     In step  190 , the computing unit produces an output (e.g., a readout display screen) with respect to the setting (or settings) that was (were) just assessed, with the output(s) displays being configured to guide the user (as described above) in making needed adjustments so as to obtain a suitable image. A suitable image is one that meets predetermined quality standards so that the image can be relied on to produce an accurate measurement, such as that object in the image is in reasonable focus, the image is substantially gray scale (i.e., no saturation nor dark current situations), etc. 
     In step  192 , the computing unit determines whether the predetermined image quality criteria are met (prospectively, looking at the focus, exposure, etc.). If the answer is “YES”, then the method proceeds to step  196 , where a “condition affirmation” is output by the computing unit indicating to the user that proper adjustments to the settings have been made (i.e., image will meet predetermined quality criteria) and the apparatus is now reading for taking a measurement. In some embodiments, the apparatus would be enabled to allow taking a measurement, if the apparatus had been previously disabled. However, if the answer is “NO” then the method branches to step  194  where the process is repeated (steps  188 ,  190 ,  192 ) after receiving further user input. 
     In step  194 , the computing unit obtains (e.g., through an interface, such as hard keys or soft keys or other adjustable dials, rings, etc., as described in connection with the apparatus in  FIGS. 6A-6B ) further user input (i.e., adjustments to the setting(s)). 
     The foregoing steps described in connection with  FIG. 12  may constitute a means or mechanism for enabling a user to adjust at least one setting for focus and exposure wherein the computing unit is configured to guide the user in adjusting such settings so as to obtain an image meeting predetermined quality criteria. 
     Still further variations are possible. For example, a measurement apparatus according to the invention can be configured with a user interface to allow a user to change the field of view (FOV) associated with the imaging system portion of the apparatus. For example, in one embodiment, a user can change the “zoom” level (i.e., the lens focal length) by adding fixed attached lens(es). The user interface is configured to allow the user to then change the zoom (or amplification) factor used in the processing logic described above. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.