Abstract:
The present invention is directed to solving the problems associated with the detection of surface defects on metal bars as well as the problems associated with applying metal flat inspection systems to metal bars for non-destructive surface defects detection. A specially designed imaging system, which is comprised of a computing unit, line lights and high data rate line scan cameras, is developed for the aforementioned purpose. The target application is the metal bars (1) that have a circumference/cross-section-area ratio equal to or smaller than 4.25 when the cross section area is unity for the given shape, (2) whose cross-sections are round, oval, or in the shape of a polygon, and (3) are manufactured by mechanically cross-section reduction processes. The said metal can be steel, stainless steel, aluminum, copper, bronze, titanium, nickel, and so forth, and/or their alloys. The said metal bars can be at the temperature when they are being manufactured.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part (CIP) of U.S. application Ser. No. 10/331,050 filed on Dec. 27, 2002 entitled “APPARATUS AND METHOD FOR DETECTING SURFACE DEFECTS ON A WORKPIECE SUCH AS A ROLLED/DRAWN METAL BAR,” now allowed, which in turn claims the benefit of U.S. Provisional Application Ser. No. 60/430,549 filed Dec. 3, 2002, the disclosures of which are each hereby incorporated herein by reference in their entirety. 
     
    
     STATEMENT REGARDING FEDERALLY FUNDED RESEARCH  
       [0002]     This invention was made with United States government support under Cooperative Agreement No. 70NANBOH3014 awarded by the National Institute of Standards and Technology (NIST). The United States government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Related Field  
         [0004]     The present invention relates generally to an imaging system that can image the surface details of a workpiece, such as a rolled/drawn metal bar.  
         [0005]     2. Description of the Related Art  
         [0006]     It is known to produce a metal bar by a mechanical process such as rolling or drawing. Such metal bar is different than a metal slab, bloom, or strip (hereafter referenced as Metal Flat) in that the cross section of such a bar has a smaller circumference/cross-section-area ratio such that the bar may rotate/twist about a longitudinal axis while moving forward longitudinally. For example, the bar shapes shown in  FIG. 2  have a ratio of circumference to cross-sectional-area that is equal to or smaller than 4.25 when the cross sectional area is unity for the given shape. The shape, when taken in cross section, of such a metal bar may be a round shape (item  102 ), an oval shape (item  104 ), or a polygonal shape, as shown as a hexagon (item  106 ), octagon (item  108 ) or a square (item  110 ) in  FIG. 2 . Furthermore, such a metal bar is substantial in length. The length to circumference ratio is typically over 10 and the length to cross-section critical dimension (such as the diameter of a round bar or the side width a square bar) is over 30. A metal bar of this type is typically referred to as “long products” rather than “flat products” in the related industries. Rolling, drawing, extrusion and the like, as used in this disclosure and hereafter referenced as a Reducing Process, describe the ways for reducing the cross sectional dimensions of the metal workpiece through mechanical contact of the applicable tools, such as rollers and drawing dies, and the workpiece. These Reducing Processes are generally continuous, or substantially continuous, in nature.  
         [0007]     In the metal production industry, the presence or absence of surface defects is a relevant criterion upon which assessments of the metal products are made. For instance, surface defects account for half of the external rejects (i.e., rejected by the customer) for the steel bar and rod industry. However, the conventional art provides no reliable means to detect such defects. There are several challenges that conventional inspection approaches have been unable to overcome.  
         [0008]     First, in the case where inspection occurs while the metal bar products are “hot,” the temperature can be as high as 1,100° C., preventing the use of many inspection technologies. Second, the traveling speed of such a metal bar along its longitudinal axis as described above can be, presently, as fast as 100 m/s, several times faster than the speed of the fastest metal strip and nearly 100 times faster than a metal slab or bloom. Further, speed increases are expected in the near future in the range of 150 m/s to 200 m/s. Conventional inspection approaches simply cannot accommodate such high traveling speeds. Third, a high temperature metal bar such as described above is typically confined in a sectional conduit so that the bar will not cobble. Cobbling is an incident wherein a hot, high speed metal bar runs freely outside the conduit. The space, therefore, for any inspection device is extremely limited. Last, the length of such a metal bar, together with the fact of its longitudinal motion, makes the handling of the bar difficult and costly.  
         [0009]     While it is known to apply various imaging approaches to the inspection of cast or rolled Metal Flats in line, visible light imaging technologies have heretofore not been used in in-line Long Products (i.e., metal bar with a substantial length) inspection. Conventional imaging systems are not believed capable for use in inspecting metal bars and the like because the geometry of the metal bars invalidate the illumination and imaging designs that are used to enhance/capture defects on flat surfaces.  FIG. 4  illustrates the differences of applying illumination and of capturing images on a flat workpiece (i.e., image line  318  converges on illumination line on flat  316 ) versus a round workpiece. As to the non-flat workpiece, the freedom in optical alignment and optical working ranges disappears when the object of interest does not have a flat surface. For instance, the image line  18  and the illumination line  18 ′ may not overlap if the light or the camera is tilted, as shown in exemplary fashion in  FIG. 4 . One prior art approach employs the use of area cameras to inspect the bar surfaces. However, it requires that the bar be stationary during imaging. Another prior art approach employs the use of line scan cameras, yet requiring the bar to spin for the scanning due to its flat lighting design. In order to cope with the high longitudinal traveling speed, photo-sensitive diodes, instead of imaging sensors, are used in yet another prior art. The use of photo-sensitive diodes limits the capability of detection to short, transverse defects on the bar surface. This approach is incapable of detecting long, thin defects such as seams on a steel bar.  
         [0010]     To avoid the lighting issue, use of infrared (IR) imaging devices is reported. In this approach, IR cameras are used to capture the self-radiated light from the long products. This approach is limited to the surface defect detection solely based on surface temperature. It is known that surface voids of a hot object appear to be hotter than their neighborhoods due to the cavity theory, even though these voids are at the same temperature as their neighborhoods. This approach is further limited to its detection capability because of the focusing resolution limit of IR radiation. It is known to those skilled in the art that the optical focusing resolution is inversely proportional to the wavelength of the radiation. Typically IR cameras are nearly 10 times more expensive than a visible one and IR cameras are limited in their imaging speed due to the sensor property. As a result, this approach would not be able to accommodate the speed of today&#39;s long products.  
         [0011]     Temperature also makes the long products different to their flat counterpart. Metal bars typically are at a higher temperature than Metal Flats. Heat dissipation of an object is proportional to the area exposed to the cooling media, such as ambient air or water spray. The area of a Metal Flat is several times larger than that of a metal bar, assuming both the flat and the bar are made of the same material and both have the same longitudinal unit density and cross section area.  
         [0012]     It is, however, known to employ imaging-based instruments for bar gauge measurement/control (shadow measurement), bar existence/presence, and bar traveling speed measurement in the Reducing Process.  
         [0013]     It is also known to employ electro-magnetic devices, such as eddy current-based instruments, in the assessment of long products. Eddy-current based sensing systems are used for the detection of surface imperfections in the Reducing Process for in-line inspection. This approach has a high response rate, able to work in a high throughput production line environment (e.g., one kilometer of hot steel bars per minute). However, this approach has several drawbacks. First, it must be very close to the hot surface (typically less than 2.5 mm). Accordingly, it is vibration sensitive and temperature sensitive. Moreover, it is not quantitative in the sense that it is NOT able to describe the nature of the detected defect. Finally, eddy-current approaches are incapable of detecting certain types of defects. As a result, the inspection outcome from eddy current devices is not used by the metal industry for a deterministic judgment on the quality of a specific product. Rather, the output of eddy current-based instruments is only used for qualitative analysis, such as “this batch of steel bars is generally worse than the batch produced last week,” in the Reducing Process for process control purposes, for example, only.  
         [0014]     Another approach attempted in the art employs ultrasonic sensing. This is an approach to replace the eddy current sensors with ultrasonic ones. However, many of the restrictions associated with eddy current-based instruments, such as the short working distance, apply with equal force.  
         [0015]     Other inspection technologies used in the art include magnetic penetrant, circumflux, and infrared imaging with induction heating. The use of these technologies, however, is restricted. First, these techniques can only be used on “cold” metal bars. That is, these technologies cannot be used for in-line inspection during or shortly after hot rolling applications. Also, the metal bars must be descaled before inspection. In addition, the use of magnetic penetrant is messy and cumbersome. This process typically relies on human observation with ultra violet illumination, instead of automatic imaging and detection. The circumflux device is an eddy-current based unit, designed with a rotating detection head. Such rotating mechanism limits the application of this device in metal bar inspection with high traveling speeds, typically used at about 3 m/s. Such device is also expensive due to the moving sensing head design. The combination of induction heating and infrared imaging is based on the fact that induction current is only formed on the surface of the metal bar, and the surface defects on the metal bar will result in higher electrical resistance. Therefore, the spots with surface defects will heat up faster than other areas. There are issues associated with this approach in that (a) such faster heat up is a transient effect and thus timing (time to take images) is very critical; and (b) infrared sensors are not available for very high data rates and therefore cannot support metal bars with high traveling speed.  
         [0016]     Of course, inspection is possible after manufacture of the metal bars. However, post-manufacturing inspection often is not possible because the product is so long and coiled up, making the bar surfaces not accessible for cold inspection technologies.  
         [0017]     Currently, real-time inspection of metal bars manufactured with Reducing Processes is very limited. Metal bars are generally shipped from the manufacturer to the customer even if defective signals are posted by a conventional in-line eddy current inspection system. Customer complaints may therefore appear 3 to 6 months later due to surface defects on the metal bar products that are not immediately apparent to the customer. Such complaints cost the metal bar suppliers (i.e., manufacturers). The metal bar suppliers will either refund the customers for the entire coil/batch or cost share the expenses of additional labor to inspect the parts made out of the metal bar coil/batch.  
         [0018]     There is therefore a need for an apparatus and method to minimize or eliminate one or more of the problems set forth above.  
       SUMMARY OF THE INVENTION  
       [0019]     It is one object of the present invention to overcome one or more of the aforementioned problems associated with conventional approaches for an imaging-based apparatus suitable to be used in-line or off-line to detect surface defects on rolled/drawn metal bars.  
         [0020]     The present invention is directed to solving one or more of the problems associated with conventional metal bar inspection systems as well as problems associated with applying metal flat inspection systems to metal bars for non-destructive inspection of surface defects on metal bars through the use of an imaging system.  
         [0021]     One advantage of the present invention is that it may be effectively employed in the production of metal bars with the aforementioned characteristics, namely, those that may be at a manufacturing temperature, perhaps even hot enough to produce self-emitted radiation, as well as those subject to rotation relative to a longitudinal axis and may potentially be traveling at a very high speed. Others advantages of the present invention include (i) effectively employed to image and detect defects on non-flat surfaces; (ii) use for inspecting metal bars regardless of its temperature; (iii) use for inspecting metal bars traveling at speeds at or faster than 100 m/s; (iv) providing an increased working distance to the metal bar surface, thus minimizing or eliminating the problems set forth in the Background for eddy-current based instruments; (v) providing an output comprising quantitative data with verifiable defective site images; (vi) inspection of the workpiece even before the scale forms on its surface; (vii) suitable for use in inspection at any stage (between the reducing stands or at the end of the line) in the reducing process, not affected by or relying upon transient effects; (viii) providing real-time or near real-time surface quality information; (ix) providing a system absent of any moving sensing heads, thus minimizing or eliminating the problems of moving components set forth in the Background; (x) providing a system needing only very small gap (less than 50 mm) capable of operating between metal bar guiding conduit sections; and (xi) requiring no additional bar handling mechanisms/apparatuses However, an apparatus and/or method need not have every one of the foregoing advantages, or even a majority of them. The invention is limited only by the appended claims.  
         [0022]     A system is provided for imaging an elongate bar extending along a longitudinal axis. The system includes an image acquisition assembly, a line light assembly, and a computing unit. The image acquisition assembly has a field of view configured to image a first predetermined width over a circumference of a surface of the bar to define an image belt. The image acquisition assembly is further configured to produce image data corresponding to the acquired image belt.  
         [0023]     The line light assembly is configured to project a light line belt having a second predetermined width onto the surface of the bar. The light line assembly is disposed, for example by alignment, relative to the image acquisition assembly such that the image belt is within the light line belt. The light line assembly is further configured such that a light intensity is substantially uniform along the image belt when the light is collected by each of the image acquisition sensors.  
         [0024]     For packaging purposes, the line light assembly may include a collection of reflecting elements such as mirrors to achieve the designed projection angle. For serviceability, the collection of the reflecting elements is designed to be detachable.  
         [0025]     Finally, the computing unit is coupled to the image acquisition assembly and is configured to receive image data for a plurality of image belts acquired by the image acquisition assembly as the bar moves along the longitudinal axis. The computing unit is further configured to process the image data to detect predetermined surface features of the bar. In a preferred embodiment, the detected features are surface defects and the image acquisition assembly includes n digital cameras, where n is an integer 3 or greater, arranged so that a combined field of view of the cameras corresponds to the image belt.  
         [0026]     A method of imaging a metal bar is also presented.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     The present invention will now be described, by way of example, with reference to the accompanying drawings, wherein like reference numerals identify identical components in the several figures, in which:  
         [0028]      FIG. 1  is a schematic and block diagram view of an embodiment of the present invention.  
         [0029]      FIG. 2  are cross-sectional views of exemplary geometries for work pieces suitable for inspection by an embodiment according to the present invention.  
         [0030]      FIG. 3  illustrates a cross-sectional geometry of a metal flat.  
         [0031]      FIG. 4  is a diagrammatic view illustrating a conventional lighting scheme as applied to a metal flat and a bar.  
         [0032]      FIG. 5  are simplified perspective views illustrating a bar constrained during its travel by a conduit, and a gap between adjacent conduits in which an embodiment according to the invention may be situated.  
         [0033]      FIG. 6  is a simplified plan view illustrating an imaging coverage on a metal bar using one camera.  
         [0034]      FIG. 7  is a simplified plan view illustrating an imaging coverage on a metal bar with one camera and a telecentric lens.  
         [0035]      FIG. 8  is a simplified plan view illustrating an arc length variation based on a projection of same size grids, such as a line of pixels, onto a bar profile.  
         [0036]      FIG. 9  is a simplified plan view illustrating a lighting arrangement for a bar surface in accordance with the present invention.  
         [0037]      FIG. 10  is a simplified plan view further illustrating, in greater detail, the lighting arrangement of  FIG. 9 .  
         [0038]      FIG. 11  is a simplified perspective view of a metal bar in connection with which the lighting arrangement of the present invention is used.  
         [0039]      FIG. 12  is a simplified plan view illustrating the lighting arrangement in the circumferential direction as directed toward a bar surface.  
         [0040]      FIG. 13A  is a plan view illustrating another embodiment of a lighting and imaging arrangement for a bar surface according to the invention.  
         [0041]      FIG. 13B  is a diagrammatic view of the lighting arrangement of  FIG. 13A  such that the collection of reflective elements can be retrieved for cleaning and restored for function easily, as shown in an installed position.  
         [0042]      FIG. 13C  is a diagrammatic view of the lighting arrangement of  FIG. 13B , including the collection of reflective elements, shown in a partially removed position.  
         [0043]      FIG. 13D  is a diagrammatic, perspective view of the embodiment of  FIG. 13B , showing a protective tube.  
         [0044]      FIG. 13E  is a diagrammatic, side view of the embodiment of  FIG. 13B .  
         [0045]      FIG. 13F  is a diagrammatic, perspective view of the embodiment of  FIG. 13B  viewed from an opposite side relative to that in  FIG. 13D .  
         [0046]      FIG. 14A  illustrates a surface defect along with some surface noise.  
         [0047]      FIG. 14B  illustrates an exemplary result of an image processing step according to the invention as applied to the image of  FIG. 14A .  
         [0048]      FIGS. 15A-15C  illustrate examples of long surface defects that may be found on metal bars and which can be detected by an embodiment according to the present invention.  
         [0049]      FIGS. 16A-16C  illustrate relatively short surface defects that may be found on metal bars and which can be detected by an embodiment according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0050]     The present invention permits automated inspection of metal bars for surface defects as the metal bars are being rolled, drawn or the like (i.e., the reducing process described in the Background of the Invention) without additional handling to the bars.  FIG. 1  schematically illustrates a preferred embodiment in accordance with the present invention.  
         [0051]     Before proceeding to a detailed description of the present invention keyed to the drawings, a general overview will be set forth. The present invention provides the following features:  
         [0052]     1. Capable of working for metal bars manufactured through reducing processes at different cross section geometry; 
        2. Capable of working for metal bars in-line at a bar temperature up to 1,650° C.;     3. Capable of working for metal bars traveling at 100 m/s or higher;     4. Capable of detecting surface defects whose critical dimensions are as small as 0.025 mm;     5. Capable of reporting the defect nature such as its size, location (on the bar), image, and the like;     6. Capable of accommodating different sizes of bars, for example only, from 5 mm to 250 mm with minimum adjustment;     7. Capable of providing real-time or near real-time inspection results;     8. Capable of working with a small access window (less than 50 mm) to the target object;     9. No moving parts while inspecting;     10. No additional handling of the bars; and     11. Continuous operation in commercial, heavy industrial metal production mills.        
 
         [0063]      FIG. 1  is a simplified schematic and block diagram of a system in accordance with the present invention.  FIG. 1  shows line light assembly which may include at least one light source  2 , a light conduit  4 , a plurality of line lights  6  and a corresponding plurality of optical boosters  8 .  FIG. 1  further shows a computing unit  10  and an image acquisition assembly that may include a plurality of cameras  12  each having a corresponding lens  14 .  
         [0064]     With continued reference to  FIG. 1 , a workpiece or object under inspection, such as an elongated metal bar  16  extending along a longitudinal axis, is shown moving along its longitudinal direction  20  at a speed up to 100 m/s or faster while bar  16  is going through a reducing process. The metal bar  16  may be formed from one selected from the group comprising steel, stainless steel, aluminum, titanium, nickel, copper, bronze, or any other metal, and/or their alloys. The bar  16  may be solid or hollow. Typically such metal bar  16  is traveling inside a conduit, as shown in greater detail as conduit  24  in  FIG. 5 , not shown in  FIG. 1 . A gap  26 , shown in  FIG. 5 , is defined between two adjacent conduits  24 , and is typically very small, for example between about 20 to 50 mm taken in the axial direction for high-speed transit of metal bars  16 . It should be understood that metal bar  16  may be at an elevated temperature, for example as hot as 1,100° C. for a hot rolling process. It should also be appreciated that metal bar  16 , given its geometry, is prone to twist/rotate about its longitudinal axis uncontrollably in the direction indicated by arrow  21  in  FIG. 1  when it travels in direction  20 . This possibility for uncontrollable rotation has, among other items, presented problems for conventional imaging systems. As will be described in greater detail below, the present invention overcomes this problem to provide an imaging system that is robust to twisting and/or rotation.  
         [0065]     In order to detect surface defects on bar  16 , an imaging system in accordance with the present invention must be provided having certain features, as described below. With continued reference to  FIG. 1 , the imaging system includes an image acquisition assembly that preferably comprises n imaging cameras  12 , wherein n is an integer 3 or greater. The parameter n is selected to be three or higher based on an analysis set forth below. Each camera  12  is arranged so as to cover a circumferential span of at least 120° in order to image the entire surface of bar  16 . That is, the image acquisition assembly has a composite or combined field of view configured to image the entire circumference of the surface of the bar  16  to define an image belt  18 . As further described below, the image acquisition assembly is further configured to produce image data based on the image belt  18 . The analysis for the parameter n for the number of cameras will now be set forth.  
         [0066]     As shown in  FIG. 6 , a regular lens  14  associated with camera  12  will have a viewing angle (field of view) formed by the two tangential lines of sight  28  extending from a focal point of lens  14  to the surface of bar  16 . This viewing angle, when projected onto a non-flat surface, such as the one shown in  FIG. 6 , will result in a circumferential coverage  30  that is less than 180° and will be insufficient to cover 360° with only two lens/camera units where the lens are not telecentric.  
         [0067]      FIG. 7  shows an arrangement with a telecentric lens  14 ′. A true telecentric lens, which collects lines of sight that are in parallel, even if used, would not practically provide for a two-lens/camera system because of arc length variation. In particular, the lines of sight  28  are parallel with the addition of telecentric lens  14 ′ to lens  14 . In this case, the circumferential coverage  30  is 360°. Theoretically speaking, the entire surface of round shaped bar  16  can be covered using only two lens/camera units. However, as alluded to above, a problem of non-uniform pixel sizes arises.  
         [0068]     As illustrated in  FIG. 8 , the evenly spaced lines of sight  34 , as derived from an evenly-spaced imaging sensor  32  having a plurality of pixels, can result in an uneven arc length  36  on the surface of bar  16 , pixel-to-pixel. Even spacing is a very typical arrangement on an imaging sensor such as a CCD chip. The arc length  36  can be calculated using equation (1) as follows: 
 
 S=p /cos(θ)  EQN (1): 
        where S is the arc length  36  mapped to the pixel in position y, p is the pitch of the pixel array or the pixel size, and θ is the projected angle that can be derived from 
 
θ=arcsin( y/r ), in which  y≦r  and  r  is the radius of the metal bar  16 .  EQN (2): 
       
 
         [0070]     From  FIG. 8  one can learn that as y→r, θ→90°. As θ→90°, S the arc length  36  will approach infinity based on EQN (1). In reality, S will still be a finite number. However, S will be substantially (several times) larger than p, the pixel size. That is, the image resolution in this area will deteriorate so much that this approach is infeasible. Note that the same arc length analysis can be applied to the bottom half in  FIG. 8 , in which case y→-r.  
         [0071]     With three cameras, θ can be established at 60°. When θ=60°, S the arc length  36  (at the 12 o&#39;clock and 6 o&#39;clock positions in  FIG. 8 ) is only 2 p, an acceptable and controllable deterioration in image resolution. If better image resolution is desired, four cameras or five cameras, or even more may be used (i.e., the parameter n referred to above can be an integer equal to four, five or greater). All the lens  14 /camera  12  combinations, as illustrated in  FIG. 1 , are preferably arranged such that all such lens/camera combinations are positioned along a circular path  22  that is concentric to the circular geometry of the exemplary metal bar  16  such that the working distances, the distance from each lens  14  to the nearest metal surface, are the same or nearly the same for all the lens/camera combinations. Note that the path  22  may stay circular if the metal bar is non-circular, say a hexagon, for the purpose of generally serving the same manufacturing line. One that is skilled in the art can appreciate that the path  22  can, if desired, be made to conform to the actual bar geometry.  
         [0072]     In order to accommodate the potentially very high traveling speed of the metal bar  16 , high data rate cameras  12  are preferably used. The cameras  12  in the system are thus preferably digital cameras, with digital outputs to computing unit  10 . This digital output format is desirable to accommodate the harsh environment for signal fidelity. This digital format image signal may be received by the computing unit  10  through standard communication channels such as IEEE-1394 (also known as FireWire), Camera Link or USB ports, or a special interface known as a frame grabber. Each camera  12  preferably is able to generate at least 10,000,000 (or 10 Mega) pixels per second such that a defect feature that is 0.025 mm×0.5 mm can be identified. It should be appreciated, however, that to detect larger features, a reduced resolution, and hence reduced data rate (in pixels per second) would be required. Line scan cameras are preferred even though progressive (non-interlaced) area scan cameras can be used when the bar  16  is not traveling fast. Line scan cameras have an advantage over area scan cameras in that line scan cameras only require a line of illumination, instead of an area of illumination. This will simplify the illumination complexity caused by the non-flat surface. In the case of using line scans, all the cameras in  FIG. 1  will be aligned such that their imaging lines will be forming a circumferential ring, an image belt  18 , on bar  16 . This alignment is necessary to address the issue of twist and/or rotation (item  21 ). If this alignment is not held, the twisting or rotating motion can result in incomplete coverage of the bar surface.  
         [0073]     Referencing back to  FIG. 1  again, each camera will have a lens  14  to collect light reflected from the bar surface. Telecentric lenses (lenses that collect parallel image light rays, as illustrated with  FIG. 7 ) are preferred for a more uniform arc length distribution, even though regular lenses can be used. In addition, cameras  12  may be configured to include a lens iris to control exposure, and further, preferably configured (if included) with the use of a predetermined lens iris setting for improved depth of focus/field in the application.  
         [0074]     With continued reference to  FIG. 1 , the imaging system according to the present invention also includes a line light assembly configured to project a light line belt onto the surface of the metal bar  16 . Preferably, the line light assembly includes a plurality of line lights  6 . These line lights  6  can be individual light sources, such as lasers, or light delivery devices, such as optical fiber lights, as shown in  FIG. 1 . The light delivery devices must work with at least one light source, as shown in  FIG. 1 . More than one light source can be used if higher light density is needed for illumination. For metal bars  16  that travel at very high speed, the cameras may be light starved due to very high line/frame rates equating to a relatively short exposure time. An optical booster  8  may therefore be used for each line light to concentrate the light and increase the light intensity. This booster  8  can be a cylindrical lens or a semi-cylindrical lens. To use the imaging system in accordance with the present invention for metal bars  16  that are at an elevated temperature, the line lights and the boosters must be made of special materials configured to withstand such elevated temperatures. Each line light  6 , for example, may be configured to have its own glass window to serve this purpose. In the case of optical fiber line light, the material that binds the fibers together must be able to withstand high temperature, such as the high temperature epoxy. The boosters  8  must be made of materials that can withstand high temperature, too. Usable materials include glass, Pyrex, crystal, sapphire, and the like.  
         [0075]      FIG. 9  is a top view of the preferred embodiment shown in  FIG. 1 . To cope with light starving, the alignment between the line lights and the cameras is important. As illustrated in  FIG. 9 , the surface of metal bar  16  after the reducing process, before, for example, a descaling process, can be treated as a reflective surface. Therefore, the optical law set forth in equation (3) applies: 
 “incident angle=reflective angle”  EQN (3):  
         [0076]     EQN (3) is preferably used in a preferred embodiment to maximize the reflected light that is captured by the plurality of cameras  12 . The line lights  6  will each emit the light ray  40 , which is boosted by a booster  8  and projected onto the surface of the metal bar  16 . The light ray  40  is reflected to the path  42  and received by the lens  14  and eventually by the camera  12 . Note that in  FIG. 9 , the metal bar  16  travels in the direction  20 . The projected and reflected light rays  40  and  42  form an angle  44 , equally divided by the normal line to the surface of the metal bar  16 . This angle  44  must be as small as possible, due to the illumination problem described above that is associated with a non-flat surface, as illustrated in  FIG. 4 . In  FIG. 4 , the light line  18 ′ and the image line  18  will not overlap on a non-flat surface. The ideal case is for the angle  44  in  FIG. 9  to be 0°. As this is only possible by using a beam splitter, it is less practical to do so when the system is light starving due to inherent power losses imposed by using a beam splitter for example. The highest efficiency a beam splitter can achieve is 25%, assuming a 0% transmission loss. Therefore, the angle  44  is preferably selected so as to be reasonably small, such as 1° or in its neighborhood. If necessary, a reflective mirror  38  can be used to assist in packing the camera and the light for a small angle  44 . This is another reason to use line scan cameras in this application. Line scan cameras only need an image path  42  with a small width, such as from 5 to 30 microns. The angle  44  can be kept very small with this small image path feature.  
         [0077]      FIG. 10  shows in greater detail a portion of the lighting setup of  FIG. 9 . As mentioned above, the angle  44  will not be 0 degrees unless a beam splitter is used. Therefore, each line light  6  must have a substantial width W (item  41  in  FIG. 10 ). One can see that in  FIG. 10  the metal bar  16  has a centerline  46 . The line  48  indicates the 60° mark on the bar surface, starting from the tangential boundary on the left hand side of the bar, as shown in  FIG. 10 , and increasing to the right. One camera must be able to image the metal bar  16  for the upper half to this 60° mark line  48 . In a three-camera embodiment, the above calculations apply. If more cameras are used, the line  48  may represent 45° for a four-camera system, at 36° for a five-camera system, and so forth. If designed symmetrically, the camera can also image the bottom half of the metal bar  16  for 60°. In order to achieve this coverage, the light line width W must be greater than a threshold based on: 
   W≧ 2 ·r ·(1−cos 60°) sin α  EQN (4):         in which r is the bar radius and α is the incident angle (half of the angle  44 ). The 60° can be replaced by another angle if a different numbers of cameras other than three are used in the inventive imaging system. This notion is further illustrated in  FIG. 11 , in which the image line  42  is clearly curved differently, yet covered by the light line  40 . In other words, the image acquisition assembly (e.g., the plurality of cameras in the preferred embodiment) captures an image belt  18  having a first predetermined width over the entire circumference of the surface of the bar  16 . The line light assembly (e.g., the plurality of line light sources in the preferred embodiment) projects a light line belt onto the surface of the bar  16  having a second predetermined width. The line light assembly is disposed and aligned relative to the image acquisition assembly such the image belt falls within the light line belt. Through the foregoing, the problem of non-flat surfaces is overcome.          
         [0079]     Additionally, these line lights must be positioned such that the light intensity as reflected from a point on the bar surface to the camera that covers that point is uniform for all the points on the image belt  18  ( FIG. 1 ). A more detailed illustration is shown in  FIG. 12 . All the illumination must follow the law described in EQN (3).  FIG. 12  illustrates this arrangement for one camera. It should be appreciate that such arrangement may be duplicated for other cameras used in the inventive imaging system. Based on EQN (3), the angle formed by the incident light ray  40  and the reflected light ray  42  must be evenly divided by the surface normal  50 . As in  FIG. 12 , an illuminator  52  preferably includes a curved surface. Illuminator  52  is a device whose emitted light rays (perpendicular to this curved surface at the point of emission) will be reflected by the surface of the bar  16  to the imaging sensor in camera  12  and lens  14  based on EQN (3). Note that curve  52  need not be a circular curve. This curve  52  depends on the distance between the curve  52  and the surface of the bar  16  (i.e., target). Curve  52  may not be a smooth curve if the bar is not circular. Even though an illuminator with curve  52  can be made with modern technologies, such illuminator can only be used with bars  16  at the designated diameter. In some applications it is not practical. An alternative is to simulate such illumination effect with an array of light lines  6  and  8 , as shown in  FIG. 12 . Each combination of light line/booster can be made adjustable such that its direction can be re-pointed as shown by item  54  to accommodate targets with different diameters. The light line approach is also beneficial in the case that the bar  16  is not circular.  
         [0080]      FIG. 13A  is a simplified schematic and block diagram view of another embodiment of a system in accordance with the present invention. This embodiment provides a very easily serviceable cassette containing reflective elements that are packaged in a relatively small space (e.g., 20 to 50 mm) so as to be operable in the small access gap  26  (best shown in  FIG. 5 ) with the workpiece/moving bar  16  being contained and longitudinally moving in direction  20  through conduit  24  or the like.  FIG. 13A  shows line light assembly  6 , optical booster  8 , camera  12 , lens  14 , reflective mirror  38  for the incident/illuminated light ray  40 , a second reflective mirror  38 ′ for the reflected (image) light ray  42  representing the image of the bar surface, and a protection device such as a tube  43  having a first part  43   a  and a second part  43   b  spaced apart and offset from the first part  43   a  along axis “A” to define an access space  43   c . The protection parts  43   a ,  43   b  are configured to protect the relatively fragile imaging and illumination components from the heat, shock (e.g., contact) and other contamination (e.g., particles) originating from moving bar  16 , which may be at an elevated temperature (as described above). Parts  43   a  and  43   b  may be circumferential. Aperture  43   c  may be configured in size and shape to allow entry/exit of illumination light rays  40  and reflected (image) light rays  42 . Protection tube  43  may be formed of metal or other durable material suitable for segregating hot steel bar  16  from the rest of the inventive system.  
         [0081]      FIG. 13B  is a diagrammatic front perspective view showing a plurality of reflective mirrors  38  (illumination directing) configured in a removable cassette  152 .  FIG. 13B  shows eight reflective mirrors  38  supported by a corresponding number of mirror seats  138 . Cassette  152  is removable and is shown in the installed position ( FIG. 13B ) and in a nearly, fully removed position (best shown in  FIG. 13C ). Cassette  152  is shown mounted onto a holder such as a plate  150 . Plate  150  can be linked to other elements of the inventive imaging system through a base plate  154 .  
         [0082]     In the illustrative embodiment, plate  150  is configured to include a sliding groove  156  around an inner perimeter of plate  150 . Cassette  152  includes a plurality of fitting tabs  158  (four shown arranged in diametrically opposed pairs) configured in size and shape to mate with groove  156 . The dimensional tolerance is such that cassette  152 , particularly the mirrors  38  thereof, will be properly aligned with components  14 / 12  and components  6 / 8  when cassette  152  is in the installed position. It should be appreciated that cassette  152  includes the passive components, namely, illumination directing mirrors  38  and image reflecting mirrors  38 ′ (best shown in  FIGS. 13E and 13F ), and thus does not require any connections via cables, power wires or the like to other elements external to cassette  152  that comprise the inventive illumination and imaging system. This provides an advantage inasmuch as the cassette  152  may be removed for cleaning and reinstalled relatively easily due to the absence of such connections.  
         [0083]     Cassette  152  may be maintained in the installed position (i.e., in alignment) through the use of a suitable locking and retention mechanism, such as a simple closure member  153  (shown in phantom line in  FIG. 13B ), having suitable mating features to also slide in groove  156 , and be retained therein (e.g., fasteners). One of ordinary skill in the art will appreciate that there are a wide variety of alternate suitable locking and retention mechanisms.  
         [0084]     In the embodiment of  FIG. 13B , four cameras  12  are used.  
         [0085]      FIG. 13C  is a simplified diagrammatic view of cassette  152  in the removed position. Cassette  152  can be easily removed by first defeating/disabling any locking and retention mechanism  152  that may be in-use, and removing the cassette in the direction of arrow  160 , for example, for servicing (e.g., cleaning, repair, re-alignment). The cassette  152  may be remounted/reinstalled easily by reversing the above-described procedure.  
         [0086]      FIG. 13D  is a diagrammatic, front perspective view of the embodiment of  FIG. 13B  showing a tube-shaped protection device  43   a  and  43   b .  FIG. 13D  also shows locking and retention mechanism  153  in the installed, locked position. In the installed position as shown, the illumination directing mirrors  38  and the image directing mirrors  38 ′ are in alignment with the light line assembly (light source  6  and booster  8 ) and lens  14 /camera  12 , respectively.  
         [0087]      FIG. 13E  is a diagrammatic, side view of the embodiment of  FIG. 13B , with the cassette  152  in the installed position.  FIG. 13E  shows image directing mirrors  38 ′ in alignment with lens  14  and camera  12 .  FIG. 13E  also shows the camera viewing gap  43   c  defined in between protection tube portions  43   a  and  43   b.    
         [0088]      FIG. 13F  is a diagrammatic rear perspective view of the embodiment of  FIG. 13B .  FIG. 13F  shows three image directing mirrors  38 ′ in cassette  152  (one mirror  38 ′ for each lens  14 /camera  12  combination). Note, one mirror  38 ′ is obscured in  FIG. 13F , as is the corresponding lens  14 /camera  12  combination.  
         [0089]     Referencing back to  FIG. 1 , a computing unit  10  is coupled to plurality of cameras  12 . The computing unit  10  is configured to receive the image data for a plurality of image belts  18  acquired successively by the cameras  12  as the bar  16  moves along the longitudinal axis in the direction  20  (direction  20  best shown in  FIG. 1 ). Frame grabbers may be used to receive the image signals. The cameras  12  in the system, however, are preferably digital cameras, as described above. The computing unit may comprise one or more computers in order to have enough computing power to process the image data. Image processing hardware may be used in conjunction with the software for faster computing speed. If multiple computers are used, these computers can be linked together through inter-computer links such as TCP/IP or the like.  
         [0090]     In any event, computing unit  10  is configured to process the image data to detect predetermined features of the surface of bar  16 . In a preferred embodiment, the features are surface defects. Thus, the image data will be processed for defects, such defects being shown in exemplary fashion in  FIGS. 14A-14B . The images typically contain both the real defects (e.g., item  302 ) and noise, such as scratch marks (e.g., item  304 ). Image processing algorithms, implemented in computer codes such as C, C++, machine languages, and the like, or implemented in hardware logic, are used to filter out the noise, and to detect the true defects, as shown in  306 . The defects to be identified can be long and have a large aspect ratio, as shown in  FIGS. 15A-15C , where item  308  may be 1000 mm long, and item  310  may indicate a width of 0.050 mm. Or, the defects can be short and have a nearly 1-to-1 aspect ratio, as shown in  FIGS. 16A-16C . These algorithms are known in the art, but will be described generally. A first layer of processing may involve a comparison of local contrast in the image, such as by comparing a first predetermined threshold to the local contrast. A second layer of processing may involve applying a second predetermined threshold to detect the nature of the defect such as size, location, length and width and the like.  
         [0091]     The preferred embodiment described and illustrated in connection with  FIG. 1  will also have protection against dust, water, vibrations, and other damaging factors in a typical metal process plant such as a hot rolling mill or a cold drawing mill.  
         [0092]     Those skilled in the art shall appreciate the possibility of further restrain the bar and separately using three or more single-camera systems in the reducing process line for inspection.  
         [0093]     Those skilled in the art shall also appreciate that covering (e.g., inspection of a portion of the bar surface less than the entire circumference may be useful enough for statistical process control purpose in the reducing process line.  
         [0094]     Those skilled in the art shall also understand that a very high speed (high data rate and high frame rate) area scan camera can be used in place of the line scan cameras if only a certain portion of each of the area scan images is used for processing.  
         [0095]     One can also understand that if the metal bars are at an elevated temperature, an optical filter can be used in conjunction with the lens such that only certain wavelengths in the reflected light rays  42  (in  FIG. 12 ) will be used to carry the surface information of the metal bars. Such wavelengths are those not emitted or not dominantly emitted by the metal bars at the said elevated temperature. For metal bars at or colder than 1,650° C., the wavelength 436 nm can be used. In this case, an interference filter at 436 nm will be used with the lens. This wavelength can vary with the temperature. If the temperature decreases, longer wavelength can be used.  
         [0096]     In a still further variation, the light line assembly may be configured to include a strobe light, wherein the computing unit  10  synchronizes the illumination (i.e., the strobing) with the image capture function performed by the image acquisition assembly (e.g., the cameras  12  in the preferred embodiment).  
         [0097]     In a yet still further embodiment, the computing unit  10  is configured to maintain a running record of the detected defects, including (i) a respective location of each detected defect relative to a “start” position, such as the leading end, on the bar  16  being manufactured through processes that mechanically reduce the cross-sectional area of the metal bars; (ii) a respective notation of the nature of the detected defect, such as the size, shape, contrast; and (iii) optionally, an actual image of the site of and surrounding the detected defect. The record may be useful to the supplier/manufacturer, for example, for determining an up-front discount, and may be provided to the customer (e.g., on a diskette or other electronic means) for use in further processing, for example, what portions of the bar to avoid or do follow-up work on.