Abstract:
An inspection system for identifying defects on the surface of an item includes an information processor mounted on a base assembly. A tray is used to move the item to an inspection station on the base assembly, and an illuminator is provided at the inspection station to illuminate the item from different visual perspectives. Importantly, the illuminator includes a plurality of different light sources. An N number of cameras and an M number of image processors are operated in concert to collect image data from the illuminated item. This image data is then analyzed using the image processors to compare the image data with a template image to detect defects in the item. In the operation of the inspection system, the tray, the illuminator, the cameras and the image processors are all centrally controlled and coordinated by an central information processor.

Description:
[0001]    This application is a continuation-in-part of application Ser. No. 09/553,986 filed Apr. 20, 2000, which is currently pending and is a continuation-in-part of application Ser. No. 09/305,608 filed May 5, 1999, which is currently pending. The contents of application Ser. No. 09/553,986 and application Ser. No. 09/305,608 are incorporated herein by reference.  
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention pertains generally to inspection systems. More particularly, the present invention pertains to inspection systems that rely on optical imaging techniques for evaluation of the item being inspected. The present invention is particularly, but not exclusively, useful as an inspection system that is controlled in its operation by a central information processor.  
         BACKGROUND OF THE INVENTION  
         [0003]    Like most everything, the visual inspection of an item has some advantages and some disadvantages. On the plus side, a visual inspection can be quick, accurate and effective for many purposes. Also, visual inspections can be accomplished without dismantling or otherwise altering the item being inspected. Visual inspections are, however, limited by the type, nature and intensity of the illumination that is used. Further, they are significantly dependent on the acuity and resolution that can be obtained using optical instruments. Nevertheless, for many applications, visual inspections are preferred.  
           [0004]    For applications where the sequential inspection of a series of items is required, optical, or visual inspection methods may be desirable for several reasons. Specifically, such methods are desirable whenever they can be used to increase the speed of the inspection process without sacrificing the quality of the inspection. Both of these factors, i.e. speed and quality, become particularly important when the human inspector is replaced by optical equipment such as cameras and image processors.  
           [0005]    When cameras are used for inspection purposes, there is an inherent trade-off between image resolution and cost. Specifically, cameras that give better image resolution are generally more costly. Further, when image processors are used to recreate and analyze an image that has been taken by a camera, there is an inherent trade-off between processing time and cost. In this case, more costly image processors realize shorter processing times. Optimally, an inspection system would use as many cameras as necessary and as many image processors as necessary. This, however, is not always possible. Therefore, there is a need for an inspection system which has the flexibility that is necessary to properly balance the requirements for cameras and image processors for a particular inspection application.  
           [0006]    As mentioned above, in order for there to be a valid inspection of an item, it is necessary that the item be properly illuminated. In some instances, this may require that the item be illuminated from different angles, in order to produce different visual perspectives for the inspection. Also, the movement of the item to and from the inspection station needs to be coordinated and properly controlled. In any event, in order for there to be an effective visual inspection of an individual item, or of a continuous series of items, there are several disparate tasks that need to be coordinated.  
           [0007]    In light of the above, it is an object of the present invention to provide an inspection system that has a central control for properly placing an item to be inspected, for properly illuminating the item, for collecting image data from the item, and for the analyzing the image data. It is another object of the present invention to provide an inspection system that has the flexibility to vary the number of cameras, or the number of image processors, to achieve a desirable inspection procedure. Still another object of the present invention is to provide an inspection system that has the flexibility to illuminate an item from different angles using a number of different light sources, to create different visual perspectives of the item. Yet another object of the present invention is to provide an inspection system that is relatively easy to manufacture, is simple to use and is comparatively cost effective.  
         SUMMARY OF THE PREFERRED EMBODIMENTS  
         [0008]    In accordance with the present invention, an inspection system for identifying defects on the surface of an item includes an information processor that is mounted on a base assembly. Importantly, the information processor functions as a central control facility for the functioning and operation of the inspection system. In this capacity, the information processor coordinates the operation of several separate subassemblies in the system. It is an important aspect of the present invention that the information processor will control each of the subassemblies so that they function, in concert with each other, to evaluate and inspect an item for manufacturing defects.  
           [0009]    In addition to the information processor, the subassemblies that comprise the inspection system of the present invention are also mounted on the base assembly. One such subassembly is a tray, or conveyor belt, that is used for holding the item that is to be inspected. Additionally, the tray is electronically connected to the information processor. With this connection, the tray can be selectively moved in response to instructions from the information processor to position the item at a specified inspection station on the base assembly.  
           [0010]    Another subassembly of the inspection system that is mounted on the base assembly, and that is also electronically connected to the information processor, is an illuminator. As used for the present invention, the illuminator includes several light sources for illuminating the item while it is at the inspection station. In order to illuminate the item from different perspectives, light from different light sources is directed toward the item along different, selected beam paths. Specifically, the illuminator has one light source that directs light along a beam path that is substantially normal to the surface of the item being inspected. The illuminator also has another light source for directing light along a low angle beam path toward the surface of the item being inspected. For purposes of the present invention, this low angle beam path is inclined from the other beam path at a predetermined angle that is in a range from approximately seventy five degrees to approximately eighty five degrees (75°-85°).  
           [0011]    In another embodiment of the present invention, the illuminator may include a plurality of light sources that are arranged as an annulus. The plurality of light sources are separated into groups, with the light sources in each group being arranged as an array of columns and rows. The arrays can then be positioned in a circle so that one row of light sources in each of the arrays will cooperate with other rows of light sources in other arrays to create an annulus of light sources. Thus, there can be a plurality of annulae in a circle, and there can be also be a plurality of circles. In any case, light from the plurality of light sources are directed toward the item along different, selected beam paths in order to illuminate the item from different perspectives.  
           [0012]    Once the item being inspected has been moved to the inspection station and illuminated, image data of the item is obtained using a plurality of cameras that are electronically interconnected with a plurality of image processors. As envisioned for the inspection system of the present invention, depending on the degree of resolution and the speed of data acquisition that is desired, there can be an N number of cameras and an M number of image processors. In any case, it is preferable that the N number of cameras are focused onto the surface of the item along paths that are substantially normal to the surface. More specifically, this is accomplished using a lens assembly that focuses the cameras to predetermined portions of the surface of the item. Thus, depending on which light source of the illuminator is being used, the cameras are able to collect image data from the item being inspected from different perspectives.  
           [0013]    Importantly, like the tray and the illuminator, the cameras and the image processors are also directly responsive to instructions from the information processor. Accordingly, with instructions from the information processor the inspection system of the present invention is capable of positioning an item, illuminating the item to establish a visual perspective of the item, and then using the cameras and image processors to collect image data from the item. Specifically, the image processors will use the image data to analyze and evaluate the item by comparing it with a template image or in accordance with predefined rules to detect defects in the item.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0015]    [0015]FIG. 1 is a perspective view of the inspection system of the present invention;  
         [0016]    [0016]FIG. 2 is a perspective view of the gantry assembly of the inspection system;  
         [0017]    [0017]FIG. 3 is a cross-sectional schematic view of the subassemblies in the gantry assembly as seen along the line  3 - 3  in FIG. 2;  
         [0018]    [0018]FIG. 4 is a perspective view of a system of an alternative embodiment of the present invention with portions broken away for clarity;  
         [0019]    [0019]FIG. 5 is a plan view of a circle of light sources of the alternative embodiment as would be seen along a line  5 - 5  in FIG. 4;  
         [0020]    [0020]FIG. 6 is a schematic view of an incident light beam path and a reflected light beam path;  
         [0021]    [0021]FIG. 7 is a schematic view of light beams from different light sources being reflected from different points on a curved surface along substantially the same reflected beam path;  
         [0022]    [0022]FIG. 8 is a schematic block diagram of the electronic connections between the various subassemblies and components of the inspection system;  
         [0023]    [0023]FIG. 9 is a perspective view of an item in FIG. 7 having a contoured surface, with light rays in a beam from a common light source being reflected from points on the surface having the same gradient; as shown, light reflected from these points will travel along substantially the same reflected beam path; and  
         [0024]    [0024]FIG. 10 is an image of points on the item shown in FIG. 9 which all have the same gradient and which therefore have reflected light along the reflected beam path.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    Referring initially to FIG. 1, an inspection system in accordance with the present invention is shown and is generally designated  10 . As shown, the inspection system  10  includes a base assembly  12  and at least one gantry assembly  14  that is mounted on the base assembly  12 . The gantries  14   a - c  are only exemplary as it will be appreciated there can be one or several such gantries  14 . FIG. 1 also shows that the inspection system  10  includes a tray  16  that is used for holding an item  18  on the base assembly  12 . It should be noted that tray  16  may hold a plurality of items  18  depending on a particular need. System  10  also includes a tray positioning motor  20  that is used for moving the tray  16  back and forth along the guide rails  22   a  and  22   b.  More specifically, the motor  20  moves the tray  16  between a loading position (as shown in FIG. 1) and an inspection position wherein the tray  16  is located under the gantries  14 . The system  10  may also include a vacuum system (not shown) which will assist in holding the item  18  on the tray  16 .  
         [0026]    It is an important aspect of the present invention that the inspection system  10  include various control components and support equipment. Specifically, it is intended that these components and equipment be modular, that they be individually and selectively incorporated into the system  10  and that, when so incorporated, they can be interconnected for centralized control. For these purposes, it is to be appreciated that such components can be mounted either on the base assembly  12  or on one of the gantry assemblies  14 . For the moment, considering only the base assembly  12 , it is seen in FIG. 1 that a variety of individual modular components can be mounted on the base assembly  12 . By way of example, components that are to be mounted on the base assembly  12  can include: an information processor  24 , a general power supply  26  (See FIG. 8), a tray motor controller  30 , a display power supply  32 , a vacuum valve controller  34 , a part loader  36 , and an image processor  38 . Actually, the inspection system  10  contemplates there will be a relatively large number (M) of image processors  38  mounted on the base assembly  12 . Importantly, the plurality of M image processors  38  will share low data rate information for operation in concert with the image processors  38 , and with the controller/power supply  26 .  
         [0027]    As indicated above, in addition to the components that are mounted on the base assembly  12 , other components of the system  10  are mounted on the gantries  14 . For example, FIG. 2 shows that the gantry  14   a  includes a frame  40 , and that a camera assembly  42  and a lens assembly  44  are mounted on the frame  40 . In particular, the camera assembly  42  and lens assembly  44  are mounted on the gantry  14   a  such that the camera assembly  42  is positioned directly above the item  18  whenever the tray  16  moves the item  18  into an inspection position. Further, FIG. 2 shows that when the item  18  is located in its inspection position under gantries  14 , it is preferable that item  18  be selectively illuminated by three different light sources, from three different perspectives. This is only exemplary as there may be fewer or more light sources depending on the particular need. It also should be noted that the type of light sources used for the present invention can be different from each other, such as using a polarized light source with a monochromatic light source. In any case, these light sources include a coaxial lighting assembly  46  for directing light in a direction toward the item  18  that is substantially perpendicular to the surface of the item  18 . Additionally, there are two low angle lighting assemblies  48   a  and  48   b  that direct light toward the item  18  at a slant angle, α. As envisioned for the present invention, the angle α will preferably be in a range that is from about seventy five degrees to eighty five degrees (75°-85°). The interaction of the components of inspection system  10  that are mounted on the gantry  14   a  will be best appreciated with reference to FIG. 3.  
         [0028]    In FIG. 3 it can be seen that, in addition to the components mentioned above, a beam splitter  50  and an iris  52  are also mounted on the gantry  14   a.  Thus, the beam splitter  50 , in combination with the coaxial lighting assembly  46  and the low angle lighting assemblies  48   a  and  48   b  will function in concert, or individually, as an illuminator for the item  18 . Specifically, light from the coaxial lighting assembly  46  is directed as a beam  54  toward the beam splitter  50 . At the beam splitter  50 , a portion of the light in beam  54  is redirected toward the item  18 . This light, in turn, is reflected from the item  18  as a beam  56  that travels toward the camera assembly  42  on a path that is coaxial with the focusing axis of cameras in the camera assembly  42 . On the other hand, light from the low angle lighting assembly  48   a  will travel as a light beam  58  on a path that is inclined at a slant angle, α, to the focusing axis of cameras in the camera assembly  42 . Light from the light beam  58 , however, will be reflected from the item  18  as part of the light beam  56 . In substantially the same manner, light from the low angle lighting assembly  48   b  will travel toward the item  18  as a light beam  60  on an inclined path. The light beam  60  will then also join light beam  56 . In any event, the light in light beam  56  will be used by cameras  62  in the camera assembly  42  to create images of the item  18 . As will be appreciated by those skilled in the art, depending on the source of light (i.e. coaxial lighting assembly  46 , or low angle lighting assemblies  48   a,b ), different perspectives of the item  18  can be visualized.  
         [0029]    In an alternative embodiment of the present invention, referring to FIG. 4, the system  10  includes at least one light circle  64  and, preferably, will include additional light circles, such as the light circles  66  and  68 . For purposes of the present invention, the light circles  64 ,  66  and  68  will be vertically stacked and lie in respective planes that are substantially parallel to each other. As shown, the light circles  64 ,  66  and  68  will have progressively increasing diameters, with light circle  64  having the smallest diameter, light circle  66  having an intermediate sized diameter and light circle  68  having the largest diameter.  
         [0030]    As best appreciated by reference to FIG. 5, where the light circle  66  is generally shown for exemplary purposes, each of the light circles  64 ,  66  and  68  will include a plurality of light arrays  70 . For the present invention, each light array  70  will include a plurality of light sources  72 , and the light sources  72  in each group will be arranged in groups of columns  74  and rows  76 . As so arranged, each row  76  of each array  70  will cooperate with a respective row  76  in each of the other arrays  70  to establish an annulus (ring)  78  of light sources. By cross referencing FIG. 5 with FIG. 4, it will be appreciated that the light circles  64 ,  66  and  68  of the system  10  generally form a hemispherical dome of light sources  72  which is made up of a plurality of substantially parallel annulae (rings)  78 . Preferably, each light source  72  is a light-emitting diode (LED) that is capable of generating a beam  82  of semi-collimated light.  
         [0031]    In FIG. 4, a particular light source  72  in light circle  64  has been selected for discussion purposes. It is to be appreciated from the above disclosure, however, that the system  10  actually includes a plethora of light sources  72  which are all arranged in a plurality of annulae  78 . Further, it is to be appreciated that, with this arrangement, all of the light sources  72  in a selected annulus  78  will lie in a plane. More specifically, the light source  72  in the annulus  78  will be equidistant from an axis  80  that is perpendicular to the plane of the annulus  78 . Thus, the light source  72  will generate a cone of light when directed toward a common point on the axis  80 .  
         [0032]    With the above in mind, and returning to the consideration of a single light source  72 , it is seen in FIG. 4 that a light source  72  generates a semi-collimated beam of light  82  which is directed toward an item  18  along an incident beam path  84 . The beam  82  is then reflected from the item  18  along a reflected beam path  86  (axis  80 ) toward a camera  62 . As shown in FIG. 4, the angle between the incident beam path  84  and the reflected beam path  86  is designated θ. It is an important aspect of this alternative embodiment that all of the light sources  72  in the system  10  are directed toward the item  18 . Thus, although the respective azimuthal angle will be different for each light source  72  in a particular annulus  78 , the incident beam paths  84  from all light sources  72  in a particular annulus  78  will be at the same angle θ with respect to the reflected beam path  86 . There are, of course, a plurality of annulae  78 . Accordingly, light sources  72  in different annulae  78  will have a slightly different angle θ between their respective incident beam paths  84  and the reflected beam path  86 . It is another important aspect of the present invention that all of the semi-collimated light beams  82  from all of the light sources  72  in the system  10 , regardless of their respective angles θ, will have substantially the same reflected beam path  86  toward a particular camera  62 . As indicated above, to accomplish this, each annulus  78  of light sources  72  will lie in a respective plane which is substantially perpendicular to the reflected beam path  86 . Further, all of the light sources  72  in the same annulus  78  will be substantially equidistant from the reflected beam path  86 . If more than one camera  62  is employed, the position of each camera  62  will define its own particular reflected beam path  86 , and will establish a respective angle θ for the incident beam paths  84  from which it will receive light beams  82  from the various light sources  72 .  
         [0033]    For purposes of disclosure, the phenomenon of the specular or quasispecular reflection of light is schematically depicted in FIG. 6. From FIG. 6 it is to be understood that on any surface  88 , a normal  90  can be established at any arbitrary point  92  on the surface  88 . By definition, the normal  90  at a point  92  is perpendicular to the surface  88  at the point  92 . Further, in accordance with the laws of physics, a light beam  82  that is directed toward the point  92  along an incident beam path  84 , will be reflected from the point  92  on surface  88  along a reflected beam path  86  such that the incident beam path  84 , the normal  90 , and the reflected beam path  86  are coplanar. Also, it must happen that the incident beam path  84  and the reflected beam path  86  make equal angles θ/2 with the normal  90 .  
         [0034]    In FIG. 6, the surface  88  is shown to be flat. In FIG. 7, however, the surface  88  is shown to be contoured. In each case, regardless whether the surface  88  is flat or contoured, each point  92  on the surface  88  will have its own respective normal  90 . As shown in FIG. 7, normals  90  at different points  92  on a contoured surface  88  will have different orientations. For example, the normal  90 ′ at point  92 ′ has a different orientation in space than does the normal  90 ″ at point  92 ″. On the other hand, for purposes of discussion, if the surface between point  92 ′ and point  92 ″ was flat (i.e. in the same plane) the normal  90 ′ would be parallel to the normal  90 ″. It happens, as discussed above, that the orientation of a normal  90  at any particular point  92  on a surface  88  can be mathematically expressed as a gradient, ∇θ, where ∇θ=(i∂/∂x+j∂/∂y+k∂/∂z)θ. With this in mind, the physical laws of reflection of light are employed by the system  10  to image gradient profiles on a surface  88  for all points  92  which have the same gradient, ∇θ.  
       OPERATION  
       [0035]    The overall operation of the inspection system  10  will, perhaps, be best appreciated with reference to FIG. 8. There it will be seen that the information processor  24  is effectively connected to operate in concert with all of the other components of the system  10 . Thus, once the information processor  24  instructs the part loader  36  to position a part (e.g. item  18 ) on the tray  16 , the information processor  24  will activate the vacuum valve  34  and instruct the tray motor controller  30  to reposition the tray  16  as required. After the item  18  has been moved to its inspection position under the gantry(ies)  14 , the information processor  24  will instruct the light controllers  28  to appropriately activate the coaxial lighting assembly  46  and the low angle lighting assemblies  48   a,b.  As indicated above, the lighting assemblies  46  and  48   a,b  can be selectively activated individually, or in various combinations, to establish different visual perspectives for the item  18 . Once the item  18  has been properly illuminated, cameras  62  in the camera assembly  42  can be operated with instructions from the information processor  24  to collect image data from the item  18 .  
         [0036]    For purposes of the present invention, the cameras  62   a  and  62   b  shown in FIG. 8 are only exemplary and, it is to be appreciated that there can be a relatively large number, N, of cameras  62 . Further, it is to be appreciated that the number of cameras  62  (N) need not be the same as the number of image processors  38  (M). Instead, N and M can be selected according to the functional and operational specifications that are set for the inspection system  10 .  
         [0037]    When the image data has been collected from the item  18  by camera assembly  42 , the image data will be transferred from the camera assembly  42  to predetermined image processors  38 . The image processors  38 , in turn, compare and evaluate the image data with respect to pre-selected standards, such as an image template. Based on these comparisons the inspection system  10  is able to provide valuable information about the structure, integrity, configuration and constitution of the item  18 .  
         [0038]    In the operation of the system  10  of the alternative embodiment of the present invention, a camera  62  is positioned at a fixed predetermined distance  94  from an item  18 . The light sources  72  in the various annulae  78  of light circles  64 ,  66 , and  68  are then selectively activated to illuminate the item  18 . For example, in FIG. 7, a light source  72   a  (perhaps from light circle  66 ) and a light source  72   b  (perhaps from light circle  68 ) are shown directing semi-collimated light beams along respective incident beam paths  84 ′ and  84 ″ toward the surface  88  of item  18 . In the context of the present invention, although the individual effect of only one light source  72  may be discussed, it is to be understood that the discussion applies equally to all of the other light sources  72  in the same respective annulus  78 . Also, as indicated above, the light beams  82  from each light source  72  are preferably semi-collimated. For some applications, however, it may be desirable for the light from the light source  72  to be perfectly or nearly perfectly collimated. With this in mind, the term “semi-collimated” contemplates nearly parallel light rays which diverge through a relatively small angle. In order to control the diffusion of semi-collimated light, diffusers  96   a  and  96   b,  as shown by way of example in FIG. 7, are respectively associated with light sources  72   a  and  72   b.  Specifically, the diffuser  96   b  of light source  72   b  is shown to spread light from the light source  72  through an angle α. For the present invention, the maximum value for the angle α, is preferably in the range of from ten to sixty degrees (10°- 60°). Recall, however, that collimated light may be useful for some applications. If so, α will be equal to zero.  
         [0039]    By being slightly diffused, the semi-collimated light from light source  72   a,  which travels along the incident beam path  84 ′, will be incident on both the point  92 ′ and  92 ″ of surface  88  (see FIG. 7). Likewise, semi-collimated light from light source  72   b,  which travels along the incident beam path  84 ″, will be incident on both the point  92 ′ and  92 ″ of surface  88 . As intended for the present invention, however, in order for the camera  62  to image either of the points  92 ′ or  92 ″, the camera  62  must receive light that is reflected from the particular point  92  along the reflected beam path  86 ′ or  86 ″ (note: for purposes of the present invention, the reflected beam paths  86 ′ and  86 ″ can be considered to be coincident). As shown in FIG. 7, the point  92 ′ has a normal  90 ′ and the point  92 ″ has a normal  90 ″. Due to the curvature and contour of the surface  88 , however, the normals  90 ′ and  90 ″ are not parallel to each other. In accordance with the physical laws discussed above, this means that in order for camera  62  to receive light from point  92 ′, the normal  90 ′ at point  92 ′ will dictate the angle θ′ that is required between the reflected beam path  86 ′ and the incident beam path  84 ′. In turn this will determine where the light source  72   a  should be located. Similarly, in order for camera  62  to receive light from point  92 ″, the normal  90 ″ at point  92 ″ will dictate the angle θ″ that is required. In turn this will determine where the light source  72   b  should be located. For most applications the reflection angle θ will be somewhere in the range between zero and ninety degrees (0°- 90°).  
         [0040]    The consequence of the fact that each point  92  on the surface  88  will have its own normal  90  is that with a predetermined reflected beam path  86  which is established by the location of the camera  62 , the orientation of the normal  90  will determine the reflection angle θ for the point  92  and, hence, the required orientation for the incident beam path  84 . For example, in FIG. 7, a light beam  82  from light source  72   a  will be reflected toward the camera  62  along only a reflected beam path  86  from the point  92 ′. But the angle between the reflected beam path  86  and the normal  90 ′ at point  92 ′ is θ′/2. Thus, reflected light from point  92 ′ will travel along reflected beam path  86  only when the incident beam path  84 ′ is at an angle θ′/2 from the normal  90 ′. Stated differently, the camera  62  will image point  92 ′, but not the point  92 ″, with light from the light source  72   a.  Likewise, light from light source  72   b  will be reflected toward the camera  62  along the reflected beam path  86  from the point  92 ″ only for a particular orientation of the incident beam path  84 ″. Again, the orientation of the incident beam path  84 ″ is dependent on the orientation of the normal  90 ″ at the point  92 ″ and the magnitude of the consequent angles θ″/2. In this case, the camera  62  will image the point  92 ″, but not the point  92 ′, when only the light source  72   b  is illuminated. Recall, the light source  72   a  and the light source  72   b  will be in different annulae  78  of the system  10  of the alternative embodiment. Consequently, by selectively operating the light sources  72  in the different annulae  78 , points  92  on surface  88  can be imaged in isolation. Specifically, all points  92  on the surface  88  which have normals  90  with the same orientation (i.e. the same gradient ∇θ) will be imaged. The result is an image of a gradient profile of the surface  88 .  
         [0041]    In order to appreciate how an image profile of an item  18  can be obtained in accordance with the present invention, consider FIGS. 9 and 10. For purposes of inspecting the item  18 , it may be of utmost importance to determine the magnitude of the distance  98  across the item  18 , or to determine the integrity and continuity of ridges, crests, shoulders, edges, corners or other portions of a surface  88  where there is a change in contour. For the moment, however, consider the distance  98 .  
         [0042]    By way of example, the distance  98  across the item  18  can be determined in accordance with the present invention by measuring the distance between specular reflections from the edge  100  and edge  102  of item  18 . This, however, requires that points on edge  100  (e.g. points  92   a  and  92   b ) have the same gradient ∇θ as points on the edge  102  (e.g. point  92   c ). If the gradient ∇θ at the points  92   a - c  is the same, all of the reflected beam paths  86   a - c  will be directed from these points  92   a - c  on the item  18  toward the camera  62 . From this it follows that all of the normals  90   a - c  are substantially parallel to each other, and each of the normals  90   a - c  is inclined at a same angle θ/2 from their respective reflected beam path  86   a - c.  Consequently, the points  92   a - c  and all other points on the edges  100  and  102  having the same gradient ∇θ will be imaged by camera  62 . This happens when the incident beam paths  84   a - c  all come from light sources  72  such that the angle between each of the incident beam paths  84   a - c  and their respective normals  90   a - c  is equal to θ/2. Thus, in summary, by knowing the location of camera  62 , and by selecting a particular annulus  78 , a reflection angle θ between the incident beam paths  84  and the reflected beam path  86  can be established. Then, upon activation of the light sources  72  in the annulus  78 , all points  92  on the surface  88  of item  18  which have the same gradient ∇θ will be imaged by the camera  62 .  
         [0043]    [0043]FIG. 10 is a representation of a gradient profile from the item  18  under the conditions which were established above during the discussion of FIG. 9. Importantly, from the lines  104  and  106  in FIG. 10 it is possible to measure the distance  98  with an accuracy that may not be possible with other means. Accordingly, compliance with predetermined standards can be ascertained for inspection purposes. Furthermore, it can be appreciated that a blemish  108  (see FIG. 9) will cause discontinuities in the gradient profile, due to the irregular disruption of normals  90 , and can be easily detectable.  
         [0044]    It should be noted that in the example given above, all points  92  having the same gradient ∇θ showed up as lines  104  and  106 . This was because the points  92  were all located on edges where the gradient ∇θ was changing. Recall, a flat surface will have the same gradient ∇θ at all points on the surface. A specular reflection from a flat surface would then be imaged as an area rather than a line.  
         [0045]    While the particular Linked Cameras and Processors for Imaging System as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.