Abstract:
An illumination system including at least one reflector subtending an angle with respect to a location on a surface of an article, and first and second light sources, the first and second light sources each providing a light output, the light outputs from both of the first and second light sources being directed to impinge on the location on the surface of an article within the angle, at least one of the light outputs being reflected by the reflector.

Description:
RELATED APPLICATION  
       [0001]    The present application is a division of U.S. patent application Ser. No. 09/565,500, filed May 5, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to visual inspection of surfaces of articles, and more particularly to an illuminator for the automated optical inspection of ball grid array substrates, lead frames and printed circuit boards.  
         BACKGROUND OF THE INVENTION  
         [0003]    Apparatus and methods useful for illuminating substantially flat patterned surfaces of articles, such as electrical circuits on printed circuit boards (PCBs), ball grid array substrates (BGAs) reticles, semiconductors and other similar articles, during the automated optical inspection thereof are well known in the art.  
           [0004]    During automatic optical inspection of flat patterned surfaces of articles, such as electrical circuits on PCBs and BGAs, the surface is illuminated by intense broad spectrum illumination while the article is transported beneath a sensor, such as a CCD or TDI camera. Conventionally, the sensors acquire scanned gray level images of the surface. Various materials which appear on the surface of an article each have different reflective properties and reflect illumination at a different level of intensity. For example, copper which defines conductors, various metal platings on the conductors, and the substrate itself each have different reflective properties. The reflected intensities in the image are sensed and automatically processed and analyzed to determine the presence of defects in the patterns on the surfaces.  
           [0005]    The surfaces of electrical circuits being inspected, although substantially flat, generally exhibit a topographical relief that results both from the cross-sectional configuration of conductors as well as the surface microstructure thereof. Typically, very intense illumination impinging on a surface of an article being inspected over a solid angle of incidence is employed to mitigate negative affects of the topographical relief  
           [0006]    The following patents are believed to represent the state of the art in high intensity illumination for the inspection of substantially flat patterned article surfaces such as electrical circuits on printed circuit boards (PCBs), ball grid array substrates (BGAs), lead frames, reticles and semiconductors:  
           [0007]    U.S. Pat. No. 4,421,410 to Karasaki describes an illuminator comprising a half reflecting mirror disposed above a printed wiring board. Concentrated light is reflected off the mirror and directed onto the wiring board at an angle substantially normal to the surface of the board and diffuse light emanating from fiber optics is simultaneously directed onto the surface at a large angle of incidence. An image of a line on the surface is transmitted through the mirror to a sensor.  
           [0008]    U.S. Pat. No. 4,877,326 to Chadwick describes a high intensity illuminator providing focused quasi lambertian illumination to a region of the surface of an article to be inspected. The illuminator includes a half reflecting mirror, first and second and third elliptical cylindrical reflectors and first, second and third lamps, all having mutually parallel elongate axes. Two of the reflectors and two of the light sources are spaced from each other and illuminate the surface with focused light at a large angle of incidence. The half reflecting mirror, the third reflector, and the third lamp are arranged to reflect focused light along an axis normal to the surface to fill the gap between the first and second reflectors. Each lamp is located at one focus of a reflector, and the illuminated region is located at the second focus of the reflectors. Forth and fifth planar reflectors are provided at the longitudinal ends of the first, second and third reflectors, and a sensor is provided to image the illuminated region by sensing light reflected therefrom which passes through the half reflecting mirror.  
           [0009]    Israel patent 81450 in the name of Orbotech Ltd. describes a high intensity illuminator similar in structure to that described in U.S. Pat. No. 4,877,326, but employing light supplied via fiber optics, and effectively having a numerical aperture substantially smaller than the numerical aperture of the apparatus described in U.S. Pat. No. 4,877,326.  
           [0010]    U.S. Pat. No. 5,058,982 to Katzir describes a high intensity illuminator comprising a beam splitter cube and first, second and third elliptical cylindrical reflectors, two of which are spaced from each other. Light received via fiber optics is provided at one focus of each reflector. The reflectors are oriented so that the second focus of each reflector illuminates a region of a surface to be inspected. A third illuminator and a condensing lens are provided and oriented to direct light through the beam splitter and onto the surface to be inspected along an axis normal thereto. A sensor is oriented to receive light reflected from the surface via the beam splitter cube.  
           [0011]    U.S. Pat. No. 5,153,668 to Katzir describes an illuminator for illuminating an area to be inspected on the surface of an article, in which the illumination is configured to be substantially circularly symmetric over a solid angle around an optical axis normal to the surface. A sensor is provided to image the surface through a gap between the illuminators.  
           [0012]    U.S. Pat. No. 4,801,810 to Koso describes an elliptical reflector, comprising approximately one half of an elliptical cylinder, which is used to illuminate the surface of a printed circuit board. The axis of the elliptical cylinder is oblique to the surface of the printed circuit board. A lamp is disposed under the reflector at one focus of the ellipse, while the region illuminated is located at the other focus. An imaging system images the illuminated region through an aperture formed in the reflector.  
           [0013]    The illuminator employed in Inspire™ automated optical systems manufactured and sold by Orbotech Ltd., and described in copending PCT application PCT/IL98/00285 (unpublished), is a high intensity illuminator comprising a first light source that emits light over a continuous wide angle of illumination toward a surface of an article to be inspected and has a blocking element that blocks a portion of the continuous angle of illumination to form two separate portions of illumination. A second light source is employed to supply illumination to the region blocked by the blocking element. Concentrating optics are provided to concentrate the illumination onto the article. The illumination is provided at a first angle to the normal, and an imaging sensor is provided to image the illuminated region of the article at a second angle to the normal.  
           [0014]    Systems which image a surface to be inspected along an imaging axis that is normal to the surface being inspected typically employ at least three separate sources of illumination. In systems that employ less than three sources of illumination to provide an intense solid angle of illumination, the surface is imaged along an axis which is oriented at a non-normal angle thereto.  
           [0015]    Additionally, conventional systems that illuminate and image a surface to be inspected along an axis that is normal to the surface being illuminated acquire images via beam splitting apparatus that introduces undesired aberrations into the image.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention generally seeks to provide high intensity illumination for the automated optical inspection of patterned articles in which two light sources together provide a solid angle of illumination, the spatial uniformity of which is adjustable.  
           [0017]    One aspect of a preferred embodiment of the invention provides illumination of a region on the surface of an article over a solid angle using two independently adjustable sources of illumination, wherein the axis of at least one source of illumination is substantially normal to the surface. Preferably, the region illuminated is imaged by a sensor whose axis of imaging is also substantially normal to the surface.  
           [0018]    According to another aspect of a preferred embodiment of the invention, a region of an article to be inspected is illuminated with a first illuminator, including a reflective surface which is apertured at a location which overlies the region to be inspected. Supplemental illumination of the region is provided through the aperture and a sensor images the region to be inspected by sensing light reflected therefrom through the aperture.  
           [0019]    According to still another aspect of a preferred embodiment of the invention, an imaging system is provided in which the surface of the article is illuminated by a solid angle of illumination to illuminate a line, and wherein a sensor is provided to image the illuminated region in a manner that avoids transmitting reflected light from the region through a beam splitter or through a partially reflected mirror en route to the sensor.  
           [0020]    Preferably, a partially reflective planar mirror is oriented along an imaging axis normal to the surface. Illumination is transmitted through the partially reflective planar mirror to provide illumination along an axis normal to the surface. Light reflected from the surface is reflected by the partially reflective mirror into the sensor for imaging the surface..  
           [0021]    In accordance with still another aspect of a preferred embodiment of the invention, an illuminator is provided to illuminate a linear region along a surface of an article during automated optical inspection thereof. The illuminator is configured to provide a broad angle of illumination from a multiplicity of illumination sources. At least one illumination source provides illumination along an axis which is substantially normal to the surface. Illumination from that source is passed through an optical element that introduces astigmatism into the illumination along an axis of astigmatism that is substantially colinear to the linear region, thereby smearing the illumination along the illuminated linear region.  
           [0022]    Another aspect of a preferred embodiment of the invention provides an illuminator for illuminating a linear region along a surface of a substrate with a broad angle of illumination. The illumination is provided by a plurality of sources, wherein a first source of illumination provides illumination along an axis that is substantially normal to the surface of an article. The illumination provided by the first source is passed through an optical assembly configured to introduce astigmatism into the illumination provided by the first source. The astigmatism is oriented along an axis that is substantially colinear to the linear region being illuminated. Preferably, an imaging sensor is provided to image the linear illuminated region at least partially along an axis that is normal to the surface of the article.  
           [0023]    According to still another aspect of the invention, there is provided a substantially telecentric imaging system for imaging a region of a surface of an object during automated optical inspection thereof. The system comprises a plurality of lenses and sensors operative simultaneously to acquire a plurality of images of mutually overlapping regions. Preferably, the maximum angle at which any pixel is imaged is less than approximately 5° and preferably less than 3.7° to the normal with respect to the surface. The imaging path of at least one of the sensors is preferably folded by a folding mirror which is located in a region adjacent to a waist in the imaging paths of the other sensors.  
           [0024]    According to another aspect of the invention, there is provided an inspection system for automatically inspecting patterned articles. Illumination in a first spectral range is provided on one surface of the article and illumination in a second spectral range, distinguishable from the first range, is provided on the opposite surface of the article. Sensors are employed to sense the intensity of reflected light in the first spectral range, and to sense the intensity of transmitted light in the second spectral range. Patterns on the surface of the article are inspected based on reflected light intensity in the first spectral range. The presence and pattern of desired and undesired apertures in the article are inspected based on transmitted light in the second spectral range.  
           [0025]    There is thus provided in accordance with a preferred embodiment of the present invention an illumination system including at least one reflector subtending an angle with respect to a location on a surface of an article, and first and second light sources, the first and second light sources each providing a light output, the light outputs from both of the first and second light sources being directed to impinge on the location on the surface of an article within the angle, at least one of the light outputs being reflected by the reflector.  
           [0026]    Further in accordance with a preferred embodiment of the present invention the reflector is formed with a light transmissive region to permit a light output from a first one of a the first and second light sources to pass therethrough. Preferably the light transmissive region is an aperture formed in the reflector. Preferably the illumination system also includes an optical element disposed intermediate the first one of the first and second light sources and the aperture for directing the light output of the first one of the first and second light sources through the aperture. Additionally the optical element also includes at least one reflecting surface.  
           [0027]    Alternatively the illumination system may also include an optical assembly disposed along an optical path extending between the optical element and the location on the surface of an article for directly light reflected from the location on the surface of an article to a light sensor. Preferably the optical assembly is generally transmissive to the light output of the first one of the first and second light sources directed through the aperture onto the location on the surface of an article and generally reflective of light reflected from the location on the surface of an article.  
           [0028]    Furthermore the illumination system may also include a reflective surface disposed intermediate a second one of the first and second light sources and the reflector for directing the light output of the second one of the first and second light sources onto the reflector.  
           [0029]    Additionally in accordance with a preferred embodiment of the present invention the at least one reflector is an elliptical reflector and the second one of the first and second light sources and the location on the surface of an article are each located at or near a locus of the elliptical reflector. Preferably the at least one reflector includes two spaced apart reflectors which together define a section of an ellipse. The spaced apart reflectors are preferably separated by an aperture.  
           [0030]    Alternatively the first and second light sources may also be independently controllable light sources.  
           [0031]    There is also provided in accordance with a preferred embodiment of the present invention an illumination system including a cylindrical reflector extending along a first longitudinal axis and having an axial aperture formed therein which extends along the longitudinal axis, a first elongate light source extending along a second longitudinal axis, generally parallel to the first longitudinal axis and being arranged to illuminate the cylindrical reflector, such that light from the first elongate light source is directed onto a plane generally along a third longitudinal axis, generally parallel to the first and second longitudinal axes, a second elongate light source extending along a fourth longitudinal axis, generally parallel to the first, second and third longitudinal axes, the second elongate light source being arranged with respect to the axial aperture of the cylindrical reflector such that light from the second elongate light source is directed through the axial aperture onto the plane generally along the third longitudinal axis, and an optical element disposed along a light path between the second elongate light source and the plane and being operative to pass light from the second elongate light source to the plane and to reflect light from the plane to an optical sensor.  
           [0032]    Further in accordance with a preferred embodiment of the present invention the first elongate light source comprises an elongate light generating element and an elongate reflector extending along a fifth longitudinal axis, generally parallel to the first, second, third and fourth longitudinal axes and being operative to reflect light from the elongate light generating element onto the cylindrical reflector.  
           [0033]    There is also provided in accordance with yet another preferred embodiment in accordance with a preferred embodiment of the present invention an illumination system including at least two elongate light sources extending along at least two light source longitudinal light source axes, each of the at least two elongate light sources being arranged to illuminate a plane generally along a longitudinal illumination axis, generally parallel to the at least two light source longitudinal axes, and an optical element disposed along a light path between at least one of the at least two elongate light sources and having astigmatism, the optical element being operative to cause light from the at least one of the at least two elongate light sources to be smeared along the longitudinal illumination axis.  
           [0034]    There is also provided in accordance with another preferred embodiment of the present invention an illumination system including a light source arranged to illuminate an object, a plurality of lenses imaging light from the light source reflected by the object onto a plurality of light receivers, the physical sizes of the plurality of lenses and the spacing of the plurality of light receivers being such that the plurality of lenses cannot all lie in a single plane, folding optics arranged along light paths between the plurality of lenses and the plurality of light receivers and being arranged such that notwithstanding different physical separations between ones of the plurality of lenses and corresponding ones of the plurality of light receives the light paths between the ones of the plurality of lenses and the corresponding ones of the plurality of light receivers are substantially identical.  
           [0035]    Further in accordance with a preferred embodiment of the present invention the plurality of lenses have substantially identical focal lengths.  
           [0036]    Additionally or alternatively the plurality of lenses image at least partially overlapping regions on the object.  
           [0037]    There is further provided in accordance with a preferred embodiment of the present invention an inspection device including first and second light sources arranged to illuminate a substrate having reflective features thereon, the first and second light sources providing light outputs having wavelength spectra which are at mutually distinguishable, and at least one light receiver receiving light from the first light source reflected from the substrate and receiving light from the second light source transmitted via the substrate.  
           [0038]    There is further provided in accordance with a preferred embodiment of the present invention imaging apparatus including an illuminator illuminating a region on the surface of an article. A plurality of sensors is associated with a plurality of quasi-telecentric lenses, and the sensors and lenses are arranged to simultaneously image mutually overlapping portions of the illuminated region. Preferably, principal rays imaging each part of the portion deviate less than ±10° from the normal, and more preferably they deviate less than or equal to 3.5° from the normal.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings, in which:  
         [0040]    [0040]FIG. 1 is a simplified schematic illustration of an illumination and imaging system constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0041]    [0041]FIG. 2 is a simplified side view of the illumination and imaging system shown in FIG. 1;  
         [0042]    [0042]FIG. 3 is a simplified schematic illustration of a quasi-telecentric imaging system for simultaneously imaging mutually overlapping regions using multiple lenses in accordance with a preferred embodiment of the present invention;  
         [0043]    [0043]FIG. 4 is a simplified ray diagram of the quasi telecentric imaging system shown in FIG. 3;  
         [0044]    [0044]FIG. 5 is a simplified side view of a preferred embodiment of a quasi-telecentric imaging system of the type shown in FIG. 3 and FIG. 1; and  
         [0045]    [0045]FIG. 6 is a simplified pictorial illustration of an imaging system for imaging a patterned object having voids, using illumination in distinguishable spectral ranges in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]    Reference is now made to FIG. 1 which is a simplified schematic illustration of an illumination and imaging system  10  constructed and operative in accordance with a preferred embodiment of the present invention, and to FIG. 2 which is a simplified side view of the system of FIG. 1. Illumination and imaging system  10  typically forms part of an automated optical inspection system which comprises a first image processing subsystem running in hardware, preferably as described in copending Israel Patent Application 131092, and a second image processing subsystem running in software as described in copending Israel Patent Application filed concurrently herewith.  
         [0047]    The illumination and imaging system of FIG. 1 can be used for the inspection of patterns on flat surfaces of articles such as electrical circuits. Such electrical circuits, include for example, printed circuit boards, ball grid array substrates, multi-chip modules and semiconductors. Other examples of articles that can be inspected using the system of FIG. 1 include reticles, lead frames and finely engraved and etched metal substrates, as may be found, for example, on medical implants. Reference herein to a patterned article is deemed to include any suitable patterned article.  
         [0048]    Illumination and imaging system  10  is preferably operative to illuminate a portion of a surface  12  of a patterned article which is being inspected. A first reflector  14 , preferably formed from a section of a cylinder having a substantially elliptical cross-section, is oriented with respect to surface  12  so as to subtend an angle α, preferably, approximately ±24° with respect to a location  13  thereon, location  13  being at or near a first focus of first reflector  14 . Reflector  14  includes a centrally disposed light transmissive region, preferably an aperture  16 , overlying location  13 .  
         [0049]    A first illuminator  18  is situated at or near a second focus of first reflector  14 , and is configured and arranged to direct illumination onto first reflector  14  and therefrom onto surface  12  at location  13  to define a linear illuminated portion  20  of surface  12 . First illuminator  18  preferably includes a bundle of 80 micron optical fibers available from Schott Corporation of Germany which are fanned out to form a fiber optic emitter preferably having dimensions of 1 mm by 100 mm. First illuminator  18  is preferably fed by a halogen projection lamp.  
         [0050]    Preferably a planar folding mirror  22 , operative to fold illumination emitted by first illuminator  18 , is disposed between reflector  16  and first illuminator  18  as shown in FIG. 1.  
         [0051]    A second reflector  24 , preferably formed from a section of a cylinder having a substantially elliptical cross-section, is disposed and arranged to overlie aperture  16  so that a first focus of reflector  24  lies at or near linear illuminated portion  20 .  
         [0052]    A second illuminator  26 , the intensity of which preferably is adjustable independently of the intensity of first illuminator  18 , is provided at a second focus of reflector  24 , and is arranged to direct illumination onto second reflector  24  and from second reflector  24  through aperture  16  onto linear illuminated region  20 . Second reflector  24  and second illuminator  26  preferably are configured and arranged so that illumination from second illuminator  26  slightly overfills aperture  16 . As a result, a solid angle of illumination, part of which is provided by illuminator  18  and part of which is provided by illuminator  26 , fills the entire extent of angle a.  
         [0053]    At least one, and preferably three, imaging sensors  30 , are arranged to image linear illuminated region  20  via aperture  16 . Quasi-telecentric imaging optics  32 , described hereinbelow in greater detail with reference to FIG. 4-FIG. 6, preferably are provided. Sensors  30  may be CCD or TDI sensors, and are preferably KLI 2103 Red, Green and Blue sensor units, available from Eastman Kodak. These sensors are preferably operative to acquire polychromatic images and output reflective intensities for Red, Green and Blue spectra in the linear illuminated region  20 . Preferably, patterned surface  12  is positioned on a stage (not shown) which is transported below reflector  14  in a scan direction  34 , while sensors  30  image the linear illuminated region  20 .  
         [0054]    An optical element  40 , preferably a highly optically flat light transmissive plane parallel plate, preferably approximately equal in length to second reflector  24 , is disposed between aperture  16  and second reflector  24 , defines an optical path, part of which extends along an axis  42  normal to surface  12 , along which sensors  30  image linear illuminated region  20 . Preferably, optical element  40  is arranged at a 45° angle relative to the plane of surface  12 , and is provided with a partially silvered surface  44  on the underside thereof. Optical element  40  preferably transmits light received from second illuminator  26  via reflector  24  onto linear illuminated region  20  and directs light that is reflected from linear illuminated region  20  to sensors  30 .  
         [0055]    It is readily appreciated from the preceding discussion that the present invention provides a solid angle of intense illumination to a linear region of a workpiece using only two separately adjustable sources of illumination. First illuminator  18  and second illuminator  26  each provide illumination that is oriented along axis  42 . The solid angle of the illumination provided by illuminator  26  is narrower than that of the illumination provided by illuminator  18 .  
         [0056]    It is a particular feature of the present invention that optical element  40  serves two disparate functions. Principally, it reflects light received from linear illuminated region  20  onto sensors  30  substantially without astigmatism. Secondarily it introduces astigmatism into light transmitted therethrough from reflector  24  onto linear illuminated region  20 . This astigmatism is perpendicular to the longitudinal axis of linear illuminated region  20  and produces desired smearing of the illumination from reflector  24 . The extent of the desired smearing is controlled by suitable orientation and the thickness of the optical element  40  relative to reflector  24  and surface  12 .  
         [0057]    Reference is now made to FIG. 3, which is a simplified schematic illustration of a quasi-telecentric imaging system for simultaneously imaging mutually overlapping regions using multiple lenses in accordance with a preferred embodiment of the present invention.  
         [0058]    In systems for automated optical inspection of surfaces having a non-even surface topography, as is generally present in printed circuit boards, ball grid array substrates, lead frames, reticles, and other etched and engraved surfaces, it is generally desirable to employ an imaging system which has a high degree of telecentricity. A telecentric imaging system is an optical system having an entrance pupil located at infinity. In telecentric imaging systems, the principal ray emanating from each point on a surface being imaged is substantially parallel to the optical axis of the optical system. Consequently, all of the points on the surface are viewed at the same perspective. In order to achieve a high degree of telecentricity, the front element of a lens in the optical system is typically at least as large as its field of view.  
         [0059]    In conventional imaging systems a high degree of telecentricity is typically obtained either by employing very large and expensive lenses to image an entire region, or by employing a plurality of smaller and less expensive lenses wherein each individual smaller lens images a relatively small region and the plurality of lenses together image the entire region.  
         [0060]    Because of the physical size requirements of telecentric lenses as described hereinabove, in conventional multiple telecentric lens configurations, the individual lenses are spaced apart from each other and image non-mutually overlapping regions. Thus, in conventional telecentric imaging systems, in order to image an entire surface, it is necessary either to scan the surface in multiple passes or to stagger multiple rows of telecentric lenses.  
         [0061]    As seen in FIG. 3, a quasi-telecentric imaging system  310  is provided for simultaneously imaging mutually overlapping regions using multiple lenses which are configured and arranged in accordance with a preferred embodiment of the present invention.  
         [0062]    Quasi-telecentric imaging system  310 , includes at least two, and preferably three or more, RGB sensor units  332 ,  334  and  336 , which together make up sensor  30  (FIG. 1). In the preferred arrangement shown, each of the three sensors  332 ,  334  and  336  is operative to simultaneously view a corresponding one of a mutually overlapping viewing region  352 ,  354 , and  356  along surface  12  (FIG. 1), via respective lenses  362 ,  364  and  366  associated therewith. Preferably lenses  362 ,  364  and  366  are Macrosimar 120 mm, F5.6 lenses.  
         [0063]    Sensors  332 ,  334  and  336  and lenses  362 ,  364  and  366  are preferably arranged so that overlapping viewing regions  352 ,  354  and  356  intersect linear illuminated region  20  (FIG. 1). It is readily appreciated that only those parts of viewing regions  352 ,  354  and  356  which fall along linear illuminated region  20  are imaged by sensors  332 ,  334  and  336 .  
         [0064]    In order to explain the structure of FIG. 3, reference is made additionally to FIG. 4, which is a simplified optical diagram of quasi telecentric imaging system  310 . Rays  400  are shown in the diagram as subtending an angle β which is exaggerated for the sake of simplicity and clarity of illustration. In a quasi telecentric imaging system such as that of the present invention, the maximum deviation from the optical axis  42  (FIG. 1), normal to surface  12  (FIG. 1) of the principal rays reaching sensors  332 ,  334  and  336  from portions of respective viewing regions  352 ,  354  and  356  is less than ±5°, and more preferably less than or equal to ±3.7°.  
         [0065]    Each of telecentric lenses  362 ,  364  and  366  is preferably held by a respective mounting, designated by respective reference numerals  462 ,  464  and  466 , and is located an equal optical distance from surface  12  and sensors  432 ,  434  and  436 . Together with their respective mountings, lenses  362 ,  364  and  366  are typically larger than respective viewing regions  352 ,  354  and  356 .  
         [0066]    In order to provide simultaneous viewing of mutually overlapping viewing regions  352 ,  354  and  356  and to accommodate lenses  362 ,  364  and  366  a first planar folding mirror  470  is preferably interposed between telecentric lens  366  and viewing region  354 . A second planar folding mirror  478 , interposed between first planar folding mirror  470  and telecentric lens, is optionally provided. Planar folding mirror  470  is preferably sufficiently large to provide an unhindered view of viewing region  354 , and is arranged to be situated intermediate rays  472  and  476  entering respective lenses  362  and  364  so as not to interfere with their view of mutually overlapping viewing regions  352  and  356 . It is noted that first planar folding mirror  470  folds the rays that are shown in FIG. 4 as extending between overlapping portion  354  and sensor unit  334 . Thus it is appreciated that sensor unit  334  and telecentric lens  366  and mountings  464  lie in a different plane than sensor units  332  and  336 , telecentric lenses  362  and  364 , and mountings  462  and  466  respectively.  
         [0067]    Reference is made to FIG. 5 which is a simplified side view illustration of an illumination and imaging system  510  for illuminating and imaging a linear illuminated region on a flat patterned article  512 . The embodiment of FIG. 5 is an alternative to that of FIGS.  1 - 4  and is believed to be the preferred embodiment. The illumination and imaging system  510  preferably employs quasi-telecentric imaging combined with an illumination system which is identical to that described hereinabove with reference to FIG. 1 and FIG. 2.  
         [0068]    Illumination and imaging system  510  preferably includes a first reflector  514 , preferably formed from a section of a cylinder having a substantially elliptical cross-section, which is oriented with respect to a surface  512  of an article so as to subtend and angle α, preferably ±24°, with respect to a location thereon  513 , which is arranged to be at or near a first focus of reflector  514 . First reflector  514  preferably includes a centrally disposed light transmissive region, preferably an aperture  516 , overlying location  513 .  
         [0069]    A first illuminator  518  is situated at or near a second focus of first reflector  514  and is configured and arranged to direct illumination onto first reflector  514  and therefrom onto surface  512  at location  513  to define a linear illuminated portion  520  of surface  512 . First illuminator  518  is preferably configured and arranged similarly to illuminator  18  as described hereinabove with reference to FIG. 1. Preferably a planar folding mirror  522 , operative to fold illumination emitted by first illuminator  518 , is disposed between reflector  514  and first illuminator  518 .  
         [0070]    A second reflector  524 , preferably formed of a section of a cylinder having a substantially elliptical cross-section, is disposed and arranged to overlie aperture  516  so that a first focus of reflector  524  lies at or near linear illuminated portion  520 .  
         [0071]    A second illuminator  526 , the intensity of which preferably is adjustable independently of the intensity of first illuminator  518 , is provided at a second focus of reflector  524 , and is arranged to direct illumination onto second reflector  524  and from second reflector  524  through aperture  516  onto linear illuminated region  520 . Second reflector  524  and second illuminator  526  preferably are configured and arranged so that illumination from second illuminator  526  slightly overfills aperture  516 .  
         [0072]    Preferably three imaging sensors  530  (of which only two are seen), are arranged to image linear illuminated region  520  via aperture  516 . Sensors  530  may be CCD or TDI sensors, and are preferably KLI 2103 Red, Green and Blue sensor units, available from Eastman Kodak. These sensors are preferably operative to acquire polychromatic images and to output reflective intensities for Red, Green and Blue spectra in the linear illuminated region  20 .  
         [0073]    An optical element  540 , preferably a highly optically flat light transmissive plane parallel plate, is disposed between aperture  516  and second reflector  524 , defines an optical path, part of which extends along an axis  542 , normal to surface  512 , along which sensors  530  image linear illuminated region  520 . Preferably, optical element  540  is arranged at a 45° angle relative to the plane of surface  512 , and is provided with a partially silvered surface  544  on the underside thereof. Optical element  540  preferably transmits light from second illuminator  526  via reflector  524  to linear illuminated region  520  and directs light that is reflected from linear illuminated region  520  to sensors  530 .  
         [0074]    Each sensor  530  is preferably associated with a quasi-telecentric imaging lens  558 . Each sensor  530  is operative to simultaneously image a mutually overlapping portion of linear illuminated region  520 .  
         [0075]    A first planar folding mirror  580  is disposed along a first optical path  582 , shown in broken lines, extending from linear illuminated region  520  to a first sensor  530  via a first lens  558 . A second planar folding mirror  584  is disposed in a second optical path  586 , shown in solid lines, extending between linear illuminated region  520  and a second sensor  530  via a second lens  558 . A third planar folding mirror (not seen in FIG. 5) is disposed in a third optical path (not seen in FIG. 5) which is hidden by optical path  586 , extending between linear illuminated region  520  and a third sensor  530  via a third lens  558 , both of which are not seen in FIG. 5.  
         [0076]    It is noted that first planar folding mirror  580  is preferably sufficiently large to provide an unhindered view of a second portion of linear illuminated region  520  and is arranged to be situated intermediate optical paths entering respective first and third lenses  558  so as not to interfere with their view of mutually overlapping portions of linear illuminated region  520 .  
         [0077]    It is additionally noted that despite the different locations of the first, second and third mirrors, the respective optical paths through the first, second and third lenses  558  are each situated at an equal optical distance from the respective sensor  530 , and each lens  558  is situated at an equal optical distance from linear illuminated region  520 .  
         [0078]    Reference is now made to FIG. 6, which is a simplified pictorial illustration of an imaging system  610  for imaging a patterned object having apertures, using illumination in mutually distinguishable spectral ranges in accordance with a preferred embodiment of the present invention. The imaging system  610  is described herein with reference to the inspection of lead frames, for which its is particularly suited. It is readily appreciated that the imaging system  610  is not limited to use in inspecting lead frames and is generally useful for optically inspecting any suitable patterned surface that is characterized by apertures, voids and through holes.  
         [0079]    Lead frames typically comprise a plurality of leads  612 , which have a width and height dimension and which are typically formed via a suitable pressing, engraving or etching process, and which are typically coated with a metal coating. A typical lead frame is formed of copper, and coated at various locations thereon with silver and gold. Leads  612  are generally optically inspected to determine that each of the leads is separated one from another by a separation  614 , and that no pair of adjacent leads is connected by a bridge  616 .  
         [0080]    It is readily appreciated due to the depth dimension of leads  612 , and due to the existence of non-vertical edges between adjacent leads, which are generally the result of pressing, engraving and etching processes used to manufacture lead frames, the intensity of reflected light in regions  618  between leads  612  is typically considerably less than the intensity of light reflected from the horizontal regions of the leads. Accordingly, the intensity of light reflected from a bridge  616  may be very close to zero as is characteristic of regions having a separation  614  between adjacent leads.  
         [0081]    In a preferred embodiment of the present invention, an illuminator is preferably provided to illuminate a first side  640  of leads  612  in or more spectral ranges which facilitate the detection of combinations of materials commonly present on lead frames. For example, a first illuminator  642  is preferably configured to provide substantially red and green illumination, which together generally facilitate the identification of regions of copper, gold and silver.  
         [0082]    A second illuminator, preferably emitting in a spectral range which is distinguishable from the spectral range of first illuminator  642 , is provided to illuminate a second side  644  of leads  612 . For example, a second illuminator  646  is configured to provide substantially blue illumination, or illumination in a non-visible spectrum, to second side  644 .  
         [0083]    A sensor  650  is preferably provided to view a portion of first side  640 , and is operative to separately sense the intensity of light in each of the spectral ranges emitted by first illuminator  642  and by second illuminator  646 . Thus, sensor  650  is operative to sense the intensity of light provided by first illuminator  642  and reflected by the first side  640 , and to separately sense light provided by second illuminator  646  and transmitted via apertures located in and between leads  612 , for example light transmitted through separations  614 .  
         [0084]    It is readily appreciated that because the spectrum of reflection of light provided by first illuminator  642  is distinguishable from the spectrum of light provided by second illuminator  646 , it is possible to simultaneously provide an output indicating the existence of voids and apertures in and between leads  612 , and to inspect leads for defects which are detectable by analyzing reflected light, for example, defects in coatings on leads  612 .  
         [0085]    It is appreciated that various features of the invention which are, for clarity, described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable combination. The scope of the present invention is not limited to what has been described hereinabove but rather also includes modifications and variations thereof as would occur to persons skilled in the art upon reading the foregoing description and which are not in the prior art.