Abstract:
A method and an apparatus for an optical inspection of an object, having upper and lower faces, so as to detect defects existing on the object. First and second beams of an incident radiation are produced and directed onto the object. A light component of the first incident beam, which is reflected from one face of the object, and a light component of the second incident beam, which is transmitted through the upper and lower faces of the object, are simultaneously sensed. First and second images, formed, respectively by reflected and transmitted light components are acquired and analyzed so as to provide data indicative of the defects.

Description:
FIELD OF THE INVENTION 
     The present invention is in the field of optical inspection techniques and relates to a method and a system for inspecting patterned objects such as, for example, photomasks, printed circuit boards (PCBs) or the like. 
     BACKGROUND OF THE INVENTION 
     There is a great variety of optical inspection systems having a common goal for locating defects existing on the patterned surface of an inspected object. The term “patterned surface” signifies such a surface which is formed with regions having different optical properties in respect of an incident radiation. 
     An inspection system of the kind specified typically comprises means for illuminating an object to be inspected, acquiring images formed by light reflected from the illuminated object, and image processing. However, if the inspected object is a photomask, flexible printed circuit board (PCB) or the like, whose patterned surface typically comprises transparent and opaque regions, the acquired images formed of light reflected from the illuminated surface are not indicative of such ‘defects’ as foreign particles, for example, of dirt or dust, which may occasionally be located in the transparent regions. Indeed, it is known that a surface of such particle is not mirror like, and, accordingly, light returned from the particle is irregularly reflected, scattered light. The problem is very essential when using the photomask as a phototool in PCB, graphic arts and printing industries. 
     There have recently been developed methods and systems wherein the inspection is performed by means of illuminating an object and acquiring and processing images formed of reflected and transmitted beams of light. Such systems are disclosed, for example, in U.S. Pat. Nos. 5,572,598 and 5,563,702. The systems in both patents employ a so-called ‘scanning technique’, wherein an illuminating laser beam is generated and focused onto a pixel defining spot on the surface of an object to be inspected. The illuminated beam is deflected in an oscillatory fashion so as to sweep the spot across the inspected surface. The system is adapted for three different modes of operation. According to the first and second modes, so-called “Transmitted Light Inspection Mode” and “Reflected Light Inspection Mode”, the object is point-by-point inspected by means of detecting either transmitted or reflected light, respectively. These modes of operation are timely separated. The third mode of operation, which is aimed at defects classification, is based on detecting both reflected and transmitted beams of light. A single laser beam of incident radiation is directed onto the patterned surface of an object through light deflection means and is either reflected or transmitted, or partly reflected and partly transmitted by the object. This intensity of the incident beam is determined before its interaction with the object. Two separate detectors are accommodated at opposite sides of the object and detect transmitted and reflected beams resulting from this interaction. To this end, the system comprises separate directing optics for receiving the transmitted and reflected beams, respectively, and directing them onto the detectors. 
     This approach is based on that the interaction of an incident beam with an object to be inspected causes changes in beam&#39;s intensity, which changes depend on reflectivity and transmission of the respective region of the object. Hence, by appropriately detecting the intensities of the incident beams and reflected and transmitted beams, respectively, before and after the interaction, each inspectable point, or pixel, on the surface can be represented in a so-called ‘T-R space’, namely by a point with coordinates corresponding to the transmitted and reflected signal values produced at that point. 
     However, the system requires very complicated arrangements for illuminating and collecting optics. Indeed, the illuminating arrangement should be provided with the light deflection means and detector appropriately accommodated in the optical path of the incident beam so as to determine the beam intensity prior to the interaction with the object. This complicates and extends the optical path of the incident beam. Moreover, the use of a single beam of incident radiation results in an unavoidable requirement for locating the collecting optics, as well as the detectors for sensing the reflected and transmitted beams, at opposite sides of the object. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide novel method and apparatus for automatic optical inspection of an object by means of detecting reflected and transmitted light components of an incident radiation. 
     There is provided, according to one aspect of the invention, a method for an optical inspection of an object, having upper and lower face, so as to detect defects existing on the object, the method comprising the steps of; 
     (a) providing first and second beams of an incident radiation; 
     (b) directing the first beam of the incident radiation onto the object and sensing a light component reflected from one face of the object; 
     (c) directing the second beam of the incident radiation onto the object and sensing a light component transmitted through the upper and lower faces of the object; 
     (d) simultaneously acquiring first and second images of the object, wherein the first image is formed by the reflected light component and the second image is formed by the Transmitted light component; and 
     (e) analyzing said first and second images so as to provide data indicative of said defects. 
     The term “defects” used herewith signifies certain undesirable conditions of the object such as, for example, existence of foreign particles located on the object. 
     Thus, the idea of the present invention is based on the following main features. The first and second beams of incident radiation are produced and directed towards the object for focusing them onto the upper face. It is understood that, generally, each of the incident beams can be both reflected and transmitted by the different regions of the object. In other words, each of the incident beams, depending on the region of its interaction with the object, may be partly transmitted and partly reflected resulting in, respectively, transmitted and reflected light components. To this end, what is actually detected by two image sensors are, respectively, that light component of the first incident beam which is reflected from the object and that light component of the second incident beam which is transmitted through the object. 
     Generally, the first and second incident beams can be directed onto the object from the same side thereof, that is from either the upper or the lower face. In this case the sensors and associated directing optics are located. at opposite sides of the object. It should be noted that it is advantageous to illuminate the object from opposite faces. This enables the sensors to be placed at one side of the object and, accordingly, a common directing optics to be employed for receiving both the reflected and transmitted light components and directing them onto the respective sensors. 
     Thus, the reflected and transmitted light components are, preferably, directed onto the different sensors via a common optical system appropriately accommodated in the optical paths of the both light components. It is understood that, accordingly, means should be provided for successfully separating the different light components so as to be sensed by the different image sensors. To this end, two alternating embodiments of the invention are exemplified. 
     According to one embodiment, the first and second beams of the incident radiation are simultaneously directed onto different portions of the object. More specifically, they illuminate, respectively, first and second spaced-apart, parallel, identical strips of the upper face. The relationship between the two illuminated strips and the common optical system is such that the strips extend symmetrically relative to the optical axis of the common optical system. The optical system actually projects the strips onto the first and second image sensors, which are, preferably, line sensors. Hence, a pair of spaced, parallel lines of the object is imaged, It is understood that the dimensions of the line are defined by a field of view of the image sensor, the width a of the line being substantially smaller than that of the illuminated strip. The space between the two illuminated strips is adjusted so as to minimize an overlap region between the two images. The space d between the two imaged lines satisfies the following condition: 
     
       
         
           d=n·a  
         
       
     
     wherein n is an integer such that not. 
     According to an alternative embodiment of the invention, the first and second beams of the incident radiation illuminate the same portion of the upper face, which portion is in the form of a strip. To this end, the first and second beams of the incident radiation are formed of light having either different wavelengths, or different polarizations. In the case of different wavelengths, the common optical system comprises a suitable beam splitter, for example, a dichroic beam splitter. In the case of the different polatizations, the common optical system is provided with appropriate beam polarizer device based, for example, on a double refraction effect. 
     The first and second beams of the incident radiation may be produced by either two light sources, or by a single light source adapted for generating a beam of light. If the single light source is employed, the generated beam is directed towards the object via a beam splitter, which splits it into the first and second beams of the incident radiation. 
     Preferably, the image sensors are of a kind adapted to receive a light signal and provide an electric output representative thereof. For example, charge coupled device (CCD) cameras, or bidirectional time delay integration (IDI) sensor may be employed. 
     In order to successively inspect the whole object, it is supported for sliding movement within an inspection plane along two orthogonally oriented axes. It is appreciated that in order to allow for so-called “double side” illumination, the object may be supported on a transparent slab, or, alternatively, on a frame supporting solely the periphery region of the object. As a result of the inspection, each point on the upper face of the object is represented by so-called ‘reflected’ and ‘transmitted’ images. Comparing these images to each other enables to locate the defects, if any, on the object. For that purpose, the output signals provided by the image sensors are transmitted to a processor operated by a suitable software for comparing the first and second images to each other. 
     According to another aspect of the present invention there is provided an apparatus for an optical inspection of an object, having upper and lower faces, so as to detect defects existing on the object, the apparatus comprising: 
     (i) an illumination system for producing first and second beams of an incident radiation and simultaneously directing them onto the object; 
     (ii) a sensing system mounted for sensing a light component of the first incident beam reflected from the upper face and a light component of the second incident beam transmitted through the upper and lower faces of the object, and for providing output signals representative thereof, 
     (iv) a light directing system accommodated in optical paths of the reflected and transmitted light components for directing them onto the sensing system; and 
     (v) a processor coupled to the sensing system for receiving and analyzing said output signals representative of the reflected and transmitted light components and for providing data indicative of said defects. 
     Thus, the present invention enables to inspect an object by simultaneously illuminating it by two incident beams of light and detecting reflected and transmitted light components of these incident beams, respectively. In other words, in comparison to the above U.S. patents, each point of the inspected object is represented by both “T-map” and “R-map”, i.e. “transmitted image” and “reflected image”. This simplifies the analyzing procedure. Additionally, owing to the provision of the common optical system for directing the reflected and transmitted beams onto the different sensors, and the above solutions for successful separation between the reflected and transmitted beams, the construction and operation of the apparatus can be significantly simplified. 
     More specifically the present invention is used for inspecting a photomask which is typically in the form of a polished transparent substrate whose upper surface has a plurality of spaced-apart regions coated by a thin opaque layer such as, for example, chromium. The upper surface of the photomask represents a pattern in the form of transparent and opaque regions. Defects, which can be detected by the method according to the invention, may also be in the form of through-holes in the opaque regions and/or width variation of these regions. It is understood that the opaque and transparent regions of a photomask would be represented by bright and dark regions in the ‘reflected image’ and by dark and bright regions, respectively, in the ‘transmitted image’. If a foreign particle is located in the transparent region, it will appear as a bright spot on a dark background in the reflected image and vice versa in the transmitted image. If a foreign particle is located in the opaque region, solely the image sensor responsive to the reflected light component will detect it. Such particle will appear as a dimmer spot on the bright background in the reflected image. The other kinds of defects such as, for example, through-holes in the opaque regions or width variation of these regions will be detected by both the reflected and transmitted beams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how the same may be carried out in practice, several preferred embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram illustrating the main components of an apparatus for optical inspection of a patterned object, according to one embodiment of the invention; 
     FIGS. 2 and 3 more specifically illustrate the main principles of operation of the apparatus of FIG. 1; 
     FIG. 4 is a graphic illustration of the main principles of operation of an optical system of the apparatus of FIG. 1; 
     FIGS.  5   a  to  5   f  are schematic illustrations of images of the illuminated portion of an upper surface of the object during sequential steps of operation of the apparatus of FIG. 1; 
     FIGS.  6   a  and  6   b  are schematic illustrations of two images of a region of the upper surfaces of the object; and 
     FIG. 7 is a block diagram illustrating the main components of an inspection apparatus, according to another embodiment of the invention; and 
     FIG. 8 is a block diagram illustrating the main components of an inspection apparatus, according to yet another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is illustrated an apparatus, generally designated  10 , for inspecting a photomask  12 . The propagation of beams of light is shown here schematically solely in order to facilitate the understanding of the main principles of the construction of the apparatus  10 . The photomask  12  is typically in the form of a polished transparent substrate having upper and lower surfaces  12   a  and  12   b,  respectively. The upper surface  12   a  is formed with a pattern (not shown) having a plurality of spaced-apart regions coated by a thin opaque layer such as, for example, chromium. In other words, surface  12   a  is formed with transparent and opaque regions. The photomask  12  is supported at its periphery region on a frame  14  mounted for sliding movement along mutually orthogonal axes x and y. Alternatively, a sliding base formed of a transparent material may be employed for the same purpose of slidingly displacing the photomask  12  within an inspection plane in a manner to provide illumination access to the lower surface  12   b.    
     The apparatus  10  comprises an illumination assembly, generally at  16  mounted at the upper side of the photomask  12  for illuminating its upper surface  12   a.  The assembly  16  includes a light source  18 , producing a beam of light  18   a,  and an optical system  20 , accommodated in the optical path of the beam  18   a.  The optical system  20  includes an anamorphic optics  22  typically comprising a cylindrical lens or plurality of such lenses, which are not specifically shown, a beam splitter  24  and an objective lens  26 . All these components of the optical system  20  are well known per se and, therefore, need not be described in more detail, except to note that they enable a desired shape of the beam  18   a  to be obtained and focussed onto the photomask  12 . As shown, the beam  18   a  illuminates a strip S r  of the upper surface  12   a.    
     Further provided is an illumination assembly, generally designated  28 , mounted at the lower side of the photomask  12  for illuminating its upper surface  12   a.  Similarly, the assembly  20  includes a light source  30  producing a beam of light  30   a,  and an optical system  32  accommodated in the optical path of the beam  30   a.  The optical system  32  comprises an anamorphic optics  34 , a mirror  36  and a condenser lens  38 . The beam  30   a,  being transmitted through the transparent lower surface  12   b,  illuminates a strip S t  of the upper surface  12   a.  It will be understood that the provision of the mirror  36  is optional and depends solely on the location of the light source  30  relative to the surface  12   b.    
     As further schematically shown in FIG.  1  and more specifically in FIG. 2, the beam  18   a  impinges onto the surface  12   a  and is reflected from reflective regions, if any, disposed within the strip S r , resulting in a reflected beam  40 . The incident beam  30   a,  being transmitted through the transparent lower surface  12   b  of the photomask  12 , impinges onto the upper surface  12   a  and is transmitted through transparent regions, if any, within the strip S t , producing a transmitted beam  42 . 
     An optical system  44 , typically comprising a collecting lens or plurality of such lenses (not shown), is located at the upper side of the photomask  12  so as to be in the optical paths of both the reflected beam  40  and transmitted beam  42 . The system  44  has its optical axis shown in dotted line OA. The system  44  directs the beams  40  and  42  onto line sensors  46  and  48 , respectively, thereby projecting two geometrically separated strips S r  and S t  into two imaged lines L r  and L t . The image L r  is formed by light reflected from the strip S r  illuminated by the beam  18   a,  while the image L 1  is formed by light transmitted through illuminated strip S t . As more specifically illustrated in FIG. 3, in order to render the image quality at both sensors equal, the configuration is such that the illuminated strips S r  and S t  extend symmetrically relative to the optical axis OA. 
     It is understood, although not specifically illustrated, that the dimensions of the imaged lines L r  and L t  are defined by the field of view of each of the sensors  46  and  48  and are substantially smaller than those of the strips S r  and S t . The sensors  46  and  48  are of a kind adapted for receiving light signals and generating electrical outputs corresponding thereto such as, for example, a conventional line-type CCD camera. 
     FIG. 4 illustrates the intensity distributions of the beams  40  and  42  which are in the form of two lobes  50  and  52 , respectively. It is appreciated that the spacing between the two illuminated strips S r  and S t  is arranged so as to minimize an overlap region  54 , thereby reducing crosstalk between the two images. 
     Turning back to FIG. 1, coupled to the sensors  46  and  48  is a processor  56  receiving the electrical outputs of the sensors  46  and  48 . The processor  56  is operated by suitable software carrying out an image processing technique capable of analyzing the imaged lines L r  and L t  by means of comparing the electrical outputs to each other and of providing information indicative of defects, if any, on the photomask  12 . The electrical outputs may also be compared with corresponding reference data which may be stored in a database of the processor  56  or derived from another photomask or from another part of the same photomask being inspected. The construction and operation of the processor  56  do not form a part of the present invention and therefore need not be more specifically described. The information generated by the image processor  56  is output to a computer device  58  and displayed on its screen  58   a.    
     Alternatively, although not specifically shown, the processor  56  and the computer device  58  may be combined in one integral unit. The light sources  18  and  30  may be replaced by a single light source for generating a beam of radiation, in which case the generated beam is directed onto the inspected photomask via a beam splitter so as to be split into two separate beams for illuminating the photomask from opposing sides. 
     The operation of the apparatus  10  will now be described with reference to FIGS.  5   a - 5   f,  partly illustrating images of the upper surface  12   a  of the photomask  12  during the inspection. Initially, two strips are simultaneously illuminated (not shown) in the above described manner and two lines Lr 1  and Lt 1  are imaged. The lines Lr 1  and Lt 1  are identical having the same width a and length b and are aligned in a spaced-apart, parallel relationship along the y axis. A space d between the lines Lr 1  and Lt 1  is defined as follows: 
     
       
           d=n·a   (1)  
       
     
     wherein n is an integer n≧t, being equal to 1 in the present example. 
     At a next operational stage, the support frame  14  moves the photomask a certain preset step H 1  in a direction D 1  along the axis y, which step satisfies the following condition: 
     
       
           H   1   =n   1   ·a   (2)  
       
     
     wherein n 1  is an integer n 1 ≧1, being equal to 1 in the present example. A further pair of lines Lr 2  and Lt 2  is imaged by the sensors  46  and  48 , respectively, and corresponding electrical outputs are transmitted to the processor  56 . Meanwhile, the sliding movement of the frame  14  in the direction D 1  results in a further displacement of the photomask the same step H 1 , and a pair of lines Lr 3  and Lt 3  is imaged. As clearly seen in FIG.  5   c,  the lines Lt 1  and Lr 3  coincide, which means that the corresponding strip of the surface  12   a  has now been sequentially illuminated by the beams  30   a  and  18   a.  FIGS.  5   d  and  5   e  illustrate, in a self-explanatory manner, the sequential increase of the number of such imaged lines corresponding to those strips illuminated by both beams of the incident radiation. 
     Hence, a slice, generally at B i , of the surface  12   a  is strip-by-strip inspected by step-by-step displacing the photomask  12  in the direction D 1  along the axis y. It is understood that the beginning of the inspection is stipulated such that the lines L r ′-L r ″ and L t ′-L t ″ corresponding to those strips illuminated by either of the beams  18   a  or  30   a,  respectively, are associated with a so-called ‘margin’, non-patterned region of the surface  12   a.    
     In order to inspect an adjacent slice B i+1  of the surface  12   a,  the sliding frame  14  is moved a preset step H 2  in a direction D 2  along tile axis x, which step H 2  is defined as follows: 
     
       
         H 2 =b  (3)  
       
     
     Thereafter, the photomask  12  is step-by-step displaced the same distance H 1  in a direction D 3  along the axis y. As shown, in the pair of simultaneously imaged lines L r  and L t  of the slice B i+1  the ‘reflected’ and ‘transmitted’ images are located in a reverse relationship relative to the direction of the displacement of the photomask, in comparison to that of the pair of simultaneously imaged lines L r  and L t  of the slice B i . To this end, the image processor  56  is provided with a suitable software for controlling its operation so as to consider the respective changes in the direction of movement of the photomask  12 . Additionally, although not specifically shown, optical sensors may be appropriately accommodated at either side of the frame  14 . 
     It is important to note that if a pair of time delay integrated (IDI) sensors is employed as the imaging sensors  46  and  48 , they should be of the so-called ‘bi-directional’ kind. The construction and operation of such a ‘bi-directional’ TDI sensor are well known per se and do not form a part of the present invention. 
     Turning now to FIGS.  6   a  and  6   b,  there are more specifically illustrated the imaged lines Lt 1  and Lr 3 , which correspond to the same illuminated strip on the surface  12   a,  which strip is sequentially illuminated by the beams  30   a  and  18   a.  It is assumed that the portion of the upper surface within the illuminated strip includes both transparent and opaque regions, generally designated  60  and  62 , and that foreign particles  64  and  66  are located, respectively, in the transparent and opaque regions  60  and  62 . As clearly shown, the transparent and opaque regions  60  and  62  are in the form of bright and dark areas, respectively, in the ‘transmitted’ image Lt 1  (FIG.  6   a ), while being in the form of dark and bright areas, respectively, in the ‘reflected’ image Lr 3  (FIG.  6 b). As for the foreign particles, it is known that a surface of such particle is not mirror like, and, accordingly, fight returned from the particle is irregularly reflected, scattered light. Therefore, both the transmitted and reflected beams  40  and  42  are indicative of the existence of the particle  64  located within the transparent region. The particle  64  appears as light fall-off, i.e. dark spot on the bright background  60 , in the ‘transmitted’ image Lt 1  and as a brighter spot on the dark background  60  in the ‘reflected’ image Lr 3 . The existence of the particle  66  located on the opaque region  62  may be detected solely by the reflected beam  40 , appearing as a dimmer spot on the bright background in the ‘reflected’ image Lr 3 . 
     It is also appreciated that, should the plane of location of the particle  64  be identified, namely the upper or the lower surface of the photomask  12 , this may be achieved by slightly shifting the upper Surface  12   a  along the axis OA, so as to be out of the focal plane, and detecting the changes in the electrical output. Additionally, it is understood, although not specifically shown, that both the ‘reflected’ and ‘transmitted’ images will be indicative of such ‘defects’ as through-holes in the opaque regions and missed chromium coating presenting a so-called ‘width variation defect’. 
     During the movement of the photomask  12  along the axes y and x as described above, the processor  56 , to which the electrical signals generated by the sensors  46  and  48  are continuously fed, analyzes these electric signals and produces data indicative of the condition of the photomask  12 . The processed data may be in the form of a list showing in respect of each ‘defect’ its type and coordinates, which list is displayed on the screen  58   a.    
     Reference is now made to FIG. 7 illustrating the main components of an apparatus, generally designated  100 , which is constructed and operated according to another embodiment of the present invention. Those components which are identical in the apparatuses  10  and  100  are indicated by the same reference numbers, in order to facilitate understanding. The apparatus  100  inspects the photomask  12  supported on the sliding frame  14 . Two illumination assemblies  116  and  128  are provided for illuminating the upper surface  12 a of the photomask  12  from its opposite sides. The assemblies  116  and  128  are generally similar to those of the apparatus  10 , each comprising a light source for emitting a beam of incident radiation and a suitable optical system accommodated in the optical path of the emitted beam. In distinction to the apparatus  10 , the light sources  118  and  130  produce, respectively, light beams  118   a  and  130   a  of different wavelengths λ 1  and λ 2 . The beam  118   a  is directed through the optical system  20  onto the surface  12   a  so as to illuminate a strip St and be reflected from opaque regions, if any, producing a reflected beam  140 . The light beam  130   a,  in turn, passes through the optical system  32  so as to impinge onto the surface  12   a  and illuminate the same strip S rt , producing a transmitted beam  142 . The reflected and transmitted beams  140  and  142  are projected via an optical system  144  onto the image sensors  46  and  48 , respectively. To this end, the system  144 , in addition to the collecting lens  44 , comprises a dichroic beam splitter  145 . The dichroic beam splitter is a well known color-selective device which is widely used for transmitting a particular band of spectral energy and reflecting any other. 
     It will be readily understood, although not specifically shown, that the operation of the apparatus  100  is generally similar to that of the apparatus  10 . Each illuminated strip St is projected by the optical system  144  into two imaged lines (not shown). The photomask  12  is sequentially displaced along the axis y a certain preset step. It is appreciated that this step is, preferably, equal to the width of the imaged line so as to, on the one hand, avoid an overlap between the images and, on the other hand, speed up the inspection. Upon inspecting a slice of the photomask, the latter is displaced along the axis x a certain step which is, preferably, equal to the length of the imaged line. 
     Referring to FIG. 8, there is illustrated an apparatus  200  constructed and operated according to yet another embodiment of the invention. Similarly, those components which are identical in the above is described embodiments and the apparatus  200  are indicated by the same reference numbers. The apparatus  200  is capable of illuminating a strip S of the upper surface  12   a  of the photomask  12  by two beams of incident radiation  218   a  and  230   a  having different polarizations. To this end, the optical systems  220  and  232  comprise beam polarizer devices  234  and  236  accommodated in the optical paths of the beams  218   a  and  230   a,  respectively. Alternatively, each of the light sources  218  and  230  may be of a kind adapted for producing a polarized light beam. Hence, reflected and transmitted beams  240  and  242  are of different polarizations. The dichroic beam splitter  145  of FIG. 7 is replaced by a beam polarizer device  245  of a kind capable of splitting the different polarizations. Such beam polarizer devices are known, typically comprising a polarization sensitive medium, for example, in the form of a birefringent cell or multi-layered dielectric structure. It is appreciated that light component returned from a foreign particle located in the opaque region of the upper surface of the photomask, would be, due to reflection and diffraction effects, a depolarized scattered forward light. This increases the contrast of particle&#39;s appearance on the bright background in the ‘reflected’ image. 
     Those skilled in the art will readily appreciate that various modifications and changes may be applied to the preferred embodiments of the invention as hereinbefore exemplified without departing from its scope as defined in and by the appended claims. In the method claims which follow, characters which are used to designate claim steps are provided for convenience only and do not apply any particular order of performing the steps.