Abstract:
An illumination system shares portions of an objective of an optical inspection system. A plurality of beam-shaping optics collects light from a plurality of effective light sources and directs the light through a portion of the objective for illuminating an object under inspection. The objective includes a front relay lens, a rear relay lens, and an objective stop disposed between the front and rear relay lenses for collecting light scattered from the object and forming an image of the object with the collected light. The beam-shaping optics, which surround the objective stop, are arranged together with the associated effective light sources for non-uniformly distributing light within a range of angles required for illuminating the object.

Description:
TECHNICAL FIELD 
     The invention arises within the field of metrology as an improvement to illumination systems for optical comparators and relates particularly to illumination systems that share optical paths with imaging systems of optical inspection machines. 
     BACKGROUND OF THE INVENTION 
     Optical comparators project images of objects under inspection onto display screens for comparison against a reference datum. Comparisons to the reference datum can be made in association with different types of illumination, including direct lighting, back lighting, and oblique lighting of the objects. 
     The comparators include optical imaging systems responsible for projecting the images of objects under inspection onto display screens. Objectives can be used to form intermediate images of the objects, and the intermediate images can be magnified by projectors for producing display images capable of comparison to the reference datum. 
     So called “through-the-lens” illuminating systems have been used for illuminating the objects under inspection by directing light through the objectives to the objects. The illuminating light is generally produced by light sources located remote from the optical imaging systems but producing light beams that generally intersect light paths through the optical imaging system. Inclined mirrors of such illuminating systems surrounding aperture stops of the illuminating systems fold the illuminating beams into alignment with the light paths of the imaging systems. 
     Light sources of the type used for such illuminating systems, such as mercury arc lamps, tend to be large and expensive, and can raise safety concerns. Some countries, for example, have banned mercury arc lamps. The invention includes among its objects the replacement of such large light sources while providing through-the-lens illumination of objects under inspection. 
     SUMMARY OF THE INVENTION 
     The invention, as may be practiced within certain preferred embodiments of through-the-lens illumination systems, replaces large single light sources located remote from the shared pathways of coextending portions of imaging systems with a plurality of smaller light sources and associated beam-shaping optics located coaxially with the shared pathways. The plurality of light sources can include light-emitting diodes (LEDs) whose output is shaped by the beam-shaping optics for filling portions of an effective illuminator aperture surrounding an aperture stop of an objective of the imaging system. Although propagating in opposite directions, both the imaging light filling the objective stop and the illuminating light filling portions of a surrounding space can pass through a common optical element of the objective for directing light both to and from an object under inspection. Light distributions from the beam-shaping optics are preferably arranged for more efficiently conveying generated light to the object under inspection and for more uniformly distributing the light over the object field. 
     One example of an optical inspection system arranged in accordance with the invention includes both an illuminating system for illuminating an object under inspection and an imaging system for imaging the object under inspection within an object field. An objective is shared in part by both the illuminating system and the imaging system and includes a front relay lens, a rear relay lens, and an objective stop disposed between the front and rear relay lenses. The objective, which is preferably at least approximately telecentric, collects light scattered from the object and forms an image of the object with the collected light. The image can be a final image but is preferably in intermediate image that is projected onto a comparator screen. A multiplexed beam generator of the illuminating system includes a plurality of effective light sources and associated beam-shaping optics surrounding the objective stop for illuminating the object. Each of the beam-shaping optics is arranged together with its associated effective light source for non-uniformly distributing light within a range of angles required for illuminating the object field. 
     Each of the beam-shaping optics and its associated effective light source is preferably arranged for distributing light differently within the range of required angles so that the object field is more efficiently and uniformly illuminated. For example, the beam-shaping optics can be distributed in pairs symmetrically about an optical axis of the objective and the non-uniform distributions of light within the pairs can be substantially mirror symmetrical. The range of angles through which light is distributed from the beam-shaping optics can include angles at which the light both converges toward and diverges from an optical axis of the objective and the beam-shaping optics preferably distribute more light into the angles that converge toward the optical axis. 
     The effective light sources are preferably relatively sized for producing together with the beam-shaping optics a limited range of angularly related light beams that are converted by the front relay lens into a range of spatially distributed beams over the object field. The beam-shaping optics include optical axes, and in one arrangement, the associated effective light sources can have centers that are offset from the optical axes of the beam-shaping optics. Preferably, the centers of the effective light sources are offset from the optical axes of the beam-shaping optics in directions that extend radially of an optical axis of the objective. 
     In another arrangement, the beam-shaping optics include optical axes that are inclined to an optical axis of the objective. The axes of the beam-shaping optics are preferably inclined in axial planes containing the axis of the objective. 
     For cost considerations, the relay lenses of the objective preferably have a limited size. The numerical aperture of the objective through the front relay lens is preferably limited by the objective stop in accordance with the general requirements for approaching telecentricity. The illumination system operates through the front relay lens of the objective at a higher numerical aperture, and as such, only the illuminating light closest radially to the objective stop approaches the object field as near telecentric light. Illuminating light farther radially from the objective stop progressively departs from telecentricity. For example, an aperture of the front relay lens can block angles that would otherwise have contributed to more telecentric illumination. 
     In another arrangement, the beam-shaping optics can have peripheries that are truncated adjacent to the objective stop. The optical axes of the beam-shaping optics are positioned closer to the objective stop through radial distances that approximately correspond to amounts that the peripheries of the beam-shaping optics are truncated in common radial directions. With the illuminating light concentrated closer radially to the objective stop, angular uniformity at the object field can be increased along with spatial uniformity and overall efficiency. Spatial uniformity can be increased at some cost to angular uniformity by increasing the distribution of light among certain of the angles that converge toward the optical axis of the objective. 
     An illumination system in accordance with another example of the invention is particularly adapted for use with an optical inspection system having an objective for forming an image of an object under inspection. A plurality of effective light sources and associated beam-shaping optics are arranged for collecting light from the plurality of light sources and directing the light through a portion of the objective for illuminating the object. A common housing for the beam-shaping optics has a central aperture about which the beam-shaping optics are mounted and within which a stop of the objective is defined. An optically transmissive plate located within the central aperture of the common housing blocks transmissions of heat through the objective stop. 
     Locating the plurality of effective light sources proximate to the imaging pathway can produce heat disturbances within the imaging system (e.g., heat waves) particularly if one or the other of the relay lenses is located along a convection pathway. Locating the optically transmissive plate within the central aperture of the common housing blocks the propagation of heat along the imaging pathway. The optically transmissive plate is preferably inclined out of a normal orientation to an optical axis of the imaging system to avoid producing spurious images of the object. A seal between the optically transmissive plate and the common housing prevents even minor air flow through the housing&#39;s central aperture. In addition, the optically transmissive plate is preferably thermally coupled to the common housing for cooling the plate. The common housing itself can be actively cooled, such as by flowing coolant through or around the housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
         FIG. 1  is a diagram of an optical comparator showing imaging pathways of an imaging system from an object under inspection to a screen for comparing the object under inspection to a datum. 
         FIG. 2  is a diagram of a portion of the imaging system shared by an illumination system for illuminating the object under inspection showing illumination pathways from a multiplexed beam generator shown in cross section to the object. 
         FIG. 3  is a relatively enlarged axial view of the multiplexed beam generator showing a distribution of beam-shaping optics surrounding an aperture stop of the imaging system. 
         FIG. 4  is a diagram of an alternative illumination system for the optical comparator in which effective light sources are radially offset with respect to axes of beam-shaping optics within a modified multiplexed beam generator. 
         FIG. 5  is a diagram of another alternative illumination system for the optical comparator in which beam-shaping optics together with their associated effective light sources within a modified multiplexed beam generator are inclined toward a common axis of the imaging and illumination systems. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , an optical comparator  10 , as an example of an optical inspection system in accordance with the invention, includes an objective  12  for forming an intermediate image  16  of an object  14  under inspection and a projector  18  for forming a magnified image  20  of the object  14  on a screen  22 . The magnified image  20  can be compared to a datum, such as a template, for checking if the object  14  is within certain tolerances or taking other measurements. While the objective  12  can be regarded as an imaging system through the formation of the intermediate image  16 , in a larger sense, the objective  12  combined with the projector  18 , which together form the magnified image  20 , constitutes a more complete imaging system of the comparator  10 . Although not shown, the comparator  10  can include other conventional features (not shown) including one or more stages for mounting and moving the object  14 , other illumination systems for bright-light or back-light illumination, gaging apparatus for measuring or otherwise comparing the object to a datum, and controls for the various comparator functions and for linking the comparator to other systems such as processing, work flow, or communications systems. 
     The objective  12  includes a front relay lens  24  and a rear relay lens  28  straddling an aperture stop  26 , which is preferably located at the back focal plane of the front relay lens  24  and at the front focal plane of the rear relay lens  28 . The front relay lens  24  collects light from an object field  30  in which the object  14  is located and the rear relay lens  28  forms the intermediate image  16  of the object  14  within an intermediate image field  32 . The aperture stop  26  preferably constrains a range of field angles collected form the object field  30  so that the objective  12  is at least approximately telecentric for avoiding distortions of the intermediate image  16  accompanying variations in the focal depth of the object  14  throughout the object field  30 . Preferably, the objective  12  has a one-to-one magnification and is doubly telecentric, i.e., telecentric at both the object field  30  and the intermediate image field  32 . 
     The projector  18 , which can provide various amounts of magnification, projects the intermediate image  16  onto the comparator screen  22 . Fold mirror  34  within the objective  12  and fold mirror  36  between the projector  18  and screen  22  exemplify a redirection of imaging light within the comparator  10  for interconnecting locations convenient for mounting the object  14  and observing the magnified image  20  of the object  14 . 
     A through-the-lens illumination system  38  in accordance with the invention includes a multiplexed beam generator  40  surrounding the objective aperture stop  26 . The illustrated multiplexed beam generator  40 , which is also shown by  FIG. 2  and further enlarged in  FIG. 3 , includes four effective light sources  44  and associated four beam-shaping optics  46  all mounted within a common housing  50 . A central aperture within the common housing  50  functions as the stop  26  of the objective  12 . The exit faces of the beam-shaping optics  46  are preferably located in or near a plane containing the objective stop  26  so that the output of the beam-shaping optics  46  fills at least portions of an effective illuminator aperture stop  58  within the same plane. The four effective light sources  44  preferably comprise an equal number of light-emitting diodes (LEDs). The beam-shaping optics  46  preferably comprise collector or collimating lenses for converting output from the effective light sources  44  into an angular spread of beams for collectively illuminating the object field  30  through the front relay lens  24  of the objective  12 . 
     The effective light sources  44  can be shaped and sized for achieving the desired angular spread of light beams through the beam-shaping optics  46  by combining the LEDs with respective diffusers (not shown) between the LEDs and collimating lenses. In the embodiment shown, the effective light sources  44  are aligned with axes  48  of the beam-shaping optic  46 , and the axes of the beam-shaping optics are evenly distributed around an axis  42  of the objective  12  (which axis is also common to the illumination system  38 ). Preferably, the beam-shaping optics  46  are arranged in mirror symmetrical pairs for delivering balanced angular distributions of light across the object field  30 . 
     Although shown as four (e.g., two pairs) of beam-shaping optics  46 , more or less beam-shaping optics and associated light sources can be used (preferably in mirror symmetrical pairs). The beam-shaping optics preferably fill as much as possible of an annular periphery surrounding the aperture stop and are limited in overall radial dimension in accordance with a desired diameter of an effective illuminator aperture stop setting the maximum field angle for illuminating the object field  30 . 
     As also shown in the illustrations of  FIGS. 2 and 3 , the beam-shaping optics  46  have peripheries that are truncated (see flats  54 ) adjacent to the objective stop  26 , which allows the axes  48  of the beam-shaping optics  46  to be moved radially closer to the stop  26 . The radial moves of the axes  48  through the amount the optics  46  are radially truncated allows more light from the beam-shaping optics  46  to pass through the front relay lens  24  while concentrating light within field angles closer to the field angles collected by the objective  12  for forming the intermediate image  32 . The radially truncated form of the beam-shaping optics  46  effectively flattens a portion of an otherwise circular periphery, but other truncated shapes could also be used including concave shapes to more closely match the form of the objective stop  26 . 
     The effective light sources  44  and associated beam-shaping optics  46  surrounding the objective stop  26  are referred to as a multiplexed beam generator because the spatial and angular contributions from each of the light sources  44  and associated beam-shaping optics  46  combine to illuminate at least partially overlapping portions of the object field  30  over a spread of field angles. The front relay lens  24  operating through a first effective numerical aperture collects light from the object field  30  through a range of field angles limited by the size of the objective stop  26 . As a part of the illuminating system  38 , the front relay lens  24  operates through a higher numerical aperture not limited by the objective stop  26  for conveying light to the object field  30  through a higher range of angles, which can be limited by either the aperture of the front relay lens  24  or the effective illuminator aperture  58  encompassing the beam-shaping optics  46 . The truncated form (e.g., flats  54 ) of the beam-shaping optics  46 , which moves the axes  48  of the beam-shaping optics  46  closer to the objective stop  26 , allows more light to pass through the front relay lens  24  and concentrates the light within field angles that reach more of the object field  30 . 
     The common housing  50  and its adaptations also reduce potentially undesirable effects from heat generated by the effective light sources  44  close to the imaging pathway of the comparator  10 . For purposes of further isolating the effective light sources  44  and blocking heat convection along the optical axis  42  toward the front relay lens  24 , an optically transmissive plate  52  is located within the central aperture of the housing  50  covering the aperture stop  26 . Preferably, the optically transmissive plate  52 , which can be made of optical glass, is mounted slightly tipped (e.g., the optical axis of the transmissive plate  52  is inclined to the objective axis  42  by approximately 10 degrees) to avoid producing spurious reflections along the imaging pathway from the object field  30  to the intermediate image field  32 . Anti-reflective coatings can be applied to the plate to reduce reflections and enhance transmissivity. The plate  52  is preferably sealed to the central aperture of the common housing  50 . Conventional sealing materials can be used for this purpose, such as room-temperature vulcanizing (RTV) silicones. The sealing material preferably (a) blocks heat flows around the plate  52 , (b) provides a thermally conductive pathway between the plate  52  and the housing  50  to uniformize temperatures, and (c) provides a secure and steady mounting for the plate within the housing  50 . The effective light sources  44 , which are also mounted within the common housing  50 , tend to transfer heat into the housing  50 . A circulating cooling system  60  connected to the common housing  50  extracts the excess heat from the housing  50 . For example, a coolant such as water can be circulated between the housing  50  and a heat exchanger. A fan (not shown) can also be used for conveying heat from the light sources  44 . 
     An alternative multiplexed beam generator  70  is shown in  FIG. 4 . Similar to the multiplexed beam generator  40 , the multiplexed beam generator  70  includes a plurality of effective light sources  74  and associated beam-shaping optics  76  arranged within a common housing  80  in mirror symmetrical pairs (as is generally preferred) around the aperture stop  26  of the objective  12 . However, instead of truncating the peripheries of the beam-shaping optics  76 , the effective light sources  74  are radially displaced from axes  78  of the beam-shaping optics  76 . Centerlines  82  of the effective sources are displaced radially outwardly from the axes  78  of the beam-shaping optics  76  through the distance dR. 
     The radial offset dR of the effective sources  74  concentrates light within a range of aperture angles (within the illumination aperture) prone to reaching the object field  30  through the front relay lens  24 . The mirror symmetry between the effective sources  74  and associated beam-shaping optics  76  on opposite sides of the objective stop  26  provides for balancing illumination across the object field  30 . That is, the object field positions disfavored by the effective sources  74  and associated beam-shaping optics  76  on one side of the objective stop  26  are favored by the mirror symmetrical effective sources  74  and associated beam-shaping optics  76  on the opposite side of the objective stop  26 . The shapes and sizes of the effective light sources  74  are preferably limited in relation to the beam-shaping optics  76  for generating light beams through the range of aperture angles required for illuminating the object field  30  as imaged by the objective  12  or the projector  18 . Preferably, the radial offset dR is limited to relatively increasing concentrations of light within certain of the aperture angles within the range required for illuminating the object field  30 . 
     Another alternative multiplexed beam generator  90  is shown in  FIG. 5 . Here, a plurality of effective light sources  94  remain aligned with axes  98  of associated beam-shaping optics  96  within a common housing  100 , but the axes  98  of the beam-shaping optics  96  are inclined to the objective optical axis  42  through an angular offset dθ (shown measured with respect to parallel axes  102 ) for concentrating light within aperture angles prone to reaching the object field  30  through the front relay lens  24 . The inclinations of the axes  98  are preferably taken within axial planes that include the objective axis  42 . Similar to the preceding embodiment, mirror symmetry is preferably provided between the effective sources  94  and associated beam-shaping optics  96  on opposite sides of the objective stop  26  for balancing illumination across the object field  30 . Also similar to the preceding embodiment, the shapes and sizes of the effective light sources  94  are preferably limited in relation to the beam-shaping optics  96  for generating light beams through the range of aperture angles required for illuminating the object field  30  as imaged by the objective  12  or the projector  18 . The angular offset dθ is limited to relatively increasing concentrations of light within certain of the aperture angles within the range required for illuminating the object field  30 . 
     Various combinations of offsetting the effective light sources and inclining the beam-shaping optics can be used along with optimally sizing and shaping of the effective light sources to control distributions of light among aperture angles required for illuminating the imaged object field  30 . In addition, the peripheries of the beam-shaping optics can be truncated to shift the inclined or relatively offset axes of the beam-shaping optics closer to the objective aperture for concentrating light within field angles that are closer to the field angles at which the object field  30  is imaged. 
     Although described with respect to an optical comparator, as an example of the invention&#39;s preferred use, the teachings of this invention are expected to apply in general to optical inspection systems with combined imaging and illumination systems, particularly where the minimum numerical aperture through which the illuminator operates is beyond the numerical aperture within which the imager operates.