Patent Publication Number: US-11044412-B2

Title: Dark field illumination for laser beam delivery system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 16/401,100, filed May 1, 2019, the entirety of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to laser beam delivery systems such as laser welders and laser cutters, and more particularly to camera systems for observing delivery of laser beams to a substrate by such laser beam delivery systems. 
     BACKGROUND 
     Infrared lasers are used in various industrial applications to perform operations on substrates such as metal. Such operations include cutting a monolithic substrate into two or more pieces, or welding a substrate consisting of multiple pieces together. 
     The design of most laser beam delivery systems used in such industrial applications provides for the delivery of a powerful infrared laser beam to a target point on the substrate with precise focus and position. In addition, a pilot laser, which typically emits light in a range visible to the human eye and at an intensity that does not affect the substrate, is usually integrated into the laser beam delivery system as an indicator for the operator as to where the infrared laser will appear on the substrate. The pilot laser, in general, is a red laser with a wavelength range in the visible spectrum between 620 nm to 690 nm. 
     The main infrared laser beam enters the laser beam delivery system from a laser fiber cable via a beam delivery connector and is projected onto a target point by the laser beam delivery system optics, that is, the internal optics of the laser beam delivery system. The laser beam delivery system optics typically include a combination of specialized optical mirrors, lenses, filters and other optical devices that provide a focused, directed light beam of a specific wavelength. Typically the laser beam delivery system is designed to suit the type of welding to be performed. The same connector is also used for delivery of the pilot laser source through the laser beam delivery system optics. 
     On many laser beam delivery systems, a video camera port is included to allow for the optical coupling of a video camera to the system to allow the operator to visually observe the setup of the welding process (including material alignment and location) prior to activating the laser that will operate on the substrate. In such systems, the laser beam delivery system optics, that is, the internal optics of the laser beam delivery system, are configured, typically by way of a beam splitter, to direct light reflected from the substrate to the camera port. The camera port may take the form of a transparent window in a housing of the laser beam delivery system. 
     Typically, conventional video cameras are coupled to the camera port of a laser beam delivery system. They provide an adequate image of the work area through the laser beam delivery system optics prior to activating the laser that will operate on the substrate, and allows use of the pilot laser for initial positioning. However, once the operating laser is activated, it provides too much illumination and conventional video cameras become saturated with light, seriously inhibiting their ability to monitor operations on the substrate. Stated another way, because conventional video cameras have a low dynamic range, they cannot be effectively used during infrared laser welding or cutting processes as the image is too bright for the camera. 
     It is also known to use high dynamic range (HDR) cameras in welding operations. The term “dynamic range” refers to the ratio between the largest and smallest values that a signal (e.g. a light signal) may assume, and in base-10 is measured in decibel (dB). The term “high dynamic range” or “HDR”, as used herein, means an imaging capability of a dynamic range of at least 140 dB. Specialized HDR camera systems for welding operations include those offered by Xiris Automation Inc. having an address at 1016 Sutton Drive, Unit C5, Burlington, Ontario, Canada L7L 6B8. HDR cameras offer a good image when the operating laser (e.g. the infrared laser used to operate on the substrate) is active. 
     Thus, a logical solution for the problem of light saturation of conventional video cameras during infrared welding operations would seem to be replacement of the conventional video camera with an HDR video camera to be able to see the laser spot and substrate during operation of the infrared laser. Unfortunately, this merely substitutes one problem for another. While an HDR video camera provides very good imaging when the operating laser is active, when the operating laser is not active, there is insufficient light for the HDR video camera to effectively resolve the work surface. 
     One option that has been explored in an effort to resolve this latter problem is to use a co-axial illuminator in conjunction with an HDR video camera to illuminate the work surface with enough light to be seen by the HDR camera through the laser beam delivery system optics. In reality, however, because the optical components and specialized optical coatings of the laser beam delivery system optics are optimized for the delivery of the powerful operating infrared laser beam, there is substantial back reflection or “ghosting” in the optical path that results from the co-axial illumination light and from the pilot laser. This ghosting appears in the view seen by the HDR video camera, substantially impeding its effectiveness. 
     Thus, one was left with the dichotomous choice of either effectively observing the substrate before activation of the operating laser, or during the welding or cutting operation, but not both. 
     SUMMARY 
     The present disclosure describes a combination of an HDR video camera and a dark field illuminator configured so that dark field illumination can be delivered through the camera port, via the laser beam delivery system optics, to a substrate while the HDR video camera simultaneously observes the substrate through the camera port. 
     In one aspect, an illuminated camera system comprises a dark field illuminator, a high dynamic range (HDR) camera system comprising an HDR video camera and a lensing system, a beam splitter, and a support mounting that carries the dark field illuminator, the HDR camera system and the beam splitter. The beam splitter is optically interposed between the dark field illuminator and the HDR camera system. The dark field illuminator, the HDR camera system and the beam splitter are arranged relative to one another so that the dark field illuminator and the HDR camera share a common field of view. When the illuminated camera system is arranged so that the common field of view is in registration with a camera port on a laser beam delivery system having laser beam delivery system optics and positioned to deliver a laser beam over a substrate, light from the dark field illuminator is transmitted through the beam splitter, through the common field of view, through the camera port and through the laser beam delivery system optics to the substrate, and reflected light from the substrate is transmitted through the laser beam delivery system optics, through the camera port, through the common field of view, and reflected from the beam splitter to the HDR camera system. 
     In some embodiments, the HDR camera system further comprises an attenuating filter optically interposed between the beam splitter and the lensing system of the HDR camera system. The attenuating filter is substantially completely transparent to wavelengths of at least one light source of the dark field illuminator and transmissive to wavelengths of a pilot laser of the laser beam delivery system, but is substantially less transmissive to the wavelengths of the pilot laser than to the wavelengths of the at least one light source of the dark field illuminator. In such embodiments, the HDR camera system may further comprise a path-defining filter optically interposed between the beam splitter and the lensing system of the HDR camera system. The path-defining filter is substantially completely opaque to the wavelengths of the pilot laser, and is shaped to provide a peripheral off-axis optical path between the beam splitter and the lensing system of the HDR camera system. The peripheral off-axis optical path bypasses the path-defining filter rather than passing through it. 
     In another aspect, a method for observing laser operations on a substrate comprises simultaneously directing dark field illumination through a camera port of a laser beam delivery system, whereby the dark field illumination is transmitted through laser beam delivery system optics of the laser beam delivery system onto the substrate, and using a high dynamic range (HDR) camera system to observe, through the camera port of the laser beam delivery system, light reflected from the substrate and transmitted through the laser beam delivery system optics of the laser beam delivery system to the camera port of the laser beam delivery system. 
     The method may comprise attenuating intensity of wavelengths of a pilot laser of the laser beam delivery system reaching a lensing system of the HDR camera system, and may further comprise confining the wavelengths of the pilot laser of the laser beam delivery system reaching the lensing system of the HDR camera system to a peripheral, off-axis optical path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features will become more apparent from the following description in which reference is made to the appended drawings wherein: 
         FIG. 1  is a schematic representation of an illustrative illuminated camera system according to an aspect of the present disclosure; 
         FIG. 1A  is a schematic representation of the illuminated camera system of  FIG. 1 , showing light paths from a dark field illuminator thereof to an HDR camera thereof; 
         FIG. 1B  is a schematic representation of the illuminated camera system of  FIG. 1 , showing light paths from a pilot laser of a laser beam delivery system to the HDR camera of the illuminated camera system of  FIG. 1 ; 
         FIG. 2  is an isolated elevation view showing the relative positioning and function of an attenuating filter and a path-defining filter of the illuminated camera system of  FIG. 1 ; 
         FIG. 2A  shows a plan view of the attenuating filter of  FIG. 2 ; 
         FIG. 2B  shows a plan view of the path-defining filter of  FIG. 2 ; 
         FIG. 2C  shows a plan view of an alternate embodiment of a path-defining filter; 
         FIG. 3  is a schematic representation of an illustrative dark field illuminator of the illuminated camera system of  FIG. 1 ; 
         FIG. 3A  is a schematic representation of a first alternative embodiment of a dark field illuminator; 
         FIG. 3B  is a schematic representation of a second alternative embodiment of a dark field illuminator; 
         FIG. 3C  is a schematic representation of a third alternative embodiment of a dark field illuminator; 
         FIG. 3D  is a schematic representation of a fourth alternative embodiment of a dark field illuminator; 
         FIG. 4  is a schematic representation of an opaque diaphragm for the dark field illuminator of the illuminated camera system of  FIG. 1 ; 
         FIG. 4A  is a schematic representation of an opaque diaphragm for the first alternative dark field illuminator of  FIG. 3A ; 
         FIG. 4B  is a schematic representation of an opaque diaphragm for the second alternative dark field illuminator of  FIG. 3B ; 
         FIG. 4C  is a schematic representation of an opaque diaphragm for the third alternative dark field illuminator of  FIG. 3C ; and 
         FIG. 4D  is a schematic representation of an opaque diaphragm for the fourth alternative dark field illuminator of  FIG. 3D . 
     
    
    
     DETAILED DESCRIPTION 
     Broadly speaking, the present disclosure describes a system and method in which dark field illumination is applied to the substrate to be operated on so that the material can be imaged by an HDR video camera during the alignment procedure using the pilot laser, with minimal “ghosting” reflection from the laser beam delivery system optics. The HDR video camera can also observe the substrate during operation of the main infrared laser. 
     Reference is now made to  FIG. 1 , which shows an illustrative implementation of an illuminated camera system, indicated generally by reference  100 , according to an aspect of the present disclosure. 
     The illuminated camera system  100  comprises a dark field illuminator  102 , a high dynamic range (HDR) camera system  104  comprising an HDR video camera  106  and a lensing system  108 , and a beam splitter  110 . The HDR video camera  106  and lensing system  108  are shown schematically for illustrative purposes and are not intended to be limiting; e.g. a wide range of suitable lensing systems and HDR video cameras may be used. A support mounting  112  carries the dark field illuminator  102 , the HDR camera system  104  and the beam splitter  110 , which is optically interposed between the dark field illuminator  102  and the HDR camera system  104 . 
     The dark field illuminator  102 , the HDR camera system  104  and the beam splitter  110  are arranged relative to one another so that the dark field illuminator  102  and the HDR camera system  104  share a common field of view  114 . Preferably, the support mounting  112  comprises a substantially opaque enclosure that includes a window  116  positioned in registration with the common field of view  114 . In the illustrated embodiment, the respective optical axes  118 ,  120  of the HDR camera system  104  and the dark field illuminator  102  are arranged substantially normal to one another, and share the common field of view  114  by way of direct passage through the beam splitter  110  for the dark field illuminator  102  and by way of reflection off of the beam splitter  110  for the HDR camera system  104 . The substantially normal relative arrangement of the dark field illuminator  102  and the HDR camera system  104  within a substantially opaque enclosure is merely one illustrative embodiment and is not intended to be limiting; other configurations are also contemplated. 
     Where, as in the illustrated embodiment, the support mounting  112  comprises a substantially opaque enclosure that includes a window  116  positioned in registration with the common field of view  114 , the illuminated camera system  100  may be positioned with the window  116 , and hence the common field of view  114 , in registration with a conventional camera port  122  of a laser beam delivery system  124 , which may be positioned to deliver a laser beam onto a target substrate  126 . Although not shown, one skilled in the art will appreciate that the position of the HDR camera system  104 , relative to the beam splitter  110 , can be adjusted linearly (i.e. perpendicular to the optical axis  118 ) and by axial displacement (i.e. longitudinal displacement along the optical axis  118  of the HDR camera system  104 ). 
     The laser beam delivery system  124  may be of conventional design and may be, for example, a laser welder or a laser cutter. The laser beam delivery system  124  has laser beam delivery system optics  128  which form a main optical channel  130  of the laser beam delivery system  124 . The laser beam delivery system optics  128  will typically comprise a sophisticated multi-element optical design incorporating special optical coatings for the main operation laser beam (typically a powerful infrared (IR) laser beam), which is delivered by an optical fiber  132 , through the main optical channel  130  formed by the laser beam delivery system optics  128 , to the surface of the target substrate  126 . In addition to the main operation laser beam, the laser beam delivery system  124  will typically also be provided with a pilot laser, which is projected co-axially, as a narrow beam, through the main optical channel  130  to provide provides a precise indication of the focus point on the surface of the target substrate  126  where the main operation laser will be projected. The pilot laser is typically a red laser in the visible spectrum, with a wavelength range from about 620 nm to 690 nm. As can be seen, the laser beam delivery system optics  128  may include a laser system beam splitter  134  so that the surface of the target substrate  126  is reflected through the camera port  122 . Examples of commercially available laser beam delivery systems include those offered by HIGHYAG Lasertechnologie GmbH, having an address at Hermann-von-Helmholtz-Str. 2, 14532 Kleinmachnow, Germany, those offered by Laser Mechanisms, Inc. having an address at 25325 Regency Drive, Novi, Mich. 48375, U.S.A., those offered by Precitec, Inc., having an address at 28043 Center Oaks Ct., Wixom, Mich. 48393 U.S.A., those offered by TRUMPF GmbH+Co. KG having an address at Johann-Maus-Strasse 2, 71254 Ditzingen, Germany, those offered by Laserline GmbH, having an address at Fraunhofer Straße, 56218 Mülheim-Kärlich, Germany and those offered by Rofin-Sinar Laser GmbH, having an address at Berzeliusstraße 87, 22113 Hamburg, Germany. 
     In the illustrated embodiment shown in  FIGS. 1 to 1B , the dark-field illuminator  102  comprises at least one light source  140 , a first condenser  142 , a second condenser  144 , and an opaque diaphragm  146  having off-axis optical apertures  148  (see  FIGS. 3 and 4 ). The first condenser  142  may include a diffuser  150  (see  FIG. 3 ). The light source(s)  140  may be, for example, one or more light-emitting diodes (LEDs), such as a single LED or a ring of LEDs, or a light source incorporating an optical fiber, or another suitable light source (or sources). In one embodiment, the light source(s)  140  emit a specific narrow band light in the blue or green range of the visible spectrum, for example from about 450 nm to about 570 nm. The specific blue or green wavelength range for the light source(s) may be selected in consideration of the internal optical components and coatings that exist within the laser beam delivery system optics  128  so as to provide high transmittance through the main optical channel  130  of the laser beam delivery system  124  while limiting “ghosting” reflections from inner optical surfaces. Certain illustrative implementations of particular embodiments of the dark-field illuminator  102  will be described in detail further below, with reference to  FIGS. 3 to 4D . 
     Referring now to  FIG. 1A , the light source(s)  140  emit light  150  (shown with hatched lines as a sectional projection from a point, for ease of illustration) toward the first condenser  142 . The first condenser  142  projects the light  150  toward the opaque diaphragm  146 , where a portion of the light  150  passes through the off-axis optical apertures  148  while the remainder is obstructed. Then, the second condenser  144  projects the light  150  which has passed through the opaque diaphragm  146  and therefore images the opaque diaphragm  146  from a position plane thereof, through the beam splitter  110 . The light  150  continues through the window  116 , and hence the common field of view  114 , through the camera port  122  of the laser beam delivery system  124  into the main optical channel  130  thereof, where it is reflected by the laser system beam splitter  134  onto the surface of the target substrate  126 . The light  150  illuminates the surface of the target substrate  126 , co-axially with the main optical channel  130 , as a nearly collimated inclined beam. 
     The incident light  150  is directly reflected from the surface of the target substrate  126 , as reflected light  152 , into the main optical channel  130 , where it is reflected by the laser system beam splitter  134  and projected back through the camera port  122  of the laser beam delivery system  124 . The reflected light  152  continues through the window  116 , and hence the common field of view  114 , and is then reflected by the beam splitter  110  toward the HDR camera system  104 , substantially co-axially therewith. 
     Thus, when the illuminated camera system  100  is arranged so that the common field of view  114  is in registration with the camera port  122  on a laser beam delivery system  124  having laser beam delivery system optics  128  and positioned to deliver a laser beam over a substrate  126 , light  150  from the dark field illuminator  102  is transmitted through the beam splitter  110 , through the common field of view  114 , through the camera port  122  and through the laser beam delivery system optics  128  to the substrate  126 , and reflected light  152  from the substrate  126  is transmitted through the laser beam delivery system optics  128 , through the camera port  122 , through the common field of view  114 , and reflected from the beam splitter  110  to the HDR camera system  104 . 
     The light  150  from the dark field illuminator  102  reaches the substrate  126  as an incident inclined beam  150 , and the direct reflected beam of light  152  produces a conjugated diaphragm image  154  at the lensing system  108  after passing through a set of filters  160 ,  162  (described further below). Because the light  150  images the opaque diaphragm  146 , the conjugated diaphragm image  154  at the lensing system  108  is a shadow image, masking severe direct “ghosting” reflections from the inner optical surfaces of the laser beam delivery system optics  128 . The incident inclined beam  150  and then the direct reflected beam of light  152  do not form a real image at the lensing system  108 ; the conjugated diaphragm image  154  presented at the lensing system  108  represents a real image of the surface of the substrate  126  by way of reflections/shadows from the surface features of the substrate  126 . The reflected light  152 , in the form of the “ghost” reflection from the inner optical surfaces of the of the laser beam delivery system optics  128 , is also inclined and does not form a real image at the lensing system  108 . 
     Thus, the use of the dark field illuminator  102 , rather than conventional illumination, allows the HDR camera system  104  to image the surface of the substrate  126  when the main operating laser is not operating while obviating “ghosting” effects. 
     While this approach is sufficient to obviate the “ghosting” effect resulting from conventional illumination through the camera port  122 , preferred implementations according to the present disclosure also incorporate narrow bandwidth filters to compensate for “ghosting” in the optical path that results from the pilot laser, so that the HDR camera system  104  can also image the surface of the substrate  126  when the pilot laser is projected thereon. This is achieved by way of first and second narrow bandwidth filters  160 ,  162  interposed between the lensing system  108  and the beam splitter  110 , which are now described with particular reference to  FIG. 1B . 
     The first filter  160  is an attenuating filter  160 , which is optically interposed between the beam splitter  110  and the lensing system  108  of the HDR camera system  104 . The attenuating filter  160  has a transmission range of the spectrum optimized according to the specific range of the light source(s)  140  of the dark field illuminator  102 , and also has a proportionally lower transmission range at the main wavelength of the pilot laser  164 . Thus, the attenuating filter  160  is substantially completely transparent, that is, preferably at least 90% transmissive, more preferably at least 92.5% transmissive, and most preferably at least 95% transmissive to the wavelengths of the light source(s)  140  of the dark field illuminator  102 , and transmissive to wavelengths of the pilot laser  164  and its reflection  168  (shown with hatched lines in  FIG. 1B ) of the laser beam delivery system  124 . However, the attenuating filter  160  is substantially less transmissive to the wavelengths of the pilot laser  164  (e.g. 620 nm to 690 nm) than to the wavelengths of the light source(s)  140  of the dark field illuminator  102 . Preferably the attenuating filter  160  is at least 50% less transmissive, more preferably at least 60% less transmissive, still more preferably at least 70% less transmissive, yet still more preferably at least 80% less transmissive and even more preferably at least 90% less transmissive to the wavelengths of the pilot laser  164  than to the wavelengths of the light source(s)  140  of the dark field illuminator  102 . One skilled in the art, now informed by the present disclosure, can select a difference in transmissivity based on a comparison of the relative intensities of the focal point from the pilot laser  164  to “ghosting” in the optical path. 
     The second filter  162  is a path-defining filter  162 , which is also optically interposed between the beam splitter  110  and the lensing system  108  of the HDR camera system  104 . The path-defining filter  162  filter also has a transmission range of the spectrum tuned, and preferably optimized, according to the specific range of the light source(s)  140  of the dark field illuminator  102 , but substantially fully blocks/absorbs the main wavelength of the pilot laser  164  and its reflection  168  (e.g. absorption range within the visible spectrum between 620 nm to 690 nm). Thus, the path-defining filter  162  is at least transmissive, and preferably substantially transparent, to the wavelengths of the light source(s)  140  of the dark field illuminator  102 , but is substantially completely opaque to the wavelengths of the pilot laser  164 , and is shaped to provide a peripheral off-axis optical path  166  between the beam splitter  110  and the lensing system  108  of the HDR camera system  104 . The peripheral off-axis optical path  166  bypasses the path-defining filter  162  rather than passing through it. Preferably, the path-defining filter  162  is at least 80% transmissive, more preferably at least 85% transmissive and still more preferably at least 90% transmissive to the wavelengths of the light source(s)  140  of the dark field illuminator  102 , and at least 90% opaque, preferably at least 95% opaque and still more preferably at least 97% opaque wavelengths of the pilot laser  164 . 
     The filters  160 ,  162  are selected and positioned, relative to one another and to the optical axis  118  of the HDR camera system  104 , in such a way that the attenuating filter  160  is closest to the lensing system  108  of the HDR camera system  104  and covers the full aperture of the lensing system  108  of the HDR camera system  104 , while the path-defining filter  162  is positioned in front of the attenuating filter  160  (i.e. closer to the beam splitter  110 ) and only partially covers the aperture of the lensing system  108 . The peripheral off-axis optical path  166  is in registration with the portion of the lensing system  108  that is not covered by the path-defining filter  162 . 
     Reference is now made to  FIG. 2 , which shows the relative positioning of the illustrative filters  160 ,  162  in greater detail. A plan view of the illustrative attenuating filter  160 , which is generally disk-shaped, is shown in  FIG. 2A . A plan view of a first illustrative embodiment of the path-defining filter  162 , which forms a major portion of a disk but with a segment cut away, is shown in  FIG. 2B ; the omitted segment (denoted by dashed lines) defines the peripheral off-axis optical path  166 .  FIG. 2C  shows a plan view of a second illustrative embodiment of the path-defining filter  162 C, which is similar to the first embodiment in that it forms a major portion of a disk, but has a parabolic portion rather than a segment cut away, with the omitted parabolic portion (denoted by dashed lines) defining the peripheral off-axis optical path  166 C. The generalized disk and disk-like shapes of the filters  160 ,  162  are merely illustrative and are not intended to exclude other shapes. Likewise, the embodiments  162 ,  162 C shown in  FIGS. 2B and 2C  are merely illustrative and not limiting, and other shapes and configurations for defining a peripheral off-axis optical path are also envisaged (for example, a de-centered aperture may be used instead of omitting an edge portion). In this sense, the term “peripheral off-axis” refers to a configuration in which the relevant path is not in registration with the optical axis  118  of the HDR camera system  104 . Preferably, the path-defining filter  162 ,  162 C can be adjusted linearly, i.e. perpendicularly to the optical axis  118 , and by rotation about the optical axis  118 , as shown by the arrows in  FIGS. 2B and 2C . 
     Referring again to  FIG. 1B , the path of the pilot laser  164  and its reflection  168  are shown with hatched lines. For clarity of illustration, the pathways of the “ghosting” reflections of the pilot laser  164  and its reflection  168  from the laser beam delivery system optics  128  are not shown. 
     As noted previously, the pilot laser  164  will typically be a red laser in the visible range of the spectrum, between about 620 nm and about 690 nm, and is emitted from the same optical fiber  132  as the main operation laser beam (typically a powerful infrared (IR) laser beam) and travels through the same main optical channel  130 , along the optical axis  133  of the laser beam delivery system  124 , to a focal point on the surface of the substrate  126 . Thus, the pilot laser  164  will provide an indication of where the main operation laser beam will contact the surface of the substrate  126 . The pilot laser  164  produces a direct reflection  168  from the surface of the substrate  126  back to the main optical channel  130  of the laser beam delivery system  124 , where it is reflected by the laser system beam splitter  134  through the camera port  122 . The reflected pilot laser  168  continues through the window  116 , and hence the common field of view  114 , and is then reflected by the beam splitter  110  toward the HDR camera system  104 , substantially co-axially therewith  114 . As the reflected pilot laser  168  traverses this path, the laser beam delivery system optics  128  produce a significant amount of “ghosting” reflections inside the optical pathway of the pilot laser  164  and its reflection  168 . 
     The filters  160 ,  162  are selected and co-axially positioned, relative to one another and to the optical axis  118  of the HDR camera system  104 , in such a way that only a portion of the reflected light  168  from the pilot laser  164  reaches the lensing system  108 . Specifically, only that portion of the reflected light  168  from the pilot laser  164  that is in registration with the peripheral off-axis optical path  166  (i.e. where the lensing system  108  is not covered by the path-defining filter  162 ) reaches the lensing system  108 . This arrangement allows the focal point from the pilot laser  164  on the surface of the substrate  126  to be imaged at the lensing system  108  of the HDR camera system  104 , without a significant image of the “ghosting” reflection, while also permitting imaging of the surface of the substrate  126  while illuminated by the dark field illuminator  102 , as well as imaging of a welding or cutting operation of the main operation laser on the surface of the substrate  126 . 
     Returning to  FIG. 2 , because the path-defining filter  162  is at least transmissive, and preferably substantially transparent, to the wavelengths λ illum  of the light source(s)  140  of the dark field illuminator  102 , these wavelengths pass through the path-defining filter  162  to reach, and pass through, the attenuating filter  160 , which is also transparent to the wavelengths λ illum  of the light source(s)  140 , to reach the lensing system  108 . However, because the path-defining filter  162  is substantially completely opaque to the wavelengths λ Plas  of the pilot laser  164 , these wavelengths are blocked by the path-defining filter  162  except at the peripheral off-axis optical path  166 , which allows the wavelengths λ Plas  of the pilot laser  164  to bypass the path-defining filter  162  to reach the attenuating filter  160 . The wavelengths λ Plas  of the pilot laser  164  that reach the attenuating filter  160  are transmitted therethrough to reach the lensing system  108 . However, because the attenuating filter  160  is substantially less transmissive to the wavelengths λ Plas  of the pilot laser  164  and its reflection  168  than to the wavelengths λ illum  of the light source(s)  140  of the dark field illuminator  102 , the wavelengths λ Plas  of the pilot laser  164  that reach the lensing system  108  do so with attenuated intensity (as shown by the smaller arrow for λ Plas  on the right hand side of  FIG. 2 ). Thus, the filters  160 ,  162  enable the reflection  168  from the pilot laser  164  through the main optical channel  130  to be partially projected onto the lensing system  108 , via the peripheral off-axis optical path  166 , to provide a balance between the contrast of the illuminated surface of the substrate  126  and the reduction of “ghosting” for the image of the pilot laser  164  from the laser beam delivery system optics  128 . Moreover, the filters  160 ,  162  are also transmissive, and preferably substantially transparent, to the visible wavelengths associated with welding or cutting operations by the main operation laser, typically from about 400 nm to about 590 nm, so that the HDR camera system  104  unit provides a high-quality, high-dynamic range image of the laser cutting or laser welding process (e.g. an image of the weld melting pool through the bright glare of the proximate plasma jet or arc created during the laser welding process). 
     While the Figures show the attenuating filter  160  disposed between the lensing system  108  and the path-defining filter  162 , this is merely one illustrative embodiment. In other embodiments, the relative positions of the filters may be reversed, with the path-defining filter  162  disposed between the attenuating filter  160  and the lensing system  108 . 
     While any suitable dark field illuminator may be used in association with the embodiments described herein, certain illustrative implementations of dark field illuminators will now be described, with reference to  FIGS. 3 to 3D and 4 to 4D . 
     Reference is first made to  FIGS. 3 and 4 , which show in greater detail the illustrative dark field illuminator  102  shown in  FIGS. 1 to 1B . This embodiment of a dark-field illuminator  102  comprises a single LED  140  as the light source  140 , a first condenser  142  including a diffuser  150 , a second condenser  144 , and an opaque diaphragm  146  having off-axis optical apertures  148 . The first condenser  142  is closer to the light source  140  than the second condenser  144 . In this embodiment, the opaque diaphragm  146  comprises an opaque central disk  170  surrounded by an annular rim  172 , with spokes  174  extending between the central disk  170  and the rim  172  so that the off-axis optical apertures  148  are defined between the central disk  170 , the rim  172  and the spokes  174 . The opaque diaphragm  146  is positioned strategically between the two condensers  142 ,  144 , and the dark field illuminator  102  is positioned relative to the window  116  and camera port  122 , in such a way that the light from the light source  140  is projected as substantially annular, axially symmetrical beam that ultimately produces a conjugated shadow image  154  on the lensing system  108 , masking severe direct “ghosting” reflections from the laser beam delivery system optics  128 . 
     Reference is now made to  FIGS. 3A and 4A , which show a first alternate embodiment for a dark field illuminator  102 A. The first alternate dark field illuminator  102 A is similar to the illustrative dark field illuminator  102  described above, and like references refer to like features, but with the suffix “A”. The first alternate dark field illuminator  102 A differs from the illustrative dark field illuminator  102  described above in that the opaque diaphragm  146 A comprises the major portion of a disk with a parabolic portion  148 A cut away, with the omitted parabolic portion  148 A (denoted by dashed lines) defining the off-axis optical aperture  148 A. The opaque diaphragm  146 A can be axially rotated to adjust the position of the off-axis optical aperture  148 A, as shown by the arrows, and is positioned strategically between the two condensers  142 A,  144 A so that when the dark field illuminator  102  is correctly positioned relative to the window  116  and camera port  122 , the light from the light source  140 A produces a conjugated shadow image  154  on the lensing system  108 . 
     Now referring to  FIGS. 3B and 4B , a second alternate embodiment for a dark field illuminator  102 B is described. The second alternate dark field illuminator  102 B includes only a single condenser  144 B. In the second alternate dark field illuminator  102 B, the opaque diaphragm  146 B comprises a disk with a non-concentric opening  148 B formed therethrough; this non-concentric opening  148 B is the off-axis optical aperture  148 B. The light source  140 B is arranged in registration with the non-concentric opening  148 B, preferably coaxially therewith, and coupled with a reflector-concentrator  178 B. The opaque diaphragm  146 B can be axially rotated to adjust the position of the off-axis optical aperture  148 A, as shown by the arrows, and the light source  140 B and reflector-concentrator  178 B are mechanically coupled to the opaque diaphragm  146 B so as to orbit the rotational axis thereof to maintain registration with the non-concentric opening  148 B. A diffuser  150 B is disposed between the light source  140 B and the non-concentric opening  148 B, and the opaque diaphragm  146 B is disposed between the diffuser  150 B and the condenser  144 B. This arrangement is also configured to produce a conjugated shadow image  154  on the lensing system  108 . 
       FIGS. 3C and 4C  show a third alternate embodiment for a dark field illuminator  102 C. The third alternate dark field illuminator  102 C includes only a single condenser  144 C, with an opaque diaphragm  146 C similar to the opaque diaphragm  146  of the illustrative dark field illuminator  102  shown in  FIGS. 3 and 4 . Instead of a single LED as the light source, the third alternate dark field illuminator  102 C has a ring of LEDs  140 C, each coupled with a respective reflector-concentrator  178 C and arranged in registration with respective ones of the off-axis optical apertures  148 C defined between the central disk  170 C, the rim  172 C and the spokes  174 C of the opaque diaphragm  146 C. A diffuser  150 C is disposed between the light source  140 C and the opaque diaphragm  146 C. The light from the light sources  140 C will produce a conjugated shadow image  154  on the lensing system  108  when suitably positioned. 
     With reference now to  FIGS. 3D and 4D , a fourth alternate embodiment for a dark field illuminator is indicated generally at reference  102 D. The fourth alternate dark field illuminator  102 D is similar to the illustrative dark field illuminator  102  described above, with like references referring to like features but with the suffix “D”. However, the fourth alternate dark field illuminator  102 D has a pair of opposed concentric conical mirrors, namely an inner mirror  180 D and an outer mirror  182 D. The light source  140 D is single LED  140 , which is coupled with a reflector-concentrator  178 D, and the LED  140 D, reflector-concentrator  178 D, inner mirror  180 D and an outer mirror  182 D are configured so that light emitted by the LED  140 D is formed by the mirrors  180 D,  182 D into an annular beam. Like the other embodiments for dark field illuminators described herein, this arrangement is also configured to produce a conjugated shadow image  154  on the lensing system  108 . 
     For each of the above-described illustrative dark field illuminators  102 ,  102 A,  102 B,  102 C,  102 D, the light emitted from the light source(s)  140 ,  140 A,  140 B,  140 C,  140 D, projected towards the operational surface of the substrate  126 , travels around the central portion of the opaque diaphragm  148 ,  148 A,  148 B,  148 C,  148 D and the optical axis  120 ,  120 A,  120 B,  120 C,  120 D of the dark-field illuminator  102 ,  102 A,  102 B,  102 C,  102 D, and then, in general, travels parallel to the optical axis  133  of the main optical channel  130  of the laser beam delivery system  124  (see  FIG. 1A ). 
     While the above-described embodiments for dark field illuminators contemplate opaque diaphragms in which the entire diaphragm is formed from an opaque material and the off-axis optical aperture(s) are formed by physical gaps in the opaque material, it is also contemplated that the opaque diaphragms may comprise transmissive or transparent materials with opaque coatings, with the off-axis optical aperture(s) formed by gaps in the opaque coating. More generally, the dark field illuminators  102 ,  102 A,  102 B,  102 C,  102 D and the opaque diaphragms  148 ,  148 A,  148 B,  148 C,  148 D are merely illustrative and are not intended to exclude other configurations, shapes or arrangements. 
     Thus, as illustrated in  FIG. 1A , the present disclosure enables a method for observing laser operations on a substrate  126 . This method comprises directing dark field illumination  152  through a camera port  122  of a laser beam delivery system  124 , whereby the dark field illumination  154  is transmitted through laser beam delivery system optics  128  of the laser beam delivery system onto the substrate  126  and simultaneously using an HDR camera system  104  to observe, through the camera port  122  of the laser beam delivery system  124 , light (including the dark field illumination  152 ) reflected from the substrate  126  and transmitted through the laser beam delivery system optics  128  of the laser beam delivery system  124  to the camera port  122  of the laser beam delivery system  124 . 
     As shown in  FIG. 1B , the method may further comprise attenuating intensity of wavelengths of a pilot laser  164  of the laser beam delivery system  124  reaching a lensing system  108  of the HDR camera system  104 , and may yet further comprise confining the wavelengths of the pilot laser  164  of the laser beam delivery system  124  reaching the lensing system  108  of the HDR camera system to a peripheral, off-axis optical path  166 . 
     The optical parameters for the optical components of the illuminated camera system  100  should be designed to be compatible with the laser beam delivery system optics  128  of the laser beam delivery system  124 , and one skilled in the art, now informed by the present disclosure, can select appropriate parameters. Magnification and other conventional optical features may also be included within the embodiments described herein; these are within the capability of one skilled in the art, now informed by the present disclosure, and hence are not described further. 
     Certain currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the claims.