Patent Publication Number: US-2022236413-A1

Title: Laser scanner apparatus and method of operation

Description:
TECHNICAL FIELD 
     Various embodiments of a laser scanner apparatus and a method of operating a laser scanner apparatus, as disclosed herein, include the use of a variably blocked aperture or a controlled defocusing in relation to receiving backscattered light. 
     BACKGROUND 
     A typical laser scanner apparatus, or simply “scanner.” emits a laser pulse into a surrounding physical environment and detects one or more “return” or “reflection” pulses, as backscattered from one or more objects in the surrounding environment. By way of example, a scanner may “sweep” a defined angular range within a horizontal plane, e.g., 180 degrees, or it may sweep through defined horizontal and vertical ranges, emitting one or more pulses at each angular step and correspondingly monitoring for backscattered light. Monitoring for return reflections with respect to each emitted laser pulse may be confined to an interval corresponding to minimum and maximum detection distances of the scanner—i.e., a working “detection” range”—according to time-of-flight (ToF) principles. 
     An example scanner includes a transmitter arrangement operative to emit laser pulses and a receiver arrangement operative to detect corresponding backscattered light, where the optical paths for transmission from the scanner and reception by the scanner may be coaxial. A scanning mirror may be used to receive backscattered light and direct it towards a photodetector of the scanner, where the angle subtended by the mirror is greater for closer objects and lesser for more distant objects. 
     Consequently, backscattered light from objects tends towards a paraxial approximation with increasing object distance, and the differences in ray divergence of backscattered light as a function of object distance influence the performance of the optical receive path in a typical scanner. Other distance-related factors that are recognized herein as affecting such performance further include poor focusing performance regarding objects that are closer than a defined threshold. 
     SUMMARY 
     Various embodiments of a laser scanner apparatus and a method of operating a laser scanner apparatus, as disclosed herein, include the use of a variably blocked aperture or a controlled defocusing in relation to receiving backscattered light. One or more embodiments combine both variable-blocking and defocusing and may use a lens design that complements the blocking and defocusing. Among the various advantages offered by one or more embodiments disclosed herein is a laser scanner apparatus that exhibits a flatter response curve to backscattered light over a defined range of distances. That is, among other advantages of the configurations and operating methods disclosed herein, a laser scanner apparatus experiences less variation in the optical power delivered to its photodetector arrangement, in relation to detecting an object at different distances within a defined range. 
     A laser scanner apparatus according to one embodiment includes an optical transmitter arrangement configured to transmit a laser pulse into a surrounding physical environment of the laser scanner apparatus. Further included, an optical receiver arrangement is configured to receive backscattered light at a mirror and project the received backscattered light as a projected beam towards an aperture interposed between a lens and the mirror. The lens is configured to focus backscattered light passed by the aperture towards a photodetector, and the aperture configured to impart no blocking of the projected beam with respect to the lens, for beam sizes that do not exceed a fixed central region of the aperture. 
     However, the aperture imparts a variable blocking of the projected beam with respect to the lens, for beam sizes that are larger than the central region of the aperture. Variable blocking is provided by a fixed annular region of the aperture surrounding the central region. The amount of blocking increases within the annular region as a function of radial distance from the optical axis of the lens, on which the central region of the aperture is centered. As such, the annular region may be considered as providing progressively more blocking, with increasing radius. Progressive blocking in this manner prevents some of the light associated with larger, more divergent beams from reaching the lens, with the amount of blocking becoming more aggressive (greater) with increasing beam size, for beams that are larger than the diameter of the central portion of the aperture. Here, “beam size” may be understood as beam diameter taken in the plane defined by the surface of the aperture facing the mirror, where that plane is transverse to the optical axis of the lens. 
     In another embodiment, a method performed by a laser scanner apparatus includes transmitting a laser pulse into a surrounding physical environment of the laser scanner apparatus. Further, the method includes the laser scanner apparatus receiving backscattered light and projecting it towards a lens as a projected beam centered on the optical axis of the lens, wherein the lens operates as a focusing lens for a photodetector of the laser scanner apparatus that is used to sense backscattered light. Still further, the method includes blocking the backscattered light with respect to the lens, for beam sizes of the projected beam that exceed a first beam size, wherein the blocking is progressive as a function of radial distance from the optical axis of the lens, for beam sizes between the first beam size and a larger, second beam size. 
     As noted above, various embodiments disclosed herein offer the advantage of flattening the sensitivity curve of a laser scanner apparatus over its operating (distance) range. That advantage is gained in whole or in part by any one or more of the following operations or configurations: (a) a controlled defocused position of the photodetector with respect to the ideal focal plane of the receiver lens; and (b) implementation of the laser scanner apparatus according to a mathematical model of the receiver lens, aperture, and object-distance ranges that leads to the definition of light breaking structures at the inner edge (circumference) of the aperture, with a geometry aimed at equalizing the optical power delivered from the lens to the photodetector, for backscattered coming from an object at different distances from the laser scanner apparatus. 
     The contemplated sawtooth profile of the light breaking structures of the aperture in at least one embodiment is symmetric about the optical axis of the lens, with defined angles and thicknesses for the tooth portions. Further, the receiver lens has a geometry that in tandem with the geometry of the aperture and yields further smoothing of the sensitivity curve of the laser scanner apparatus. 
     Of course, the present invention is not limited to the above features and advantages. Those of ordinary skill in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of one embodiment of a laser scanner apparatus. 
         FIG. 2  is a block diagram of example details for a laser scanner apparatus, according to one embodiment. 
         FIGS. 3-5  are block diagrams of an aperture for use in a receive optical path of a laser scanner apparatus, according to one or more embodiments. 
         FIG. 6  is a block diagram of examples of backscattered light returned to a laser scanner apparatus, for objects at different distances from the laser scanner apparatus. 
         FIGS. 7-9  are diagrams of further example details for an aperture for use in a receive optical path of a laser scanner apparatus, according to one or more embodiments. 
         FIG. 10  is a cutaway, side view of an example optical subassembly for a receive optical path of a laser scanner apparatus, according to one embodiment. 
         FIG. 11  is a block diagram of an example arrangement for defocusing a photodetector with respect to a lens, within a receive optical path of a laser scanner apparatus, according to one embodiment. 
         FIG. 12  is a diagram illustrating an example of shadowing by a laser transmitter module in an optical transmitter arrangement of a laser scanner apparatus, onto an aperture in an optical receiver arrangement of the laser scanner apparatus. 
         FIG. 13  is a plot of example sensitivity curves of a laser scanner apparatus, depicting sensitivity as a function of object distance, with and without sensitivity compensation as contemplated herein. 
         FIG. 14  is a logic flow diagram of one embodiment of a method performed by a laser scanner apparatus. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts an example laser scanner apparatus  10 , which may also be referred to as “apparatus  10 ” or “scanner  10 .” In at least one example, the apparatus  10  determines distances to objects detected in its surrounding physical environment, based on the time-of-flight principle, according to which the apparatus  10  measures the time elapsed between its transmission of a laser pulse into the environment and its detection of the return reflection(s). 
     Here, the “reflections” are backscattered light from the object(s) illuminated by the transmitted laser pulse. Detecting the reflected pulses comprises, for example, monitoring a photodetector signal output by a photodetector of the apparatus  10  over an interval referenced to the transmission event, detecting signal pulse(s) within the monitored photodetector signal, and determining an elapsed time between the transmission event and the occurrence(s) of the detected signal pulses. Of course, “detection” in this regard may involve relatively complex filtering and waveform processing, for rejection of noise, separation of closely-spaced reflections, etc. 
     Casting the above operations against the implementation details of the example apparatus  10  depicted in  FIG. 1 , the apparatus  10  includes an optical transmitter arrangement  12  that is configured to transmit a laser pulse  14  outward into its surrounding environment. Assuming the transmitted laser pulse  14  strikes a reflective object that falls within the detection capabilities of the apparatus  10  in terms of object size, reflectivity, and distance from the apparatus  10 , the apparatus  10  receives backscattered light  18  comprising one or more return reflections, referred to as a “return pulses” or “reflected pulses.” Generally, an optical receiver arrangement  16  of the apparatus  10  receives only a portion of the light backscattered by the object, and “backscattered light  18 ” refers to the reflected pulses incoming to the optical receiver arrangement  16 . 
     Further elements of the example apparatus  10  include an internal test/calibration arrangement  20 , which may include one or more types of reflective targets and associated circuitry within the apparatus  10 . The apparatus  10  uses such an arrangement to verify ongoing detection capabilities of the apparatus  10 , e.g., for use of the apparatus  10  in safety-critical monitoring applications, such as where the apparatus  10  scans a two-dimensional area or a three-dimensional volume, for object intrusions. 
     Other example elements include processing circuitry  22 , input/output (I/O) circuitry  24 , and communication interface circuitry  26 . The processing circuitry  22  comprises fixed circuitry, programmatically-configured circuitry, or some combination of both. Example processing circuitry includes any one or more of Field Programmable Gate Arrays (FPGAs). Complex Programmable Logic Devices (CPLDs). Application Specific Integrated Circuits (ASICs). System-on-a-Chip (SoC) modules, Digital Signal Processors (DSPs), microcontrollers, or microprocessors. In at least some embodiments, such circuitry includes or is associated with one or more types of computer-readable media used for storing one or both of configuration data, operating logs, and computer-program instructions, the execution of which at least partially configures the apparatus  10  to operate in the manner(s) described herein. 
     I/O circuitry examples include solid-state or mechanical (“dry”) relay outputs. e.g., for gating power to machinery, triggering external events, activating alarms, activating visual or audible annunciators, etc. Examples of the communication interface circuitry  26  include network interface cards (NICs), such as for Ethernet or other data-networking protocols. The communication interface circuitry  26  may implement more than one physical interface and more than one set or type of communication protocols, depending upon operational requirements, factory-floor network types, etc. Similarly, the power supply  28  comprises, for example, an AC/DC converter that receives mains power and provides the various DC voltages needed within the apparatus  10 . Of course, other power-supply configurations are contemplated. 
     The apparatus  10  may be housed in a dustproof and splash-resistant housing, to prevent contamination of its optical components and may include an optical window  30  for emitting laser pulses  14  and receiving backscattered light comprising reflected pulses  18 . After passing through any such window  30 , the backscattered light “enters” the optical receiver arrangement  16 , which includes an optical receive path. 
       FIG. 2  illustrates example details for an optical receive path  40 , with the example arrangement including a scanning mirror  42  that is configured to project the backscattered light  18  as a projected beam  44 , towards an aperture  46 . The projected beam  44  passes completely or partly through the aperture  46 , such that the amount of backscattered light  48  that impinges on a lens  50  depends on whether or to what extent the projected beam  44  is blocked by the aperture  46 . Correspondingly, the backscattered light  48  that impinges on the lens  50  is focused towards a photodetector  54  of the apparatus  10 , as focused light  52 . In at least one embodiment, the photodetector  54  is an avalanche photodiode, which is denoted in  FIG. 2  as an “APD.” 
     A photodetector signal  56  output from the photodetector  54  is an electrical signal that responds to backscattered light impinging on its active surface area. Detailed later herein are embodiments of the apparatus  10  where the photodetector  54  is positioned at an offset relative to the focal plane of the lens  50 —i.e., it is “defocused” with respect to the lens  50 . The offset is along the optical axis of the lens  50 , either towards the lens  50  or away from the lens  50 . As such, some of the rays of the focused light  52  may not strike the active surface of the photodetector  54 . 
     In at least one embodiment, the photodetector signal  56  is an analog electrical signal that increases in amplitude in proportion to the optical power received at the active surface of the photodetector  54 . Return reflections of the transmitted laser pulse  14  that are received at the apparatus  10  as backscattered light  18  are manifested in the photodetector signal  56  as signal pulses having a peak amplitude corresponding to the peak optical power impinging on the photodetector  54 . One transmitted laser pulse  14  may produce multiple reflections, and the photodetector signal  56  may exhibit multiple signal pulses over the interval of interest, along with spurious movements and other noise. 
     Filter circuitry  60  provides some noise rejection and bandwidth limiting of the photodetector signal  56 , in advance of analog-to-digital converter (ADC) circuitry  62 , which outputs a series of digital samples over the interval of interest, for temporary storage in a buffer circuitry  64 . Waveform processing circuitry  66  evaluates the series of digital samples held in the buffer circuitry  64 . e.g., for peak detection and corresponding pulse identification. Time-of-flight (ToF) processing circuitry  68  performs ToF calculations, using the temporal position(s) of the detected pulse(s) within the series of digital samples, and system processing circuitry  70  responds to the ToF determinations. e.g., by carrying out various actions in dependence on whether an object was detected or at what distance. Such operations may include qualification operations, for more reliable detection, and may be repeated at high speed over one or more angular scanning ranges. 
     Turning back to illustrated details, the optical receiver arrangement  16  is configured to receive backscattered light  18  at a mirror  42  and project the received backscattered light  18  as a projected beam  44  towards the aperture  46 , which is interposed between a lens  50  and the mirror  42 . The lens  50  is configured to focus backscattered light  48  passed by the aperture  46  towards the photodetector  54 . Particularly, the aperture  46  in one or more embodiments is configured to impart no blocking of the projected beam  44  with respect to the lens  50 , for beam sizes that do not exceed a fixed central region of the aperture  46  and impart a variable blocking of the projected beam  44  with respect to the lens  50 , for beam sizes that are larger than the central region of the aperture  46 . A fixed annular region of the aperture  46  surrounds the central region and provides the progressive blocking, with the amount of blocking increasing within the annular region as a function of radial distance from the optical axis of the lens  50 , on which the central region of the aperture  46  is centered. 
       FIG. 3  illustrates an example configuration of the aperture  46 , shown in a plan view—i.e., looking towards the lens  50 , along the optical axis of the lens  50 . In other words,  FIG. 3  depicts the “face” of the aperture  46  seen from the perspective of the mirror  42 . 
     A central region  80  of the aperture  46  is open and free of obstruction, with the circumferential boundary that defines the central region  80  at a radius r 1  from the optical axis of the lens  50 , where, in assembled form, the aperture  46  is centered around the optical axis of the lens  50 . An annular region  82  surrounds the central region  80  of the aperture  46  and occupies the circular area lying between the radius r 1  and the radius r 2 . The annular region  82  imparts a variable blocking for beam sizes of the projected beam  44 —see  FIG. 2 —that have beam diameters in the range of 2×r 1  to 2×r 2 . Here. “variable blocking” denotes a progressive blocking, wherein the amount of light blocking increases as a function of increasing radius from the optical axis of the lens  50 , for radii in the range between r 2 −r 1 . 
     The intensity of the backscattered light  18  varies as a function of distance, for an object having a given reflectivity. Consequently, the optical power delivered to the photodetector of a typical conventional scanner may vary sharply as a function of object distance. There may a close-in range of distances that are too close for focusing by the typical scanner, meaning that the photodetector receives comparatively little optical power. Beyond that close-in range lies “near-field” range of distances that begin falling into the focusing capabilities of the typical scanner, and the scanner delivers increasing optical power to its photodetector as object distance increases and focusing performance improves. Beyond the near-field range of distances and out to a maximum detection distance of the scanner lies a “far-field” range, and the typical scanner tends to experience decreasing optical power with increasing object distance over the far-field range. 
     One way to appreciate the configuration and resulting operation of the aperture  46  depicted in  FIG. 3  is to recognize that the radius r 1  may be dimensioned for passing beam sizes associated with a first range of object distances, e.g., a far-field range, whereas the radii between r 1  and r 2  correspond to beam sizes associated with a closer, second range of object distances. e.g., a near-field range. As such, the amount of beam blocking varies as a function of beam size, with the larger, more intense beams associated with decreasing object distance in the near-field range experiencing progressively greater light blocking, with respect to that portion of the beam that impinges on the annular region  82  of the aperture  46 . The further surrounding region  84  of the aperture  46  may be opaque and include structural features for assembly and mounting. Thus, the radius r 2  that defines the outer circumference of the annular region  82  may be set according to a maximum beam size associated with the closest distance in the near-field range. 
     With the annular region  82  effectively providing a variable transmissivity between radius r 1  and radius r 2 , one or more embodiments of the aperture  46  form the annular region  82  using a transparent medium that darkens with increasing radius or otherwise exhibits decreasing transmissivity with increasing radius, for radii between r 1  and r 2 . The decrease may be continuous or non-continuous, e.g., stepped increases in light blocking.  FIG. 4  illustrates one embodiment of stepped increases in light blocking, where concentric rings  92 - 1 ,  92 - 2 ,  92 - 3 , and  92 - 4  form the annular region  82 , with each next (larger) ring  92  having more light blocking.  FIG. 5  illustrates the other approach, where the annular region  82  provides continuously increasing blockage. The term “progressive blocking” encompasses both stepped increases in blocking and continuous or gradual increases in blocking. 
       FIG. 6  offers additional understanding of the blocking effect, where the mirror  42  subtends a larger angle a 1  for an object at a distance d 1 , as compared to the angle a 2  subtended for the same object at a distance d 2 . The beam projected by the mirror  42  is larger for the object at the distance d 1  and the outer extents or portion of the beam that fall onto the annular region  82  of the aperture  46  experience progressive light blocking, while that portion of the beam corresponding to the central region  80  of the aperture  46  experiences no blocking. The beam size associated with the object at distance d 2  falls entirely within the central region  80  of the aperture  46 . 
       FIG. 7  illustrates another embodiment of the aperture  46 , where light breaking structures  94  ring the central region  80  in a radial array and form the annular region  82 . Here, the light breaking structures  94  comprise “saw teeth” that point towards the center of the open central region  80  of the aperture  46 , meaning that the width of the saw teeth increases with increasing radius from the center of the central region  80 —i.e., from the optical axis of the lens  50 . As such, the aggregate or overall amount of light blocking provided by the saw teeth depends on the radial distance from the center of the central region  80 , which is aligned on the optical axis of the lens  50 . The taper of the saw teeth provides progressively more light blocking for increasing radii between radius r 1  and radius r 2 . 
       FIG. 8  offers a close-up view of the light breaking structures  94  in one embodiment, where only a small portion of the aperture  46  is shown. The depicted embodiment shows a stepped progression with respect to the taper, rather than a smooth, continuous progression. Using a stepped progression of the sort illustrated defines concentric rings, with each next ring in order of increasing radius from the optical axis providing a greater amount of light blocking. As such, the stepped taper seen in  FIG. 8  represents another approach to achieving the concentric-ring arrangement shown in  FIG. 4 . 
       FIG. 9  depicts the aperture  46  according to one embodiment, with the aperture  46  shown in perspective.  FIG. 10  illustrates an optical subassembly  110  according to one embodiment of the optical receiver arrangement  16  introduced in  FIG. 1 , for the apparatus  10 . Included in the optical subassembly  110 —which is shown in cross-section from a side view—are the aperture  46 , the lens  50 , and the photodetector  54 , in an assembled arrangement. 
     Notable details in the example embodiment depicted in  FIG. 10  for the optical subassembly  110  include a clear illustration of the “shadowing” arrangement of the aperture  46  with respect to the lens  50 . The central region  80  of the aperture  46  aligns on the optical axis and is open or otherwise 100% transmissive, meaning that it imparts no blocking or reduction of light rays passing through the central region  80 . The annular region  82  that surrounds the central region  80  provides a progressive blocking or shadowing of light rays, with increasing radius from the optical axis of the lens  50 , meaning that progressive light blocking occurs with respect to the lens surface area(s) that are shadowed by the annular region  82  of the aperture  46 . 
     In one or more embodiments, at least for a defined range of distances, the variable blocking provided by the aperture  46  reduces distance-related variations in the optical power delivered to the photodetector  54  that would otherwise arise because of the beam size of the projected beam  44  being dependent upon the distance between the apparatus  10  and an object in the surrounding environment that provides the backscattered light  18  received by the apparatus  10 . 
     In an example arrangement, in cases where the beam size of the projected beam  44  does not exceed the size of the central region  80  of the aperture  46 , the aperture  46  imparts no reduction in the optical power delivered to the photodetector  54 . In cases where the beam size of the projected beam  44  exceeds the size of the central region  80  of the aperture  46 , the aperture  46  imparts a reduction in the optical power delivered to the photodetector  54 , with the amount of the reduction depending on the beam size (at least for beam sizes that fall within the size range defined by the annular region  82  of the aperture  46 ). 
     For example, the size—e.g., diameter—of the central region  80  is dimensioned for beam sizes of the projected beam  44  that correspond to a first range of distances, and an overall size of the central region  80  plus the annular region  82  is dimensioned for beam sizes of the projected beam that correspond to a second range of distances. The second range is closer to the apparatus than the first range. Backscattered light  18  returned to the apparatus  10  from objects at distances in the first range is substantially paraxial with the optical axis of the optical receiver arrangement  16  and backscattered light  18  returned to the apparatus  10  from objects at distances in the second range is not substantially paraxial with the optical axis of the optical receiver arrangement  16 . Because the optical axis of the optical receiver arrangement is the optical axis of the lens  50 , as “projected” onto the mirror  42 , one may refer to the backscattered light  18  being paraxial, or not paraxial, with the optical axis of the lens. 
     A circumferential edge of the aperture  46  surrounds the central region  80  of the aperture  46 , which central region  80  is open. The circumferential edge in one or more embodiments has a sawtooth contour that provides progressive light blocking. As an example, the central region  80  of the aperture  46  is open and ringed by plurality of circumferentially-arrayed tapered projections having tips extending towards the center of the aperture  46  and terminating at a first radial distance (r 1 ) from the center of the aperture  46 . The first radial distance defines the size of the central region  80  and the tapered projections provide increased beam blocking with increasing radial distance from the center of the aperture  46 . 
     In at least one embodiment of the optical receiver arrangement  16 , the arrangement  16  includes a photodetector  54  that provides an output signal responsive to the projected beam  44 , as directed towards the photodetector  54  by the lens  50 . That is, the photodetector  54  is responsive to the backscattered light that is passed by the aperture  46  and focused by the lens  50  towards the active surface of the photodetector  54 . In one or more embodiments, the active surface of the photodetector  54  is positioned along the optical axis of the lens  50  at an offset from the focal plane of lens  50 . The offset positions the active surface of the photodetector closer to the lens (or further from the lens), so that less than all the focused light, as redirected by the lens  50  towards the photodetector  54 , falls on the active surface of the photodetector  54 , for beam sizes greater than a defined size. The defined size corresponds to a near-field range of distances from the apparatus  10 , for example. 
     Broadly, in one or more embodiments of the apparatus  10 , the spacing between the lens  50  and the photodetector  54  partially “defocuses” the photodetector  54  with respect to the lens  50 , to flatten a sensitivity curve of the apparatus  10  that is associated with characteristically higher beam powers of the backscattered laser beam, for an object that is within a near-field distance range from the apparatus, as compared with a far-field distance range. The “amount” or degree of defocusing is configured to provide a certain amount of flattening of the sensitivity curve, while preserving a minimum sensitivity of the optical receiver arrangement  16 . 
     In one or more embodiments, the lens  50  is a bi-convex aspheric lens, and the unobstructed central region  80  of the aperture  46  is a circular area having a diameter that is less than a width of a reflecting surface of the mirror  42 , which projects the backscattered light  18  towards the aperture  50  as a projected beam  44 . Such a lens design complements the imposition of progressive light blocking by the aperture  46 , as described herein. 
       FIG. 11  illustrates another technique for implementation of the optical receiver arrangement  16  of the apparatus  10 , for reducing variations in the amount of optical power delivered to the photodetector  54 , for an object at different distances from the apparatus  10 , at least for distances within a defined detection range. Referring to reducing the variations in optical power delivered to the photodetector  54  as a function of object distance is another way of describing the flattening of the sensitivity curve of the apparatus  10 . 
     The technique comprises operating the photodetector  54  at a defocused position with respect to the lens  50 , meaning that the active surface of the photodetector in a plane transverse to the optical axis is not aligned with the focal plane of the lens  50 . Instead, the active surface of the photodetector  54  is offset along the optical axis. One embodiment uses a negative offset, towards the lens  50 , while another embodiment-emphasized by showing the photodetector  54  in dashed lines-uses a positive offset, away from the lens  50 . Here. “towards” and “away” refer to displacement of the active surface of the photodetector along the optical axis of the lens  50 , either closer to the lens  50  or further from the lens  50 . 
     Another way to describe the technique illustrated in  FIG. 11  is “defocusing” the photodetector  54  with respect to the lens  50 . Defocusing is used in combination with progressive blocking via the aperture  46  in some embodiments, with the design parameters of the aperture  46  and the amount of defocusing determined in a complementary or joint fashion. Other embodiments use progressive blocking without defocusing, and still other embodiments use defocusing without progressive blocking. 
     “Defocusing” does not mean a significant defocusing but rather implies a slight or small defocusing, in which the photodetector  54  is purposefully offset from the ideal focal plane of the lens  50 , where the offset is along the optical axis of the lens  50 . Defocusing via the purposeful offsetting of the photodetector  54  results in a more uniform delivery of optical power to the photodetector  54  for a given object illuminated by the apparatus  10 , occupying different distances within the detection range of the scanner. 
     More particularly, the photodetector  54  does not experience a sharp peak in delivered optical power at the transition point between near-field optical performance of the apparatus  10  and far-field optical performance of the apparatus  10 . That transition point corresponds to the object distance at which the backscattered light  18  still has a high intensity as compared to the intensity seen at greater distances for the same object reflectivity but where the rays of the backscattered light  18  begin taking on a paraxial approximation with respect to the optical axis of the optical receiver arrangement  16 . That axis is a projection of the optical axis of the lens  50 . 
       FIG. 12  illustrates another aspect of the aperture  46  in one or more embodiments, wherein the optical axis of the optical transmitter arrangement  12  is coaxial with that of the optical receiver arrangement  16 , which results in a partial “shadowing.” That shadowing projects onto the aperture  46  and thus onto the underlying lens  50 . In other words, the transmitter-module shadowing imposes a certain amount of light reduction with respect to the projected beam  44  produced by the mirror and, correspondingly, with respect to the focused light  52  produced by the lens  50 . The amount and progressivity of light blocking provided by the annular region  82  of the aperture  46  and/or the amount of defocusing of the photodetector  54  relative to the lens  50  accounts for the “lost” optical power associated with the shadowing caused by the transmitter module of the optical transmitter arrangement  12 . 
       FIG. 13  is an example plot of a “sensitivity” curve  120  for an example apparatus  10  without use of the curve-flattening techniques disclosed herein—i.e., any one or more of progressive blocking or defocusing, along with complementary lens design. The sensitivity curve  122  illustrates the flattening effects provided by the combination of progressive blocking and defocusing, along with a complementary design of the lens  50 . 
     Both curves  120  and  122  show the optical power delivered to the photodetector  54  of the apparatus  10 , with respect to a given object moved through a range of distances, e.g., out to some maximum detection distance of the apparatus  10 . The curve  122  exhibits a more uniform power delivery over the range—i.e., the curve  122  is flatter than the curve  120 . Improvements are seen in the closest and next-closest ranges of distance, labeled A and B in the diagram, at the expense of lower power delivery across all three ranges A. B, and C. 
     Range A may be understood as including distances for which the apparatus  10  exhibits poor focusing performance, whereas Range B may be understood as a near-field range of distances, where focusing performance of the apparatus  10  begins improving, although the backscattered light  18  received by the apparatus  10  is not paraxial with the optical axis of the optical receiver arrangement  16 . Range C may be understood as a far-field range of distances, out to some maximum distance, wherein the backscattered light  18  received by the apparatus  10  tends towards the paraxial approximation with increasing distance. 
       FIG. 14  illustrates a method  1400  performed by a laser scanner apparatus  10  according to one or more embodiments. The method  1400  includes transmitting (Block  1402 ) a laser pulse into a surrounding physical environment of the apparatus  10 , receiving (Block  1404 ) backscattered light and projecting it towards a lens as a projected beam centered on the optical axis of the lens, wherein the lens operates as a focusing lens for a photodetector of the apparatus  10  that is used to sense backscattered light. Further, the method  1400  includes blocking ( 1406 ) the backscattered light with respect to the lens, for beam sizes of the projected beam that exceed a first beam size, wherein the blocking is progressive as a function of radial distance from the optical axis of the lens, for beam sizes between the first beam size and a larger, second beam size. 
     As one example, the first beam size corresponds to a first object distance that represents the beginning of a far-field distance range of the apparatus  10  for which the backscattered light returned to the apparatus  10  takes on a paraxial approximation. Correspondingly, the second beam size corresponds to a minimum specified object distance, with object distances between the minimum specified object distance and the beginning of the far-field distance range being a near-field detection range of the apparatus  10  for which the paraxial approximation does not hold. 
     The method  1400  in one or more embodiments further includes operating (Block  1408 ) the photodetector at a defocused position with respect to the lens. In this regard, the apparatus  10  may include an internal structure or subassembly that purposefully locates the photodetector at a fixed position along the optical axis of the lens, where that fixed position is offset from the focal plane of the lens. That is, the active surface of the photodetector does not lie in the focal plane of the lens. The offset position may be towards or away from the lens. 
     Among the various goals or benefits associated with the method  1400  and other embodiments detailed herein is a flattening of the sensitivity curve of a laser scanner apparatus, where the sensitivity curve refers to the optical power delivered to the photodetector of the apparatus as a function of the distance from the apparatus to the object being detected. As an added benefit, the associated circuit design and signal processing downstream from the photodetector is simplified because of the curve flattening. Metrics may be used to quantify the amount of flattening gained, and the metrics may be subdivided into far-field and near-field metrics. As an example, the near-field detection range of an example apparatus  10  spans the range of 0 to 1 meters of distance from the apparatus  10 . An example far-field distance range then spans from the 1-meter mark out to a maximum distance of 6 meters. Of course, the apparatus  10  may be designed for detection over different distance ranges. 
     Thus, a working definition of the near-field sensitivity of an apparatus may be expressed as: 
     
       
         
           
             
               Near 
               ⁢ 
               
                   
               
               ⁢ 
               field 
               ⁢ 
               
                   
               
               ⁢ 
               sensitivity 
             
             = 
             
               
                 
                   
                     
                       Max 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       power 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       on 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       photodetector 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       when 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       target 
                     
                   
                 
                 
                   
                     
                       moves 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       between 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       0 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1000 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                   
                 
               
               
                 
                   
                     
                       Power 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       on 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       the 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       photodetector 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       when 
                     
                   
                 
                 
                   
                     
                       target 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       at 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1000 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                   
                 
               
             
           
         
       
     
     Here. “target” refers to the given object being detected. 
     A corresponding definition of the far-field sensitivity of the apparatus may be expressed as: 
     
       
         
           
             
               Far 
               ⁢ 
               
                   
               
               ⁢ 
               field 
               ⁢ 
               
                   
               
               ⁢ 
               sensitivity 
             
             = 
             
               
                 
                   
                     
                       Max 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       power 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       on 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       photodetector 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       when 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       target 
                     
                   
                 
                 
                   
                     
                       moves 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       between 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1000 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       and 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       6000 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                   
                 
               
               
                 
                   
                     
                       Power 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       on 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       the 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       photodetector 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       when 
                     
                   
                 
                 
                   
                     
                       target 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       is 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       at 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       6000 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       mm 
                     
                   
                 
               
             
           
         
       
     
     The ideal values for both near field and far field sensitivity metrics is one (“1”), which means a constant horizontal sensitivity curve. Without the use of one or more of the compensating features disclosed herein, the sensitivity-curve metrics may exceed a value of 8. As detailed herein, an apparatus  10  includes one or more features that flatten the sensitivity curve across the far-field and near-field distance ranges, bringing the metric values into the range of 2 to 4, for example. 
     For example, with respect to the defocusing feature described herein, the defocusing technique positions the photodetector  54  at a location along the optical axis of the lens  50  that makes the optical power delivered to the photodetector  54  less strongly dependent on the extent to which the backscattered light  18  incoming to the optical receiver arrangement  16  deviates from the paraxial approximation. The amount of defocusing is calculated, for example, as a trade-off between sensitivity-curve flattening and the need to preserve detectable signal levels out to the maximum detection distance, for a defined range of object sizes and reflectivity. 
     Regarding the design of the lens  50  to complement defocusing of the photodetector  54  and/or progressive blocking by the aperture  46 , the lens geometry is determined in accordance with a desired back focal length and focusing performance. As noted, the lens  50  may be configured as a bi-convex aspheric lens, and the size of the central region  80  of the aperture  46  may be set to a diameter just slightly smaller than the horizontal width of the scanning mirror  42 . 
     The central region  80  of the aperture  46  is fully open in one or more embodiments, with the diameter of that opening mainly responsible for passing backscattered light from objects in the far field (where paraxial approximation is valid). Correspondingly, the annular region  82  of the aperture  46  may be regarded as a circular crown region with variable clarity or otherwise variable light-blocking, in dependence on radial distance from the optical axis of the lens  50 . The annular region  82  is mainly responsible for controlling the light coming from near-field objects, where the paraxial approximation is not valid any longer. Central-region and annular-region dimensioning flows, for example, from considering the characteristic beam sizes of the projected beam  44  for the apparatus  10 , over the far-field and near-field distance ranges. 
     For example, with reference back to  FIG. 6 , having the characteristic beam sizes and knowing the width of the scanning mirror  42  allows for the calculation of the subtended angles a 1  and a 2  corresponding to an object at distances d 1  and d 2 . The subtended angle a is inversely proportional to “d” (when “d” increases, a decreases and vice versa). For example, considering two target positions d 1  (in near field) and d 2  (in far field), the subtended angle a 1  is greater than the subtended angle a 2 . 
     Knowing the subtended angle a for each position d allows for the calculation of the fraction of beam power P enclosed in a and backscattered towards the mirror  42 . For d 1 , the power backscattered towards the mirror  42  is P 1 , and, for d 2 , the power is P 2 . The ratio between P 1  and P 2  depends on the actual distances, where P 1  can be either higher or lower than P 2  depending on the specific positions d 1  and d 2 . Further, P is only a fraction of the total power backscattered by the object at the distance d because it is only that portion of the backscattered light enclosed by the subtended angle a—i.e., that portion of the backscattered light that is returned to the mirror  42  of the apparatus  10  (labeled as backscattered light  18  in  FIG. 1 ). 
     Knowing the angle a for each distance d, the area “b” of the backscattered beam projected by the mirror  42  onto the top surface of the aperture  46  may be calculated. As the subtended angle a varies with object distance d, these beam areas b also vary according to the distance d (i.e., the size of the projected beam  44  varies with object distance). For example, consider two target positions d 1  (in near field) and d 2  (in far field): the subtended angle a 1  for d 1  is greater than the subtended angle a 2  for d 2 , and the beam area bi for d 1  is greater than the beam area b 2  for d 2 . 
     The foregoing geometric considerations provide for calculation of the average irradiance I (power per unit area) for each beam area b, which is assumed to be uniform within the beam area. For example, the projected-beam area b 1  has irradiance I 1  given by P 1 /b 1 , while the projected-beam area b 2  has irradiance  12  given by P 2 /b 2 . Because the clear central region  80  of the aperture  46  is smaller than the projected-beam sizes b for near-field objects, the power P′ encircled within the central region  80  may be calculated for each area b, knowing its average irradiance I. That is P′=I*CA, where “CA” denotes the unobstructed clear area of the aperture  46 , as provided by the central region  80 . The power calculations must, however, also account for the transmitter module shadowing, as shown in  FIG. 12 . That adjusted power may be expressed as P“and it is based on an adjusted beam area b”, where b″=CA−area shadowed by the transmitter module). The power P″ reflects the power “conveyed” to the lens  50  via the central region  80  of the aperture, with the TX-module shadowing account for. 
     For an object at a distance d 1  from the apparatus, the power delivered to the photodetector  54  may be expressed as P 1 ″ and the power delivered to the photodetector  54  for an object at a distance d 2  may be expressed as P 2 ″. (Note, the P 1 ″ and P 2 ″ values may be as calculated above—i.e., they may be the optical power delivered to the lens  50  and then focused onto the photodetector  54 , or they may deviate from the above calculations by amounts related to any defocusing applied for the photodetector  54  with respect to the lens  50 .) 
     For a flat sensitivity curve, then for each object distance d, the power P″ delivered to the photodetector  45  must be the same, that is P 1 ″=P 2 ″ for distances d 1  and d 2 . Because the optical power incoming to the apparatus  10  changes with distance, one mechanism for reducing the variation in the optical power delivered to the photodetector  54  over a range of optical distances is to operate on the beam shape of the resulting projected beam  44 . That is, the aperture  46  can be understood in some sense as reshaping or controlling the projected beam  44 , to provide beam areas b″ that vary less over object distance. The central region  80  is, for example, dimensioned in dependence on the optical power incoming to the apparatus  10  for far-field object detection. Further, the inner and outer diameters of the annular region  80 , the amounts and progressiveness of the light blocking provided by it, may be determined based on dividing the annular region  82  into a series of small concentric circular crowns, such as seen in  FIG. 8 , with crown providing a certain amount of light blocking. There may be one concentric ring (crown) per each distance d considered in near field, each one with fixed width. For example, ringing the central region  80  with a sawtooth profile provides for a correspondingly greater reduction in the amount of light passing through the aperture  46  in the annular region  82 , with increasing radius from the optical axis. 
     This progressive light reduction yields a corresponding reduction in transferred optical power, as between the mirror  42  and the lens  50 , for that portion of the projected beam that falls onto the annular region  82 . As such, the photodetector  54  does not experience as much variation in the optical power delivered to it, over the range of object distances corresponding to beam sizes affected by the annular region  82 . 
     Notably, modifications and other embodiments of the disclosed invention(s) will come to mind to one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention(s) is/are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of this disclosure. Although specific terms may be employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.