Patent Publication Number: US-11385350-B2

Title: Tilted image plane lidar

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation patent application claiming priority to, and thus the benefit of an earlier filing date from, U.S. patent application Ser. No. 14/869,448 (filed Sep. 29, 2015), which claims priority to, and thus the benefit of an earlier filing date from, U.S. Provisional Patent Application No. 62/056,945 (filed Sep. 29, 2014), the entire contents of each of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The inventive aspects herein generally related to lidar systems 
     BACKGROUND 
     The interrogating laser of a lidar system is used to cause emissions that are detected by a receiver of the lidar system. The detected emissions can be used to determine spatial distributions and other parameters and materials. The emissions may result from any number of processes (e.g., Mie scattering, Raleigh scattering, nonlinear scattering, scatter from hard targets, parametric processes, Raman scattering, fluorescence, laser induced breakdown etc.). 
     Lidar systems are either monostatic or bistatic. Monostatic optical arrangements share the same aperture for transmitting and receiving. These arrangements have the advantage of optical returns being focused onto relatively fast and small detectors regardless of the range of the interrogated material. Moreover, a single scan mirror can be used for directing the interrogating laser beam and the receiver&#39;s field of view. This common scanner for transmitting and receiving allows the system to have a relatively small field of view that results in relatively low background signatures. However, when emissions are produced near the receiver, they generally provide much larger detected signals than emissions generated at more distant ranges, thus making detection of materials farther away more difficult. 
     Bistatic optical arrangements use separate apertures for the laser and receiver. In these arrangements, the focusing optics for the receiver are generally configured so that near range emissions do not come to focus at the receiver. The field of view of the receiver should be large enough to accommodate imaged illumination patterns from the laser beam path. However, an enlarged field of view generally results in more background radiation being collected, which reduces the signal to noise ratio. 
     SUMMARY 
     Systems and methods presented herein provide for improved range response in lidar systems. More particularly, the embodiments herein relate to lidar systems that operate in distributed mediums (e.g., water, air, tissues with embedded materials, etc.) having emissions ranging from a near field of view to a far field of view. In one embodiment, a lidar system includes a laser, a detector, and first optics operable to direct light from the laser on a beam path along a first optical axis of the first optics. The lidar system also includes second optics having a second optical axis that differs from the first optical axis. The first and the second optical axes lie in a same first plane. The second optics image the light onto a second plane that is substantially normal to the first plane. A first line in the first plane intersects a second line in the second plane at an acute angle. The first line is perpendicular to the first optical axis. The lidar system also includes a spatial filter configured in or near the second plane to filter the light from the second optics onto the detector. 
     The various embodiments disclosed herein may be implemented in a variety of ways as a matter of design choice. For example, some embodiments herein are implemented in hardware whereas other embodiments may include processes that are operable to implement and/or operate the hardware. Other exemplary embodiments, including hardware, software, firmware, and various combinations thereof are described below. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Some embodiments of the present invention are now described, by way of example only, and with reference to the accompanying drawings. The same reference number represents the same element or the same type of element on all drawings. 
         FIG. 1  is a block diagram of an exemplary lidar system. 
         FIG. 2  is a block diagram of an exemplary spatial filter. 
         FIG. 3  is a block diagram of an exemplary tilted image plane of the lidar system of  FIG. 1 . 
         FIG. 4  is a block diagram of another exemplary lidar system. 
         FIGS. 5A-5E  exemplarily illustrate a lidar system&#39;s sensitivity to isotropic emissions from various distances. 
         FIG. 6  is a block diagram of an exemplary lidar system employing a two channel receiver. 
         FIG. 7  is a block diagram of an exemplary lidar system receiver employing a prism. 
         FIG. 8  is a block diagram illustrating exemplary design considerations of the prism of  FIG. 7 . 
         FIG. 9  is a block diagram of an exemplary range response filter. 
         FIGS. 10-13  are graphs illustrating exemplary features of a Linear Variable Neutral Density Filter (LVND). 
         FIG. 14  is a block diagram illustrating a shutter being employed with a prism in an exemplary receiver of the lidar system. 
         FIG. 15  is a block diagram of an exemplary spatial filter having variable widths. 
         FIGS. 16-18  are graphs illustrating exemplary features of the range response filter of  FIG. 15 . 
         FIG. 19  is a graph illustrating exemplary design considerations of a bandpass filter operable within a lidar receiver. 
         FIGS. 20 and 21  are block diagrams of an exemplary imaging spectrometer employing a tilted image plane. 
         FIG. 22  is a block diagram illustrating an exemplary surface scanning embodiment of a lidar system. 
         FIG. 23  is a flowchart of an exemplary process of the lidar system of  FIG. 1 . 
         FIG. 24  is a block diagram of an exemplary computing system in which a computer readable medium provides instructions for performing methods herein. 
     
    
    
     DETAILED DESCRIPTION OF THE FIGURES 
     The figures and the following description illustrate specific exemplary embodiments of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within the scope of the invention. Furthermore, any examples described herein are intended to aid in understanding the principles of the invention and are to be construed as being without limitation to such specifically recited examples and conditions. As a result, the invention is not limited to the specific embodiments or examples described below. 
       FIG. 1  is a block diagram of an exemplary lidar system  100 . The lidar system  100  comprises a laser  101  having optics that are operable to direct the light along the beam path  109 . In this regard, the optics of the laser  101  comprise a first optical axis that coincides with the beam path  109 . The laser  101  may be implemented in a variety ways as a matter design choice. For example, the laser  101  may be operable to pulse laser light along the beam path  109 . Alternatively, the laser  101  may continuously direct light along the beam path  109 . Moreover, the laser  101  may be operable to direct light in any variety of wavelengths and/or polarizations depending on, for example, environmental conditions, desired return emissions, etc. Accordingly, the laser  101  is combination of software, hardware, firmware, and optics, operable to direct light along the beam path  109 . 
     The lidar system  100  also includes a receiver configured from optics  102  (e.g., a lens), a spatial filter  103 , and a detector  105 . The optics  102  comprise an optical axis  116  that generally lies in the same plane as the beam path/optical axis  109 . For example, the optical axis  109  and the optical axis  116 , while different, generally lie in the same plane as the view of the page representing the lidar system  100  and, in this embodiment, generally run parallel to one another, neglecting certain design tolerances. The optics  102  and thus the optical axis  116  are offset some distance  120  from the optical axis  109 , generally referred to herein as the “bistatic offset”. 
     The optics  102  are operable to receive light from the beam path  109  and to image that light onto the spatial filter  103 . To illustrate, the optics  102  receive light from a near position  107  ranging through a far position  108 . The light from the near position  107  is illustrated with the ray  111  whereas the far position  108  is illustrated with the ray  110 . 
     The spatial filter  103  comprises an aperture that is operable to filter off a portion of the light from the beam path  109 . For example, to improve the far field of view range of the lidar system  100 , the spatial filter  103  filters off the light from the near position  107  on the beam path  109  because the stronger emissions from the near position  107  tend to obscure the emissions from the far position  107 . As illustrated, the ray  110  from the far position  108  is allowed to pass through the spatial filter  103  whereas the ray  111  from the near position  107  is rejected by the spatial filter  103 . 
     The spatial filter  103  is configured in or near a second different plane at an acute angle and at a distance  115  from the optics  102 , generally referred to herein as the focal distance  115  of the optics  102 . To illustrate, the spatial filter  103  is configured in a plane that is normal to the plane illustrating the view of the lidar system  100 . The second plane intersects the plane of the lidar system view on a line  118 . Then, assuming a line  119  is drawn in the plane of the lidar system view roughly perpendicular to the optical axis  116  of the optics  102 , an acute angle  112  is formed. Thus, the plane in which the spatial filter  103  lies (or is configured proximate to) is configured at an acute angle of a plane normal to the plane of the lidar system view intersecting at the line  118 . This plane of the spatial filter  103  is generally referred to herein as the “tilted image plane”. 
     Other optics  104  (e.g., a lens) are optionally configured behind the spatial filter  103  to direct the ray  110  from the far position  107  onto the detector  105 . The optics  102  and  104  may be configured in a variety ways as a matter design choice. For example, the optics  102  is generally configured as a lens having a diameter  113 . However, the optics  102  may be representative of one or more optics depending on design considerations, details of which are shown and described below. The detector  105  may also be configured in a variety ways as a matter design choice. For example, in some embodiments, the detector  105  comprises a Photomultiplier Tube (PMT). Alternatively or additionally (e.g., in a dual receiver embodiment), the detector  105  may be a PIN diode detector. Other exemplary embodiments are shown and described below. 
     The embodiments herein have elements and advantages of both bistatic and monostatic lidar systems without many of the disadvantages of those lidar systems (e.g., bright near-field emissions competing with dim far-field emissions, complex and unwieldy designs, etc.). For example, the lidar system  100  can adaptively customize range response which, in turn, reduces dynamic range constraints of the lidar system  100 . The lidar system  100  can also narrow the field of view and attenuate background signals. In this regard, the lidar system  100  can selectively block near range emissions and improve dynamic range control. The lidar system  100  may also be operable to employ a shared scan mirror that aligns the receiver and transmitter portions of the lidar system  100 , as can be seen in the alternative embodiment of  FIG. 4 . 
     It should be noted that it is not necessary for the two optical axes  109  and  116  to be parallel as the position of the tilted image plane may be determined for non-parallel optical axes. For example, the laser beam path  109 , even if not parallel to the optical axis  116 , comes to focus along the tilted image plane. The mathematical formulation of the emission range to the focal position along the tilted image plane can be modified as a matter of design choice while operating in the same manner. 
       FIG. 2  is a block diagram of an exemplary spatial filter  103 . In this embodiment, an aperture or slit  122  is configured in the spatial filter  103 . Generally, the spatial filter  103  is configured from an opaque material that blocks light. When the spatial filter  103  is placed at or near the tilted image plane, the edge  121  of the aperture  122  that is closest to the focal point of the optics  102  blocks the light that is emitted from the more distant ranges (e.g., the light emitted beyond the far field of view point  108 ). The edge  123  of the aperture  122  that is furthest from the focal point of the optics  102  blocks the light emanating from the beam path  109  in the near range (e.g., the light emitted from the near field of view point  107  to the laser  101 ). In some embodiments, the detector  105  may be at the tilted image plane so that the spatial filter is defined by the spatial extent of the detector  105  itself. 
     The aperture  122  of the spatial filter  103  can be dynamically altered depending on a desired range response. For example, if a narrower field of view is desired, the length of the slit between the edges  121  and  123  may be decreased to block more light outside the desired range. The angle  112  of the spatial filter  103  can also be dynamically changed to alter the range response of the lidar system  100 . For example, the spatial filter  103  lies in and or is proximate to the tilted image plane. When that tilted image plane is changed, the point on the beam path  109  which the spatial filter  103  views is also changed. Coupled with the ability to dynamically change the field of view, the lidar system  100  can effectively “close in” on a point of the beam path  109  and image that point via the detector  105 . 
       FIG. 3  is a block diagram of the exemplary tilted image plane of the lidar system  100 . Generally, a mapping between position on the aperture  122  and the range of an emission point can be established as follows: 
                 ξ   ⁡     (   r   )       =       F   ⁢       (       F   2     +     B   2       )           r   -   F         ,         
where r is the range (e.g., the distance between the points  107 - 108 ), F is the focal length  115  of the optics  102 , B is the bistatic offset  120 , and ξ is a distance along the tilted image plane from the focal point of the optics  102 . By choosing the width of the slit of the aperture  122  based on desired range, a customized range response can be established with the lidar system  100 . The near range edge  123  of the aperture  122  is operable to reduce dynamic range requirements that would arise from the undesirable detection of near range emissions.
 
     Additionally, the spatial filter  103  itself may be moved in the tilted image plane (e.g., along the line  118 ) to tailor the range response based on a static aperture size. For example, when a predetermined range is desired and the dimensions of the aperture  122  (e.g., the slit between the edges  121  and  123 ) are statically configured, the field of view of the lidar system  100  can be changed by sliding the spatial filter  103  along the tilted image plane. In this regard, the range response can also be tailored based on the tilt of the spatial filter  103 . With this in mind, the following table illustrates one exemplary set of design considerations of the lidar system  100 . 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Design Specification: 
                 Source/Rationale: 
                 Implications: 
               
               
                   
               
             
            
               
                 Lens diameter 113 (D) = 4 inches 
                 Radiometric analysis 
                 Scanner design, window size, and 
               
               
                   
                   
                 angle range 
               
               
                 Bistatic offset 120 (B) = 3 inches 
                 B &gt; D/2 including room for mounts 
                   
               
               
                   
                 and laser spot. Larger distances may 
                   
               
               
                   
                 require a larger scan mirror 
                   
               
               
                 Focal length 115 (F) = 8 inches 
                 Overall “packaging” constraints, 
                 A lower distortion lens design 
               
               
                 Image angle 125 (α) = 69.44° 
                 α = arctan (F/B) 
                 Collection optics design and tailored 
               
               
                   
                   
                 range response 
               
               
                 10.5 mm &lt; Aperture 122 
                 When near range cutoff (e.g., point 
                 Collection optics design and tailored 
               
               
                 length &lt; 12.1 mm 
                 107) is 5 mm, distance along 
                 range response 
               
               
                   
                 aperture 122 from focal point is 
                   
               
               
                   
                 given by 
                   
               
               
                   
               
               
                   
                 
                   
                     
                       
                         
                           
                             ξ 
                             ⁡ 
                             
                               ( 
                               r 
                               ) 
                             
                           
                           = 
                           
                             
                               F 
                               ⁢ 
                               
                                 
                                   ( 
                                   
                                     
                                       F 
                                       2 
                                     
                                     + 
                                     
                                       B 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               
                                 r 
                                 
                                   near 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   range 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     ( 
                                     107 
                                     ) 
                                   
                                 
                               
                               - 
                               F 
                             
                           
                         
                         , 
                       
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     In this embodiment, the optics  102  is a lens with a 4 inch diameter. Assuming a circular shape, the optical design of the lidar system  101  may be readily implemented when the bistatic distance  120  is greater than the radius of the imaging lens. This avoids mechanical and/or optical interference between the transmit and receive channels. To provide an inch of margin for mechanical mounts, a bistatic distance of 3 inches may be selected. A normal from the tilted image plane makes a tilt angle  125  relative to the optical axis  116  given by α=arctan (F/B). 
     Collection of the light transmitted through the aperture  122  onto the detector  105  is more challenging for larger tilt angles. Accordingly, design considerations generally drive the use of a shorter focal length  115 . However, lower focal number (F/#) systems are generally more difficult to design. Accordingly, a balance is struck between the lower F/# and the light collection for steeper image planes. In this embodiment, an F/2 imaging system was implemented resulting in an 8 inch focal length  115  with a tilted image angle  125  of about 69° (this angle  125  is shown and described in greater detail below). 
       FIG. 4  is a block diagram of another exemplary lidar system  100 . In this embodiment, the lidar system  100  is configured with a scan mirror  30  that is operable with both the transmitter portion of the lidar system  100  (e.g., the laser  101 ) and the receiver portion of the lidar system  100  (e.g., the optics  102 / 104 , the spatial filter  103 , and the detector  105 ). Generally, the scan mirror  30  has little bearing on the imaging and range considerations and is an optional feature that allows the lidar system  102  rotate. In other words, the scan mirror  30  is operable to provide the lidar system  100  with scanning imaging capabilities without affecting the range characteristics of the lidar system  100 . 
     Although shown with respect to the scan mirror  130  rotating in one particular direction, the invention is not intended to be limited as such. Rather, the scan mirror  130  may be configured to allow the lidar system  100  to scan in one or more directions. Moreover, the scan mirror  130  may be representative of multiple mirrors and may even allow the lidar system to scan in virtually any axis of rotation without affecting the operational aspects of the lidar system  100  (e.g., the range response). 
     One effect of the above optical arrangement of the lidar system  100  is that receiver sensitivity to emissions in the outside volume differs from traditional optical systems.  FIG. 5A  exemplarily illustrates the lidar system  100  and  FIGS. 5B-5E  illustrate the lidar system  100 &#39;s sensitivity to isotropic emissions from various distances. Sensitivity to isotropic emissions is proportional to the emitted power passing through the aperture  122  of the spatial filter  103  lying in the tilted image plane. The graphs  171 ,  181 ,  191 , and  201  of  FIGS. 5B-5E  show the power received at the optics  102  from various distances (i.e., 4 m, 5 m, 10 m, and 30 m, respectively). The points  172 ,  182 ,  192 , and  202  illustrate the position of the optical axis  116  of the optics  102 . The graphs  171  and  181  show that near range fluorescence is blocked by the spatial filter  103 . However, the graphs  191  and  201  show that near range fluorescence has passed albeit with larger amount of far range fluorescence. 
     In this example, the aperture  122  (e.g., slit) in the spatial filter  103  is designed to cut off light emitted from the beam path  109  at ranges less than 5 m. The graph  171  shows that light emitted from the beam path  109  is outside the field of view corresponding to the aperture  122  of the spatial filter  103 . The other graphs  181 ,  191 , and  201  show the collection sensitivity for 5 m, 10 m, and 30 m respectively as well as the range dependent field of view with the laser beam path  109  within the highest sensitivity volume. 
     In general, the dimension of the aperture  122  may be selected to match the laser divergence to ensure efficient collection of laser stimulated emissions from the beam path  109 . In some instances, the lidar system  100  may produce laser stimulated emissions at multiple wavelengths of polarizations. These emissions may occur from multiple and/or simultaneous processes including, for example, Raleigh scattering, Mie scattering at the wavelength of the laser  101 . Additional processes include fluorescence, Raman scattering, and laser induced breakdown at wavelengths other than those of the laser wavelength. 
       FIG. 6  is a block diagram of exemplary two channel receiver employed with the lidar system  100 . In this embodiment, the lidar system  100  comprises a pair of turn mirrors  250  and  251  that are used to direct the beam path  109  along a line that is substantially parallel to the optical axis  116  of the optics  102 . This allows the transmitter and the receiver to be implemented in a compact form. The receiver portion comprises a scan mirror  130  that rotates about an axis normal to the optical axis of the optics  102 . The scan mirror  130  may be tilted at or about 45° to the optical axis  116  of the optics  102 . A “window” configured between the scan mirror  130  and the distributed medium interrogated by the laser  101  (a.k.a. the “interrogated volume”) allows for a deviation from 45° to be used to control the direction of the window&#39;s back reflections. Of course, additional scanning mirrors or other scanning mechanisms may be used to provide scanning in additional directions. 
     The lidar system  100  also comprises dichroic mirror  252  that is used to reflect light at the same wavelength as the laser  101  while transmitting light resulting from fluorescence along the beam path  109 . Alternatively or additionally, a polarizing beam splitter may be used to separate the polarizations of light. One or more bandpass filters  253  may be used to block additional light at the laser wavelength leaking through the dichroic mirror  252 . The fluorescence light emitted from the beam path  109  comes to focus on the tilted image plane where the spatial filter  103  lies in or is proximate to. The spatial filter  103  transmits the light onto the detector  105  which, in this embodiment, comprises a PMT to collect the light. Again, the spatial filter  103 , positioned at or near the tilted image plane, limits the receiver&#39;s field-of-view and reduces the detection of background light. 
     Light received from the beam path  109  that is reflected from the dichroic mirror  252  is propagated to a second detector  256 . For example, the light from the beam path  109  comes to focus on a second different tilted image plane and is refocused by a reimaging optics  255  (e.g., a lens after reflection from an in band tuning mirror  254 . A spatial filter may also be inserted at this tilted image plane as a matter of design choice. The reimaging optics  255  demagnifies the image so that it can fit on a smaller detector  256  (e.g., a PIN diode detector or an avalanche diode). This change in magnification generally changes the tilt angle of the reimaged tilted image plane. However, the detector  256  does not need to be parallel to the tilted image plane. 
     As mentioned, the F/# of the imaging optics (e.g., the optics  102 ) depends generally on the length of the aperture  122  of the spatial filter  103 . For example, if the detector  105  has a collection area that is comparable to a slit size of the spatial filter  103 , the detector  105  may be placed directly behind the spatial filter  103  in the relatively same orientation of the tilted image plane. However, some detectors may not be designed to efficiently collect light at such steep angles of incidence (e.g., due to surface reflectance and other detector properties). 
     To increase the detector  105 &#39;s light collection efficiency, the detector  105  may be arranged to reduce the angle between the optical axis  116  and the detector  105 &#39;s aperture. In this regard, the standoff between the aperture  122  of the spatial filter  103  and the detector  105 &#39;s aperture is increased. Depending on the F/# of the imaging optics, the additional standoff from the tilted image plane may prove inoperable. However, additional reimaging optics may allow the light from the beam path  110  to be projected onto the detector  105  through the spatial filter  103 . 
       FIG. 7  is a block diagram of exemplary lidar system receiver employing a prism  270  in one example of the reimaging optics that allows the light from the beam path  110  to be projected onto the detector  105  through the spatial filter  103  with the additional standoff distance from the tilted image plane. The detector  105  in this embodiment is a PMT and the prism coupler  270  is configured with the aperture of the detector  105 . Light passing through the spatial filter  103  impinges on a surface of the prism  270  and passes to the detector  105 . 
     The prism surface may be coated with an antireflective coating designed for anticipated optical bandwidth that is to be detected. The antireflective coating may also be designed based on the angle of incidence of the surface of the prism  270  and the F/# of the imaging optics  102 . This provides an advantageous solution because it is generally easier to place an antireflective coating on a custom optic than it is to modify input windows for detectors, such as those found in commercially purchased PMTs. 
     The prism  270  serves multiple purposes. Light that enters the tilted surface of the prism is refracted away from the optical axis  116  of the optics  102 . The new optical axis direction within the prism  270  reduces the constraints on the position of the detector  105 . More specifically, the detector  105  may be configured so that light entering the detector  105  is incident upon the aperture of the detector  105 . Due to the higher index of refraction within the prism  270 , the divergence angles of light within the prism material are less than they would be in air. This further minimizes the impact of the standoff range between the detector  105  and the aperture  122  of the spatial filter  103 . Total internal reflection from the prism surfaces may also be used to minimize the impact of the standoff distance between the aperture  122  of the spatial filter  103  and the detector  105 . 
     Also illustrated in this embodiment is an optional laser line blocking filter  260  and a range response filter  280  configured with or near the tilted image plane. The filter  260  is operable to filter out unwanted wavelengths (i.e., laser lines) of the laser  101 . The range response filter  280  is discussed in greater detail below. 
       FIG. 8  is a block diagram illustrating exemplary design considerations of the prism  270 . In this embodiment, the angle  279  between the optical axis  116  and a normal from the tilted image plane  275  is about 69° as described above. However, the angle  277  between the optical axis  116  and a normal from the surface  271  of incidence of the prism  270  can be designed for about 60°. In fact, the prism  270  can be designed for angles of incidence ranging from 45° to 75° for both polarizations depending on the antireflective coating on the surface  271  of the prism  270 . 
     The optical axis within the prism  270  is directed at an angle  273  of approximately 25.18° relative to the incident optical axis  116 . The other dimension of the prism  270  regards the angle  274  being configured at approximately 30°. The exit surface  276  of the prism  270  has a width of approximately 10.7 mm and is oriented to be approximately normal to the internal optical axis direction. Thus, a portion light entering the prism  270  is reflected from the top and bottom surfaces (i.e., those surfaces parallel to the view and illustrated as  278 ) prior to exiting the prism  270  via total internal reflection. 
     Mounts for the prism  270  generally make contact on the top and bottom surfaces, respectively, with a plastic having a low index of refraction so as to ensure total internal reflection for the expected illumination patterns at the contact points of the mounts. Additionally, the material of the mounts may be chosen so as to absorb light that is transmitted out of the side surfaces of the prism  270  as a means for reducing signal background from the internally scattered light. 
       FIG. 9  is a block diagram illustrating an exemplary range response filter  280  of the lidar system  100 . For example, the lidar system  100  provides a means for shaping the response of the system to emission sources of varying ranges. In particular, the lidar system  100  suppresses the response from emission sources at near ranges (e.g., from the near range point  107 ). By tailoring the system range response function to suppress the response of near range emission sources, the gain of the lidar system  100  affecting all ranges may be increased without near range saturation effects. 
     As illustrated in  FIG. 9 , the range response filter  280  attenuates light in a range dependent fashion. To illustrate, cones of light impinge the tilted image plane and the spatial filter  103  at various points along the spatial filter  103 . The cone of light  290  emitted along the ray  110  from the far range point  108  along the beam path  109  shows the illumination path of light that is focused to the tilted image plane and the spatial filter  103  at one point. Similarly, the cone of light  291  emitted along the ray  111  from the near range point  107  along the beam path shows the elimination path of the light focused at a different point along the tilted image plane and the spatial filter  103 . The range response filter  280  can be configured to further attenuate light depending on the desired range. 
     The range response filter  280  may be implemented in a variety of ways as a matter of design choice. For example, the filter  280  may be positioned at the tilted image plane or displaced from the tilted image plane. The filter  280  may run roughly parallel to the tilted image plane, although not required. Transmission through the filter  280  may be implemented as a gradient transmission, an electronically controlled spatial light modulator, and/or an opaque mask with limited spatial extent. The spatial variation in the filter  280  may be in a direction perpendicular to both the optical axis  116  and a normal from the filter surface. Alternatively or additionally, the spatial variations in the filter  280  may be in a direction within the bistatic plane of the lidar system  100 . 
     In one embodiment, the filter  280  is a gradient filter that is positioned so that emissions from the near range positions (e.g., point  107  on the beam path  109 ) are transmitted with less efficiency than emissions from far range positions (e.g., point  108  on the beam path  109 ). Linear Variable Neutral Density (LVND) filters are examples of gradient filters suitable for such an embodiment. LVND filters can have optical density factors that linearly vary from 0 to 5 over a distance of about 40 mm along the component. It should be noted that the linear variation in the optical density factor corresponds to an exponential spatial dependence for transmission. 
       FIGS. 10-13  are graphs  301 ,  303 ,  305 , and  307  illustrating exemplary features of an LVND filter of one exemplary embodiment. The graph  301  illustrates the use of an LVND filter  280  with an attenuation that ranges from an optical density of “0” to an optical density of 1.6 at about 8 mm. The graph  305  shows the optical systems range dependent transmission, with the LVND configured at the tilted image plane with the gradient along a direction coinciding with the bistatic plane. Calculations were made for a medium that attenuates light with an exponential attenuation ranging between 0.01 1/m and 0.08 1/m. The graph  303  takes into account the exponential attenuation and the 1/r 2  collection efficiencies along with the system&#39;s range dependent transmission to present an overall relative system collection efficiency for laser stimulated emissions at varying ranges. 
     For any given exponential attenuation, the ratio the maximum to minimum relative system collection efficiency may be taken as a required dynamic range ratio. The graph  307  shows that the system using the LVND filter  280  may measure laser stimulated signals between 5 m and 35 m in a medium having an attenuation of 0.08 at 1/m, with the minimum and maximum signals differing by factor less than  300 . 
     Alternatively, the range response filter  280  may be configured to provide a single obscuration. This is similar to a gradient filter except that the filter is fully transmissive on one side and fully obscuring on the other. One implementation of such a filter is a single obscuring shutter.  FIG. 14  is a block diagram illustrating a shutter  284  as the range response filter  280  being employed with the prism  270  in one exemplary embodiment. 
     In this embodiment, the edge of the shutter  284  may be translated over the tilted image plane  282  (and the spatial filter  103 ) some relatively close distance  277  (e.g., about 1 mm) to dynamically change the range response. The shutter  284  may be operable to translate along a shutter plane positioned in close proximity to the tilted image plane. This translation capability may be used to dynamically adjust the system range response to adaptively varying environmental or measurement conditions. For example, the shutter  284  may allow the system to increase gain in varying attenuating mediums and/or varying efficiency in the processes resulting in light at various ranges (e.g., the various forms of scattering and other processes mentioned above). 
     The relative positions and orientations of the spatial filter  103 , the shutter  284 , and the tilted image plane may be determined as a matter of design choice. For example, if the shutter edge is positioned at a location where the tilted image plane coincides with the shutter plane, a rapid gradient in the range response may be achieved. However, if the shutter edge is positioned at some distance away from the tilted image plane, the range dependent attenuation from obscuration of the shutter  284  is generally more gradual. And, more gradual range dependent attenuation is a consequence of a larger illumination region near the shutter edge corresponding to light emitted at positions within the laser beam path  109 . 
     In this exemplary embodiment, the tilted image plane  282  is the plane where the light from the laser beam path  109  comes to a focus. The spatial filter  103  is configured at a slight angle relative to the tilted image plane  282 . The tilted image plane is at an angle  279  of 69° relative to the optical axis  116 . The spatial filter  103  and the range response filter  284  were set to 60 degrees relative to the optical axis  116 . The separation between the range response filter  284  and the spatial filter  103  is a distance  277  of 1 mm. The 2 mm distance  285  is a separation between the range response filter and the spatial filter along the direction of the optical axis  116 . And, 2 mm×cos(60)=1 mm, the relationship between the distance  285  and the distance  277 . Although, each of these distances and angles were implemented as a matter of design choice for a particular application. 
     The shutter  284  is translated along its plane (e.g., at a 9 degree angle with relative to the tilted image plane  282 ) with the shutter  284  a distance of 2.8 mm away from the focal point of the optics  102 . The distance from the prism  270 , in this embodiment, to the optics  102  is approximately 209.5 mm. And the distance  285  is approximately 2 mm. 
       FIG. 15  is a block diagram of an exemplary spatial filter  103  having an aperture with variable widths. In this embodiment, light emissions from locations within the beam path  109  at far ranges (e.g., point  108 ) come to focus near the filter at the position near the region  321 . As the width of the aperture is greater at this region, the far range light is largely transmitted by the spatial filter  103 . On the other hand, light emissions from locations in the near range (e.g., point  107 ) of the beam path  109  come to focus near the region  320  of the spatial filter  103 . Consequently, the near range light “over fills” the aperture and is largely blocked by the spatial filter  103 . 
     Intensity of light returning to an active system generally has a range response that is proportional to a reciprocal of the square of the range (or a distance from the receiver to the scattering or re-emission range). This is, in general, a consequence of the proportion of the solid angle extended from the scattering or the re-emission range subtended by the receiver aperture. Additionally, there may be an exponential range dependence due to scattering and/or absorption. 
       FIGS. 16-18  are graphs  330 ,  332 , and  334 , respectively, illustrating exemplary features of the range response filter of  FIG. 16 . For example, the graph  330  shows the collection efficiency of an aperture that is a function of range where the outgoing light is transmitted in a medium with an attenuation between 0.01 at 1/m and 0.08 at 1/m. If the lidar system  100  is to detect emissions from ranges between 5 m and 35 m, the lidar system  100  should have a dynamic range of nearly 4 orders of magnitude. 
     However, if the system efficiency is made to balance the collection efficiency so that a flat range response is achieved at the detector  105  at ranges between 5 m and 15 m, then only about two orders of magnitude of dynamic range may be needed as illustrated in the graph  332 . Thus, the shape of the aperture may be calculated according to the desired range response for each attenuation value as illustrated in the graph  334 . Generally, the aperture of the spatial filter  103  may be constructed from two opposing “blades” such that near ideal performance may be achieved by reducing separation between the blades at higher attenuation values. Such may be useful in circumstances where the medium which the lidar system  100  is interrogating comprises varying attenuation. 
     As mentioned, the range response filter may also be implemented as an electronically addressable spatial light modulator. A dynamically controlled amplitude mask may be used to dynamically modify the lidar system  100  &#39;s response to light emitted from various ranges. This minimizes the dynamic range requirements of the lidar system  100 . 
     To implement such, the lidar system  100  may vignette near range emissions. For example, because the lidar system  100  uses a common aperture, off axis angles correspond to near range emission ranges. Consequently, optical vignetting may be used to suppress light emanating from the near range positions (e.g., from the point  107  on the beam path  109 ). In this regard, the apertures for spectral filters or other optical elements may be specifically designed to suppress the near range returns. 
       FIG. 19  is a graph illustrating exemplary design considerations of a bandpass filter operable to suppress the near range returns. In this example, illumination patterns sizes are shown at the plane of the bandpass filter for light emanating from multiple ranges, including 3 m, 4 m, 5 m, etc. The bandpass filter inserted between the optics  102  and the tilted image plane may include a mask or an aperture that significantly attenuates light emanating from the near range positions. However, this filter fully transmits light emanating from the far range positions (e.g., from the point  108  on the beam path  109 ). Also, it is not necessary that the embodiments include a bandpass filter. Rather, this embodiment merely illustrates one alternative in which the aperture of the optical components herein can have different transmissions depending on the range of the emissions. 
       FIGS. 20 and 21  are block diagrams of an exemplary imaging spectrometer employing a tilted image plane. In this embodiment, the imaging spectrometer  400  is implemented with the detector  105  and may be implemented as a variant of a Czerny-Turner spectrometer. The tilted image plane of the spatial filter  103  may be imaged onto a sensing plane such that spectral components of the light emanating from the beam path  109  may be spectrally dispersed in a direction different from the long direction of the aperture of the spatial filter  103 . For example, assume that the spectral dispersion is in the direction of the plane of the view for the imaging spectrometer  400  of  FIG. 20 . The optical axis of the optics  102  is not positioned within that plane. Rather, it is lifted out of that plane, perpendicular to that plane and to that of the tilted image plane. Light exiting the spatial filter  103  is collimated by optics  409  prior to traversing through a transmission grating filter  401 . Light from a first diffraction order is then focused by imaging optics  104  to form an image on a plane of the detector  105 . At the detector plane, the position of the image provides information about both the spectrum of the collected light and the range at which the light was emitted. 
       FIG. 21  provides a roughly orthogonal view of this embodiment in which light propagation is illustrated through the system from emissions at two different ranges (e.g., points  107  and  108  along the beam path  109 ). Light from these two emission points comes to focus at two separate locations on the tilted image plane of the spatial filter  103 . The light from these points is collimated by the collimating optics  405 , propagated through the grating  401 , and then focused on the detector  105  by the imaging optics  104 . 
     The grating  401  may be tilted about a grating axis. For the purposes of this representation, the imaging optics  104  and the detector  105  of the tilted image spectrometer are illustrated in the same plane as the other elements. However, the optical axis of the imaging optics  104  and a normal from the detector are at an angle to the plane of the view. This angle is designed to capture a first-order diffraction light from the grating  401 . 
     The detector  105  may be implemented as an array of time resolved detectors or an imaging array. For example, detectors placed at different positions in the detector plane can gather different spectral components of received emissions. Time resolved detectors may be used to extract range distributions of spectral source components based on a lidar time-of-flight delay. For emission processes that have relatively long timescales, a time-of-flight based range detection may be impractical and the elimination position on a two-dimensional array may be used to obtain the range instead. 
       FIG. 22  is a block diagram illustrating an exemplary surface scanning embodiment of a lidar system. Optically, the surface scanning embodiment is similar to the lidar system  100  embodiments above. This embodiment employs an additional scanner to scan the laser within the bistatic plane. The optical system utilizes a tilted image plane to image a line of emissions induced by laser interactions where the optical axis  116  is along a separate, possibly parallel, path. 
     For example, multiple laser beam paths  109 - 1 - 109 - 3  may intersect an extended surface along a beam emission line  460 . This beam emission line  460  is optically the same as a beam emission axis formed by a single laser beam passing through a distributed medium. At each interception point ( 450 ,  451 , and  452 ), light is emitted through either linear or nonlinear processes. In this example, the bistatic plane is the plane intercepting the beam emission axis  460  and the imaging optical axis  116 . Just as the beam emission axis is formed by a single laser, beam traversing a distributed volume in the above embodiments, the optical system images the multiple beam emission axis onto a tilted image plane. 
     Though some of the above embodiments have been depicted for a case where a single detector  105  is positioned to receive incoming light traversing the tilted spatial filter  103 , it should be understood that the detector  105  may comprise an array of detectors. In fact, the detector  105  may be representative of a detector array that provides multiple detector pixels. The spatial distribution of incoming light at the tilted image plane is, in general, mathematically related to the range dependent distributions of emission sources. Consequently, a detector array may be used to capture the spatial distribution of incoming light near the tilted image plane and additional processing may be performed on the signals obtained from each detector pixel to extract emission spatial distribution data. Spatial data captured from an array of detector pixels may be combined with temporal data from detector pixels to provide improved emission distribution data. For example, the response or gain of pixel detectors located at different positions in close proximity to the tilted image plane may be selected or adjusted to compensate range-dependence in the processes of emissions generation and collection. 
       FIG. 23  is a flowchart of an exemplary process  150  of the lidar system of  FIG. 1 . In this embodiment, light is first directed from the laser  101  using first optics on a beam path  109  that coincides with the optical axis of the first optics (e.g., as configured with the laser  101 ), in the process element  151 . Light emitted from the beam path  109  is received through second optics  102  having a second different optical axis  116 , in the process element  152 . The light is imaged onto a second plane that is substantially normal to the bistatic plane containing the first and second optical axes  109 / 116 , in the process element  153 . Again, the first and second optical axes  109 / 116  lie substantially in the same first plane (e.g., neglecting design tolerances). And, a first line in the first plane intersects a second line in the second plane at an acute angle. The first line is perpendicular to the first optical axis  109 . Light projected onto the second plane is then filtered via the spatial filter  103  that lies in or near the second plane, in the process element  154 . The filtered light is projected onto the detector  105 . 
     The invention can be embodied in various forms of hardware and software. For example, optical components such as lenses are generally preconfigured “hard” components. However, other optical devices, including lenses may be electronically controlled. In this regard, certain features of the invention may employ various combinations of hardware and software. In one embodiment, such software includes but is not limited to firmware, resident software, microcode, etc.  FIG. 24  illustrates a computing system  500  in which a computer readable medium  506  may provide instructions for performing any of the methods and processes disclosed herein. 
     Furthermore, some aspects of the invention can take the form of a computer program product accessible from the computer readable medium  506  providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, the computer readable medium  506  can be any apparatus that can tangibly store the program for use by or in connection with the instruction execution system, apparatus, or device, including the computer system  500 . 
     The medium  506  can be any tangible electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device). Examples of a computer readable medium  506  include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Some examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
     The computing system  500 , suitable for storing and/or executing program code, can include one or more processors  502  coupled directly or indirectly to memory  508  through a system bus  510 . The memory  508  can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code is retrieved from bulk storage during execution. Input/output or I/O devices  504  (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the computing system  500  to become coupled to other data processing systems, such as through host systems interfaces  512 , or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters.