Abstract:
Active imaging devices can include a camera and an illuminator that provides light to the scene under observation. Most often, a laser beam combined with projector optics is used to generate a field of illumination while a telescope and a camera are use to acquire the images in its field of view. This specification demonstrates the production of a rectangular field of illumination having a highly uniform intensity distribution matching and aligned with a rectangular field of view of the camera.

Description:
BACKGROUND 
       [0001]    Active imaging devices have both a camera and an integrated light source to illuminate the scene under observation. They can thus be said to include both an emission and reception channel. The emission channel typically uses an illuminator and its associated projection optics to produce, in the far field, a field of illumination (FOI). The reception channel typically uses a camera sensor and its associated reception optics (e.g. a telescope) giving a field of view (FOV). Active imaging devices typically offer independent control over the FOI and FOV by controlling the dedicated projection and reception optics. 
         [0002]    Given the format of camera sensors, the camera aspect ratio is typically rectangular and the camera sensor typically has a uniform sensitivity across its surface area. However, previously known illuminators were non-rectangular and many even had non-uniform intensity distribution. For instance, typical micro-collimated laser diode arrays illuminators coupled to a projector produce, in the far field, a field of illumination having a Gaussian-like intensity distribution. An example of such a non-uniform and non-rectangular field of illumination  110  is shown in  FIG. 1A  on which a typical camera field of view  112  is superimposed. An exemplary intensity distribution is illustrated at  FIG. 1B  in which the Y-axis represents the relative intensity and the X-axis represents the horizontal angular position. 
         [0003]    From  FIG. 1A , it will be understood that a portion of the field of illumination exceeds the field of view and is thus of no use to the camera sensor. In covert applications, the excess illumination reduces the stealthiness of the imaging device by allowing its detection from outside its field of view. Further, in the case of active imaging devices used with limited energy sources, the excess illumination represents undesirably wasted energy. From  FIG. 1B , it will be understood that the intensity distribution further did not match the sensitivity distribution of the camera sensor. There thus remained room for improvement. 
       SUMMARY 
       [0004]    It was found that the field of illumination could be matched to the field of view by using a fiber illuminator having an illumination area with a rectangular cross-sectional shape that matches the aspect ratio of the sensor, and consequent field of view of the camera. 
         [0005]    In accordance with one aspect, there is provided an active imaging device having: a fiber illuminator having a rectangular illumination area; a projector lens group having a focal plane coupleable to the rectangular illumination area to project a corresponding rectangular field of illumination on a scene located at far field of the projector lens group, a camera having a camera sensor and a rectangular field of view alignable with the rectangular field of illumination, the field of view and the field of illumination having matching rectangular aspect ratios. 
         [0006]    In accordance with another aspect, there is provided an active imaging device having: a frame; a camera mounted to the frame, having a camera sensor, and a field of view having a camera aspect ratio; a fiber illuminator mounted to the frame and having a rectangular cross-section light output path corresponding to the camera aspect ratio; and a projector lens group mounted to the frame, the projector lens group being optically coupleable to the light output path of the fiber illuminator for projection into a field of illumination aligned with the field of view of the camera. 
         [0007]    In accordance with another aspect, there is provided an active imaging device having: a frame; a telescope mounted to the frame, a camera mounted to the frame, having a sensor, and a field of view having a rectangular aspect ratio; a fiber illuminator mounted to the frame and having a rectangular cross-section corresponding to the camera aspect ratio; and a projector lens group mounted to the frame, the projector lens group being optically coupled to the output of the fiber illuminator projecting a field of illumination corresponding to the field of view of the camera. 
         [0008]    Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0009]    In the figures, 
           [0010]      FIG. 1A  shows a field of illumination overlapped by a field of view, in accordance with the prior art,  FIG. 1B  showing an intensity distribution thereof; 
           [0011]      FIGS. 2A and 2B  schematically demonstrate corresponding imperfect matches between circular field of illumination and a rectangular field of view; 
           [0012]      FIG. 3  shows an example of an active imaging device having a field of illumination and a field of view with matching aspect ratios; 
           [0013]      FIG. 4  shows a field of illumination of the active imaging device of  FIG. 3 ; 
           [0014]      FIG. 5A to 5D  show several fiber illuminator embodiments for the active imaging device of  FIG. 3 ; and 
           [0015]      FIG. 6  shows a variant to the active imaging device of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    A circular field of illumination can be produced by a light source coupled to a circular core optical fiber which, in turn, is injected into projection optics. However, as demonstrated on  FIG. 2A , the intersection area between a circular field of illumination  110  and a typical rectangular 4:3 aspect ratio FOV  112  will yield only 58% of surface overlap. Alternatively, as shown in  FIG. 2B , if the circular FOI  110  is made smaller to fit inside the FOV  112 , then part of the FOV  112  becomes completely dark and unusable. This is solely based on geometrical considerations. 
         [0017]    In  FIG. 3 , an active imaging device  10  is shown having a fiber illuminator  12  having an illumination area  18  schematically depicted as having a rectangular aspect ratio. The active imaging device  10  further has a camera  20  having a field of view  22  with a rectangular aspect ratio, and a projector lens group  14  having a focal plane  40  coupled to the rectangular illumination area  18 , in the sense that the rectangular illumination area  18  is positioned at the focal plane  40  of the projector lens group  14  for the projector lens group to produce, in the far field  42 , a field of illumination  24  having an aspect ratio corresponding to the aspect ratio of the field of view  22  of the camera  20 . Examples of how such a rectangular shape  18  can be obtained from a fiber illuminator  12  will be described below. 
         [0018]    The projector lens group  14  can include a tiltable alignment lens group for instance, to align the optical axis of the fiber illuminator  12  with the optical axis of the projector lens group  14 . The field of illumination  24  can then be boresighted with the field of view  22  by the use of Risley prisms used at the output of the projector lens group  14  or by mechanically steering the coupled fiber illuminator  12  and projector lens group  14  assembly, for instance. The projector lens group  14  projects, on a scene  28  located in the far field  42 , the rectangular image of the rectangular illumination area  18 . 
         [0019]    Light is reflected by the scene  28 . In this embodiment, the reception channel has a camera  20  which includes both a telescope lens group  26  and camera sensor  30  positioned at a focal plane of the telescope lens group  26 . The camera  20  can thus have a field of view  22  with a rectangular aspect ratio which matches the rectangular aspect ratio of the field of illumination  24  and thus receive the reflected light with the camera sensor  30 . The divergence of the illumination can be adjusted using the projector lens group  14  to scale the rectangular field of illumination  24  with the field of view  22 , for instance. The field of view  22  of the camera  30  can thus be fully illuminated by a field of illumination  24  which does not, at least significantly, extend past the field of view  22 . In practice, the fiber illuminator  12 , camera sensor  30 , and the optical components  14 ,  26  can all be mounted on a common frame  32  to restrict relative movement therebetween. The illumination channel and reception channel can be provided in a common housing, or in separate housings and be independently steered towards the same point under observation, for instance. 
         [0020]    An example of a rectangular field of illumination  24 , in the far field, is shown more clearly in  FIG. 4 . This rectangular shape was obtained using a fiber illuminator  12  as shown in  FIG. 5A , having a light source  34 , such as a laser, a LED or another convenient source, optically coupled to the input end  36  of a highly multimode optical fiber  38  having a rectangular core  44 . As shown schematically in  FIG. 5A , the rectangular core  44  reaches the output end where it generates a rectangular illumination area  18  which can have the same shape and aspect ratio as the rectangular aspect ratio of the camera sensor  30 . The cladding of the optical fiber  38  can be circular, in which case the optical fiber  38  can be drawn from a corresponding preform for instance. Alternately, the cladding of the optical fiber  38  can have another shape, such as rectangular for example and be either drawn from a corresponding preform, or be pressed into shape subsequently to drawing, such as by compressing an optical fiber between flat plates and subjecting to heat for instance. 
         [0021]    In alternate fiber illuminator embodiment schematized at  FIG. 5B , an output section  46  of an optical fiber has been shaped into a rectangular cross-section  48  by compressing and subjecting to heat, thereby shaping the core into a rectangular cross-section leading to a rectangular illumination area. An input section  50  of the optical fiber was left in its original circular shape  52 . A tapering section  54  can bridge both sections progressively, for instance. The input section  50  is optional. 
         [0022]    An other alternate fiber illuminator embodiment is schematized at  FIG. 5C , having a circular cross-section optical fiber  56  forming an input section  50  fusion spliced  58  to a rectangular cross-section optical fiber  60  forming an output section  46 . In this embodiment, it can be practical to have an input section  50  having a smaller core than the output section  46  to minimize losses. 
         [0023]    In the embodiments schematized in  FIGS. 5B and 5C , the output section  46  of the optical fiber can be referred to as a light pipe having the matching aspect ratio. 
         [0024]    When using fiber illuminator embodiments such as schematized in  FIGS. 5A ,  5 B and  5 C, the projector lens group  14  can have its focal plane  40  coupled to coincide with an outlet end tip of the optical fiber. The optical fiber end tip is thus magnified and projected on the scene in the far field according to the required field of illumination. 
         [0025]    In an alternate embodiment schematized at  FIG. 5D , the fiber illuminator can have an optical fiber  62  having a core other than rectangular, but being subjected to an opaque mask  64  having a rectangular aperture  66  of the matching aspect ratio, coupled at the focal plane  40  of the projector lens group  14 . The mask thus imparts a rectangular shape to a formerly circular (or other) cross-sectioned light output  68 , thereby forming a rectangular illumination area at the focal plane  40 . 
         [0026]    All the fiber illuminator embodiments described above can further include an optical relay or the like to offset the rectangular illumination area from the output tip or mask, for instance. 
         [0027]    Embodiments of fiber illuminators such as described above can produce rectangular field of illuminations  24  in the far field such as shown in  FIG. 4 . It will be understood that the aspect ratio shown in  FIG. 4  is a 4:3 horizontal:vertical aspect ratio, but alternate embodiments can have other aspect ratios, depending on the camera aspect ratio, such as 3:2, 16:9, 1.85:1 or 2.39:1 for instance. Further, it will be noted that camera sensors could be provided in other shapes than rectangular, in which case the shape of the light output can be adapted accordingly to match the shape of the camera sensor. 
         [0028]    In most uses, the field of illumination can be precisely matched and aligned to the camera field of view. In other instances, the field of illumination can be adjusted to be smaller than the field of view to obtain a higher light density on a portion of the target to obtain a better signal to noise ratio in an sub-area of the image. Either way, the field of illumination is aligned with the field of view. 
         [0029]    The optical design of the projector lens group  14  can be appropriately scaled for the projection sub-system (illuminator dimensions/projector focal length) to be matched with the reception channel (sensor dimensions/telescope focal length). For instance, the field of view (reception channel) of a system based on a sensor (H×V) of 10 mm×7.5 mm and a variable focal length of 1000 mm to 2000 mm telescope will produces images that correspond from 10×7.5 mrad to 5×3.75 mrad field of view. To illuminate the scene using a rectangular fiber of 200 um×150 um, the projector focal length will range from 20 mm to 40 mm for the field of illumination to match the field of view. The projector focal length can exceed 40 mm to obtain a smaller field of illumination than the smallest field of view. 
         [0030]      FIG. 6  shows an alternate embodiment of an active imaging device  70  having a field of view matching the field of illumination. In this embodiment, the fiber illuminator  72  and the sensor  74  share a common set of lens  76  which acts as both the projector lens group and a telescope lens group, i.e. the telescope is used as both the emission and the reception channel. 
         [0031]    To achieve this, the illumination area can be scaled using an optical relay  78  between an optical fiber  80  and the focal plane to match the optical fiber physical dimension to the actual the sensor dimensions. A typical magnification of 10 would be required to scale a typical 1 mm fiber core to a 10 mm apparent size at the focal plane of the telescope. The magnified fiber image can then be injected in the telescope-projector  76  using a prism  82  or beamcombiner with a 50-50% transmission/reflection, for instance, in which case the emitter light is transmitted through the beamcombiner (or prism  82 ) with an transmission of 50% into the telescope up to the target  84  and the light coming back through the telescope  76 , is reflected by the beamcombiner to the sensor  74  with again a reflection of 50%, for a global efficiency of 25%, which may nevertheless be sufficient for certain applications. 
         [0032]    An active imaging device configuration such as shown above in relation to  FIG. 3  can be used in a range gated imaging device for instance, where a precise flash of light can be sent to a distant target at the scene of observation, reflected, and the camera sensor gated to open and close as a function of the target range. Active imaging device configurations such as taught herein can also be used in any other application where it is convenient. 
         [0033]    As can be understood, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.