Abstract:
A scanned imaging system includes a lens system for directing radiation from a scene to be scanned onto external reflecting surfaces of a rotating polygon and onto a detector, the external reflecting surfaces including primary and secondary flat facets which rotate together on a common axis, the lens system including an image space lens system for focusing an image of the scene to be scanned in the vicinity of a primary facet, and further including an optical system for directing radiation from a primary facet to a focus at a point on or adjacent an associated secondary facet and to reflect the radiation from the secondary facet to the detector, the arrangement being such that a beam of radiation is incident on substantially the whole width of a primary facet and sweeps round with that facet as the polygon rotates.

Description:
This invention relates to the improvement of scanning efficiency in mechanically scanned imaging systems incorporating one or more scan mirrors. 
     Mechanically scanned imaging systems are used to obtain an image of a scene using a comparatively small number of detectors. A linear array of detectors may be scanned across the scene using a single scan mechanism such as a flapping mirror interposed in the optics of the system or the detector array may be scanned in both azimuth and elevation using a rotating polygon with various angled facets or a combination of a rotating polygon with identical facets and a flapping mirror. 
     The scanning efficiency of a flapping mirror may be reasonably high while that of a rotating polygon operating with parallel or near parallel incident radiation is usually no better than 50% because of the dead time between adjacent facets. The dead time arises because incident light beams are usually stationary whilst a facet is swept across the beam. The time taken for a pool of light to move between adjacent facets is unusable and is known as dead time. 
     BACKGROUND OF THE INVENTION 
     A scanning system for reducing dead time is described in G.B. Pat. No. 1,419,940. This system comprises two polygons having the same number of facets and rotating on a common axis. One polygon, a prescanner, has convex facets, while the other or main polygon, has flat facets. Laser light is focussed onto a prescanner facet and reflected off a concave mirror back to a main polygon facet. The beam of light on the main facet occupies the whole width of the facet and sweeps round with it as the main polygon rotates. This system is also suggested for use with a detector replacing the laser to form a flying aperture system. 
     Other known systems use a double reflection off a rotating polygon; the two reflections may be off different ends of the same facet or the polygon may be split into two with two facets behaving as a single facet rotating together. These known systems suffer from dead time because the region where the incident light reflects off the facet remains substantially stationery while the facet rotates. 
     Most prior art systems operate in what is known as object space, where an image of the scene being swept is focussed far away from the rotating facets. These use a lens system, such as an afocal telescope, so that the facets operate on parallel or near parallel beams of light. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a system which operates in image space where the reflecting facets operate on widely diverging or converging light. An image of the scene is focussed near a facet. 
     It is an object of this invention to provide an optical and mechanical configuration for a scanning polygon with an improved optical scanning efficiency while maintaining good radiometry at the detector. 
     According to this invention a scanned imaging system comprises a lens system for directing radiation onto a rotating polygon from where radiation is directed onto a detector or light emitter, characterized by a polygon having flat primary and secondary facets which rotate together, an image space lens system for focussing an image of the scene to be scanned in the vicinity of a facet, and an optical system for directing radiation from a primary facet to its associated secondary facet for reflection to the detector, the arrangement being such that radiation is focussed to a point Amor adjacent to a secondary facet and a beam of radiation incident on substantially the whole width of a primary facet sweeps round with that facet as the polygon rotates. 
     The primary and second facets may be one and the same, or extensions of one another, or separate from one another, and may be at a non-zero angle to one another. In one form the facets may be at a nominal 45° to their axis of rotation and at 90° to one another. To obtain a banded scan successive facets may be at slightly different angles. 
     The primary and secondary facets may be of roof prism or reflector form. In one form the primary facets are faces at about 90° facing one another and on successive facet pairs the apex is displaced in a direction parallel to the axis of rotation to give a vertical scan. 
     The optical system may include at least one lens arranged in front of a mirror which may be a plane mirror or two 90° surfaces forming a roof reflector. 
     In one form the imaging system is a thermal imaging system incorporating a thermal detector, e.g. a liquid nitrogen cooled detector of the known alloy material cadmium mercury telluride (C.M.T.). Alternatively the detector may be replaced by a modulated light source, e.g. a light emitting diode, so that an image is raster scanned e.g. onto a screen. 
     Additionally the thermal detector may modulate the light output of the light emitting diode. The light emitting diode light is directed from the secondary and primary facets to an eyepiece for direct viewing of the thermal scene. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example only with reference to the accompanying drawings of which: 
         FIGS. 1 and 2  are diagrammatic plan and side views respectively of a thermal imager system using two similar scanning polygons; 
         FIG. 3  is a side view of a thermal imager having two 45° polygons; 
         FIG. 4  is an optical equivalent of part of  FIG. 3  to explain its operation; and 
         FIG. 5  is diagrammatic side view of another form of thermal imager incorporating a light emitting diode viewer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As seen in  FIGS. 1 and 2  a thermal imager for forming a visible image of a scene  1  on a video monitor  2  comprises an optical component  3 , a polygon  4  having plane facets  5  to  10 , an optical system  11  and a thermal detector  12  which supplies a signal to the monitor  2 . The detector  12  may be a single detector or an array of detectors. A flapping mirror  13  provides a vertical scan. As shown the polygon  4  is a single element but could be two separate but equivalent (i.e. primary and secondary) polygons rotating together. 
     The radiation  14  originating from the scene  1  is incident at the first polygon  6  at position  23  (i.e. the primary facet) at or near the center of the facet  6  and is arranged to track round with that facet as the scene  1  is scanned. The reflected radiation  15  passes to the optical focussing arrangement  11  where it is directed back along path  16  onto the secondary facet  6  and brought to a focus at or near its surface  17 . 
     The radiation undergoes a further reflection at  17  along direction  18  and is brought to a focus either directly or indirectly at the radiation detector  12 . Ideally the efficiency of the scan is controlled by the passage of the next reflecting polygon facet  5  across the cross-section of the beam  18 . 
     To achieve good radiometry at the detector  12  the central ray of the beam  18  which has undergone a second reflection from the scan mechanism should have little or no angular variation as the scanning polygon  4  rotates. This defines the position and direction of the beam  16 . 
     The incident radiation  14  is assumed (for this particular example) to have a pupil at position  19  (inside the polygon  4 ). On reflection at  23  the radiation appears to have originated beyond the polygon. It is the function of the optical focussing arrangement  11  to relay the first reflected beam  15  along the required path  16 . 
     In practice the beams would be controlled by an aperture stop somewhere within the optics, most probably near the detector  12 , and a degree of pupil wander would be tolerated elsewhere within the system. 
     With identical facets on the polygons the system achieves a one dimensional scan (i.e. single line scan). A flapping mirror  13  is required for vertical scanning or a further polygon used. Alternatively with facets  5 - 10  variously inclined to the axis of rotation  21  a two dimensional banded scan may be achieved. 
     For good optical performance the image of the scene at the detector would have to be in focus and free from significant aberrations throughout the scan. 
     A further  FIG. 3  shows a particular optical and mechanical configuration which has high scan efficiency, good optical performance, and is insensitive to lateral displacement of the polygon pair along their axis of rotation. 
     The imager of  FIG. 3  assumes a pupil at infinity (long focal length objective lens). The optical focussing arrangement may however with suitable adjustment accommodate an arbitrarily positioned pupil. 
     The scanner of  FIG. 3  comprises an objective lens  22  which directs radiation  33  from a scene  1  onto a scanning arrangement and thence via a detector lens  20  to a detector or detector array  12 . In this case the scanning is performed by a primary  24  and a secondary polygon  25  mounted on a common axis  26  for rotation together. These polygons  24 ,  25  are identical and have their twelve facets  27 ,  28  inclined at 45° to the axis  26 . Above each polygon  24 ,  25  is an identical lens  29 ,  30  and above them a 90° roof mirror pair  31 ,  32 . 
     In this configuration with the pupil at infinity and the center of the incident beam  33  parallel to the common axis  26  of rotation, reflected radiation from facet  27  appears to arise from a region  34  near the common axis of rotation and is a reflection of the image position  35 . The function of the lenses  29 ,  30  and mirrors  31 ,  32  is to relay an image at the axis of rotation of the primary polygon  24  to a position  36  close to the face  28  of the secondary polygon  25 . 
     The central ray of beam  37  arriving at the lenses  29 ,  30  and mirrors  31 ,  32  and the beam  38  leaving them must lie in planes parallel to one another and the axis  26  to achieve good radiometry, since the entrance pupil is at infinity. 
     In the arrangement of lenses  29 ,  30  and mirrors  31 ,  33  an image at  34  is relayed to an image at  36  with unit magnification. 
     An example of this optical arrangement is illustrated in  FIG. 4 . In  FIG. 3  in the plane perpendicular to the axis of rotation  26 , the mirrors  31  and  32  are equivalent to a single plane mirror at the line of intersection of  31  and  32 . This equivalent mirror is shown as  39  in  FIG. 4 . The two lenses of  FIG. 3  are shown as a single lens in  FIG. 4 . The optical separation between lens  29 ,  30  and mirror  39  is equal to the focal length f of the lens  29 ,  30 . 
     An image on the axis  26  at position  34  is re-imaged to the position  36  in  FIG. 4 , the positions  34  and  36  being spaced either side of the focal point of lens  29 ,  30 . With this optical focussing arrangement the central rays to and from the focal positions  36  and  34  are parallel but displaced as required. 
     Alternatively if the entrance pupil of the scanner were not at infinity but at a finite position such as an objective aperture then the rays  37  and  38  would not be parallel in  FIG. 4 . This may be accommodated through altering the optical separation between the lens  29 ,  30  and the mirror  39  in  FIG. 4 . 
     The optical focussing arrangement as shown in  FIG. 4  is by means of example only. The lens  29 ,  30  and mirror plane  39  could be replaced by a mirror or lens-mirror combination so long as it fulfilled the required optical function. 
     The particular optical focussing arrangement shown in  FIG. 3  employs a roof mirror pair  31 ,  32  whose function is to produce a flat one dimensional scan with the illustrated polygon configuration and enables the scanning mechanism to be insensitive to lateral displacements of the polygon shaft along the axis of rotation  26 . 
     The optical and mechanical configuration shown in  FIG. 3  employs separate polygons  24  and  25  and separate lens  29  and  30 . The lenses  29 ,  30 , polygons  24 ,  25  and the pair of mirrors  31  and  32  may be replaced as a unit by single components if required. 
     The use of the roof mirrors  31  and  32  of  FIG. 3  has been described with reference to a particular optical and mechanical arrangement but could however be used in other optical and mechanical arrangements. 
     The particular optical and mechanical arrangement shown in  FIG. 3  is capable of producing a one dimensional scan of a flat image at  35  with little optical distortion when employing appropriately corrected lenses  29  and  30 . 
     The polygon  24 ,  25  facets have been described as lying at ±45°; this gives a one dimensional scan, which requires an additional polygon pair or a flapping mirror to give a vertical scan. Alternatively or additionally the facets  27 ,  28  may have different inclinations to give vertical scanning. 
     As shown in  FIG. 5  a thermal image  41  is scanned onto an infra red detector  42 . This detector  42  may be of the alloy Cd x Te 1-x Hg as described in U.K. Pat. No. 1,488,258 where the detector has a strip form and thermal radiation is scanned along the strip at a velocity matched to an electrically imposed ambipolar carrier velocity. 
     The thermal image  41  is directed through a lens  43  onto a primary polygon  44  having  12  similar length sides or facets. Each side is formed by two inwardly directed facets  45 ,  46  inclined at about 45° to the polygon&#39;s axis of rotation  47 , i.e. about 90° to one another thus forming a mirror roof pair. The position of the apex between the two facets varies from facet pair to facet pair along the axis to give a vertical scan of the image. 
     A secondary polygon  48  is fixed co-axially with the primary polygon  44 . This secondary polygon  48  also has  12  sides, each side being formed of two inwardly directed facets  49 ,  50  forming a mirror roof pair. To one side and between the two polygons  44 ,  48  is a fixed mirror roof prism  51  and two lenses  52 ,  53  which direct radiation from the primary polygon  44  to the secondary polygon  48 . An angled mirror  54  and lens  55  direct radiation from the secondary polygon to the detector  42 . 
     The detector  42  output is amplified by an amplifier  56  and used to modulate the visible light output of a light emitting diode (L.E.D.)  57 . Light from the light emitting diode  57  is directed to an eye piece  58  via a path similar to that of incoming infra red (I.R.) radiation, i.e. mirror  59 , lens  60 , roof mirror  61 , and lenses  62 ,  63 . 
     Each mirror roof pair  45 ,  46 , and  49 ,  50 , reflects as a flat mirror at the apex but with -a displacement along the axis  47 . This displacement prevents the various system components e.g. lens  52 ,  53 , mirrors  51 , etc., obstructing the rays of radiation. The effect of flat mirror scanning normal to the scene gives a good linear scan. 
     A thermal image of the scene  41  is incident on the primary facet  45  where it reflects via the primary facet  46 , lens  52 , mirror pair  51 , and lens  53  to focus close to the secondary facets  49 ,  50 . The apparent image position is about half way between the apex of the primary facets  45 ,  46 , and axis  47 . From the secondary facet  49  radiation reflects off the secondary facet  50  and mirror  54  to be focussed through the lens  55  onto the detector  42 . 
     As the polygons  44 ,  48  rotate a different horizontal scan is made of the thermal scene by each facet pair  45 ,  46 , and  49 ,  50  due to the varying position of the primary facet  45 ,  46  apex. An advantage of having the primary and secondary facets in the form of roof mirror pairs is the linearity of the horizontal scan and its constant width for all vertical bands. 
     The detector  42  output modulates the L.E.D.  57  light output. An observer  64 , due to retinal persistance, sees a visible image of the thermal image  41  as the L.E.D. is raster scanned across the eye piece  58 . 
     As an alternative to a scanned L.E.D. display the detector output could be used to modulate the scan of a cathode ray tube (C.R.T.) whose screen then presents an image of the thermal scene. 
     Instead of, or in addition to, vertical scanning by the primary polygon a flapping mirror may be arranged before the detector. 
     The roof mirror prism may be two mirror surfaces, or a prism reflector, forming a roof reflector.