GB2123575A - Opto-mechanical scanner - Google Patents

Abstract

An opto-mechanical scanner in the form of a polygonal reflective rotor 3 and used in say an aircraft-borne line-scanner, has its edges or corners truncated by non-reflective surface portions 9 and a masking baffle 10 is placed between the rotor and a fixed reflector element (5, Fig. 1) of the scanner so that undesirable spurious input and detector re-imaging effects are reduced. Optional masking baffles 12 are also shown. <IMAGE>

Description

SPECIFICATION Reconnaissance apparatus Optical radiation-sensitive reconnaissance apparatus, for example an infra-red line-scanner for use on board an aircraft or other vehicle, can comprise a detector containing a radiationsensitive detector element or an array of such elements, and a mechanical scanning system (comprising movable mirrors say) via which, at any instant, radiation from a small portion of the viewed scene is received by the detector. As the mirror(s) move, the viewed portion moves over the overall scene to give a scanning effect and the detector produces a corresponding video signal representative of the scene.In a line scanner, a single movable scanning element may be used to give a line scanning effect since the vehicle itself is moving and hence advancing the lines over the scene (e.g. the ground beneath an aircraft.) This element may take the form of a rotating, mirror- faced prism of regular polygonal (e.g. triangular) cross-section as disclosed in U.S. patent specification No.3211046. The prism or "scanning rotor" rotates about an axis parallel to the direction of movement of the aircraft or other vehicle containing it and infra-red radiation from the scene is reflected from the rotor, collected by one or two fixed parabolic mirror strips and directed via a fixed flat mirror to the detector element(s).

With such apparatus, it may happen that at about the middle of each scan-line, the rotor comes to such a position relative to the other mirror elements and the detector that the detector is imaged back onto itself. This results in a sudden drop in background radiation reaching the detector, a corresponding drop in the level of the video signal produced by the detector, and hence a blemish on the eventual reproduced picture.

Also, particularly if the geometry of the arrangement has been made such as to attempt to maintain the useful aperture of the overall optical system constant or nearly constant throughout each scan line, then it may also become such that, during part of each scan, the detector sees radiation from two different directions at once.

The radiation from only one of these directions is useful, the rest being spurious and producing a kind of scrambling effect in the video signal.

Accordingly, it is an object of the invention to provide an arrangement and geometry for a scanning rotor in a mechanical scanning system for reconnaissance apparatus, particuiarly but not exclusively a linescanner, which at least reduces the above-noted effects.

According to the invention, there is provided a mechanical scanning system containing a rotating reflective scanning rotor for receiving radiation from a viewed scene and collecting mirror means for receiving radiation reflected from the rotor and directing it to a radiation detector, the scanning rotor defining a plurality of reflective faces extending along and regularly spaced around the axis of rotation of the rotor, and further defining between each two adjacent reflective faces, a non-reflective face.

Advantageously, radiation stop means, e.g. an interposed screen or baffle, is interposed between the rotor and a fixed ridge-shaped mirror element forming part of the scanning system. Further screens or baffles may be introduced at strategic positions within the system to stop spurious reflections.

For a better understanding of the invention, reference will be made, by way of example, to the accompanying drawings in which figure 1 is a diagrammatic view of a previously proposed scanning arrangement, and figure 2 is a similar view of a scanner in accordance with the invention.

Figure 1 shows part of an infra-red line scanner wherein a detector assembly 1 containing focussing lenses and radiation sensitive element(s) (not separately shown) receives radiation 2 from a scene viewed, say from the ground scene beneath an aircraft (not shown) carrying the scanner, by way of a triangular reflective rotor 3, two fixed parabolic collecting mirror strips 4 positioned to respective sides of the rotor and a fixed triangular-section reflecting element 5. As the rotor 3 rotates, the portion of the scene imaged on the detector element(s) moves linearly across the scene. Thus, when the rotor is at the position shown by dashed lines, radiation including the ray 6 is imaged. At the same time, however, the spurious ray 7 is also imaged at the detector.Also, with the rotor in the mid-scan position shown in solid lines, the detector may be imaged on itself via the dash-dot path 8 going from one side of mirror 5, one of the mirrors 4, the uppermost surface of rotor 3, the other mirror 4 and the other side of mirror 5.

It may be of course that the illustrated arrangement, relative positions and such could be played with until the above effects of re-imaging and spurious input are reduced or eliminated.

Then, however, the system would probably be deoptimised from the point of view of other factors in particular regarding the maintenance of a reasonably constant effective system aperture throughout the scan.

In the figure 2 scanner, the sharp corners or edges between each two adjacent mirror faces are not present. Instead, each two faces are joined by a slightly rounded or flat surface portion 9 which is coated with non-reflective material. As a result, it has been possible to place between the rotor 3 and fixed mirror element 5 a screen or baffle 10 having a concave lower face 11 which is just cleared by the surface portions 9 as the rotor 3 rotates. The baffle extends around the axis of rotor 3 just sufficiently so that, by a combination of the masking effect of this baffle and the effect of the truncating of the rotor corners, the aforementioned re-irnaging and spurious input effects are reduced or eliminated. Additional masking baffles 1 2 may be placed beneath the mirror strips 4 so as to further reduce spurious input. Alternatively, the mirror strips 4 may have non-reflective portions extending down to the positions of the baffles 11 and performing the same function.

Provided the figure 2 scanner is properly designed, it can achieve the elimination of the above undesirable effects while still providing reasonably constant effect aperture throughout each scan line. In addition, however, the scanner is smaller and lighter for the same effective area of reflecting face, less power is required to drive it and, by removing stress concentration near the rotor edges during high speed rotation, may help the rotor faces to remain more accurately flat during such high speed rotation.

Claims (4)

1. An opto-mechanical scanning system for use in optical radiation sensitive reconnaissance apparatus, the system including: a reflective rotor defining a plurality of reflective faces extending along and regularly spaced around the rotor axis, at least one collecting reflector strip positioned alongside the rotor for receiving radiation reflected therefrom, a further reflective element positioned alongside the rotor for receiving radiation from the collecting reflector strip, and radiation detector means for receiving radiation from said further reflective element, said rotor further defining a plurality of nonreflective surface portions respectively between each two adjacent reflective faces and effectively truncating the corners which would otherwise be present at the respective positions and a masking baffle being positioned between the rotor and said further reflective element.

2. A system according to claim 1, wherein said masking baffle has a concave face extending partly around the rotor axis and positioned for being just cleared by the non-reflective faces of the rotor as the latter rotates.

3. A system according to claim 1 or 2 including further masking baffle means extending alongside the or each said collecting reflector strip.

4. An opto-mechanical scanning system substantially as hereinbefore described with reference to figure 2 of the accompanying drawings.