GB1567319A - Optical apparatus - Google Patents

Description

(54) OPTICAL APPARATUS (71) We, THE RANK ORGANISATION LIMITED, of 11 Hill Street, London, W1X 8AE, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention concerns improvements in optical scanning systems, with particular reference to mechanical scanning in which line scanning of a scene is effected by reflecting radiation from the scene onto a detector from successive faces of a rotating polygon, and "frame" scanning, in a direction perpendicular to the line scan, is effected by means of an oscillating reflector which oscillates about an axis perpendicular to the axis of rotation of the polygon, and by means of which radiation from the faces of the polygon is directed onto the detector.

An optical scanning system of the above type is described in our U.K. Patent Specification 1537483. An optimum arrangement of such a system employs a relay optical system for directing a beam of radiation from the successive faces of the rotating polygon onto the oscillatory frame scanning reflector, this optical system including at least one optical component which is offaxis with respect to the beam from the rotating polygon. For example, a concave imaging miror may be arranged with its radius of curvature substantially equal to the distance between it and the centre of each polygon face when the latter is reflecting a beam of radiation into the imaging mirror at the centre of a line scan, a beam deflecting mirror being arranged off-axis with respect to the said beam from the polygon in a focal plane of imaging mirror to direct radiation reflected by the imaging mirror onto the oscillatory reflector, which is positioned at substantially the same optical path length from the imaging mirror as the said polygon face. The off-axis tilt introduced into the optical path in such an arrangement results in a small angular displacement of the image of the scene provided at the detector. Such an angular displacement is of little consequence when the scanning system is used with a single radiation detector. If, however, a multiple detector or a detector array is employed, as may be necessary in certain applications, the angular displacement of the image of the scene at the detector may result in the scanner failing to scan the scene in precise cartesian coordinates, resulting in an overlap or underlap when a matrix or extended detector is employed with the scanning system.

The present invention provides a simple means of compensating for the effect of rotation introduced by off-axis optical components in a scanning system as aforesaid.

The invention provides an optical scanning system in which successive line scans of a scene are made across a radiation detector which is arranged to receive radiation from the scene reflected via successive faces of a rotating polygon, and in which a frame scan of slower speed than the line scan, and transverse the direction of the line scan, is effected by a frame scanning reflector which makes rotational oscillations about an axis perpendicular to the axis of rotation of the polygon, a beam of radiation being directed onto the frame scanning reflector from the rotating polygon by optical components which include at least one element which is off-axis with respect to the axis of the beam from the rotatong polygon when the latter is reflecting a beam of radiation at the centre of a line scan, and a prismatic element being included in the optical path between the polygon and the said frame scanning reflector for the purpose of effecting angular displacement of the image produced by said beam such as to compensate for angular displacement introduced by the offaxis element or elements.

The optical components directing the beam of radiation from the rotating polygon onto the oscillating frame scanning re flector may include an off-axis reflector arranged to receive radiation reflected by a further reflector from the said beam of radiation from the polygon and to direct it onto the oscillating frame scanning reflector.

The said further reflector may comprise a concave mirror arranged with its radius of curvature substantially equal to the distance between it and the centre of each polygon face when the latter is reflecting a beam of radiation into the said concave mirror at the centre of a line scan.

The prismatic element may be interposed between the off-axis reflector and the said further reflector. Alternatively, the prismatic element may be incorporated in the off-axis reflector, for example by having an internally reflecting face which constitutes the offaxis reflector.

The invention will be further described, by way of example, with reference to the drawings accompanying the Provisional Specification, in which: Figure 1 is a diagrammatic plan view of an optical scanning system according to one embodiment of the invention and Figure 2 is a diagrammatic section taken on the line Il-Il of Figure 1.

Referring to the drawings a rotating polygon 1, in this example a hexagon, is shown, mounted for rotation about its longitudinal axis 0, for effecting line scanning of a scene as "viewed" by an infrared radiation detector array D or other extended two-dimensioned detector. The radiation from the scene is directed onto a face of the polygon 1 by an afocal telescope system (not shown) two forms of which are described in our U.K. Patent Specification No. 1537483.

The rotating polygon 1 shown in Figure 1 effects scanning of a scene in one direction only, namely the line scanning direction. To effect two-dimensional scanning by means of the detector array D a frame scan is superimposed on the line scan transverse the direction of and at a slower speed than the line scanning. For this purpose a flat flame scan mirror M5 is provided which makes rotational oscillations about an axis X perpendicular to the axis of rotation 0 of the polygon 1. After reflection at each successive face of the rotating polygon 1 a beam B of radiation is directed onto the reciprocating mirror by a relay optical system which is such that it does not introduce aberrations.

One form of such a relay optical system is illustrated in Figures 1 and 2 and includes a single concave imaging mirror M3 which is located with its radius of curvature substantially equal to the distance of the mirror M3 from the centre of a reflecting face of the polygon 1 when the latter is at the centre of a given line scan. A beam deflecting flat mirror M4 is located in the focal plane of the concave mirror M3, that is, one half the radius of curvature of the mirror M3 from the surface of the latter, with its length parallel to the axis of rotation 0 of the polygon 1. The mirror M4 is located off-axis with respect to the axis of the beam B when the polygon 1 is positioned in correspondence with the centre of a line scan. The mirror M4 reflects radiation onto the reciprocating mirror M5, the axis of rotational oscillation of which is indicated by X. The radiation reflected by the oscillating mirror M5 is gathered by a lens L3 and imaged onto the detector array D.

The oscillation rate of the mirror M5 is related to the rotational speed of the polygon 1 so that the combined oscillation of the mirror M5 and rotation of the polygon 1 causes the detector to "see" successive areas of the scanned scene in a succession of line scans each occupying a "frame" so that a "picture" is formed by the successive frames.

The frame scanning rate will be determined by the rate of oscillation of the mirror M5 and the line scanning rate by the rate of rotation of the polygon 1.

It is arranged that the optical path length between the concave mirror M3 and the oscillating mirror M5 is substantially the same as that between the mirror M3 and the reflecting face of the polygon 1-that is, the distance M4--M5 is equal to one half of the radius of the concave mirror M3. In this way first order optical aberrations are eliminated, since the effective pupil is at the centre of curvature of the mirror M3, which therefore produces no coma or astigmatism.

The mirror M3 does, however, produce field curvature, but this is compensated by the reciprocating mirror M5. Any residual spherical aberration in the reflecting system can be corrected either by suitable asphericity of a surface in the associated telescope which directs radiation onto the polygon 1 or by asphericity of the lens L3.

It will be noted that since the surface of the polygon is imaged by the concave mirror M3 onto the reciprocating mirror M5 the mirror MS is also located at an effective system pupil. Thus no pupil scan occurs at the reciprocating mirror M5 or at the detector D.

The radiation paths in the system herein described correspond to those by which the scene would be illuminated if the detector were a source of radiation. It will be apparent from Figure 1 that, as a result of the off-axis component of the radiation path represented by M3, M4, the image of the detector array D in the scene being scanned, and conversely the image at the detector array D of the scene being scanned, will suffer a small rotation. Such rotation is clearly undesirable in the case where the detector array is two-dimensional or consists of a multiple detector, as may be the case for some applications, since it would result in the detector array failing to scan the scene accurately in two-dimensional cartesian coordinates.

In order to compensate for the image rotation introduced by the off-axis optical components a prismatic element is introduced in the optical path between M3 and M4. The prismatic element may take the form of a prism P as shown in Figures 1 and 2 with an apex angle, refractive index and dimensions calculated experimentally or by computation so as to compensate for the image rotation to enable the detector array D or other extended detector to scan the scene rectilinearly in two dimensions.

Alternatively, the prismatic element may be combined with the mirror M4 by providing a totally internally reflecting prism replacing both the miror M4 and the prism P.

Although described with reference to a rotating polygon optical scanner the invention is applicable to optical scanning systems generally. Accordingly, in its broadest aspect the invention comprehends an optical scanning system in which a linear or twodimensional scan of a scene is made by directing radiation onto a radiation detector from a scene by way of at least one rotating and/or oscillating reflecting scanning element along an optical path including at least one optical component which is off-axis with respect to the direction of the scanned beam when in the centre of scan position, and at least one prism element located in said optical path to compensate for image rotation introduced by the off-axis component or components.

WHAT WE CLAIM IS:- 1. An optical scanning system in which successive line scans of a scene are made across a radiation detector which is arranged to receive radiation from the scene reflected via successive faces of rotating polygon, and in which a frame scan of slower speed than the line scan, and transverse the direction of the line scan, is effected by a frame scanning reflector which makes rotational oscillations about an axis perpendicular to the axis of rotation of the polygon, a beam of radiation being directed onto the frame scanning reflector from the rotating polygon by optical components which include at least one element which is off-axis with respect to the axis of the beam from the rotating polygon, when the latter is reflecting a beam of radiation at the centre of a line scan, and a prismatic element being included in the optical path between the polygon and the said frame scanning reflector for the purpose of effecting angular displacement of the image produced by said beam such as to compensate for angular displacement introduced by the off-axis element or elements.

2. A scanning system according to Claim 1, in which the optical components directing the beam of radiation from the rotating polygon onto the oscillating frame scanning reflector include an off-axis reflector arranged to receive radiation reflected by a further reflector from the said beam of radiation from the polygon and to direct it onto the oscillating frame scanning reflector.

3. A scanning system according to Claim 2, in which the prismatic element is interposed between the off-axis reflector and the said further reflector.

4. A scanning system according to Claim 2, in which the prismatic element has an internally reflecting face which constitutes the off-axis reflector.

5. A scanning system according to Claim 2, Claim 3 or Claim 4, in which the further reflector is a concave reflector arranged with its radius of curvature substantially equal to the distance separating it from the centre of each polygon face when the latter is reflecting a beam of radiation into the said further reflector at the centre of a line scan.

6. An optical scanning system in which a linear or two-dimensional scan of a scene is made by directing radiation onto a radiation detector from a scene by way of at least one rotating and/or oscillating reflecting scanning element along an optical path including at least one optical component which is off-axis with respect to the direction of the scanned beam when in the centre of scan position, and at least one prism element located in said optical path to compensate for image rotation introduced by the off-axis component or components.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. some applications, since it would result in the detector array failing to scan the scene accurately in two-dimensional cartesian coordinates. In order to compensate for the image rotation introduced by the off-axis optical components a prismatic element is introduced in the optical path between M3 and M4. The prismatic element may take the form of a prism P as shown in Figures 1 and 2 with an apex angle, refractive index and dimensions calculated experimentally or by computation so as to compensate for the image rotation to enable the detector array D or other extended detector to scan the scene rectilinearly in two dimensions. Alternatively, the prismatic element may be combined with the mirror M4 by providing a totally internally reflecting prism replacing both the miror M4 and the prism P. Although described with reference to a rotating polygon optical scanner the invention is applicable to optical scanning systems generally. Accordingly, in its broadest aspect the invention comprehends an optical scanning system in which a linear or twodimensional scan of a scene is made by directing radiation onto a radiation detector from a scene by way of at least one rotating and/or oscillating reflecting scanning element along an optical path including at least one optical component which is off-axis with respect to the direction of the scanned beam when in the centre of scan position, and at least one prism element located in said optical path to compensate for image rotation introduced by the off-axis component or components. WHAT WE CLAIM IS:-

1. An optical scanning system in which successive line scans of a scene are made across a radiation detector which is arranged to receive radiation from the scene reflected via successive faces of rotating polygon, and in which a frame scan of slower speed than the line scan, and transverse the direction of the line scan, is effected by a frame scanning reflector which makes rotational oscillations about an axis perpendicular to the axis of rotation of the polygon, a beam of radiation being directed onto the frame scanning reflector from the rotating polygon by optical components which include at least one element which is off-axis with respect to the axis of the beam from the rotating polygon, when the latter is reflecting a beam of radiation at the centre of a line scan, and a prismatic element being included in the optical path between the polygon and the said frame scanning reflector for the purpose of effecting angular displacement of the image produced by said beam such as to compensate for angular displacement introduced by the off-axis element or elements.

2. A scanning system according to Claim 1, in which the optical components directing the beam of radiation from the rotating polygon onto the oscillating frame scanning reflector include an off-axis reflector arranged to receive radiation reflected by a further reflector from the said beam of radiation from the polygon and to direct it onto the oscillating frame scanning reflector.

3. A scanning system according to Claim 2, in which the prismatic element is interposed between the off-axis reflector and the said further reflector.

4. A scanning system according to Claim 2, in which the prismatic element has an internally reflecting face which constitutes the off-axis reflector.

5. A scanning system according to Claim 2, Claim 3 or Claim 4, in which the further reflector is a concave reflector arranged with its radius of curvature substantially equal to the distance separating it from the centre of each polygon face when the latter is reflecting a beam of radiation into the said further reflector at the centre of a line scan.

6. An optical scanning system in which a linear or two-dimensional scan of a scene is made by directing radiation onto a radiation detector from a scene by way of at least one rotating and/or oscillating reflecting scanning element along an optical path including at least one optical component which is off-axis with respect to the direction of the scanned beam when in the centre of scan position, and at least one prism element located in said optical path to compensate for image rotation introduced by the off-axis component or components.