Optical system

Abstract: Catadioptric optical systems are generally arranged to view a fixed large field of view. Apparatus for steering a field of view includes a pair of Risley prisms (15, 16), which are disc shaped prisms having a generally shallow wedge like side cross-section, arranged in the path of the incoming radiation. The prisms are rotatable with respect to each other and to the optical axis of the system so as to steer the field of view. By utilising such steering in a dual waveband system then a system having two fields of view and two magnifications may be achieved in which an image over a smaller field of view but of high magnification may be steered within a larger image of large field of view but low magnification.

Description: This invention relates to an optical system including transmission paths for respective first and second co-axial optical beams having a common optical axis and each having a field of view, the first field of view being magnified relative to the second field of view. In particular it relates to such an optical system of the catadioptric type which includes both reflective and refractive elements. Optical systems are known which include a first relatively large reflector haying a central aperture and arranged to reflect radiation towards a second reflector from which radiation is again reflected through the aperture within the first reflector. One or more refractive elements then collimate or focus the radiation as desired. Such systems are generally designed to operate with one particular chosen frequency waveband and tend to be of a fixed field of view. Accordingly, they are useful for looking at small fixed fields of view and must be physically moved to survey a larger field of view. Infra-red thermal imagers often use such systems. EP-A-0128815 describes a system which allows for the simultaneous transmission of two fields of view which are fixed relative to one another. It is an object of the present invention to provide an optical system in which the two fields of view are not fixed and which can obviate the need for the system to be physically moved. According to the present invention there is provided an optical system including transmission paths for respective first and second co-axial optical beams, having a common optical axis and each having a field of view, the first field of view being magnified relative to the second field of view, characterised by one or more prisms disposed only in to the transmission path of the first optical beam and arranged so as to adjustably steer the first field of view relative to the second field of view. The use of prisms for steering a field of view is known from International Publication No. WO83/00382. Preferably, steerability is achieved by means of a pair of Risley prisms mounted for relative rotation with respect to each other and to the optical axis, the Risley prisms having circular main faces and shallow prism angles, typically as shown in Figure 2. The prism angles are perhaps 4° or so. It is seen that if such a prism is rotated around its central axis then a field of view can be steered around a radius which is dependent upon the angle of the prism and its refractive index. By using a pair of such prisms, co-axially mounted then the field of view can be steered to anywhere within the radius determined by the two prisms. The system may comprise a pair of Risley prisms as described, passing light to a concave reflector having a central aperture, which reflector reflects the radiation onto a secondary reflector and from there through the aperture within the first reflector, where may be situated further refractive elements for collimating or focusing the radiation beam and directing it to, for instance, a scanning element and detector. Typically, the apparatus is adapted for use with infra-red radiation, although other types of electromagnetic radiation may also be suitably used. The applicants UK Patent Application No. 8727856 (GB-A-2212936, published on 02/08/89) describes a system which is capable of imaging two wavebands simultaneously, one of which wavebands can be of high magnification-small field of view and the other waveband can be of low magnification-large field of view. This is achieved by means of an optical element which reflects one waveband and transmits the other. The present invention enables the provision of an apparatus which combines this idea with beam steerability. Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 shows an optical system according to the invention, Figure 2 shows in cross section a type of prism, and Figure 3 shows diagrammatically a typical scene which can be viewed with the optical system shown in Figure 1. Referring to Figure 1, an optical system comprises a primary concave reflector 11 arranged to reflect radiation towards an element 12 which has the property of reflecting radiation in waveband 1 and transmitting radiation in waveband 2, which latter radiation is transmitted first through a group of elements 13. The primary reflector is provided with an aperture 14 at its centre where there is situated a further group 17 of lenses which serve to collimate radiation reflected from reflector 12 as well as the radiation transmitted by the first group of elements 13, and to pass it towards, for instance a scanning element or elements and a detector (not shown). If the system is an infra-red one then the scanning element may be for instance a polygon and the detector an array of individual infra-red detectors. A pair of Risley prisms 15 and 16, having central apertures 21, are mounted co-axially, one in front of the other in the radiation path before the concave reflector and also in co-axial alignment with aperture 14. The prisms are shown mounted, for convenience, generally co-planar with the element 12. Figure 2 shows a side view in cross-section of such a prism, which is a disc when viewed from above, and shows more clearly the structure which is of gently wedge-shaped cross-section, tapering gently from its first larger side 18 to its second smaller side 19. The properties of Risley prisms are well known; by rotating the prism a beam is steered to any position on a circle whose radius is determined by the prism angle and refractive index. By co-axially mounting two such prisms and independently rotating the prisms relative to each other the beam may be steered to any position within or on the radius determined by the combination of both prisms. This is a well know phenomenon. Apparatus according to the invention is therefore useful when a small field of view and high magnification is required since the small field of view can be steered around a larger area for, perhaps, object detection or similar processes. Typically, the radiation may be in the 3-5 or 8-12 micrometre wavebands. Consider the situation in which waveband two is of low magnification and large field of view and waveband one is of high magnification and small field of view. In these circumstances a scene such as that shown in Figure 3 can be viewed in which radiation in waveband two is used to obtain an overall image of a large field of view. Radiation in waveband one represents a small field of view but of higher magnification and is shown as a smaller box within the larger one. Since the radiation in waveband one is steerable then the small area of this image is movable within the the larger field of view to any desired position, such as that shown in dotted lines by position 22. In this way, a large field of view can be continually observed whilst smaller portions of interest may be viewed at higher magnification for easier identification of any objects within that smaller region, and these smaller regions may be steered around the larger scene to follow whatever objects are present. By careful selection of the optical components used both wavebands can be fully optically corrected. The wavebands may be for example 3-5 micrometres and 8-12 as one of the wavebands, more commonly as the large field of view low magnification waveband so that the operator can visually align the system to look at any desired large field of view. Infra-red radiation may then be used as the high magnification low field of view waveband or alternatively, after initial setting up and alignment, both wavebands one and two could be set to be infra-red wavebands if the use of visual radiation is not required after setting-up. In such a case the optical components may be selected such that the two infra-red wavebands are optically correct to a high degree and the visual waveband to perhaps a slightly lower degree, although perfectly adequate for initial setting up purposes. The invention thus provides the facility of having two telescopes of different fields of view and magnification within one unit and hence reduces space requirements. In addition, there is a fixed bore sight relationship between both fields of view and hence the setting up procedure is relatively simple. It will of course be noted that other beam steering arrangement than Risely prisms are also possible. In some circumstances one prism may be suff8icient for a limited degree of steerability. In other circumstances three or more prisms can be utilised. This can have advantages in that more complicated steering patterns can be achieved. For instance a beam can be generally steered by the first two prisms within an overall radius equal to the field of view and then steered by the third prism within a small radius positioned anywhere within the larger one. Other patterns can be envisaged. To achieve such flexibility in steerability is difficult with only one or two prisms.

Claims: An optical system including transmission paths for respective first and second co-axial optical beams (1, 2) having a common optical axis and each having a field of view, the first field of view being magnified relative to the second field of view, characterised by one or more prisms (15, 16) disposed only in the transmission path of the first optical beam (1) and arranged so as to adjustably steer the first field of view relative to the second field of view. An optical system as claimed in claim 1, wherein said one or more prisms are constituted by a pair of co-axially mounted prisms (15, 16) each having a central aperture (21), which apertures (21) are aligned and through which the transmission path of the second optical beam (2) extends, said prisms being rotatable relative to one another for adjustably steering the field of view of the first optical beam (1) relative to the field of view of the second optical beam (2). An optical system as claimed in claim 1, wherein said one or more prisms are constituted by a plurality of Risley prisms mounted for relative rotation with respect to each other and to the common optical axis. An optical system as claimed in claim 2 or claim 3, comprising an optical element (12) disposed in the transmission path of the second optical beam (2) and through which said second optical beam (2) is transmitted, a reflector (11) disposed in the transmission path of the first optical beam (1) after said prisms (15, 16) for reflecting said first optical beam (1) onto said optical element (12), said optical element (12) being adapted to reflect said first optical beam (1) whereby both said optical beams are directed to a common output.

Inventors: SMITH BRIAN FREDERICK, SMITH, BRIAN FREDERICK
Applicants: MARCONI GEC LTD, GEC-MARCONI LIMITED
Application year: 1989
Main classification: G02B17
IPC codes: G02B26, G02B17