GB2198858A - Optical scanning surveillance apparatus - Google Patents

Abstract

Surveillance apparatus having an optical system for directing received radiation onto a scanning rotor 5 comprising a series of reflective roof mirror portions 6, 7 and thence to at least one detector 11, the scanning rotor being operable to give a vertical line scanning effect of the field-of-view of the optical system and the apparatus being mounted for rotation or oscillation about a vertical axis YY so as to advance the line scans around the axis, ie so as to rotate the azimuth direction of the field-of-view. The apparatus may also be pivoted, within limits, about a horizontal axis XX. <IMAGE>

Description

SURVEILLANCE APPARATUS This invention relates to surveillance apparatus in which an optical system scans a field-of-view and the received optical radiation is passed to a suitable radiation-sensitive detector for producing a video signal representative of the scanned field. More particularly, but not exclusively, the invention relates to apparatus operating in the infra-red spectral band of radiation, i.e. to 'thermal imagery According to the invention, there is provided surveillance apparatus comprising an optical system which is turnable to vary its azimuth direction and which is operable for receiving radiation from a field-of-view and for directing said radiation towards a scanning rotor, the rotor comprising reflective peripheral surface portions defining a plurality of roof-mirrors such that the radiation is folded by the roof mirrors as they respectively come to the position of incidence of the radiation on the rotor, and the radiation is then directed towards a radiation sensitive detector element, the arrangement being such that, as each roof mirror moves past said position of incidence, the radiation seen by the detector element is from a portion of the field-of-view of which the elevation varies linearly through the field-of view to give a line-scan effect.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, in which: figures 1 and 2 are a sectioned elevation and a sectioned plan view respectively of part of a first thermal imager, figures 3 and 4 are a sectional elevation of part of a second thermal imager and a section on the line AA of figure 3 respectively, figure 5 is a sectional plan view of a third thermal imager, figures 6, 7 and 8 are respectively a sectional elevation, a sectional side-view and a sectional plan-view of a fourth imager, figures 9 to 12 are respectively a sectional elevation, side-view, plan-view and a detail drawing relating to a fifth imager, figure 13 is a sectional elevation of a sixth imager, figure 14 is a diagram of a scan pattern executed by the sixth image, figures 15, 16 and 17 are respectively a side-view an elevation, and a detail drawing relating to an imager with a laser range-finder, figure 13 is a sectional elevation of another imager with a laser rangefinder, and figures 19 to 2 are diagramnatic views of respective further imager embodiments.

Each of the thermal inagers to be described may be constructed to operate as a threat detection device for being mounted say beneath a helicopter and for then operating to scan over a field-of view extending through 360" in azimuth and through some desired angle in elevation. The field may be scanned by executing a series of near vertical lines, each line extending through say 10 in elevation, which lines are advanced in the azimuth direction by oscillating or better still rotating the imager about a vertical axis. To extend the elevation coverage, the imager may be able to tilt about a horizontal axis.

The imager of figures 1 and 2 comprises a telescope including a conical tube or shroud 1 with an objective lens 2 at its wider end and an 'eyepiece' lens or lens arrangement 3 at its narrower end.

The 'eyepiece' end of the tube communicates with the interior of a housing 4 containing a scanning rotor 5. The rotor 5 has a shape something like a vee-groove pulley except that each side of the vee-shape is formed by a series of six flat, radiation-reflective facets 6 and 7 each set at 45" to the axis of rotation SS of the rotor. The facets 6 on one side of the vee-shape are faced by respective corresponding ones of the facets 7 on the other side of the vee-shape so that each pair of corresponding facets 6 and 7 forms a 90" roof mirror. The rotor is rotated about axis ZZ by a motor (not shown).Whilst any pair of facets 6 and 7 is adjacent the narrow end of tube 1, radiation received via the telescope is folded by that facet pair so that it becomes incident on the first of two fixed folding mirrors 8, which mirrors fold it back again and direct it into a detector unit 9 including, in a housing, a focussing lens 10 and a radiation sensitive detector element 11 along with any suitable cryogenic means (not shown) for maintaining the detector element 11 at a low temperature.

The entire assembly of the telescope, rotor housing 4 and detector unit 9 is mounted in gymbals (not shown) so that it can be continuously rotated about a vertical axis YY and so that it can be pivoted, within limits, about a horizontal axis XX. The axes YY and XX intersect with each other and with the optical axis of lens 2 at a point within the tube 1 near the eyepiece end thereof.

Whilst the rotor 5 rotates, as any one pair of the facets 6 and 7 moves past the narrow or exit end of tube 1, the deflection of the received radiation by that pair so varies in a vertical plane that the detector element 11 will 'see' radiation of which the direction of incidence on lens 2 varies about the optical axis of that lens, i.e. a vertical line-scan effect is achieved. Since there are six pairs of facets on rotor 5, each revolution of the rotor will effect six line scans. Meanwhile, the assembly is rotated at a constant speed within its gymbals about the vertical axis YY so as to advance each line-scan with respect to the preceding one in azimuth.

The overall result is to scan an area extending all round the imager in azimuth by a series of elevation line scans. These lines are of course not quite vertical (since the assembly continues to rotate while each is executed) - the actual scanning path is in fact helical. Depending upon the detailed design of the optical elements of the imager, particularly the rotor 5, each line scan might extend through say 10 in elevation. The assembly can be turned about the horizontal axis XX to vary the direction of this elevation coverage.

The signals from the detector element 11 may be passed to a suitable display monitor to provide a display of part or all of the view seen by the imager and/or the signals can be passed to processing apparatus giving automatic target or threat detection and position measurement. The detector element 11 could be replaced by a linear array of say eight detector elements so as to achieve simultaneous scanning of a swathe of eight lines. Preferably, the or each detector element is of the kind known in the art as a 'TED' detector.

By way of example, the rotor 5 might rotate at about 30,000 r.p.m. while the assembly is rotated about axis YY at say one revolution per second.

As will be appreciated, it is not essential that the imager be gymbal mounted to provide, as well as azimuth rotation, a change in elevation coverage. Where the elevation coverage is to be fixed, i.e. to the extent of the scan lines, the assembly can be simply mounted on a suitable rotating platform (not shown). If gymbals are provided, it will generally be preferable to provide also some form of stabilising arrangement to maintain the elevation setting in the face of motion of the carrier, for example some type of gyroscopic or momentum stabilisation.

Instead of a refracting telescope as shown, a suitable reflecting telescope can be used. Also instead of rotating the imager continuously in azimuth it could be oscillated through some required azimuth range.

As shown in figures 3 and 4, a single rotor like the rotor 5 in figures 1 and 2 could be used as the line scanning element associated with two (or more) telescope/radiation detector combinations. In figures 3 and 4, two telescopes 12 and 13 each comprising an objective lens 14 and eyepiece lens 15 are respectively positioned above and below the rotor 5 which is driven to rotate, by a motor (not shown), about a horizontal axis XX.Radiation received via the two objective lenses 14 is folded by respective mirrors 16 so as to be rendered incident via the eyepiece lenses 15 onto respective diametrically opposite sides of the rotor 5, the radiation from one of the telescopes being initially incident on the facets 6 on one side of the vee-shape of the rotor and being reflected across to the facets 7 on the other side while the radiation from the other telescope is initially incident on facets 7 and is folded across to the facets 5. In each case, the radiation is further folded by a respective pair of fixed folding mirrors 17 into a respective one of two detector units 9 each similar to the detector unit 9 of figures 1 and 2. The two telescopes in figures 3 and 4 face in opposite directions.As the imager is rotated in azimuth however, respective helical scans throughout the 360" areas covered by the respective telescopes are executed. The two telescopes could be arranged to cover the same area in elevation in which case the effect of using two telescopes is to double the information update rate compared with the case where only one telescope is used or to permit improvement in the signal to noise ratio by averaging of the detector outputs.

Instead of covering the same area however, the two telescopes could cover different areas in elevation hence doubling the elevation coverage. As with the embodiment of figures 1 and 2, the elevation direction could be made variable, either by pivoting the telescopes in elevation together, e.g. so that they remain aligned and when one 'looks up' the other 'looks down', or they could be independently controllable in elevation. Alternatively or in addition to any of the options relating to elevation control, one of the telescopes could have a greater magnification than the other and/or the detector element(s) for one could be responsive to a different spectral bandwidth than the element(s) for the other telescope.

Where elevation variation is provided, whether for the embodiment of figures 1 and 2 or for one or both telescopes of figures 3 and 4, such elevation control could be by command of an operator or it could be made automatic, i.e. the elevation direction could be repetitively varied, either in steps or smoothly, by some automatic control means so as give an overall elevation coverage longer than the extent of the scan lines. One possibly particularly advantageous embodiment would comprise two telescopes of which one is automatically varied in elevation so as to give an overall field-of-view while the other perhaps with a higher magnification, is operator comnandable to look at areas covering particular items of interest initially detected via the first telescope.

Figure 5 shows an embodiment which, like the embodiment of figures 3 and 4, uses two telescopes and a single scanning rotor.

The difference is that a further folding mirror 20 is provided between the objective and eyepiece lenses of each telescope to provide 1800 folding of the received radiation and this permits the telescope objectives to be positioned at respective sides of the rotor 5. Operation is otherwise similar to that of the figure 3 and 4 embodiment and similar items are given the same reference numerals in each.

A further embodiment (not shown) could comprise a single rotor with two telescopes respectively positioned above and below the rotor as in figures 3 and 4 and two more telescopes positioned at respective sides of the rotor as in figure 5, thus giving a total of four simultaneously scanned fields-of-view.

Figures 6, 7 and 8 show an embodiment where the axis of rotation of the scanning rotor is in a vertical plane so that the rotor will give a gyroscope effect contributing to the momentum stabilisation of the imager. This embodiment comprises a dome-shaped housing 29 mounted on a platform (not shown) for rotation about axis YY to give azimuth scanning and for turning about axis XX to vary the elevation of the scanned area. A telescope including objective lens 30, a series of four folding mirrors 31, 32, 33 and 34 and an eyepiece lens assembly 35 is positioned within the housing so that the optical axis of lens 30 intersects the point of intersection between axes YY and XX and is able to receive radiation from the field-of-view via an opening 36 formed in the housing.The folding mirrors are arranged to fold the received radiation down and around so that, via the eyepiece lens assembly, it becomes incident on the facets 6 of a rotor 5 similar to the rotors of the previously described embodiments but here driven by a motor 37 to rotate about axis SS parallel to axis YY. As before, the radiation is reflected by rotor facets 6 across to rotor facets 7 and then into a detector unit 9. As before, the vertical extent of the line scans might be 10 say and this 10 area can be moved within an overall elevation coverage by turning the imager about axis XX. The stability of the imager position with respect to this axis is augmented by the gyroscopic action of rotor 5. Of course, additional gyroscopic stabilising means may be provided.These, like the two-axis suspension platform, torque motors and such for varying the azimuth and elevation directions, are not shown.

Like the embodiment of figures 1 and 2, the embodiment of figures 6, 7 and 8 can be further developed to include more than one telescope operating with a single scanning rotor. Figures 9, 10, 11 and 12 show one such development, this including four telescopes. It comprises. a cylindrical housing 40 arranged on a platform (not shown) for rotation about a vertical axis Y and turning about horizontal axis XX as before. Four openings or windows are provided in the housing each admitting radiation to a respective telescope objective lens 41, the optical axes of the lenses being at right angles to one another within a plane which also contains axis XX. Here, each objective lens axis is displaced to one side of the axis YY.

Meanwhile, the rotor 5 driven by motor 37 rotates about axis YY. The four folding mirrors 42 to 45 of each telescope are therefore re-positioned, compared with the previous embodiment, so as to fold the received radiation down and around and now also across as shown best by figure 12, to line up with the axis YY. From these mirrors, the radiation passes via a respective telescope eyepiece lens assembly 46 and the rotor 5 to a respective detector unit 9 as before.

The embodiment of figures 9 to 12 exhibits a further development, which could also be made to the embodiment of figures 6 to 8, namely the coupling of a gyro wheel 47 to the output shaft of rotor drive motor 37 so as to further augment the momentum stabilisation of the imager.

Figure 13 shows an embodiment with a rotor 5 rotating about a horizontal axis and two telescopes with objective lenses 50 respectively above and below the rotor somewhat similar to the embodiment of figures 3 and 4. Here, however, each telescope contains a beamsplitter 51 for dividing the radiation received through each objective lens into two parts, which parts are directed via respective additional folding mirrors 52, eyepiece lens assemblies 53 and the rotor 5 into a respective one of four detector units (not shown). Thus, for each telescope, there are two detector channels. These can be made responsive to different spectral bands either by using bean splitters with appropriate characteristics or by using appropriate detector elements in the detector units.

The geometry of each channel pair is such that the optical lengths of the channels are equal but the scanning actions therein are in opposite directions (one up and one down) half a scan period out-of-phase. The resultant overall scan pattern is thus a diamond lattice as shown in figure 14. Thus, if symmetrical interlaced scanning in object space is to be achieved, the sample timing of the detected signals must be matched to the lattice.

The use of a beam splitter reduces the sensitivity of the imager in each channel but this reduction is offset by the use of two telescopes and by summing the signals from each channel associated with one telescope and the corresponding channel of the other telescope. If the sensitivity loss is not a problem, then each of the total of four channels could be made responsive to a different spectral band. This might be useful in helping to discriminate objects in the viewed field.

Figures 15, 16 and 17 show an embodiment including a laser range-finder. This comprises a rotor 5 which is similar to the rotors of the previous embodiments and which is driven to rotate about a horizontal axis SS by a drive motor (not shown). Three telescopes are provided, all covering the same field-of-view, and comprising respective objective lenses 60, 61 and 62, and eyepiece lens asemblies 63, 64 and 65. The optical axes of the lenses 60 and 61 lie respectively above and below the side of the Vee-shape defined by rotor 5 which carries the facets 6 and these axes are folded by respective single folding mirrors 66 and 67 to become incident vertically on those facets.The axes are folded across to the facets 7 on the other side of the Vee-shape and then, in the case of the axis of objective lens 60, to the exit pupil of a laser beam generator 68 and, in the case of the axis of lens 61, to a laser beam detector 69. The third objective lens 62 has its axis positioned to one side of rotor 5 and in the plane containing axis SS but at right angles thereto. The axis extends past the rotor and is then folded through 180 by two folding mirrors 70 and 71, to become incident on the facets 7 via eyepiece lens assembly 65 and thence across to facets 6 and into a detector (not shown) similar to each unit 9 of the previous embodiments. The telescope comprising lens 62 and associated detector unit acts as a surveillance imager in the manner described for previous embodiments.When an item of interest is detected, the laser beam generator emits a laser pulse which is emitted via the telescope comprising objective lens 60, this telescope in fact acting as a projector. Laser radiation reflected from the item is then received via lens 61 and sent to the laser beam detector 69. From a consideration of the drawing figures, it will be realised that the scanning of the laser transmit and receive fields via lens 60 and 61 respectively is in synchronism while the thermal imager scan, via lens 62, is half a line period out-of-phase relative to that of the laser fields. The mechanical arrangement and/or the imayer signal processing is made such that the thermal imager scan leads the laser field scan by this half-line period.The period then provides for the inevitable delay between detection of an item of interest and the output of a laser pulse without auxiliary steering of the laser beam. Thus, once an item has been designated for range finding by signal processing equipment (not shown) coupled to the output of the thermal imager detector unit, the laser can be fired half a line period after the item is next detected, the rotation of the whole apparatus in azimuth and the spin of the rotor (and the angular position measurements thereof) being such that the laser beam will then be directed at the item automatically.For example, with a rotor comprising six roof mirrors as shown, i.e. six facets on each side of the Vee-shape, and a rotor speed of 10,000 * IT revolutions per minute, a half-line period will equal: 0.5 * 60/(10,000 * IT * 6), .e. about 166 microseconds. The range to the target item can be deduced by measuring the transit time of the laser pulse either directly or indirectly, i.e. by measuring the angular displacement of the rotor during the transit time or by measurement of a displacement or length in the image plane of the laser detector 69. Thus, the laser transit time t in seconds for a range of x metres is 2x/3.108 and the angular displacement of the rotor during this time would be: (10,000 * n * 360/60) * t.

Doubling this displacement gives the angular shift at the laser detector. Since the embodiment of figures 15 to 17 comprises three separate sets of optics for the respective functions of thermal imaging, beam projection and return beam detection, each set can be designed to operate in the spectral band most appropriate to its function. Economies can be effected, if imaging and range-finding are performed in the same spectral band, as shown in figure 18. This embodiment comprises laser beam projecting optics including objective lens 90 and an imaging objective lens 91, the optical axes of these lenses being folded onto respective diametrically opposite sides of the rotor 5 via respective eyepiece lens assemblies 92 and 93. The folding of the axis of lens 91 however is done by means of a beam splitter mirror 94 which permits the return laser radiation to pass through it and thence via an eyepiece lens assembly 95 to a laser detector 96 directly, i.e. not via the rotor 5. The beam splitter could be a notch band pass beam splitter, for example if the imager operates in the 8 to 14 micrometre band and the laser at 10.6 micrometres, the beam splitter characteristics would be such that a narrow band of wavelengths centred on 10.6 micrometres are transmitted through to laser detector 96 while other wavelengths are reflected onto the rotor 5 and thence to the imager detector unit (not shown).With the arrangement as shown, the imager and beam projection field scan lines are in phase so the projection and imaging optics are positioned such that the imager scanning leads the projection scanning by one full line and the delay between detection of an object to be ranged and the emission of a laser pulse is made equal to a line period. If this delay is too long, the embodiment can be modified so that the optical axes of the imaging and projection fields are incident on the rotor at positions 90" apart as in the embodiment of figures 15 to 17.

It may be desirable to provide a capability for making a closer examination of objects of interest detected by one of the imagers described earlier herein. For this, an appropriate separate piece of imaging apparatus may be provided, for example, a helicopter say could be provided with an imager as described earlier mast mounted below the helicopter to give an all-around surveillance function plus say a thermal imaging sight mounted on the roof of the helicopter, the sight being usable to execute a frame scan, ie in the manner of a normal thermal imager, of a small area containing the object of interest.

Figure 19 shows an embodiment of a surveillance imager which may be suited for use on board a helicopter and which comprises two detecting channels, one of which may be used for locating objects of interest and the other of which can be used to examine such objects.

The embodiment comprises a wide field-of-view telescope comprising an objective lens 100 and two folding mirrors 101 and 102 arranged to fold the received radiation around into an eyepiece lens assembly 103 from whence it is incident on a scanning rotor 5 similar to that shown in earlier embodiments. The rotor folds the radiation around into a detector assembly 104 comprising a linear array 105 of ten photosites (not separately shown). A narrow field-of-view telescope is also provided, this comprising an objective lens 106, two folding mirrors 107 and 108 and an eyepiece lens assembly 109 which direct radiation received through lens 106 onto the rotor 5 at a position diametrically opposite the position at which it receives the radiation from lens 100.The radiation from lens 106 is directed by rotor 5 into a detector assembly 110 comprising an array 111 of twenty-six photosites. Each array 105 and 111 may be what is known in the art as a 'Sprite' detector. The rotor 5 is rotated by a drive motor (not shown) about horizontal axis RR while the whole of the imager is rotatable, driven by a motor (not shown) about a vertical axis 00 extending, perpendicular to the plane of the figure, through the narrow field-of-view telescope so that, as in previous embodiments, the wide field-of-view telescope and its associated detector assembly can execute an all-around surveillance function with vertical scan lines proceeding around axis 00. Axis RR intersects axis 00.The whole of the narrow field-of-view telescope, ie lens 106, mirrors 107 and 108 and eyepiece lens assembly 109, and the detector assembly 110 are mounted in bearings 112 for turning movement about axis RR and coupled via any suitable transmission 113, a a belt, sprocket-chain or gear drive, to a drive motor 115. The motor is energised by a controller (not shown) which is operable to turn the narrow field-of-view telescope about axis RR, thereby changing the elevation of its field-of-view. When an object of interest has been picked up via the wide field-of-view telescope, the rotation of the imager about axis 00 can be stopped with the azimuth direction of the narrow field-of-view telescope aimed at that object of interest.The elevation of the telescope can then be changed so that there is received via the narrow field-of-view a view of the object, this being scanned by a twenty-six line vertical swathe. The imager could now be oscillated about axis 00 to give a frame scan effect.

For each telescope, a position pick-off 114 is provided, this giving a signal indicative of the position of rotor 5, which signal is used to ensure correct synchronisation of the video signals from the associated detector assembly. At least, the pick-off 114 of the narrow field-of-view telescope is fixed to the eyepiece lens assembly 109 of that telescope so that it rotates with the telescope about the axis RR when the telescope elevation is changed. This maintains the correct synchronisation in the face of such elevation changes.

Similar rotor position pick-offs may be positioned appropriately in the other imager embodiments described herein.

The figure 19 embodiment could be modified by arranging it to rotate about axis RR instead of axis 00 and replacing mirrors 102 and 108 by respective roof mirrors to ensure that the scan lines are again vertical despite the changed rotation axis of the imager.

Variation of the elevation direction of the narrow field-of-view telescope is now done by rotating the whole imager about axis 00.

The motor 115 now controls the azimuth direction of the associated telescope and can be used to oscillate this telescope to give the aforementioned frame scanning of the swathes of scan lines executed by detector assembly 110. The modified embodiment may be best, firstly because then rotation of rotor 5 gives a momentum stabilising effect amd secondly because the frame scanning effect in the close-examination mode may be able to be done at a faster rate given that this frame scanning now involves oscillation of only the narrow field-of-view telescope and detector assembly 110 rather than the whole imager.

A scanning rotor such as the rotor 5 but divided into. two halves so that the respective sets of facets can be rotated at slightly different speeds and with different numbers of facets in each set, ie in each 'half' of the rotor, can execute a frame scan this is disclosed in UK Patent Specification No. 2,110,897.

The embodiment of this invention shown in figure 20 comprises a telescope 120, a scanning rotor 5, a folding mirror 121 and a twelve photosite detector assembly 122 generally arranged to operate in the same way as the other embodiments described herein. Positioned adjacent the rotor 5 however, and coaxial therewith is a further rotor 123, this comprising two halves 124 and 125 respectively with eight and seven facets. The rotor 5, with its two sets of eight facets, and the half 124 of rotor 123, which half may be fixed to or integral with rotor 5, are rotated art a speed of say 15700 r.p.m by a motor (not shown) which is also coupled to rotate rotor half 125 in the same direction as the rotor 5 and rotor half 124 but at a speed of 17952 r.p.m. The rotors are supported by an assembly which permits them to be moved (up and down in the figure) so that either rotor 5 registers with the radiation from telescope 120 or, as shown at X in the figure, so that rotor 123 is in such registry. With rotor 5 in the registry position, the imager is rotated, preferably about the axis of the rotors but not necessarily so, to scan the azimuth direction of the telescope and give an all-around surveillance function. When an item of interest is detected, the imager is stopped with the telescope looking at the area containing the item and then the rotors are moved so that rotor 123 comes to the registry position and now a frame or raster scan of the area is carried out.The given rotor rotation speeds and facet numbers are of course exemplary - information for enabling a different choice of such parameters to be made is given in the aforementioned UK patent specification.

Figures 21 and 22 also show embodiments comprising a 'line scanning' rotor and a 'frame scanning' rotor.

Figure 21 illustrates a compact means for rotating the horizontally generated scans through 90" to provide the vertical line scanning mode required. This 'means' is a 90" roof mirror sloped at 45" to provide a fold in the optical path which is also rotated through 45" to rotate the optical ray bundle through 90". The roof mirror, if placed close to the eyepiece as shown in figure 21, leads to a compact layout in which the optical axis is folded back over the rotors.

Figure 21 has only one telescope-detector channel with which to perform two functions which results in design compromises with respect to resolution, FOV, frame rate etc. The switching of functions is achieved by hoisting the rotor assembly over a distance of some 55mm which by comparison with electrical switching is not so elegant even if it is robust. Note that the line scanning part of the rotor has roof angles (not 90 ) to mimic the mean angle of the co-axial scanner section which may be 97" or more. Also, if required, the speed could be changed so that the speed and the function in operation are matched; there would be a delay to be accommodated.

The folding/rotating roof mirror would cause the lines in the framing mode to be vertical. If this is disliked, the roof mirror could be either 'derotated' or rolled over. In the latter case a flat would have to be provided across the ridge at the back.

Figure 22 is similar to figure 21 except for the provision of two detector-lens assemblies, two telescopes. Hence there is the option to design each channel to suit its function. The line scan roof mirros can now be 90" if this is otherwise convenient. It is not now necessary to move the rotors linearly with respect to the telescopes.

By using a 16 element SPRITE detector in the line scan channel a lower rotor speed can be used whilst retaining the 1Hz rotations rate and the 0.25 milli radian line pitch. A 12 element SPRITE In the co-axial channel will provide 672 lines of which only 625 could be used if a 625 line standard is needed. As shown a 28Hz frame rate is used as a result of the 0.25 milli radian line pitch chosen for the line scan channel. A 25Hz 625 line frame and a 0.25 milli radian pitch can be provided if the rotation rates of the rotors are 12,000 RPM and 10,500 RPM with a line scan update rate of 0.78Hz or 1.28 secs for 360". The folding-rotating roof mirror is required in the line scan channel but a plain (flat) mirror in the co-axial channel will provide horizontally scanned lines in the frames, which is normal practice. The assembly of figure 22 is larger than that of figure 21 but the packing is more dense because space is not wasted to provide for rotor hoisting. The option of providing a high manification telescope for the co-axial channel is useful. Note also that it is only 1800 behind the line scan optical axis and thus a detected object can be recognised in a shorter time than is practicable with the design of figure 21 (assuming 'forward' motion during function switching).

Claims (2)

1. Surveillance apparatus comprising an optical system which is turnable to vary its azimuth direction and which is operable for receiving radiation from a field-of-view and for directing said radiation towards a scanning rotor, the rotor comprising reflective peripheral surface portions defining a plurality of roof-mirrors such that the radiation is folded by the roof mirrors as they respectively come to the position of incidence of the radiation on the rotor, and the radiation is then directed towards a radiation sensitive detector element, the arrangement being such that, as each roof mirror moves past said position of incidence, the radiation seen by the detector element is from a portion of the field-of-view of which the elevation varies linearly through the field-of view to give a line-scan effect.

2. Surveillance apparatus substantially as hereinbefore described with reference to the accompany drawings.