GB2202105A - Thermal imaging system - Google Patents

Abstract

A thermal imaging system, especially for missile guidance, comprises electronic video line processing means 7 for combining signals in pairs from respective elements of an IR detector device 6 in different ways to produce selectability of the displayed field-of-view without modifying the magnification of an optical system 1-4 used for receiving radiation from the scene. The scene may be scanned, in each of a series of successive frame periods, by a series of successive swathes of simultaneously executed scan lines. In one mode of operation sensor signals associated with adjacent scan lines in the same swathe are combined; in another mode the swathes of scan lines in each frame overlap and sensor signals associated with scan lines in successive swathes are combined. <IMAGE>

Description

TITLE: THERMAL IMAGING.

This invention relates to thermal imaging and display systems more particularly but not exclusively to such systems intended for use as part of a missile guidance system.

A thermal imager may comprise an infra-red.

radiation detection device and an optical scanning system, including moving mirrors for example, for in effect causing the detection device to scan the radiation image of a viewed scene and to produce a video signal which, after suitable processing, can be displayed on t.v. monitor for example.

It is desirable to be able to control the fieldof-view of the imager. For example, particularly in connection with missile guidance, three field-ofview affecting operation modes have been identified as useful.

In the first mode, the full field-of-view is displayed to enable an operator to detect or acquire a target say. Having acquired the target, it is desirable to provide an enlarged display of that portion of the field-of-view which includes the target.

This constitutes the second mode of operation. In a third mode which has been found useful, the target containing portion only of the full field is scanned as in the second mode but the displayed image is not enlarged. Instead the scanning and the display updating is done at an increased rate.

The field-of-view can of course be modified by the inclusion of an optical system having controllable magnification or by providing a dual optical system and providing means for selecting one or the other part of the dual system to give different magnifications.

According to the present invention, there is provided a thermal imaging system including electronic control means for achieving a change in the displayed portion of the field-of-view of the system without modifying the magnification of an optical system used for receiving radiation from the scene viewed.

For a better undestanding of the invention, reference will now be made, by way of example, to the accompanying drawings, in which: figure 1 illustrates diagrammatically some of the parts of a thermal imager and display system, figures 2 and 3 are two diagrams illustrating the combination of scanning signals respectively formed during a first and a second mode of operation of the figure 1 imager, and fiqures 4 and 5 are two diaras illustrating

the respective sequences of readindisplaying scanning signals during the first and second operating modes.

Figure 1 shows, diagranmatically, some of the parts of a thermal imaging and display system. A receiving system, for example a telescopic viewer comprising lenses 1 and 2 as shown, receives radiation from the scene viewed and focuses it onto a polygon mirror 3 which is rotated about its axis by a motor (not shown). The surfaces of mirror 3 reflect the radiation towards a plane mirror 4 which is oscillated back and forth about an axis within the plane of the mirror by an electronically controlled drive 5. From mirror 4, the radiation reaches an infra-red detector device 6 comprising a linear array of eight photosites (not shown individually).The signals formed by the respective photosites of the device 6 are passed to a control and picture generating unit 7 which converts each into a series of signals representing a line of picture elements and suitable for display on a t.v. monitor 8.

The rotation of mirror 3, in effect, causes the device 6 to scan azimuth lines, i.e. across the fieldof-view of the imager while the oscillation of mirror 4 gives a frame or elevation scanning effect.

It will be appreciated that figure 1 only sets out to show some of the parts of the imager to illustrate its principle of operation and that, in a practical arrangement, various focussing lenses and subsidiary reflecting surfaces will generally be provided.

The parts shown are not intended to be in a practical positional arrangement and are not drawn to scale.

Because device 6 comprises an array of eight photosites, the thermal imager is capable of scanning a swathe of eight azimuth lines simultaneously. The unit 7 incorporates storage and/or delay means for enabling the respective simultaneously formed lines of picture information to be displayed consecutively by monitor 8 and for enabling these lines to be combined or added as shown in figures 2 and 3. In figure 2, the lines A, B, C, D, E, F, G and H from the respective photosites are combined in pairs so that each scan in the line direction gives four composite lines W, X, Y and Z withW= A+B, X = C+D, Y = E+F and Z = G+H.In figure 3, the lines ABC and D are combined with respective ones of the lines E, F, G and H formed in the next preceding line-scan so that, during the first scan, W = A, X = B, Y = C, and Z = D while, during the second and each subsequent scan W = A+E( 1), X = Y = C+G (-1) and Z = D+H(-1) r the symbol (-1) indicating that the relevant line was formed during the next preceding azimuth scan.

Thus, in either case, four composite lines are produced at a time. Each azimuth scan is performed while a face of mirror 3 is moving through a range in which it so reflects radiation from positions within the field-of-view of the imager that this radiation becomes incident on detector device 6. Then, as the next following face of mirror 3 moves to the beginning of this range, the next azimuth scan starts to be executed and so on. The scans do not follow directly on one from another but are instead separated by time intervals during which no face of mirror 3 is in the scanning range and the length of which determines a parameter called the scanflingefficiency of mirror 3. This might have a value of 50% which means that the intervals between scans are equal to the time taken for each scan.Thus, with the mirror 3 rotating at 39,000 rpm, each line scan occupies 128 rA Secs and is followed by a non-scanning interval also of 128 l28KSecs. Thus, 64,ASecs are available for displaying each of the four composite lines produced during each scan, this time being appropriate to a normal 625 line standard CCIR t.v. monitor with 1:1 interlace, the frame rate, i.e. the rate at which mirror 4 is oscillated, being 50 Hz.

The imager is operable in two modes. In the first mode, suited for target acquisition say, the full field of view of the imager is displayed by monitor 8. For this mode, the picture information lines formed at the photosites of device 6 are combined in the way shown in figure 2 and the operation is as shown in figure 4. During the first scan, the composite lines W, X, Y and Z, equal to A+B, C+D, E+F and G+H respectively as described earlier, are received and stored in the unit 7. After this first scan, there follows a 128 rSec.

non-scanning interval during which the first two composite lines W and X of the first scan are displayed in succession by monitor 8. Then, while the second scan is taking place and the second set of information lines WXY and Z are being read and stored in unit 7, the lines Y and Z of the first scan are displayed in succession. The second set of lines W, X, Y and Z then start to be displayed in succession and so on.

In the second mode of operation, suitable for viewing the target and a missile being flown towards it say, the central part of the displayed image is enlarged by twice in both the elevation and azimuth directions. Accordingly, the drive 5 is controlled so as to give only half the movement of mirror 4 as occurs when in the first mode, although the movement retains the same frequency and is about the same median position. Meanwhile, the picture information lines from the device 6 are read by the unit 7 for only half the time of each line scan and are delayed in the unit 7 so as to become combined and displayed as shown in figures 3 and 5. Thus, during a central 64 A Sec. portion of the first scan, the lines A to H are each read separately while signal A is also displayed.

The other lines B to H read during the first scan are passed to monitor 8 for display after respective delays equal to successively greater multiples of 64Secs.

Thusi the monitor will have just ended its display of line D as the unit 7 starts to read the lines A to H of the next scan. Thus, the line A of this next scan is added to the line E of the first scan and the sum is displayed by monitor 8. Thereafter, the line B of the second scan and the line F of the first scan are summed and displayed and so on. It will be realised that this summation of the lines is correct because, in the second mode of operation, the movement of mirror 4 and hence also the elevation scan have been reduced to half of what they were in the first mode of operation and therefore the swathes of lines scanned during the successive azimuth scans overlap, the lines A to D in each case corresponding to the same linear regions of the scene as the lines E to H of the next previous scan.In effect, a desired object of reducing the size of each photosite during the second mode is achieved while the reduction in sensitivity that would normally follow from this reduction is obviated by the summation.

The illustrated imager and display apparatus could be adapted for operability in a third mode which, like the first and second modes, has been identified as useful. In this third mode, the oscillation amplitude of mirror 4 is again reduced to half of the amount in the first mode but the frequency of oscillation is doubled to 100 Hz.

Meanwhile, the monitor 8 is made controllable so that it can provide a picture having 312 lines instead of 625 with a field rate of 100 Hz. The result is a display of the same width as in the first mode but of half the height and updated at twice the frequency.

The availability of the third operating mode, particularly the 100 Hz field rate aspect thereof, may be desirable in connection with certain kinds of missile control system.

As will be appreciated, the principles set out above can be extended to systems wherein any or all of the t.v. line standard, the polygonal mirror speed, the number of photosites of detector, the scan efficiency and the electronic magnification factor are different from that described in connection with the particular embodiment. Also, although the embodiment comprises t.v. monitor 8 and reference is made to line standards and such, it is of course not essential that the signal derived from the thermal imager be actually displayed.

Instead, it may for example pass direct to an automatic missile guidance system. The modifiable parameters mentioned above are to some extent interdependent and so, as will be clear, any modifications desired may require consideration of the whole context. By way of example only some possibilities and associated factors are considered below: TV line standard - the CCIR T.V. standards mention systems wherein there are 405, 525, 625 and 819 lines per frame, a T.V. frame being comprised of two fields of scan of half the TV lines. Some of these systems use 1/25 second for a frame and others use 1/30 second.

This frame time difference is to enable the matching of the field rate to domestic power supply frequency e.g.

USA use 525 for their 60 Hz supply and UK use 625 for the 50 Hz supply. Thermal imager manufacturers have also produced equipment that use i TV (625 line) standard at 312 line, standard 625 line and there is a proposal for a high resolution thermal imager of approximately 1000 line standard. The number of lines to be displayed in the frame period decides the length of time to display the line (i.e. for a 1/25 second frame period, 1 line at 625 = 64us, 1 line at 312 = 128us, etc.).

Polygon Speed - for a 625 line 25 frames/sec. system, the reflective or refractive line scan system has to scan the scene 937,500 times per minute, (i.e. 625 lines x 25 frames x 60 seconds). For a given number of scans per minute, increasing the number of surfaces of a polygonal scanning mirror reduces the polygon speed but also increases the size of the polygon. This increase of size causes increased air drag and as a result a sixface polygon as used in figure 1; appears to be optimum and is hence preferred. Six faces are not essential however. Nor is it essential that a polygonal mirror be used.Instead, line scanning could be carried out by a different kind of reflective element, or by say a refractive element (which may or may not be a polygonal rotor element) Number of Detectors - for a six face polygon and if only one detector with one discrete photosite is used, the rotational speed would be 156,250 r.p.m.

To reduce the polygon speed, additional detectors/ photosites can be used and the information stored until required for display. This reduces the polygon speed and also increases the dwell time of the detector on the scene, i.e.

No. of detectors Dwell time per line Rotational Speed (100W scan efficiency) 1 64 u sec 156,250 rpm 2 128 u sec 78,125 rpm 4 256 u sec 39,625 rpm 8 512 u sec 19,531 rpm 16 1024 u sec 9,765 rpm The stored information is then read out of store to correspond to the line time, i.e. 64 u sec.

Scan Efficiency - with some polygon systems it is not possible to optically obtain more than 50% scan efficiency, therefore, the dwell time -for this is only half of the full scan time.

Electronic Magnification Factor - the electronic magnification factor can be a variable if no account is taken of increased spatial resolution with magnification. The increased spatial resolution is accomplished in the present patent by using 2 x the number of detectors for a 2 x magnification. It would be possible to have other magnifications with matched spatial resolution by increasing the number of detectors by the magnification factor, i.e. 2 x mag. = 2 x N detectors, 4 x mag. = 4 x N detectors.

Claims (3)

WE CLAIM:

1. A thermal imaging system including electronic control means for achieving a change in the displayed portion of the field-of-view of the system without modifying the magnification of an optical system used for receiving radiation from the scene viewed.

2. A system according to claim 1, wherein a radiation detector used in the system includes a plurality of sensitive elements for providing respective lines of video signal and wherein said control means is selectively operable for combining the signals from pairs of the elements to provide a first form of picture signal and for combining the signals from different pairs of elements to provide a second form of picture signal.

3. A thermal imaging system substantially as hereinbefore described with reference to the accompanying drawings.

3. A thermal imaging system substantially as hereinbefore described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows CLAIMS 1. A thermal imaginy system comprising an array of image sensors and scanning means for so directing radiation from a viewed scene to the sensors that, in each of a series of successive frame periods, the scene is scanned by a series of successive swathes of simultaneously executed scan lines, the scan lines extending in one direction and the successive swathes being advanced over the scene in a direction transverse to said one direction, the apparatus further comprising combining means for causing there to appear, at an output of the apparatus, a series of video signal lines each comprising a combination of a plurality of sensor signals associated with respective different ones of said scan lines, and control means for controlling the scanning means and the combining means to cause the system to operate in either of two modes as selected by the system operator, in one of the two modes the scanning means operating such that the scanned area has a first dimension in said transverse direction, and each said video signal line comprises a combination of sensor signals respectively associated with adjacent scan lines in the same swathe while in the other mode the scanned area has a second smaller dimension in said transverse direction, the swathes of scan lines in each frame overlap, and each said video signal line comprises ,a combination of sensor signals associated with respective scan lines in successive swathes.

2. A system according to claim 1, wherein the control means is operable such that a third mode of operation of the system can be selected, in which third mode the scanned area has said second dimension in the transverse direction and the repetition rate of the frame scans is increased so that, for each frame, fewer video signal lines are produced at said output terminal than is the case for the first and second modes.