GB2339355A

GB2339355A - Laser rangefinder

Info

Publication number: GB2339355A
Application number: GB8233556A
Authority: GB
Inventors: Glenn Mcpherson Cuthbertson; Leslie George Shadrake; Neil James Short
Original assignee: GEC Marconi Ltd; Marconi Co Ltd; Marconi Avionics Ltd
Current assignee: BAE Systems Electronics Ltd; Allard Way Holdings Ltd
Priority date: 1982-11-24
Filing date: 1982-11-24
Publication date: 2000-01-19
Anticipated expiration: 2002-11-24
Also published as: GB2339355B; GB8233556D0

Description

2339355 Rangefinder This invention relates to rangefinders which utilise a

laser beam to determine the range of distant objects. In some circumstances, the need arises to determine therange of a distant object in addition to or in combination with the generation of an image of the thermal content of a viewed scene in which the distant object is present. The provision of separate rangefinders and thermal imagers results in some duplication of expensive equipment and adds significantly to its bulk and vulnerability, but is has proved difficult to integrate the two functions into a single system. The present invention seeks to provide an improved rangefinder in which these difficulties are reduced.

Accokding to this invention, a rangefinder is incorporated into an imaging device having a two dimensional field of view and in which a sensor is caused to scan across the field of view in a raster pattern, the sensor being an elongate device extending in the line direction of the raster pattern; the rangefinder including a laser arranged to emit a pulse of light towards an object within said field of view, the reflection pulse of which is received at said elongate device; and means utilising the position along the length of sakd sensor at which the reflection pulse is received to determine the range of said object.

A coarse indication of the ranae is obtained from a knowledge of the position of the sensor relative to said field view at the instant the pulse of light is emitted, as compared to the position to which it has moved by the time the reflection puls e is received. This information is derived from the tate at which the device is scanned across the field of view. In order to obtain the coarse indication of range a number of pulses of light are transmitted sequentially, typically 2 one per frame of the raster pattern with the instant of emission of each pulse having a different phase relative to that of the raster Pattern, until a reflection from one of the pulses is received. In one embodiment of the invention, the precise indication of range is given by the actual position along the length of the sensor at wftich the reflecion pulse is received. This is conveniently determined by noting the precise instant at which the reflection pulse is received; the fact that it is received at all gives the range to within a predetermined value (typically 1 kmJ.

In another embodiment of the invention, a first sequence of pulses is transmitted, as before, to obtain a coarse indication of ranqe when a reflection of one of the pulses is recieved at said sensor, but then a second sequence of pulses which are staggered in time is transmitted so that the reflection pulse of each is received at a different location along the elongate sensor. The precise range can be calculated when one of these reflections of the second sequence of pulses just strikes the end of the sensor, so that its actual position of reception is known.

The elongate device is the sensor, or one of the sensors, which is/are used to build up a thermal image of a viewed scene as this permits the same raster scanning arrangement to be used, and avoids the need to provde more than just a single optical aperture through which the external scene is viewed. The arrangement allows the position of the object whose range is found to be clearly and precisely identified within the same field of view, so that the object whose range is to be found can be identified on an imag e of the thermal content of the viewed scene.

The invention is further described by way of example with reference. to the accompanying drawings, in which Figure I shows a rangefinder in accordance with the 3 invention, Figure 2 shows part of it in greater detail and Figure 3 is an explanatory diagram.

Referring to Figure 1, there is shown therein a rangefinder which is incorporated into a thermal imaging system which is capable of generating an output renresentative of the thermal content of a two dimensional field of view. A sensor which generates the video signal is also utilised to produce a sigria! indicative of the range of a distant object lying within the field of view. The thermal imager is of generally conventional form and consist of a number of elongate sensors which are mounted side by side and scanned across the field of view by means of an optical system comprising a continuously rotating polygon mirror I and an oscillating or flapping mirror 2. Although in a thermal imager, a number of elongate detectors would be mounted side by side to trace out a wide swath in the line direction of the raster pattern, only the single sensor 3 which is utilised by the rangefinder is illustrated.

The field of view scanned is reDresented in a diagrammatic fashion by the area 4, and it is scanned by means of a raster pattern, which is somewhat analogous to a television raster pattern. The pattern consists of a number of parallel line scans 5(represented by thick broken lines), which are separated by flyback intervals 6. As the sensors which are suitable for generating electrical signals in response to thermal images are relatively small, and of a very localised nature, it is necessary to cause the field of view to be scanned across just a few of these sensors. The instantaneous position of the sensor 3 upon the field of view 4 is represented diagrammatically by the rectangular outline

7 to illustrate the way in which it is in effect traced over the viewed scene. For thermal imaging purposesf as 4 previously mentioned, a number of these sensors are mounted side by side, and the oDtical system is arranged such that the sensors are scanned together in the line direction of the raster pattern, so as to sweep out a relatively broad swath having a width corresponding to the number of sensors. In this way, relatively few individual line scan movements are needed. This is an important practical consideration, since although scanhing in the line(X) direction is achieved by means of the continuously rotating polygon mirror 1, scanning in the f ield M direction is obtained by means of a torque motor 8, which drives the flapping mirror 2. The use of a polygon mirror and a flapping mirror to scan a viewed scene across a themal. sensor, are in themselves well known. The thermal radiation of the field of view 4 is gathered by a lens system 9, which passes the radiation via the mirror system and brings it to a focus at the sensor 3.

A laser 10 is mounted along side the optical system 9 and arranged to point in the same direction. The laser 10 is controlled by a laser control circuit 11, which initiates the emission of a pulse of light at required instants. The duration of the pulse of light is very short, and the range of an object which is struck by the pulse of light from the laser is measured by determining the'time which elapses before a reflection pulse (i.e. an echo) is received. The reflection pulse will, of course, only be detected if the object lies in that portion of the field of view 4 which is currently imaged upon the sensor 3.

The line and field deflections of the raster pattern which are generated by the mirrors 1 and 2 respectively are controlled by a line scan circuit12 and a field scan circuit 13, which are themselves synchronised by a common timing circuit 14 having an accurately stable reference frequency. This timing circuit 14 is also 1 used to control the firing of the laser 10, so that the pulse of light is transmitted at an instant in time which is known relative to that of the raster scanning pattern. The exact time of transmission is determined by sensing the emmission from the laser this information is Drovided from the laser 10 via lead 19. A number of pulses of light are transmitted in sequence, one in each frame of the raster scanning pattern, and off-set in time from each other so that each IQ is transmitted while a different region of the field of view is imaged upon the sensor 3. The timing is chosen such that the whole of the possible range within which the object lies is covered by the sequence of pulses as this means that at least one of the reflection

Ci.e. echo) pulses will be recieved by the sensor 3. The number of pulses needed to cover a particular total range will depend upon the physical length of the sensor 3 and the rate at which it scans across the raster pattern. These parameters are set in this example such that the time taken to scan along the length of the sensor 3 is equal to the time taken for light to travel out and back I km i.e. 6.6 microseconds.

Thus when a reflection pulse is detected bythe sensor 3, the range of the object is known to an accuracy of 1 km. This is unlikely to be sufficient for most purposes as generally a much finer degree of spatial resolution is required.

The nature of the sensor 3 is shown in greater detail in Figure 2. It consists of an elongate rectangular strip of sensitive material, typically having a length of about I mm., a width of 62.5 pm and 10 Um thick. The sensor is conventiently of the kind t ermed a "SPRITE" detector and described in the paper by C.T. Elliott, page 1 et. seq. of the IEE international Conference on Advanced Infra- Red Detectors and Systems (Londonl, 20th October, 1981 IEE Conference Publication No. 204.

6 Sensors of this kind are intended to operate in an integration mode when being used to generate a signal representative of the thermal content of a viewed scene. As the image 20 of the viewed scene is scanned along the lergthof the sensor 3 in the direction of the arrow 21, the charges generated within the body of the sensor by the presence of the image are arranged to migrate along the length of the sensor at the same velocity. Thus the image relating to an individual point within the viewed scene under goes constructive integration for the period of time taken for the image to traverse the length of the sensor, and this greatly enhances the signal-to-noise ratio of the thermal imager.

In a first enbodiment of this invenion, the sensor is, however, not used in its integration mode as an output signal is obtained by connecting an output amplifier 23 connected to electrodes at the opposite ends of the sensor 3, via leads 24 to 25. At the instant in time that the reflection of the transmitter laser r)ulse is received 2a at the sensor 3, a voltage pulse is generated at the innuts to the amplifier 23, and a corresponding electrical signal is provided at the output terminal 25. This is fed to the echo Drocessor 15 shown in Figure 1, which is operative to clean the profile of the output pulse and to remove excess background noise. The echo processor 15 also receives a signal direct from the laser sensing circuit, to indicate the instants at which laser pulses are emitted, so that it has a knowledge of the coarse range of the distant object to within an accuracy of 1 km.

Additionally, a further timing circuit 16 determines the instant in time at which,the echo signal is received in relation to the period of time T 1 taken to scan along the length of the sensor 3 - this period T 2 gives the fractional Dart of the range in terms of the period Ti and from this the actual range within the kilometre interval can be determined. The range is then displayed 7 on the range display 17, the number of whole kilometres being supplied by the echo processor 15, and the fract ional part of the range being supplied by the timing circuit 16.

Figure 3 diagrammatically represents the transmiss ion of a sequence of pulses of light, each pulse being emitted at a different point in successive frames of the raster pattern. An echo of the fifth pulse is detected by the sensor 3, and thus the total range is represented by the length of the line 18, which in the time domain has a length of (5 x T 1 1 + T 2F where T 1 is 6.6 VS With further reference to Figures I and 2, the operation of the rangefinder has so far been described with reference to the use of amplifier 23, which receives a signal which is generated across the length of the sensor 3. The particular merit of the sensor 3 as a thermal detector is that it can be operated in the integration mode, which most effectively removes the effect of background noise. This mode of operation is utilised in a second emb6diment of the invention, which is an alternative to that previously described. In the integration mode of operation the output signal is taken from a oair of electrodes positioned at the far end of the sensor 3, and the output signals are fed via leads 25 and 26 to an amplifier 27, which provides an output signal at terminal 28. As previously, a number of pulses of light are transrnitteO by the laser 10, one pulse occurring in each of a number of consecutive frames of the raster pattern, but displaced relative to the other pulses so that the reception of a reflection pulse at the sensor 3 indicates in which;bf a number of contiguous I kilometre intervals the distant object lies. However, no output signal is generated by the sensor 3 at the actual instant that the reflection echo is received by it. Instead, the locally generated charge carriers (which are generated by the incidence of 8 of the reflection pulse of light) are swept along the length of the sensor 3 in the direction of the arrow 21, and an output signal is provided at terminal 28 only when the charge carriers have been passed to the read out region to which the leads 25 and 26 are connected.

When the output signal is present at terminal 28, the echo processor 15 generates an output signal which is fed to the laser control circuit 11, which causes the laser to emit a second sequence of pulses of light. As with the f irst sequence of pulses, a single pulse is transmitted in each of a number of consecutive frame periods of the raster pattern, but this time the relative phase off-set of each pulse is made very much smaller, so that the corresponding reflection pulses are received at different locations along the length of the sensor 3. Thus the range of the object can be determined to a fairly high degree of precision by noting which of the reflection pulses are received at the sensor 2 and which are not. Those which are not received, are those which are returned to the rangefinder at periods of time which cause them to miss the far end of the sensor 3. The reflection pulse which strikes the read-out region of the sensor has a particularly hiliamplitude and this aids its detection.

To minimise the number of light pulses which are emitted subsequently to the first sequence, the second sequence can be separated in time into subsidiary sequences which progressively establish the range with increasing accuracy, and these Pulses are generated under the overall control of the echo processor 15 in response to the received reflection pulses. The range value is then passed to the range display 17.

9

Claims

1. A rangefinder which is incorporated into an imaging device having a two dimensional field of view and in which a sensor is caused to scan across the field of view in a raster pattern, the sensor 'being an elongate device extending in the line direction of the raster pattern; the rangefinder including a laser arranged to emit a pulse of light towards an object within said field of view, the reflection pulse of which is received at said elongate device) and means utilising the position along the length of said sensor at which the reflection Dulse is received to determine the range of said object. 2. A rangefinder as claimed in claim 1 and wherein means are provided for causing the laser to emit a sequence of pulses of light, with the instant of emission of each pulse of the sequence having a different phase relative to that of the raster pattern. 3. A rangefinder as claimed in claim 2 and wherein each pulse of the sequence is emitted in a different frame period of the raster pattern. 4. A rangef inder as claimed in claim 2 or 3 and wherein the precise indication of range is given by the position of reception along the length of the sensor of the first reflection pulse to be received by the sensor. 5. A rangefinder as claimed in claim 4 and wherein the sensor is arranged-to generate instantaneously an output pulse in resonse to the reception of the reflection pulse. 6. A rangefinder as claimed in claim 2 or 3 and wherein a second secruence of nulses which are staggered in time is transmitted so that the reflection oulse of each is received at a different location along the elongate sensor. 7. A rangefinder as claimed in claim 6 and wherein a precise indication of range is deter-mined by establishing which relection pulse of said second sequence is received at a predetermined region of said sensor. 8. A rangefinder substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.

Amendments to the claims have been filed as follows Claims I. A rangefinder which is incorporated into an imaging device having a two dimensional field of view and in which a sensor is caused to scan across the field of view in a raster pattern, the sensor -being an elongate device extending in the line direction of the raster pattern; the rangefinder including a laser arranged to emit a DUlse of light towards an object within said field of view, the reflection pulse of which is received at said-e-tongate device; and iTeans utilising the time of reception of the reflection pulse at a position along the length of said sensor to determine the range of said object.

2. A rangefinder as claimed in claim 1 and wherein means are provided for causing the laser to emit a sequence of pulses of light, with the instant of emission of each pulse of the sequence having a different phase relative to that of the raster pattern.

3. A rangef-inder as claimed in claim 2 and wherein each pulse of the sequence is emitted in a different frame period of the raster pattern.

4. A rangefinder as claimed in claim 2 or 3 and wherein the precise indication of range is given by the position of reception along the length of the sensor of the first reflection pulse to be received by the sensor.

5. A rangefinder as claimed in claim 4 and wherein the sensor is arranged to generate instantaneously an output pulse in response to the reception of the reflection pulse.

G. A rangefinder as claimed in claim 2 or 3 and wherein a second sequence of pulses which are staggered in time is transmitted so that the reflection nulse of each is received at a different location along the elonaate sensor.

7. A rangefinder as claimed in claim 6 and wherein a precise indication of range is determined by establishin<.:T which relection pulse of said second sequence is received 410 i'2, at a predetermined region of said sensor.

8. A rangefinder substantially as illustrated in and described with reference to Figure 1 of the accompanying drawings.