GB2316253A

GB2316253A - Combined rangefinding and imaging system

Info

Publication number: GB2316253A
Application number: GB7935039A
Authority: GB
Inventors: Brian Blandford
Original assignee: Rank Organization Ltd
Current assignee: Rank Organization Ltd
Priority date: 1979-10-09
Filing date: 1979-10-09
Publication date: 1998-02-18
Anticipated expiration: 1999-10-09
Also published as: GB2316253B; GB7935039D0

Abstract

A combined laser rangefinder receiver and thermal imaging system comprises a rotating multifaceted reflector (10) constituting the line scanner in the thermal imaging system, the returned laser rangefinding signal (12) also being incident on this reflector (10) at a first facet (101) thereof, thereby to be reflected to a mirror (13) forming part of the rangefinder receiver which in turn reflects said signal to a second facet (102) of the line scan reflector (10) in such a manner that the returned laser signal is reflected from the second facet (102) along approximately the same optical Path (14) for all angular positions of the line scan reflector (10).

Description

Combined Rangefindings Thermal Imagine System This invention relates to a thermal imaging system incorporating a laser rangefinder.

A thermal imaging system, for example sensitive to radiation in the wavelength range of from about 8 to 12.5 micrometres, typically consists of a front optical subsystem, in practice a telescope system or zoom objective, a scanning mechanism, a detector, electronic signal processing circuitry and a ORT display.

A laser rangefinder, for example operating at 10.6 microns wavelength, typically consists of a pulsed carbon dioxide laser and associated optics as the transmitter, and a receiver comprising a front optical subsystem (generally similar to the transmitter optics), a detector, and timing circuitry to measure the time period elapsing between pulse firing and detection of the pulse reflected from the target.

A thermal imaging system and a laser rangefinder are commonly used in conjunction with one another, and it would obviously be advantageous if the two devices could be in part combined, using components or services in common however, a major difficulty in combining trite two devices, especially to incorporate the rangefinder receiver, lies in the presence of the scanning mechanism in the thermal imaging system, This scanner, which lies in the optical path between the front optical subsystem and the detector of the thermal imaging system, comprises one or more mechanically scanned optical components, effecting a raster scan of the detector across the image formed by the system optics, including the front optical sub-system, One of the scan motions, the line scan, is usually provided by a multi-faceted reflector which is rotatable at high speed. 2he other, perpendicular, scan motion, which in some systems employing parallel scanning can be omitted, often consists of an oscillating mirror, The problem of incorporating the laser rangefinder in the thermal imaging system lies, more particularly, in the high speed of rotation of the multifaceted line scan reflector.

Thus, when the returned laser pulse is received through the front optical subsystem of the thermal imaging system, the time difference between signals received from targets at the upper and lower limits of the desired operational range of the laser rangefinder corresponds to a significant difference in the position of the multifaceted line scan reflector. Thus, when in addition the returned laser pulse is incident on the line scan reflector, the returned laser pulse tends with increasing target distance to change its point of incidence on the rangefinder detector, increasingly moving away from the sensitive area of the detector, In order to overcome this problem, three solutions have been proposed. Two of these involve the use of optical components moving rapidly through the optical path at or behind the front optical subsystem to intercept the returned laser pulse, enabling this pulse to by-pass the line scan reflector. Of these two solutions, the first employs a reflecting shutter within the front optical subsystem and the second employs a reflecting germanium prism behind this optical subsystem, A third solution accepts the movement of the laser rangefinder image by providing a succession of rangefinder detector elements extending along the direction of line scan.

It is an object of the present invention to provide an alternative and improved solution to the problem of combining a thermal imaging system with a laser rangefinder, more especially with the rangefinder receiver, According to the invention, there is provided a combined rangefinding, thermal imaging system, including a multifaceted reflector which is rotatable to effect line scanning for thermal imaging, and a laser rangefinding receiver which includes a reflecting means for reflecting a rangefinding signal received from the first facet of the multifaceted reflector to cause said signal to be incident on a second facet of the multifaceted reflector and to be reflected therefrom along approximately the same optical path for all angular positions of the rotatable multifaceted reflector.

Further features and advantages of the invention will be apparent from the following description of a practical arrangement, referring to the accompanying drawings, in which: Figure 1 illustrates the principle of the invention, and Figure 2 shows in diagrammatic form a combined rangefinder receiver and thermal imaging system.

In Figure 1, the reference 10 designates a six faceted mirror drum rotating at high speed about the axis 11 to effect line scanning in a thermal imaging system, The reference 12 designates the optical path of a returned laser pulse which has been fired by a laser rangefinder transmitter and reflected from the target. This pulse is swot: tG be incident at 103 on a first facet 101 of the rotating mirror drum 10.

From the drum facet 101, the pulse is reflected to a curved mirror 13 to be incident at 104 on a second facet 102 of the drum, thereby to be reflected towards a rangefinder detector (not shown) along an exit path 14. The path of the pulse between the first and second facets of the drum is indicated by full line 15.

The rotational position of the mirror drum 10 at the moment the laser pulse is returned is not predetermined. Even when the laser transmitter is always fired, as is desirable, at the moment when the thermal imaging system is scanning the target in the centre of the thermal image display, the rotational position of the drum 10 at the moment of incidence of the returned pulse will vary with the target distance.

Accordingly, Figure 1 also shows two further rotational positions of the drum, in broken lines 10A and lOB corresponding to its positions before and after the position represented by 10. In accordance with the invention, the position of the curved mirror 13 is chosen in relation to its curvature so that, when the drum is in the position 10A or 10B, the returned laser pulse is reflected by the curved mirror 13 to be incident on the second facet 102 of the drum at approximately the same spatial point as the point of incidence 104 for the full line position of the drum 10, The paths of the laser pulse between the first and second facets, for the positions 10A and 10B of the drum, are indicated in dash-dotted line 15A and in dashed line 15B, respectively. In each case, the changed angle of incidence at the second facet 102 substantially compensates for the changed angle of reflection at the first facet 101, so that the laser pulse takes approximately the same exit path 14, More generally, the curvature and position of the curved mirror 13 are selected to maintain an approximately unchanging exit path 14 of the laser pulse for all rotational positions of the line scanning drum 10, The invention thus enables a laser rangefinder receiver to be incorporated in a thermal imaging system, without introducing movable optical components and without employing a line of rangefinder detector elements, It is to be noted that although Figure 1 shows a six faceted mirror drum, the invention can be employed with any multifaceted reflector, and the second reflection of the laser pulse on the multifaceted reflector may take place at any of the facets, and not necessarily at the next facet following the one on which the pulse is first incident, as is the case in the illustration.

Figure 2 shows a practical arrangement of combined rangefinder receiver and thermal imaging system, incorporating a six faceted rotating mirror drum similar to that shown in Figure 1. In this arrangement, the thermal imaging system comprises a front objective 20 (in practice this may comprise a zoom objective or telescope), the line scanning drum 21, a plane mirror 22, a curved mirror 23, a plane mirror 24, a perpendicular scanner comprising an oscillating mirror 25 providing the frame scan motion, a rear lens subsystem 26 and a detector 27. The path of the beam which forms the thermal image is shown in dotted line 28. The laser rangefinder receiver comprises the front objective 20, the line scanning drum 21, a curved mirror 29 equivalent to the curved mirror 13 in Figure 1, a further curved mirror 30, a plane mirror 31, the frame scan mirror 25, the rear lens subsystem 26 and a detector 32. The path of a returned laser pulse is shown in full line 33.

In the thermal imaging system, the purpose of the curved mirror 23 is to image the pupil at the point of incidence 103 (Figure 1) on the line scanning drum 21 on the frame scan mirror 25, and at the same time to form an intermediate image of the object at or adjacent to the plane mirror 24. This intermediate image is finally imaged on the thermal image detector 27 by the lens subsystem 26.

In the rangefinder receiver, the curved mirror 29 acts, in the same manner as previously described with reference to the mirror 13 in Figure 1,- to minimise lateral shift of the optical path of the returned laser pulses after reflection at the second facet of the line scanning drum 21, thereby to ensure that these pulses are always incident on the sensitive area of the rangefinder detector 32.

The curved mirror 29 may be spherical or toric, and it forms an intermediate image of the object between the mirror 29 and the second facet of the drum 21, The curved mirror 30 forms a second intermediate image between the plane mirror 31 and the frame scan mirror 25, close to the intermediate image of the thermal imaging system at the mirror 24, The lens subsystem 26 then forms the final image on the rangefinder detector 32. It should also be mentioned that the curved mirror 30 may also be spherical or toric, and its effect may be supplemented either by additional lenses between the mirrors 30 and 31 or between the mirror 31 and the frame scan mirror 25, o by using a spherical or toric mirror instead of the plane mirror 31, The above described imaging systems enable the detectors 27 and 32 to be located close together and, in practice, to be carried on a common cooled substrate 34, Thus, in practice, the detectors 27 and 32 each comprise one or more wafers of lead tin telluride or cadmium mercury telluride, bonded to the common substrate 34, thus enabling the usual cooling services, electrical services, cold shrouding and the like also to be common to both detectors.

In a modification of the above-described arrangement, the geometry of the optical relay system of the laser rangefinder receiver is modified to bring the curved mirrors 23 and 30 substantially into colncidence, thus enabling them to be replaced by a single curved mirror serving to form the appropriate intermediate image both in the laser rangefinder receiver and in the thermal imaging system, It is to be noted that the optical path for the thermal imaging system must transmit wavelengths in the 8 to 12,5 micrometre band, whereas the performance of the laser rangefinder, assumed to empioy a pulsed carbon dioxide laser, will be greatly enhanced if a narrow band filter transmitting substantially only at 10,6 microns wavelength is incorporated in the optical path for the rangefinder receiver. The positioning of the filter is important, as a selective filter will both itself radiate and reflect heat from other parts of the system unless it is correctly designed and positioned.

Preferably, as shown in Figure 2, a narrow band filter 35 for the rangefinder receiver is located directly above the rangefinder detector 32, and is bonded to the substrate 34 so as to be cooled to about the same temperature as the detector 32, If it is not possible to achieve such positioning of the filter 35 without obscuring the thermal imaging beam, an equivalent filter may be located between the mirrors 30 and 31, or between the mirror 31 and a point of the laser optical path adjacent the mirror 24, said filter being designed to give the desired characteristic of reflecting only the low temperature region around the detector, while transmitting the laser radiation returned from the target, The above-described arrangement has various advantages additional to those already mentioned.

Thus, the use of a common front objective 20 for both thermal imaging and rangefinder results in material cost savings, especially since, for thermal radiation in the range of interest, it is necessary for all lenses to be principally made of germanium. Full utilisation of the front element, usually the largest element, of this objective 20 is possible to maximise the performance both of the thermal imaging system and the rangefinder receiver, while minimising the area unprotected by armour which is presented to the target, In general, the use of components common to both devices, and the utilisation of common services, results in material savings in size, eight and cost, In this connection, it is also to be appreciated that, although the laser rangefinder transmitter is not illustrated, it will often be possible to incorporate certain components into the arrangement of Figure 2, enabling this transmitter to share a common path through one or more components of the rangefinder receiver or of the thermal imaging system.

Claims

1. A combined rangefinder, thermal imaging system, including a multifaceted reflector which is rotatable to effect line scanning for thermal imaging, and a laser rangefinding receiver which includes a reflecting means for reflecting a rangefinding signal received from one facet of the multifaceted reflector to cause said signal to be incident on a second facet of the multifaceted reflector and to be reflected therefrom along approximately the same optical path for all angular positions of the rotatable multifaceted reflector.

2. A system according to claim 1, wherein the reflecting means comprises a curved mirror receiving the rangefinding signal at a point of incidence which is movable around the mirror according to a variable angle of reflection at the first facet of the multifaceted reflector, said curved mirror being located to cause the signal to be incident on the second facet at an approximately constant spatial point and at an angle of incidence which is variable substantially to compensate for the variable angle of reflection at the first facet,

3. A system according to claim 1 or claim 2, including a common, front optical subsystem through which both the thermal imaging beam and the rangefinding signal are transmitted before incidence on the multifaceted reflector.

4, A system according to claim 1 or claim 2 or claim 3, including a detector for thermal imaging, a detector for laser rangefinding, an optical relay subsystem for thermal imaging and an optical relay subsystem for laser rangefinding, wherein the two relay subsystems are adapted to form respective intermediate images in sufficiently close proximity for the two detectors to be carried by a common substrate.

5. A system according to claim 4, wherein the two relay subsystems include at least one common component.

6. A system according to claim 5, wherein said at least one common component is a spherical or toric mirror.

7. A system according to any one of claims 1 to 6, including a narrow band transmission filter positioned in the path of the laser rangefinding signal, out of the path of the thermal imaging beam.

8. A system according to claim 7, wherein the said filter is bonded in thermal contact with the rangefinder detector substrate.

9. A system according to any of claims 1 to 8, including a laser rangefinding transmitter utilising one or more optical components which are also used for thermal imaging.

10. A system according to any of claims 1 to 9, including a laser rangefinding transmitter having one or more optical components in common with the rangefinding receiver.

11. A combined rangefinding, thermal imaging system substantially as hereinbefore described with reference to the accompanying drawings.