GB2165957A - Checking aiming apparatus alignment - Google Patents

Abstract

Aiming apparatus 10' for observing the direction of a designating beam from laser 15 by thermal imaging means 12 or optical sighting means 35 (in the form of a tv camera) is checked for alignment of the nominally parallel 'thermal', optical and laser sightlines 14, 37 and 16 when focussed at infinity by means of alignment checking means 40 disposed in front of the aiming apparatus. An enclosure 41 contains a body 26 at the focal point of a concave mirror 25 and a periscope arrangement 29' to divert the collimated laser beam on sightline 16 via mirror 25 to heat body 26. Thermal radiation given off is reflected and collimated by mirror 25 into a beam parallel to the laser sightline 16 and within the field of view of thermal imaging means 12. Visible laser radiation is passed by beam splitting reflector 30' of the periscope and reflected by roof prism 43 towards the tv camera 35. If either received image is offset from the respective sightline 14 or 37 this indicates misalignment between that and the laser sightline. Non-visible laser radiation, e.g. from a CO2 laser at 'thermal' wavelengths, may be caused to generate a discharge at the focus of the mirror including thermal and optical radiation at the body and a further periscope arrangement employed to direct the optical radiation generated to the sighting means (See Fig. 3.) <IMAGE>

Description

SPECIFICATION Checking aiming apparatus alignment This invention relates to aiming apparatus and in particular to means for checking alignment thereof. The sort of aiming apparatus with which the present invention is concerned is arranged to direct a collimated beam of continuous or pulsed laser radiation onto a target in order to designate or mark the target to weapons or projectiles equipped to detect laser radiation scattered therefrom or to compare reflected radiation with the transmitted beam to determine the range of the observed target.

The principle of operation of such aiming apparatus is to define an axis, or sightline, with respect to the apparatus by which an operator can keep a target object in view and to align the laser transmission path with the sightline so that this transmission path, or laser sightline as it is called hereinafter for convenience, is directed towards the target object.

Such aiming apparatus is in practice often employed in conditions of poor visibility, e.g.

smoke or darkness, and thermal imaging means, responsive to radiation in the far infrared part of the spectrum, is employed which converts such 'thermal' radiation into visible light for observation.

'Thermal radiation', as the term is used in this specification, has a wavelength of the order of 8-10 microns and consequently is outside the band which can be transmitted by normal optically transmissive materials. Laser radiation may be produced in a similar part of the spectrum, for example 10.6 microns from a carbon dioxide laser, or, on the other hand, may be in the visible, or in the near infra-red, part of the spectrum and transmissible by conventional optical elements. To distinguish the radiation emitted by laser from similar radiation when received by the aiming means it is also referred to herein as 'laser radiation' and radiation transmissible by conventional optical elements is generally referred to as 'optical radiation'.

Furthermore, in this specification 'thermal aiming apparatus' is defined as comprising thermal imaging means operable to be positioned to receive thermal radiation from a direction defining a thermal sightline and to produce a visible image therefrom, laser designating means operable to produce a collimated beam of laser radiation and elements operable to direct said collimated beam along an axis defining a laser sightline at least parallel to the thermal sightline.

To cater for operation in conditions where visibility is adequate the aiming apparatus may include optical sighting means operable to define an optical sightline parallel to the thermal sightline and laser sightline.

The optical sighting means may be arranged for receipt of visible radiation and direct viewing by the operator, but motions required of the apparatus and measures to prevent eye damage by receipt of laser radiation complicate the formation of a directly observable image. Preferably the optical sighting means includes a television camera on which the optical image is formed, which image is processed and displayed remotely to the operator. Use of a camera receiver permits a wider optical bandwidth including greater sensitivity than the eye to reception of radiation at the red or near infra-red part of the spectrum, shared by some lasers.

Irrespective of whether thermal aiming apparatus, as defined above, has additional optical sighting means, operation relies upon the maintenance of accurate parallel alignment between the laser sightline and thermal and, if appropriate, optical sightlines.

Checking such alignment is difficult in many respects. Firstly, the laser and thermal/optical sightlines are normally arranged to be parallel to each other in order to engage distant target objects, said sightlines ideally never deviating by greater than their separation in the apparatus. Furthermore, the sighting devices operate in different wavelength bands from each other, and, possibly from the laser and cannot share the radiation paths or detection means of the other. Alignment between (thermal and optical) sightlines is checked by providing a common target at a suitably distant range for the laser beam to be detected at and from which thermal and optical radiation is emitted. However this is operationally inconvenient to implement and may be the source of additional errors.

Refocusing involves the movement of lenses and any imperfection in the movement will affect the alignment at other focussed distances. Furthermore because the finite range of the target does not check alignment with the sighting means focussed at infinity it requires compensation for parallax between the sightline axes.

It is an object of the present invention to provide alignment checking means for thermal or thermal/optical aiming apparatus as herein defined which mitigates disadvantages of known alignment checking.

According to the present invention alignment checking means for thermal aiming apparatus (as herein defined) comprises a housing adapted to be operatively located adjacent the apparatus and containing a thermal window therein for the passage of thermal radiation from the housing and a laser window for the passage of laser radiation into the housing, said windows being disposed so as to be operatively substantially in alignment with the thermal and laser sightlines, respectively, of the apparatus, a concave conic section mirror facing the thermal window, a body located substantially at the focal point of the mirror, and a periscope arrangement one reflector of which has an effective area less than that of the mirror and is located between the mirror and the thermal window and the other reflector of which is displaced from said one reflector by a distance equal to the separation of said thermal and laser sightlines of the aiming apparatus and operatively disposed substantially in alignment with the laser window of the housing, whereby laser radiation of the collimated beam from the aiming apparatus incident on said other reflector is directed by way of said one reflector along a parallel transposed laser sightline to the concave mirror and focussed onto the body to cause heating thereof, thermal radiation from the body being reflected by the concave mirror along a path, parallel to the laser beam, to the thermal window.

Where the thermal aiming apparatus also includes optical sighting means defining an optical sightline the alignment checking means may include an optical window in the housing for optical radiation, means within said housing to derive from said collimated beam of laser radiation a collimated beam of optical radiation and optical means, aligned with the periscope arrangement, operable to divert said beam of optical radiation along a path, parallel to the laser sightline, towards said optical window.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a section through thermal aiming apparatus showing some of the principal components thereof and alignment checking means according to the present invention, Figure 2 is a section through thermal aiming apparatus, similar to Figure 1, but including optical sighting means and alignment checking means, also according to the present invention, employed when the laser of the aiming apparatus emits optical radiation, and Figure 3 is a section through alignment means, similar to that shown in Figure 2, but showing an operating scheme suited to when the laser of the aiming apparatus emits pulsed thermal wavelength radiation.

Referring to Figure 1, known thermal aiming apparatus 10 comprises a structure 11 which is directable as to its orientation by an operator (not shown) and may be remotely steered in direction or maintained in a particular direction by gyro-stabilising means (not shown) as is well known in the art.

The aiming apparatus contains thermal imaging means 12 which is located to receive thermal radiation, that is, in the far infra-red region of the spectrum at 8-10 microns wavelength, by way of suitable radiation collection and processing elements, depicted by lens system 13, formed of material, such as germanium, which is transparent to radiation at these thermal wavelengths but opaque to radiation at visible and near infra-red wavelengths.

The thermal imaging lens system 13 defines for the thermal imaging means a field of view centred on an axis 14 which may be considered as a thermal sightline and the imaging means produces at a remote viewing station (not shown) a visible representation of the field of view.

The aiming apparatus also contains a designating laser 15 producing a pulsed beam of visible radiation. The laser may typically be of the ruby, Nd:YAG or Nd:glass type. The laser beam is collimated and transmitted along an axis 16 by conventional optical elements 17.

For convenience of description this axis 16 may be considered as the laser beam sightline.

The thermal imaging means and laser and/or their associated sightline defining elements are arranged to ensure that the sightlines 14 and 16 are parallel.

The aiming apparatus thus far described is conventional, although precise constructional details vary according to manufacturers' preferences.

The alignment checking means of the present invention shown at 20 comprises a housing which takes the form of an enclosure 21 adapted to be releasably attached to the aiming apparatus structure, for example, by clips 22. The enclosure has windows 23, 24 therein disposed so as to be substantially in alignment with the thermal and laser sightlines 14, 16 respectively when the enclosure is operatively attached to the apparatus 10. These windows may be considered as the thermal and laser windows respectively.

The enclosure 21 also includes a concave mirror 25 facing the thermal window 23 and a body 26 located at the focal point of the mirror. The mirror should have good focussing capability over a relatively large aperture and is preferably of parabolic section, although other conic sections, such as spherical, may be employed.

The body comprises a thermally conductive substrate 27 on the surface of which facing the mirror is carried a layer of thermally insulating material 28, such as a ceramic having suitable absorption/emission coefficients. If an insulating material is available having suitable thermal conduction and mechanical properties the conductive substrate may not be required.

The enclosure 21 also contains a periscope arrangement, shown generally at 29, which comprises effectively a pair of plane reflectors 30, 31. One reflector 30 is located between the body 26 and the thermal window 23 and the other reflector 31 is displaced from it by a distance equal to the separation of the thermal and laser sightlines 14 and 16 and is substantially aligned with the laser window 24.

The reflector dimensions and dispositions are such that when the enclosure 21 is attached to the housing 10 the laser sightline 16 extends through reflector 31 whose reflective surface is arranged to be disposed at approximately 45" to this line. It has an effective reflective area greater than the beam such that substantially the whole beam is intercepted by the reflector without accurate alignment between the enclosure and housing.

Laser radiation emitted in a beam centred on the laser sightline 16 is thus directed by the reflectors 31 and 30 into a beam centred on a parallel transposed sightline 32, the collimated radiation of which beam is incident upon the mirror 25 and focussed onto the thermally insulating surface of body 26. The body is heated by the focussed laser radiation, the conduction of excess heat from the focal point by the substrate leaving a well defined 'hot' spot at the insulating layer.

The body surface emits radiation at a longer wavelength, including the 8-10 micron region, principally from the heated spot at which the laser radiation is focussed. Because of the juxtaposition of the insulating layer and the mirror this thermal radiation is emitted principally towards the mirror 25 which reflects and collimates the thermal radiation into a parallel sided beam, indicated by broken lines 33, which is directed towards the thermal window 23 on an axis which is at least parallel to the incident axis 32 of the transposed laser beam and thus parallel to the laser sightline 16.

With the thermal imaging means 12 focussed to infinity, that is, to accept a collimated thermal beam, the beam 33 is thus within the field of view of the aiming apparatus which produces on the display means an image the centre of which should coincide with the centre of the displayed field of view, that is, thermal sightline 14.

Any deviation from coincidence is an indication of a lack of parallelism between the laser and thermal sightlines for a target at infinity.

It will be appreciated that it is not necessary to mount the enclosure 21 of the alignment checking means on the structure 11 nor even to mount them in close physical proximity, but irrespective of the precise relationship it is nevertheless a feature of the invention that alignment is checked with the aiming means ostensibly directed at an infinite object by means of local checking means. Furthermore, there is no need for precise relative juxtaposition of them, providing the laser sightline 16 is directed into the laser window 24 and the thermal sightline 14 is directed into the thermal window 23.

It will be appreciated that in general the laser beam will be substantially circular in section having a diameter typically only 30% of the aperture of the thermal imaging 'optics' 13 so that the reflector 30 may be sufficiently large to reflect substantially the whole beam without significantly obscuring the thermal radiation between the mirror 25 and the thermal imaging means.

For convenience the reflector 30 is disposed such that the transposed laser sightline 32 passes through the body 26. The body 26 then has to be smaller than the laser beam section such that the portion thereof not obscured is sufficient when focussed by mirror 25 onto the layer 28 to effect the necessary heating. While this configuration results in some obscuration of the collimated laser beam by the body, the body causes no additional obscuration of the thermal radiation from mirror 25 to that caused by the reflector 30.

It will be appreciated that alternatively the reflector 30 may be disposed such that the transposed laser sightline is not partially obscured by the body 26 but in such an arrangement the thermal radiation beam between the mirror 25 and thermal window 23 will be partially obscured by both the reflector 30 and the body 26.

As mentioned above thermal aiming apparatus may also include optical sighting means defining an optical sightline aligned with the laser sightline similarly to the thermal sightline.

Referring to Figure 2 this shows aiming apparatus 10' including thermal imaging means 12 and laser 15 as described above contained in structure 34. The structure also contains optical sighting means 35, comprising a television camera, and focussing optical elements 36 defining a field of view centred on an optical sightline 37. The optical elements 26 are opaque to the thermal radiation but transparent to visible and near-infra red radiation, such as provided by the laser 15.

Alignment checking means 40 contains many elements corresponding to those of the means 20 shown in Figure l and like reference numbers are employed.

The checking means comprises a housing in the form of enclosure 41 having thermal and laser windows 23 and 24 respectively, concave mirror 25 and body 26 onto which laser radiation is focussed by way of a periscope arrangement 29'.

The periscope arrangement 29' is similar to that 29 shown in Figure 1 but the one reflector 30 is replaced by a beam splitting reflector 30' which is arranged to reflect some of the collimated beam of laser radiation diverted by reflector 31 onto the transposed laser sightline 32 as described above and to transmit some of the collimated beam on path 42 to a deflecting element 43, such as a roof prism.

The deflecting element 43 is aligned with respect to the other reflector 31 of the periscope arrangement such that they form a corner reflector and the still-collimated beam of laser radiation is deflected along a path 44 parallel to the laser sightline 16 but in the opposite direction. The separation of the roof prism 43 and periscope reflector 31 is accurately set to give a separation of retroreflected beam axes equal to the nominal separation of laser and optical sightines 16 and 37 respectively of the aiming apparatus. An optical window 45 is substantially aligned with the path 44, and when the enclosure 21 is attached to the aiming apparatus, is substantially aligned with the optical sightline 37.

In operation, it will be appreciated that with the thermal and optical sighting means focussed at infinity, an 'image' of thermal radiation is received from thermal sightline 14 and presented for visual display by thermal imaging means 12 and a spot of optical radiation is received from optical sightline 37 and presented for visual display by camera 35.

If there is any misalignment between the optical sightline and laser sightline the spot of laser radiation received will be displaced from the part of the display corresponding to the sightline 37.

By displaying the received optical and thermal images in relation to each other, for example, superimposed on a C.R.T. display, the degree of alignment between the optical and thermal sightlines can also be checked.

The use of a television camera as the receiving element in the optical sighting means, rather than direct vision, permits the use of emitted laser radiation of relatively high intensity for alignment checking.

In some cases it may be desired to employ direct viewing in the optical sighting means, stringent safety measures being taken to ensure that such laser radiation is not directly viewed, or the laser radiation may be emitted at a wavelength which is not readily detected by the television camera or transmitted by its optical elements, for example, the 10.6 micron pulsed carbon dioxide laser.

An alternative arrangement of alignment checking means in which the laser radiation is not directed back onto the optical sightline is illustrated in Figure 3.

Again, elements corresponding to those shown and described with reference to Figures 1 and 2 are given iike reference numbers.

The aiming apparatus 10" is as shown in Figure 2 but the laser 15' is a carbon dioxide laser emitting laser radiation at a 'thermal' wavelength of 10.6 microns. The alignment checking means 50 comprises a housing in the form of an enclosure 51 which is releasably attached to the aiming apparatus by clips.

The enclosure 51 has thermal, laser and optical windows 23, 24 and 45 respectively substantially aligned with the thermal, laser and optical sightlines 14, 16 and 37 respectively.

The enclosure also contains a concave mirror 25 and body 26 as shown in Figures 1 and 2 and a periscope arrangement 29 as shown in Figure 1.

In addition the enclosure 51 contains a further periscope arrangement 52, one further reflector 53 of which is located on the transposed laser sightline 32, that is, between body 26 and said one reflector 30 of the periscope arrangement 29. This further reflector 53 is disposed in the path of the radiation collimated and reflected by the mirror 25 towards the thermal window 23 and is dichroic in order to pass thermal radiation of the laser wavelength but to reflect and divert radiation at visible wavelengths. The reflector may be formed of a slab of thermally transparent material, such as germanium, polished to act as a reflector for the optical radiation.

The other further reflector 54 is disposed so as to re-divert visible radiation deflected by further reflector 53 along an optical path 55 parallel to the transposed laser sightline 32 and thus the laser sightline 16.

The target 26 is arranged adjacent the focal point of the mirror such that in response to the focussed pulsed carbon dioxide laser radiation it is not only heated but also the focussed electromagnetic energy of the beam exceeds the dielectric strength of the air and causes a breakdown resulting in both the generation of thermal radiation and flashes of incoherent optical (including visible) radiation which optical radiation is presented to the television camera 35 by way of mirror 25 and further periscope arrangement 52.

As the optical radiation originates at the focal point of the mirror 25 the alignment of elements within the enclosure 51 is such that the observable spot pulses should be coincident with the optical sightline 37 and any deviation represents a misalignment of the optical sightline with respect to the laser sightline 16.

It will be appreciated that although the longer wavelength carbon dioxide laser radiation is detectable within the operating bandwidth of the thermal imaging means such imaging means produces a two dimensional image by mechanical scanning and the short pulse duration of the laser emission makes it difficult to synchronise their operation to ensure direct detection of the laser radiation by the thermal imaging means in a manner similar to that shown in Figure 1 for optical radiation. However, the continuous emission of thermal radiation from the body 26, heated by successive pulses of laser radiation, provides a suitable source for the thermal imaging means.

It will be appreciated that the embodiments of alignment checking means described above only exemplify the configurations of path defining elements and paths which may be varied partly as a matter of choice and partly as constrained by the layout of the elements of the aiming apparatus.

For instance, in the alignment checking means shown in Figure 3 the further periscope arrangement 52 is shown with the one further reflector 53 in the path 52 of thermal wavelength laser radiation passing from reflector 30 to mirror 25 and in the path of thermal and optical radiation passing from the mirror 25.

The reflector 53 of the further periscope arrangement may be located to one side of the path 32, as shown ghosted at 53', so as not to interfere with the passage of transposed laser radiation. The further reflector 53' need not be thermally transmissive provided it obscures a sufficiently small portion of the mirror 25 from thermal window 23. The further reflector of the further periscope arrangement may be disposed at other positions between the mirror and the thermal window.

It will be appreciated that the form of alignment checking means shown in Figure 3 is equally suited to aiming apparatus in which the optical sighting is performed by direct viewing along the optical axis irrespective of the laser type or the wavelength of laser radiation employed.

Constructional variations of choice are equally applicable to the alignment checking means described above with reference to Figure 1 and 2. For example, in Figure 2 the one reflector 30' of periscope arrangement 29' may be arranged to achieve beam splitting in a number of ways, such as half-silvering, a multilayer partially transmitting coating, multilayer polarisation sensitive coating or a nonreflective central portion corresponding to the part of the beam that, if reflected, would be blocked by the body 26.

Other variations common to all embodiments are the use of non-planar reflectors in the periscope arrangement, the use of a concave mirror conforming to a different conic section and addition optical and 'thermal' radiation beam forming and directing elements.

It will be appreciated that the precise layout of elements within the alignment checking means is a function of the disposition of the various sightlines in the aiming apparatus.

A different layout may require additional reflecting elements for some or all of the radiations but as a guiding principle the fewer additional elements that are introduced the greater the inherent alignment accuracy is maintained.

In all of the above described embodiments, the housing of the alignment checking means had been described as an enclosure having laser, thermal, and where appropriate, optical windows 23, 24 and 45 respectively. It will be appreciated that such windows may comprise simple apertures on the wall of the enclosure but if it is desired to seal the enclosure to prevent the ingress of dirt or other harmful materials then the windows may be formed of suitable solid materials transparent to the appropriate wavelength radiation.

If the housing is not to comprise a sealed enclosure then the housing may take any form, even an open framework, provided the positional relationships are maintained between the reflective elements and the radiation paths offered between them and the windows.

Alignment checking means of the present invention will be seen to be operationally convenient in permitting the sightline axes to be checked with the sighting means at infinity focus without any compensation for parallax errors, does not require accurate alignment in relation to the aiming apparatus and is totally passive, requiring no external power other than that derived by way of the laser aiming beam.

Claims (9)

1. Alignment checking means for a thermal aiming apparatus (as herein defined) comprising a housing adapted to be operatively located adjacent the apparatus and containing a thermal window therein for the passage of thermal radiation from the housing and a laser window for the passage of laser radiation into the housing, said windows being disposed so as to be operatively substantially in alignment with the thermal and laser sightlines, respectively, of the apparatus, a concave conic section mirror facing the thermal window, a body located substantially at the focal point of the mirror, and a periscope arrangement one reflector of which has an effective area less than that of the mirror and is located between the mirror and the thermal window and the other reflector of which is displaced from said one reflector by a distance equal to the separation of said thermal and laser sightlines of the aiming apparatus and operatively disposed substantially in alignment with the laser window of the housing, whereby laser radiation of the collimated beam from the aiming apparatus incident on said other reflector is directed by way of said one reflector along a parallel transposed laser sightline to the concave mirror and focussed onto the body to cause heating thereof, thermal radiation from the body being reflected by the concave mirror along a path, parallel to the laser beam, to the thermal window.

2. Alignment checking means as claimed in claim 1 in which the transposed laser sightline passes through the focal point of the concave mirror, said body being arranged to have a cross sectional area less than that of the transposed collimated laser beam.

3. Alignment checking means as claimed in claim 1 or claim 2 in which the body comprises a thermally conductive substrate on the surface of which facing the mirror is carried a layer of thermally insulating material.

4. Alignment checking means as claimed in any one of the preceding claims adapted also for checking alignment between the laser sightline and an optical sightline of optical sighting means of the aiming apparatus, said means including an optical window in the housing for the transmission of optical radiation, means within said housing to derive from said collimated beam of laser radiation a collimated beam of optical radiation and optical means, aligned with the periscope arrangement, operable to divert said beam of optical radiation along a path, parallel to the laser sightline, towards said optical window.

5. Alignment checking means, as claimed in claim 4, for aiming apparatus in which the laser radiation is emitted at a wavelength detectable within the optical sighting means, said means for defining a collimated beam of optical radiation comprising a beam-splitting arrangement at said one reflector of the periscope arrangement operable to pass some of the collimated beam of laser radiation diverted from the laser sightline by said other reflector and a deflecting element aligned with said other reflector of the periscope arrangement to form a corner reflector whereby said portion of the radiation is directed to the optical aperture along an opposite but parallel path to the laser sightline.

6. Alignment checking means as claimed in claim 5 in which the deflecting element is a roof prism.

7. Alignment checking means as claimed in claim 4 in which said means for deriving a collimated beam of optical radiation comprises means to generate from the laser radiation an electrical breakdown discharge in the atmosphere at the focal point of the concave mirror producing optical radiation at a wavelength detectable within the optical sighting means and a further periscope arrangement one further reflector thereof being located between the focal point of the mirror and the thermal window and operable to deflect generated optical radiation, reflected by the mirror along the same path as the thermal radiation, towards another further reflector disposed with respect to said one further reflector to direct said optical radiation to the optical window along a path parallel to the thermal radiation.

8. Alignment checking means as claimed in claim 7 in which the one further reflector of the further periscope arrangement comprises a dichroic reflector located between the mirror and the said one reflector of the periscope arrangement in the transposed laser sightline and operable to transmit said laser radiation but to reflect the generated optical radiation received from the mirror.

9. Alignment checking means for thermal aiming apparatus, substantially as herein described with reference to, and as shown in, any one of Figures 1 to 3 of the accompanying drawings.