WO1987006718A1

WO1987006718A1 - Laser ignition device

Info

Publication number: WO1987006718A1
Application number: PCT/AU1987/000103
Authority: WO
Inventors: Michael D. Waterworth
Original assignee: The University Of Tasmania
Priority date: 1986-04-21
Filing date: 1987-04-15
Publication date: 1987-11-05

Abstract

Apparatus comprising a laser beam generating device for generating a laser beam, and beam focusing means (14) operable to focus the generated laser beam at a distance remote from the apparatus so as to provide intense heat in a selected small area at that distance, the beam focusing means (14) comprising a focusing system (15) comprising focusing optical elements (21, 22), e.g. lenses/mirrors located in the path of the beam and arranged to diverge the beam, and a reflecting system comprising reflective elements (19, 18) for reflecting the divergent beam from the focusing system (15) and directing it along the focusing path so as to converge at the selected area.

Description

"LASER IGNITION DEVICE"

This invention relates broadly to the generation of localised heat, and more particularly to apparatus for providing intense heat at a selected small area remote from the apparatus. The apparatus is applicable for safe and controlled point ignition of combustible materials, such as logging slash or other ground vegetation, at a distance from the apparatus, and it will be convenient to hereinafter disclose the apparatus in relation to that exemplary application. However, i.t is to be appreciated that the apparatus may be equally suited to other applications including de-icing or thawing of snow on ground, structures, and travel craft.

For successful controlled regeneration burning of vegetation in a ground area, such as with logging slash burning, it is necessary that burning be rapid and intense to ensure complete combustion so that regeneration growth can take place. Such burning can be achieved, for example, by igniting the area in an accurate predetermined pattern. Traditionally, that ignition has been conducted in situ with personnel located on the ground within the area. Whilst successful burning may be achieved, ignition can be time and labour consuming and consequently expensive. In addition, it can sometimes be difficult to coordinate ignition and maintain contact between numerous personnel required. That can lead to dangerous, and life threatening, situations should burning become at all uncoordinated.

In an effort to alleviate these disadvantages, aircraft, particularly helicopters, have been used to drop ignition devices into the area. However, that may still be expensive and dangerous, and may not be entirely successful should the ignition devices land out of position or fail to cause ignition. It is an object of the present invention to provide a relatively simple and safe apparatus for providing intense heat at a remote area which, in the above exemplary application, may alleviate the foregoing difficulties.

It is a further object of the present invention to provide a focusing mechanism for focusing a beam of electromagnetic radiation, e.g. a laser beam, at a predetermined location remote from the mechanism and which overcomes the problem of "thermal-blooming" which normally occurs when a small diameter, high intensity laser beam is transmitted through the atmosphere.

In one of its forms therefore, the apparatus of the present invention broadly includes a laser beam generating device for generating a laser beam, and beam focusing means operable to focus the generated laser beam at a distance remote from the apparatus .so as to provide intense heat in a selected small area at that distance.

More specifically, the apparatus of this invention comprises a generating source for generating a near- collimated beam of electro-magnetic radiation, for example a laser beam, and beam focusing means arranged to focus the beam at a distance remote from the apparatus so as to provide a spot of intense heat in a selected small area at that distance, said beam focusing means comprising a focusing system comprising two or more focusing elements located in the path of the beam and arranged to diverge or expand the beam, at least one of said elements being movable along the axis of the beam relative to another said element whereby the axial distance between the elements can be adjusted and in turn the extent of beam divergence (and thereby the focal distance), and a reflecting system comprising spaced apart primary and secondary reflective elements, said secondary reflective element having a reflective surface arranged to reflect the incident divergent beam from the lens system and deliver it co-axially to the surface of said primary reflective surface of said primary reflective element which is arranged to forward reflect the beam in a convergent manner along the focusing path, in the direction of the selected small area whereat the intense heat spot is required.

The laser beam generating device and beam focusing means are preferably selected and coupled together such that, in apparatus operation, the intense heat can be focused at large distances from the apparatus. Such

I I' distances preferably include up to about 1500 metres. The area at focus is preferably sufficiently small to be considered a point in relation to that distance. In that regard, this small area preferably has dimensions transverse of the beam of up to about 10 centimetres. The laser beam generating device may be of any suitable structure that will provide sufficient beam power density at the remote distance, having regard to the required heat intensity. Carbon dioxide lasers operating at a wavelength of 10.6 micrometres in single non-guassian mode are preferred because of the absorption properties of naturally occurring materials at that wavelength.

Preferably, the laser beam generating device includes a^' beam discharge head (with collimator) from which the near collimated laser beam is discharged, during apparatus operation, to the beam focusing means. The generation device preferably also includes a power supply unit connected to the discharge head, at least during apparatus operation, for generation of the laser beam.

The apparatus preferably also includes a support frame on which are mounted the laser beam discharge head and beam focusing mechanism, the mounting being adapted to provide stable coupling of the discharge head and focusing mechanism relative to one another. In addition. it is desirable that the mounting provide for at least the beam focusing mechanism to be maintained in a stable - condition for accurate alignment of the beam, emitted from the apparatus, with the selected area during apparatus operation.

In a preferred form of the apparatus of the present inention, the laser beam discharge head is mounted on the support frame relative to the beam focusing mechanism. That relationship may be fixed or movable, such that the laser beam discharged^', from: the head passes to the focusing mechanism along either a straight or , bent delivery path. A bent path may permit more convenient relative mounting between the discharge head and focusing mechanism. Where the path is bent then the laser beam generating device further includes one or more beam reflecting elements arranged in fixed relation to the discharge head along the path so as to reflect the beam along that path into the main optical system. The reflecting element(s) may include one or more mirrors mounted directly or indirectly on the support frame.

In this preferred form, the power supply unit may be mounted on the support frame or discharge head or remote therefrom. The power supply unit may be fully self-contained, this being particularly convenient where the apparatus is operated in isolated environments, as could be expected in the exemplary application.

In another preferred form of the apparatus of the present invention, the focusing system of beam focusing mechanism comprises a pair of focusing lens elements located in the path of the beam and positioned to receive the laser beam as it is discharged from the laser beam discharge head, one of the lens elements being fixed, the other adjustable so as to permit the axial separation of the lens elements to be varied, and in turn the rate of divergence of the emergent beam from the lens system. In this form, it is the lens system that acts to vary the distance from the apparatus at which the beam will focus.

In another preferred form, the lens system comprises a pair of spaced apart lens subsystems each of which includes a plurality of lens elements, the sub-systems being elements movable within the focusing path relative to one another in order to vary the focus distance. In that regard, the individual lens elements of one sub- system may be movable along the path toward and away from the other sub-system. That movement may be achieved by movably mounting those lens elements, such as in a rack and pinion focusing assembly, included within the lens system. Alternatively, one or more lens elements of a respective sub-system may be replaceable, so that a plurality of lens elements may be selectively positioned within and withdrawn from the focusing path within the lens system. That positioning may be achieved by mounting the lens element(s) within a turret assembly included within the lens system.

The individual elements may be of any suitable shape and composed of any suitable material. There may be a first concavo-concave shaped lens element followed along the focusing path by a second concavo-concave shaped lens element. Moreover, the elements may be zinc selenide lens elements with multi-layer anti-reflection coatings on each surface to permit 99.6% transmission at 10.6 urn wavelength.

In yet another preferred embodiment, the focusing system contains reflecting focusing optical elements

(in lieu of lens elements); and may for example comprise a forward reflecting beam projection mirror and a reverse reflecting beam mirror spaced forwardly thereof and co- axially aligned therewith. Preferably, at least one pair of reflecting elements are provided within the reflecting system, which are fixed in relation to one another and also in relation to any fixed lens elements in this form. The reflecting elements may be mirror elements of any suitable shape and composed of any suitable material. In that regard, a convex shaped secondary mirror element reverse reflecting the laser beam received from the lens system, followed along the focusing path by a concave shaped primary mirror element forward reflecting the laser beam and converging it to the point of focus may be provided. Such a reflecting system may be in the nature of a Cassegrain telescope (in a reversed mode). The primary mirror may be a double-sided coated metal mirror so as to provide two working surfaces. In another preferred form, the beam focusing mechanism may also include a cooling system arranged to ^* prevent overheating of any focusing elements which otherwise may overheat. In particular, that cooling system may function to prevent overheating of the lens element(s). That cooling system may cool the element(s) by circulating cooled gas, such as air, past those element(s). The system may be closed so that the gas is recirculated, flow past the element(s) heating the gas whereupon it is subsequently cooled for flow past the element(s) again.

In yet another preferred form of the apparatus of the present invention, the support frame provides for stable mounting of the apparatus on a foundation. The frame may provide a mounting in which the apparatus is generally fixed as a whole in relation to that foundation. Such a mounting may be suitable where the foundation is stationary at least during apparatus operation, so that the apparatus as a whole is stationary and thus stable. That foundation may include ground or a platform fixed thereto, or travel craft such as vehicles stationary during apparatus operation. Alternatively, the frame may provide a mounting in whichl the apparatus as a whole and the foundation can move ₍

or about a plurality of relatively perpendicular aligning

In this preferred form, the support frame may include a frame assembly comprising a base part by which the apparatus is stably mounted on the foundation, and a support part mounted on the base part and on which at least the beam focusing mechanism is directly or indirectly mounted. The support part may be mounted on the base part for rotary movement about one aligning axis whilst the beam focusing mechanism may be directly or indirectly mounted on the support part for rotary movement about another aligning axis. The one axis may extend vertical and the other axis horizontal.

To assist in accurately aligning the laser beam with the selected area, the apparatus may also include beam alignment means with which the beam can be visually aligned during apparatus operation. That alignment means may be mounted on the beam focusing mechanism. Any suitable alignment means may be used. In that regard, the alignment means may include a sighting telescope or an alignment system giving visual access via the beam focusing mechanism, a suitable beam splitter, and visible wavelength optics incorporating aberration corrections. A laser range finder can also be used in conjunction with automatic focusing to control the steering of the beam., i ' ,' . ^'•

In usinjg,' a(,preferred form of the apparatus, as outlined above,¹ in the exemplary application, the apparatus may be conveniently mounted on a truck tray or in a cargo bay of a helicopter for ready and rapid transportation to an area toi'be burnt. At that area, the apparatus is operated to generate laser beams providing intense heat, at selected point areas within the area causing ignition at those point areas. The apparatus may be shifted about that area to be burnt during or between operations as necessary.

Acording tα another aspect of this invention, there is provided a laser optics focusing mechanism for focusing a beam of electro-magnetic radiation at a distance in the range of 30-1500 metres remote from the mechanism so as to provide a spot of intense heat in a selected small area at that distance, comprising a lens system for receiving a near-collimated e.m. beam from a laser generating source and including two or more axially aligned focusing lens elements located in the focusing path of the beam and arranged to diverge or expand the beam, at least one of said lens elements being movable along the axis of the beam relative to another said lens element whereby the axial distance between the lens elements can be adjusted and in turn the extent of beam divergence (and thereby the focal distance), and a reflecting system comprising spaced apart primary and secondary reflective elements, said secondary reflective element having a reflective surface arranged to reflect the incident divergent beam from the lens system and deliver it co-axially to the surface of said reflective surface of said primary reflective element which is arranged to forward reflect the beam in a convergent manner along the focusing path, in the direction of the selected small area whereat the intense heat spot is required. For assistance in arriving at an understanding of the present invention, examples of the apparatus incorporating the present invention are illustrated in the attached drawings. However, as the drawings illustrate examples only, their particularity is not to be understood as superceding the generality of the preceding description. In the drawings:

Fig. 1 is a side elevational view of one example of the apparatus incorporating the present invention and mounted on a vehicle;

Fig. 2 is a part section fragmentary side elevational view of the apparatus of Fig. 1 showing the laser generating device and its mounting.

Fig. 3 is a schematic view of the apparatus incorporating the invention showing the path of the laser beam passing through the apparatus;

Fig. 4 is a schematic view similar to Fig. 3 but showing yet another example of the apparatus incorporating the present invention; Fig. 5 is a partial side elevational view of the apparatus of Fig. 1 showing the support frame assembly and its base mounting to the vehicle tray;

Fig. 6 is a fragmentary end view of the assembly showin in Fig. 5; Fig. 7 is a part sectional side elevational view of the apparatus showin in Fig. 1 wherein the lens system is located between the primary and secondary mirrors of the reflecting system (compare Figs. 3 and 4), and Fig. 8 is a part sectional view of the apparatus shown in Fig. 7.

Referring to the drawings, specifically Figs. 1, 2, 5, 6, 7 and 8, the laser ignition apparatus 10 is shown mounted on the tray 11 of a truck vehicle 12. The apparatus comprises a laser box 13 in which is housed a laser generating device 13', and a beam focusing unit 14 which includes a lens system 15 and a reflecting system which itself comprises a primary beam projection mirror 18 and a secondary mirror 19, the lens system 15, and the primary and secondary mirrors 18, 19 all being co-axially aligned. The lens system 15, as shown in Fig. 7, is housed in a focusing canister20 and contains two lens elements 21, the large lens element 21 being fixed whilst the smaller lens element 22 is movably mounted to allow the axial separation between the lens elements to be adjusted. In this embodiment the axial movement of the smaller lens element 22 is effected by a stepping motor (not shown) under computer control to achieve the desired distance to the beam focal point in front of the primary projecting mirror 18. This can be from 30 metres up to 1500 metres, the latter being a diffraction limit set by the primary mirror diameter, that is, the larger the primary mirror the greater maximum useful focal distance. The stepping motor drives a precise lead screw to which is attached the mounting (not shown) containing lens element 22. This mounting attaches via a teflon impregnated nut held down by a leaf spring. In this embodiment, the lens elements 21, 22 are made of zinc selenide with multi-layer anti-reflection coatings on each surface to permit 99.6% transmission at 10.6 um wavelength. Of course the lens element within the lens system 15 can be widely varied with a view to improving or altering the aberration correction states. The powers or focal lengths of the lens elements (or groups thereof), whether it be a singlet, doublet or triplet lens sub-system, will remain unchanged. The shapes of individual lenses may change but the focusing effects will not - it is only the aberration correction states which will change.

The reflecting system in this embodiment is provided in the form of a reverse Cassegrain telescope, this

the backing plate 26 which forms the inner end of a cylindrical container 27 which shrouds the telescope system and the focusing unit. A cover 29 closes off the other end of the container 27 and provides protection for the optical system during transport of the apparatus.

The primary mirror 18 is mounted on the backing plate 26 by means of four "push pull" screws 31 designed to allow adjustment of mirror tilt about two perpendicular axes. All mirrors within the system are similarly mounted to assist optical alignment.

As will be evident, the secondary mirror 19 acts to reverse reflect the incident divergent laser beam and delivers it co-axially to the primary mirror 18 which in turn forward reflects the beam and converges it in the direction of the selected small area whereat the intense heat spot is required.

Although the secondary mirror 19 is shown as having a convex reflective surface 25, with suitable adjustment to the optical elements in the canister 20, the secondary mirror could be flat; however, a flat mirror would be of a greater diameter and hence be further from the primary mirror 18. This has the disadvantage in that a greater secondary mirror diameter increases the obstruction ratio and in turn decreases the optical efficiency of the system.

In this embodiment, the primary mirror 18 is a double sided coated metal mirror which provides two working surfaces accessible by switching the mirror cell through 180°. Each mirror half is machined from solid aluminium, and the two halves are epoxy resined together. The concave mirror surfaces themselves are smoothed to the precise radius of curvature and plated with electroless nickel. Final polishing of the nickel produces the required mirror profile.

The secondary mirror 19 has a glass substrate and a coating of aluminium or gold. Preferably a small spike is located at the centre of the mirror to prevent the direct reflection of laser radiation back into the focus control canister 20.

To enable the beam focusing unit 14 to be quickly pointed in the desired direction, such unit is carried on a elevation axis shaft 30 which is journalled in angular contact bearings 33 and carried in a casing or box 34 (refer Fig. 8). The backing plate 26 is attached to adaptor box 35 (refer Fig. 7) which is in turn attached to the elevation axis shaft 30. The case or box 34 rotates as a whole on the azimuth axis shaft 37. An azimuth tube 38 carries the azimuth bearings 40 and via the combination of horizontal support plates 41, 42, the tube 38 is mounted on the laser box 13. With this arrangement beam pointing is achieved by a combination of azimuth and elevation motions on the angular contact bearings 33, 40. Both motions can be achieved by means of geared motors and toothed belts (not shown). Elevation torque requirements are minimised by near static balancing achieved by means of a counter weight 44.

The precise distance focused from the lens system is determined using a laser range finder 64 the output of which can be either read manually or sent to a microprocessor. In the former case the focusing lens systems may be adjusted manually whilst in the latter case focusing can be controlled by the microprocessor, and is thus controlled automatically. Visual acquisition is obtained by means of a sighting telescope attached to the laser range finder, the output of which may be coupled to a television display. The accuracy of focusing of the emitted radiation is determined by the laser range finder. Outside a small range either side of focus, the emitted radiation has a smaller effect due to the angles of convergence.

As shown in Figs. 1 and 5, the laser box 13 is mounted on a base frame in the form of a transport skid 44 via vibration mounts and location control arms 45. A cooling water chiller 48 is similarly mounted on vibration mounts 49 to minimise the vibration level during transport. Items such as the gas supply bottles 31 are carried in a cradle (no shown) fixedly mounted on the base frame 44.

Referring to Fig. 2, the laser generating device 13' has its laser head assembly rail 13" supported along its length by means of a plurality of clamped constant force mounting units 43. During site operation, the alternator 50 is operated using the truck engine. Hydraulic fluid is pumped from cubical 51 to activate the elevating legs 52 which lift the laser unit from its base frame 44 and its vibration sources. Undesirable twisting forces tending to distort the laser box 13 and causing optical misalignment are alleviated by pivoting the forward leg mounting cross arm 53 on pivot bolt 53'. This provides a synthesised three point support for the laser box 13 minimising any torsional forces acting upon it.

Referring now to the examples of the invention illustrated schematically in Figs. 3 and 4, Fig. 3 shows the arrangement where the laser beam 57 follows a bent path from the laser beam discharge head 54 and the lens system 55, whilst Fig. 4 illustrates the arrangement where the laser beam 57' follows a linear path between the discharge head 54' and the lens system 55*. With reference to both examples, the near collimated laser beam (for example, with beam divergence of 7.5 x 10^"^ radians) discharged by the laser beam discharge head 54, 54' is diverged by the optical lens system 55, 55' to fill the aperture of the primary mirror element 56, 56' which together with the secondary mirror 58, 58' combine to form a reverse Cassegrain telescope. The output laser beam from the primary mirror 56, 56' converges to the preselected point of focus. With reference to Figs. 2 and 8, the laser beam is arranged to enter the side below the base 41 and is turned to the vertical by mirror 59 located in tube 62 which is co-axial with tube 38. The laser beam travels vertically co-axially through the axis shaft 37 and is again turned through 90° by means of mirror cell 60 to travel horizontally co-axially along the elevation axis shaft 30. A further 90° turn is made into the primary mirror axis by the mirror 63 (refer Fig. 7). The beam is now co-axial with the primary mirror axis and enters the focusing lens system 15 of the focus unit 14.

In operating the apparatus, the telescope system is activated by the axis motor drive unit until the desired object is identified. The laser range finder 64 continually outputs the distance to the object as the telescope moves, and feeds this information to the microprocessor which in turn controls the drive of the movable lens 22 of the lens system 15. In this mode, scanning of the areas to be irradiated or detected can be achieved. Of course, with the aid of the sight , ι ι telescope 65¹, jthe beam can be readily aligned with a

I s ^■elected are*a. The laser beam can be discharged for as ,

J lnb as is necessairy ' M¹ to! ptovide intense heat at that area causing ignition of ground material to commence area burning.

I

The, apparatus according to the present invention provides safe _|'and effective heat generation remote from ' I' l

I l the apparatus. As such, in the exemplary application the apparatus cam provide safei and effective ignition of areas to be burnt.

¹ The apparatus of the present invention requires I I a minimum number of personnel to provide remote heat i ¹ gejneration_t Moreover, that heat generation can be provided quickly to a large number of objectives, particularly where the apparatus is mobile during or between operations. As such, heat generation may be _' less expensive compared to previous heat generation arrangements.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. Apparatus for generating heat at a distance remote from the apparatus comprising a generating source for generating a near-collimated beam of electro-magnetic radiation, for example a laser beam,, and beam focusing means arranged to focus the beam at a distance remote from the apparatus so as to provide a spot of intense heat in a selected small area at that distance, said beam focusing means comprising (i)¹ a focusing system comprising two or more focusing elements located in the path of the beam and arranged to diverge or expand the beam, at least one of said elements being movable along the axis of the beam relative to another said element whereby the axial distance between the elements can be adjusted and in turn the extent of beam divergence (and thereby the focal distance), and (ii) a reflecting system comprising spaced apart primary and secondary reflective elements, said secondary reflective element having a reflective surface arranged to reflect the divergent beam from the lens system and deliver it co-axially to the surface of said primary reflective surface of said primary reflective element which is arranged to forward reflect the beam in a convergent manner along the focusing path, in the direction of the selected small area whereat the intense heat spot is required.

2. Apparatus according to claim 1 wherein said focusing system comprises a lens system containing two or more axially aligned lens elements.

3. Apparatus according to claim 2 wherein said reflecting system comprises a reversed Cassegrain telescope wherein said primary reflective element comprises a large concave mirror surface and said secondary reflective element comprises a relatively small convex mirror surface, said primary and secondary mirror elements being coincident with the axis of the beam and co-axially aligned with said lens elements, said secondary mirror being arranged to reverse reflect the incident beam received from the lens system.

4. Apparatus according to claim 3 wherein said lens system is bodily located between the primary and secondary reflective elements.

5. Apparatus according to claim 3 wherein said lens system is bodily located rearwards of said reflecting system so that the primary mirror element is positioned between the lens system and the secondary reflective element, said primary reflective element having a central aperture through which the divergent beam travels after emerging from the lens system.

6. Apparatus according to any one of claims 2 to 5 wherein said lens system comprises a fixed lens element and a movable lens element having a diamter less than the diameter of the fixed lens element, said fixed lens element being spaced from the secondary reflective element by a predetermined distance.

7. Apparatus according to any one of claims 2 to 5 wherein said lens system comprises spaced apart first and second lens sub-systems each comprising a plurality of lens elements co-axially aligned with the lens elements of the other sub-system, the lenses of one of said sub-system being movable along the axis of the beam relative to the other sub-system to thereby enable the axial distance separating the sub-systems to be varied.

8. Apparatus according to any preceding claim wherein said primary and secondary reflective elements are fixed with respect to one another.

9. Apparatus according to any preceding claim wherein said generating source is a laser beam generator containing a laser beam discharge head.

10_* Apparatus according to claim 9 wherein the laser beam discharged from the discharge head passes to the focusing system along a straight delivery path.

11. Apparatus according to claim 9 wherein the laser beam discharged from the discharge head passes to the focusing system along a bent delivery path, there being one or more beam reflecting elements arranged in fixed relation to the discharge head along the path so as to reflect the beam along that path.

12. Apparatus according to any preceding claim further comprising beam alignment means carried on said focusing means by which the beam from the apparatus can be visually aligned with a selected area.

13. Apparatus according to claim 12 wherein said beam alignment means comprises a sighting telescope.

14. Apparatus according to any preceding claim further comprising a laser range finder carried on said focusing means for determining the distance of the selected area (i.e. the focal distance) from the focusing system.

15. Apparatus according to claim 14 wherein the output signal from the laser range finder is sent to a microprocessor control means which in combination with a stepping motor means automatically adjusts the axial separation between the focusing elements or sub-systems to that required for the focal distance of the selected area.

16. Apparatus according to any preceding claim/ wherein the laser beam may be focused at selected areas in the range of 30-1500 metres.

17. Apparatus according to any preceding claim further comprising mounting means for supporting galιd|beam focusing means for movement in both vertical and horizontal planes to enable alignment of J the' focused beam at the selected area.

18. Apparatus according to

a support, frame assembly for stably supporting' said mounting means and said laser generator. >

I

19. Apparatus according to claim 18 when mounted on the tray of a vehicle truck. ' '

20. Apparatus according to claim 19 further comprising a base frame securable on the tray of Said truck, said support frame assembly being supported on said base frame, and elevating means operatively connected between the base frame and the support frame assembly for lifting the latter clear of the base frame and holding it in a stable condition in such elevated position during operation of the apparatus.

21. A laser optics focusing mechanism for focusing a beam of electro-magnetic radiation at a distance in the range of 30-1500 metres remote from the mechanism so as to provide a spot of intense heat in a selected small area at that distance, comprising a lens system for receiving a near-collimated e. . beam from a laser generating source and including two or more axially aligned focusing lens elements or lens sub-systems located in the path of the beam and arranged to diverge or expand the beam, at least one of said lens elements or sub-systems being movable along the axis of the beam relative to another said lens element or sub-system whereby the axial distance between the lens elements or sub-systems can be adjusted and in turn the extent of beam divergence (and thereby the focal distance), and a reflecting, system comprising spaced apart primary and secondary reflective elements, said secondary reflective element having a reflective surface arranged to reflect the divergent beam from the lens system and deliver it co-axially^* to the surface of said primary reflective surface of said primary reflective element which is arranged to forward reflect the beam in a convergent manner along the focusing path, in the direction of the selected small area whereat the intense heat spot is required.

22., Apparatus according to claim 21 wherein said reflecting system comprises a reversed Cassegrain telescope wherein said primary reflective element comprises a concave mirror surface and said secondary reflective element comprises a convex mirror surface, said primary and secondary mirror elements being co-axially aligned with the axis of the beam and with said lens elements or sub-systems, said secondary mirror being arranged to reverse reflect the incident beam received from the lens system.

23. Apparatus according to claim 22 wherein said lens system comprises a fixed lens element and a movable lens element having a diameter smaller than the diameter of the fixed lens element, said fixed lens element being spaced from the secondary reflective element by a predetermined distance.