COMPOSITE MIRROR

Abstract: A composite mirror (1) comprising a reflective surface (2) bonded to a syntactic foam backing layer (3) using adhesive (5). The syntactic foam layer (3) comprises a multiplicity of inclusions consisting of micro-cellular material, such as a prill and/or micro-spheres, bound in a binding agent, such as a ceramic.

Description: Composite Mirror The present invention relates to a composite mirror and in particular to one suitable for use as either a flat precision optic or as a deformable optic of either flat or curved form and which, for example, may be used in high power laser systems. The amount of glass in mirrors of some laser systems can weigh several tens of tonnes and can present severe mounting problems. Such traditional mirrors have a ratio (R) of largest surface dimension to thickness of between 5:1 and 8:1. There exists a need to reduce this weight and to provide mirrors having an R in excess of 10: 1 in order to reduce both weight and volume. Moreover, any rnirror used in these laser systems must be capable of being finished to provide a surface having an operational variation from flatness, which includes self weight distortion and distortion due to adjusting the mounted mirror, of less man λ/4 where λ is the wavelength of laser light to be reflected. Currently available monolithic glass structures, employed as a flat precision optic, are typically 400mm square by 55mm thick (R~7:l) show self weight distortions of between 0.14μm and 1.8μm even under the best mounting conditions. It is known to form lightweight mirrors from all glass structures which may consist of a reflecting surface behind which voids are provided in the glass in order to reduce the overall weight of the rnirror. These voids are made either by drilling holes in a glass monolith structure from the surface rearward of the reflecting surface or by providing a sandwich arrangement comprising t* Ό sheets of glass separated by a plurality of glass cylinders fused to form a sound structure. However, with either of these arrangements the thickness of the reflecting surface needed in order to achieve an R approaching 10:1, with a suitably low mass, is unable to support itself sufficiently, particularly during the polishing process, and deflects in the voids to produce unacceptable variations in surface flatness, so called "quilting". Composite mirrors are also known having a reduced weight over monolithic glass mirrors and comprise a reflectively coated sheet, formed from glass or a suitable ceramic material, bonded to a less dense honeycombed backing layer, usually made from a ceramic material in which a plurality of evenly sized and spaced holes have been made, for example by drilling. A problem with the use of such a honeycombed layer is that unless the reflecting surface is made sufficiently thick then, when polished, the reflecting surface exhibits the quilting effect, as with the all glass structures, reflecting the honeycombed backing. In the cases of the modified glass monolith, the composite glass structure and the honeycombed ceramic structure the quilting effect may be reduced either by reducing the cross sections of the individual voids or by increasing the thickness of the reflecting surface. However both solutions are at the expense of an increased mirror weight and lead to a reduced R value. According to the present invention there is provided a composite mirror having a reflective sheet bonded to a backing layer characterised in that the backing layer comprises a multiplicity of inclusions consisting of micro-cellular material and a binding agent adapted to bind the inclusions therein. The syntactic foam material formed by the inclusion/binding agent mixture provides a backing layer in which the void size compared with those of the honeycomb backing is significantly reduced to thereby eliminate quilting. Consequently thinner refecting sheets may be used formed from, for example, glass or ceramic. Most usefully, the micro-cellular materials may be selected to have a lower density than the binding agent, which may for example be a ceramic, a rubber, a plastic or a resin material. This provides a light weight mirror system having a backing layer with a higher specific stiffness (defined as ε/p where ε is Young's Modulus and p is the density of the backing layer) than a honeycombed structure of the same binding agent. This is due to the presence of material where, in a honeycomb, there would exist a void. The higher specific stiffhes provides a the further advantage in that a smaller ' thickness of backing layer may be employed to support any given thickness of reflective sheet without significant distortions occurring due to twisting as a result of gravity effects. Most advantageously the binding agent with micro-cellular inclusions may be chosen to provide a backing layer with a density less than a glass sheet reflector and with a greater specific stiffness. This would allow a mirror to be made which is lighter and which has a higher specific stiffness than a monolithic glass or a sandwich glass mirror and which has an R of 10: 1 or greater. Obviously, the composite mirror may be made stiffer by applying a high stiffness material, such as an aluminium sheet, to the face of the backing layer opposite the reflecting surface. The inclusions may be formed from so called micro-spheres or from elongate foamed ceramic pellets, for example a prill such as TECPRLL (TM), available from FILTEC Limited, Widnes, Cheshire. These elongate pellets, largely due to their shape, produce a higher specific stiffness backing than using just the micro-spheres in the same binding agent. Usefully, a combination of micro-spheres and pellets may be employed, the micro-spheres being able to fill voids between the pellets to reduce the free spaces in the binding agent itself and thereby increase its specific stiffness. Additionally when ceramic is selected as the binding agent the presence of micro- spheres makes it more flowable. Typically, when making a light weight mirror system, the weight percentage of micro-spheres and prill in the ceramic bonding agent is between 2-20 % and 8-15% respectively, particularly between 14-16% and 9-11% and most particularly 14.9% and 9.9% respectively. Usefully, the coefficient of thermal expansion of the backing layer may be made substantially that of the reflecting sheet by varying the constituent weights and curing conditions in ways clear to a person skilled in the art. Advantageously where the reflecting sheet comprises a polished glass it may be bonded to the backing layer using a glass net (a so called frit) to maintain an adhesive film of substantially uniform thickness, the adhesive being selected to have a curing temperature lower than the distortion temperature of the glass sheet However, other bonding techniques, being common in the art, may be employed. Embodiments of the present invention will be described by way of example only with reference to the drawings of the accompanying figures of which: Figure 1 shows a part sectional view of a precision optic mirror according to the present invention. Figure 2 shows a mounting point for use with the mirror of Figure 1 with 2a showing a ceramic insert used in the mounting point and 2b showing the insert and anchor assembly. Figure 3 shows a flat deformable mirror assembly. Referring now to Figure 1 , a lightweight precision optic 1 comprises a flat, polished glass sheet 2 bonded to a syntactic ceramic foam backing layer 3 using a glass frit 4 and adhesive 5. The backing layer 3 has a number of mounting points 6 made in it, for example by foπning threaded sockets after the layer 3 has solidified or by setting the mounting points into the layer 3 during its solidification. The layer thickness of the adhesive 5 is controlled by the frit 4 since it is spread over the frit 4 to fill in the spaces between the individual glass strands. Thus the layer thickness is maintained at substantially that of the glass frit 4. The backing layer 3 is formed from a micro-cellular material comprising mixture of micro-spheres and a ceramic prill dispersed in a ceramic binder to form a syntactic foam. The density of this backing layer will have a lower density than one formed from the ceramic binder. Typically the backing layer 3 may comprise: MATERIAL WT% Alumina powder 25 Glass Ceramic 15.7 Silica Sol 34.5 micro spheres 14.9 prill 9.9 to which a wetting agent will normally be added and which is then poured into a mould and cast using methods common in the art. For best results the alumina powder is chosen to have an average grain size of typically between 48 mesh and 50 μm. When adhered to a glass reflector 2 to form a composite mirror 1 then the self weight deflection of a 400 mm square by 50 mm thick mirror is 0.05μm which is between 3 and 36 times better than the glass monolith. A simple drilled out mounting point may be used in the composite mirror 1 but this is found to have a relatively low pull out force of about 0.8kN. In order to increase this pull out force a mounting point as shown in Figures 2 may be used. A removable insert (not shown), for example made of silicone rubber, is cast into a conical ceramic plug 7. On removal of the insert an undercut hole 8 is left in the ceramic plug 7. The required number of such plugs 7 are arranged in the mirror backing layer mould and the backing layer mixture is added which is then freeze cast and fired after drying. The plugs 7 co-fire with and bond into the backing layer An anchor comprising a bolt 9 and retaining spring 10 arrangement is then inserted in the plug 7 as shown in Figure 2b to provide a mounting point with a pull out force of about 2kN. The external mount may be secured directly to the bolt or may be screwed into a female threaded collar 11. A curved rnirror (not shown) may be made using these above described components relatively simply. The backing layer 3 could be moulded in the required shape and the glass reflector 2 heated to its distortion temperature and allowed to slump on to the curved backing layer. The resulting mirror would be much more temperature stable than a conventional curved mirror which has been shaped by pohshing. This is because a curved mirror according to the present invention would have a substantially uniform reflector thickness as compared with a conventional curved mirror where the thickness varies with the curvature so that its thermal properties will vary correspondingly. Turning now to the deformable mirror system 12 shown in Figure 3. This comprises a reflector 13 bonded to a compliant syntactic foam layer 14. Actuators 15 a . Λ are positioned within the layer 14, arranged depending on the deformed profile required of the reflector, and pass into a stiff backing plate 16 which will tend to resist deformation caused by movement of the actuators 15a„ n . Associated with each actuator 15a.. n are individual drives (not shown) which can be operated to push or pull an activator 15 in order to achieve the required distortion.

Claims: Claims 1. A composite mirror having a reflective sheet bonded to a backing layer characterised in that the backing layer comprises a multiplicity of inclusions consisting of micro-cellular material and a binding agent adapted to bind the inclusions therein. 2. A composite mirror as claimed in Claim 1 characterised in that the binding agent and micro-cellular material are selected to have different densities. 3. A composite mirror system as Claimed in Claim 1 or Claim 2 characterised in that the binding agent is a material selected from the group ceramic, rubber, plastic and resin. 4. A composite as claimed in any preceding claim characterised in that the micro-cellular material comprises one or both of micro-spheres and prill. 5. A composite mirror as claimed in Claim 4 characterised in that both micro-spheres and prill are present in a ceramic binding agent in weight percentages of 2-20% and 8-15% respectively. 6. A composite mirror as claimed in Claim 5 characterised in that the backing layer is of flat form. 7. A composite mirror as claimed in Claim 5 characterised in that the backing layer is of curved form. 8. A composite mirror as claimed in any of the Claims 5 to 7 characterised in that the backing layer additionally comprises one or more mounting point inserts each comprising a co-operating plug and bolt arrangement. 9. A composite mirror as claimed in Claim 1 characterised in that the inclusions and binding agent are selected to together provide a compliant backing layer and in that there is additionally provided a resistive plate in bonded contact with the layer and actuator means in operable connection with the layer, for deforming the layer.

Inventors: NUNN JONATHON PHILIP, NUNN, JONATHON, PHILIP
Applicants: NUNN JONATHON PHILIP, SECR DEFENCE, NUNN, JONATHON, PHILIP, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1998
Main classification: G02B7
IPC codes: G02B5, G02B7