Infra red filter

A phosphate glass doped with copper in an amount of less than 2% by weight is formed to produce a lens for use on external aircraft lights. The glass attenuates infra red radiation to a degree where it does not pose a danger of temporarily blinding a pilot wearing night vision goggles yet remains visible. The glass has little appreciable effect on light transmission in the visible spectrum. Conveniently, the glass can be formed by slumping at a softening temperature against a mold to produce intricate lens shapes. In addition, the glass is able to withstand conventional toughening processes.

BACKGROUND OF THE INVENTION
 The present invention relates to a phosphate glass for use as an infra red
 (IR) filter and to a method of forming the same, and is particularly but
 not exclusively directed towards filters for use on external lights for
 aircraft, ships and the like.
 Aircraft lights, whether fitted to civil or military aircraft, generally
 comprise navigation lights at the wingtips (red port and green starboard)
 and white tail lights. High intensity anti-collision strobe lights are
 also fitted at the top and bottom of the fuselage.
 Strict regulations operate to govern the color and intensity of aircraft
 lights. In the United Kingdom, the Civil Aviation Authority ("CAA") has
 responsibility for ensuring that all aircraft adhere to the regulations.
 For example, at least in relation to civil aircraft, the red and green
 wingtip lights must fall within stipulated color bandwidths, so that an
 aircraft whose lights emit an orange or a blue hue, rather than red or
 green, would fail to meet CAA regulations and hence be refused a licence.
 Although military aircraft are exempt from these regulations, it is
 obviously desirable that lights on military aircraft conform as closely as
 possible to the specified standards.
 To improve visibility under low light conditions, it is now common for
 pilots to fly wearing night vision goggles. These goggles are fitted with
 a filter to exclude light in the visible spectrum and operate by detecting
 radiation in the infra red ("IR") region (650 nm to 1000 nm). To maintain
 a good output image under varying light levels, the goggles are generally
 provided with automatic gain control.
 While the automatic gain control operates satisfactorily in most
 conditions, it is unable to compensate when a very bright light is
 introduced into the field of view. Such a situation occurs, for example,
 when the pilot approaches another aircraft at night and results in the
 output image becoming very bright and occasionally the whole display is
 "bleached".
 The inability of the goggles to cope with bright lights at night can
 therefore make it difficult for the pilot accurately to locate other
 aircraft in the vicinity and, at worst, may temporarily blind the pilot.
 In such circumstances, it will be appreciated that both aircraft can be
 placed in an extremely dangerous situation.
 On the other hand, if other aircraft are to remain clearly visible and
 identifiable as such when viewed through night flying goggles, it is
 preferred that at least some IR radiation is permitted to emerge from the
 lights. In other words, the level of IR emission should ideally be
 suppressed but not entirely eliminated.
 It will be appreciated that there is a fine balance to be struck between
 filtering sufficient IR radiation to avoid temporarily blinding the pilot
 yet transmitting enough IR radiation to allow the pilot to recognize
 lights on other aircraft in the vicinity.
 To the best of our knowledge, no-one has been able to quantify the
 intensity of the IR transmission which meets the above criteria. Experts
 in the field do, however, recognize when an appropriate balance has been
 achieved. In this regard, the balance is considered to be about right when
 the effect around each aircraft light as viewed through the goggles
 resembles a "football". This phenomenon is the commonly used indicator by
 which the optimum IR transmission level is judged.
 Of course, while the aforementioned problems are most acute for pilots
 flying close to other aircraft, it is also important that pilots are able
 clearly to view other structures in low visibility conditions, for example
 when approaching naval vessels such as aircraft carriers, marine
 structures such as off-shore oil and gas rigs, or land-based constructions
 such as airports or even high-rise buildings.
 A further factor which must be taken into account when filtering IR
 radiation is the potential for the filter medium to affect the light
 emitted in the visible spectrum. In this regard, it can happen that
 attenuation of IR radiation is accompanied by a color shift in the visible
 range. With regard to wingtip lights, the amount of color shift can mean
 the difference between compliance with or failure to meet CAA regulations.
 Attempts to solve the aforementioned problems of filtering out a proportion
 of the IR radiation yet avoiding significant color shifts have only been
 partially successful as outlined below.
 For example, lenses for aircraft lights have hitherto been made only from
 conventional silicate glass because this has been the only glass suitable
 for forming into curved sections which are then bonded together to form
 the lenses. In this regard, silicate lenses are generally formed by
 molding or pressing while the glass is in the molten state.
 Although IR filter glass of the silicate type is commercially available, it
 is of limited use for aircraft lights because the light emitted in the
 visible spectrum is also affected. In this regard, it produces a
 measurable color shift in the red region making it difficult to comply
 with the CAA regulations.
 Rather than using the aforementioned IR filter glass, an alternative has
 been to provide a coating of an IR filter material on conventional
 silicate glass. The high temperatures involved in forming the glass
 sections for subsequent assembly into the lens means that such coatings
 can only be effectively applied after the forming stage. Moreover, to
 avoid accidental damage to the coating, it is preferable to apply the
 coating after the individual sections have been bonded together to form
 the complete lens.
 However, coating with an IR filter material after assembly of the lens has
 still proved unreliable because of the highly contoured surfaces involved.
 In particular, it has been difficult to coat either the interior or
 exterior surfaces of the lenses uniformly.
 In order to be effective, IR filter coatings must be evenly applied; too
 thin a coating will result in inadequate IR attenuation with potentially
 disastrous consequences and too thick a coating may block out the IR
 spectrum completely. Coating of lenses made from conventional silicate
 glass is therefore problematic.
 As far as other types of glass are concerned, it is known to dope phosphate
 glass with copper in order to achieve low transmission in the IR range and
 such a glass has been used to shield illuminated color displays such as
 those in aircraft cockpits. IR filter glass of this type, for example as
 is documented in U.S. Pat. No. 5,173,212 commonly assigned to Schott
 Glaswerke, is commercially available. However, for external lighting
 applications the glass would need to be only 1 mm thick, rendering it
 useless mechanically; in particular, because of its inability to withstand
 the treatment involved in known glass toughening processes. Such treatment
 is essential if it is to achieve the strength and durability required for
 the demanding physical conditions encountered when used on aircraft
 exteriors.
 Moreover, in its known applications phosphate glass is characterized by its
 brittle nature and hence it has not hitherto been possible to form into
 substantially non-planar components.
 SUMMARY OF THE INVENTION
 It is evident from the above that the problem of finding a material which
 can be toughened to make it suitable in particular for aviation use, which
 is able to reduce IR transmission to a satisfactory level yet not
 eliminate it entirely, which does not result in adverse color shifts in
 the visible range, and which is capable of being formed to make lenses
 remains to be solved.
 With the above objectives in mind, we have set out to produce such a
 material. Accordingly, it is an object of the present invention to provide
 a material suitable for use in aircraft and other lighting which not only
 provides attenuation of IR radiation but also has no appreciable affect on
 the visible spectrum and which is able to withstand conventional glass
 toughening processes both mechanically and optically.
 Despite the prior art indicating the apparent unsuitability of phosphate
 glass as a material for forming into lenses such as for aircraft lights,
 we have undertaken extensive research and development into this material.
 As a result of our efforts, we have not only invented a material
 comprising phosphate glass which is able to attenuate IR radiation to the
 desired levels, but have also invented a method by which phosphate glass
 can be formed into non-planar elements, such as aircraft lenses or
 components thereof.
 From a first aspect, the present invention resides in a phosphate glass for
 use as an IR filter, wherein the glass is doped with copper (II) oxide in
 an amount of less than 2% by weight based on the total weight of the
 glass.
 Expressed in another way, the invention comprises an IR filter comprising a
 phosphate glass doped with copper, wherein the copper is present in the
 glass as copper (II) oxide in an amount of less than 2% by weight based on
 the total weight of the glass.
 The actual amount of copper included in the glass of the invention will
 vary according to a number of factors, the main ones being the thickness
 of the glass itself and the light source which is to be filtered. For
 instance, the emission characteristics of a high intensity strobe light
 will differ from those of the tail lights and those of the red and green
 wingtip lights.
 In addition, the thickness of the glass will vary for controlling the
 optical properties to suit a particular application, for example,
 according to whether it is to be used as an auxiliary filter in
 conjunction with an existing lens or as a replacement lens.
 However, by means of this invention, it is possible to attenuate the IR
 radiation to a level which does not have an adverse effect on the
 automatic gain control of the pilot's goggles yet allows the pilot to
 readily identify the light source. Moreover, the glass composition has
 little or no appreciable effect on the visible spectrum.
 As will be understood, in order to be suitable for use on an exterior
 aircraft light, especially when used as a replacement lens, the filter
 material must be mechanically very strong. In this regard, we have found
 that when phosphate glass has a thickness of from about 2 mm to 6 mm it
 can acquire sufficient strength for such use.
 While commercial phosphate glass has heretofore been available in slabs,
 sheet, etc in appreciable thicknesses, because of its high attenuation in
 the IR and lower visible range, it has had to be machined to a thickness
 of about 1 mm in order to provide acceptable transmission characteristics.
 At such a thickness, the glass has no strength and is incapable of
 surviving a toughening process.
 From another aspect therefore, the invention resides in an IR filter for
 use as lens comprising a copper doped phosphate glass having a thickness
 of from about 2 mm to 6 mm. The expression "from about 2 mm to 6 mm"
 includes any range or specific value therebetween. More especially, the
 invention resides in a toughened copper doped phosphate glass having such
 a thickness.
 In a preferred embodiment, the phosphate glass has a thickness of between
 about 3 mm and 5 mm. However, it will be appreciated that the thickness
 may be varied within the aforementioned range according to the type of
 light source involved.
 While the IR filter according to the invention is generally used at
 thicknesses between about 2 mm and 6 mm, the filter is preferably
 manufactured in sheets or slabs having a thickness of about 10 mm or more.
 Thus, we have found that only the central or middle core of the
 manufactured glass possesses the required optical properties for use as a
 filter. Grinding, polishing and other known forms of cold working are used
 to arrive at the desired thickness. Once at the desired thickness, the
 glass may then be subjected to forming processes.
 Unlike previously known IR filter phosphate glass, the phosphate glass of
 the invention is mechanically well able to withstand the stresses which
 are induced during subsequent glass toughening processes. Being able to
 withstand a toughening process is of course a prerequisite for achieving
 the necessary strength for use as lenses on aircraft lights.
 Significantly, the phosphate glass of the invention retains its optical
 properties during toughening.
 In order to achieve the desired attenuation of IR yet maximize transmission
 of visible light, particularly at the red end of the visible spectrum, the
 IR filter having a thickness in the range specified preferably comprises
 copper doped phosphate glass having a copper (II) oxide content of less
 than 2% by weight based on the total weight of the glass.
 Advantageously, the copper (II) oxide content of the phosphate glass is
 from about 1.5 to 1.8% by weight or any value or range in between; more
 preferably between about 1.6 to 1.7% by weight.
 Apart from limiting the copper (II) oxide content to below 2% by weight,
 which is essential for achieving the desired optical properties, other
 components are present in varying amounts to contribute to the overall
 properties of the phosphate glass.
 MgO is one such component which is preferably present in the phosphate
 glass of the invention. Advantageously, MgO is included in an amount of
 from 5.0% to 7.0% by weight, more preferably between 5.4 to 6.6% by
 weight.
 K.sub.2 O is a further component preferably contained in the glass, most
 conveniently in amounts of from about 0.2% to 0.4% by weight, more
 preferably between 0.24% to 0.37% by weight.
 SiO.sub.2 is also preferably included since it is believed to contribute
 beneficially to the weathering properties of the glass. In this regard, it
 is ideally present in an amount of between about 1.5% to 2.4% by weight,
 more preferably between 1.9% to 2.2% by weight.
 Other components which are advantageously included are Al.sub.2 O.sub.3
 (preferably between 10.0% to 14.0% by weight), CaO (preferably between
 about 0.1% to 0.5% by weight) and Na.sub.2 O (preferably between about
 3.0% to 6.0% by weight).
 Of course, the bulk of the glass composition will comprise P.sub.2 O.sub.5
 and this is usually present in amounts of between 65.0% to 80.0% by
 weight, more preferably between 69.0% to 72.0% by weight.
 It should be understood that all of the ranges expressed herein are
 intended to embrace any range or specific value therebetween and are based
 upon the total weight of the glass.
 As is well known in the art of glass making, the final composition of the
 glass is influenced by many factors. For example, the addition of
 identical quantities of starting materials will not result in the same
 product if the same process conditions are not used. Such factors as
 furnace temperature, atmosphere, wind speed in the furnace chimney,
 composition of the refractory chamber, etc. all have an influence on the
 resulting glass composition. In addition, a change in batch size will also
 have an effect so that a simple extrapolation of the amount of starting
 materials to increase or decrease the volume of glass to be produced will
 not necessarily give reproducible results even if other external factors
 are kept constant.
 However, by routine investigation, it is within the capabilities of a
 person skilled in the art of glass making to produce an end product having
 the required copper (II) oxide content and indeed the desired quantities
 of other components.
 By careful balancing of the copper (II) oxide content and the thickness of
 the filter within the parameters specified, we have been able to achieve
 high attenuation of IR radiation, particularly in the 700 nm to 900 nm
 range, yet without sacrificing spectral quality in the visible spectrum,
 especially towards the red end.
 In optical terms, based on a 4 mm thick sample, the phosphate glass
 according the invention has a peak transmission of preferably
 82.5%.+-.5.0% at a peak wavelength of 512 nm.+-.4 nm. More preferably, at
 such a thickness, the glass has a transmission of 60.0%.+-.7.0% at 400 nm
 and an optical density of 2.65.+-.10% at 700 nm.
 Hence the invention further resides in a copper doped phosphate glass
 having a peak transmission of 82.5%.+-.5.0%, a peak wavelength at 512
 nm.+-.4 nm, a transmission of 60%.+-.7.0% at 400 nm and an optical density
 of 2.65.+-.10% at 700 nm, when measured on a 4 mm thick sample.
 Such characteristics represent the optimum values to achieve the right type
 of light emission to avoid temporarily blinding a pilot wearing night
 vision goggles.
 It will be appreciated that by means of this invention, we have made a
 major contribution to improving air safety in poor visibility conditions.
 Apart from this significant contribution to safety, our research in this
 area has resulted in another important advance in phosphate glass
 technology. In particular, we have found that it is possible to form or
 bend phosphate glass without causing cracking or affecting the long term
 stability of the glass. Furthermore, when the process of forming is
 applied to the IR filter glass of the invention it has no deleterious
 affect on the optical properties of the glass.
 Whereas the shape of some lenses can be created simply by machining blocks
 or slabs of cast glass, for other shapes the only practical way of
 producing the lenses is by forming the glass.
 Again, the ability of phosphate glass in accordance with the invention to
 undergo forming without detriment to the optical properties is surprising.
 In experiments conducted by us on known phosphate IR filter glass, heating
 up to the softening point or plastics state temperature has resulted in
 the known glass undergoing an unacceptable color change, the glass turning
 darker green thus causing more visible light attenuation. Such a marked
 shift in the visible light transmission further demonstrates the
 unsuitability of the prior art glass for aircraft lighting applications.
 From a yet further aspect therefore, the invention comprises a process for
 forming phosphate glass comprising the steps of heating the glass until
 the glass reaches a plastic state and forms under its own weight, cooling
 the formed glass to an intermediate temperature, maintaining the glass at
 or about the intermediate temperature for an extended period, and allowing
 the glass to cool to ambient.
 The above process is particularly suitable for forming phosphate glass
 having a thickness of from about 2 mm to 6 mm.
 In order to achieve a desired shape, the glass is preferably placed in
 contact with a mold such that when the glass reaches a plastic state it
 forms by collapsing or slumping against the mold and adopts the same
 contours thereof. For example, the glass may be "slumped" into a generally
 concave mold surface or over an upwardly convex mold surface.
 While it is preferred to allow the glass to bend or form "naturally" under
 its own weight, shaping may be alternatively achieved by pressing the
 glass into a mold, for example, by means of a two-part mold. However,
 pressing incurs an increased risk of creating stresses in the glass and
 hence requires exercise of all due care.
 Ideally, the mold against which the glass is bent, "slumped" or pressed is
 made of a ceramic material. This is believed to be because ceramic has a
 similar expansion coefficient to the glass so that when cooling occurs it
 contracts at the same or similar rate to the glass.
 Also, ceramic has a lower thermal conductivity than the traditionally used
 cast iron so that it does not chill the glass surface when the glass and
 mold are in contact. In this way, deterioration in surface quality and
 cracking can be successfully avoided. Another reason for choosing ceramic
 over cast iron is that cast iron oxidizes readily at the forming
 temperatures involved thus further reducing surface quality of the glass.
 Generally, copper doped phosphate glass according to the invention reaches
 a plastic state at a temperature between 500.degree. C. and 600.degree.
 C., more usually between about 540.degree. C. and 570.degree. C., and
 therefore it is preferred that the temperature of the glass is not allowed
 to rise much beyond these temperatures during the forming process. The
 plastic state or softening temperature will differ according to the
 precise composition of the glass and it is within the ability of the
 skilled person to select the ideal temperature for slumping according to
 the composition in question.
 The rate at which the glass is heated to the plastic state temperature is
 ideally controlled to avoid creating differential heating which can set up
 stress in the glass. Preferably the glass is heated at a rate of about
 80.degree. C. to 120.degree. C./hour, more preferably at about 100.degree.
 C./hour.
 Actual melting of the glass takes place at much higher temperatures,
 usually around 1200.degree. C., and it is important that the glass does
 not reach this temperature, otherwise the optical properties will almost
 certainly be affected. Ideally, heating of the glass is stopped once the
 plastic state temperature has been reached or is at least reduced so that
 the temperature does not rise above the plastic state temperature.
 Once the glass has reached the plastic state, the heating is maintained at
 about the same temperature until the slumping is finished. Depending on
 the thickness of the glass, the bending or "slumping" of the glass is
 usually achieved over a period of 15 minutes to 1 hour; 30 minutes being
 about average.
 Furthermore, after the glass has been formed, it is preferred that the
 annealing or cooling is allowed to proceed with the glass freed from the
 mold. To this end, the glass is preferably removed from the mold once it
 is rigid enough to maintain its shape.
 After removal from the mold, the unsupported but formed glass is
 advantageously maintained at a temperature approaching but lower than the
 softening temperature for a further period before being allowed to cool to
 an intermediate temperature. For example, the formed glass is maintained
 at a temperature of between 20.degree. C. to 100.degree. C., preferably
 about 50.degree. C., below the softening temperature for a period of
 between about 1 to 3 hours.
 After forming, and ideally after being held close to but below the
 softening temperature for a period, the glass is allowed to cool to an
 intermediate temperature which is advantageously between about 250.degree.
 C. and 350.degree. C. An intermediate temperature of about 300.degree. C.
 has been found to be particularly suitable.
 Cooling to the intermediate temperature is most conveniently carried out at
 the same or similar rate as the heating takes place. A cooling rate of
 around 100.degree. C./hour is therefore preferred.
 The glass is then held at or close to the intermediate temperature for a
 period sufficient to release any stresses created in the glass during the
 slumping. A period of about 3 hours is generally considered sufficient,
 although this may be increased or decreased depending upon the extent to
 which the glass has been bent and its thickness. If the glass is not
 annealed in the correct manner, for example, if it is not allowed
 sufficient time to cool, the resulting glass is likely to be unstable at
 ambient and unworkable.
 The formed glass is returned to ambient in a controlled manner. In other
 words, it should not be subjected to rapid or uneven cooling rates.
 Preferably the glass is allowed to cool at a rate of around 40.degree.
 C./hour over a period of about 8 hours. Once cooled to ambient, it is
 preferred to remove by grinding or other known processes a portion of the
 glass around its lateral edges thereby to produce the ultimate shape for
 use.
 The aforementioned process of forming phosphate glass may be employed
 either to form a complete filter or to form sections thereof for
 subsequent bonding together. Where bonding of various sections is
 required, conventional high temperature semi-rigid adhesives for optical
 use are generally employed.
 After forming, the surface of the filter is usually subjected to a final
 grinding and polishing to perfect the surfaces ready for toughening.
 Toughening processes are well known to those in the glass making field and
 require no further explanation here. It will be appreciated that after
 toughening, no further glass modification is generally possible. Thus all
 grinding, polishing, etc must be completed before the toughening stage.
 Where a lens is comprised of several sections, for example to achieve
 highly irregular shapes, the sections may be bonded together after the
 toughening stage.
 Such complex shapes as those illustrated hereinafter have heretofore been
 impossible to achieve using known IR phosphate filter glass doped with
 copper without causing deterioration in the optical properties. This is
 because heat treatment of commercially available phosphate glass causes
 chemical reactions which adversely affect the optical transmission
 properties.
 The composition of the glass according to the invention and the method for
 forming the same have together made it possible to form intricate shapes.
 In particular, because the phosphate glass of the invention is chemically
 more stable when heated than previously available glass and the method
 used to form the glass minimizes the extent to which it is exposed to high
 temperatures.

DETAILED DESCRIPTION OF THE INVENTION
 A number of phosphate glasses manufactured into thick sheets in accordance
 with known glass-making techniques were ground to a thickness 3 mm. The
 ground sheets, representing the middle core of the manufactured glass, had
 compositions as set out in Table I. The quantities of the various
 components were calculated using XRF analysis and the percentages are
 expressed by weight on an oxide basis.
 TABLE I
 Ex. 1 Ex. 2 Ex. 3
 SiO.sub.2 2.02 1.89 2.14
 TiO.sub.2 &lt;0.01 &lt;0.01 &lt;0.01
 Al.sub.2 O.sub.3 11.8 11.8 12.1
 Fe.sub.2 O.sub.3 0.04 0.04 0.04
 CaO 0.42 0.41 0.43
 MgO 5.58 5.39 5.62
 K.sub.2 O 0.31 0.23 0.30
 Na.sub.2 O 4.39 4.38 4.50
 B.sub.2 O.sub.3 (wet) 0.09 0.12 0.10
 Li.sub.2 O (wet) 1.12 1.10 1.12
 F(wet) 0.02 0.01 0.06
 OEF 0.01 &lt;0.01 0.03
 P.sub.2 O.sub.5 71.7 71.7 70.8
 Cr.sub.2 O.sub.3 &lt;0.01 &lt;0.01 &lt;0.01
 ZrO.sub.2 &lt;0.02 &lt;0.02 &lt;0.02
 PbO &lt;0.02 &lt;0.02 &lt;0.02
 ZnO 0.03 0.05 0.02
 BaO &lt;0.01 &lt;0.01 &lt;0.01
 SrO &lt;0.01 &lt;0.01 &lt;0.01
 CuO 1.62 1.54 1.67
 As.sub.2 O.sub.3 &lt;0.01 &lt;0.01 &lt;0.01
 CeO.sub.2 &lt;0.01 &lt;0.01 &lt;0.01
 Nd.sub.2 O.sub.3 &lt;0.01 &lt;0.01 &lt;0.01
 CoO &lt;0.01 &lt;0.01 &lt;0.01
 L.O.I. 0.07 0.07 0.02
 TOTAL 99.19 98.73 98.95
 An optical analysis of the glass of Example 1 showed a peak transmission of
 82.88% at a peak wavelength of 508 nm, a transmission at 400 nm of 66.77%
 and an optical density at 700 nm of 2.12.
 A similar analysis of the glass of Example 2 revealed a peak transmission
 of 81.24% at a peak wavelength of 518 nm, a transmission at 400 nm of
 58.77% and an optical density at 700 mn of 1.814.
 While both glass samples of Examples 1 and 2 exhibited desirable optical
 properties in terms of usefulness as an IR filter for an external aircraft
 light, the sample of Example 1 was particularly effective in the visible
 range.
 In order to further demonstrate the improved optical properties of the
 phosphate glass according the invention, there is shown in FIG. 1 a
 transmission graph comparing a prior art filter (A) with an IR filter
 according to the invention (B). The results were taken using 1 mm samples
 for each glass.
 It can be seen from FIG. 1 that the IR filter according to the invention
 (B) offers consistently higher transmission in the visible range than the
 prior art filter (A). At the red end of the spectrum, the difference is
 most marked. For example, at 650 nm the prior art filter (A) displays only
 about 10% transmission whereas the filter in accordance with the invention
 (B) displays about 50%.
 Moreover, it can also be seen that the filter according to the invention
 (B) permits at least some transmission in the IR region whereas with the
 prior art filter (A) it is almost completely blocked.
 The difference in the properties of the two glasses (A, B) in the IR region
 is perhaps more clearly seen from FIG. 2 which is a graph showing optical
 density for the same 1 mm thick samples (A, B) as used for FIG. 1.
 In particular, it is evident that the optical density of the filter
 according to the invention (B) is much lower across the IR wavelengths
 than the prior art filter (A). Indeed, the attenuation in the IR region by
 the prior art filter (A) is such that a pilot wearing night vision goggles
 would not be able to see a light which incorporated such a filter.
 There now follows an example of the forming process:
 EXAMPLE 4
 A piece of copper doped phosphate glass of approximate dimensions 100
 mm.times.50 mm.times.4 mm thick was machined from the raw sheet of 10 mm
 thick by reducing the thickness equally on each face. The piece was then
 profiled on its edges to the pre-slump shape. The glass was subsequently
 placed on the ceramic mold in a furnace and the temperature raised from
 ambient to 550.degree. C. at a rate of 100.degree. C./hour. This
 temperature was maintained for 30 minutes by which time the glass had
 formed itself to the shape of the mold. The temperature was then allowed
 to fall to 500.degree. C. over a period of 30 minutes and held at this
 temperature for 2 hours at which stage the glass was then removed from the
 mold inside the furnace. The furnace temperature was then reduced to
 300.degree. C. over a period of 2 hours and then held at 300.degree. C.
 for 3 hours. After this intermediate stage, the furnace temperature was
 again reduced to ambient (25.degree. C.) over a period of 8 hours. The
 slumped glass was then profiled to the final shape and polished to remove
 surface defects.
 After completion of the forming process, the phosphate glass is finally
 toughened before being ready for fitting on an aircraft light, either as a
 replacement lens or as an auxiliary filter in conjunction with an existing
 lens.
 Examples of various types of aircraft lights fitted with IR filters
 according to the invention are illustrated in FIGS. 3 to 10.
 In particular, an aircraft tail light shown in FIGS. 3 to 5 comprises an
 opaque housing or cast mount (1) in which is housed a light source (not
 shown). At one end of the housing (1) there is provided an opening which
 is covered by IR filter (2) having an outwardly convex surface and through
 which the light is filtered. IR filter (2) may be produced by simply
 machining from a block form of the phosphate glass, rather than by forming
 using the "slumping" process.
 A modified navigation light as illustrated in FIGS. 6 to 8 comprises a
 curved cowl (4) onto which is fitted auxiliary frame (5) retaining an IR
 filter (6) of the invention. As seen most clearly from FIG. 8, the IR
 filter (6) of the invention is fitted over the outer convex surface of
 existing lens (7) retained by the existing lens frame (8). The IR filter
 (6) is slightly curved when viewed in one plane (section X--X) yet is
 greatly curved when viewed in an orthogonal plane.
 A high intensity strobe light as illustrated in FIGS. 9 to 11 comprises a
 filter mounting frame (10) on which is supported a filter made up of four
 sections of glass; end glasses (12a, 12b) and side glasses (14a, 14b).
 While end glasses (12a, 12b) are curved appreciably in one plane with a
 constant radius of curvature along the section, it may be seen from FIG.
 10b that side glasses (14a, 14b) are of a more complex shape and include
 portions having different radii of curvature.
 While particular embodiments have been described, it should be appreciated
 by those skilled in the art that various modifications may be made without
 departing from the broad scope of the invention. For example, while the
 invention has been most commonly described in terms of external aviation
 lighting, it will be appreciated that it is not limited thereto. In this
 regard, it will be understood that the invention is also applicable to
 internal aviation lighting, such as internal cockpit lighting and map
 reading lighting, to marine lighting, such as for ships and marine
 structures (oil rigs, lighthouses, buoys, etc), and to external lighting
 on high-rise buildings. Additionally, IR filter glass according to the
 invention may also find application in landing and other navigational
 lights at airfields.
 It will be appreciated by those skilled in the art that changes could be
 made to the embodiment(s) described above without departing from the broad
 inventive concept thereof. It is understood, therefore, that this
 invention is not limited to the particular embodiment(s) disclosed, but it
 is intended to cover modifications within the spirit and scope of the
 present invention as defined by the appended claims.