GB2104528A - Infra-red transmissive plastics - Google Patents

Abstract

A polymer composition containing a dispersion of infra-red transmissive particulate inorganic material is proposed as a material for making infra-red transmissive optical components in place of the presently used heavy and usually expensive solid crystalline materials.

Description

SPECIFICATION Infra-red transmissive optical components The optical components, for example windows, lenses and soon, of an infra-red optical system are made of heavy and expensive materials such as Germanium and Zinc Sulphide, ordinary glass and piastics material, as used in visible-light optical sys tems, being insufficiently transmissive.

According to one aspect of the invention there is provided a composite material for use in making infra-red transmissive optical components, the com posite material comprising a dispersion of infra-red transmissive inorganic material in a polymeric material.

The composite material may be made into lenses, windows and such by any suitable forming method.

Such components will generally not be as efficient as ones formed of solid l.R. transmissive material but may be much less heavy and expensive and, provided the components of the material are chosen correctly, may be good enough for particular, perhaps many, applications.

One measure, which may be taken to improve transmissivity, is to make the thickness of the element as small as possible.

Thus lenses made of the composite material are preferably of the Flat-optic kind, ire Fresnel lenses or Kineforms.

The composite material may also be used as a cement for bonding together two elements, e.g.

lenses of an I.R. optical system.

According to a second aspect ofthe invention, there is provided apparatus including means for sensing or generating infra-red radiation, especially far-infra-red radiation such as the 8-14y band, and an optical component for passing such radiation to or from said sensing or generating means, the optical component comprising composite material as described earlier herein.

The composite material may comprise, in a polymeric material, a dispersion oftwo or more l.R.

transmissive inorganic materials, the two or more inorganic materials preferably being in the form of particles of a solid solution of the materials. By using two or more inorganic materials, the combined refractive index of the inorganic part of the composite material can be matched with that of the polymer.

Inorganic materials which may be useful for making the composite material, include, by way of example, Thailium Halide, GaAs, AgCI, AgBr/CI compounds, CuCI, BaF2, SrF2, Germanium, Thallium, As2Se3, NaCI, Kl, KCI, Csl, Tellurium, Selenium, Sillicon, AgCI, KBr, ZnS, ZnSe, CdT, CdSe, CdS, Tli, CsBr, CdTe, CdS, ThBr, proprietary compounds such as the KRS materials, KrS5, KRS6 for example, which are various solid-solutions including Thallium salts, and As/GeiTe compounds.

The polymer material may be polyethylene or polytetrafluorethylene (P.T.F.E.) for example.

Thus, in one method of making an optical element for an infra-red optical system, a solid solution of AgCI and NaCI is made by mixing the molten com pounds together and the resultant solid is ground to produce a fine powder. The powder, possibly after treatment with a surfactant, is then weil mixed with polyethylene powder and the resultant material moulded to the required shape. The solid solution powder may have a fineness in the Grade G range around one micron. The proportion of organic to inorganic material may be around 50:50. The mould ing of the' polyethylene matrix might be done at a temperature of say 1500C and at low pressure.

The refractive index of polyethylene varies accord ing to the particular mix being used but generally the refractive indices of the two inorganic salts used in the described example will be respectively above and below that of the polyethylene. The refractive index of the solid solution has an intermediate value dependent on the respective proportions of the two salts. Thus, these proportions are chosen, by calculation and/or experiment, so that the resultant refractive index of the solid solution matches that of the particular polyethylene mix used with a view to obtaining optical homogeneity of the final component.

As mentioned earlier, the polymer material could be P.T.F.E., the inorganic material(s) being chosen to suit and the component forming process being adapted accordingly-generally P.T.F.E. is formed by a known but specialised three-stage process including rough forming at very high pressure, then sintering and forming a gel of the material at a temperature around 320"C and finally annealing the formed component.

The polymer may be any suitable halogenated, fluorinated or partially fluorinated hydrocarbon type material preferably having a plasticity of at least 50%.

1. A composite material for use in making infrared transmissive optical components, the material comprising a dispersion of infra-red transmissive inorganic material in a polymeric material.

2. A composite material according to claim 1 comprising, in a polymeric material, a dispersion of at least two inorganic materials in particulate form.

3. A composite material according to claim 2, wherein the combined refractive index of said at least two inorganic materials at least substantially matches that of said polymeric material.

4. A composite material substantially as hereinbefore described.

5. Apparatus including means for sensing or generating infra-red radiation and an optical component for passing said radiation, the optical component comprising composite material according to claim 1,2,3or4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Infra-red transmissive optical components The optical components, for example windows, lenses and soon, of an infra-red optical system are made of heavy and expensive materials such as Germanium and Zinc Sulphide, ordinary glass and piastics material, as used in visible-light optical sys tems, being insufficiently transmissive. According to one aspect of the invention there is provided a composite material for use in making infra-red transmissive optical components, the com posite material comprising a dispersion of infra-red transmissive inorganic material in a polymeric material. The composite material may be made into lenses, windows and such by any suitable forming method. Such components will generally not be as efficient as ones formed of solid l.R. transmissive material but may be much less heavy and expensive and, provided the components of the material are chosen correctly, may be good enough for particular, perhaps many, applications. One measure, which may be taken to improve transmissivity, is to make the thickness of the element as small as possible. Thus lenses made of the composite material are preferably of the Flat-optic kind, ire Fresnel lenses or Kineforms. The composite material may also be used as a cement for bonding together two elements, e.g. lenses of an I.R. optical system. According to a second aspect ofthe invention, there is provided apparatus including means for sensing or generating infra-red radiation, especially far-infra-red radiation such as the 8-14y band, and an optical component for passing such radiation to or from said sensing or generating means, the optical component comprising composite material as described earlier herein. The composite material may comprise, in a polymeric material, a dispersion oftwo or more l.R. transmissive inorganic materials, the two or more inorganic materials preferably being in the form of particles of a solid solution of the materials. By using two or more inorganic materials, the combined refractive index of the inorganic part of the composite material can be matched with that of the polymer. Inorganic materials which may be useful for making the composite material, include, by way of example, Thailium Halide, GaAs, AgCI, AgBr/CI compounds, CuCI, BaF2, SrF2, Germanium, Thallium, As2Se3, NaCI, Kl, KCI, Csl, Tellurium, Selenium, Sillicon, AgCI, KBr, ZnS, ZnSe, CdT, CdSe, CdS, Tli, CsBr, CdTe, CdS, ThBr, proprietary compounds such as the KRS materials, KrS5, KRS6 for example, which are various solid-solutions including Thallium salts, and As/GeiTe compounds. The polymer material may be polyethylene or polytetrafluorethylene (P.T.F.E.) for example. Thus, in one method of making an optical element for an infra-red optical system, a solid solution of AgCI and NaCI is made by mixing the molten com pounds together and the resultant solid is ground to produce a fine powder. The powder, possibly after treatment with a surfactant, is then weil mixed with polyethylene powder and the resultant material moulded to the required shape. The solid solution powder may have a fineness in the Grade G range around one micron. The proportion of organic to inorganic material may be around 50:50. The mould ing of the' polyethylene matrix might be done at a temperature of say 1500C and at low pressure. The refractive index of polyethylene varies accord ing to the particular mix being used but generally the refractive indices of the two inorganic salts used in the described example will be respectively above and below that of the polyethylene. The refractive index of the solid solution has an intermediate value dependent on the respective proportions of the two salts. Thus, these proportions are chosen, by calculation and/or experiment, so that the resultant refractive index of the solid solution matches that of the particular polyethylene mix used with a view to obtaining optical homogeneity of the final component. As mentioned earlier, the polymer material could be P.T.F.E., the inorganic material(s) being chosen to suit and the component forming process being adapted accordingly-generally P.T.F.E. is formed by a known but specialised three-stage process including rough forming at very high pressure, then sintering and forming a gel of the material at a temperature around 320"C and finally annealing the formed component. The polymer may be any suitable halogenated, fluorinated or partially fluorinated hydrocarbon type material preferably having a plasticity of at least 50%. CLAIMS

1. A composite material for use in making infrared transmissive optical components, the material comprising a dispersion of infra-red transmissive inorganic material in a polymeric material.

2. A composite material according to claim 1 comprising, in a polymeric material, a dispersion of at least two inorganic materials in particulate form.

3. A composite material according to claim 2, wherein the combined refractive index of said at least two inorganic materials at least substantially matches that of said polymeric material.

4. A composite material substantially as hereinbefore described.

5. Apparatus including means for sensing or generating infra-red radiation and an optical component for passing said radiation, the optical component comprising composite material according to claim 1,2,3or4.