Pilkington Pe Limited

Michael Roberts

Philip J Rogers

Michael J Tuck

This invention is concerned with a thermal imaging optical system finding utility as an individual weapon sight. The system comprises an infra-red objective lens including a chopper blade together with front and back elements with an adjustable element therebetween. Infra-red radiation is received by the objective lens which also incorporates an internal substantially collimated plane polarized visible light source which illuminates a liquid crystal. The path of the infra-red and visible light which is directed to the liquid crystal cell, is repeatedly interrupted by the chopper blade. Changes in the liquid crystal cell due to the infra-red radiation transmitted by the objective lens are caused to modulate the plane polarized visible light transmitted through the liquid crystal cell to generate a visible image. This image passes through a relay inverting system and is detected by a visible light detector and viewed through an eyepiece.

Thermal imaging optical system

An optically athermalized infra-red lens has a group of a least three closely spaced lens elements of different respective materials having positive thermal coefficients of refractive index. Two of the elements are of materials whose refractive indices are relatively temperature insensitive, one, e.g. of zinc selenide or chalcogenide glass, being positively powered and the other, e.g. of zinc sulphide, being negatively powered to effect achromatization and correct spherical aberration. The third element is negatively powered and of a relatively temperature senitive refractive index material, e.g. germanium, of lower dispersion and higher refractive index than the other two. The lens may be the objective of an afocal telescope having an eyepiece and associated scanner and detector arrangement and may effectively athermalize the whole system.

Infra-red lenses

An infra-red objective zoom lens has a fixed front positive singlet, a movable negative component, a movable positive singlet and a fixed rear component. The zoom lens can be relatively short and can be used with an 'eye-piece' lens to form an infra-red afocal telescope. The system can be further shortened by using a rear group of telephoto form. The fixed front positive singlet is shaped so that at high magnification it gives undercorrected third and fifth order spherical aberration, and the moveable negative component has at least one lens element shaped so that at high magnification the negative component gives compensating overcorrected third and fifth order spherical aberration.

A biocular magnifying lens suitable for use as an eyepiece in a night vision device comprises from front to back a positive element which is bi-convex or meniscus convex towards the front, a negative meniscus element concave towards the front, a positive singlet or doublet element which is convex towards the front, a positive element which is bi-convex or meniscus convex towards the front, and a positive meniscus element convex towards the front.

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Clwyd, United Kingdom

Michael Roberts