Collimation lens system

Claim 2

An eyepiece or collimation lens system formed by three optically-powered lens elements aligned on a common optical axis and arranged to accept radiation in the infrared wavelength region from a real image and to provide a bundle of parallel rays at an exit pupil, each said lens element being positively powered and made of a material which has a useful spectral bandpass in the infrared wavelength region, the refractive surface of the lens element adjacent said real image being flat and the five other refractive surfaces of said lens elements being substantially spherical.

Claim 3

A system as claimed in claim 1, including a graticule mounted on or adjacent said flat refractive surface.

Claim 4

A system as claimed in claim 1, wherein one or more of said refractive surfaces is anti-reflection coated.

Claim 5

A system as claimed in claim 1, wherein the two lens elements adjacent said exit pupil have refractive indices of not less than 4.0 at a wavelength of 10 microns and a temperature of 20° C.

Claim 6

A system as claimed in claim 1, wherein said lens element adjacent the real image is made of Zinc Selenide and the remaining lens elements of said system are made of Germanium.

Claim 7

A system as claimed in claim 1, wherein at least one of said lens elements is movable along the optical axis with respect to at least one other of said lens elements whereby the system can be compensated for large ambient temperature variations.

Claim 8

A system as claimed in claim 1, wherein the radius of curvature of each refractive surface is given together with the aperture diameter of each surface and of the exit pupil the position of which exit pupil is used as a datum from which the separation of successive refractive surfaces is defined, together with the nature of the material relevant to each such separation interval in the following table: ______________________________________ Separa- Radius of Aperture Item Surface tion Curvature Material Diameter ______________________________________ Exit -- -- Flat Air 15.50 pupil 1st lens 1st 46.63 -254.02 Air 55.54 element surface 2nd 8.05 -117.35 Ge 57.74 surface 2nd lens 1st 30.30 54.61 Air 56.12 element surface 2nd 22.91 38.10 Ge 38.23 surface 3rd lens 1st 12.92 113.54 Air 36.96 element surface 2nd 3.50 Flat ZnSe 36.56 surface ______________________________________ wherein numerical data is given in millimeters and negative radius of curvature denotes that the centre of curvature is on the exit pupil side of the surface whereas positive radius of curvature denotes that the centre of curvature is on the real image side of the surface.