The present invention relates to a beam splitter block and in particular to an image combiner using such a beam splitter block. Further, the present invention relates to night vision apparatus utilising such an image combiner.

Night vision apparatus which is provided integrally with a helmet provides an enhanced luminosity image of a remote scene superimposed on the image on the direct line of sight from the eye of the wearer of the helmet and the remote scene. The enhanced luminosity image is generally superimposed using a beam splitter which is placed in front of the eye. Such an arrangement is disclosed in EP0134116.

For military applications, to allow space for the donning of protective NBC (Nuclear Biological Chemical) suits, the minimum clearance between the eye and the beam splitter must be at least 22mm. In view of this spacing, the field of straight line vision of the eye is limited by the field of view through the beam splitter. Generally, it is not possible to increase the size of the beam splitter in order to provide a wider field of view since this increases the weight of the night vision apparatus.

It is therefore an object of the present invention to provide a beam splitter for use in an image combiner such as night vision apparatus and which can provide a wider field of view than conventional beam splitters.

The present invention provides a beam splitter block formed of an optical material having a refractive index of at least 1.8, and preferably at least 2.

The present invention also provides an image combiner comprising optical means defining a direct view line of sight which is substantially free of optical aberrations and which includes a partially reflective surface on said line of sight to superimpose on said line

of sight an image of a predetermined object located off the line cf sight, wherein said optical means comprises a beam splitter block formed of an optical material having a refractive index of at least 1.8, and preferably of at least 2.

In an embodiment of the present invention the image combiner includes a further reflective surface located off said line of sight to direct light from said object to said partially reflective surface so that light from said object is reflected twice only to produce said superimposed image.

In another embodiment of the present invention the partially reflective surface and the further reflective surface are angled so that a centre line from the object to the further reflective surface is parallel to the line of sight. Preferably the partially reflective surface is at 45° to the line of sight and the further refractive surface is at 45° to the centre line.

Conveniently, the further reflective surface comprises an internal surface of a prism, which is preferably formed of an optical material having a refractive index of at least 1.8.

In one embodiment of the present invention the image combiner further includes imaging means comprising at least one refractive optical element having positive power to present the superimposed image substantially at infinity. Preferably the imaging means includes a positive power optical element located externally and adjacent to a surface of said beam splitter block, wherein the optical element has a refractive index of at least 1.75. In one embodiment said positive power optical element comprises a first planar concave element formed of a material having a refractive index of at least 1.9 and a dispersion coefficient of greater than 20, and a second convex element formed of a material having a refractive

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index of at least 1.75 and a disperson coefficient of greater than 55. Also, preferably the imaging means includes additional refractive optical elements so as to substantially eliminate chromatic aberration in the superimposed ir.age.

The present invention also provides night vision apparatus comprising means presenting at an image plane an enhanced luminosity image of a remote scene along a line of sight, said image plane being located off said line of sight, which includes an image combiner as hereinbefore described to superimpose said image on the direct view line of sight.

An example of the present invention will now be described with reference to the accompanying drawing which is a schematic representation of an image combiner combining the image from an image intensifier tube with the image on a direct line of sight.

In the drawing there is a direct line of sight 1 from the eye 2 to a remote scene. The image of the remote scene observed by the eye is through an optical beam splitter block 3 which has a partially reflective surface 4 at 45° to the line of sight 1.

An image intensifier tube 5 is provided at a position off the line of sight 1, which views a remote scene on a centre line 6, which is parallel to the line of sight 1. The image intensifier tube 5 provides an enhanced luminosity image which is transmitted through imaging means comprising lenses 7, 8 and 9 to a prism 10. The enhanced luminosity image is transmitted to an internal face of prism 10 which has a reflective surface 11 lying at 45 to the centre line 6. The reflected enhanced luminosity image is then transmitted through lenses 12 and 13 to the partially reflective surface 4 in the beam splitter block 3 such that the enhanced luminosity image arrives at the partially reflective

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surface 4 at 5 ^w and upon reflection is superimposed on the line of sight 1 to provide a combined line of sight and enhanced luminosity image at the eye 2.

Since the partially reflective surface 4 in the beam splitter block 3 and the reflective surface 11 of the prism 10 are parallel and both are at 45° to the line of sight 1, the centre line 6 of the enhanced luminosity image is parallel to the line of sight 1.

The beam splitter block 3 is formed of a material having a high refractive index n, at 587nm which is at least 1.8. For example it can be formed of LaSF35 available from Schott which has a refractive index of 2.03. Alternatively, SF59 can be used which has a refractive index n, of 1.964 at 587nm. The upper surface 4a of the beam splitter block 3 is convex to provide power. The high refractive index of the material forming the beam splitter block enables the radius of curvature to be small to provide the required power thus keeping the overall dimensions of the arrangement small. Although in the drawing the beam splitter block 3 is illustrated as including a convex upper surface 4a, alternatively the upper surface could be planar and a separate planar convex optical element used.

The prism 10 is formed of lightweight glass having a specific gravity of 3.53 and a refractive index n, of 1.855. Such a material is SF57 available from Schott. The use of this type of prism allows the image to be projected down from a flat surface.

The eye 2 is arranged to be at least 22mm from the beam splitter block 3 and the lenses 7, 8, 9, 12 and 13 are arranged to present the enhanced luminosity image substantially at infinity when superimposed on the line of sight 1. Lens elements 7, 8 and 9 are designed to control coma, astigmatism and distortion and control the Petzval sum to match the curved output window of the image

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intensifier tube. The distortion is matched to a suitable objective lens. In order to control chromatic aberration and accomplish this with minimum optical material thickness, so that the vertical distance from the eyeline of sight to the tube line of sight is kept to a minimum, lens 13 is formed from a low dispersion high index crown such as LaF21 whose refractive index n, is greater than 1.75 and whose dispersion coefficient v, is greater than 40 and lens 12 from a very high dispersion high index flint, e.g. SF59 whose refractive index n, is greater than 1.9 and whose dispersion coefficient v. is less than 21.

The use of high refractive index material for the beam splitter block material provides for greater refraction of light entering the beam splitter from the remote scene and thus provides the eye 2 with a wider field of view than would be available from a beam splitter block made of material of lower refractive index. Further, the use of the beam splitter block as described provides an on axis arrangement which allows for even control of aberrations, which is not possible in the arrangement disclosed in EP0134116.

The image combiner illustrated in the drawing is ideally suited for use in night vision apparatus such as night vision goggles wherein an enhanced luminosity image is provided by the image intensifier tube 5 and superimposed on the direct view line of sight 1. The simple construction utilising only two reflections from the reflective surface 11 and the partially reflective surface 4, provides a lightweight arrangement which can interface directly with ANVIS (Aviators Night Vision Imaging System) tube. The use of lightweight high refractive index optical components greatly reduces not only the weight but also the dimensions of the optical elements.

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It is believed that the arrangement for incorporating the image combiner in night vision apparatus would be evident to a skilled person in the art. Two image combiners, one per eye can be mounted for attachment to a user's helmet in a similar manner to that disclosed in EP0134116, the disclosure of which is hereby incorporated by reference.

The image combiner provides a 40 field of view and images 40° onto 18mm format of the standard ANVIS tube. Within the compact volume of the image combiner it is possible to achieve a 40 field of view with a greater than 22mm eye-relief.

By using the described optical arrangement the current ANVIS image intensifier tube can be used unmodified thus providing cost benefits. Further, the optical arrangement described hereinabove matches the distortion of the current ANVIS objective. This objective has barrel distortion of 8% and the combiner eye piece has matching pin cushion distortion of 8%. Thus by suitable design any standard goggle can be easily upgraded to a combiner system.

Thus the present invention provides a beam splitter block which has a refractive index at 587.6nm of at least 1.8 which can be used in an image combiner for combining an enhanced luminosity image with a direct view line of sight image in night vision apparatus. The optical arrangement to provide such a combined image is both lightweight and simple and is substantially free of chromatic aberration.