GB2049118A - Improved collimator gun sight - Google Patents

Abstract

A collimator gun sight has a partially reflecting concave surface (18) which is axially symmetrical and a reflecting graticule (23) located on the optical axis (19) so that an image of the pattern can be seen at infinity superimposed on the target when viewed along the optical axis. The sight can be formed as a solid glass block (10) with a prism (24) cemented to one side to deflect light from above the target onto the graticule. <IMAGE>

Description

SPECIFICATION Improved collimator gun sight The present invention relates to collimator gun sights. With such collimator sights, light from a suitably illuminated graticule pattern or other aiming mark is collimated for presentation to an eye of the user of the weapon as an image of the aiming mark at infinity, or at a predetermined distance where appropriate. In one arrangement, the other eye of the user is free to view the target so that, when the brightness levels of the aiming mark and the target are correctly balanced, the user will perceive the aming point superimposed on the viewed target. It is also known to provide various arrangements for presenting both the collimated light from the image of the aiming point and light from the target to one or both eyes of the user simultaneously.A simple arrangement for doing this which is known is to provide a partially reflecting concave mirror suitably placed to reflect back to the aiming eye of the user of the weapon light from means defining the graticule pattern. If the distance between the concave mirror and the graticule pattern corresponds to the focal length of the concave mirror, the image seen in the concave mirror will appear to be substantially at infinity. The user of the weapon can then view the target field through the partially reflecting mirror whilst simultaneously seeing the image of the graticule pattern at infinity in the mirror. It is usual to ensure that the partially reflecting mirror provides substantially no deflection to light passing through the mirror from the target field so as to minimise any distortion of the target as seen by the user.Thus, it is known to employ a parallel faced meniscus glass for the partially reflecting mirror with one surface of the meniscus glass suitably coated to provide the reflecting surface.

It is also known to form a collimator sight as a block or rod of optically transparent material with the partially reflecting mirror formed at one end of the rod. The means defining the graticule pattern is embedded in the rod so that the light path from the graticule pattern to the partially reflecting surface is entirely within the material of the rod. An example of this type of sight is disclosed in United States Patent Specification No.

3,942,901.

However, in the above mentioned United States Patent Specification, and in all other arrangements known to the Applicants, the sight is made with the graticule pattern defining means displaced off the line of sight through the partially reflecting surface. In known arrangements where the partially reflecting surface is symmetrical about its opti cal axis, the gun sight is arranged so that the line of sight is angled relative to the optical axis so that the graticule pattern defining means can be placed off the line of sight. In arrangements such as that disclosed in the above referred U.S.Patent Specification, the partially reflecting surface is not symmetrical about its optical axis so that, althugh the graticule defining means can be placed on the optical axis, the line of sight, i.e. the line substantially through the centre of the field of the partially reflecting surface, is spaced parallel to the optical axis.

It will be appreciated that such arrangements with the graticule pattern defining means spaced away from the line of sight are apparently desirable so that the graticule pattern defining means does not obscure the view through the sight both of the target and of the image of the graticule pattern.

According to the present invention, a collimator gun sight comprises means defining a partially reflecting transparent concave surface which is substantially symmetrical about its optical axis, means defining a reflecting graticule pattern on a transparent field, means for illuminating said pattern, and means locating said pattern on said optical axis so that an image of the illuminated pattern, forming an aiming mark, can be seen reflected in the partially reflecting surface superimposed on a target viewed through the surface substantially along said optical axis, the pattern being sufficiently fine so as not to obscure said image or the target when viewed with an eye positioned sufficiently close to the pattern defining means.The invention stems from the realisation, that, if the eye of the user of the gun sight is placed sufficiently close to the pattern defining means, this pattern defining means does not obscure the view of the target or of the graticule pattern image even though the defining means is in fact located directly in the line of sight. The gun sight of the present invention can thus be formed with the concave surface symmetrical about its optical axis but with the graticule pattern defining means located on the optical axis. Of course, the graticule pattern defining means provides a graticule pattern on a transparent field so that only the elements of the graticule pattern itself could potentially obscure the view through the sight.It is much easier and cheaper to form a partially reflecting concave surface which is symmetrical about its optical axis, than an asymmetrical surface such as that used in the arrangement disclosed in the above referred U.S. Patent Specification. Further, by locating the graticule pattern defining means on the optical axis, the problem of the various aberrations in the reflected image of the graticule is much alleviated. Also the aberration problem is reduced compared with the prior art arrangement using an asymmetri cal concave surface, because, for the same aperture of partially reflecting surface, the present invention employs only portions of the reflecting surface relatively close to the optical axis. Because aberrations can be reduced with the present invention, the resulting sight can be made shorter for the same errors.

In summary, a sight in accordance with the present invention can be made smaller, lighter and shorter than those known previously.

It should be understood that the sight of the present invention is designed to be used with the graticule pattern defining means relatively close to the eye. For gun sights, a typical graticule pattern appears when viewed through the gun sight to have a line thickness subtending an angle of 0.001 radians to the eye. When the collimator sight is arranged to present the image of the graticule pattern at infinity, i.e. when the graticule pattern is located substantially at the focal point of the concave partially reflecting surface, then the actual thickness of the lines of the graticule pattern in the pattern defining means should be approximately equal to f X 10-3 where f is the focal length of the concave surface.A typical focal length of a concave surface used in the sight of the present invention is about 20 mm, so that the actual thickness of the lines of the graticule pattern in such a sight should be about 0.02 mm. It will be appreciated that lines of such thickness would be quite unnoticeable, even though lying on the line of sight through the gun sight, when viewed with the eye relatively close to the gun sight so that the distance from the graticule pattern defining means is only a few centimetres.

The graticule pattern is made reflecting so as to reflect light incident upon it back into the aperture of the partially reflecting concave surface. It will be appreciated that, when the sight is mounted in an opaque tube, light from the target field entering the sight through the partially reflecting surface will normally be inadequate to illuminate the graticule pattern sufficiently for it to be seen reflected in the concave surface. Accordingly, normally, said means for illuminating the graticule pattern is located clear of the field of view along the optical axis through the partially reflecting surface and is arranged to direct illuminating light towards said reflecting graticule pattern from off said optical axis.

Then, said graticule pattern defining means may comprise a reflecting surface or surfaces acutely angled relative to said optical axis to reflect said illuminating light towards said partially reflecting surface.

The reflecting surface or surfaces of the graticule pattern may lie in a single plane or the surfaces may lie in a set of parallel planes and said means for illuminating may then be arranged to direct illuminating light towards said reflecting surface or surfaces substantially parallel to a plane containing said optical axis and perpendicular to the plane or planes of said surface or surfaces. Then, the graticule pattern can be illuminated from a source of illuminating light located to one side of the line of sight through the gun sight and the reflecting surface or surfaces of the pattern is/are arranged to make optimum use of the illuminating light falling upon it/them by specularly reflecting this light into the aperture of the concave partially reflecting surface.

In a preferred arrangement, the gun sight is formed as an elongate solid block of optically transparent material, the block having said partially reflecting surface defining means at one end with the optical axis extending from the concave surface along the length of the block, the block being no shorter than the focal length in air of the partially reflecting concave surface, and the block or a portion thereof constituting said graticule pattern locating means. Forming the sight as a solid elongate block of material, such as glass or transparent plastics, provides an extremely convenient arrangement for constructing the gun sight and ensuring that the graticule pattern defining means is located rigidly on the optical axis of the partially reflecting concave surface, and in particular at the focal point of the surface if the image of the graticule pattern is to appear at infinity.The partially reflecting concave surface may be embedded in said block adjacent said one end and the block may then be formed at each end with optically plane end faces perpendicular to the optical axis of the concave surface.

Thus, the entire sight is made substantially optically neutral sofar as concerns the target field viewed through the sight. The block may have adjacent said other end a plane cut across the length of the block at an acute angle to said optical axis, and the reflecting surface or surfaces of the graticule pattern may then be provided in the plane of said cut, the two portions of the block formed by the cut being cemented together. Thus, the graticule pattern defining means may be applied as a silver coating to one of the abutting surfaces of the cut through the block before the two portions are subsequently cemented together so that the graticule pattern defining means is embedded in the body of the block.

The means for illuminating the graticule pattern may then comprise a prism cemented to a side face of the block between the partially reflecting surface and the graticule pattern defining means, the prism being arranged to deflect ambient light from a predetermined sector relative to said optical axis into said block through the side face towards the reflecting graticule pattern so as to be reflected thereby into the partially reflecting concave surface. The predetermined sector may be selected so that, when the sight is used on a suitable weapon, the graticule pattern is illuminated by ambient light from just above target. This is especially convenient so that the illumination level of the graticule pattern is varied automatically with the ambient light level and in particular with the brightness of the target.

An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a view in elevation from one side of the optical elements of a collimator gun sight embodying the present invention; Figure 2 is a corresponding view from the left hand end of the sight of Fig. 1 and Figure 3 illustrates a typical graticule pattern for the gun sight of Figs. 1 and 2.

Referring to the drawings, the sight is formed as a solid block of optically transparent material, such as a suitable glass. Although the sight, when assembled as shown in Fig. 1, is a single block, the block is formed of several parts cemented together.

Thus, the block comprises an elongate bar 10 of square cross-section as shown in Fig. 2.

The bar 10 is itself divided by a plane cut 11 into a first portion 1 2 and second portion 1 3.

A partially reflecting doublet optical element 1 5 is cemented centrally to the left hand end face 14 of the bar 10. The doublet element 1 5 comprises a plano concave lens 1 6 and a plano convex lens 1 7 which are cemented together at their curved faces which are substantially spherical and of the same radius. A partially reflecting layer 1 8 is provided between the two lenses 1 6 and 1 7 so as to form a partially reflecting concave mirror surface when viewed from the right hand side of Fig.

1. Various materials may be employed to provide the partially reflecting layer 1 8. For example, one of the curved surfaces of the lenses 1 6 and 1 7 may be coated with for example, silver, aluminium, gold, copper or chrome. The choice of coating material is dependent on the colours required to be preferentially reflected and transmitted.

The partially reflecting concave surface formed by the layer 1 8 is symmetrical about its optical axis, indicated by the dotted line 1 9 in Fig. 1, and is arranged so that the optical axis 1 9 extends substantially centrally along the axis of the bar 10.As shown in Fig. 1, the bar 10 has end faces 14 and 20 which are perpendicular to the axis 1 9 and the doublet element 1 5 is formed with its outer plane faces parallel to one another so that, when the element 1 5 is cemented to the end face 14 of the bar 10, the exposed plane face 21 of the element 1 5 is perpendicular to the opposite face 20 of the bar 1 0. Thus, the entire block with the element 1 5 at one end is substantially optically neutral when viewed through the length of the sight, for example by an eye 22 located to the right of the sight in Fig. 1.

The partially reflecting surface formed by the layer 1 8 is arranged to have a radius curvature which is less than twice the length of the bar 10 so that the focal length in air of the concave surface is less than the length of the bar. In the present example, the plane cut 11 through the bar 10 intersects the optical axis 1 9 of the concave surface at the focal point 23 of the concave surface. A graticule for the signai is located centred at the focal point 23 in the cut 11 between the portions 1 2 and 1 3 of the bar 10.Conveniently, the graticule is formed by applying a silvered layer in the form of the graticule pattern, to the surface of the portion 1 2 of the bar 10 before the two portions 1 2 and 1 3 are cemented together. The graticule pattern is therefore formed of a reflecting surface or surfaces which can reflect light illuminating the graticule from the left of the focal point 23 in Fig. 1, towards the concave surface formed by the layer 1 8. The reflecting surfaces of the graticule pattern lie in the plane of the cut 11 and the cut 11 is orientated so as to reflect towards the partially reflecting concave surface illuminating light provided from a prism 24 which is cemented to a top surface 25 of the bar 10.Prism 24 has an upper light collecting surface 26 which is angled relative to the surface 25 of the bar 10 so as to deflect ambient light entering the surface 26 from the left in Fig. 1, downwards through the prism and into the bar 10 through the surface 25. As shown in the figure, the plane of the cut 11 is perpendicular to the side faces 27 and 28 of the bar 10 (Fig. 2). Similarly, the face 26 of the prism 24 is perpendicular to the side faces 27 and 28 so that the ambient light deflected by the prism 24 into the bar 10 is directed generally parallel to a plane containing the optical axis 1 9 and perpendicular to the plane of the cut 11.

The angle of the cut 11 to the optical axis 19, the angle of the face 26 of the prism 24 to the top surface 25 of the bar 10 and the position of the prism 24 along the length of the bar 10 between the focal point 23 and the doublet element 1 5 are selected so that the illuminating light, reflected by the reflecting surfaces of the graticule pattern substantially parallel to the optical axis 1 9 towards the partially reflecting concave surface formed by the layer 18, comprises light entering the surface 26 of the prism 24 from a sector which is slightly above the target field viewed through the sight by the eye 22. In this way the brightness of the graticule pattern is automatically adjusted in accordance with the brightness of the target.

In use, the sight is enclosed in an opaque tube with the prism 24 protruding through one side of the tube so that the surface 26 is exposed to ambient light. The tube may form part of a mechanical mounting arrangement for mounting the sight on a gun or other weapon and the mounting arrangement may provide for zeroing of the sight in the usual way. No further details of the mounting ar rangement are given herein because it forms no part of the present invention.

When using the sight, the aimer of the weapon places his eye 22 relatively close to the end face 20 of the bar 10. The user then views the target through the length of the block i.e. the bar 10 and element 15, and can see superimposed on the target field, an image of the graticule pattern reflected in the partially reflecting surface formed by the layer 18.

Fig. 3 illustrates a typical graticule pattern which may be used in the sight of Figs. 1 and 2 comprising a simple circle. The graticule pattern is formed in the plane of the cut 11 and therefore in order for the image of the pattern reflected in the concave reflecting surface of the layer 1 8 to appear as a circle, the actual pattern in the plane of the cut 11 should be somewhat elongated in the vertical direction to account for the angle of the cut to the optical axis 19. The pattern is formed.as a silvered reflecting surface and the image of the pattern seen by the eye 22 appears as a bright circle superimposed on the target field.

The sight may be made very small and in one example the overall length from the surface 20 to the surface 21 is about 32 mm and the bar 10 has a square cross-section of side 8 mm with the doublet element 1 5 being circular with a diameter of slightly less than 8 mm. The partially reflecting concave surface formed by the layer 18 has a radius of curvature of 54 mm so that the focal point 23 is 27 mm from the optical centre of the layer 1 8. The actual graticule pattern at the focal point 23 is extremely fine. For the example of Fig. 3, the circle has an outer diameter of 0.142 mm providing a line thickness of .01775 mm. Typically the eye 22 is placed only about 5 cm behind the surface 20 of the sight and at this distance the graticule pattern at the focal point 23 is quite invisible and in no way obscures the view through the sight.

However, it is straightforward to calculate that the graticule pattern of Fig. 3 with these dimensions produces reflected in the layer 18, an image at infinity having a diameter which subtends 6 milliradians at the eye 22, which provides an aiming mark of a useful size. It will be appreciated that the size of the image of the aiming mark depends on the refractive index of the material of the block. Preferably, all the transparent components of the sight, i.e. the two portions 1 2 and 1 3 of the bar 10, the doublet element 1 5 and the prism 24 are formed of the same material with the same refractive index. In the dimensioned example above, the refractive index is taken to be about 1.52 in order to produce an aiming mark having an outer diameter of about 6 milliradians.

By forming the sight as a solid block of transparent material as described the effective magnification of the graticule pattern is increased by the factor of the refractive index of the material of the sight. This permits the actual graticule pattern at the focal point 23 to be made that much smaller and finer for the same size of image.

The angle of the cut 11 to the optical axis 1 9 is not critical. However, the angle should not be so small that the vertical extend of the graticule pattern at the focal point 23 causes it to extend significantly on either side, along the optical axis 19, of the actual focal point.

This would result in the upper and lower limits of the image of the graticule pattern viewed by the eye 22 to appear to be out of focus. Provided the graticule pattern has only a very Small extent, then the vertical defocusing of the image is negligible and an angle between the cut 11 and the optical axis of about 67.5 degrees has been used successfully. The prism 24 is then located and arranged to direct light appropriately to the graticule surface to be reflected into the aperture of the partially reflecting surface. With the prism located as shown in Fig. 1, it has been found convenient to form the surface 26 Of the prism at an angle of 1 5 degrees to the optical axis.

If however, it is desired to form the graticule pattern with a greater vertical extent, for example in order to provide for greater elevation on the graticule itself, then it may be necessary to increase the angle of the cut 11 to the optical axis so that it is more nearly normal to the axis. In which case, it will be necessary to form the sight with the prism 24 positioned further along the length of the bar 10 towards the doublet element 1 5 and the angle of the surface 26 should be altered accordingly to ensure that ambient light from the desired sector is deflected towards the graticule pattern at the required angle. It my also be necessary to make the prism rather larger to ensure that the required amount of illuminating light reaches the graticule pattern.

Claims (9)

1. A collimator gun sight comprising means defining a partially reflecting transparent concave surface being substantially symmetrical about its optical axis, means defining a reflecting graticule pattern on a transparent field, means for illuminating said pattern, and means locating said pattern on said optical axis so that an image of the illuminated pattern, forming an aiming mark, can be seen reflected in the partially reflecting surface superimposed on a target viewed through the surface substantially along said optical axis, the pattern being sufficiently fine so as not to obscure said image or the target when viewed with an eye sufficiently close to the pattern defining means.

2. A collimator gun sight as claimed in claim 1, wherein said means for illuminating is located clear of the field of view along the optical axis through the partially reflecting surface and is arranged to direct illuminating light towards said reflecting graticule pattern from off said optical axis.

3. A collimator gun sight as claimed in claim 2 wherein said graticule pattern defining means comprises a reflecting surface or surfaces acutely angled relative to said optical axis to reflect said illuminating light towards said partially reflecting surface.

4. A collimator gun sight as claimed in claim 3, wherein the reflecting surface or surfaces lie in a single plane or the surfaces lie in a set of parallel planes and said means for illuminating is arranged to cirect illuminating light towards said reflecting surface or surfaces substantially parallel to a plane containing said optical axis and perpendicular to the plane or planes of said surface of surfaces.

5. A collimator gun sight as claimed in any preceding claim, and formed as an elongate solid block of transparent material, the block having said partially reflecting surface defining means at one end with the optical axis extending from the concave surface along the length of the block, the block being no shorter than the focal length in air of the partially reflecting concave surface, and the block or a portion thereof constituting said graticule pattern locating means.

6. A collimator gun sight as claimed in claim 5, wherein the partially reflecting concave surface is embedded in said block adjacent said one end and the block is formed at each end with optically plane end faces perpendicular to the optical axis of the concave surface.

7. A collimator gun sight as claimed in either of claims 5 or 6 as appendent to claim 4, wherein the block has adjacent said other end a plane cut across the length of the block at an acute angle to said optical axis, and the reflecting surface or surfaces of the graticule pattern are provided in the plane of said cut, the two portions of the block formed by the cut being cemented together.

8. A collimator gun sight as claimed in claim 7 wherein said means for illuminating comprises a prism cemented to a side face of the block between the partially reflecting surface and the graticule pattern defining means, the prism being arranged to deflect ambient light from a predetermined sector relative to said optical axis into said block through the side face towards the reflecting graticule pattern so as to be reflected thereby into the partially reflecting concave surface.

9. A collimator gun sight as claimed in claim 8, wherein said predetermined sector is selected so that, when the sight is used, the graticule pattern is illuminated by ambient light from just above the target.

1 0. A collimator gun sight substantially as herein before described with reference to and as illustrated in the accompanying drawings.