GB2271860A

GB2271860A - Aspheric vehicle mirror

Info

Publication number: GB2271860A
Application number: GB9222429A
Authority: GB
Inventors: Ivor Maldwyn Hugh Pagden
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-10-26
Filing date: 1992-10-26
Publication date: 1994-04-27
Anticipated expiration: 2012-10-26
Also published as: GB2271860B; GB9222429D0

Abstract

A forward viewing periscope, typically for use in vehicles, incorporating an adjustable optical system using one or more aspheric mirrors. The optic axes of the mirror are skewed by an angle theta with respect to the geometric axes. The tangential alpha sagittal radii of curvature may be in the ratio 2:1. <IMAGE>

Description

VIEWING DEVICE This invention relates to a vehicle viewing system where the driver, because of his position in the vehicle, requires a view in a specified direction, frequently forward, from another position.

Presently available devices commonly involve periscope type systems using plane mirrors, giving unit magnification. Simple systems require large mirrors to provide a sufficient field of view. Designs avoiding large plane mirrors compromise, for example, with a location significantly offset from the driver, such that use diverts attention from the road ahead.

The provision of an adequate field of view is governed by, among other things, the widely differing geometries of vehicle interiors in the vicinity of the windscreen/fascia. These factors set a constraint on the size of the viewing mirror which, in a system with unit magnification, limits the angular field available. Efficiency of the viewing aperture is important.

Efficient viewing mirrors approximate to rectangular outline, though they may have rounded corners and gently radiused edges, as such, they have major and minor geometric axes, X and Y, substantially at right angles.

It is possible to devise a system using spherical mirrors which will produce the wide field of view required without the need for excessively large mirrors. It is known that in periscopes of this type two images may be perceived, one for each eye. The images do not fuse unless the mirrors have only very weak curvatures and then the field of view is restricted. Methods have been described to reduce this effect, using supplementary optical devices. but these are inconvenient and may interfere with normal vision. It is often possible for the driver to suppress one of the images mentally, but the retained image is not always the required one and it is then necessary to try again.

The two images may be separated vertically and horizontally.

Vertically separated images are much more difficult to fuse due to the behaviour of the human visual system. For an erect observer, the muscles of each eye operate almost identically when locating vertical targets, but in a slightly dissimilar manner for horizontal targets, so that vertical fusion would involve unnatural movements of the eye muscles.

The present invention eases these problems providing a near optimised wide laterally displaced view, without recourse to large components or the need for distracting head movements.

In its broadest form the invention relates to a system using an aspheric mirror in which the optic and geometric axes are skewed with respect to each other. More particularly the invention is defined by the independent claims, to which reference should now be made.

The invention will now be described by way of example and with reference to the attached drawings. In a particular embodiment, the invention comprises a periscope incorporating one or more of the mirrors described above.

Figure 1 is a representation of an aspheric optical surface, as used in the invention where the optical axes oo' and o o" are substantially at 45 deg to the mean optical surface and the plane ABCD is the tangential section.

Figure 2 shows the appearance, as seen through a conventional periscope, of the objective mirror in the viewing mirror if the optic and geometric axes of the aspheric mirror be aligned and the mirrors adjusted to eliminate the vertical offset. The effective viewing aperture is severely reduced.

Figure 3 shows a mirror, as used in the invention, in which the optic planes are inclined at some suitable angle e to the geometric axes, X and Y.

Figure 4 shows the corresponding view to Fig2 but using a mirror of the type shown in Fig3. The viewing aperture is optimised.

Figure 5 shows a mirror,of the type used in the invention, mounted in an adjustable mount, held by its rim in a split ball joint.

In a particular embodiment, the invention comprises a lateral periscope employing a convex objective mirror and a concave viewing mirror, arranged so as to be substantially parallel to each other and inclined to an axis joining their optical centres.

typically close to 45 deg. To avoid misleading impressions of speed and distance, the system should have near unity magnification. This requirement is usually met when both optical elements have powers close in magnitude. For private motor vehicles the greater radius of curvature employed is in the vicinity of 10 inches. The mirrors are made severely aspheric as in FigS, that is, the surface form is such that the tangential radius of curvature is markedly different from the sagittal radius of curvature. Due to the difference in radii, the mirrors have directional properties so that by rotating a mirror about its optical axis, the two perceived images may have their displacements altered. In particular, the vertical displacement may be set to an insignificant level.

If the optic planes of the mirrors be aligned with the geometric axes, then the appearance of the objective mirror in the viewing mirror, when adjusted for minimum vertical separation, would be typically as shown in Fig2. In these circumstances, the useful viewing aperture is severely reduced.

When the tangential and sagittal planes of the aspheric mirror are inclined at some angle e to the geometric axes as shown in Fig3, then the appearance in the viewing mirror. with a suitable choice of e, would be as shown in Fig4. The useful viewing aperture is considerably improved over Fig 2. Typically, the value of e would lie between lOdeg and 15deg.

The most suitable viewing aperture is thought to have upper and lower field limits horizontal, with an image that has had minimal rotation about the optic axis. As the optic axis changes direction after each reflection, different angles ei 42 may be required for each mirror. The angle h may have opposite sense to that shown in order to achieve optimum performance.

In a preferred embodiment, the mirrors are mounted at an angle of about 45 deg to the axis joining their optic centres and their tangential and sagittal powers along the axis cf reflection are substantially equal. For small mirrors and an angle of exactly 45 deg, the two radii of curvature would be in the ratio 2:1. Ideally the profiles of the mirror in the two planes would be parts of conic sections other than circular, however, circular profiles are sufficient for the invention.

The required orientations of the mirrors may be secured in an arrangement where the mirrors are held in a lockable-ball. or split-ball, joint, attached to some suitable surface of the vehicle interior. Fig 5 shows a section through a typical arrangement comprising a mirror (1) held by its rim (2) in a split ball joint (3 & ).

Claims

1 An aspheric mirror whose optic and geometric axes are skewed with respect to each other.

2 A periscope incorporating one or more mirrors according to Claim 1, comprising a convex objective mirror and a concave viewing mirror arranged so as to be substant ially parallel to each other and inclined to an axis through their optical centres.

3 A periscope according to Claim 2 wherein the mirrors are inclined to said axis at an angle of substantially

45 deg.

4 A periscope according to Claim 2 or 3 wherein the tangential and sagittal powers of at least one of the mirrors are substantially the same.

5 A periscope according to Claim 2,3 or 4 wherein the mean powers of the mirrors lie within 10% of each other.

6 A mirror according to Claim 1 wherein the tangential and sagittal surface profiles are both circular.

7 A mirror according to Claims 1 and 6 wherein the ratio of the tangential and sagittal radii of curvature lies between 1.5 and 2.5.

8 A periscope according to Claim 2,3,4 or 5 incorporating one or more mirrors according to Claim 6.

9 A periscope according to Claim 2,3,4 or 5 incorporating one or more mirrors according to Claim 7.

10 A mirror substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11 A periscope substantially as hereinbefore described with reference to and as illustrated in the accompany ing drawings.