GB2282236A - Periscope having a convex mirror and a concave cylindrical mirror - Google Patents

Abstract

A periscope, for use in vehicles comprises a convex objective mirror 10 and a concave cylindrical mirror 11, substantially parallel to each other and inclined to an axis through their optical centres, typically at an angle of about 45 degrees. <IMAGE>

Description

VIEWING DEVICE This invention relates to a vehicle viewing system where the driver, due to his position in the vehicle, requires a view in a specific 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 their use diverts attention from the road ahead.

The provision of an adequate field of view is governed by, among other things, the widely different geometries of vehicle interiors in the vicinity of the windscreen/fascia, these factors set a constraint on the size of the viewing mirrors, which limits the angular field available. Efficiency of the viewing apertures then becomes important. Efficient viewing mirrors approximate to rectangular outlines and, as such, have major axes X and Y, substantially at right angles.

It is possible to devise a system using spherical mirrors, which will produce the width of field required, without the need for excessively large mirrors; such a system is described in the co-pending British Patent Application No.9222429.4. A disadvantage of such a system is, 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 curvature and then the field of view is restricted. Methods have been described to reduce this effect, using supplementary optical devices, but these may be inconvenient and interfere with normal vision. It is possible for the driver to suppress one of the images mentally, but the retained image is not always the one required 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 movement of the eye muscles.

The situation may be eased by the use of aspheric mirrors in systems which retain close to unit magnification in both horizontal and vertical planes, this is attributed to the existence of non-parallel optical axes for the eyes, hence these systems need to be used monocularly.

The present invention seeks to overcome this difficulty in the viewing conditions. In its broadest form, the invention relates to a system, one element of which is a cylindrical concave viewing mirror. Hore 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.

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. A second plane perpendicular to ABCD and approximately at 45deg to the optic axis 00' is referred to as the quasi-sagittal plane.

Figure 2 shows a mirror, as used in the invention, in which the optic planes are inclined at some suitable angle to the geometric axes.

Figure 3 shows a mirror, of the type used in the invention, mounted in an adjustable mount, held by its rim in a split ball joint, Figure 4 shows a lateral periscope embodying the invention.

In a particular embodiment,see Fig. 4, the invention comprises a lateral periscope employing a convex objective mirror (10) and a concave viewing mirror (11) of the type shown in Fig.1, arranged so as to be substantially parallel to each other and inclined to an axis joining their optical centres, typically close to 45 degrees. In operation the convex mirror (10) forms an extended spatial virtual image, which is magnified in the vertical direction by the substantially cylindrical mirror (11) viewed by the driver.

Impressions of speed and distance perceived in a system become more reliable as the magnification approaches unity. In a system with magnification m, less than one, the apparent distance of an object seen through the system will be increased in the ratio m-1:l. Thus for m = 0.5 the apparent visual distance is increased by a factor of 2. The mentally appreciated distance may not quite approach this as part of the system visual field should overlap the direct visual field, but it does indicate the desirability of keeping m close to unity. Values of horizontal magnification and vertical magnification are each dependent on the various radii chosen for the optical surfaces.

As it is preferable that the concave cylindrical mirror has substantially zero power in the horizontal plane, the overall horizontal magnification is principally set by the horizontal radius of curvature in the objective mirror. The assessment of distance is better if the two magnifications are approximately the same. It is found that, if both radii of curvature in the vertical plane are the same at about 48 inches, with a typical mirror separation of some 36 inches then the vertical magnification will be close to 0.9, depending a little on the separation between the driver and the viewing mirror. Using this radius in a viewing mirror 2.5 inches x 5.5 inches which is comparable in size to rear view mirrors, a vertical field of about 6deg is achieved which has been found to be satisfactory. The same radius, in conjunction with a 700 inch horizontal radius of curvature in the objective mirror provides a horizontal magnification of approximately 0.8. An objective mirror 3 inches x 9 inches with the long dimension horizontal, provides a lateral field which may just exceed 10 degrees if the dimensions previously mentioned be used.

There is some latitude in the various values which may be used. These figures typically allow the system visual fields to overlap with direct vision, which is important. Changes in the radii of curvature to enhance the field reduce the magnification and vice versa. The vertical radii are not necessarily equal and subject to the previous statement are a compromise affected by the value chosen for the viewing mirror vertical height. Due to the differences in the orthogonal radii the mirrors have directional properties. Under suitable viewing conditions it is possible, with conscious effort to see two images whose vertical displacements may be reduced to an insignificant level by rotating either of the mirrors about its optical axis However, this may lose effective aperture. For this reason the planes of the radii of curvature may be inclined with respect to the geometric axes of the mirrors as a means of enhancing the efficiency of the apertures.

The sizes of the mirrors are related to the radii employed in such a manner that the apertures used are matched for best efficiency. Preferably the mirrors are moulded from shatter resistant plastic for lightness and strength.

Claims (6)

1 A periscope comprising a convex objective mirror and a concave cylindrical mirror arranged so as to be sub stantially parallel to each other and inclined to an axis through their optical centres.

2 A periscope according to Claim 1 wherein the mirrors are inclined to said axis at an angle of substantially

45 degrees.

3 A periscope according to Claims 1 or 2, wherein the ratio of the tangential and quasi-sagittal radii of curvature of the convex mirror is greater than 6.

4 A periscope according to Claims 1,2 or 3, wherein the ratio of the quasi-sagittal radii of curvature of the convex and concave mirrors lies in the range of 0.

5 to 2.0 5 A periscope according to Claims 1,2,3 or 4, wherein the magnification of the system is approximately unity.

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