GB2111713A - Improvements in mirrors and devices incorporating mirrors - Google Patents

Abstract

A convex mirror wherein the convexity of the mirror is such that a cross-section of the mirror across the major dimension thereof is in the form of a segment of a circle in which the ratio of the major chord length to the depth of the segment is in the range of 6:1 to 3.75:1. Where such a mirror is used at a T junction or other location where it is mounted flat on a vertical wall, an observer approaching the mirror from ahead will be afforded a clear view to both sides of the mirror without undue distortion and without undue inclusion in the reflected image of unwanted views. A device for inspecting inaccessible locations is also provided, comprising an upwardly- facing convex mirror 15 mounted on wheels 17, 20 and provided with a handle 21. <IMAGE>

Description

SPECIFICATION Improvements in mirrors and devices incorporating mirrors The invention relates to mirrors and devices incorporating mirrors and, in particular, to convex mirrors for providing a wide field of view. The wide field of view provided by a convex mirror may be required for security purposes, for example the mirror may be disposed at a fixed location in a shop or similar premises to permit observation of parts of the premises which would otherwise be hidden. Thus such mirrors may enable shop assistants to observe customers and thus guard against shoplifting. Alternatively the convex mirror may require to be portable, being mounted on a device permitting ready inspection, where required, of locations which are not otherwise readily accessible.

The wide field of view provided by a fixed convex mirror may, for example, be required at a corridor or road junction to enable persons approaching the junction to see whether any other vehicle or person is approaching the junction from another direction. In the case of a simple angled junction between two roads or corridors, the mirror may be placed symmetrically across the apex of the junction, and such an arrangement can provide an adequate field of view from each road or corridor into the other. A particular problem may arise, however, at a T junction where a person approaching the junction from the leg of the T wishes to have a clear view along both arms of the T. Hitherto, such a requirement has been met by providing two convex mirrors arranged back-to-back, substantially at right angles to one another, opposite the leg of the T.While this may be fairly effective, it can be an expensive solution due to the necessity of providing two mirrors, and also the assembly of two mirrors arranged back-to-back may be too unsightly for an indoor location.

Over recent years there have been developed convex security mirrors which are substantially hemispherical in configuration, being formed, for example, by metallising the interior surface of a vacuum-formed acrylic dome. Such mirrors provide an all-round view and are commonly mounted on a ceiling or across the junction between two walls and a ceiling. Such hemispherical mirrors have also been used at T junctions instead of the backto-back mirrors referred to above, and if the hemispherical mirror is mounted on the vertical wall of the junction opposite the leg of the T, it will provide a view along both arms of the T. However, it is found that, due to its configuration, the view reflected by the mirror shows large areas of the wall, on each side of the mirror, on which the mirror is secured.For this reason the mirror presents an observer approaching the junction with a highly distorted and confusing view of the corridors forming the arms of the T. The present invention sets out to provide a form of convex mirror where the dimensions of the mirror are so selected that this disadvantage is avoided.

According to the invention there is provided a convex mirror wherein the convexity of the mirror is such that a cross-section of the mirror across the major dimension thereof is in the form of a segment of a circle in which the ratio of the major chord length to the depth of the segment is in the range of 6:1 to 3.75:1. Preferably the ratio of the major chord length to the depth of the segment is in the range of 4.75:1 to 5.25:1.

It is found that where such a mirror is used in the situation described above, at a T junction or other location where it is mounted flat on a vertical wall, an observer approaching the mirror from ahead will be afforded a clear view, through approximately 180 , to both sides of the mirror without undue distortion and without undue inclusion in the reflected image of unwanted views, such as large areas of the wall on which the mirror is mounted.

The mirror may be circular, in which case the major chord length referred to above will be equal to the diameter of the mirror. However, the mirror may comprise other portions of a spherical surface. For example the mirror may comprise half of a circular mirror cut along the diameter thereof, or a generally triangular sector of a circular mirror. The latter two arrangements may be suitable for use where the mirror requires to be located at the junction with a surface, such as a ceiling, which extends at right angles to the surface on which the mirror is mounted.

As previously mentioned, convex mirrors may also be useful in portable arrangements for inspecting inaccessible locations. Accordingly, the invention also provides a device for inspecting inaccessible locations comprising an upwardly facing convex mirror mounted on ground-engaging wheels, and having connected thereto an upwardly extending handle by means of which the mirror may be manoeuvred along the ground.

Such a device may be used for inspection beneath vehicles, furniture and similar structures where the ground clearance is such that direct access cannot readily be obtained. The device is suitable, for example, for finding faults beneath vehicles, but is also particularly applicable to the location of explosive devices which may be planted in inaccessible locations beneath a vehicle or elsewhere.

Preferably the mirror is tilted slightly towards the handle of the device at such an angle that the person holding the handle will see in the mirror a view of the location vertically above the mirror.

Preferably the device includes a lamp for illuminating the location reflected in the mirror. The lamp may be mounted on the handle.

The lamp may be arranged to illuminate directly an area above the mirror. Preferably, however, the lamp directs a beam of light downwardly towards the mirror so that the beam is reflected onto the location being inspected.

To facilitate manoeuvring of the mirror over the ground, at least certain of the groundengaging wheels may be castor wheels.

The following is a more detailed description of embodiments of the invention, reference being made to the accompanying drawings in which: Figure 1 is a diagrammatic horizontal section through a convex mirror mounted on a vertical wall, Figure 2 is a side elevation of a mirror incorporating an electric lamp, Figure 3 is a front elevation of the mirror shown in Fig. 2, and Figure 4 is a diagrammatic side elevation of an inspection device incorporating a convex mirror.

Referring to Fig. 1 there is shown a convex part-spherical mirror 10 mounted on a vertical wall 11. The mirror may be formed from transparent acrylic plastics metallised on the interior surface thereof. The metallic film may comprise high purity aluminium deposited by conventional methods. In the particular example shown the mirror is a circular mirror, and the major chord length L of the segment seen in bross-section is 30 inches (76.2 cm) diameter. The depth D of the segment, i.e.

the maximum distance to which it stands away from the wall 11, is 6 inches (15.2 cm).

Thus, the ratio of the major chord length to the depth of the segment is 30: 6, that is to say 5:1. However, as previously mentioned, the advantages of the invention can be given by mirrors where this ratio is anywhere in the range from 6:1 to 3.75:1.

Preferably the ratio is in the range from 4.75:1 to 5.25:1, and examples of specific dimensions are as follows: Major chord length (L) Depth of segment (D) 12 inches (45.7 cm) 2.5 inches (6.4 cm) 18 inches (61.0 cm) 3.5 inches (8.9 cm) 24 inches (76.2 cm) 5.0 inches (12.7 cm) 36 inches (91 .4 cm) 7.5 inches (19.1 cm) Each mirror is preferably integrally formed with a flat annular rim around the periphery thereof, the rim being from 2 to kinches(1.3 to 1.9 cm) in width, depending on the diameter of the mirror. A flat circular aluminium back plate, provided with mounting brackets, may be seated on to the rear of the mirror by means of a steel lined PVC edging strip which extends around the periphery of the mirror and secures the rim of the mirror to the back plate.The mirror may be arranged for mounting on a wall or on support posts by means of suitable brackets.

Fig. 1 shows diagrammatically the paths of light beams reflected from the mirror, the paths to the left of the centre line 1 2 representing light received by an observer in front of the mirror and approximately 10 feet therefrom, as indicated at 1 3. The light paths to the right hand side of the centre line 1 2 represent the light received by an observer approximately 6 feet from the mirror, as indicated at 14.

It will be seen that the mirror provides a wide angle of view to each side of the mirror (i.e. through approximately 180"), but no image is reflected of the parts of the wall 11 to each side of the mirror. As the distance of the observer from the mirror increases the reflected light beams incident upon the mirror become more nearly parallel to the wall 11, thus reducing the amount of the wall which is visible to the observer.

Figs. 2 and 3 show an arrangement where the convex mirror 25 incorporates an electric lamp 26. The lamp may be a self-contained unit mounted on the exterior of the mirror, or it may be mounted within the concave interior of the mirror and project through an aperture in the surface thereof.

The lamp may be a flashing warning lamp, to draw attention to the mirror and thus ensure its use at a junction, or it may provide steady illumination of the junction.

In some locations the mirror may be prone to misting, and in order to overcome this the mirror may incorporate a heater. For example, the heater may be located within the concave interior of the mirror, and may be in the form of an electric heating foil extending across the concave side of the mirror. The heater may, if required, be of sufficient power to provide some heating of the surroundings of the mirror.

Fig. 4 shows a device incorporating a convex mirror for inspecting inaccessible locations. The device comprises a relatively flat convex mirror 15 mounted on a frame 16.

One or more castor wheels 1 7 are mounted on the underside of the mirror adjacent the periphery thereof, and at the side of the frame remote from the castors 1 7 the frame is connected by crank arms 1 8 to an axle 1 9 supporting a pair of spaced wheels 20. The wheels 20 are rotatably mounted on the lower end of an upwardly extending handle 21.

The arrangement of the crank arms 1 8 is such that the mirror 1 5 is tilted slightly from the horizontal towards the handle 21.

A lamp assembly 22 is mounted on the handle 21 and may be arranged to direct a beam of light onto a part of the mirror adjacent the crank arms 1 8 so that the beam is reflected upwardly from the mirror towards the location to be inspected. The lamp assembly 22 may also incorporate a lamp which illuminates generally the area above the mirror 15.

The mirror 1 5 and frame 1 6 may be folded up to a position where they lie upwardly along the handle 21, and a releasable detent 23 is provided for retaining the mirror in the folded position.

In use, with the device in the operative position shown in Fig. 4, the mirror may be readily manoeuvred beneath a vehicle or article of furniture so that a person holding the handle of the device sees the reflected image of the underside of the vehicle or reflected image of the underside of the vehicle or furniture, which may be illuminated, if required, by the lamp assembly 22.

A version of the wide angle mirror described in relation to Fig. 1 may be of particular use in buses and other passenger-carrying vehicles where it is necessary for the driver or conductor to check various parts of the vehicle before setting off to ensure that, for example, all passengers have alighted and boarded and also, perhaps, that all doors are closed. For example, the mirror may be mounted above the driver's head so that by looking upwardly he can see all parts of the interior of the vehicle. A particularly suitable form of mirror for this purpose is a half circular mirror cut across it diameter and fitted at the junction between the ceiling and wall of the vehicle.

Claims (24)

1. A convex mirror wherein the convexity of the mirror is such that a cross-section of the mirror across the major dimension thereof is in the form of a segment of a circle in which the ratio of the major chord length to the depth of the segment is in the range of 6:1 to 3.75:1.

2. A convex mirror according to claim 1, wherein the ratio of the major chord length to the depth of the segment is in the range of 4.75:1 to 5.25:1.

3. A convex mirror according to claim 1, wherein the major chord length is approximately 12 inches (30.5 cm), and the depth of the segment is approximately 2.5 inches (6.4 cm).

4. A convex mirror according to claim 1, wherein the major chord length is approximately 18 inches (45.7 cm), and the depth of the segment is approximately 3.5 inches (8.9 cm).

5. A convex mirror according to claim 1, wherein the major chord length is approximately 24 inches (61.0 cm), and the depth of the segment is approximately 5 inches (12.7 cm).

6. A convex mirror according to claim 1, wherein the major chord length is approximately 30 inches (76.2 cm), and the depth of the segment is approximately 6 inches (15.2 cm).

7. A convex mirror according to claim 1, wherein the major chord length is approximately 36 inches (91.4 cm), and the depth of the segment is approximately 7.5 inches (19.1 cm).

8. A convex mirror according to any of claims 1 to 7, wherein the mirror is circular and the major chord length is equal to the diameter of the mirror.

9. A convex mirror according to any of claims 1 to 7, wherein the mirror comprises half of a circular mirror cut along a diameter thereof.

10. A convex mirror according to any of claims 1 to 7, wherein the mirror is a generally triangular sector of a circular mirror.

11. A convex mirror according to any of the preceding claims, wherein the mirror is formed from transparent plastics material having a reflective metallic film deposited on the interior, concave surface thereof.

1 2. A convex mirror according to claim 11, wherein the metallic film is formed from high purity aluminium.

1 3. A convex mirror according to any of the preceding claims, incorporating an electric lamp.

14. A convex mirror according to claim 11, wherein the mirror is formed with an aperture, and the electric lamp is mounted within the concave side of the mirror and projects through said aperture.

1 5. A convex mirror according to any of the preceding claims, incorporating a heater for demisting the surface of the mirror and /or heating the surroundings of the mirror.

1 6. A convex mirror according to claim 15, wherein the heater is located within the concave side of the mirror.

1 7. A convex mirror substantially as hereinbefore described with reference to Fig. 1 or Figs. 2 and 3 of the accompanying drawings.

1 8. A device for inspecting inaccessible locations comprising an upwardly facing convex mirror mounted on ground-engaging wheels, and having connected thereto an upwardly extending handle by means of which the mirror may be manoeuvred along the ground.

1 9. A device according to claim 18, wherein the mirror is tilted towards the handle of the device at such an angle that the person holding the handle will see in the mirror a view of the location vertically above the mirror.

20. A device according to claim 1 8 or claim 19, wherein the device includes a lamp for illuminating the location reflected in the mirror.

21. A device according to claim 20 wherein the lamp is mounted on the handle.

22. A device according to claim 20 or claim 21, wherein the lamp directs a beam of light downwardly towards the mirror so that the beam is reflected onto the location being inspected.

23. A device according to any of claims 1 8 to 22. wherein at least certain of the ground-engaging wheels are castor wheels.

24. A device for inspecting inaccessible locations substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.