GB2077673A - Automobile day-night mirror - Google Patents

Abstract

A day-night rear view mirror for an automobile comprises two separate reflecting elements, both of planar form and set at an acute angle to each other. The front element A which may be of clear polycarbonate is transparent and the rear element B is opaque and light reflective. The rear element is used for normal day-light viewing and for night driving, the front element is employed, its front surface being sufficiently relative to reflect bright images. <IMAGE>

Description

SPECIFICATION Day-night rear view mirrors for automobiles Day-night rear view mirrors for automobiles' are well known and have been in general use for several years.

Such mirrors have usually consisted of a wedge of high quality glass silvered on the rear surface and highly polished on the fronts surface.

For daytime viewing the high optical resolution of the reflected image is required and this is provided by the rear silvered or mirrored surface. For night time viewing reduced reflectivity is the requirement and a reduction in optical quality is acceptable. This is provided by the polished front surface of the glass wedge. The angle of the front and rear surfaces of the wedge to each other is the angle through which the mount can move from the daytime viewing to the night time viewing positions.

Such mirrors possess at least the following disadvantages: 1. They must be produced from a high quality and correspondingly expensive glass; 2. They require a high degree of working to produce the required high surface polish and precise angular relationship between daytime and night time surfaces.

3. They are relatively heavy and are a potential safety hazard on impact.

In accordance with the present invention there is provided a day-night rear view mirror for an automobile, comprising a planar front element of transparent material having a light reflective front surface and a separate planar rear element which is opaque and light reflective the two elements being supported at an actue angle to each other.

The front element, which is preferably injection moulded from polycarbonate, water-clear, may be abrasion resistant coated on its front surface, and may be coated on its rear surface with an anti-reflection layer (555nm preferred). Such lightweight thin section moulding can be produced at a rate of 500,000 per annum from a single cavity mould, and any gradual out-of flatness that may be present initially or develop after a period of time will not affect the integrity of the bright day-light image viewed through it nor be discernible when used in its night time, front surface mode when only bright objects can be imaged.

The rear element may be thin, plano glass conventionally silvered or may for example be a thin polyester film metallized on its front surface.

The following considerations will normally apply in combinating these two separate elements in a suitable housing: 1. Since the two inner surfaces are inaccessible for cleaning, access for contaminants or moisture (condensation) should be excluded by the chosen design; 2. If the day-light viewing surface can be flexed by atmospheric pressure changes and thereby affect optical flatness, then pressure equalizing channels should be provided; 3. The joining of the optics sub-assembly to the mirror housing must be achieved in a manner whereby any out-of-flatness of the latter must not affect the integrity of the former.

The proposed optical arrangements intro duce one or two additional surfaces through which images are viewed, as compared with the existing single element glass prism. The theoretical effect is: 1. Day-light viewing. Reduction of reflec tion intensity by 4 to 5% per additional surface.

2. Night-time mode: additional internal reflections may lead to ghost repeats verti cally, of diminishing intensity.

The thinner the light transmitting sections are, the more tolerable the anticipated condi tion. Thereafter the most cost-effective step is for the day-light reflecting surface to be front reflectant. An additional, more costly step is to provide an anti-reflection coating on the second surface of the front element. Cost effectiveness would dictate a single layer coat ing in which case one that is effective at 555nm (greatest visual perception) is likely to be most beneficial.

Some preferred forms of day-night mirror in accordance with the invention are described below, by way of example, with reference to the accompanying drawings, in which each of the four figures is a vertical cross-section of the mirror.

In each of the illustrated embodiments, the day-night mirror comprises a rear element A and a front element B, both planar and set at a predetermined acute angle to each other in a suitable frame or support structure. The rear element A in each case constitutes the day light viewing element and the front element constitutes the night-time viewing element when only bright objects will be viewed.

In Fig. 1 the rear element A consists of mirror glass having a metallized reflective front or rear surface, and the front element consists of transparent polycarbonate, prefera bly having its rear (inner) surface treated with an anti-reflective coating preferably effective at 555 nm. As examples the rear element may be 0.5 mm thick and the front element 1 mm thick.

The two elements are securely set in a mirror frame 2 to which is also secured a mirror drum 3 by which the assembly is suitably mounted in a vehicle.

The mirror of Fig. 2 is generally similar, except that the front element preferably of polycarbonate or polymethyl methacrylate, is formed as part of a moulding which makes snap-fitting engagement into a forward socket portion on a mirror housing/drum moulding 3, e.g. of polypropylene, the rear element A being held securely clamped against the front moulding with the assistance of a number of resilient pressure pads 4.

In Fig. 3, the front element B again forms part of a moulding, and the rear element A consists of a stretched metallized plastics film, e.g. of polyester, secured about its periphery for example by ultrasonic welding, to the rear open side of the moulding. In this case a vent hole 6 is made in the moulding side wall to maintain the otherwise sealed internal space at ambient pressure.

In the mirror of Fig. 4, the rear element A is again of stretched, metallized plastics film secured to the rear of the front moulding, which in this case is held by snap-fitting engagement in a mirror backing, or frame 7 conveniently formed as a plastics moulding, e.g. of polythene. The film may have a pin hole through it to equalize pressure to either side.

In each case the front, outer surface of the front element B is preferably treated with a transparent coating to increase abrasion resistance of the surface.

Claims (9)

1. A day-night rear view mirror for an automobile comprising a planar front element of transparent material having a light reflective front surface and a separate planar rear element which is opaque and light reflective, the two elements being supported at an actue angle to each other.

2. A mirror according to claim 1, wherein the front element is formed of a transparent synthetic plastics material.

3. A mirror according to claim 2 wherein the front element is made of polycarbonate or polymethyl methacrylate.

4. A mirror according to claim 1 or 2, wherein the front element has a non-reflective coating on its rear surface.

5. A mirror according to claim 2, 3 or 4 wherein the front element is moulded integrally with a peripheral frame portion which supports or assists in supporting the rear element.

6. A mirror according to any preceding claim, wherein the rear element comprises mirror glass.

7. A mirror according to any one of claims 1 to 5 wherein the rear element consists of a film of metallized plastics material held in a stretched planar condition.

8. A mirror according to claim 7, wherein the space between the two elements is vented to atmosphere.

9. A day-night rear view mirror for an automobile substantially as herein described with reference to Fig. 1, or Fig. 2, or Fig. 3, or Fig. 4 of the accompanying drawings.