GB1595907A

GB1595907A - Mirrors

Info

Publication number: GB1595907A
Application number: GB1890577A
Authority: GB
Original assignee: BFG Glassgroup GIE
Current assignee: BFG Glassgroup GIE
Priority date: 1977-05-05
Filing date: 1977-05-05
Publication date: 1981-08-19
Also published as: FR2389908A1; DE2817957A1; FR2389908B1; BE866288A

Description

(54) MIRRORS (71) We, BFG GLASSGROUP, a Groupement d'Interet Economique., .estäblished under the laws of France (French Ordinance dated 23rd September 1967), of-Rue Caumartin 43, 75009 Paris, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, tQ be particularly described in and by the following statement: This invention relates to mirrors comprising a support which is coated with light' reflecting material to reflect light images on each side of such support, the light-reflecting power being greater on one side of the mirror than on the other.

Such mirrors are useful e.g. as reversible rear-viewing mirrors for vehicles. The mirrors can be reversed for night-time use to reduce light-reflectance and the harmful effects of glare when the mirror is illuminated by light from the headlamps of oncoming vehicles.

Reversible mirrors are known wherein surfaces formed by light-reflecting coatings of different light-reflecting power are formed on different supports arranged back to back.

The present invention provides a reversible mirror which as compared with such known mirrors affords the advantage that there is less risk of the mirror performance being adversely affected by damage to a light-reflecting coating. This is an important advantage because coatings having adequate light-reflecting power for rear viewing purposes generally do not have the desired resistance to damage which impairs their optical qualities. The invention moreover achieves this advantage in a mirror of relatively simple construction incorporating a single coated support.

According to the present invention, there is provided a mirror comprising a transparent support which is coated with light-reflecting material to cause specular light-reflection on each side of the mirror, the light-reflectivity of the mirror being greater on one side thereof than on the other, characterised in that the specular light-reflection on both sides of the mirror is wholly or mainly caused by an opaque metal light-reflecting coating on one side of said support, and in that the side of such coating which is opposite said transparent support is overall covered by a transparent modulating coating which reduces the reflectivity of the mirror as viewed from that side, the reflectivity of the mirror being in excess of 35% as viewed from the side opposite said modulating coating and being less than 25% as viewed from the side of said modulating coating.

The light reflectivity of a mirror is the intensity of a reflected light ray expressed as a percentage of the intensity of the corresponding incident ray. The reflectivity values herein specified are mean reflectivities at normal incidence over the visible light wavelength range using "Illuminant A" specified by the International Commission on Illumination.

When a mirror according to the invention is used as a rear-viewing mirror for day time driving the mirror is orientated with the opaque light-reflecting coating behind the transparent support so that such coating is viewed through this support. For night-time use the mirror is turned through 1800 so that the opaque light-reflecting coating is viewed through the modulating coating. In this position the mirror has a lower effective light-reflectivity and permits head-lamps of on-coming vehicles to be viewed without discomfort or very dangerous glare.

The modulating coating, in addition to attenuating light-reflection at that side of the mirror, gives the mirror a better resistance to damage.

A mirror according to the invention can be easily manufactured using well established coating techniques, e.g. vacuum coating or chemical deposition.

In preferred embodiments of the invention, the reflectivity of the mirror as viewed from the side of said modulating coating is not more than half the reflectivity of the mirror as viewed from the opposite side. Such a relationship between the light-reflectivities for alternative use makes the mirror particularly useful for various purposes, particularly as a rear-view driving mirror as aforesaid.

The reflectivity of the mirror as viewed from the side of said modulating coating is preferably less than 20%. By observing that condition, uncomfortable glare due to reflection of light from vehicle headlamps or other sources of similar light-emitting power can be avoided. However, the reflectivity of the mirror viewed from the side of the modulating coating is preferably at least 4%. In many cases the reflectivity of the mirror viewed from that side is at least 10%.

It has been found that it is sufficient for a rear view driving mirror to have only a moderate reflectivity in order to give fully adequate visibility and image brightness under day-time conditions. The reflectivity of the mirror viewed from the side opposite the modulating coating can have any value in excess of 35%. Preferably however the reflectivity of the mirror as viewed from that side does not exceed 85%. Most preferably the reflectivity from that side is from 40% to 65%.

The choice of the composition of the opaque metal light-reflecting coating for a given mirror will take account of the intended conditions of use and the particular properties which it is desired to confer on the mirror.

A particularly preferred metal for use in forming the opaque light-reflecting coating is chromium. A chromium coating is especially useful in a rear-view driving mirror for providing adequate reflectivity for rear-viewing purposes under day-time driving conditions.

In certain advantageous embodiments of the invention, the opaque light-reflecting coating is formed from metal selected from nickel and nickel-chromium alloys. Other suitable metals which can be used include silver, aluminium, copper gold, germanium and tin.

The metals named as examples above when forming opaque light-reflecting coatings on glass, have the following reflectivity values in respect of light incident upon the coating from its exposed side and having a wavelength of 5,500A, which is in the range of maximum sensitivity of the human eye: chromium 56inc nickel 58% tin 37% silver 98% aluminium 91% copper 67% gold 82% germanium 45% The reflectivities of gold and copper are selective within the visible wavelength range in a way which can be of interest in certain specific applications of the invention.

The opaque metal light-reflecting coating can be a single-layer coating or a plural-layer coating, i.e. an opaque coating comprising two or more metal layers which together have the requisite reflectivity. A very good example of a plural-layer opaque light-reflecting coating is one comprising a combination of an opaque light-reflecting layer (e.g. a chromium layer) and a transparent light-reflecting metal layer (e.g. a silver layer) which is disposed between that opaque layer and the support and gives the mirror a higher reflectivity, as viewed through the support, than it would have in the absence of such transparent layer.

The opaque metal coating can be formed on the transparent support by evaporation in vacuo.

The modulating coating can likewise be a single-layer coating or a plural-layer coating.

The modulating coating can be one which attenuates the reflectivity of the mirror at the corresponding side thereof wholly or mainly by creating an interference effect. As an alternative the modulating coating can affect reflectivity due to some other phenomenon.

For example, the invention includes mirrors wherein the modulating coating exerts its modulating effect wholly or mainly by absorbing light. Such a coating can be formed by simultaneous evaporation under vacuum of a metal and a metal compound, e.g. chromium and magnesium fluoride.

The modulating coating can be formed from a metallic compound. In preferred embodiments of the invention, the modulating coating is a metal oxide coating. Such a coating can also be formed by using techniques known per se. A particularly preferred metal oxide is zirconium oxide. A zirconium oxide coating is a very effective modulator and can afford very good protection to the underlying light-reflecting metal coating.

In certain advantageous mirrors according to the invention the modulating coating material is selected from the oxides of titanium, zinc, silicon, tin, indium, aluminium and bismuth.

Certain metal compounds other than oxides can be used for forming a modulating coating. Examples are zinc sulphide and magnesium fluoride.

The support is preferably a piece of glass, most preferably a sheet of ordinary untinted soda-lime glass. The glass of such sheet can advantageously be tempered glass. Instead of a glass support, a piece of transparent plastics can be used. The support can be colourless or tinted. For example a sheet of so-called grey glass can be used as the transparent support.

When using a tinted support a given reflectivity of the mirror at the side opposite that of the modulating coating can be achieved by using an opaque light-reflecting coating of relatively high light reflectivity.

As already stated, it is very advantageous for the mirror to have a reflectivity at one side which is not more than half of the reflectivity of the mirror at its opposite side. It is preferable to incorporate that feature even when one or more other optional features hereinbefore or hereafter described is or are employed.

Acceptable results for some purposes can be achieved if the light-reflecting coating has a limited transparency. Therefore the present invention includes a mirror according to the invention as hereinbefore defined but with the modification that the light-reflecting coating is transparent. its transparency however being less than 4%, i.e. such that the specular light transmission of the coating averages less than 4% over the visible light wavelength range, measured in respect of light incident normally on the coating.

Preferably a mirror according to the present invention is connected by a swivel joint to a mounting means attachable to a fixture, the joint permitting rotation of the mirror through 180C relative to such mounting means. A ball and socket joint is suitable for this purpose. In another very satisfactory embodiment of the invention the mirror is carried by a supporting means in such a way that the mirror is pivotable about a predetermined axis, preferably an axis of symmetry, relative to such means. The supporting means may itself be connected or connectable to a fixture so that the position of the supporting means relative to the fixture can be adjusted. There may be releasable clamping means by which the mirror can be held in any adjusted position relative to its mounting means (of whatever type). As an alternative the swivel or pivotal joint may afford sufficient frictional restraint to displacement of the mirror to make positive clamping means unnecessary under the intended conditions of use.

Certain embodiments of the invention selected merely by way of example, will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of a mirror; Figure 2 is a cross-section on the line II-II in Figure 1; Figure 3 is a cross-section, similar to Figure 2, of another mirror; Figure 4 shows an alternative mounting of a mirror according to the invention, and The mirror 1 shown in Figures 1 and 2 is a rear-view driving mirror and is reversible in the sense that it specularly reflects light at both sides. The mirror is connected to a mounting arm 2 via a ball and socket joint 3. After mounting the mirror in a vehicle by securing the arm 2 to a suitable fixture, either side of the mirror can be brought into use by rotating the mirror to bring that side to the front, facing the driver.

The mirror comprises a colourless tranparent glass sheet 4 bearing a substantially opaque light-reflecting coating 5 of metal and on top of that coating 5, a transparent modulating coating 6.

The light reflectivity of coating 5 is such that the mirror is well suited for day time use when orientated with the glass sheet 4 towards the driver so that the reflectance is not affected by the modulating coating 6. The reflectivity of the mirror viewed in that orientation is in excess of 35%.

The modulating coating has the effect of reducing the reflectivity of the mirror at the side at which that modulating coating is present, to less than 25%. This is due to an interference phenomenon involving an out-of-phase relationship between the rays reflected at the exposed face of the modulating coating 6 and the rays reflected at the interface of that coating and the light-reflecting coating 5. Consequently, for night-time use, the rear view mirror is orientated in the reverse manner, i.e. with the modulating coating 6 towards the driver. In these conditions the driver is completely or to a considerable degree relieved of uncomfortable glare when the mirror is illuminated by beams from the lamps of oncoming vehicles. However when the mirror is in that reverse orientation the reflected image is still sufficiently clear to permit all essential features in the reflected field of view to be properly distinguished.

The theory of interference is well known. This phenonmenon can be utilized in a mirror according to the invention to reduce or increase observed reflectance of light rays of a given wave length from a light-reflecting coating by giving the modulating layer a thickness equal to an odd number multiple of a quarter of that wavelength or, as the case may be, a multiple of half of such wavelength.

Example 1 In an actual mirror as described with reference to Figures 1 and 2 the glass sheet 4 had a thickness of 4 mm, the light-reflecting metal coating 5 was a layer of chromium 600 A in thickness and the modulating coating 6 was a layer of titanium dioxide 400 A in thickness.

The reflectivity of the mirror as viewed from the uncoated side of the glass sheet was 42%.

The reflectivity of the mirror viewed from the opposite side was only 5%, thus much less than half the reflectivity of the mirror as viewed from the uncoated side of the glass sheet.

The light transmissivity of the mirror as a whole was not more than 1%.

The reflectivity of the mirror at the coated side of the glass sheet was in part a function of the thickness of the modulating coating 6 and therefore could be modified within certain limits by selecting a different thickness for that coating.

By way of modification, a sheet of tinted glass (e.g. a sheet of so-called grey glass) can be used in place of the colourless glass sheet 4. In that case the reflectivity of the mirror in its day-time orientation will be less.

Example 2 A mirror was made as described with reference to Figures 1 and 2, comprising a colourless glass sheet 2 mm in thickness, a coatipg 5 of chromium 500 A in thickness and a modulating coating 6 of zirconium oxide 450 A in thickness. The mirror had calculated reflectivities of 44.5% at the side of the glass and 9.7% at the side of said oxide coating. For a mirror comprising the same coatings on a glass sheet 4 mm in thickness the corresponding calculated reflectivities are 43.8% and 9.7% respectively.

By suitably choosing the thickness of the modulating coating 6 it is possible to select the modulated reflectivity at that side of the mirror within certain limits. By way of example, when using in combination a chromium light-reflecting coating 50 nanometers in thickness and a zirconium oxide modulating coating the modulated reflectivity of the mirror varies in function of the thickness of the modulating coating as indicated in the following table: Thickness of zirconium oxide Reflectivity of combined coating in nanometers Cr and ZrO2 coatings 40 13.0% 45 9.7% 50 9.3% Examples 3 to 6 Mirrors of the structure shown in Figure 2, i.e. comprising a piece of colourless glass carrying on one side an opaque or substantially opaque metal light-reflecting coating (next to the glass) and a transparent modulating coating (on top of such light-reflecting coating) were made and tested. In each mirror the glass support was 4 mm in thickness. The mirrors had the coating specifications and the properties set out in the following Table: TABLE Example Coating Composition Light Reflectivity Light-trans and Thickness of: of mirror at: missivity of mirror Light-reflecting Modulating Uncoated side Coated side coating coating 3 Ni (1000 ) S 10x(500) 48% 18% 0% (x=1.2) 4 Cr(600 ) SnO2(500) 42% 12% 1% 5 Cr (600) Al2O3(650) 43% 20.4% 1% 6 Ni (1000) Bi2O3(300) 47% 7.5% 0% Example 7 A mirror as represented in Figure 3 was made. The mirror comprised a colourless lass sheet 7 bearing a layer 8 of silver 30 A in thickness, a layer 9 of chromium 500 in thickness, and a coating 10 of ZrO2 450 A in thickness. The layers 8 and 9 constituted a double-layer light-reflecting coating to give the mirror a calculated reflectivity of 49.6% as viewed through the glass sheet. The calculated light-reflectivity of the mirror at the opposite side was 9.5%, a value which is much less than the reflectivity which the mirror would have at that side in the absence of the zirconium dioxide coating.

Another mirror was made having the foregoing coating specifications except that the silver coating layer 8 was 20 A in thickness. The calculated reflectivities of this mirror were at the uncoated and coated sides were 47.6% and 9.5%.

A further mirror was made having the same specifications except that the silver layer 8 was 10 A. The calculated reflectivities of this mirror at the uncoated and coated sides were 45.7% and 9.6%.

Example 8 Figure 4 shows a rear view driving mirror according to the invention having a mounting permitting the mirror to be easily reversed. The mirror 11, which can have a composition according to any of Examples 1 to 7, is mounted in a frame 12 which is mounted for pivotal movement through 1800 in a supporting stirrup 13 connected to a fixture (not shown) or to a mounting plate forming part of the mirror unit, by a ball and socket joint 14. Each of the vertical side members of the stirrup has a horizontal pivot pin such as 15 secured thereto and these pins intrude into bearings in the mirror frame so that the mirror is rotatable about a horizontal axis. A stop 16 is connected to the mirror frame and abuts against the bottom of the lower or upper one of the two notches 17 to limit the pivotal movement of the mirror to 1800.

Various modifications can be made to the mirrors described with reference to the drawings. For instance, a mirror as described with reference to Figures 2 or 3 can be laminated to another piece of glass for security purposes.

In the various examples given above the coatings were applied using coating techniques well known per se, for example evaporation in vacuum or cathode sputtering.

Claims

WHAT WE CLAIM IS:

1. A mirror comprising a transparent support which is coated with light-reflecting material to cause specular light-reflection on each side of the mirror, the light reflectivity of the mirror being greater on one side thereof than on the other, characterised in that the specular light-reflection on both sides of the mirror is wholly or mainly caused by an opaque metal light-reflecting coating on one side of said support, and in that the side of such coating which is opposite said piece of transparent material is overall covered by a transparent modulating coating which reduces the reflectivity of the mirror as viewed from that side. the reflectivity of the mirror being in excess of 35% as viewed from the side opposite said modulating coating and being less than 25% as viewed from the side of said modulating coating.

2. A mirror according to claim 1, characterised in that the reflectivity of the mirror as viewed from the side of said modulating coating is not more than half of the reflectivity of the mirror as viewed from the opposite side.

3. A mirror according to claim 1, characterised in that the reflectivity of the mirror as viewed from the side of said modulating coating is less than 20%.

4. A mirror according to claim 1 or 3, characterised in that the reflectivity of the mirror viewed from the side of the modulating coating is at least 4%.

5. A mirror according to claim 1 or either of claims 3 and 4, characterised in that the reflectivity of the mirror viewed from the side opposite said modulating coating is from 40% to 65%.

6. A mirror according to claim 1 or any of claims 3 to 5, characterised in that the said light-reflecting coating is composed of chromium.

7. A mirror according to claim 6. characterised in that the modulating coating is composed of zirconium oxide.

8. A mirror according to claim 1 or any of claims 3 to 5, characterised in that the said light-reflecting coating is composed of a metal selected from: nickel and nickel-chromium alloys.

9. A mirror according to claim 1 or any of claims 3 to 5, characterised in that said light-reflecting coating comprises an opaque light-reflecting layer and a transparent light-reflecting layer which is disposed between such opaque layer and the support. which transparent layer gives the mirror a higher reflectivity, as viewed through the support than it would have in the absence of that transparent layer.

10. A mirror according to claim 9, characterised in that said opaque light-reflecting layer is composed of chromium and said transparent light-reflecting layer is composed of silver.

11. A mirror according to claim 10, characterised in that said modulating coating is composed of zirconium oxide.

12. A mirror according to claim 1 or any of claims 3 to 10, characterised in that the modulating coating is a metal oxide coating.

13. A mirror according to claim 12, characterised in that the modulating coating is composed of zirconium oxide.

14. A mirror according to claim 12, characterised in that the modulating coating is composed of metal oxide selected from the oxides of titanium, zinc, silicon, tin, indium, aluminium and bismuth.

15. A mirror according to claim 1 or any of claims 3 to 14, characterised in that the support is a piece of colourless glass.

16. A mirror according to any of claims 3 to 15, characterised in that the reflectivity of the mirror as viewed from the side of said modulating coating is not more than half of the reflectivity of the mirror as viewed from the opposite side.

17. A mirror according to any preceding claim, but with the modification that the light-reflecting coating is transparent, its transparency however being less than 4%.

18. A mirror substantially according to any of Examples 1 to 8 herein.