DE8914938U1 - Rear-view mirrors for vehicles, especially motor vehicles - Google Patents

Description

D2009/Gbm

Rearview mirrors for vehicles, in particular Kr^äahrzeucje

The invention relates to a rear-view mirror for vehicles, in particular motor vehicles, according to the preamble of claim 1.

As is well known, such rear-view mirrors are best suited to coatings whose total reflection is not too high, i.e. at least 40% in accordance with an EC directive, and which also have a clear blue reflectance. Such reflective coatings do not have glare-reducing properties, partly because of the already reduced total reflection compared to silver or aluminum, for example, but also because of the fact that headlight light, due to its rather low color temperature, has an emission spectrum shifted into the yellow range, which is reflected less strongly by such a coating than daylight. Such coatings can be applied to both the front and back of a transparent layer carrier, which is usually made of soda silicate glass.

O For example, front-coated rear-view mirrors are known whose re

flexural layer consisting of three dielectric individual layers of approximately
Λ/4 thickness (Λ approx. 450 nm), with the two outer layers having a high refractive index and the middle layer a low one. Due to the light transmission of dielectric layers, the rear surface of such a mirror is usually provided with a dark, absorbent coating (DE-AS 1036672, DE-AS 2449763).

Furthermore, front surface mirrors are known which, in addition to a metallic layer directly on the substrate with 55 to 65% total reflection, also have a dielectric layer with a high refractive index and

I have a thickness of about Δ./2 ('λca 450 nm). Such an additional

1 interference layer, the originally neutral reflection color of the

i metallic layer into a blue one. Since the metallic

I layer on the front surface is largely opaque, can be used in

1 such a mirror on the above-mentioned dark coating

I can be waived (DIE-OS 37 28100). I Such front surface mirrors have the advantage of an absolutely

double-image-free reflection, but at the same time have the disadvantage that the reflection coating, which is usually only a few hundred nm thick, is exposed to external mechanical and chemical attacks without protection. This often leads to premature wear and the mirrors become unusable.

u On the other hand, rear-view mirrors with a blue reflective tint are also known, in the form of three different I coating types:

I In the first type, a print head is located directly on the substrate surface. I dielectric layer with high refractive index and about Δ/2 thickness (λ

1 ca 450 nm). This is followed by a metallic layer with 60 to 80 % total

I reflection and then possibly a protective layer Just as with

• dom previously described front surface mirror, the original

I Highly neutral reflection color of the metallic layer by an additional

i additional interference layer into a blue one.

I O Rear surface mirror, however, the A /2 layer borders on the substrate I glass and not in air as with the front surface mirror described above. I As a result, this interface loses its optical properties, so that a

such a rear surface mirror only has a relatively weak blue reflection color tone. This is also only achieved by reducing the reflection in the long-wave visible spectral range. Therefore, the total reflection of such a rear surface mirror decreases significantly when setting a blue reflection color tone, so that the requirement for at least 40% total reflection may no longer be met. This disadvantage can only be avoided by using materials with an extremely high refractive index > 2.5 to produce the Λ/2 layer. Such materials as

e.g. zinc sulphide, are chemically less stable and can only be applied using rather complex vapor deposition processes. The type of mirror just described does have the advantage that the metallic layer can consist of chemically resistant chromium, for example, so that a protective coating on the back is not necessary, but the disadvantages of this type of mirror are based on a very weak blue reflection colour and the simultaneously low overall reflection (DE-PS 3436016 C1).

In the second type, there is first a very thin, partially transparent metallic layer directly on the sub-surface, followed by a dielectric layer with a thickness of about 1/4 ('Λ, approx. 750 nm), and finally another metallic layer, but rather thick and with > 90% total reflection. The refractive index of the dielectric layer is irrelevant for this type of mirror, as it is only intended to create a certain distance between the two metallic layers between which the interference phenomena take place. The metallic layer with high total reflection described above consists of either silver or aluminum, but both metals must be provided with protective layers due to their low chemical resistance. In the case of silver in particular, such protection is often not sufficient, so that the metal is attacked after a while. On the other hand, the use of a highly reflective metal is essential for the type of mirror described above, otherwise the total reflection of the layer system does not exceed 40%. While this type of mirror advantageously succeeds in achieving a pronounced blue reflection color with sufficient total reflection, there is a significant disadvantage in the use of a highly reflective metal that is sensitive to chemical attack, such as silver (DE PS 34 &> 011 C1).

In the third type, there is also a system of three dielectric interference layers between the substrate and a metallic layer with 50 to 80% total reflection, but these only consist of materials with a high and low refractive index. Here, there is a layer with a high refractive index and a thickness of about 7C /4 (approx. 470 nm) directly on the back surface of the substrate, followed by a

Layer with a low refractive index and about X /4 thickness and then another layer with a high refractive index, but about Δ /2 thickness. Finally, the metallic layer mentioned above follows. The disadvantage of this layer system is the choice of materials with a high refractive index for the layer on top of the substrate. This makes it almost impossible to achieve a blue reflection color and a total reflection of > 40 % at the same time. In addition, such a layer system not only has the desired reflection maximum at about 450 nm, but also has a very pronounced non-reflection peak at about 100 nm, where the spectral reflection increases again above 650 nm. In general, however, when interference layer systems are viewed at an acute angle, such reflection maxima and minima migrate to shorter wavelengths, and in the case of this layer system, this means an undesirable change of the reflection color tone to reddish (JP-OS 602 127 04).

The invention is based on the object of creating a rear-view mirror for vehicles, in particular motor vehicles, whose reflective coating is protected against chemical and mechanical attacks and which, in addition, has a total reflection of at least 40% and a pronounced blue reflection color that is largely independent of the viewing angle.

D| _e e ₈ AMfgaKo »«HM Hufch solved the rearview mirror described in Sehyta claim 1

In the rearview mirror according to the invention, the interference layer system arranged between the transparent layer carrier and the metallic reflection layer consists of three dielectric layers, of which the first layer, which lies directly on the transparent layer carrier, has a medium refractive index, the subsequent second layer has a low refractive index and the subsequent third layer has a high refractive index. The thickness of the layers is dimensioned such that the rearview mirror has a total reflection of at least 40% and a pronounced blue reflection color when viewed from above onto the substrate side.

The triple interference layer system described above, in conjunction with the metallic layer, not only reduces reflection in the long-wave visible spectral range, but also increases it in the short-wave, blue range. This is achieved primarily by using materials with a medium refractive index for the first of the three interference layers on top of the transparent layer. This surprisingly makes it possible to achieve a clear blue reflection color that is independent of the viewing angle, as well as a stable overall reflection, while at the same time avoiding the use of highly reflective metals that are sensitive to chemical attack, such as silver. In particular, the triple interference layer enables the use of a metallic layer with only

50 - 80% total reflection as the last optically effective layer, without the risk of the total reflection falling below 40%.

Particularly high reflection values with a reflection maximum in the blue are achieved when the first layer adjacent to the transparent layer carrier has a refractive index of 1.6 - 1.85 and a layer thickness of 40 - 80 nm, the subsequent second layer has a refractive index of < 1.5 and a layer thickness of 60 - 120 nm and the subsequent third layer has a refractive index of > 1.9 and a layer thickness of 70 - 140 nm

If the layer thicknesses are reduced or increased beyond the limits given above, the reflection maximum in the first case moves into the short-wave spectral range, the blue reflection color becomes more intense, which simultaneously leads to a decrease in the total reflection, in the second case the maximum moves into the long-wave spectral range, so that the mirror receives an undesirable yellow reflection color. The interference optical effect also decreases when the individual layer thicknesses are changed beyond the limits given above.

Since the optical path of the light, which is the product of the geometric path and the refraction, is important for the interference optical effect of the layer system,

index is decisive, similar considerations can also be made for deviations of the refractive indices from the enriched values mentioned above.

A triple interference layer system is particularly preferred in which the first layer adjacent to the transparent layer carrier has a refractive index of 1.7 and a layer thickness of approximately 60 nm, the subsequent second layer has a refractive index of 1.45 and a layer thickness of approximately 95 nm and the subsequent third layer has a refractive index of ?jO5 and a layer thickness of approximately 10 nm. With such a layer system, a total reflection of up to 50% can be achieved with a suitable metallic mirror layer without the blue reflection color being impaired. Small deviations from the above-mentioned layer thicknesses due to production, which cannot be avoided in particular when coating large surfaces, generally do not lead to any deterioration in the reflection properties of the mirror according to the invention.

Because the reflective coating is located on the back of the rear-view mirror in a known manner, it is well protected from external chemical and mechanical attacks. Suitable metals for the metallic mirror layer are in particular chromium, zinc, tin or nickel or alloys of aluminum-copper, nickel-copper, tin-copper or iron-chromium-nickel. Applied in sufficient thickness, layers of these metals show the desired total reflection of 50 - 80%.

The thickness of the metallic layer should generally be such that, on the one hand, disruptive transmission and insufficient reflection caused by insufficient thickness are avoided. For this reason, a minimum layer thickness of 30 nm is required. On the other hand, the coating should also be cost-effective so that the layer thickness should not be increased unnecessarily. When using chromium for the metallic layer, which is particularly preferred due to its high chemical resistance, a pronounced blue reflection color and a total reflection of 45 to 50% can be achieved with a layer thickness of around 45 nm.

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In addition to silicate glass and other glasses, transparent plastic materials such as polymethyl methacrylate, etc. can also be used as transparent layer supports. The layer support can be either flat or slightly curved. §

A variety of dielectric interference rails are available |

tric materials, provided that they only operate in the visible spectral region '\

are essentially absorption-free and therefore do not cause an excessive ;$'

additional undesirable light attenuation is caused. For high- |·

Refracting layers are, for example, the oxides of titanium, cerium, tin, zirconium <|

or mixtures of these oxides preferred for layers with low refraction |;

Examples of materials with a high index of influence are silicon dioxide or the material obtained from vapor deposition.

Magnesium fluoride and other fluorides are suitable. |

The interference layers with medium refractive index are preferred § Mixed layers of metal oxides with high and low refractive indices;

dex, e.g. from &Tgr;&Iacgr;&Ogr;2 ^and S©2« whose ratio is chosen so that the desired average refractive index results

Methods for applying the layers to the transparent layer carrier are known per se. In the rear-view mirror according to the invention, the triple interference layer system is preferably first applied using a dipping process, using alcoholic solutions of organic compounds of the metals whose oxides are to form the interference layers. To produce mixed layers made of different metal oxides, mixtures of the corresponding solutions are used.

In the dipping process, the transparent layer carrier wetted with the corresponding solution is subjected to a tempering process in order to apply a layer, whereby a dielectric layer made of the corresponding metal oxide(s) is formed on the wetted surface.

The dipping process is particularly suitable for this purpose, as it allows a cost-effective coating of large-area substrates with dielectric materials, which is not easily possible using vacuum processes.

The use of vacuum processes for applying the dielectric layers is therefore generally recommended when smaller areas are to be coated. Vacuum processes that can be used include direct evaporation or sputtering of the corresponding dielectric materials, but also reactive evaporation or reactive sputtering of the corresponding metals.

The dipping process is preferably used to deposit dielectric layers made of the oxides of silicon, titanium, Ger, tin or zirconium or mixtures.

The metallic mirror layer, for example made of chromium, that follows the triple interference layer system is preferably applied using a vacuum process, e.g. by means of magnetron cathode atomization. In addition to vapor deposition, other deposition processes, e.g. wet chemical processes, can also be used.

To improve resistance to chemical and mechanical attacks, it may be advisable to apply a protective layer, e.g. a varnish, to the back of the reflective coating.

The invention is explained in more detail below using the figures and exemplary embodiments

Show it:

Figure 1 shows a schematic longitudinal sectional view of a rear-view mirror according to the invention;

Figure 2 shows the graphical representation of the degree of reflection of a rear-view mirror according to the invention with a flat and one with a curved transparent layer carrier;

Figure 1 shows a transparent layer carrier (1), on the back of which the triple interference layer system is located, consisting of a first dielectric layer with a medium refractive index (3a), a second

dielectric layer with a low refractive index (3b) and a third dielectric layer with a high refractive index (3c). The dielectric layer system (3a-3c) is followed by a metallic reflection layer (2), to which a protective layer (4) can be applied. For a better overview, the dimensions of the mirror, in particular the layer thicknesses, are not drawn to scale.

The rearview mirror according to the invention can be manufactured in the manner described in the following example.

Prepare the surface of a transparent substrate with a flat surface made of soda ash glass. A SiO2 /TiO2 mixed layer with a refractive index of 1.7 and a layer thickness of about 60 nm, a SiO2 layer with a refractive index of 1.45 and a layer thickness of about 95 nm and a TiO2 layer with a refractive index of 2.05 and a layer thickness of about 105 nm were applied to a 2 mm thick layer using a fired dipping process using a SiO2 or TiO2-forming solution and a mixture of both solutions in a mass ratio of 0.9. A chromium layer of about 45 nm thickness was then applied to the triple interference layer system using a vacuum process. The same process was repeated with a transparent layer carrier with a curved surface.

The course of the reflectance was measured on the coated panes when the beam was incident at an angle of 25° to the surface normal of the O reflection mirror in the visible spectral range from the glass side. The spectral course of the reflectance for the curved and the flat mirror is shown in curves 1 and 2 in Figure 2.

Both curves are largely identical. A maximum at around 460 nm is followed by the continuous decrease in the degree of reflection from the short-wave to the long-wave range, which is necessary for glare-free vision. The desired brightness level is also achieved. Both mirrors have a total reflection of around 44%.

Claims (1)

30.01.1990 D 2009 Scfoutzanso.üche . ) 1) Rear view mirror for vehicles, in particular motor vehicles, consisting of a transparent layer carrier, preferably of sodium silicate glass and a rear reflective coating formed by a metallic layer with 50 - 80 % total reflection, measured on the surface against air, and several dielectric interference layers with different refractive indices, which are located between the transparent layer carrier and the metallic layer,
thereby gkmzBkiiiiet, that the interference layer system consists of three dielectric layers, wherein the first layer (3a) lying directly on the transparent layer carrier (1) has a medium refractive index, the subsequent second layer (3b) has a low refractive index and the subsequent third layer (3c) has a high refractive index, and O the thicknesses of the dielectric layers are such that the Rearview mirror has a total reflection of at least 40 % and a blue reflection tint. 2) Rearview mirror according to claim 1,
characterized, that the first dielectric layer (3a) adjacent to the transparent layer carrier (1) has a refractive index of 1.6 to 1.85 and a layer thickness of 40 to 80 nm, the subsequent second dielectric layer (3b) has a refractive index < 1.5 and a layer thickness of 60 S tt - 2- to 120 nm and the subsequent third dielectric layer (3c) has a refractive index > 1.9 and a layer thickness of 70 to 140 nm. 3) Rearview mirror according to claim 1 or 2,
characterized,
that the first layer adjacent to the transparent layer carrier (1) has a layer thickness of ≤ 60 nm, that the subsequent second layer (3b) has a refractive index of 1.45 and a layer thickness of approximately 95 nm and the subsequent third layer (3c) has a refractive index of 2.05 and a layer thickness of approximately 105 nm. 4) Rearview mirror according to at least one of claims 1 - 3,
characterized, that the metallic layer (2) consists of chromium, zinc, tin, nickel or of an alloy of aluminium-copper, nickel-copper, tin-copper or iron-chromium-nickel. 5) Rearview mirror according to at least one of claims 1 - 4,
characterized, that the metallic layer (2) has a thickness of at least 30 nm. 6) Rearview mirror according to at least one of claims 1 - 5,
characterized, that the metallic layer (2) consists of chromium and has a thickness of approximately 45 nm. 7) Rearview mirror according to at least one of claims 1 - 6,
characterized, that the triple interference layers (3a-c) are layers produced using a dipping process. &bull;&bgr; · * &bull; * - 3 &dgr;) Rearview mirror according to at least one of claims 1 - 7, characterized in that at least one of the triple interference layers (3a-c) is a layer produced by means of a vacuum process. 9) Rearview mirror according to at least one of claims 1 - 8, characterized in that that the metallic layer (2) is a layer produced by means of a vacuum process. 10) Rearview mirror according to one of claims 1-9, characterized in that the metallic layer (2) is provided with a protective layer (4).