Optical: systems and elements – Single channel simultaneously to or from plural channels – By partial reflection at beam splitting or combining surface
Reexamination Certificate
2000-03-10
2001-09-18
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By partial reflection at beam splitting or combining surface
C359S839000, C359S580000
Reexamination Certificate
active
06292302
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to mirrors, and particularly to dichroic mirrors that are formed utilizing flat substrates such as glass and which subsequently are subjected to a heat treatment to temper or bend the mirror.
BACKGROUND OF THE INVENTION
Dichroic mirrors, that is mirrors that exhibit different colors in reflection and transmission, can be employed as rearview mirrors for motor vehicles and for other purposes. A mirror can be manufactured by applying a reflective coating to a glass substrate using magnetron sputtering techniques of the type described in Chapin, U.S. Pat. No. 4,166,018. Although a reflective metal such as chromium or silver may be employed as a reflective film for standard mirrors, dichroic mirrors commonly employ two contiguous films or films of materials having different refractive indices, and reflection occurs at the interface of these films.
Dichroic mirror assemblies that are used for rearview mirrors for vehicles may be provided with light sources such as light emitting diodes (“LEDs”) carried in housings forward of the mirror so that light from the LEDs is transmitted rearwardly through the mirror. Examples of such mirror assemblies are found in Roberts, U.S. Pat. Nos. 5,481,409, 5,355,284 and 5,207,492, and Roberts et al. U.S. Pat. No 5,361,190.
Substantial optical performance is required of these dichroic mirrors, inasmuch as the mirrors must be highly reflective so that a motor vehicle operator can clearly see what is happening near the rear of the vehicle. Transmittance, of course, is important so that sufficient LED light passes through the mirror without the necessity of using extremely powerful and hence expensive LEDs. LEDs may signal a variety of events, both to the driver and to following traffic. LEDs may signal when doors have been opened, when brakes are applied, etc. Although some LED signals need only be bright enough to be readily perceived by a motor vehicle operator, others should be sufficiently bright so as to be perceived by vehicles following the vehicle. For example, it is difficult for following traffic to see when the side door of a passenger or cargo van has been opened. The appearance of a bright LED signal in the van's exterior rearview mirror can signal to following traffic that the door is open and passengers may be about to exit.
When curved mirrors are manufactured using magnetron sputtering processes, the glass substrates for the mirrors may first be bent to the desired shape, and then can be placed on an appropriate carrier and coated by magnetron sputtering. Due to curvature of the substrates, the reflective coatings that are produced may not be precisely uniform. The manufacturing process itself is tedious and time-consuming inasmuch as it requires multiple small glass substrates to be laid by hand upon a carrier that passes through a magnetron sputtering apparatus and requires each of the resulting individual mirror pieces to be removed by hand from the carrier sheet once the sputtering operation is completed.
It would be desirable to provide a dichroic mirror utilizing a flat glass sheet as the substrate, and thereafter bending and cutting the mirror as desired, with the mirror retaining good reflectance so that objects to the rear of the driver can be seen and good transmittance so LED signals can be easily perceived through the mirror. Unfortunately, the heat bending process, which may require temperatures ranging from about 650° C. to about 800° C., can produce undesired changes in commercially available dichroic mirrors. Reflectivity of such mirrors often is greatly decreased, and the development of haze may also present a problem. Haze in excess of 1%, in particular, should be avoided.
SUMMARY OF THE INVENTION
The present invention provides a dichroic mirror that can be subjected to substantial temperatures during tempering or bending and which, after such treatment exhibits excellent reflectance and transmittance properties. In one embodiment, the invention relates to a heat treatable dichroic mirror that comprises a transparent substrate having a glass transition temperature in the range of 650-800° C. and which carries a plurality of sputtered-on films. The films form at least two and preferably at least three pairs of contiguous films, the films of each pair having disparate refractive indices (that is, indices of refraction differing by at least about 0.2 and preferably by at least about 0.4) so as to provide between them a reflective interface. Each film may be a member of one or two pairs of contiguous films having disparate indices of refraction.
A first film, which is a member of a contiguous film pair having disparate refractive indices, comprises an oxide of a metal, for example, an oxide of titanium. A second film further from the substrate than the first film comprises an oxidizable film, preferably of a metal or semi-metal such as silicon, and is also a member of a contiguous film pair having disparate refractive indices. The first and second films may be contiguous, or may have between them a third, oxygen barrier film forming a contiguous, disparate index film pair with at least one of and preferably both of the first and second films so as to provide reflective interfaces with the latter films and to restrain permeation of oxygen through it to thus protect the second film from oxidation.
A fourth protective overcoat film is provided further from the substrate than the second film and of a thickness and composition sufficient to substantially prevent penetration of oxygen therethrough during heat treatment at the glass transition temperature and to provide protection against physical damage to the films. The third and fourth films preferably sandwich between them and are contiguous to the second film.
As a fifth film, a base film may be positioned between the substrate and the first film. The fifth or base film desirably is immediately beneath (that is, contiguous to) the first film and has an index of refraction substantially different from the index of refraction of the first film. For example, if the first film is titanium oxide which has an index of refraction of about 2.4, the base film may be silicon nitride or zinc oxide, each of which has an index of refraction of about 2.0. As with the first and second films, the difference in indices of refraction between the fifth base film and the first contiguous film provides a refractive interface that contributes to reflectivity of the mirror. Moreover, the overcoat film desirably is contiguous to the second film and has an index of refraction substantially different from that of the second film
The resulting dichroic mirror, after heat treatment, exhibits a transmittance of at least 24% and preferably at least 35%, and a reflectance of at least 45%. The haze desirably is not greater than about 1% and preferably not greater than 0.5%. Of particular importance is the protective overcoat film, as it is this film that protects the oxidizable element from aid and substantially prevents oxidation of that film so that reflectance is not significantly altered by the heat treatment.
Desirably, the first film includes an element or compound that is different from the metal oxide of this film and which may be thought of as an impurity. The impurity may be a compound of the metal of that film, and is present in an amount sufficient to retard haze formation in that film upon heat treatment. The mol ratio of the different material to the oxide of that metal does not exceed 0.1 and preferably ranges from about 0.001 to 0.1.
Thus, in another embodiment, the invention involves a method of making a dichroic mirror that comprises magnetron sputter coating a plurality of sequential films upon a transparent substrate having a glass transition temperature in the range of 650-800° C., the films including at least two and preferably at least three pairs of contiguous films, the films of each pair having disparate refractive indices (that is, indices of refraction differing by at least about 0.2 and preferably by at least about 0.4) so as
Krisko Annette J.
Maxwell Scott A.
Cardinal Glass Industries, Inc.
Fredrikson & Byron , P.A.
Mack Ricky
Seyrafi Saud
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