Optical: systems and elements – Light interference – Produced by coating or lamina
Reexamination Certificate
2000-03-02
2001-06-05
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Light interference
Produced by coating or lamina
C359S589000, C428S403000, C106S415000
Reexamination Certificate
active
06243204
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention is related generally to thin film optical coatings for producing color. More specifically, the present invention is related to multilayer thin film interference coatings or flakes which exhibit color shifting properties with changes in angle of incident light or with viewing angle.
2. The Relevant Technology
Color shifting pigments and colorants have been used in numerous applications, ranging from automobile paints to anti-counterfeiting inks for security documents and currency. Such pigments and colorants exhibit the property of changing color upon variation of the angle of incident light, or as the viewing angle of the observer is shifted. The primary method used to achieve such color shifting colorants is to disperse small flakes, which are typically composed of multiple layers of thin films having particular optical characteristics, throughout a medium such as paint or ink that may then be subsequently applied to the surface of an object.
The color shifting properties of the colorant can be controlled through proper design of the optical coatings or films used to form the flakes. Desired effects can be achieved through the variation of parameters such as thickness of the layers forming the flakes and the index of refraction of each layer. The changes in perceived color which occur for different viewing angles or angles of incident light are a result of a combination of selective absorption of the materials comprising the layers and wavelength dependent interference effects. The absorption characteristics of a material are responsible for the basic color which is observed. The interference effects, which arise from the superposition of the light waves that have undergone multiple reflections and transmissions within the multilayered thin film structure, are responsible for the shifts in perceived color with different angles.
Thin film flakes having a preselected single color have been previously produced, such as disclosed in U.S. Pat. No. 4,434,010 to Ash, in which flakes composed of symmetrical layers may be used in applications such as automotive paints and the like. The flakes are formed by depositing a semi-opaque metal layer upon a flexible web, followed by a dielectric layer, a metal reflecting layer, another dielectric layer, and finally another semi-opaque metal layer. The thin film layers are specifically ordered in a symmetric fashion such that the same intended color is achieved regardless of whether the flakes have one or the other lateral face directed towards the incident radiation.
Examples of color shifting thin films are disclosed in U.S. Pat. No. 4,705,356 to Berning et al. In one embodiment therein, a three layer metal(1)-dielectric-metal(2) stack is disclosed in which metal(1) is a relatively thin, highly absorptive material, metal(2) is a highly reflecting, essentially opaque metal, and the dielectric is a low index of refraction material. In an alternative embodiment, an all-dielectric stack is disclosed. These designs, however, require the use of so-called subtractive colorant carrying superstrates which utilize dyes to achieve the desired colors. Further, the high numbers of layers used in all-dielectric stacks are known to suffer from mechanical stresses, which in turn are extremely detrimental to color characteristics, since nonuniform surfaces will themselves cause destructive interference effects which will effectively “wash out” the intended colors.
Further improvements in the optical characteristics of thin film flakes which may be used in paints and inks such as for anti-counterfeiting applications are disclosed in U.S. Pat. No. 5,135,812 to Phillips et al. According to this patent, a symmetrical optical multilayer film is composed either of transparent all-dielectric stacks, or transparent dielectric and semi-transparent metallic layered stacks. In the case of an all-dielectric stack, the optical coating is made of alternating layers of high and low index of refraction materials. Suitable materials disclosed are zinc sulfide or titanium dioxide for the high index layers, and magnesium fluoride or silicon dioxide for the low index layers.
In U.S. Pat. No. 5,278,590 to Phillips et al., a symmetric three layer optical interference coating is disclosed which comprises first and second partially transmitting absorber layers which have essentially the same composition and thickness, and a dielectric spacer layer located between the first and second absorber layers. The dielectric layer is composed of a material having a low index of refraction such as magnesium fluoride.
High chroma interference platelets for use in paints, including color shifting and nonshifting single color platelets, are disclosed in U.S. Pat. No. 5,571,624 to Phillips et al. These platelets are formed from a symmetrical multilayer thin film structure in which a first semi-opaque layer such as chromium is formed on a substrate, with a first dielectric layer formed on the first semi-opaque layer. An opaque reflecting metal layer such as aluminum is formed on the first dielectric layer, followed by a second dielectric layer of the same material and thickness as the first dielectric layer. A second semi-opaque layer of the same material and thickness as the first semi-opaque layer is formed on the second dielectric layer. For the color shifting designs, the dielectric materials utilized have an index of refraction less than 2.0, such as magnesium fluoride. For the nonshifting designs, the dielectric materials are selected to have an index of refraction greater than 2.0, such as titanium dioxide or zinc sulfide.
SUMMARY AND OBJECTS OF THE INVENTION
It is a primary object of the invention to provide color shifting flakes which have a wide range of color shifting properties.
A further object of the invention is to provide color shifting flakes which have substantial shifts in chroma and hue with a varying angle of view.
It is yet another object of the invention to provide color shifting flakes which may be easily and economically utilized in colorants such as paints and inks for various applications.
To achieve the forgoing objects and in accordance with the invention as embodied and broadly described herein, a color shifting multilayer interference film is provided which may be used to produce flakes for pigments having color shifting properties. The flakes can be interspersed into liquid media such as paints or inks which can subsequently be applied to objects or papers to achieve color variations upon shifts in angle of incident light or upon shifts in viewing angle.
In one embodiment of the invention, a five layer design of the interference film includes a first absorber layer, a first dielectric layer on the first absorber layer, a reflector layer on the first dielectric layer, a second dielectric layer on the reflector layer, and a second absorber layer on the second dielectric layer. The first and second absorber layers are preferably composed of the same materials, and the first and second dielectric layers are preferably composed of the same materials. The first and second dielectric layers have an index of refraction of greater than about 1.65 and an optical thickness at a design wavelength that provides a color shift as the angle of incident light or viewing changes.
In another embodiment, a three layer design of the interference film includes a first absorber layer, a dielectric layer on the first absorber layer, and a second absorber layer on the dielectric layer. The first and second absorber layers are preferably composed of the same materials. The dielectric layer has an index of refraction of greater than about 1.65 and an optical thickness at a design wavelength that provides a color shift as the angle of incident light or viewing changes.
In a method of fabricating a color shifting multilayer interference film according to the present invention, a web material with a release layer formed on an upper surface is utilized. The various absorber, dielectric, and reflector layers as descri
Bradley, Jr. Richard A.
Witzman Matthew R.
Flex Products, Inc.
Jr. John Juba
Spyrou Cassandra
Workman, Nydegger & Seeley.
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