I. Field of the Invention
This invention relates to optical interference structures capable of filtering the visible spectrum into one or more bands of relatively high reflectance bounded by bands of relatively low reflectance, thus exhibiting a coloured appearance when illuminated with white light. More particularly, the invention relates to optical interference structures of this kind incorporating porous dielectric films.
II. Description of the Prior Art
It is known that a variety of structures are capable of producing interference colours of the type mentioned above. One class of known optical interference structure employs multiple transparent or semi-transparent layers to achieve the filtering effect. There are two basic designs, namely the all-dielectric stack (multiple thin layers of dielectric material of alternating high and low refractive index) and metal-dielectric stacks (alternating metal and dielectric layers, with all of the metal layers being semi-transparent) except for an opaque bottom layer when used in the reflection mode.
In these structures, multiple reflections from the various layers lead to a filtering action due to constructive or destructive interference of light in different wavelength bands. These two filter types are the basis of precision optical elements for a variety of applications such as lens coatings.
One limitation of these conventional structures is that they are produced by costly techniques involving vacuum deposition methods such as evaporation and sputtering; this restricts their use to precision optics or high value coatings. In particular, dramatic colour effects arising from the filtering action of such structures in the visible region of the spectrum are normally not exploited for consumer applications.
A second limitation of conventional multilayer stacks is the instability of their optical response arising from effects due to moisture adsorption. Voids are present in dielectric layers as they are normally produced by vacuum deposition and these permit the penetration of atmospheric moisture into the film over a period of weeks which shifts the characteristics of narrow band filters from their designed response. This problem is well-known in the optical coating industry and great emphasis has been placed on finding new deposition methods to minimize the void fraction and obtain maximally dense films. With present technology, densities of dielectric films are usually in the range of 80-95% of bulk values and often in excess of 90%.
In contrast to this, in the past, we have created optical interference effects in structures which incorporate highly porous anodic films. This developed from the need to colour the surfaces of anodized aluminum articles. Anodizing is a well known technique for producing decorative and protective coatings on aluminum or aluminum alloys (see, for example, the monograph entitled "The Surface Treatment and Finishing of Aluminum and Its Alloys" by S. Wernick and R. Pinner, Robert Draper Ltd., UK, 1972). Anodizing of aluminum in selected electrolytes produces an oxide film consisting of an outer layer containing a dense array of fine pores orientated perpendicularly to the treated surface and an inner non-porous barrier layer of compact oxide separating the porous layer from the underlying aluminum. The anodic film is normally transparent or translucent depending on the film thickness. Films that are at least several micrometers thick (generally 10.mu. or more) are hard and durable and are used as protective coatings.
For decorative applications, such films have been coloured by the electrolytic deposition of a metal or metal compound (inorganic pigment) into the pores as also described in the above cited monograph by Wernick and Pinner. The colours that can be obtained in this way are rather limited, ranging from brown through bronze shades to black as the pores are increasingly filled with pigmentary deposit. The colouring effect is due to scattering and absorption by the deposit within the anodic film of light reflected from the surface of the underlying aluminum metal.
In our British Patent Specification No. 1,532,235 on Nov. 15, 1978 (the disclosure of which is incorporated herein by reference), we have described products in which a new range of colours can be obtained by electrocolouring, the colour being due to optical interference in addition to the scattering and absorption effects already noted. In this process, the pigmentary deposit is controlled so as to be thinner than in the original electrocolouring process and the heights of the deposits are made more uniform throughout the film. The generated colour is the result of interference between light reflected at the outer ends (relative to the aluminum surface) of the individual deposits and light reflected at the aluminum/aluminum oxide interface. The colour produced depends on the difference in optical path resulting from separation of the two light reflecting surfaces (the tops of the deposits and the underlying aluminum surface) and is selected by controlling the height of the deposits. Practically useful colour effects can thus be realized, though the colours tend to be pastel-like for thinner deposits and to have a somewhat muddy appearance for thicker deposits, probably due to the increased contribution of the scattering and absorption effects, noted above, to the interference effects. The appealing colour effects that can be achieved by this "first generation" interference structure are thus somewhat limited.
In our subsequent U.S. Patent No. 4,310,586 published on Jan. 12, 1982 (the disclosure of which is incorporated herein by reference), we described the achievement of significantly clearer and brighter colours by growing additional oxide film beneath relatively shallow deposits. These colours are due to the same interference effect between light reflected from the same two reflecting surfaces as in the first generation structure described above. In this case, however, the colour is selected not by varying the height of the metal deposits, but by continued anodizing after the electrodeposition stage to thicken the oxide below the deposit. The difference in optical path between light reflected from the tops of the deposits and from the aluminum surface is thus controlled, along with the colour, by moving the aluminum surface relative to the tops of the deposits rather than vice versa as in the previous case. In these "second generation" interference structures, a wider range of bright colours is accessible because the separation of the two reflecting surfaces can be varied more widely without increasing the thickness of the deposit and hence the associated scattering and absorption effects.
In all of these previously described coloured anodic film structures, the thickness of the porous anodic film is greater than 3 micrometers and typically from 10-20 micrometers. Interference effects in these films are between light reflected from the metal deposit and from the underlying aluminum substrate; the porous oxide above the deposits does not play any part in the colour generation since it has a thickness greater than 3 .mu. and is thus "optically thick". Moreover, the pores above the deposits are typically filled and sealed by the well-known boiling process to enhance the chemical and mechanical resistance of the anodic film (see the Wernick & Pinner monograph mentioned above).
Despite the improved colours produced by the second generation structures disclosed in our U.S. patent mentioned above, there is still a need for optical interference structures having extended capabilities for new applications.
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