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1. WO2018130856 - APPLICATION OF AN OPTICAL EFFECT

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

APPLIC&f ¾ON OF AN OPTICAL EFFECT

The invention relates to a method of imparting structural colour to an object and an object to which structural colour has been imparted.

Optical effects such as structural colour may be provided by an optical coating structure which is a stack of thin layers of material deposited on a substrate or object in a way that alters the way in which the object reflects and transmits light. The thin layers are deposited typically to a thickness of between 10 nm to 200 nm.

For example, a quarter-wave stack reflector is a well-known building block of optical thin-film products. Such a stack generally comprises alternating layers of two or more dielectric materials with different refractive indexes, in which each layer has an optical thickness (i.e., the geometric thickness of the layer multiplied by the refractive index of the layer material) that corresponds to one-quarter of the principal wavelength of reflection. Here, the wavelength of light reflected varies with angles of incidence and reflection, thus one can observe different optical effects/colours at different viewing angles; a visual effect known as iridescence.

In this manner, an optical coating structure can be constructed to accurately and selectively reflect certain wavelengths of light in order to impart a desired colour to an object at particular angles of incidence. Unlike paints in which the colour is determined by pigments or dyes that are held together with binders, with optica! coating structures the transition from near total reflection to maximum transmission can take place over a very short wavelength range, enabling a precise discrimination between different wavelengths. As a result, objects coated with such structures can take on a sharp and well-defined optical effect such as colour, albeit each colour is observable over a narrow range of directions only. These flat multilayers give the appearance of a spot of white (the mirror-reflection point) surrounded by a halo of the colour, in turn surrounded by black (most of the object will be black).

WO 2011/161482 and WO 2016/156863 each disclose optical effect structures which comprise a reflector deposited on profile elements. This imparts a relief into the reflector such that these optical effect structures provide a bright colour effect with minimal iridescence and/or the appearance of a more complete coverage of a surface with the optical effect.

At any given time the eye detects only a narrow range of the potential angles of reflection from an object, and global averaging of the wavelengths

gathered at the retina occurs within that narrow range of detection. As known, the colour of light observed at a particular angle will depend on the optical distance of each layer through which the light travels. When a multi-layer reflector is viewed normal to the underlying base layer the light will travel a distance through each layer which is equal to the thickness of each layer. As a result, due to interference effects and global averaging in the eye, the colour of light observed normal to the coating will be the colour of light which is determined by the geometrical thicknesses of the layers and the respective refractive indexes of the layers. In a normal quarter wavelength stack (i.e. with no underlying profile elements), when viewed from an angle, the light detected by the eye at that angle will have travelled slightly further through each layer (a distance greater than the thickness of each layer) and thus the optical thickness travelled by the light rays will be larger. Light with a longer wavelength will be observed from the broader viewing angles giving rise to iridescence.

In the structures disclosed in WO 2011/161482 and WO 2016/156863, the presence of the underlying profile elements causes the layers to vary in angle relative to the plane of the substrate/base layer (as they follow the profile of the profile elements). Consequently over a broader range of viewing angles, a significant proportion of the reflector layer surfaces producing the observed reflections will be orientated more to the observer in a way that also substantially maintains the intended thicknesses in the layers of the reflector, i.e. the normal position. As a result, the colour observed by the eye over that broader range of angles is relatively constant.

Compared to an optical coating structure with a flat reflector, the visual experience gained with these optical effect structures is a velvety, luxurious appearance and is more similar to the effect of pigment. This difference in the effect may in part be due to the perceived complete coverage of an object with colour in contrast to a flat reflector that gives the above described effect of a white spot, surrounded by a halo of the colour, in turn surrounded by a mostly black surface.

Whilst these structures with a profiled reflector can provide a good visual effect, it remains desirable to provide new methods (which in turn may produce new structures) for producing an optical coating structure capable of providing structural colour that is sufficiently bright and that exhibits a minimal or limited iridescence effect, i.e., so that the optical effect/colour remains substantially the same to an observer over a broad range of viewing angles and/or over a large amount of the surface, whilst being relatively easy, cheap and/or reliable to produce.

In an aspect, the present invention may provide a method of imparting structural colour to an object, the method comprising: providing a substrate, wherein the substrate comprises a base layer with profile elements thereon and a reflector on the base layer over the profile elements, wherein the reflector is for imparting a colour effect; attaching the reflector to a surface of the object; and removing the base layer from the reflector.

The attaching of the reflector to the object may impart an optical effect to the object to provide an article with an optical effect, i.e. to provide a structurally coloured article.

The present invention may provide an article that is made using the above method.

In another aspect, the present invention may provide an article (i.e.

structurally coloured article), the article comprising: an object; and a reflector attached to a surface of the object to impart structural colour to the article; wherein the reflector has a relief made up of a plurality of relief structures that has been made by forming the reflector on a base layer with profile elements thereon; and wherein the reflector is no longer in contact with the base layer on which it was formed, i.e. the base layer has been removed.

Structural colour may be an optical effect that is the appearance of colour to an observer due to the structure of the optical effect structure, i.e. the reflector is structured to provide a visual effect of colour.

The reflector may be formed using a plurality of profile elements that deform the reflector to provide relief structures that create the more constant structural colour effect but the final product may not comprise the profile elements used to deform the reflector.

The following described optional features may be applicable to one or more of the above aspects of the invention. Whilst the invention is described as providing structural colour, the method of forming a profiled reflector and applying it to an object may be applicable to producing structures that cause other optical effects, such as anti-reflection and transparency etc.

The reflector may have a relief made up of a plurality of relief structures that correspond to the profile elements of the base layer.

The reflector may form an optical effect structure. This optical effect structure may be for imparting an optical effect that is a desired colour to an object when applied onto its surface.

The object may be an object to which an optical effect in the form of structural colour has been imparted. The reflector and the object may together provide an article with an optical effect in the form of structural colour to provide a structurally coloured article.

The substrate may be a substrate with an optical effect such as a structurally coloured substrate.

The advantage of removing the base layer is that the total thickness of the article and/or optical effect structure created by the effect of the reflector, may be decreased. This may be particularly the case if the glue layer is thin, e.g. thinner than the base layer. Also, the strength of the bond between the reflector and the base layer does not affect the strength of the final product.

The steps of the method may or may not be performed in the order that the steps are recited in the above aspects and in the appending claims. For example, the base layer may be removed from the reflector before it is attached to the object.

Alternatively, the base layer may be removed from the reflector after it has been attached to the object.

The attachment between the reflector and the object may be stronger than the attachment between the reflector and the base layer. The attachment between the reflector and the base layer may be weaker than the material of the object, reflector and/or base layer. As a result, when force is applied to the base layer relative to the reflector (or vice versa), the base layer may detach from the reflector with minimal or no damage to the reflector.

The object may be any product or device such as a plastic case for a communication device such as a mobile telephone, watches, computers, pens, household objects, coatings for vehicles, glass or crystal ornaments, glass, crystal, metal or polymer jewellery, or plastic objects, or to smaller surfaces such as flakes for cosmetics or paints. A product or device may be referred to as an object or vice versa.

The object may be a transparent, or at least partially transparent, object. The object may for example be a glass and/or crystal object. For example the object may be a cut glass and/or crystal object.

The surface of the object to which the reflector is attached may be the underside of the glass and/or crystal object.

The reflector may be attached to the object by glue.

The glue may be a transparent and/or refractive index matching glue. This may be advantageous when the object is a transparent object.

The glue may be a two component Epoxy Resin Glue such as CG 500-35 (A) + (B).

The article may comprise no or minimal air bubbles between the reflector and the object. The method may comprise ensuring that there are no or minimal air bubbles between the reflector and the object.

Force may be applied to force the reflector and base layer apart.

The reflector may be removed from the base layer, and transferred to the object.

The profile elements may remain intact on the base layer (i.e. only the reflector is transferred to the object to form the structurally coloured article).

The reflector and the base layer may be separated by freezing the substrate, heating the substrate, applying pressure to the substrate, applying force to the substrate and/or peeling the reflector and base layer apart.

For example, in the case that the reflector and/or the base layer are formed of a flexible material, the reflector and base layer may be peeled apart.

Force may be applied substantially parallel to the plane of the substrate (i.e. applied laterally) to break the attachment between the reflector and the base layer. Additionally (i.e. after the lateral force has been applied) or alternatively, force may be applied perpendicular to the plane of the substrate to pull the reflector and the substrate apart.

The force applied to the reflector or base layer relative to the other of the two layers may be greater than the bond force between the two layers (i.e. the reflector and the base layer).

The reflector may be covered, e.g. coated, on its surface opposite to the surface from which it is, in use, to be viewed with a material which is opaque to at least visible light. This may form a back layer on the reflector. This may be to prevent light coming through the reflector. Having a black back layer may prevent stray light entering the reflector from the surface opposite to the surface that is viewed and thus allow a more brilliant colour to be produced by absorbing those wavelengths in incident light that are not desired for reflection.

For example, if the object Is transparent, the reflector may be in use viewed through the object (such that the optical effect, e.g. colour, is seen through the object) and in which case the black layer may be applied on the exposed surface of the reflector. If the object is opaque, the black cover may be provided on a surface of the reflector that faces the object.

Black material, such as black paint, may be applied to the reflector (e.g. the surface to which the base was attached). There may be no air gap between the black material and the reflector. The black layer may be applied after the base layer has been removed. This may be advantageous when the object is a transparent object. The black material may be applied to the underside of the (exposed) reflector. This may restore bright colour to the article.

It has been found that if a white or transparent backing layer is used a pearly effect may be obtained.

The base layer may be formed of acrylic.

The shape of the surface of the object may be complementary to the shape of the surface of the base layer, reflector and/or substrate. For example, the surface of the object may be the inverse of the surface of the base layer, reflector and/or substrate. This may facilitate the attaching of the reflector to the object as they may have complementary shaped surfaces.

The base layer, reflector and/or substrate may be a curved surface. For example, the base layer, reflector and/or substrate may be convex and/or concave surface. The surface of the object to which the reflector is to be attached may be the inverse/negative curve.

The profile elements on the base layer may have a width and length which are each in the range of less than 1 micron or 5 to 500 pm in size. Accordingly, the relief structures in the reflector (which were created by distorting the reflector so that it conforms to the profile elements) may have a width and length which are each in the range of less than 1 micron or 5 to 500 pm in size.

The profile elements and/or relief structures may have a width, length and/or height which are each in the range of 25nm to 500 pm in size.

The profile elements may be sub-micron in size. By this it is meant that the main dimensions of the profile elements, for example, the cross-sectional dimensions as provided by the width and height of the elements, are less than 1.5 pm, more preferably less than 1 pm. For example, the profile elements may have an average base width of between 50 nm to 500 nm, more preferably 200 nm to

450 nm and/or the profile elements may have a height in the range of 25 nm to 250 nm, more preferably 75 nm to 200 nm and more preferably still a height of 100 nm to 170 nm.

The profile elements and/or optical effect structure may be as disclosed in WO 2011/161482 for example.

The profile elements and relief structures may be arranged in non-periodic manner or a periodic manner.

The reflector may be distorted by the underlying profile of the profile elements on which it is formed. These base structures may cause the reflector that may have been deposited thereon to conform to their profile.

The relief structures (protrusions and/or indents) are introduced to an outermost layer of the reflector (this is due to the profile elements being on a base layer to which the reflector is applied) to cause a degree of scattering in the reflector. The relief structures may extend entirely through the reflector. For example, in the case of a multilayer reflector each layer may comprise bumps and/or protrusions (i.e. relief structures) that correspond to the profile elements that were used to shape the reflector.

The optical coating structure may provide a metallic, almost anodised appearance.

The structural colour may be used in place of pigments that would otherwise be used to impart colour to the object.

The reflector may have a near-identical profile to that of the profile elements, i.e. the reflector may comprise relief structures therein that correspond to the profile elements. In the case of a multilayer reflector each layer may have substantially a near identical profile. This profile may be the same as that of the profile elements. This is because the dimensions of the profile elements may be significantly larger than the thickness of the layer(s) of the reflector. For example, the reflector may have a thickness less than 1 m, or less than 200nm.

The profile elements of the base layer and hence the relief structures in the reflector may be shaped so as to result in a portion of the reflector, for example greater than 25%, greater than 30%, greater than 40% or greater than 50% being approximately normal to the incoming light when the light is incident at the normal to the plane of the reflector. The profile elements/relief structures may also be shaped so as to result in a portion of the reflector, for example greater than 25%, greater than 30%, greater than 40% or greater than 50% being approximately

normal to the incoming light when the light is incident at an angle of up to 45 degrees.

It has been found that having greater than 25%, greater than 30%, greater than 40% or greater than 50% of the surface normal of the reflector to the incoming light (even for angles up to about 45 degrees) is sufficient for just the optical effect, e.g. colour, created by the reflections at the normal to be perceived, e.g. a single rich colour may be observed.

The surface norma! of the reflector may vary in a length and/or width direction of the reflector surface.

References to the features of the profile element are equally applicable to the relief structures of the reflector and vice versa. This is because the relief structures correspond to the profile of the profile elements on which they are formed. Thus where details are provided of the profile elements these may also be the features of the relief structures and vice versa.

The profile elements/relief structures may be mainly between 5 and 500 μηι in width and length. By this it is meant that the width and length of at least 60%, 75%, 80%, 90% or 99% of the profile elements (by area), for example, may be between (and including the end values of the ranges) about 5 to 500pm, 5 to 100pm, 5 to 30pm, 10 to 30pm or about 10pm.

One preferred range for the width and length dimensions of the profile elements is 10 to 50pm, though commercial considerations may favour larger profile elements/relief structures, e.g., extending up to 500pm (particularly, but not exclusively, in case where the profile elements are in the form of recesses). This may account for 80%, 90%, 95% or 99% of the profile elements (by area).

The height of the profile elements/relief structures may be between 0.1 and

50 pm, 1 to 5pm, or 2 to 3pm. At least 60%, 75%, 80%, 90% or 99% of the profile elements (by area), for example, have a height between (and including the end values of the ranges) about 0.1 and 50 pm, 1 to 5pm, or 2 to 3pm.

The profile elements and/or optical effect structure may be as disclosed in WO 2016/156863 for example.

The height of a profile element/relief structure may be the dimension of the profile element/relief structures (in a direction perpendicular to the plane of the surface of the base layer/reflector) from the lowest point of a trough in the base layer/reflector to the furthest most point of the profile elements/relief structures from the plane of the surface. The length of a profile element/relief structure may be the dimension of the profile element/relief structure which is largest in a direction which is parallel to the plane of the substrate (i.e. base layer or reflector) and the width of the profile element/relief structure may be the dimension which is parallel to the plane of the substrate and perpendicular to the length of the profile element/relief structure.

For example, in the case of a conical profile element the height would be the distance from the plane of the substrate to the peak/tip of the cone and the width and length would both be equal to the diameter of the cone at its base, i.e. at the plane of the surface. In the case of a concave indentation profile element, the height would be the distance from the bottom of the indentation to the top.

At least 60%, 75%, 80%, 90% or 99% of the profile elements/relief structures (by area), for example, may preferably have a height of <50 μιτι, more preferably≤25 pm and more preferably still <15 pm; most preferably these profile elements are of a height≤10 pm. In some embodiments, at least 60%, 75%, 80%, 90% or 99% of the profile elements (by area), for example, have a height in the range of (and including the end values of the ranges) 1 to 10 pm, more preferably 2 to 10 pm.

The average period (this is the average distance between adjacent profile elements/relief structures (e.g. peak to peak)) between adjacent profile

structures/relief structures may be between 5 and 100pm, or about 25pm. The desired average period may depend on the size and/or shape of the profile structures.

The profile elements may comprise an upper convexly curved surface that may create a concavely curved relief structure in the surface of the reflector in contact with the base layer during formation. The curved surface may extend symmetrically either side of an uppermost point.

The profile elements may be protrusions from the surface of a base layer that form indents in a surface of the reflector in contact with the base layer during the formation of the deformed reflector.

The profile elements may be indents (hollows) formed in the surface of a base layer that form protruding relief structures from the surface of the reflector in contact with the base layer during formation.

While the profile elements/relief structures are preferably smooth, curved shapes, they may include steps or flat sides to create the overall shape of the outwardly extending projection or inwardly extending indentation.

The profile elements may be protrusions or indents formed by roughening the surface of a base layer. In one example, the roughened surface is a roughened glass surface. In another it is a roughened plastic surface. Other materials, such as ceramics and metals may also make suitable base layer materials.

The profile elements (whether protrusions or indents) and hence the relief structures may be approximately conical or frusto-conical (i.e. the projections or indents may have horizontally or diagonally flattened tops). These profile elements may be roughly shaped (and the elements may vary in height, width and length between profile elements) or the elements may be neatly shaped and be all substantially the same shape and have approximately the same dimensions.

The profile elements (whether protrusions or indents) may have an approximately part-spherical surface, for example, a convex or concave surface respectively having a substantially even radius of curvature. The curvature of adjacent profile elements may be substantially the same or may be slightly different, for example, within 50%, more preferably within 25%. The approximately part-spherical surface, as well as including smooth curved surfaces, may also comprise small steps or terraces, when viewed in close-up detail, for example, which may for example be artefacts from the manufacturing process in a 3D etching or printing process.

The profile elements/relief structures may be a series of abutting or juxtaposed hollows/indents.

The profile elements/relief structures may be randomly shaped plates (e.g. tiles) which have raised ends which overlap an adjacent plate.

The profile elements/relief structures may include a curved surface. The surface may be entirely curved and/or angled relative to the plane of the base layer (at the micron level).

The base structure and hence a surface of the reflector may comprise flat portions. These flat portions may be on the profile elements themselves and/or on the surface of the base layer between the profile elements. These fiat portions may have a width dimension which is at least 0.5pm. If present, the flat portions may have a width and/or length which is between 0.5pm and 10μιτι. When such flat portions are present the reflector may cause an increased sparkling effect. This is because there can be a strong mirror reflection from each flat plane. The flat portions may vary with respect to their position and orientation on a profile element.

The profile elements/relief structures may have height and width dimensions which are within a factor of three (0.33w < h < 3w where w is the width and h is the height of the profile element/relief structure) and/or height and length dimensions which are within a factor of three (0.33I < h < 3! where I is the length and h is the height of the profile element/relief structure) and/or width and length dimensions which are approximately the same or within a factor of three (w < 3I), two, 1.5, 1.2. The width and length of the profile elements/relief structures may be substantially the same. For example, the profile elements/relief structures may have a substantially circular cross-section (in a plane parallel to the plane of the base layer/reflector).

The profile elements/relief structures (for example at least 50%, 60%, 75% or 90% by area) may have a ratio of height and width or length dimensions between 1 :2 and 1 : 100, or 1 :5 and 1 :50, or 1 :5 and 1 :10.

The profile elements/relief structures may have variable size or a range of different sizes.

The profile elements/relief structures may be closely spaced or juxtaposed. For example, the maximum gap (i.e. fiat base layer) between profile elements/relief structures may be less than 25 m, 10pm, 5 m, 1 pm, 0.5μιη or 0.1 μιη. For example the profile elements/relief structures may appear to be an array of overlapping bumps/protrusions/indents. For example, the profile elements/relief structures may be in a form that mimics the pattern made by a plurality of closely packed bubbles on the surface of a liquid. The profile elements/relief structures may look like the tops of peaks which are closely spaced or have been 'pushed' together.

The profile elements/relief structures may be closely spaced so that there are substantially no flat portions between the profile elements/relief structures, i.e. the edges of each profile element profile elements/relief structure may be in contact.

The edges of the profile elements/relief structures may form an angle to the plane of the base layer which is less than 45 degrees, 30 degrees, 25 degrees or 20 degrees.

The profile elements/relief structures may be arranged in a random or pseudo-random manner. This may be referred to as a non-periodic manner. When the profile elements are non-periodic it may be possible to prevent significant

diffraction caused by the profile elements/relief structures. In other words, there should be no obvious periodicity.

The profile elements/relief structures may also be arranged more evenly, for example, hexagonal close-packed, where the size of the profile elements is sufficient to prevent the optical coating structure acting as a diffraction grating. Accordingly they may also be arranged in a periodic manner. However,

random/pseudo-random arrangements of profile elements/relief structures may be generally preferred, partly from a manufacturing perspective (random structures can be produced easily using acid-etching processes) and also from avoiding diffraction effects.

In this specification, "pseudo-random" is considered as a random

arrangement of several adjacent structures that might reveal some degree of order over a "larger" area, such as when examined using a Fourier analysis, so that there is some, but not exclusive, constructive interference of reflected light rays of the same wavelength from the larger area). Consequently, the profile elements/relief structures serve only to broaden the angular range of the light of wavelength reflected at the surface normal from a reflector.

By arranging the profile elements/relief structures in a pseudo-random scattering pattern, in the case of a reflector for imparting structural colour, e.g. a multi-layer reflector, the optical effect of the relief structures of the reflector will be to reduce the reflector's optical effect, e.g. property of colour change, with changing angle, so that the object may take on a single optical effect, e.g. single colour, that is visible from a range of angles with little or no perceived iridescence and/or appears to cover a large percentage of the surface. The colour imparted may be generally brighter than most pigments while possessing a subtle and rich appearance that is not glossy but instead a mesmerizingly deep, luxurious matt effect suggesting the impression of solid metal. This colour may have the advantage that it would not fade over time when exposed to light as occurs with pigments.

If the profile elements/relief structures are too ordered, particularly at the lower ends of the size ranges, they may cause some (undesirable) iridescence (e.g. significant colour change with angle through diffraction).

At any given time, the eye detects only a narrow range of the potential angles of reflection from an object (unless extremely close to the object), and global averaging of wavelengths gathered at the retina occurs within that narrow range of

detection. Therefore the profile elements described here may provide the visual effect of a rich, single colour observable over a range of angles (for instance up to 20°, more preferably up to 45°, either side of the surface normal), i.e. with minimal iridescent/colour change effect but with an appearance slightly brighter than that of a pigment or dye, and over a large amount of the surface.

In the case of a colour effect observed by a human eye, at certain wavelengths in particular, it has been found that a change of wavelength of up to 30nm is not perceived by the observer or does do have a significant effect on the viewer's experience. Thus, whilst a small shift in wavelength may occur over viewing angle, this may not be 'seen' by a human. For example, it has been found that the human eye does not distinguish different shades of blue well, but it does for green. For example, a difference of 25nm in wavelength in the blue range may lead to a different blue, but this may not affect the viewer's experience much. A 5nm change in the wavelength in the blue range may not be discernible, whereas a change of 5nm, or even 2nm in the green range and the hue of colour may appear fairly different to an observer. This means that for certain wavelengths at least, a small amount of iridescence may occur but this would not necessarily be perceived by an observer.

The method may comprise forming the profile elements.

The base layer and/or the profile elements may be formed by deposition of material such as via printing techniques, by etching (e.g. lithographic/photochemical techniques, or other known methods used on silicon chips which form a "negative" of the base structure by removing rather than adding material), by moulding, casting and/or by stamping. The base layer may for example be fine sand-blasted or acid-etched to form the profile elements. The profile elements may be formed by casting, printing, stamping and etching for example.

The reflector may be a selective reflector that only reflects certain wavelengths of light. Thus the reflector may impart a colour effect to the object.

The reflector may when flat, i.e. without relief structure(s), have a property of colour change with changing viewing angle. The profile elements may reduce the reflector's normal property of colour change with changing viewing angle. Thus the observed colour of the reflector may remain substantially the same to an observer over a broad range of viewing angles.

The reflector may be a narrow band reflector. Such a reflector may produce reflections over a limited wavelength ranges (e.g. 50nm or less). The reflector may be a narrow-band reflector that reflects only a narrow band of wavelengths such that the visual effect observed is a single colour.

The reflector may be constructed to accurately and selectively reflect certain wavelengths of light in order to impart a desired colour to an object.

The reflector may be a multilayer reflector. The multi-layer reflector may comprise one or more layers of each of a higher and lower refractive index material.

The reflector may be arranged so that it creates an optical effect observable by the human eye. The optical effect may be structural colour, i.e. in the visible light range or UV or infra-red range.

For example, the reflector may be arranged, e.g. thicknesses of the layers may be chosen, so that a colour effect is imparted. The reflector may be arranged so that it is a reflector for non-visible light, e.g. an IR or UV reflector. Such a reflector may for example be added to an object, e.g. transparent objects such as precious stones (e.g. diamonds) as an anti-counterfeiting measure. The IR or UV may be viewed easily throughout the entire stone, and the unique optical signature may be recognised by a detector.

The multilayer reflector may comprise two or more layers. For example, the multilayer reflector may comprise three to twenty layers. These layers may be alternate/alternating layers of higher and lower refractive index materials. The number of layers will determine the reflectivity of the optical coating structure. For example, twenty layers (i.e. ten pairs of layers) should achieve 100% reflectivity and three layers (if arranged high-low-high refractive index material for example) should result in about 60 to 70% reflectivity. The number of layers may be 2 to 15 layers, preferably 2 to 11 layers and more preferably 2 to 8 layers. Although in these cases the reflectivity would be less than 100%, it has been found that this reduced level of reflection may not be perceived or noticed by an observer.

It has been found that if the multi-layer reflector contains too many layers the effect of minimal or no perceived iridescence may start to be lost. This is because the uppermost layers could start to have a relief which does not closely match that of the profile structures of the base structure. Also, the number of layers affects the profile of the reflection curve (increasing layers tend to make the reflection curve narrower and therefore more sensitive).

The multilayer reflector may be a layered quarter-wave stack comprising alternate layers of two different materials with different refractive indices (n) but each with the same optical thickness (actual or geometric thickness x n = 1/4A).

The multilayer reflector may be a half-wave type. This may be the same as a quarter wave stack but each layer is double the thickness of all layers (e.g. actual or geometric thickness x n = 1/2A). This may have the effect of causing destructive interference of at least some of the incoming light. An object with a half-wave stack may be used as an antireflection/transparency device. This could for example, be used as the surface of a solar panel/cell. For example, the object may be a solar device and the half-wave reflector may be attached, e.g. glued, to a surface of the device and finally the base layer may be removed.

Alternatively, a chirped stack with dielectric layers of varying thickness may be used. As is well known, a chirped stack can be designed to reflect varying wavelengths of light between the layers. Chirped structures may for instance be preferred where the desired colour is gold, silver or copper.

The multilayer reflector may be a metal-dielectric reflector. For example, the reflector may have a metal (e.g. aluminium) coating and one or more dielectric layers (e.g. Si02 which is about 200-500nm thick). Both layers may comprise the relief structures. Such a multilayer reflector may provide vivid optical effects, such as colour effects, for fewer layers and better uniformity and less angle sensitivity.

Alternatively the reflector may be a liquid crystal (chiral/helical-type) reflector. In this case the wavelength of reflection would be equal to the distance of two twists in the helical structure multiplied by the refractive index. Such a reflector would provide a degree of circular or elliptical polarisation properties in the reflected light.

The solid liquid crystals may for example be replicated in titania. These can be nano-engineered for a wide range of resonant wavelengths. For example, the pitch may be as low as 60nm for a circular Bragg resonance at 220 nm in a Sc203 film. All colours, in a stable, solid material, can be made using this method.

There are also many other ways of producing liquid crystals in solid, stable (e.g. at room temperature) form (novel liquid crystal materials based on the porphyrin ring structure, or nanocrystalline cellulose, for example).

An advantage of using liquid crystals as the multilayer reflector is that they may be tuneable, for example, through being responsive to physical stimuli, e.g., electrical potentials, temperatures, etc., to provide a "tuneable optical effect" such as "tuneable colour". The optical properties of the liquid crystals may be adjustable by using transparent filaments within the liquid crystal.

When applied to an object, an optical coating structure comprising liquid crystals may provide tuneable colour which is observable over a broader angle with less optical effect change than with current tuneable liquid crystals.

Flat multilayers within the liquid crystal reflector may be caused to expand and contract to alter the optical effect imparted by the optical coating structure. In this way it may be possible to adjust a peak reflection of the reflector, e.g., by causing a shift of a peak wavelength in the range of 350 to 800 nm to cause a change in observed colour. This might be in response to physical stimuli such as temperature and applied electrical potentials causing a stress to be induced in one, two or three orthogonal directions of the optical coating structure. For example, the stress may be induced through one or more devices adjacent or below the optical coating structure, e.g., one or more devices having layers with different thermal coefficients to make them responsive to changes in temperature or one or more piezoelectric devices which can cause strains when electrical potentials are applied.

When the reflector is a multi-layer reflector, the method of forming an optical coating structure may comprise depositing a first layer of a first reflector material on the base layer over the profile elements, depositing a first layer of a second reflector material on the first layer of the first reflector material, then (if more layers are present) depositing a second layer of the first reflector material on the first layer of the second reflector material and so on to form the multi-layer reflector. Each layer may comprise a relief that corresponds to the profile of the profile elements.

The materials used in the layers of the multilayer reflector may be generally dielectric materials such as silicon dioxide, titanium dioxide, zinc oxide, zinc sulfide, magnesium fluoride, zirconium dioxide and tantalum pentoxide. For example, the multi-layer reflector may comprise alternating layers of a relatively (compared to the other material of the multilayer reflector) high refractive index layer such as zinc oxide and a relatively (compared to the other material of the multilayer reflector) low refractive index material such as silicon oxide.

In general, the specific dimensions of the layers in the optical coating layer will vary depending on the materials of the reflector and the desired optical effect, e.g. colour, to be imparted. In the case of a multilayer reflector, each layer of material of the multilayer reflector may have an actual thickness of the order of 50 nm to 150 nm for producing colours in the visible range. The colours may additionally or alternatively be in the non-visible range such as UV and/or SR.

For example, each layer may be about 100 nm for a red colour depending on the materials used. The optical thickness (thickness x refractive index) should equal to a quarter of the wavelength of the desired light reflected at the surface normal (e.g. that representing the desired colour observed). As will be appreciated, by varying these dimensions, different colours can be produced. For example, by reducing the dimensions, lower wavelength colours (such as violet or ultra violet) can be produced.

The reflector, e.g. multilayer reflector, may have materials and/or thicknesses to result in a certain wavelength of light being reflected. For example, the wavelength may be about 425 nm to 450 nm for "blue", about 545 nm for "green" and about 680 nm to 700 nm for "red". These values lie at the far end for each colour because they are observed at the surface normal of a flat reflector, but as the angle of viewing increases (towards glancing incidence) then wavelength reflected shifts to the near (left) part of the spectrum.

As an example, to produce a blue colour, when the stack comprises zirconium dioxide which has a refractive index of about 2.17 and silicon dioxide which has a refractive index of about 1.46, the following multilayer reflector may be used:

1. 1/8 wave Zr02 = 31 nm

2. Si02 1/4 wave = 93 nm

3. Zr02 1/4 wave = 62 nm

4. Si02 1/4 wave = 93 nm

5. Zr02 1/4 wave = 62 nm

6. Si02 1/4 wave = 93 nm

7. 1/8 wave Zr02 = 31 nm

It has been found that such a seven layer multilayer reflector achieves around 90% reflectivity of a blue wavelength, which can produce a bright and vibrant colour effect.

Ti02 may also be used as a high index material layer.

Preferably the outer layer of a stack (e.g. quarter wave stack) is a high index material to provide a stronger reflection.

Different colours can be achieved by using a different number of layers and different thicknesses, for example, in a Ti02 and Si02 stack. Seven layers in total have been found to provide good blues, violets and silvery or greenish blues.

Different colours have also been achieved using nine layers in total, including a deep orange, a rusty red/orange and a pale yellow.

In general, if the two materials used for the layers have a lower contrast in refractive index, such as Al203 and Ti02> then more layers are generally needed to achieve suitable levels of reflectivity, but some new colours may become possible, including emerald green. However, more layers equates to more time in the coating machine which means that the base layer can become hotter. Depending on the material of the base layer, in some cases this may lead to complications such as the release of air bubbles or melting. The time in the machine is dependent on the material; the deposition rate for Ti02 is about 8 nm/minute, whereas that for Si02 and Al203 is about 13 nm/minute, at an RF power of 800 W.

Due to the effect of the relief structures on the wavelength of reflection (the sloping sides of the base relief structures cause reflection of a shorter wavelength -see below), the peak wavelength of reflection may be shorter than that for a flat quarter wave stack. For example, a stack optimised at a peak reflection of 732 nm (i.e. infra-red) can provide an orange hue when formed on the profile elements.

The multilayer reflector may comprise as few as three layers. For example, it has been found that three layers in a high-low-high index arrangement can produce the blues and violets mentioned above, but their reflection peaks are shallower and broader (they are less bright and reflect a greater portion of the spectrum, which is in turn averaged by the eye). In this case the higher refractive index materia! may be the outer layer. For example the multi-layer reflector may be a layer of Zr02 on the profile elements, then Si02 and then a final layer of Zr02. In this case the higher refractive index material (the zirconium dioxide) is the outer layer.

When the layers of a multilayer reflector are deposited on the base layer that incorporates the profile elements, the reflector layers will follow the shape of the cross section of the profile elements. This is because the thickness of the reflector layers may be much smaller than the size of the profile elements.

The layers may reflect incident light into a range of directions, but averaging around 0 to 20 degrees from the normal. Therefore, also considering the global averaging of the eye, the appearance produced by the optical coating structure may be one of a single colour/effect, changing only slightly in hue with changing angle. Effectively, this may create a structural colour that appears to be a substantially uniform colour from all directions and/or over a large percentage of the surface.

Due to the fact that the size (i.e. height, length and/or width) of the profile elements may be significantly larger than the thickness of the reflector, the reflector will conform to and follow the surface profile of the base layer. For example, the height, width and/or length of (for example at least 50%, at least 60%, at least 75%, at least 90% by area of) the profile elements may be at least 10 times (e.g. 10 to 00 times) larger than the thickness of the reflector.

The various layer(s) of the reflector may be produced and applied onto the base layer using a number of fabrication steps well-known to those of ordinary skill in the art such as printing, ion beam deposition, physical vapour deposition, chemical vapour deposition, molecular beam epitaxy sputter coating, dip and spray coating or self-assembly methods.

Layers of additional materials may also be incorporated into the reflector. One or more layers of different material(s) may be applied to the layer of the reflector furthest from the base layer. For example, the method may include the step of depositing a covering (i.e. protective) layer of an optically inactive material onto the reflector and/or an opaque layer. The method may also include a further step of adhering an object such as a cut crystal or a cut glass element to the covering layer. The reflector may be located between and/or adhered to two objects. For example, the reflector may be between two cut crystal or cut glass objects. In such an arrangement, the reflector may impart colour to both objections.

In some embodiments the article may comprise an additional coating (a covering layer) over the multilayer reflector, for example, a resin or other flowable product applied to the multilayer reflector, and then a further, harder layer of a transparent material, such as a layer of glass, a layer of transparent ceramic material or a layer of transparent plastics may be applied over the top. Additionally or alternatively, the optical coating structure may comprise an additional coating (a covering layer) over the multilayer reflector on the side that was in contact with the base layer when it was formed.

Thus there may be an additional coating between the reflector and the object and/or there may be an additional coating on the reflector on the side that is opposite to the side that faces the object.

The additional coating may be a 'liquid glass', such as 'Crystalusion TM' and/or 'Alien Magic Pro 1'. The additional coating may serve as a protective coating for the reflector.

If a glass (or similar) layer is glued to the multilayer reflector of the optical coating structure using a covering layer of refractive index matched adhesive, then the angle of any light incident on the surface of the glass may be drawn towards the normal by virtue of the refractive index of the coating. The steeper angle of incidence means that the colour effect produced by the optical coating structure becomes stronger for larger angles of incidence and there may be less detectable iridescence.

A glass microscope slide pressed on to a refractive index matched layer (or substantially matched layer) of adhesive, for example, may produce an attractive effect because the microscope slides have a particularly flat, smooth (at the micron/sub-micron level) outer surface. The refractive index of the adhesive (i.e., the additional coating) may contrast with the multilayer coating - if it substantially matches the first layer of the coating then it may make the first layer optically ineffective reducing the effectiveness of the multilayer coating.

It is also possible to dispense with the top layer of glass or similar material and instead rely on a hardened surface layer of the transparent coating material itself, e.g., in the case of a thermally or chemically hardened resin, which is able to fill the spaces either in the relief structures (recesses) or between the structures (protrusions) at the base of the additional coating and provide a smooth outer surface at its top.

The covering layer may be provided so as to provide a flat surface (i.e. a surface without undulations corresponding to the relief of the relief structures). This may make it easier to attach the reflector to the surface of the object

It is important that when a covering layer is applied that no air is trapped between the covering layer and the reflector, for example in the dips/troughs which form between the relief structures of the uppermost layer, as this will affect the optical effect observed. These gaps may be filled with a material which has a refractive index which matches the uppermost layer or the covering layer.

The covering layer may be made of silicon dioxide. The covering layer could be up to around 1 or 2 μηι thick, for example, a 1 pm coating of Si02.

The covering layer may comprise silicon dioxide or other various (optically transparent) glasses. The covering layer may be made of a chemical vapour deposited poly(p-xylylene) polymer (e.g. Parylene).

As mentioned above, the optical effect may comprise an effect other than colour. For example, a half-wave stack reflector may result in an optical effect of anti-reflection and/or transparency. Thus, in another aspect, the present invention may provide a method of imparting an optica! effect to an object, the method comprising: providing a substrate, wherein the substrate comprises a base layer with profile elements thereon and a reflector on the base layer over the profile elements, wherein the reflector is a layered half-wave stack; attaching the reflector to a surface of the object; and removing the base layer from the reflector.

The present invention may also provide an article, the article comprising: an object; and a reflector attached to a surface of the object to impart an optical effect to the object; wherein the reflector has a relief made up of a plurality of relief structures that has been made by forming the reflector on a base layer with profile elements thereon, and wherein the reflector is a layered quarter-wave stack; and wherein the reflector is no longer in contact with the base layer on which it was formed.

One or more of the above described features, including optional features may be applied to these aspects.

Certain preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figures 1 to 4 show schematically a sequence of a method of imparting an optical effect such as structural colour to an object.

Figure 1 shows schematically a substrate 2 that comprises a base layer 4 and a reflector 6.

The base layer 4 comprises a plurality of profile elements 8 on its surface. The reflector 6 may be formed, e.g. deposited, on the base layer 4 over the profile elements 8. Thus the reflector 6 comprises a plurality of relief structures 9 that correspond to the profile elements 8 on which the reflector 6 is formed.

The reflector 6 may be a structure that gives the optical effect of colour to an observer.

As shown for example in figure 2, the reflector 6 may be attached to an object 10. The reflector 6 may be attached by index matching glue 12 to the object.

As shown in figure 3, the reflector 6 may be separated from the base layer 4 on which it was formed so as to leave only the reflector 6 but with the relief structures 9 caused by the profile elements 8 of the base layer 4. The reflector 6 may impart a structural colour to the object 10.

Whi!st the application method is shown schematically as first attaching the reflector 6 to the object 10 and then removing the base layer 4, the base layer 4 could be removed before the reflector 6 is attached to the object.

As shown in figure 4, the exposed surface of the reflector 6 may be coated, e.g. painted with a backing layer 14. This may be a backing layer 14 that is opaque to visible light, e.g. black. This may be beneficial when the object 10 is transparent, such as a crystal or glass object, and may result in an optical effect such as colour being seen through the object 10 when it is viewed from the surface opposite to the surface to which the reflector 6 is attached.