GB1582338A - Colour enhancement by thin film interference effects - Google Patents

Description

(54) COLOUR ENHANCEMENT BY THIN FILM INTERFERENCE EFFECTS (71) I, WILLIAM JAMES KING, a citizen of the United States of America of 9 Putnam Road, Reading, Massachusetts 01867, United States of America, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to the enhancement of colour by means of the optical interference effects which are produced by thin films. Interference phenomena in connection with thin films are well known. A summary of some of these phenomena is set forth in an article in the Scientific American entitled "Optical Interference Coatings", December 1970, pages 59-75.

Although the article starts with a display of various colors in a colored illustration and includes references to certain color effects produced in nature by thin films such as oil slicks, soap films, oyster shells, and peacock feathers, the various scientific uses of optical interference coatings described in the article do not include the controlled production of visual color effects. A major use of optical coatings is the production of reflection or non-reflection across the visible spectrum. Thus anti-reflection coatings are used on lenses, and multiple reflective coatings are used in dielectric mirrors. Applications requiring enhancement at a particular wavelength have an analytical rather than a visual purpose and require the maximum reflectivity possible, such as the laser and the Fabry-Perot interferometer. Although the Plumbicon tube separates light into primary colors, these are not viewed, but produce signals for transmission to a receiver via megacycle carrier waves. Moreover, not only do these prior scientific uses of optical interference films have no visual purpose, but the way in which the films are used to achieve a particular effect is such that, once adjusted for this effect, the optical device in question can no longer be adjusted to control other parameters.

Various methods have also been used to alter the spectral transmission and other characteristics (such as absorption and color) of materials such as glasses or plastics in order to make them useful as sunglasses, either as light absorbers to reduce and/or control the amount and nature of light reaching the eye, or for cosmetic reasons. These methods have included coloring the basic materials, adding a colored layer over the surface, adding a neutral filter to one or more surfaces, and adding a polarizing material.

However, the application of interference films to provide interference colors has not normally been used for such purposes. Such colors, although observed by many investigators, have not been used in general for cosmetic purposes because of difficulties in obtaining "predetermined" colors and because the colors lacked "depth", particularly on the transparent or partly absorbing substrates that are used for sunglasses and similar purposes. It is one purpose of this specification to show how such colors can be obtained having "depth" or color "density" under controlled conditions. In addition, this specification shows that such "high depth" colors can be obtained under conditions which allow the user to control the amount and nature of light transmitted to and through the substrate. This specification will also show how the latter control of the transmitted light can alsb be obtained while having "low depth" coloring. In fact, any practical degree and/or combination of color depth and transmitted light control can be obtained by proper use of the teaching of the present specification.

In conventional optical techniques, interference films are commonly used to fabricate band-pass light filters and to "increase" (as distinct from the invention's effect, which is always to decrease the transmitted light, as in the case of sunglasses and other light reducing devices) the amount of transmitted light (for example for lenses and binoculars) through their use as so-called quarter-wavelength anti-reflection filters, the latter being a simple form of the former. As discussed below, since any film of "optical" thickness X/4 (A being the wavelength of the radiation) is effective only around one value of X (or specific functions thereof), the application of such films having A values in the visible range causes the reflected and transmitted light components to be colored, even when the incident light is white, as is usually the case, for example, for sunglasses or windows.

By choosing film thicknesses properly, one can get a wide spectrum of reflected colors (the color of the transmitted light being the spectrum of the incident light, normally white, minus the reflected and absorbed components). This technique has not normally been used as a "coloring" mechanism primarily because of difficulties in controlling the color and very importantly because of the lack of intensity or color depth when used on transparent or partly absorbing substrates. In fact, such colors are normally observed only as a necessary adjunct to other factors such as the need for an anti-reflection filter on binoculars.

The lack of color depth (pastel shading in general) is acceptable for some purposes (e.g.

lightly tinted sunglasses) but is not adequate for others. Another reason why interference techniques have not been put to widespread commercial use is the need to put such films on the outside of the lens (or window) for the best cosmetic effect or function. In practice, this means the films themselves must be quite hard or must be covered with another harder (normally transparent) film or layer to prevent scratching or other attack, thereby complicating the manufacturing process. The use of interference films has therefore been primarily restricted to optical instruments (such as binoculars, and spectrometers) and techniques (such as band pass filters) where such factors are relatively unimportant because of the care which the optical components receive and/or the undesirability of or lack of need for coloration. In fact, in many scientific instruments which use interference effects for measurement purposes, monochromatic light must be used at some stage to provide the necessary operation. For most such purposes, conventional interference techniques are adequate.

However, in the case of plastic eyeglass (i.e. spectacle) lenses (both prescription and sunglass) there is a need for a coloring technique which can provide vivid cosmetic colors and also give protection to the soft plastic surface while providing the light reflection and/or absorption necessary to perform a worthwhile sunglass function. Similar applications exist in, (for example, plastic windows, plastic decorator panels or building materials) and also for other substrate materials (e.g. glass) in special applications (such as decorator panels or functional windows). Other applications will be obvious to those skilled in the art who become familiar with this invention. Some of such applications may simply require a color effect without the need to adjust other parameters such as light transmission. For example, such applications as plastic wall panels protected against scratching, costume jewelry, decorative dishes, and bottles, may incorporate the principles of the invention simply for a coloring effect.

In these applications, the interference coloring film must usually be extremely well bonded, to a degree not normally achieved with standard deposition techniques. Although any "appropriate" process capable of attaching the required materials in the "required" form to the substrate surface may be used in applying this invention, the invention itself has been demonstrated using ion beam sputtering and ion beam implantation sputtering techniques.

The former is disclosed, for example, in U.S. Patent No. 3,472,751. The latter can be used to deposit very tightly bonded, durable films on plastics and other difficult substrates, the film of deposited material commonly, but not necessarily, being harder than the substrate material.

The invention deals with transparent solids such as windows and eyeglasses, (and, in certain embodiments, coloring effects on solids whether transparent or not, such as wall panels, and costume jewelry), and provides means for enhancing the color of light incident upon the transparent solid while at the same time permitting further control of the radiation transmitted and reflected over a wide spectrum. The invention makes use of the discovery that the color of the reflected or the transmitted light may be enhanced in a way which does not materially affect the bulk of the visible light passing through or reflected by the transparent solid. As will be clear from the specification color enhancement may be achieved by interference between light reflected from a semi-reflecting layer on the transparent solid and light reflected from the outer surface of a'dielectric layer which is hermetically sealed over the semi-reflecting layer. The reflectivity at each of these surfaces need not be particularly large since color enhancement is achieved by a differential effect whereby the eye detects either the prominence of constructive interference at a particular band of wavelengths over the background radiation, or the color effect produced when a band-width of light is removed from the reflective light by destructive interference. In each case the bulk of the radiation is not affected by the interference phenomena, so that the light transmitted through, reflected by, or absorbed in the transparent solid may still be controlled by varying the thickness of the semi-reflecting layer and by other means.

The present invention provides a method of enhancing color effects produced by ambient light while controlling light intensity produced thereby, comprising: reflecting with a phase change substantially equal to r radians between 1 and 25% of the ambient light at an interface while permitting substantially all of the remaining ambient light to continue as transmitted light, permitted said transmitted light to travel without reflection to an absorbing layer and then reflecting a portion of said transmitted light at said absorbing layer while permitting the remaining light to continue as retransmitted light, the distance between said first interface and said absorbing layer being such that a specific color effect is produced due to interference at one, or a limited number of, specific wavelength(s) in the visible spectrum between the light reflected from said interface and the light transmitted back through said interface after reflection at said absorbing layer.

The present invention also provides optical apparatus comprising in combination a dielectric substrate, a semireflecting layer intimately bonded to one surface of said substrate, and a dielectric layer having an outer surface and an inner surface, said inner surface being hermetically sealed to said semireflecting layer, said dielectric layer having a uniform thickness such that, when ambient light is incident on the outer surface, a specific colour effect is produced due to interference at one, or a limited number of, specific wavelength(s) in the visible spectrum between light reflected from said outer surface and light transmitted back through said outer surface after reflection at said semi-reflecting layer.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic sectional view of a series of layers arranged in accordance with one aspect of the invention; Fig. 2 is a diagrammatic view showing the transmission and reflection of light rays incident upon a glass layer in air according to the prior art; Fig. 3 is a series of graphs showing the effect of superimposition of waves; Fig. 4 is a view similar to that of Fig. 2 wherein the glass layer is supported upon a plastic substrate according to the prior art; Fig. 5 is two views similar to that of Fig. 2 showing the reflection of light rays incident upon a highly reflecting layer according to the prior art; Fig. 6 is two views similar to those of Fig. 5 showing the reflection of light rays incident upon a semireflecting layer in accordance with the invention; and Fig. 7 is a view similar to those of Fig. 6 wherein glass of a relatively high index of refraction is used.

The primary object of the described embodiments of the invention is the means of obtaining on suitable substrates, optical layers which with reflected light, i.e. to the viewer on the side of the incident light, have a "colored" metallic appearance as opposed to the conventional neutral metallic appearance normally used in, for example, sunglasses and mirrors.

A second object is the means to obtain optical layers which control the characteristics of the visible light and/or other radiation reaching a viewer on the opposite side of the substrate from the incident visible light and/or other radiation (hereinafter collectively referred to as radiation), while simultaneously controlling the "color" of the composite structure as viewed by an observer on the side of the substrate upon which the radiation is orginally incident.

A third object is to "control" the transmitted radiation and colors as in the second object above while simultaneously controlling the amount and type of incident radiation which is absorbed in the orginal substrate, by controlling the amount of incident radiation which is reflected away from the substrate.

A fourth object is to obtain the functions above while simultaneously protecting the underlying substrate and deposited material from mechanical and/or chemical attack.

The physical arrangement required in accordance with the invention to obtain all of the above functions is shown in Figure 1. The important feature of this arrangement is the combination of a partially-reflecting or so called semi-reflecting (reflectivity being less than a highly polished or deposited "opaque" metal layer and more than a low reflectivity substrate such as clear glass or plastic) layer 1 and a layer 2 of transparent or "partially" absorbing material (such as clear or colored glass respectively) with index of refraction and thickness appropriate to obtain the desired features of the invention. How this combination differs from conventional interference methods and how it works in practice are described below. If necessary, a second layer 3 of transparent or partially absorbing material can be put over the interference layer 2 to provide additional protection and/or coloring effects in conjunction with those due to the interference layer. The semi-reflecting layer 1 is itself supported on a suitable underlying substrate 4.

The operation of this invention and the differences with respect to previous methods can best be understood by comparison with classical optical theory and practices related to interference effects due to thin films. This can be done in stages as shown in Figures 2-6.

The incident light may be incident at any angle 0 from 0 to 90". However, in the following discussion unless stated otherwise the light is considered to be incident normal to the surface (i.e. 0 = 0 ) to simplify and clarify the description of the invention. The light rays in Figures 2, 4, 5, and 6 are shown at an angle 0 0 for purposes of ray identification and only the reflected rays of interest are shown. The corresponding transmitted rays are the incident rays minus the reflected component. If the substrate medium is absorbing (e.g. colored glass) the final transmitted ray would also be minus the absorbed component. Unless otherwise stated, all dielectric materials shown are non-absorbing and are assumed to have indices of refraction that are constant across the visible spectrum.

The first element in defining the invention is a simple very thin (e.g. < 10,000 thick) film 5 of glass suspended in an air medium as shown in Figure 2. This is analogous to the classical soap film in which interference colors are observed corresponding to discrete film thicknesses t.

In general, there is a phase change of + ir if a light wave travelling in a medium with a given index of refraction is reflected at the interface with a medium having a higher index of refraction and the phase change is 0 if the reflecting medium has a lower index of refraction than the original medium. This assumption is not rigorously true for many cases of reflection at the boundary of two different media, for example at many air-metal interfaces, but is adequate and convenient for purposes of explanation. It is valid for the air-glass-air case shown in Figure 2. Exact phase changes of 0 or changes of t rr are used below in discussing all of the interfaces in Figures 2, 4, 5 and 6 and where this can lead to appreciable difference in operation of the invention, it is discussed. In no event does the divergence from a rigorous treatment alter the basic concepts of the invention.

Since there is a phase change of + rr at the first interface in Figure 2 and 0 at the second, it can be shown that the first ray reflected from the air-glass interface is reinforced through constructive interference effects at wavelength Xc given by (2m + 1) Xe t = m = 0,1,2,3, etc. (1) 4ng cosA where t e thickness of glass ng = index of refraction of glass 0 = angle of incidence For 0 = 0, cos 0 = 1 and equation (1) becomes (2m+1)#c (2) t= (2) 4ng (All subsequent formulae and discussions assume 8 = 0) Reflectivity at the interface between two non-absorbing media is given by the formula

for normal incidence (3) where nO = index of refraction of first medium; in this case, air.

ng = index of refraction of second medium; in this case, glass.

For no = 1(air) and ng = 1.46(fused silica) R = 3.5% Unless otherwise noted it is assumed for purposes of discussion that the reflectivity is the same at all X's of interest i.e. ng is constant across the spectrum of interest.

From Figure 2 it is seen that the component (R1) reflected from the first air-glass interface has an intensity of 3.5% of the original ray. The remaining 96.5% of the original ray is reflected from the rear glass-air interface with an intensity relative to the original ray of 96.5%x 3.5% = 3.38%. At the front surface this internal ray is again reflected (reflectivity = 3.5%) with an intensity relative to the original ray of 3.38% x 3.5% = .118% and the remaining 3.38% x 96.5% = 3.26% emerges as the second component (R2).

The part of the first internal ray (.118% of original intensity) which is rereflected at the front surface, will be rereflected from the back surface with an intensity of 0.118%x 3.5 % = .00413% and will emerge through the front surface as R3 having an intensity of .00413%x 96.5% = .0040% after an additional 3.5% loss through reflection at the front surface. The internal reflections continue with corresponding decreases in the intensity of the rays R4, Rs .... emerging through the front surface. If the first internally reflected ray R2 is in phase with the originally reflected ray R1 upon emerging as given by equation (3), the second internally reflected ray (R3 after emerging) is out of phase since the total additional path length is a 1/2 integral number of wavelengths long and there is no additional phase change at either the front or rear internal reflections. The third internally reflected ray (R4 after emerging) is thus in phase, and so on.

R1 and R2 are in phase and of much larger magnitude than the other rays, resulting in an enhancement of the color at the particular wavelength involved (assuming glass thickness t corresponding to constructive interference at visible wavelengths Xc). A rough approximation is a doubling of the energy reflected at Xc as given by equation (3) and shown (for the first two rays R1 and R2) in Figure 3(a). At other wavelengths near the coherent wavelength, the amplitudes can be partially reinforced as in Figure 3(bio where it is assumed that R2 is roughly 36 (for example 30 ) out of phase with R1. If, however, R1 and R2 are ir or near ir out of phase as in Figure 3(c), there can be almost complete annihilation of the reflected components at that wavelength. The wavelengths XD for maximum out of phase destructive interference is given by: mX, t = --- m = 1,2,3 etc. (4) 2ng Whether major constructive and destructive interference effects can occur simultaneously in the same film and to what extent is primarily a function of the film thickness and is discussed below.

The net result with respect to the film is an apparent color corresponding to wavelengths around At (if constructive interference dominates) or at that color that remains after than corresponding to wavelengths around XD are removed (if destructive interference dominates). These colors for this type of film can be reasonably intense if the film is not exposed to a lot of white light incident on the rear surface. Since the transmitted light is the complement of the reflected light, if there were white light incident on the rear surface of intensity level 100% of that incident on the front surface, the two wave trains would tend to complement each other and produce white light as viewed from either side.

However, if most of the light is incident on the front surface, the "differential" effect on the reflected light can be quite significant leading to relatively intense coloring. For example, if only constructive interference occurs, those wavelengths near Xc will have an intensity level of Rl + R2 R3 + R4 etc. =IC =R1 + R2 = 3.5 + 3.26 = 6.76%while those at wavelengths far removed from Xc, where R2 is half in phase and half out of phase with R1 will have an intensity IB roughly equal to that of R1, i.e. approximately 3.5%. A convenient measure of the differential level of the constructive color component above the background is the difference between the enhanced intensity Ic and the random background intensity IB, divided by the random background intensity IB. For the case under consideration this is approximately IC - IB 6.76 - 3.5 Differential level = = . = .93 or 93% 1B 3.5 if only the reflected components are considered. In practice, some white light is incident on the rear surface and the differential effect is much less than this.

It should be noted that the structure shown in Figure 2, although producing vivid coloring, is not adequate for most practical purposes because of the thinness of the glass layer involved.

It should also be noted that the coloring effects are due to the ability of the eye to observe and evaluate the "relative" amplitudes of the various components of the light entering the eye, so that the greater the differential height of the coherent Xc (for example above) "above background", the deeper or more vivid will be the apparent color.

Although the above discussion consists primarily of an analysis of observed facts and in that sense is trivial, it is important to a clear understanding of the present invention as discussed below.

Figure 4 gives the next stage in understanding the invention and shows a glass film 5 of index of refraction ng intimately attached by some method to a plastic substrate material 6 having index of refraction np where np : > ng and both media are non-absorbing. (The materials chosen here and in subsequent stages of the development are arbitrary and could be replaced with other "suitable" materials without altering the basic explanation.) In this case there is a phase change of + ir upon reflection at the front surface and "another" phase change of + IT upon reflection at the glass-plastic interface.

The condition for constructive interference of the first internally reflected ray R2 with the initial reflected ray R1, in this case is given by; m#c t = 2n m = 1, 2,3 etc. (5) 2ng The condition for destructive interference is given by; (2m+l)AD t = m = = 0, 1, 2, etc. (6) 4ng However, in this case the amount of light reflected from the glass-plastic interface, as given by equation (3) for ng = 1.46 and np = 1.54)plastic) is only .071 % x 96.5 % - .0686 % of the incident light with the emerging component R2 only .0686%x 96.5% = .0662%. The plastic substrate 6 is assumed to be very thick since it must provide support, and so there are no interference effects due to reflection at the rear plastic-air interface. This additional light of R2, even if satisfying equation (5), will therefore produce a differential effect of only .066/3.5 = .019 or 1.9% above background. Such combinations of materials therefore have only a very slightly observable coloring. In such a case white light penetrating from the back surface also tends strongly to wash out any net coloration since almost all of the white light incident on the back surface will emerge from the front as white light, raising the background level to approximately 100%.

It should be noted that in this case, if R1 and R2 are in phase, R3 will be out of phase; i.e. will destructively interfere with R1 and R2 because of the additional + 7r phase change at the second reflection at the glass-plastic interface. The additional path length in the glass is, of course, an integral number of wavelengths since that is the condition for the first internally reflected ray R2 to be in phase with R1. The third internally reflected ray R4 is in phase, and the fourth Rg out of phase etc. This factor is unimportant for the case shown in Figure 4 because of the small reflectivities and intensities involved, but is important in the new elements involved in the present invention.

Next consider a simple highly polished opaque reflecting metal layer 7 as in Figure 5(a) (e.g. vacuum deposited Al on glass or plastic) with a reflectivity assumed for discussion to be 90% (normally higher) and flat across the visible spectrum. The reflectivity for such an opaque absorbing medium with light incident from a dielectric of index of refraction no is given by: (nO - nm)2 + km2 R = (7) (nO + nm)2 + km2 (7) where nm index of refraction of metal km extinction coefficient for metal which reduces to (1 - nm) + km R = for no = 1(air) (8) (1 + nm) + km For some metals such as Al where the relative values of nm and km are appropriate across the spectrum (visible) the reflectivity remains fairly flat and the reflected light has a neutral gray pure metallic appearance. For other metals such as Cu, the relative values of nm and km are such that R varies across the visible spectrum (e.g. for evaporated Cu, R = 58%at 4,500 and R = 96% at 7,000 ). For the example given, the Cu therefore appears by reflected light to be reddish since more of the red end of the spectrum is reflected. As discussed later, this factor is also used in controlling coloration using the present invention.

Returning to Figure 5(a) the situation is quite simple with only those rays reflected from the first surface being viewed by the observer (i.e. a simple front surface mirror). If, however, the metal is covered by a thin layer (such as that shown at 8 in Fig. 5(b)) of glass, or other appropriate medium, the situation changes to that shown in Figure 5(b) where again the "initial" reflected ray is only 3.5 % of the incident energy. A phase change of + Ir is assumed at the glass-metal interface. In a more rigorous treatment the phase change p is given by: 2n k p = tan-l 2 2 - k 2 where the symbols have the meanings previously given. For many glass-metal combinations p is near 7r, while for others it can vary by significant factors. This divergence from an exact Ir phase change on reflection has little effect on the present invention since its effect is to slightly shift the value of the thickness t required for constructive or destructive interference at a given wavelength, through the addition of an error factor viz. (for constructive interference) m#c t - -- + Atp (10) 2ng In practising the invention, as discussed below, one simply adjusts t to compensate for the Atp error (if significant). A similar correction exists for variations in reflectivity but is of no consequence to the present invention since it is basically an angle of incidence correction to reflectivity and we are primarily concerned with normal incidence. As shown by equation (1) and similar formulae, constructive and destructive interference coloring effects will be apparent at non-normal angles of incidence which will vary from those at normal incidence, but this has no effect on the practice of the invention since the contemplated uses of the invention do not require that the color effect should be the same for all angles of view.

Referring to Fig. 5(b), for a phase change of + sT, the first internally reflected ray R2 is in phase with R1 at Xc given by equation (5) and has an intensity of (100 - 3.5)%x 90%x (100 3.5)% = 96.5%x 90%x 96.5 % = 83.81 %ofthe original intensity. Rg is out of phase with R1 and R2 and has an intensity of 96.5%x 90%x 3.5%x 90%x 96.5% = 2.64%. R4 is in phase with an intensity of 96.5 % x 90 % x 3.5 % x 90 %, x 35. % x 90 % x 96.5 % = 083 %. The sum of Rl, R2, R3 and R4 (ignoring higher components) is therefore 3.5 + 83.81 - 2.64 + .083 = 84.75% One cannot readily state what the reflected amplitudes are for wavelengths other than the coherent value since they depend critically on, for example, wavelength, and materials.

However, in general, considering R2 as the primary ray because of its intensity, (R1 + R4) and R3 will tend to cancel because of the corresponding phase differences so that the intensity variation cannot be greater than approximately 83.81 + [(3.50 + .083) - 2.64] = 84.75 to 82.87 As a rough approximation, the maximum "differential effect is given by the value for Xe minus the lowest value above, i.e. Ic = 84.75, IB = 82.87 and IC IB 84.75 - 82.87 Differential level = = = .023 or 2.3% 1B 82.87 A special case can occur if the thickness t is such (see later discussion) that destructive interference at XD can occur simultaneously with constructive interference at Xc. Should such occur. R2 R3 and R4 for XD are all in phase with each other and out of phase with R, since each additional internal reflection traversal adds 2# to the phase (additional half wavelength due to path length plus Ir phase change at glass-metal interface). The amplitude for this value of XD is given by (83.81 + 2.64 + .083) - 3.5 = 83.03% which gives less than the maximum differential effect calculated above for "random" wavelengths. Interference colors on such highly reflecting metal surfaces therefore tend to be weak or washed out to the eye because of the small differential intensities involved.

With the foregoing as background. the operation of the present invention may be readily understood. Consider the situation shown in Figure 6(a) where the arrangement is the same as that in Fig. 5(b) except that the metal layer.9 is only 20% reflecting (again assumed flat across the visible spectrum). For a thickness t corresponding to constructive interference at Xc, the intensities and phases of the reflected rays are R1 = 3.5% (initial), R2 = 18.62% (constructive), R3 = 0.13 % (destructive) with the higher orders being insignificant. The total reflected intensity at Xc is therefore 3.5 + 18.62- .13 = 22% In general, for other "normal" wavelengths the effects of R1 and R3 may be approximated (considering R2 as the main reflected ray) by assuming that they will add half their difference in intensity to R2 (i.e. 1/2(3.5 -.13) = 1.69%) so that the total intensity R1 + R2 + R3 may be estimated as 18.62 + 1.69 = 20.31. The differential effect for Xe above these wavelengths is therefore 22.0 - 20.31 = = .083 = 8.3% 20.31 compared to the ~2.2% found for the opaque reflecting metal case shown in Figure 5(b).

More importantly, if a simultaneous destructive interference occurs at XD at the same value of t, the minimum amplitude is given by (R2 + R3) - R1 or (18.62 + .13) - 3.50 = 15.25% 22 - 15.25 In this case the differential effect = = .44 or 44 % above background which is 15.25 roughly 20 times that found for the opaque reflecting metal (90%) situation in Figure 5(b).

For comparison, Figure 6(b) shows the values for a 30% reflecting layer 10. In this case the intensity at AC is given by (R, + R2)-R3.SinceR1 = 3.5 %, R2 = 96.5 % x 30 % x 96.5 % = 27.94% and R3 = 96.5% x 30% x 3.5%x 30% x 96.5% = 0.293%, then the intensity is 3.50 + 27.94 -.29 = 31.15%at Xc; 27.94 + 1/2(3.5 - .29) = 29.54% for an average noncoherent XA; and (27.94 + .29) - 3.50 = 24.75% for a destructively interfering XD.

The differential effects are therefore 31.15 - 29.54 x x x 100 = 5.45% for Xc greater than XA and 29.54 31.15 - 24.73 x x x 100 = 25.96% for #c greater than AD 24.73 These are considerably smaller than for the 20% reflecting layer case but are still much larger than the = 2.2% found for the 90% reflecting case or for the simple glass on plastic case.

One of the basic elements of this invention is therefore the adjustment of the thickness of the dielectric medium (glass in examples) used as an interference layer, and the reflectivity of the semi-reflecting metal layer to enhance and/or optimize the differential coloring effect. If one goes to reflectivities less than 20%the effect is enhanced still more, with, for example, the differential effect (both Xe and XD occurring simultaneously), being > 100 at 10% reflectivity. At higher reflectivities than 30%, the effect, of course, decreases in intensity.

In examples given, SiO2 (n = 1.46) has been used as the interference dielectric since this material has been extensively used in demonstrating the invention. The differential effect can be increased still further, however, by using other dielectrics having higher values of n, thereby affecting the reflectivities (particularly at the front surface) and ultimately the differential effect. Consider Figure 7 which gives the situation comparable to that shown in Figure 6(a) (20% reflecting metal) but with TiO2 having n = 2.60 replacing the SiO2 as the interference medium. In such a case R1 = 19.753% by equation (3), R2 = 80.247%x 20%x 80.247% = 12.8792%, and R3 = 80.247%x 20%x 19.753%x 20%x 80.247% = .5088%.

Considering only R1, R2 and R3 the enhanced intensity Ic = (R1 + R2) - R3 = (19.753 + 12.8792) - .5088 = 32.1234 and the background intensity IB = R1 - (R2 + R3) = 19.753 (12.8792 + .5088) = 6.3650. The maximum differential effect is given by IC - IB 32.1234 - 6.3650 ---- ~~~~~~~~~~~~~~~~~ = = 405% 1B 6.3650 which is nearly a factor of 10 greater than in the 20% reflecting SiO2 dielectric case. One could therefore reduce the reflectivity of the metal even more to allow much more of the light to penetrate to the inside while still maintaining a very strong coloring effect. (With a dense layer of metal, there is a glass-metal interface at the top of the layer and a metal-plastic interface at the bottom. However, as the density of the metal is decreased, there will be an increasing number of areas where (on a molecular scale) there will be no metal and therefore a glass-plastic interface. The limiting factor will be the increased reflectivity at the glassplastic interface as the metal is made less dense and the relative index of the glass and plastic becomes larger leading to increased reflectivity. Exact values depend on a given application and materials). The maximum effects, of course, exist when the sum of R1 and R2 is much greater than their difference as in the above TiO2 case. In general, this occurs when the index of refraction of the dielectric has a relatively (compared to SiO2) high value. Another example is Si3N4 which has n = 2.03 resulting in a maximum differential effect of 503%.

Other materials such as SiO (n = 1.95) and Al203 (n = 1.76) will have coloring effects which are more pronounced than those of SiO2 and others will be apparent to those skilled in the art.

The choice of material depends on the particular application. In the discussions which follow, the SiO2 situation is the one which is considered in all cases.

For the non-opaque, semi-reflecting metal cases, white light incident upon and penetrating through the rear surface and emerging through the front tends to decrease the effect. The major decrease is due to an increase in the background level since interference effects that occur in the thin glass layer (i.e. interference effects between light reflected at plastic-metal interface and that reflected at glass-air interface) will be non-coherent with those occurring due to light incident on the front and even if occurring will have a much smaller effect due to the much higher background. The latter results because the transmitted light differential effect is the complement of the reflected light effect and is superimposed on a much higher background (80% of light reaching plastic-metal interface minus 3.5% reflected at glass-air interface).

Considering only the increase in background, if white light of intensity 100 % of I (intensity of white light on front surface) is incident on the rear surface in the 20% reflectivity (metal) case, approximately 74.5% (after three reflective losses at various interfaces) will exit through the front surface. The effect in the maximum differential case (he + XD simultane ously) is a reduction from 44% to a value of 22.0 - 15.24 .075 or .075 or 7.5% 15.24 + 74.5 Although in a practical embodiment of the invention (e.g. use as sunglasses) there is much less than 100% of I coming through the rear surface, even in the worst case of 100 % of I the differential effect is much greater than that obtained ( ~2.2 O/o) for a 90 % reflecting layer case.

This negative effect of white light penetrating through the rear surface can be partly negated by having the substrate (plastic in example) made of visible light absorbing material. If, for example, the plastic in the 20% reflecting case were of a thickness and absorptivity A to be 50% absorbing in the visible, white light of intensity I entering through the rear surface would have an intensity of (1 x .965 x .5 x .8 x .965) = 0.371 on exiting through the front surface and the maximum differential effect (at + XD simultaneously) would be 22.0 - 15.24 = ------ = .129 or 12.9% 15.24 + 37 In the real case of sunglass use, the light entering through the rear is much less, say 20%of I maximum, being only that going around the frames and reflected off the skin. For this value the maximum differential effect equals approximately 22% for the non-absorbing substrate use and 29.8% for the 50% absorbing substrate case.

Since the thickness t of the interference medium (glass in example), the reflectivity R of the reflecting metal, the absorptivity A of the substrate material and the ratios thereof are infinitely variable, within the limits of minimal reflectivity (no metal) and no absorption (clear substrates) and maximum reflection (opaque polished or evaporated metal) and maximum absorption (highly absorbing substrate), the color and/or intensity of the structure as viewed by an observer on the front or incident surface and that of the light reaching a viewer behind the rear surface (wearer for sunglasses) can be varied over an extremely broad range. In the practical embodiment of the invention, this allows the user to reduce the light reaching the inside viewer to a desirable level, e.g. 30% of neutral or near neutral shading for a sunglass wearer, while obtaining the desired color and intensity level for an external viewer.

It has been demonstrated in practising the invention that neutral shading can be obtained by having a substrate having neutral absorption at the proper level. This can be used to overcome or wash out coloring effects due to light coming from the -front surface (nonreflected) which is the complement of that reflected and is therefore colored, although of much less effective density than the reflected component because of the much higher background (N 74 % of light being transmitted in 20 % reflecting and non-absorbing substrate case). The light reaching the inside receiver can also, of course, be colored if desired.

A, R and t, for example, may be adjusted to yield other values of external coloring and intensity for other purposes, e.g. for office windows. For this use, in one test of the invention the absorbing substrates were of glasses manufactured by PPG Industries, Inc. under the names solarbronze, solargray and solarex. The metal layer reflectivity was adjusted to reduce the light level penetrating to the inside to a comfortable level while maintaining the neutral characteristics (particularly for solarbronze or solargray) and changing the color as viewed from the outside to that desired but for this purpose deliberately of less intensity than in the normal sunglass case. However, it should be noted that all ranges of values for external color intensity and transmitted light intensity may be used for any and all applications. Of course, in a limited number of embodiments of the invention (such as wall panels) the transmitted light intensity may be of no consequence.

A very important factor that is observed in the practical embodiment of the invention is that the colors so formed have an extremely metallic appearance; i.e. a metallic nature similar to that obtained with a highly polished metal reflector such as Al, but with deep color shading resulting in a striking "colored metallic" appearance. This occurs because the differential effect primarily results from reflection at a very thin layer in the same way that reflection results at the surface of a neutral metal reflector. The resulting radiation is therefore space as well as time coherent and the eye perceives that the light emanates from a restricted layer or layers. (This is in contrast, for example, to absorbing glasses which have a color due to absorption and reemission of radiation at many spatially separated atomic layers in the glass and which therefore do not have a metallic appearance). This factor when optimized by proper use of the present invention gives a recognizable and distinctive appearance when A, R and t, for example, are chosen for vivid coloring.

The practice of the invention can best be understood and mastered by a full appreciation of the effect of using a partially - reflecting metal layer as discussed previously in conjunction with Table I which gives the colors observed by previous investigators (Pliskin and conrad IBM Journal, Jan. 1964) for thermallv grown films of SiOv on polished (i.e. opaque maximum reflecting) slices of silicon. The latter is the case normally observed previously, where the coloration is not enhanced nor has a strong metallic appearance as in the present invention. Similar, but not exactly the same, colors were observed in the practical demonstrations of the present invention. Exact coloring depends on the metal used as the reflecting layer and varies in each case.

Table I has been prepared specifically for this invention to explain the colors obtained to show detailed operation. It gives the calculated wavelengths for constructive interference (t ) ) and destructive interference (t = ) in association with the colors observed by Pliskin and Conrad. Note that the value oft given is the real value, not the optical thickness tng and the Xc'S and XD's having effects in the visible are outlined.

At a thickness t of 500 , there is no visible wavelength Xc or XD at which interference effects should occur if the glass (SiO2) has an index of refraction of u 1.46 (used for calculating Table 1). The tan color observed by Pliskin and Conrad can be explained by the following considerations. If the SiO2 is oxygen deficient and has an appreciable proportion of SiO having an index of refraction of 1.95 (or other oxygen-deficient SiOx compounds) as can occur at the interface for thin thermally grown SiO2 layers on Si, the XD for destructive interference (m = o) is 3,900A which is above the edge for optical interference ( ~ 3,800A) in the visible. Some of the violet component will be removed from the reflected light under these conditions, so that the remaining reflected light has a tan appearance or color as observed by Pliskin and Conrad. However, for n = 1.46 which is obtained if the silicon is deposited by ion beam sputtering or ion beam implantation sputtering techniques, no tan color is apparent when layers of 500 thickness are deposited on highly reflective metal layers such as opaque ion beam sputtered Al on smooth glass or plastic substrates. Such layers, if hermetic as in the ion beam sputtering case, can be used to protect the reflecting metal against corrosion without altering its optical characteristics at wavelengths longer than 3,000A. This has been clearly demonstrated for the present invention.

If the thickness is increased to 700 , XD becomes 4,088 , moving the removed (i.e.

destructively interfered) component farther towards the blue, producing a brown appearance. At 1,000 , XD is 5,840A which is in the yellow part or middle of the spectrum. Both ends of the spectrum therefore show up in the reflected light which is dark volet to red-violet.

At t = 1,200he the red end of the spectrum is removed and the reflected light centers around the blue region. These results are confirmed by Pliskin and Conrad's observations, and one can assume that for these thicker layers the problem of oxygen deficiency at the interface is relatively less severe.

At afpproximately 1,300 thick, a new effect occurs, i.e. "constructive" interference at 3,800A with the first corresponding value in the table being a Xe of 4,380 fort = 1,500 .

For this value of t, the coloring is primarily due to constructive interference rather than destructive effects so the reflected light has a color (light blue) dominated by Xc. In fact the royal blue observed at 1 ,200A probably has a constructive component in the deep violet due to the spread around Ac (see Figure 3(b) ) and the extension of enhancement effects to higher and lower values of X than the precise value Xc.

In addition, because of the spatial as well as time coherent nature of the reflected light, it assumes a metallic appearance which is also observed at t = 1,700he and 2,000 . However these effects for opque maximum reflecting substrates as in the Pliskin and Conrad case are very small and disappear at larger values of t, but are very prominent and continue throughout the large values of t if the present invention is practised to produce large differential effects as discussed previously.

Also beginning at t = 2,000 is a definite simultaneous occurrence of Xe and XD. At t = 2,200, Xe = 6,424Awhile XD = 4,283 so the reflected light is enhanced around Xcandhasa decreased value around XD, the resulting color being a combination of the two effects, or gold with slight yellow orange for this example. Using the present invention, the enhancement of the color through the differential effect plus the spatially coherent nature of the reflected light results in a "strong" metallic appearance for all colors corresponding to thicknesses greater than 1,300 . This metallic appearance and strong coloration continue until the thickness is such that there are so many interference effects occurring simultaneously at different Xc'S and XD's (i.e. for different values of m - see Table 1) that the resulting reflected light again tends to white (e.g. in Table 1, fort = = 15,400 , there are 6 Xc's corresponding tom = 6, 7, 8, 9, 10 & 11 and 6 XD's corresponding tom = 6,7,8, 9,10 and 11). Above > 15,000A the interference colors become hard to observe on opaque maximum reflecting substrates although still easily observed on the partially-reflecting substrates of the present invention because of the color enhancement.

From Table 1, one can also see that for the values oft which would be used in practising the invention, there are values of Xe and XD corresponding to effects in the IR (infrared) and UV (ultraviolet) regions of the spectrum. Such effects are discussed below in connection with an important variation on the invention.

Table 1 allows the user to choose the correct values of t to practise and optimize the effects of the present invention when used in conjunction with appropriate reflectivity calculations.

No precise format can be given for the latter since it depends on factors (e.g. light levels, color density, and means of depositing materials) which must be chosen for a given application. The most enhanced colorations are obtained for one or two orders of Xe combined with one or two orders of XD which in general applies for t between 1,500 and 6,000 . This is not rigid, since the coloration depends on other factors such as reflectivity, absorption in the substrate, and type of reflecting metal but serves as a guideline for easiest practice of the invention. SiO2 layers of this thickness are also found to supply adequate chemical and mechanical protection for the underlying metal and/or plastic in many applications (e.g. sunglasses or windows).

Another variation which can be used to extend the range of colors obtained by the present invention is to use an absorbing dielectric medium between the front surface and the semi-reflecting layer. The color thus obtained is a combination of the interference effect and the absorption and reemission effects in the dielectric medium. It should also be noted that light penetrating from the rear through an absorbing substrate will affect the coloring to an extent depending on the intensity of the rear light and the color of the absorbing substrate.

The latter may be used to modify the color or to "mute" the metallic effect in applications such as office building windows.

Another variation is to choose the partially-reflecting metal from those that do not have near constant reflectivity across the visible spectrum but which have varying R. An example is copper which has a reflectivity of ~ 58 % at 4,500 and ~ 97 ,zo at 7,000 . This difference in reflectivity can be used further to enhance certain colors, e.g. red tones, because of their obvious enhancement of the differential effect due to the difference in reflectivity. Gold, nickel, and brass are other examples of such metals or alloys. Others will be obvious to those skilled in the art.

A most important variation of the invention is obtained by extending its application to other wavelengths outside of the visible, in particular into the infrared (IR) region. This is of special importance for windows designed to reduce or control the amount of radiant heat (from sun, atmosphere, or other hot sources such as others buildings) entering the building in order to conserve energy by reducing the air conditioning load. In order to optimize this saving, it is desirable that any optical layers used to reflect to reject the incident IR radiation be on the outside surface of the window. If applied to the inside, much of the incident IR radiation will be absorbed in the glass itself, either on the first pass through or on the second pass after reflection, thereby heating up the glass. Much of this heat in the glass is then transferred into the interior of the building by convection currents of the internal air or by reradiation at longer X's. Applied to the outside, such reflecting layers are therefore more effective in summer but are still effective in preventing heat losses in winter since the IR energy radiated by internal objects will either be absorbed in the glass, and partially returned to the room by convection, or for the portion that passes through the glass to the metal layer, will be reflected back and absorbed in the glass or returned to the room.

With the present invention, this control of the IR radiation entering or leaving the inside of the building can be effected while still controlling the visible light entering the building and also the external and internal coloring effects. This capability results from the longer wavelengths of the IR radiation. By reducing the thickness of the partially reflecting metal layer, one can control the amount of visible light entering the building for lighting needs (e.g.

= 50% of incident light for R t 20% and 40% absorbing substrate), while achieving the condition for optimizing color effects as discussed previously, and maintaining a high IR reflectivity. In demonstrating this invention, it has been demonstrated that this combination can be achieved if the partially-reflecting metal is one of inherently high IR reflectivity which is put down by a technique or process (such as ion beam sputtering) which provides uniform dispersion of the metal without appreciable agglomeration. At thicknesses where the layer looks relatively open to visible X's, the same partially-reflecting layer looks relatively opaque to the IR X's since their size is such that they intercept more of the metal atoms on the average, leading to increas time before the hermetic interference layer (or equivalent) is applied, the Cu will oxidize and the reflectivity will decrease, affecting the visible coloaration, reflected and transmitted light intensities and IR rejection capability. If the glass is not hermetic, the characteristics of the structure will degrade with time. Au is not subject to severe degradation but is relatively expensive and for some application techniques difficult to apply.

Application of the hermetic seal immediately over the reflecting layer can be used to provide very thin (500A or less) non-colored protective layers if the applied glass is impervious to chemical attack in thin layers. As discussed previously, the coloration observed by Pliskin and Conrad at sooA may be due to inadequate characteristics of the glass layer. The use of such thin layers avoids the expense of applying thick layers ( > 2,u) to eliminate interference color effects. This innovation has been demonstrated and is of importance for protecting front surface mirrors while maintaining optical characteristics, for applications such as optical instrument mirrors and concentrators for energy conservation and generation systems, and for hermetically sealing solar cells for the terrestrial applications.

Another variation of the present invention is its use on plastic substrates, both absorbing and non-absorbing at visible X's, to provide IR rejection. Whereas glass substrates, in many practical areas of interest such as sunglasses, absorb some of the incident IR, plastics in general do not. Thus wearers of plastic sunglasses are subjected to IR heating of the eye, leading to drying out of the membranes and irritation, even if the glasses are adequate for visible radiation purposes. The present invention avoids this effect through rejection of undesirable IR radiation while controlling visible light and coloring effects at desirable levels.

With the metal applied as a thicker highly reflecting opaque layer, "plastics" can also be used as excellent visible plus IR mirrors and concentrators for solar energy generation and conservation systems, with the metal layer protected by a thin (~500 ) hermetic and mechanical seal as discussed previously. Tjese effects have been demonstrated.

Still another variation of the invention is its use to produce coloring effects in wall panels, without regard to transmission properties.

TABLE I Color t(A) Tan 500 Brown 700 Dark Violet to Red-Violet 1 ,000 Royal Blue 1,200 Light Blue to Metallic Blue 1,500 Metallic to very light Yellow-Green 1,700 Light Gold on Yellow - slightly metallic 2,000 Gold with slight yellow-orange 2,200 Orange to Melon 2,500 Red-Violet 2,700 Blue to Violet-Blue 3,000 Blue 3,100 Blue to Blue-Green 3,200 Light Green 3,400 Green to Yellow-Green 3,500 Yellow-Green 3,600 Green-Yellow 3,700 Yellow 3,900 light Orange 4,100 Carnation Pink 4,200 Violet-Red 4,400 Red-Violet 4,600 Violet 4,700 Blue-Violet 4,800 Blue 4'900 Blue-Green 5,000 Green (broad) 5,200 Yellow-Green 5,400 Green-Yellow 5,600 Yellow to "Yellowish" 5,700 Light Orange on Yellow to Pink borderline 5,800 Carnation Pink 6,000 Violet-Red 6,300 Bluish (borderline violet to bluegreen - appears greyish) 6,800 Blue-Green to Green (quite broad) 7,200 "Yellowish" 7,700 Orange (rather broad for Orange) 8,000 Salmon 8,200 Dull, light red-violet 8,500 Violet 8,600 Blue-Violet 8,700 Blue 8,900 Blue-Green 9,200 Dull Yellow-Green 9,500 Yellow to "Yellowish" 9,700 Orange 9,900 Carnation Pink 10,000 Violet-Red 10,200 Red-Violet 10,500 Violet 10,600 Blue-Violet 10,700 Green 11,000 Yellow-Green 11,100 Green 11,200 Violet 11,800 Red-Violet 11,900 Violet-Red 12,100 Carnation Pink to Salmon 12,400 Orange 12,500 "Yellowish" 12,800 SkyBlue to Green-Blue 13,200 Orange 14,000 Violet 14,500 Blue-Violet 14,600 Blue 15,000 Dull Yellow-Green 15,400 TABLE 1 (cont.J Xc t ( ) m=1 2 3 4 5 6 7 8 9 10 11

500 1460 730 487 365 700 2044 1022 681 511 1000 2920 1460 973 730 1200 3504 1752 1168 876 1500 4380 2190 1460 1095 1700 4964 2482 1655 1241 2000 5840 2920 ' 1947 1460 2200 6424 3212 2141 1606 2500 7300 3650 2433 1825 2700 7884 3942 2628 1971 3000 8760 4380 2920 2190 3100 9052 4526 3017 2263 3200 9344 4672 3115 2336 3400 9928 4964 3309 2482 3500 10220 5110 3407 2555 3600 10512 5256 3504 2628 3700 10804 5402 3601 2701 3900 11388 5694 3796 2847 4100 11972 5986 3991 2993 4200 12264 6132 4088 3066 4400 12846 6424 4283 3212 4600 13432 6716 4477 3358 2686 4700 13724 6862 4575 3431 2745 4800 14016 7008 4672 3504 2803 4900 14308 7154 4769 3577 2862 5000 14600 7300 4867 3650 2920 5200 15184 7592 5061 3796 3037 5400 15768 7884 5256 3942 3154 5600 16352 8176 5451 4088 3270 5700 16644 8322 5548 4161 3463 5800 16936 8468 5645 4234 3387 6000 17520 8760 5840 4380 3504 6300 18396 9198 6132 4599 3679 6800 19856 9928 6619 4964 ]3504 3971 7200 21026 10512 7008 5256 4205 3504 3003 7700 22484 11242 7495 5621 4497 3747 3212 8000 23360 11680 7787 5840 4672 3893 3337 8200 7981 5986 4789 3991 3421 8500 8273 6205 4964 4137 3546 8600 8371 6278 5022 4185 3587 8700 8468 6351 5081 4234 3629 8900 8663 6497 5198 4331 3713 3248 9200 6716 5373 4477 3838 3358 9500 6935 5548 4623 3963 3468 9700 7081 5665 4721 4046 3540 9900 7i27 5782 4818 4130 3614 10000 7300 5840 4867 4171 3650 3244 10200 7446 5957 4964 4255 3723 3309 10500 7665 6132 5110 4380 3823 3407 10600 7738 6190 5159 4422 3869 3439 10700 7811 6249 5207 4463 3905 3472 11000 8030 6424 5353 4589 4015 3569 11100 8103 6482 5402 4630 4051 3601 11200 8176 6541 5451 4672 4088 3634 11800 8614 6891 5743 4922 4307 3828 3446 11900 6950 5791 4964 4344 3861 3475 12100 7066 5889 5047 4416 3926 3533 12400 7242 6035 5173 4526 4023 3621 12500 7300 6083 5214 4562 4055 3650 12800 7475 6229 5339 4672 4153 3738 13200 7709 6424 5506 4818 4283 3854 1 3504 14000 8176 6813 5840 5110 4542 4088 1 3716 14500 8468 7057 6049 5292 4704 4234 3849 14600 7105 6000 5329 4737 4263 3876 15000 7300 6257 5475 4867 4380 3982 15400 7495 6424 5621 4996 4497 4088 TABLE 1 (cont.) #D t ( ) m=0 1 2 3 4 5 6 7 8 9 10 11

500 2920 973 584 417 700 4088 1363 818 584 1000 5840 1947 1168 834 1200 7008 2336 1402 1001 1500 8760 2920 1752 1251 1700 9928 3309 1986 1418 2000 11680 3893 2336 1664 2200 12848 4283 2570 1835 2500 14600 4867 2920 2086 2700 15768 5256 3154 2253 3000 17520 5840 3504 2503 3100 18104 6035 3621 2586 3200 18688 6229 3738 2670 3400 19856 6619 3971 2837 3500 20440 6813 4088 2920 3600 21024 7008 4205 3003 3700 21608 7203 4322 3087 3900 22776 7592 4555 3254 4100 23944 7981 4789 3421 4200 24528 8176 4906 3504 2725 4400 25696 8565 5139 3671 2855 4600 26864 8955 5373 3838 2985 4700 27448 9149 5490 3921 3050 4800 28032 9344 5606 4005 3115 4900 28616 9539 5723 4088 3180 5000 29200 9733 5840 4171 3244 5200 10123 6074 4338 3374 5400 10512 6307 4505 3504 5600 10901 6541 4672 3634 2973 5700 11096 6658 4755 3699 3026 5800 11291 6774 4839 3764 3079 6000 11680 7008 5006 3893 3185 6300 12264 7358 5256 4088 3345 6800 13237 7942 5673 4412 3610 7200 14016 8410 6007 4672 3823 3234 7700 14989 8994 6424 4996 4088 3459 8000 15573 9344 6674 5191 4247 3594 8200 6841 5321 4354 3684 8500 7091 5516 4513 3818 3309 8600 7175 5580 4566 3863 3348 8700 7258 5645 4619 3908 3387 8900 7425 5775 4725 3998 3465 9200 7675 5970 4884 4133 3582 9500 7925 6164 5044 4268 3699 9700 8093 6294 5150 4358 3777 9900 8259 6424 5256 4447 3854 3401 10000 6489 5309 4492 3893 3435 10200 6619 5415 4582 3971 3504 10500 6813 5575 4717 4088 3607 10600 6878 5628 4762 4127 3641 10700 6943 5681 4807 4166 3676 11000 7138 5840 4942 4283 3779 11100 7203 5893 4986 4322 3813 11200 7268 5946 5031 4361 3848 3443 11800 7657 6265 5301 4594 4054 3627 3281 11900 7722 6318 5346 4633 4088 3658 3309 12100 7852 6424 5436 4711 4157 3719 3365 12400 8046 6583 5570 4828 4260 3811 3448 12500 8111 6636 5615 4867 4294 3842 1 3476 12800 8306 6796 5750 4983 4397 3934 3560 13200 7008 5930 5139 4535 4057 3671 14000 7433 6289 5451 4809 4303 3893 3554 14500 7698 6514 5645 4981 4457 4032 3682 14600 7751 6559 5684 5015 4488 4060 3707 15000 7964 6738 5840 5153 4610 4171 3809 15400 8176 6918 5996 5290 4733 4283 3910

Claims (17)

WHAT WE CLAIM IS:

1. A method of enhancing colour effects produced by ambient light while controlling light intensity produced thereby, comprising; reflecting with a phase change substantially equal to Ir radians between 1 and 25 % of the ambient light at an interface while permitting substantially all of the remaining ambient light to continue as transmitted light, permitting said transmitted light to travel without reflection to an absorbing layer and then reflecting a portion of said transmitted light at said absorbing layer while permitting the remaining light to continue as retransmitted slight, the distance between said first interface and said absorbing layer being such that a specific colour effect is produced due to interference at one, or a limited number of, specific wavelength(s) in the visible spectrum between the light reflected from said interface and the light transmitted back through said interface after reflection at said absorbing layer.

2. A method in accordance with Claim 1, wherein some of said transmitted light is absorbed prior to reaching said absorbing layer.

3. A method in accordance with Claim 1 or 2, wherein some of said retransmitted light is further absorbed after emerging from said absorbing layer.

4. A method in accordance with Claim 1,2 or 3, wherein, much infrared light is reflected at said absorbing layer while much visible light is permitted to continue as retransmitted light.

5. Optical apparatus comprising a dielectric substrate, a semireflecting layer intimately bonded to one surface of said substrate, and a dielectric layer having an outer surface and an inner surface, said inner surface being hermetically sealed to said semireflecting layer, said dielectric layer having a uniform thickness such that, when ambient light is incident on the outer surface, a specific colour effect is produced due to interference at one, or a limited number of, specific wavelength(s) in the visible spectrum between light reflected from said outer surface and light transmitted back through said outer surface after reflection at said semireflecting layer.

6. Apparatus in accordance with Claim 5, wherein said dielectric substrate is absorbing.

7. Apparatus in accordance with Claim 5 or 6, wherein the reflectivity of said semireflecting layer is colour-dependent.

8. Apparatus in accordance with Claim 7 wherein said semireflecting layer comprises copper, gold or brass.

9. Apparatus according to Claim 5 or 6, wherein the reflectivity of said semireflecting layer is greater for infra-red wavelengths.

10. Apparatus in accordance with any of Claims 5 to 9, wherein said apparatus comprises a lens for sunglasses.

11. Apparatus in accordance with any of Claims 5 to 9, wherein said apparatus comprises glazing for a window.

12. Optical apparatus comprising a first interface for reflecting with a phase change substantially equal to Ir radians between 1 and 25 %of the ambient light at the interface while permitting substantially all of the remaining ambient light to continue as transmitted light, an absorbing layer spaced from said interface for receiving said transmitted light, said absorbing layer being adapted to reflect a portion of said transmitted light while permitting the remainder to continue as retransmitted light, the distance between said first interface and said absorbing layer being such that a specific colour effect is produced due to interference at one or a limited number of, specific wavelength(s) in the visible spectrum between light reflected from said inteface and light transmitted back through said interface after reflection at said absorbing layer.

13. Apparatus in accordance with Claim 12 wherein some of said transmitted light is absorbed prior to reaching said absorbing layer.

14. Apparatus in accordance with Claim 12 or claim 13 wherein some of said retransmitted light is further absorbed after emerging from said absorbing layer.

15. Apparatus in accordance with any of claims 12 to 14, wherein, in use, the absorbing layer reflects at least some incident infrared radiation whilst transmitting a major portion of the visible light.

16. A method as claimed in Claim 1 substantially as hereinbefore described with reference to Figs. 1, 6 and 7 of the accompanying drawings.

17. Optical apparatus according to Claim 5 and substantially as hereinbefore described with reference to Figs. 1, 6 and 7 of the drawings.