The present invention relates to an optical device for augmented reality applications and a method for its fabrication.

More specifically, the present invention relates to glasses (a wearable display) capable of superimposing onto the field of view of a user images coming from a display, in the following referred to as digital images or images of the CGI (Generated Computer Image) type, preferably integrated into the glasses.

These images are generated for example by a mobile phone, a tablet, a personal computer or by other sources preferably external to the glasses and are sent to the display so that from this, as described in the following, they can be re-directed towards the field of view of the user. The information carried by the images and coming from the micro-view relate, for example, to SMS, to e-mail, to information from a browser, or else to functions of the mobile phone such as Internet connection, address book, calendar, diary, etc.

Alternatively, for dedicated applications such as glasses for poor-sighted people (low vision aids), the images coming from the micro-display are generated by a video camera placed on the glasses and able to acquire images of the real scene located in front of the glasses. These images of the real scene are acquired and reprocessed by the video camera, which outputs corresponding images of the CGI type. In this case, the user will see superimposed on the real scene a second representation of it, as captured by the video camera, but reproduced and displayed in a digital form.

This process is extremely functional, and can be a solution in cases of seriously poor vision if used as a visual aid. In particular, for example, in some forms of ametropia, the poor- sighted person who experiences great difficulties in the direct vision of the real scene, or even its impossibility, can have access to a exact reproduction of this scene, if suitably processed and represented to the eye in suitable modes.

Systems are known, referred to as Head Mounted Displays (HMD), of the "see-through" type, glasses that are capable of enabling the viewing of scenes with augmented reality through the superposition of images coming from a display onto the panorama of a real scene. US 2012/0120365 describes transmission optical systems adapted to present to a observer optical information coming from a display superimposed onto optical information coming from the surrounding real environment. Specifically, US 20122/0120365 teaches an implementation of such optical systems integrating the display into a structure disposed in front of the eyes of a observer and employing associated contact lenses on a surface of which are placed nano-filaments adapted to selectively transmit the optical information coming from the display or from the surroundings. Unfortunately, the implementation of a system in which the display is situated in the field of view of the user greatly reduces this field of view due to the necessity of supporting the display with the relevant control electronics for the generation of the images, which is disposed on the edge of the structure bearing the display, thus masking the peripheral vision of the user. Furthermore, the display in the field of view of the user is not totally transparent whereby it allows the transmission of a reduced light intensity from the real surrounding environment and is necessarily selective with respect to some wavelengths of the optical radiation coming from the real surrounding environment. In the case - not infrequent - of a user with visual defects that may need ophthalmic lenses, the system in US 2012/0120365 requires an ad hoc construction integrating the ophthalmic lens with the display or with the associated contact lens.

In general, thanks to the HMD of the "see-through" type, the user is able to see images coming from a display, which are superimposed on a real image observable through the lenses themselves of the glasses being thus able to display a combined image, more properly referred to as "augmented", from which the specific term "augmented reality glasses" is derived. The combined or "augmented" image is composed of the real scene, normally observed by the user, "augmented" by that of the CGI type, which on the contrary comes from the display, also known as "augmentation".

The user is thus able to see, at the same time, a real and a virtual scene. In these glasses, the digital images coming from a micro-display are conveyed in a manner known per se into the field of view of the user through an optical path that depends on the specific design implemented. This optical path may also include between its elements the lenses of the glasses, if these are provided.

The digital image coming from display is displayed in the space in front of the user: the distance and the dimensions of this image from the eyes of the latter are strictly correlated parameters, and depend on the whole optical layout. In particular, an appropriate focal distance of the virtual scene enables the user to bring into focus the real and the virtual scene at the same time. In this way, the two scenes of different nature are integrated by superposition obtaining the augmented image.

The element responsible for this integration is known as a "combiner", a term derived from its specific function of combining the aforementioned scenes: this element has a key role inside the layout of the glasses.

The combiner typology has a direct impact not only on the quality of the combined image, but also on the weight of the glasses and on its distribution, on its form factor, on the quality of the experience from the ergonomic point of view and, lastly, it greatly affects the cost of the glasses.

The nature of the combiner varies with the approach chosen for the optical design (refractive, diffractive, holographic) and with the choice of the relevant technology adopted for its insertion into the general layout.

A combiner may be used alone in a free-space optical propagation scenario (with a functionality analogous to that of a beam splitter/coupler) or else it may be integrated into a waveguide, contributing in each of the cases to the definition of a specific optical path. Consequently, its behaviour as part of the whole layout can be described by the theory of free-space or of guided optical propagation, or else, often, by a model which combines appropriately both theories. In summary, combiners may be:

simple semi-reflecting mirrors whose operation is based on refractive optics: they are the first that have been introduced and are elements adapted to deviate the images coming from the display in order to convey them up to the eye of the user along a pre- established optical path. These mirrors, however, aside from producing the desired deviation of the image coming from the display, have the secondary effect of also partially deviating the image coming from the real scene, screening it, with the result that transparency is not satisfactory and the reflectivity is low, making high-power sources with a high power consumption necessary. Often, furthermore, these are elements that are difficult to miniaturize;

free-form prisms: these are asymmetric prisms; these are also elements of a refractive nature which often take advantage of the interactions of the optical field with more than two interfaces in order to deviate it in an appropriate manner; consequently, they are extremely bulky, heavy and aesthetically unsightly;

elements of a diffractive or holographic nature: these have the functionality of reflecting elements with a spectrally selective mode. The transparency within the spectral band complementary to that of reflection is generally optimal, but the spectral filter property and the consequent dispersive characteristics of the field can have a blurring effect on the image displayed in that the prism effect produces a deviation with different angles for different wavelengths. If adopted in an RGB configuration, the diffractive/holographic elements can present the further limitation of 'spectral cross-talk', i.e. of communication between different (chromatic) channels. This phenomenon is due to the fact that the sources of red, green and blue are not ideal monochromatic sources (not having infinitesimal spectral width) and that the holographic/diffractive elements have a finite spectral bandwidth.

Typically, these elements may be inserted into the general layout as stand-alone elements or as parts of a system in a waveguide.

In the configuration with stand-alone elements, the diffractive/holographic elements are installed in off-axis mode (they are also known as off-axis combiners). The accentuated spectral selectivity has its counterpart in a selectivity of the angular type that greatly limits the field of view of the glasses and the dimensions of the eye-box, a volume of space inside of which the eye can move without resulting modifications to the field of view. In the guided configuration, the combiners are used in the entry and exit of a transparent waveguide which is part of the optical path carrying the images generated by the display up to the field of view of the user, said waveguide being often made of glass or of a polymer material. In this case, the diffractive or holographic element has the function of coupling the field coming from the display into the guide and decoupling it at the exit before the end part of the projection system conveys it towards the eye of the user. In a similar manner to what happens in the systems for integrated optics, the presence of the waveguide is used to guide the electromagnetic field, while conserving it, which in this case conveys the information on the image generated by the display in order to project it into the field of view of the user. The quality of the image and the power consumption do not substantially depend on the length of the waveguide; this characteristic therefore represents an interesting degree of freedom, rendering the system particularly versatile. Inside the guide, structures of various nature are often present with a distributed operation (beam expanders), which may also coincide with the combiner itself, with the function of increasing the field of view and the eye box of the glasses, which in this technology are in fact normally good. Except in exceptional cases, the HMD that implement waveguides use laser sources, and accordingly exhibit some limits that are fairly well known in the literature: ghost images due to coherent light interference between multiple reflections of the image, speckles, fragility of the element when the material used for the waveguide is glass and, in general, a rather heavy aesthetic aspect.

The object of the present invention is to provide a new optical device for applications of augmented reality that has a enhanced field of view, that is transparent, that exhibits a high spectral selectivity, a wide angular range of reflection and that is easily miniaturizable and of limited cost, thus solving the problems raised in the preceding analyses of the devices of the prior art.

Another object of the present invention is to provide an innovative method for the fabrication of such an optical device.

This and other objects are achieved by an optical device whose characteristics are defined in Claim 1 and by a method as claimed.

Particular embodiments form the subject of the dependent claims, whose content is intended to form an integral part of the present description.

In summary, the optical device of the present invention are glasses display comprising lenses obtained by applying a series of lithographic techniques for fabricating, on transparent surfaces, a controlled number of metal nano-structures having a high value of the coefficient of reflection at predetermined wavelengths and for radiation incident at a predetermined angle, in particular in the visible spectrum, as a consequence of a localized surface plasmonic resonance.

The phenomenon of plasmonic resonance is a phenomenon that occurs when nano- structures with a variable shape and dimensions typically less than 100 nm are excited by a electromagnetic field in the visible spectral region or in the near-ultraviolet. Surface plasmons are a particular collective oscillation of the charge density (electrons) that is generated on the surface of a noble metal, such as for example gold and/or silver, when it is illuminated with radiation, for example laser radiation, in the visible or in the near-ultraviolet, and which affects maximum thicknesses of penetration of the electromagnetic field down to the skin depth, which in the aforementioned nano-structures has dimensions typically in the range between 30 nm and 50 nm. This effect is due to the fact that these metals no longer behave in an ideal manner, but the electrons that they comprise acquire an oscillation frequency (plasma frequency), close to that of the external illuminating (excitation) field. The free electrons of the noble metals composing the nano- structures enter into oscillation forced by the external electric field and re-emit light in the same spectral range, although with a slight shift towards different wavelengths, as a function of the dimensions of the nano-structures in question. Because of the participation of a large number of electrons in the (collective) phenomenon, the intensity of the re- emitted radiation is remarkably high and also visible to the naked eye.

The metal nano-structures, in particular of noble metals (gold and silver), when formed into a suitable geometry and illuminated by a external source of light exhibit a high scattering coefficient due to the resonance of the surface plasmons excited by this source (localized surface plasmonic resonance, LSPR).

This value may be varied and controlled from 1% to 90% over the whole range of visible radiation, by controlling the shape and the dimensions of the nano-structures with an accuracy of a few nanometres. The spectral characteristics and the angular distribution of the re-emitted radiation can be calibrated by choosing and fabricating in a consistent manner the nanometric geometry and their spatial distribution, and by choosing appropriately the composition of the material employed. The fabrication of the nano-structures can be undertaken on materials containing Silicon or Silica (SiO _x ) and, in this case, a deposition of the electroless type known per se is used, which is characterized by a reduction of the metal starting from a ionic solution without the need for an external electric current, or else on any other dielectric or semiconductor material or on oxide and, in this case, for the deposition, evaporation or sputtering techniques are used.

The definition of the nano-structures takes place in a manner known per se by way of lithography (electron lithography, imprinting lithography, hot embossing, X-ray lithography, etc.), and is thus compatible with the presence of polymers that are photo- activatable or not, on the surface on which the nano-structures are deposited.

The deposition can take place on plane or curved surfaces and flexible this films.

Further characteristics and advantages of the invention will become apparent from the detailed description that follows, presented solely by way of non-limiting example, with reference to the appended drawings, in which:

Figure 1 is a view from above of the optical device of the present invention; Figure 2 is an enlarged view from above of a portion of a lens of the device according to the invention; and

Figure 3 is a flow diagram of the operation according to the method of the invention.

In Figure 1, an optical device (preferably a pair of glasses) according to the present invention is generally indicated 1.

Such glasses 1 comprise a frame provided with two temple bars 2a, 2b adapted to accommodate respective opto-electronic devices 4.

The frame, even though it comprises the opto-electronic devices 4 integrated into the temple bars 2a, 2b, is similar to the frame of a traditional pair of glasses. These opto-electronic devices 4 comprise a display 6, a device 6a for illuminating the display and an optical device 6b for projecting an image coming from the display 6 (said image being projected towards the lens of the glasses described in the following). The display 6, the device 6a for illuminating the display and the optical projection device 6b are known per se.

The devices 6a for illuminating the display may for example be formed:

- by means of laser diodes with emissions in the range of frequencies corresponding to red, green and blue: the exit beam of each laser diode is collimated in a known manner by an optical system in such a manner that the dimensions of the beam on the display 6 have the dimensions of the display 6 itself;

- by means of three LEDs: a first LED with emission in the frequency range corresponding to red, a second LED with emission in the frequency range corresponding to green, a third LED with emission in the frequency range corresponding to blue. The exit beam of each LED is collimated in a known manner by an optical system in such a manner that the dimensions of the beam on the display 6 have the dimensions of the display 6 itself. Each beam coming from the LEDs may furthermore be filtered in frequency by means of a narrowband filter, in such a manner as to reduce the emission spectral bandwidth to a few nanometres;

- by means of a white LED: the exit beam of the LED is collimated in a known manner by an optical system in such a manner that the dimensions of the beam on the display 6 have the dimensions of the display 6 itself. The beam coming from the LED may be filtered in frequency by means of three narrowband filters: a filter for the frequencies corresponding to red, a filter for the frequencies corresponding to green, a filter for the frequencies corresponding to blue, in such a manner as to reduce the RGB emission spectral bandwidths to a few nanometres. The display 6 may be fabricated in a manner known per se, for example with LCD or LCOS technology or micro mirror technologies based on MEMS.

The optical projection device 6b comprises for example:

- lenses made of glass that may be coated with an A/R coating; the materials of these lenses are selected so as to minimize the chromatic aberration;

- lenses made of plastic that may be coated with a A/R coating; the materials of these lenses are selected so as to minimize the chromatic aberration;

- reflecting elements such as for example mirrors or reflecting holographic elements. The display 6, the device 6a for illuminating the display and the optical projection device 6b cooperate in a manner known per se such that the digital image is emitted from the display 6.

Alternatively, only one opto-electronic device 4 is present in a temple bar 2a, 2b: The glasses of the present invention are connected by means of connection systems 7 (preferably receiver/transmitter systems) such as for example a cable, or a wireless or bluetooth connection, with the electronic devices which generate the digital images that are then sent to the display 6, and from the latter re-directed towards the field of view of the user.

In particular, through the connection systems 7, the display 6 is connected to a processing unit 8 configured for controlling in a manner known per se the images to be sent to the display 6. Advantageously, the processing unit 8 is included within a personal computer 8a, a tablet 8b, a mobile telephone 8c or within another source known per se and the images generated by the processing unit 8 and supplied to the display 6 via the connection systems 7 relate for example to SMS, e-mail, information from a browser, functions of the mobile telephone such as Internet connection, address book, calendar, diary, etc.

The glasses 1 of the present invention furthermore comprises at least one lens 10 (preferably two), adapted to convey the digital image coming from the display 6 towards the eyes of the user, which is implemented as described in the following.

The lens 10 is formed with a technology so as to allow the reflection of the image coming from the optical projection device 6b into an ocular axis.

In particular, the lens 10 is formed by means of a deposition of nano-structured material (in particular, antennas) so as to allow the reflection into the ocular axis of the digital image coming from the display 6, according to a suitable angle.

The solution guarantees a degree of transparency so as to be able to make the lens 10 thus formed, correspond to a normal clear lens for glasses, not for sunglasses.

The technological solution allows the user to view the images coming from the display 6 and, at the same time, to observe the images coming from the external reality.

The lens 10 may be a curved lens, possibly with optical power, or also an element of an optically transparent material of uniform thickness and no curvature, such as for example a plane plate or slab of an optically transparent homogeneous material.

Advantageously, the glasses 1 include at least one video camera 12 preferably integrated into the front part of the frame, adapted to acquire, in a manner known per se, images of a real scene in front of the user and to send it to the display 6, from which they will emerge as digital images directed to the eye of the user. The frame 2 furthermore comprises a light intensity sensor 14 for the detection of the level of ambient light intensity in real time which will determine the regulation of the illumination system 6a for the display 6 in such a manner as to optimize it relative to the external lighting conditions. This light intensity sensor 14 is preferably situated in the front part of the frame of the glasses 1.

Advantageously, the glasses 1 furthermore comprise, preferably installed in the temple bars 2a, 2b:

- regulators 16, which may be mechanical or electro-mechanical, for enabling the dioptric compensation, where necessary;

- regulators 18, which may be mechanical or electro-mechanical, of the inter-pupillary distance;

- regulators 20, which may be mechanical or electro-mechanical, of the height of the projection system for the image (devices 6a and 6b) with respect to the ocular axis of the user.

Advantageously, the glasses 1 furthermore comprise, preferably installed in the temple bars 2a, 2b, power supply devices 22 and connection ports 24 (for example a micro-USB port).

The glasses 1 furthermore comprise, installed in the temple bars 2a, 2b, devices 26 for the management of information present on the display 6. These devices 26 are adapted to manage the information the display 6 (for example the scrolling of the text of an email) and may consist of buttons or of a touchscreen. The commands may alternatively also be of the vocal type or make use of movements of the head of the user or use "eye-tracking".

Finally, the glasses 1 comprise an integrated microprocessor 28 for managing the electronic and opto-electronic components of the glasses 1. The microprocessor 28 is configured for controlling:

- the light-intensity sensor 14;

- where present, the electromechanical regulators 16, 18, 20;

- the connection systems 7 with external equipment; - the power supply devices 22;

- the management devices 26;

- the management of the software for processing the images acquired via the video camera 12 and the sending to the display 6 of the images thus processed.

The processing unit 8 communicates, in a known manner and by means of the connection systems 7, with the display 6 in transmission/reception mode.

The unit 8 is configured for:

- transmitting the information, generated by said unit, to the display 6, which projects it, in the form of images, towards the eye of the user;

- receiving and processing information coming from devices present on the glasses, such as preferably the stream of video data coming from the video camera 12 or possibly state parameters of the glasses system, such as for example, but not limited to, the state of charge of the power supply system 22, the state of the connection of the connection systems 7, the GPS position, or from other possibly installable devices/sensors.

The lens 10 of the glasses of the present invention is formed as it is here described in the following with reference to Figures 2 and 3.

In Figure 2, a portion of a lens 10 is shown, which includes a substrate 52, for example glass or a flexible and/or stretchable transparent polymer, on which a plurality of metal nano-structures 54a and 54b are present. In particular, the nano-structures 54a, 54b may be either single small discs, preferably with a diameter of around 50 nm, or dimers (two small discs placed with a separation distance of less than 10 nm), or else nano-antennas known per se or dimers of nano-antennas, whose geometrical dimensions, in particular the lengths (preferably included within the range 100-500nm), determine the plasma frequency, hence the resonance frequency and, consequently, the emission wavelength.

In the case in which the substrate 52 is flexible and stretchable, the mutual distance between the metal nano-structures 54a, 54b changes and thus their resonance and the wavelength, hence the colour, of the emitted light change. In this way, it is possible to mechanically change the emission colour of the nano-structures 54a, 54b. Figure 3 illustrates a flow diagram of the operations carried out in order to obtain a lens 10 according to the present invention.

The first step 100 is the preparation of the substrate 52. The materials used for the substrate 52 are insulators whereby, in order to be able to apply an electron lithography for the definition of the nano-structures, it is necessary, at step 102, to deposit onto the substrate 52 a resist, for example by performing a spinning, and subsequently, at step 104, to coat the substrate 52 with a layer of conductive material, for example aluminium or gold, for a thickness preferably in the range between 2 and 5 nm.

Subsequently, at step 106, a stage of electron lithography is carried out in order to define the nano-structures 54a, 54b.

The lithography is run by sending electrons onto the substrate 52 with an energy preferably in the range between 30 and 50 keV.

After the lithographic exposure 106, the layer of conductive material needs to be removed. This is done at step 108 by immersing the specimen in a solvent which does not interact with the substrate of underlying resist.

At this point, at step 110, the resist is developed, preferably with a solution of IPA:MIBK=3:1, and then, at step 112, dried for the definition of the metal lithographic nano-structures 54a, 54b. In order to obtain a precise definition of the nano-structures 54a, 54b the following steps are carried out. At step 114, the substrate 52 after lithography, in the following denoted as specimen, is immersed in a predetermined aqueous solution of hydrofluoric acid, for example 0.15M, for a predetermined time and at a predetermined temperature, in particular for one minute at 50°C.

At step 116, the specimen is rinsed in de-ionized water in order to eliminate the residues of hydrofluoric acid.

At step 118, the specimen is immersed in a predetermined metal solution, for example an aqueous solution of silver nitrate, for example of the ImM type, for a predetermined time and at a predetermined temperature, in particular for 30s at 50°C. Alternatively, it is possible to immerse the specimen in a solution of a gold salt.

At step 120, the specimen is again rinsed in de-ionized water so as to block the reaction for production of the nano-structures 54a, 54b.

Lastly, at step 122, the specimen is dried with a flow of nitrogen.

The immersion 114 of the specimen in hydrofluoric acid after lithography has the purpose of removing the oxide naturally present on the substrate 52, leaving a surface inert to reactions with oxygen and its compounds, for example 0 ₂ , C0 ₂ or CO, and thus available for the successive steps of the self-aggregative deposition.

If the substrate 52 is made of silicon, which becomes, on the surface, silicon oxide, owing to the presence of oxygen, the reaction between the hydrofluoric acid and the silicon oxide is as follows:

Si0 ₂ + 6HF→ 2H ⁺ + SiF ₆ ^2' + 2H ₂ 0 (1) However, it should be noted that although the Si-F bond is thermodynamically favoured with respect to the Si-H bond, the latter prevails on the surface owing to the high polarization of the Si ^" F ^" bonds which are formed as soon as the reaction between the surface of the substrate 52 and the hydrofluoric acid is initiated. The said Si ^s+" F ^6" bonds weaken the Si-Si bonds of the layers of the substrate 52 that are located underneath the said surface rendering them more vulnerable to the nucleophilic attack of the hydrofluoric acid, according to the following reaction:

Si _hulk - Si - Si ^s+ -F ^s~ + 4HF→ Si _Mk - Si - H + SiF ₄ (2) where Sib _U ik-Si-Si ^5+" F ^5" represents the substrate 52, the surface of which has already been attacked by the hydrofluoric acid with consequent formation of Si ^8+" F ^S" , bonded to this surface. The term Sibuik represents the portion of the substrate 52 which is underneath the surface layer.

The reaction of further hydrofluoric acid with this surface layer yields as a product Sibuik- Si-H (layer of hydrogenated silicon) and leads to the formation of SiF ₄ , a volatile molecule which escapes from the substrate 52.

The immersion 118 of the substrate, now having a surface layer of hydrogenated silicon, into the solution of silver nitrate leads, respectively, to the formation of the nano-structures 54a, 54b of silver (or of gold in the case of immersion into a gold salt solution).

In the vicinity of the nano-structures 54a, 54b, two electrochemical reactions take place which respectively produce the oxidation of the silicon and the reduction of the silver:

Si + 2H ₂ 0→Si0 ₂ + 4H ⁺ + 4e ^~ (3)

Ag ⁺ + e-→Ag° (4) or, for the case of gold:

Au ⁱ⁺ + 3e ^~ →Au° (5)

The nitrogen does not react but remains in the solution as N0 ₃ ^" . As far as the substrate 52 is concerned, the surface layer of hydrogenated silicon initially reacts and, subsequently, the silicon of the underlying layers Si _bu i _k also reacts.

The sub-reactions (3)-(4), which as a whole represent a oxidation-reduction reaction, take place by virtue of their difference in potential. The standard oxidation-reduction potentials for the reactions (3) and (4) are:

E ₀ _reaction3 = -0.9 V

Eo_reaction4 = 0.8 V

Starting from the standard oxidation-reduction potentials it is possible to calculate, the Nerst equation, the equilibrium constant K _e of the oxidation-reduction reaction:

RT where n is the number of electrons transferred in the oxidation-reduction reaction, F is the Faraday constant, T is the temperature at which the reaction takes place.

In the reaction for formation of the nano-structures 54a, 54b of silver, the temperature preferably in the range 45-50°C.

The mechanism for formation of the nano-spheres of silver initially sees one Ag ion, in the vicinity of the surface of silicon, capturing an electron from the valence band of the silicon itself and getting reduced to Ag°. The nucleus of silver thus being formed, being very electronegative, tends to attract other electrons from the silicon, becoming negatively charged and thus catalyzing the reduction of other Ag ⁺ ions, which will grow the grain size. The reaction must accordingly be blocked, eliminating the availability of other silver ions, by way of rinsing in de-ionized water, and/or reducing the temperature, thus rendering the process thermodynamically unfavourable.

In the case of the pair of sub-reactions (3) and (5), the standard oxidation-reduction potentials are:

Eo_reaction3 0.9 V

E ₀ _reaction5 ⁼ 1.52 V

The mechanism of reaction is similar to that of silver, but changes the reaction kinetics in that the gold reacts forming a higher number of particles having smaller dimensions with respect to the silver. For this reason, it is necessary to increase the reaction time in the step of formation of the nano- structures 54a, 54b.

In the reaction for formation of the nano-spheres of gold, the temperature is preferably in the range 40-50°C.

The nano-structures 54a, 54b thus formed all have the characteristic of exhibiting high scattering coefficients due to the generation of the surface plasmons at the visible wavelengths: from 450 nm to 680 nm.

The angular distribution of the scattering of the light has been experimentally measured and covers a wide range with a maximum intensity between 25 and 60 degrees with respect to the normal to the surface of the specimen.

By varying the dimensions of the nano-structures 54a, 54b (diameters and lengths), their shape (spheres or nano-antennas) and their separation distance, it is possible to change the percentage of reflected and transmitted light and the dominant colour in a significant manner and to make the reflected component dominant or balanced with that transmitted or to have a chromatic dominance from blue to red.

Generally speaking, it is possible to have the three RGB components reflected in an appropriate manner for the applications of the glasses.

Naturally, while still keeping the principle of the invention, the embodiments and their detailed implementations may be widely varied with respect to what has been described and illustrated purely by way of non-limiting example, without going outside the scope of protection of the present invention defined by the appended claims.