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DESCRIPTION

FIELD OF APPLICATION

The present invention relates to an opto-electronic system for augmented reality applications based on holographic optical devices.

In particular, the present invention relates to an opto-electronic system for augmented reality applications comprising a holographic "combiner" lens and the description that follows makes reference to this field of application in order to simplify the description thereof.

Combiner lens means an optical element that is partially transmissive relative to electromagnetic radiation coming from the outside and which is capable, at the same time, of superimposing digital images coming from a display.

The combiner is an optical element that redirects the digital image projected from a display towards the field of vision of a user, in such a way that the user can simultaneously view the background scene and the image projected from the display, combined in what is called an 'augmented reality' scene.

PRIOR ART

There are known systems called head-mounted displays (hereinafter HMDs) of the "see-through" type, i.e. glasses that can enable an augmented reality scene to be viewed by superimposing images coming from a display over a panorama of a real scene.

Thanks to such systems, the user is able to see images coming from a display, which are superimposed on an image of the real scene observable through the lenses of the glasses and thus view a combined image, more properly referred to as "augmented", the term from which the specific expression of "augmented reality glasses" derives. The combined or "augmented" image is made up of the real scene, normally observed by the user, "augmented" by one of a CGI (computer-generated imagery) type, which comes instead from the display, also referred to as "augmentation".

The user is therefore able to see a real scene and a virtual scene simultaneously.

The undersigned has found that, in the case of augmented reality glasses of a see- through type, the lenses are rarely completely transparent, as would be desirable, but can rather be an obstacle to natural, comfortable viewing of the reality present in the background, because of their specific conformation. Depending on the technology adopted, in fact, these lenses can be dark lenses, often necessary to bring the image to an optimal contrast, or contain inside them, in a punctual and discrete or distributed form, optical elements such as prisms, plates or other interfaces that alter vision, resulting in an unclear overall perception.

The undersigned has found that, in these glasses, the digital images coming from a display are brought into the user's field of vision in a manner that is known per se, through an optical path which depends on the specific design created. This optical path can also include, among its various elements, the glass lenses, when the latter are provided for.

The image coming from the display is viewed in the space in front of the user.

The undersigned has found that the distance of this image from the user's eyes and its dimensions are closely correlated parameters defined on the basis of the overall optical layout. In particular, an appropriate focus distance of the virtual scene enables the user to put the real scene and virtual one into focus simultaneously. In this manner, the two scenes of a different nature are integrated by superimposition, obtaining an augmented image that is in focus overall.

The element responsible for this integration is called a "combiner" lens, a term deriving from its specific function of combining the above-described scenes: this element has a key role within the layout of the glasses.

In particular, holographic combiner lenses with non-monochromatic light incident on them have the functionality of reflecting elements, but with working angles that can be defined at the design stage and whose functioning is independent of the known process of reflection along an interface in the presence of a change in the refractive index, where the angle of reflection is equal to the incidence angle according to Snell's law, as for the known mirrors.

Disadvantageously, the intrinsic dispersive characteristics of the diffracted field can have a blurring effect on the reflected image.

Holographic combiner lenses made as volume HOEs (i.e. holographic optical elements) are also known.

Disadvantageously, despite providing a high reflection efficiency in a narrow interval of wavelengths, centred on a predetermined value λ and, at the same time, a high transmissivity which is maintained throughout the remaining spectrum complementary to the wavelength, such combiner lenses can give rise to bothersome chromatic aberrations, since non-monochromatic light is propagated at different angles depending on the wavelengths according to the diffraction grating equation m λ = Λ (sin a + sin β), where m is the order of diffraction, λ the wavelength of the incident light, Λ the grating pitch of the hologram, and a and β the entrance and exit angles, respectively.

The problem of compensating for chromatic aberrations remains one of the main unsolved problems in the field of holographic techniques applied to HMD glasses and an efficient solution has not yet been found.

Typically, holographic combiner lenses can be inserted in the general layout of the HMD in the form of one or more separate units, with a discrete distribution, or as parts of a waveguide-integrated system.

In the waveguide configuration, the combiners are used at the inlet and outlet of a transparent waveguide, normally made of glass or polymeric material, which is part of the optical path, and carries the images generated by the display to the user's field of vision.

It is well known that, in general, HMDs that implement waveguides use laser sources. Disadvantageously, such HMDs are subject to well-known specific limits, such as ghost images, due to interferences of coherent light among multiple reflections of the image, speckle, limitation of lateral vision (which is important for the balance of the user during movement; this also prevents objects positioned laterally from being seen properly), fragility of the element when the material used for the waveguide is glass, and in general a rather heavy aesthetic appearance. Another problem for augmented reality glasses in general and for HMDs in particular, is energy autonomy.

Good energy autonomy is normally difficult to obtain if one wishes to maintain an approach with a lightweight design of the glasses.

It is fundamental to bear in mind that a low energy autonomy still today represents one of the aspects that mostly greatly contributes to hindering the development of the field of wearable technologies, also taking into account the unavailability of batteries that have a sufficiently long life and are at the same time miniaturised.

Document WO2016/051325, partially authored by one of the applicants, describes an optical device for augmented reality applications and a method for the manufacture thereof.

Document WO2014/1 15095 describes a transreflective holographic film for a display worn on the head of a user.

It can be understood that the two documents are incompatible because of their structural and functional features. A general object of the present invention is to provide an opto-electronic augmented reality system that overcomes the problems of the prior art.

A specific object of the present invention is to propose a novel opto-electronic system for augmented reality applications that has a large field of vision compatible with its form factor, which is transparent, which has a high spectral selectivity characterised by a considerable image sharpness, with an angular selectivity representing a compromise between an exit pupil of fairly large dimensions and the possibility of inhibiting any spurious reflections coming from outside the glasses, which can be easily miniaturised, has a modest cost and whose low consumption enables a considerable battery lifetime. SUMMARY OF THE INVENTION

In a first aspect of the invention, these and other objects are achieved with an optoelectronic system for augmented reality applications in accordance with the appended claim 1 .

The opto-electronic system for augmented reality applications comprises a glass frame adapted to be worn by a user.

The glass frame comprises a front portion, placed in front of the head of the user, adapted to support at least one lens, and at least one side supporting portion, placed along the side of the head of the user, and connected with the front portion so as to allow a support of the frame on the head of the user;

The glass frame is adapted to be coupled with an opto-electronic device comprising: a LED lighting device configured to output a first electromagnetic beam of LED light; a display configured to receive encoding information of digital images and to modulate the first electromagnetic beam, coming from the LED lighting device, as a function of the encoding information received, determining a second electromagnetic beam configured to carry the information of the image;

a projection optical device configured to project the second electromagnetic beam;

a narrow-band bandpass interference filter configured to filter spectrally the electromagnetic beam according to a narrow spectral band.

The glass frame comprises at least one combiner lens, mounted on the front portion, comprising a holographic element interposed between a first protective layer proximal to the eyes of the user and a second protective layer distal to the eyes of the user,

Preferably, the combiner lens is adapted to:

receive the second electromagnetic beam incident on the first protective layer;

reflect, within the field of view of the user (U), the second electromagnetic beam; let an image pass through from the real scene incident on the second protective layer so that it enters the field of view of the user, combining with said second electromagnetic beam;

The overlapping of the second electromagnetic beam and the real image determines an augmented reality vision for the user.

Advantageous aspects are disclosed in dependent claims 2 to 1 1 .

In a second aspect of the invention, these and other objects are achieved with a method of displaying augmented reality images in accordance with the appended claim 18. The method of displaying augmented reality images comprises the steps of:

providing a glass frame adapted to be worn by a user, wherein the frame comprises a front portion, placed in front of the head of the user, adapted to support at least one lens, and at least one side supporting portion, placed along the side of the head of the user, and connected with the front portion so as to allow a support of the frame on the head of the user;

coupling to the glass frame an opto-electronic device comprising a LED lighting device configured to output a first electromagnetic beam of LED light;

in the opto-electronic device, carrying out the steps of:

generating the first electromagnetic beam of LED light;

receiving encoding information of digital images and modulating the first electromagnetic beam coming from the LED lighting device, as a function of the encoding information received, thus determining a second electromagnetic beam configured to carry the information of the image;

projecting the second electromagnetic beam;

filtering spectrally the electromagnetic beam, at a narrow spectral band;

providing at least one combiner lens mounted on the front portion comprising a holographic element interposed between a first protective layer proximal to the eyes of the user and a second protective layer distal to the eyes of the user;

in the combiner lens carrying out the steps of:

receiving the second electromagnetic beam incident on the first protective layer;

reflecting, within the field of view of the user, the second electromagnetic beam;

letting an image pass through from the real scene incident on the second protective layer so that it enters the field of view of the user, combining with the second electromagnetic beam;

wherein the overlapping of the second electromagnetic beam and the real image determines an augmented reality vision for the user.

In a third aspect of the invention, these and other objects are achieved by a combiner lens, in accordance with the appended claim 12.

The combiner lens comprises a first protective layer proximal to the user;

a second protective layer distal to the user;

a holographic element interposed between the first and second protective layers.

The holographic element is configured to:

receive a second electromagnetic beam, configured to:

carry the information of an image, at an incidence angle;

reflect towards the user the second electromagnetic beam at an angle of reflection programmable with respect to the incidence angle.

Advantageous aspects are disclosed in dependent claims 13 to 17.

In a preferred embodiment of the invention, the invention describes augmented reality glasses comprising a non-waveguide opto-electronic projection device, preferably integrated in the temples of the glasses, and a holographic combiner lens with excellent transparency to the real image coming from the outside and with a very high efficiency in reflecting the image coming from the projection optical device, wherein the combination of the two images determines the effect of augmented reality.

In a preferred embodiment of the invention, the combiner lens and the opto-electronic projection device are configured to project the augmented image within the field of view of the user, thus simultaneously permitting a view at infinity of the real scene by the user.

The technology introduced by the present invention enables the perception of augmented reality scenes with sharp images, while simultaneously assuring a lightweight design with a high quality overall experience, in terms of both aesthetics and perception during use (user experience).

The glasses of the invention achieve the following technical effects:

- absence of chromatic aberrations and distortions;

- minimal, light form factor which makes it possible to provide a pleasing, comfortable design;

- long battery life, as no laser light sources are required;

- simplicity of the layout and thus high reliability of the system;

- no lateral narrowing of the user's field of vision in the area of insertion of the image coming from the display in the lens; - no obstruction of the user's field of vision;

- the user is not required to turn his or her eyes in directions other than the natural one identified by the ocular axis ("look at" or "look around" configuration) in order to be able to observe the augmented image, thus enabling the user to be able to entirely appreciate the scene before him or her ("see through" configuration).

- possibility of configuring a correct matching between the refractive indexes of the holographic component and of the adjacent substrates, thus assuring the obtainment of good optical characteristics of the image, and the possibility of integrating the holographic component in lenses habitually used in the production of glasses, which are based on the use of polymeric or plastic materials.

In particular, compared to the known lighting systems with laser sources, the use of a LED lighting device allows the following technical effects to be obtained:

- much lower consumption;

- smaller size;

- superior image quality, as the absence of speckle is assured.

Additional features and advantages of the invention will become apparent from the detailed description that follows, provided purely by way of non-limiting example, with reference to the appended drawings.

The invention as a whole achieves the technical effect of a high efficiency in terms of the quality of the image projected in the user's field of view.

The technical effect is obtained through:

- the modulation of a first electromagnetic beam of LED light and generation of a second electromagnetic beam of an image;

- the joint action of a narrow-band interference filter and a combiner lens with a defined angular band and a narrow spectral band synchronised with that of the interference filter, wherein the interference filter spectrally filters either the first electromagnetic beam coming from the LED or the second electromagnetic beam of the image, wherein the first or second beam is incident on the combiner lens.

The combination of a LED source, interference filter and lens assures a high diffraction efficiency and high transparency of the holographic element, produced in the combiner lens, in the spectral regions complementary to the operating ones.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 A is a top view of the opto-electronic system of the present invention in a first embodiment of the present invention. Figure 1 B is a schematic view of the components and of the electromagnetic beams operating in the system of figure 1 A.

Figure 2A is a top view of the opto-electronic system of the present invention in a second embodiment.

Figure 2B is a schematic view of the components and of the electromagnetic beams operating in the system of figure 2A.

Figure 3A is a top view of the opto-electronic system of the present invention in a third embodiment of the present invention.

Figure 3B is a schematic view of the components and of the electromagnetic beams operating in the system of figure 3A.

Figure 4A is a top view of the opto-electronic system of the present invention in a fourth embodiment of the present invention.

Figure 4B is a schematic view of the components and of the electromagnetic beams operating in the system of figure 4B.

Figure 5 is a schematic representation of a combiner lens according to the invention. Figures 6-1 1 represent diagrams simulating the structural and functional configurations of the opto-electronic system of the present invention.

DETAILED DESCRIPTION

In summary, the opto-electronic system of the present invention is a pair of glasses comprising an opto-electronic device which projects images from a display towards lenses including a holographic element (HOE, holographic optical element), defined "combiner" lens, which enables the projection of the images coming from the display towards the user's field of view superimposed on the image of the real world which emerges from the combiner lens.

With reference to the group of figures 1 -4, an opto-electronic system 1 for augmented reality applications is shown.

The description that follows will be made with reference to figure 1 -4, unless otherwise specified.

The system comprises a glass frame 2 adapted to be worn by a user.

The glass frame 2 comprises a front portion 3, placed in front of the head of the user, adapted to support at least one lens 10.

In a preferred embodiment, the front portion 3 supports two lenses 10.

The glass frame 2 further comprises at least one side supporting portion 2a,2b, placed along the side of the head of the user, and connected with the front portion 3 so as to allow a support of the frame 2 on the head of the user.

In one embodiment the at least one side supporting portion 2a, 2b comprises two temples.

According to the invention, the glass frame 2 is adapted to accommodate at least one combiner lens 10 mounted on the front portion 3.

According to the invention, the glass frame 2 is adapted to be coupled with an optoelectronic device 4.

In particular, the opto-electronic device 4 can be mounted on the glass frame 2 or on an element solidly constrained thereto.

Alternatively, the opto-electronic device 4 can be accommodated in the glass frame 2 or in the element solidly constrained thereto.

In detail, with reference to figures 1 -4, the opto-electronic device 4 comprises a LED lighting device 6a configured to output a first electromagnetic beam F _L ED of LED light. In general, the technical effects assured by the use of a LED light source are many: reduced energy consumption (fundamental in instruments such as glasses, which cannot be constantly supplied with power), reduced size, given the intrinsic dimensions of LEDs and, therefore, easy integration into electronic platforms, easy control, a temperature that is not high (and thus no cooling system is required, as is the case when a laser source is used as the lighting system), in complete compatibility with glasses wearable at the temples.

According to the invention, the opto-electronic device 4 is configured to receive as input the first electromagnetic beam FLED of LED light.

The opto-electronic device 4 comprises a display 6 configured to receive encoding information lnf_COD of a digital image ImDGT and to modulate the first electromagnetic beam F _L ED _, coming from the LED lighting device 6a, as a function of the encoding information lnf_COD received.

The display 6 thus determines a second electromagnetic beam FIMM configured to carry the information of the image ImDGT.

The opto-electronic device 4 further comprises a projection optical device 6b configured to project the second electromagnetic beam F _!M M-

The opto-electronic device 4 further comprises a bandpass interference filter 6C configured to filter spectrally the first electromagnetic beam FLED, or the second electromagnetic beam F _!M M according to a spectral band BP.

According to the invention, the bandpass interference filter 6C is configured to filter a narrow spectral band BP.

The narrow spectral band BP characteristic of the bandpass interference filter 6c is comprised between 1 and 10 nm FWHM (full width half maximum); further below in the description it will be seen how this value is also linked to other specific technical features of the invention.

In a preferred embodiment of the invention, the narrow spectral band BP is comprised between 1 nm and 3nm FWHM.

The technical effect of a narrow spectral band should be interpreted not on its own, but rather in combination with the technical effect assured by the use of a LED lighting device.

In fact, when a LED light source and a narrow-band bandpass interference filter are combined, as described here, in the place of laser source configurations, the image obtained as output from the opto-electronic device 4, i.e. the second electromagnetic beam FIMM, appears as a sharp image, without speckle or any appreciable chromatic aberrations.

In fact, the combination of the LED light source and the narrow-band bandpass interference filter enables the emission of a quasi-monochromatic light beam, very close to the monochromatic beam generated by a laser source.

From what has been said, it is clear that in the present invention, though it was chosen not to use a laser source for reasons of technical convenience - first of all because of the insufficient energy autonomy assured by solutions that exploit such a source - a quasi-monochromatic beam has been reproduced, i.e. one that is comparable to that of a laser source, but with all the advantageous technical effects of LED sources.

According to the invention, the first electromagnetic beam F _L ED is intrinsically collimated. In other words, the LED lighting device 6a is configured to output a first electromagnetic beam F _L ED of LED light that is already intrinsically collimated.

Alternatively, the opto-electronic system 1 comprises at least one collimation lens upstream of the bandpass interference filter 6C and downstream of the LED lighting device 6a.

Preferably, the at least one collimation lens is at the inlet of the bandpass interference filter 6C.

Preferably, the second electromagnetic beam FIMM is collimated before being transmitted to the combiner lens 10 via a suitable collimation device, in particular the projection optical device 6b. It follows that the second electromagnetic beam F _!M M strikes the combiner lens 10 as a collimated beam.

The opto-electronic system of filtration and collimation of the incident light beam further assures the technical effect of a narrow pass band.

The technical effects described are assured in all the embodiments of the present invention, which will now be described in detail.

With particular reference to figures 1 A and 1 B, in a first embodiment of the invention, the bandpass interference filter 6C is placed after the LED lighting device 6a.

With particular reference to figures 2A and 2B, in a second embodiment of the invention, the bandpass interference filter 6C is placed after the projection optical device 6b.

With particular reference to figures 3A, and 3B, in a third embodiment of the invention, the bandpass interference filter 6C is placed after the LED lighting device 6a.

Moreover, the opto-electronic processing device 4 comprises a beam diverting means 6d interposed between the LED lighting device 6a and the display 6, in particular between the LED lighting device 6a and the bandpass interference filter 6c.

Furthermore, the opto-electronic device 4 comprises a reflective relay means 6f configured to receive the second electromagnetic beam FIMM and redirect it in reflection towards the projection optical device 6b.

With particular reference to figures 4A and 4B, in a fourth embodiment of the invention, the bandpass interference filter 6c is placed after the projection optical device 6b.

Moreover, the opto-electronic device 4 comprises a beam diverting device 6d interposed between the LED lighting device 6a and the display 6.

Furthermore, the opto-electronic device 4 comprises a reflective relay means 6f configured to receive the second electromagnetic beam F _!M M and redirect it in reflection towards the projection optical device 6b.

In all the embodiments of the invention, the opto-electronic system 1 of the invention can comprise accessory devices placed in at least one of the side supporting portions 2a,2b, or in compartments solidly constrained thereto.

Preferably, the opto-electronic system 1 comprises power supply devices 22 and/or connection ports 24, in particular a micro USB port, preferably mounted in at least one of the side supporting portions 2a,2b or in compartments solidly constrained thereto. Preferably, the opto-electronic system 1 comprises management devices 26 mounted in at least one of the side supporting portions 2a,2b, or in compartments solidly constrained thereto, wherein said devices are adapted to manage the information present on the display 6.

Preferably, the opto-electronic system 1 comprises an integrated microprocessor 28, mounted in at least one of the side supporting portions 2a,2b, or in compartments solidly constrained thereto, and configured to manage the electronic and opto-electronic components of the system 1 , wherein the microprocessor 28 is configured to control one or more among:

- the lighting system 6a;

- a lighting sensor 14;

- the display 6;

- optional electromechanical regulators 16, 18, 20;

- connection systems 7 for connecting with external equipment;

- the power supply devices 22;

- the management devices 26;

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

Preferably, the opto-electronic system 1 is coupled with the processing unit 8.

The processing unit 8 is configured to communicate with the display 6.

In particular, the processing unit 8 is configured to send encoding information lnf_COD of digital images ImDGT to the display 6, which projects the information, in the form of images, towards the eyes of the user.

The processing unit 8 is further configured to:

- receive and process information coming from devices present on the glasses, such as, preferably, the streaming of video data coming from the camera 12 or, optionally, status parameters of the system-glasses, such as, for example, but not limited to, the charge status of the power supply system 22, the status of the connection of the connection systems 7, the GPS position, or any other devices/sensors that may be installed.

The images can be generated, for example, by a mobile phone 8c, a tablet 8b, a personal computer 8a or other sources that are preferably external to the glasses, generically indicated as a processing unit 8, and they are sent to the display so that, as described below, they can be redirected towards the field of vision of the user. The information carried by the images and coming from the micro-display relate, for example, to SMS text messages, e-mails, information of a navigator, or mobile phone functions such as the Internet connection, contacts, calendar, agenda etc. Alternatively, in dedicated applications such as those of glasses for visually impaired persons (low vision aids), the images coming from the display are generated by a camera 12 placed on the glasses themselves and adapted to acquire images of the real scene situated in front of the glasses. Such images of the real scene are acquired and processed by the camera, which outputs corresponding images of the CGI type.

In all the embodiments of the invention, the opto-electronic device 4 is configured to project images FIMM generated by the display 6, as a function of the encoded information lnf_COD, towards at least one combiner lens 10 including a holographic element 100.

In the present invention, the "combiner" lens, a term deriving from its specific function of combining an image received from a source with a real image set in the background, has a key role within the layout of the glasses. According to the invention, the optoelectronic system 1 for augmented reality applications comprises the at least one combiner lens 10 mounted on the front portion 3 of the glass frame 2.

With particular reference to figure 5, the combiner lens 10 comprises the holographic element 100 interposed between a first protective layer ST1 proximal to the eyes of the user and a second protective layer ST2 distal to the eyes of the user.

The combiner lens 10 is adapted to receive the second electromagnetic beam FIMM incident on the first protective layer ST1 ,.

The combiner lens 10 is adapted to reflect, within the field of view of the user U, the second electromagnetic beam FIMM-

The combiner lens 10 is further adapted to let an image pass through ImR from the real scene incident on the second protective layer ST2 so that it enters the field of view of the user (U), combining with the second electromagnetic beam F _!M M- The described overlapping of the second electromagnetic beam F _!M M and the real image

ImR determines an augmented reality vision for the user U.

In other words, the combiner lens and the opto-electronic projection device are configured to project the digital image within the field of view of the user, thus simultaneously permitting a view at infinity of the real scene by the user.

In one embodiment, the combiner lens and the opto-electronic projection device are configured to carry the augmented image into the field of view of the user as if the augmentation came from infinity.

In another embodiment, the combiner lens and opto-electronic projection device are configured to carry the augmented image into the field of view of the user as if the augmentation were coming from a point at a fixed distance; this embodiment is particularly useful in the case of glasses for visually impaired persons.

Using a holographic lens in combination with a LED light source with a specific narrow pass band, i.e. the above-described narrow pass band BP, in general assures the technical effect of a high efficiency of the opto-electronic system of the present invention, as will be discussed below with a mathematical explanation.

According to the invention, the holographic element 100 is off axis.

In particular, the holographic combiner lens 10 of the invention has the functionality of a reflecting element, but with working angles that can be defined at the design stage and whose functioning is independent of the known process of reflection along an interface in the presence of a change in the refractive index, where the angle of reflection is equal to the incidence angle according to Snell's law, as for the known mirrors.

As is known, such combiner lenses are optical elements characterised by modes that are selective both from a spectral and angular viewpoint, while maintaining at the same time a generally very high transparency in the spectral band complementary to that of reflection.

In other words, the holographic element 100 is configured to:

- receive the second electromagnetic beam FIMM at an incidence angle (a) and

- reflect towards the user the second electromagnetic beam FIMM at an angle of reflection β programmable with respect to the incidence angle a.

In particular, depending on the design required for the specific augmented reality glasses, the angle of reflection β may have a specific value differing from the incidence angle a.

Alternatively, if required by the design, the two angles may have the same value, so that the holographic lens can reproduce the characteristics of a simple mirror.

Preferably, the second electromagnetic beam F _!M M must strike the holographic element 100 according to a specific diffraction efficiency.

In order for the beam output by the combiner to be perceived by the user at the working angles of the holographic element 100, it is necessary to optimise the angular response in efficiency of the element itself.

With reference to figure 6, since the incidence angle a has a value comprised between 50° and 70°, the efficiency of the combiner must be centred in this angular range.

Figure 6: first order diffraction efficiency curve.

In a preferred embodiment of the invention, the angle a is close to 60°, the value on which the peak of the efficiency curve is centred.

The range of values allowed for the hologram acceptance angles a is dependent on the efficiency curve of figure 6, and is defined specifically across the interval for which the efficiency is not negligible: for values of acceptance angles falling outside that range, the efficiency takes on zero diffraction values; consequently, the field is not diffracted there. Figure 6 shows a peak for a value of a close to 60°. The field incident on the lens at angles a less than 50° or greater than 70° will not be diffracted with sufficient intensity to generate spurious diffractions in the eyes of the user.

Given the minimum width of the spectral and angular responses, the holographic element will be effective only in defined angular and spectral intervals. Therefore, there is a minimal likelihood that light coming from the outside will strike at the correct angle and with the "correct" wavelength for reconstructing a ghost image or a spurious reflection falling within the field of vision of the user in such a way that the latter would be able to perceive it.

In other words, there is a minimal likelihood that external light, with a wavelength and angles such as to produce a non-negligible illumination of the eyes by the combiner, will be visible to the user.

In other words, the angular response of the hologram is designed to substantially match the user's field of view, thereby preserving a high quality in the overall perception.

In this manner, the vignetting of the field of view of the image (angular response that is too small) is avoided on the one hand, while the likelihood that ambient light can be efficiently diffracted in the eyes of the user (angular response that is too wide) is minimised on the other.

The width of this angular band is chosen so narrow in order to filter out the spurious reflections coming from peripheral areas of the field of view, thus preventing the contamination of the images perceived with unwanted visual artifacts and contributing to the overall sharpness of the image.

One technical effect assured by the configuration of the holographic lens described is that the projection device can be off axis according to design needs; according to the principles of diffraction, the constraint of symmetry between the incident and reflected angles, typical of refraction, is overcome.

In one embodiment, the opto-electronic device 4 is mounted on at least one side supporting portion 2a,2b so that the projection optical device 6b projects the second electromagnetic beam F _!M M towards the combiner lens 1 0 according to the incidence angle a.

In particular and advantageously, the incidence angle a is configurable on the basis of the position of the opto-electronic device 4 on at least one side supporting portion 2a,2b.

In another embodiment, the opto-electronic device 4 is mounted on the front portion 3, above or below the combiner lens 10.

In one embodiment of the invention, the front portion 3 is configured to support a protection lens 10b placed distally from the eye of the user and having a function of protecting the combiner lens 10.

In other words, in this embodiment the protection lens 10b is coupled to the combiner lens 10.

The invention envisages a particular conformation of the combiner lens 10, as shown in figure 5, which makes it possible to obtain the technical effects already described and others that will be detailed here below.

According to the invention, the combiner lens (10) comprises a first protective layer ST1 proximal to the user, a second protective layer ST2 distal to the user and a holographic element 100 interposed between the first protective layer ST1 and the second protective layer ST2.

According to the invention, the holographic element 100 is off axis.

The holographic element is configured to receive a second electromagnetic beam F _!M M, configured to carry the information of an image ImDGT, at an incidence angle a and to reflect towards the user the second electromagnetic beam FIMM at an angle of reflection (β) programmable with respect to the incidence angle a.

In particular, the angle of reflection β can have a specific value differing from the incidence angle a.

Alternatively, the two angles may have the same value, so that the holographic lens can reproduce the characteristics of a simple mirror.

According to the invention, the second protective layer ST2 comprises a photochromic material PF proximal to the holographic element 100.

In particular, the holographic element 100 is laminated onto the photochromic material

PF.

Preferably, the photochromic material PF is a polymeric material.

The technical effect of the photochromic part of the lens of the glasses is to decrease the light intensity necessary to have an optimal contrast, above all under bright light conditions, when the projected image must be very clear and this entails considerable consumption of the battery. Therefore, with the use of photochromic lenses, the contrast between the projected image and real scene is favoured.

By blocking out ambient light with dark lenses, it is possible to maintain the intensity of the source at lower levels than in the absence of such lenses.

Thus the use of a holographic component associated with an outer photochromic substrate makes it possible to decrease the overall energy consumption of the battery of the system, in particular in relation to the ambient light.

In some cases, on the surface of the second protective layer ST2 it is also possible to provide for the presence of a non-reflective coating AR1 distal with respect to the holographic element 100.

According to the invention, the first protective layer ST1 comprises a transparent material PT proximal to the holographic element 100 and a non-reflective coating AR2 distal with respect to holographic element 100.

The transparent material PT is configured to allow a passage in the direction of the user U of the second electromagnetic beam F _!M M reflected by the holographic element 100 and of the light coming from the real scene frontally to the second protective layer ST2, so that it enters the field of view of the user, combining with the second electromagnetic beam (FIMM).

In other words, the layer ST2 lets an image ImR pass through from the real scene incident on it so that it enters the field of view of the user U, combining with the second electromagnetic beam FIMM-

Preferably, the transparent material PT is a polymeric material.

The combiner lens 10 of the invention further comprises a coupling means MA interposed between the first protective layer ST1 and the holographic element 100, in particular a specifically prepared glue.

Set forth below is a mathematical explanation provided by one of the applicants in order to enable a better understanding of the technical effect of a high efficiency of the system of the invention in terms of the quality of the image entering the user's field of view reflected by the holographic combiner described.

The technical effect is obtained, besides through other components, by means of the synchronised joint action of the narrow-band interference filter, which spectrally filters the image entering the combiner lens, and the combiner lens with a defined angular band and a narrow spectral band synchronised with that of the interference filter; the combination of filter and lens assures a high diffraction efficiency in line with figure 6, and a high transparency of the holographic element in the spectral regions complementary to the operating ones.

In the same explanation, the band values understood to be representative of the narrow band BP considered in the present invention will be identified.

The holographic element of the present invention is a reflection diffraction grating that reconstructs the image coming from a projector according to the known grating equation:

mG /n=sin(a)+sin(P) with a and β entrance and exit angles, G line density per millimetre, λ wavelength, m relative integer that defines the diffraction order, n mean refractive index of the medium.

An optical design of the holographic component strictly derives from the limits imposed by the dimensions of the glass frame. Therefore, the geometric parameters have been optimised so as to reach an aesthetic, dimensional and functional compromise in the optical layout of the glasses.

The optical projection components were chosen with the aim of directing and forming a beam image which, once focused, would have dimensions and a field of view adapted to be contained in the eyes of a user.

The geometric optical design thus uniquely defines the incidence angle a which can be used for the projection of the image on the lens of the glasses.

The nominal value of this working angle is, for example, equal to 60° relative to the normal at the lens; consequently, the exit angle is equal to 6° in order to enable the output beam (diffracted) to coincide with the centre of the field of view.

Once these two angles are defined, the parameters of the grating equation are uniquely determined in the case of a monochromatic beam having a wavelength λ = 532 nm.

Said lens 10, in other words, reflects, within the field of view of the user, the image in a preferably central position relative to the field of view.

The nominal entrance and exit angles having been fixed as above, i.e. a=60° and β=6° respectively, for a wavelength of λ = 532 nm, it follows that the number of lines of the grating is G = 1824.35 l/mm, as per the diffraction grating equation.

The combiner lens 10 consists of a series of layers, as previously described.

In one example, the transparent polymeric layer PT, preferably 1 mm thick, placed on the side of the lens 10 close to the eyes, being transparent in the visible, permits the passage of both the light coming from the projection system, reflected by the HOE element, external, and the light coming from the real scene. Said layer PT is coupled with the holographic element HOE, about 62 urn thick, by means of glue MA.

The holographic element HOE 100 comprises a 12 micron layer of holographic film coupled with its 50 micron polymeric protective substrate.

At the time of recording, the holographic element HOE must be deposited by lamination on a glass substrate to prevent phenomena of shrinkage from compromising the process of transformation. Glass is in fact rigid enough to counter the shifting of the holographic film during recording.

After the recording, the holographic layer HOE can be removed from the glass substrate and thus be laminated, preferably directly, on the final photochromic polymeric layer PF. The assembly of the lens 10, which has a 'sandwich-like' structure, is concluded by adding the glue MA on the free side of the film HOE in order to glue the layer of transparent polymeric material PT, preferably 1 mm thick.

Alternatively, after the removal from the vitreous substrate, the holographic layer HOE can be laminated directly onto the transparent polymeric layer PT and subsequently assembled to the photochromic polymeric layer PF by adding the glue MA on the free side of the film HOE.

After the time necessary for the layer of glue to become uniformly spread, the system is 'cross-linked' by exposure to UV light (curing) and made solid and mouldable into the shape of a glass lens or another shape.

The diffraction grating produced by means of the holographic recording process is an optical element that reflects light in a non-conventional direction compared to a normal mirror, which reflects according to the laws of refraction. By means of the holographic recording process, one obtains a volume holographic element which contains within it sinusoidal modulations of the refractive index. These index modulations occur laterally along the whole 'recorded' sample; if viewed in cross section, they can be schematised with 'lines' of modulation of the index, with an inclination relative to the normal at the surface of the sample which determines the previously mentioned entrance and exit angles a and β. Depending on their angulation, in fact, the hologram will respond with different entrance and exit angles.

The response of a volume hologram is typically described by its efficiency curve, which normally has a peak whose value will be greater the higher the change in the refractive index and a bandwidth that will decrease with decreases in the thickness of the active material (figure 7).

Figure 7: Diffraction efficiencies of a volume diffraction grating in transmission with G = 2000 l/mm for different thickness pairs and index modulation.

By using the RCWA (rigorous coupled wave analysis) model, one can simulate the spectral and angular response of the hologram to be produced, and determine, with the geometric parameters defined by the optical design, the values of the refractive index and film thickness that make it possible to have a suitable response in efficiency of the holographic element.

With a variation in the refractive index of about Δη = 0.025 and a thickness of 12 μιτι, the material, recorded with the number of lines determined for operation at a=60° and β=6°, can have a diffraction efficiency reaching 90%.

The two parameters - variation in the refractive index and thickness - are crucial in determining the holographic element since, for example, an excessively small thickness value would produce an excessively wide spectral response curve, which would result in nearly all the visible light being diffracted (Figure 8), thus blocking out the outside scene as well, contrary to what is desired. A holographic element of such scant thicknesses, moreover, would have to envisage variations in the refractive index that are technologically unfeasible in order to be able to reach efficiencies greater than 80%. Figure 8: Grating in reflection with G = 1836 l/mm, thickness 2 urn and Dn=0.1.

On the other hand, in order to be able to possess a very narrow band, a grating with a large thickness must have a very small variation in the refractive index Δη (Figure 9) so as not to "saturate the efficiency".

Figure 9: Grating in reflection with 1836 l/mm, thickness 28 urn and An=0.012.

In the present invention, a modulation of the refractive index of about 0.025 and a thickness of the photoactive material of about 12 μιη are used, since these values ensure a very narrow pass band (approx. 10 nm FWHM), centred at λ = 532 nm and with a diffraction efficiency greater than 80% (without considering the losses due to the other layers of the lens).

During recording, the response angles of the grating are defined by positioning the beams that create interference according to the diagram in Figure 10.

Figure 10: diagram of the recording setup for producing the holographic lens.

Although theoretically the angles A and B of the system should be the same as the ones used in the reconstruction step (60° - 6°), it was necessary to proceed with an optimisation of A and B in order to ensure that the response curve was centred in efficiency.

In fact, the recording process not only directly induces an absolute variation in the refractive index Δη, but it also modifies the mean value of the index present within the photosensitive material, which in turn influences the spectral response of the holographic element; this value is the refractive index n which appears in the equation of the grating and is fundamental in the geometric definition of the working angles of the holographic element.

The mean value of the index depends both on the type of material used and the value Δη selected during the simulation step.

As a consequence, the recording angles A and B which would have determined an efficiency curve centred at the working angles are shifted after the recording (Figure 1 1 ).

Figure 11: shift in the efficiency response due to the change in the entrance angle (the pair of recording angles is shown for each curve).

In order to optimise the component so that it would respond with the maximum efficiency at the working angles, it was necessary to record the hologram with the following angles: A =3°; B: 66.6°.

These angles were defined experimentally after numerous recording trials in order to maximise efficiency in the working position of the hologram.

The pairs of angles were selected so as to constantly maintain the number G of lines per millimetre of the grating defined at the design stage, so that.

sm(cx) ~ sin( 5) wr — arcsin(G * λ + sin(a )) where a, β, were previously defined and where the recording angles A and B correspond, respectively, to aour and βωη

After the recording of the hologram and the fixing thereof in the holographic material, the transparency of the holographic element in the spectral regions complementary to the operating one exceeds 90% in the visible, as a result of the ad-hoc type of material used, or the coupling of active material with a suitable substrate.

The invention also describes a method of displaying augmented reality images comprising the operating steps implemented by the above-described components.

The method of displaying augmented reality images comprises the steps of

providing a glass frame 2 adapted to be worn by a user, wherein the frame comprises:

- a front portion 3, placed in front of the head of said user, adapted to support at least one lens 10;

- at least one side supporting portion 2a,2b, placed along the side of the head of said user, and connected with said front portion 3 so as to allow a support of said frame 2 on the head of said user;

The method further comprises coupling to the glass frame 2

an opto-electronic device 4 comprising a LED lighting device 6a configured to output a first electromagnetic beam FLED of LED light;

The method further comprises carrying out, in the opto-electronic device 4, the steps of:

- generating the first electromagnetic beam F _L ED of LED light;

- receive encoding information lnf_COD of digital images ImDGT and modulating the first electromagnetic beam FLED coming from the LED lighting device 6a, as a function of said encoding information lnf_COD received, determining a second electromagnetic beam F _!M M configured to carry the information of said image ImDGT;

- projecting said second electromagnetic beam FIMM;

- filtering spectrally said electromagnetic beam FLED FIMM, with a narrow spectral band BP;

The method further comprises providing at least one combiner lens mounted on the front portion 3 comprising a holographic element 100 interposed between a first protective layer ST1 proximal to the eyes of the user and a second protective layer ST2 distal to the eyes of the user.

The method further comprises carrying out, in the combiner lens 10, the steps of:

- receiving the second electromagnetic beam F _!M M incident on the first protective layer ST1 ;

- reflecting, within the field of view of the user U, the second electromagnetic beam FIMM;

- letting an image ImR pass through from the real scene incident on the second protective layer ST2 so that it enters the field of view of the user, combining with the second electromagnetic beam FIMM;

wherein the overlapping of the second electromagnetic beam F _!M M and of the real image ImR determines an augmented reality vision for the user. A system and method for augmented reality applications, as well as a combiner lens comprised in the system, have been disclosed.

The technology introduced by the present invention enables the perception of augmented reality scenes with sharp images, while simultaneously assuring a lightweight design with a high quality overall experience, in terms of both aesthetics and perception during use (user experience).

The glasses of the invention achieve at least the following technical effects:

- absence of chromatic aberrations and distortions;

- minimal, light form factor which makes it possible to provide a pleasing, comfortable design;

- long battery life, as no laser light sources are required;

- simplicity of the layout and thus high reliability of the system;

- no lateral narrowing of the user's field of vision in the area of insertion of the image coming from the display in the lens;

- no obstruction of the user's field of vision;

- the user is not required to turn his or her eyes in directions other than the natural one identified by the ocular axis ("look at" or "look around" configuration) in order to be able to observe the augmented image, thus enabling the user to be able to entirely appreciate the scene before him or her ("see through" configuration);

- possibility of configuring a correct matching between the refractive indexes of the holographic component and of the adjacent substrates, thus assuring the obtainment of good optical characteristics of the image, and the possibility of integrating the holographic component in lenses habitually used in the production of glasses, which are based on the use of polymeric or plastic materials.