Abstract:
A system for projecting a virtual image within an observer&#39;s field of view comprises a support element, a transparent element mounted on the support element and suitable for being placed in front of the observer&#39;s eyes, the transparent element comprising a first face and a second face, an image display device suitable for forming an additional image, and a projection and focusing device for projecting the additional image in a manner such as to present it superimposed on the image of the outside world. The display device comprise light-emitting devices disposed on one of the faces of the transparent element, and the projection and focusing device comprise optical elements associated with respective emitting devices and arranged on the other face. Each of the optical elements creates a virtual image of the emitting device associated therewith. The virtual images of the emitting devices together form the additional image.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a system for projecting a virtual image within an observer&#39;s field of view. 
   There are various known systems for projecting virtual images which are used in display devices wearable by a user and are known in the field as Head Mounted Displays, or HMDs. Some of these, which are known as see-through displays, enable a virtual image to be seen superimposed on the image of the real world. Others, which are known as non-see-through displays (for example, virtual-reality helmets or viewers for palmtops such as the commercial models produced by Sony, Canon and Olympus) enable solely a virtual image to be seen. Yet others, which are known as see-around displays (for example, MicroOptical&#39;s products) have a display screen inserted within the user&#39;s field of view. In general, all of the known systems are constituted basically by three subsystems: a micro-display which generates the real image, an optical system which focuses it at the desired distance (typically at infinity), and an optical system for projection within the user&#39;s field of view. However, the incorporation of these subsystems in a single wearable device, for example, of the spectacles type, involves problems of complexity, size and weight. 
   SUMMARY OF THE INVENTION 
   The object of this invention is to provide a system for projecting virtual images which has performances similar to that of known devices but with relatively small size and weight and with minimal energy consumption. 
   This object is achieved, according to the invention, by a system for projecting a virtual image within an observer&#39;s field of view having the characteristics defined in the claims. 
   In the system according to the invention, the need for a conventional micro-display and the respective image projection system is eliminated since the three subsystems for generating images, focusing, and projection are incorporated substantially on the surfaces of a transparent substrate such as, for example, the visor of a helmet, an ophthalmic lens, a foil in contact with a lens, etc., considerably reducing overall complexity, weight and size. 
   A further subject of the invention is a display device (or HMD) wearable by a user, comprising a projection system according to the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred but non-limiting embodiment of the invention will now be described with reference to the appended drawings, in which: 
       FIG. 1  is a schematic representation of a basic structure of the projection system according to the present invention; 
       FIGS. 2   a  and  2   b  show variants of the projection system according to the invention; 
       FIG. 3  is a schematic representation of the operating principle of the projection system according to the invention; 
       FIGS. 4   a  and  4   b  are further variants of the projection system according to the invention; 
       FIG. 5  shows a further variant of the projection system according to the invention in a schematic side elevational view; 
       FIG. 6  is a front view of the variant of  FIG. 5 , and 
       FIG. 7  is a view of a pair of spectacles provided with the projection system according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , a basic structure  1  of a projection system according to the invention comprises a source  10 , that is, a light-emitting device, for example, of an organic type such as an OLED or a PLED, or of an inorganic type such as a microLED, a quantum dot laser, a nanostructured metallic source, a carbon nanotube source, a metal or polymer nanowire source, or a field-emission source, having a transverse linear dimension d, and a micro-optical element  20 , for example, a lens, a mirror, or a diffractive optical element (DOE) having a diameter Φ, for example, of about 20-50 microns. An optical axis of the optical system formed by the emitting device  10  and by the micro-optical element  20  is indicated z in  FIG. 1 . The light source is disposed on the inner side, near the eye, or on the outer side, according to the type of optical configuration for projecting the luminous pixel onto the retina, which will be of the transmission type ( FIG. 2   b ) or of the reflection type ( FIG. 2   a ). 
   The emitting device  10  and the micro-optical element  20  are arranged on respective opposed surfaces  30   a ,  30   b  of a substrate  30  that is transparent to light. The transparent substrate has a thickness f, for example, of the order of 2 mm. 
   The transparent substrate  1  is, for example, the visor of a helmet, a spectacle lens or, in a more general case, a foil in contact with a lens. 
   The basic structure  1  is a subunit of an orderly set  40 , for example, a matrix, shown in  FIGS. 2 , and  4 - 7 , formed by a plurality of such basic structures  1  in which, in the case of a projection system of the transmission type, the emitting devices  10  are arranged in an orderly manner on the face  30   a  of the transparent substrate and the corresponding micro-optical elements  20  are arranged in a correspondingly orderly manner on the other face  30   b.    
   The substrate  30  is suitable to be placed in front of at least one eye (indicated E in  FIG. 3 ) of an observer, at a distance of substantially between 25 and 50 mm from the eye with one of the faces  30   a ,  30   b  facing towards the eye and the other of the faces  30   a ,  30   b  facing towards the portion of the outside world which extends in front of the observer with respect to his direction of observation. 
   According to a possible variant, the transparent substrate  30  is a solid element made of transparent material. 
   According to another variant, the transparent substrate  30  is a hollow element formed by a pair of films of transparent material between which a cavity of the substrate  30  is interposed. In this case, the faces  30   a  and  30   b  are a face of one film and a face of the other film of the pair of films or foils, respectively. 
   Alternatively, as a combination of the two preceding variants, the transparent substrate  30  is a solid element made of transparent material to the faces of which are applied films of transparent material, on which the emitting devices  10  and the micro-optical devices  20  are disposed, respectively. 
   The micro-optical devices  20  are preferably formed directly on the face  30   b  of the substrate  30 , for example, by known micro-machining and mass-production replication techniques. 
   The emitting devices  10  are preferably positioned on the face  30   a  of the substrate  30  by localized, in situ production techniques or alternatively by bonding techniques, in any case on a surface that is prepared electrically for the energy supply of the source. 
   According to the invention and as shown in  FIGS. 2 ,  4  and  5 , the light sources  10  of the matrix  40  that are disposed on the face  30   a  of the substrate  30  together emit a beam of light rays LB towards the micro-optical elements  20 . Each of the micro-optical elements  20  that are disposed on the optical path of the rays emitted creates a virtual image of the corresponding source  10  (see  FIG. 3 ), placing the virtual image at an infinite distance and directing the rays towards the pupil P of the observer&#39;s eye E. This is achieved in practice when the source  10  is in the vicinity of the focus of the corresponding micro-optical element  20  and leads to the collimation of the beam emitted towards the eye E. The image created by each individual source  10  is disposed beside those created by the surrounding sources  10 , creating the virtual image VI as a whole, disposed within the observer&#39;s field of view FOV. 
   For simplicity and clarity, the substrate  30  is shown in  FIGS. 2   a  and  2   b  as having a flat shape with the optical axes z of the subunits  1  of the matrix  40  arranged perpendicularly relative to the plane in which the substrate  30  extends. In the embodiment of  FIG. 2   a , the micro-optical elements  20  are formed as mirrors so that the face  30   a  of the substrate  30  on which the emitting devices  10  are disposed faces towards the observer&#39;s eye. In the embodiment of  FIG. 2   b , the micro-optical elements  20  are formed as lenses or optical elements which operate by refraction or by diffraction so that the face  30   a  of the substrate  30  on which the emitting devices  10  are disposed faces the portion of the outside world which extends in front of the observer, with respect to his direction of observation. 
   To prevent the light emitted by a source  10  falling not only on the corresponding micro-optical element but also on the adjacent micro-optical elements, thus creating parasitic images, micro-sources  10  with highly collimated emission, for example, with an overall divergence δ of less than 10° may be used. Alternatively, the projection system according to the invention may be constructed with a “fly&#39;s eye” structure. According to this variant, each subunit  1  of the matrix  40  is formed as a tube portion of transparent material, but with opaque side walls, which extends along the optical axis z and in which the source  10  is fixed at one end and the micro-optical element  20  is formed at the other end. More generally, slits, apertures or stops may be disposed between the emitter and the collimation optical element to limit the propagation of parasitic rays. 
     FIGS. 4   a ,  4   b  and  5  show three possible variants of the invention for projecting the virtual image of each individual source  10  in its correct location within the observer&#39;s field of view FOV. 
   In  FIG. 4   a , the optical axes z of the individual subunits  1  of the matrix  40  are oriented towards the observer&#39;s eye. To achieve this orientation in the flat substrate  30 , the individual sources  10  are no longer aligned with the corresponding micro-optical elements  20  with respect to an axis perpendicular to the plane of the substrate  30  as shown in  FIGS. 2   a  and  2   b , but each source  10  is displaced laterally relative to the alignment axis perpendicular to the substrate  30 , with respect to the corresponding micro-optical element  20 . This displacement becomes greater towards the periphery of the observer&#39;s field of view FOV. Moreover, the micro-optical elements  20  are inclined to the axes perpendicular to the plane of the substrate  30  and this inclination varies as their position varies from the centre to the periphery of the observer&#39;s field of view FOV. 
   The optical axes z of the individual subunits  1  of the matrix  40  are also oriented towards the observer&#39;s eye in  FIG. 4   b . In contrast with the variant of  FIG. 4   a , the orientation of the axes  5  is achieved by curvature of the substrate  30  on which the subunits  30  of the projection system according to the invention are disposed. 
   In the variant of  FIG. 5 , the substrate  30  is again flat and the optical axes  5  of the subunits  1  of the matrix  40  are parallel to one another and perpendicular to the plane of the substrate  30 . The individual micro-optical elements  20  comprise a further, refractive or diffractive, prismatic component, so that the rays which emerge from the substrate  30  and are directed towards the observer are deflected by the prismatic components of the micro-optical elements  20  in the correct direction within the observer&#39;s field of view.  FIG. 5  shows schematically the prismatic component to be added to the profile of the respective micro-optical element  20  so as to create the required field of view: a flat profile for a source  10  in a central position (requiring no deflection) and a profile which varies gradually towards the edge in order to have optimized deflection. 
   With reference to the matrix of micro-optical elements  20 , the phase function of an individual micro-optical element can be expressed (in a generalization which encompasses both the refractive and the diffractive case) as:
 
Ψ i,j =Ψ lens +Ψ prism (i,j)  
 
where Ψ lens  represents the phase function of the focusing optics (whether it is refractive, reflective or diffractive), and Ψ prism (i,j)  represents, for each i,j-th element of the matrix, the additional prismatic component that is suitable for correcting the optical path of the collimated beam.  FIG. 6  shows, in a front view, a possible appearance of the optical elements arranged in accordance with this principle. Each square represents a single micro-optical element  20  of the matrix. The concentric circles drawn within each square give an idea of the deflection imparted to the light rays by the prismatic component of the individual micro-optical element  20 . In the central micro-optical element, the prismatic component has a flat profile: the central square of the matrix is concentric with the circles within it. In the peripheral micro-optical elements, the prismatic component has a more inclined profile: towards the periphery of the matrix, the centres of the circles are displaced, within the respective squares, towards one of the sides of the squares, according to the coordinates of those squares within the matrix. A wholly similar arrangement would be adopted for a reflection optical system as shown in  FIG. 2   a . In that case, the phase function of the reflector varies gradually from the centre of the FOV to its periphery.
 
     FIG. 7  shows a projection system according to the invention applied to a pair of spectacles. 
   In this case, the substrate  30  is, for example, one of the lenses of the spectacles, mounted in a spectacle frame  50 .  FIG. 7  also shows, on an enlarged scale, a portion of the substrate  30  enclosed in a rectangle B 1  drawn in continuous outline. The region of the substrate  30  occupied by the matrix  40  is represented as enclosed in a rectangle B 2 , drawn in broken outline. 
   The individual emitting pixels  10  are supplied electrically by a conventional control unit (not shown), independently of one another, for example, by means of a pattern of electrical connections  60  formed by photolithography directly on the outer surface of the lens  30  or on the film which contains the sources  10  and which is caused to adhere to the outer surface of the lens. These connections  60  are invisible to the eye since they are transparent and/or are too close to be brought into focus. 
   The size of an emitting pixel is typically sub-micrometric and the supply tracks are substantially transparent. The entire assembly of an emitter and of the electrical tracks is therefore substantially invisible to the naked eye. The overall portion of area B 2  which is occupied by the matrix  40  of micro-optical elements/micro-sources depends on the number of pixels and therefore on the resolution of the display or on the selected density of the micro-optical elements/micro-sources on the substrate  30 . Both of the standard configurations with 320×240 pixels or 640×480 pixels occupy an overall opaque area of a few square mm and are typically arranged on an overall lens area of about 2×2 cm 2 . The matrix of emitters can be placed in a peripheral position within the observer&#39;s field of view FOV or in its centre (with a configuration similar to that shown in  FIG. 6 ), whereas a more sparse arrangement may occupy the whole spectacle lens. In general, the size of the display will differ according to whether spectacles or a visor are involved. 
   The optical system of the projection matrix as a whole, whether it operates by transmission or by reflection, covers a minimal portion, typically less than one percent of the entire lens or substrate and does not therefore affect the see-through system as a whole. 
   Whereas, in a conventional projection system, the image-generating subsystem, the system for focusing the images into a virtual image, and the system for projecting the virtual image within the user&#39;s field of view constitute distinct and separate units, in the system according to the invention, the three subsystems are incorporated in a single unit which is formed on a transparent substrate (for example, a spectacle lens or a helmet visor) and which simultaneously generates, focuses and projects the virtual image. 
   Moreover, whereas, in conventional systems, the field of view is determined naturally and depends solely on the overall size of the micro-display and on the focal length of the optical system, in the system according to the invention, the field of view is created “as a mosaic” by projecting the virtual image of each individual source in the correct location within the user&#39;s field of view. 
   Naturally, the principle of the invention remaining the same, the details of construction and forms of embodiment may vary widely with respect to those described and illustrated, without thereby departing from the scope of the invention.