Patent Description:
An apparatus of this kind, as disclosed in <CIT> of the same applicant, is commonly used in AR glasses, AR helmets or head-up displays for a broad range of applications like navigation, training, entertainment, education or work. The MEMS mirror deflects the light beam into subsequent directions (angles), one direction (angle) per pixel of the image. For example, the MEMS mirror oscillates fast about a vertical axis and slowly about a horizontal axis to sweep the directions and, thus, scan the pixels row by row and line by line. The deflected light beam passes a waveguide which enlarges the beam in cross-section - for a so-called "exit pupil" or "eye box" expansion - onto the display area viewed by the user.

In VR applications, the display area is typically a (miniature) reflective screen in front of the user's eye. In AR applications the display area is a semi-transparent combiner which redirects the light beam/s towards the user's eye while concurrently superposing them with light field from a surrounding. In either case the cornea and the lens of the user's eye focus each light beam from a specific direction onto one image point on the retina, so that all image points on the retina then form the image perceived by the user. In head mounted displays the light projector may even project the scanned light beam/s directly into the user's eye, without any reflective screen or semi-transparent combiner. In video beamer applications the light projector module can be used to project an image onto an external screen like a reflective wall or movie screen.

<CIT> describes a bulky display system which has, for coupling the light beam into the waveguide, multiple MEMS mirrors arranged in series and a reflective surface therebetween.

For light projector modules used in VR or AR glasses, helmets and other head-mounted displays it is desirable to build the module as small as possible to increase user comfort and wearability. Also in portable video beamers miniaturisation is key.

It is thus an object of the present invention to provide a light projector module which is as compact as possible for optimal wearability and portability.

This object is achieved with a light projector module as defined at the outset which has a static mirror in the beam path from the light source to the MEMS mirror to fold the beam path about an angle of folding, which static mirror lies beside said first side of the waveguide and wherein the support is a housing enclosing at least the light source, the MEMS mirror, the waveguide, and the static mirror, wherein the housing has a window exposing the out-coupling area of the waveguide, the housing is attached to or integrated into a temple of a spectacle frame, and the beam path from the light source to the static mirror runs substantially parallel to the longitudinal axis of the temple.

The inventive arrangement of the light source, static mirror, MEMS mirror and waveguide leads to a tight integration and compact design while guiding the light beam in an optimised path through the module. A particularly slim design is achieved as the beam path from the light source to the static mirror runs substantially parallel to the longitudinal axis of the temple. In particular, the angle of incidence of the light beam onto the MEMS mirror - as measured with respect to a normal onto the mirror plane - can be minimised to minimise geometrical distortions, without increasing installation space. The light projector module of the invention is therefore particularly suited for integration into VR and AR devices and miniaturised beamers.

For a particularly compact design, the static mirror preferably overlaps the out-coupling area when seen in a direction substantially perpendicular to the first and second sides of the waveguide.

In a preferred embodiment, the angle of folding is in the range of <NUM>° to <NUM>°, preferably about <NUM>°. When the MEMS mirror is configured to oscillate about a resting position the folded beam path may preferably impinge on the MEMS mirror in the resting position under an angle of incidence of <NUM>° to <NUM>°, particularly preferably about <NUM>°. Both measures, individually and in particular when combined, yield a tightly integrated design while meeting the operating constraints of the MEMS mirror.

In a further preferred embodiment of the invention the waveguide comprises a transparent cover arranged at a distance from the first side of the waveguide and the static mirror is mounted on the transparent cover. In this way, the transparent cover, e.g. a thin glass plate used to protect the waveguide while maintaining the air gap necessary for total internal reflection within the waveguide to work, is co-used as a support for the static mirror. The transparent cover is at the optimum position for folding the beam path, contributing to the miniaturisation of the design.

The static mirror can be mounted on either side of the transparent cover as long as its reflective side faces away from the waveguide. It is, however, particularly preferred when the mirror is mounted on that side of the transparent cover that faces away from the first side of the waveguide so that it is directly exposed to the beam path, i.e., the light beam does not need to pass through the transparent cover to reach the static mirror.

The transparent cover could be mounted directly on the support. Preferably, the transparent cover and the waveguide are attached to one another via a circumferential rim or set of studs. This ensures the necessary air gap between the transparent cover and the waveguide on the one hand, and on the other hand only one structure, that is the transparent cover carrying the static mirror and the attached waveguide, needs to be aligned with respect to the support and all other components carried by the support.

In all embodiments the light source may comprises three LEDs or laser diodes for emitting red, green, and blue beams of light, respectively, and a beam combiner for combining the beams of light to said light beam, to project a colour image.

The invention will now be described by means of exemplary embodiments thereof with reference to the enclosed drawings, in which show:.

<FIG> shows a pair of augmented reality (AR) glasses <NUM> comprising a spectacle frame <NUM>, a pair of eye glasses <NUM> and a pair of temples <NUM>. Attached to each temple <NUM> is a light projector module <NUM> which projects an image <NUM> onto a semi-transparent combiner <NUM>. The semi-transparent combiner <NUM> is supported by the spectacle frame <NUM> or an eyeglass <NUM> or integrated into the latter. The semi-transparent combiner <NUM>, e.g., a waveguide or a holographic combiner, superposes the image <NUM> projected by the light projector module <NUM> with the light field from a surrounding <NUM> so that the wearer of the AR glasses <NUM> can see the image <NUM> overlaying ("augmenting") the surrounding <NUM>.

The image <NUM> can, e.g., be monochromatic or coloured, a single image or part of a video sequence of images. The image/s <NUM> can augment any surrounding <NUM> such as a landscape, an urban environment, a road, a classroom, a workplace etc. so that the user can perceive additional information, e.g., for navigation, work, education, training or entertainment as an overlay ("AR image") of the light field ("view") of the surrounding <NUM>.

In the example of <FIG>, the light projector module <NUM> (here: two modules <NUM>, one per temple <NUM>) is built into AR glasses and used in combination with a semi-transparent combiner <NUM>. A similar application of the light projector module <NUM> could be in an AR helmet worn by a user, a handheld AR device like a smartphone with a camera, or an AR head-up display for a vehicle which all use a semi-transparent combiner <NUM> as the display area of the light projector module <NUM>. If desired, suitable relay optics can be interposed between the light projector module <NUM> and the semi-transparent combiner <NUM>.

Instead of the semi-transparent combiner <NUM> the light projector module <NUM> could be used with any other display area, e.g., a conventional reflective projection screen such as a miniature screen mounted on the frame <NUM> of virtual reality (VR) glasses, or a projection wall or a movie screen, for example when the light projector module <NUM> is used as a miniature (or full-scale) video beamer. The light projector module <NUM> could even be used to project the image <NUM> directly into the user's eye, optionally with suitable optics therebetween.

The light projector module <NUM> can be built into a separate housing <NUM>, illustrated in <FIG> and <FIG> by solid lines and the horizontal broken line, or be directly integrated into the spectacle frame <NUM> or one of its temples <NUM>, i.e., use the spectacle frame <NUM> or a temple <NUM> as its housing <NUM>.

As shown in <FIG> and <FIG>, the housing <NUM> forms a support for the primary components of the light projector module <NUM>, that are: a light source <NUM>, a static mirror <NUM>, a micro-electro-mechanical-system (MEMS) mirror <NUM>, and a waveguide <NUM>.

The light source <NUM> emits a collimated light beam <NUM> which carries the image <NUM> in a time-multiplexed manner, i.e. the intensity values of the image pixels one after the other, e.g., row-by-row and line-by-line per image <NUM> comprised of a grid of pixels, and image-by-image per video comprised of a sequence of images <NUM>.

For this purpose the light source <NUM> can be of any type known in the art configured to emit a collimated light beam <NUM>. In most embodiments, the light source <NUM> is a semiconductor light source such as a light emitting diode (LED), microLED (µLED), or laser diode, e.g., edge-emitting laser diode or surface-emitting laser diode. For colour images <NUM>, the light source <NUM> may be a polychromatic light source <NUM>, e.g., comprise three LEDs or laser diodes <NUM><NUM>, <NUM><NUM>, <NUM><NUM> of the three primary colours red, green and blue which emit a red, green and blue beam of light <NUM><NUM>, <NUM><NUM>, <NUM><NUM>, respectively, that are fed - e.g. via lenses <NUM>, <NUM> - to a beam combiner <NUM> that combines the beams <NUM><NUM>, <NUM><NUM>, <NUM><NUM> to the light beam <NUM>. The lenses <NUM>, <NUM> may be common to all beams <NUM><NUM>, <NUM><NUM>, <NUM><NUM> or comprise one or more individual lense/s (not shown) for each beam <NUM><NUM>, <NUM><NUM>, <NUM><NUM>.

The beam path P of the light beam <NUM> downstream of the light source <NUM>, drawn in dotted lines in <FIG> and <FIG>, is folded (diverted) by an angle of folding δ, here: away from the user towards the outside of the temple <NUM>. The angle of folding δ is in the range of <NUM>° to <NUM>° and in the present example about <NUM>°.

Downstream of the static mirror <NUM> the folded beam path P with the light beam <NUM> impinges on the MEMS mirror <NUM>. The MEMS mirror <NUM> deflects the light beam <NUM> as a (collimated) "deflected" light beam <NUM> into subsequent directions (angles), one direction per pixel of the image <NUM>, towards the waveguide <NUM>. The MEMS mirror <NUM> can, e.g., oscillate fast about a vertical axis and slowly about a horizontal axis to sweep the directions and, thus, reproduce the pixels of an image <NUM> row-by-row and line-by-line, and image-by-image for a sequence of images <NUM>. Alternatively, the MEMS mirror <NUM> can sweep ("scan") the directions by any other movement, e.g., by means of Lissajous curves, to reproduce the image/s <NUM>.

The MEMS mirror <NUM> has a resting position (shown in <FIG>) about which it oscillates, and the folded beam path P of the light beam <NUM> impinges on the MEMS mirror <NUM> in that resting position under an angle of incidence φ of <NUM>° to <NUM>°, in the present example about <NUM>°. The angle of incidence φ is measured with respect to a normal N on the mirror plane of the MEMS mirror <NUM>.

The waveguide <NUM> receives the deflected light beam <NUM> from the MEMS mirror <NUM> and guides it towards the semi-transparent combiner <NUM>. On its way through the waveguide <NUM> the deflected light beam <NUM> is expanded (enlarged) in its cross section so that it exits the waveguide <NUM> as an "expanded" light beam <NUM> with a cross section A<NUM> which is larger than the cross section A<NUM> of the deflected light beam <NUM>. For example, the cross sections A<NUM>, A<NUM> of the deflected and expanded light beams <NUM>, <NUM> can be <NUM> - <NUM><NUM> and <NUM> - <NUM><NUM>, respectively.

Downstream of the waveguide <NUM>, the semitransparent combiner <NUM> redirects the expanded light beam <NUM> as an "image" light beam <NUM> towards the user's eye <NUM> for superposing the image <NUM> with the light field of the surrounding <NUM>. To this end, the semitransparent combiner <NUM> not only re-directs the expanded light beam <NUM> impinging on its one side <NUM> facing the user's eye <NUM> but also lets pass the light field of the surrounding <NUM> impinging on its opposite side <NUM>, i.e., its far side with respect to the eye <NUM>, so that the user perceives both the AR image <NUM> as well as the surrounding <NUM>.

<FIG> shows the arrangement of the components light source <NUM>, static mirror <NUM>, MEMS mirror <NUM> and waveguide <NUM> on the support, here: the housing <NUM>. As discussed before, the housing <NUM> can be the temple <NUM> itself or any other structure, even a lattice- or scaffold-like structure.

To achieve a slim and compact design of the module <NUM> while keeping the angle of incidence φ on the MEMS mirror <NUM> as low as possible, the space underneath the rear side of the waveguide <NUM> is used as a mounting position for the static mirror <NUM>. This allows the beam path P from the light source <NUM> to the static mirror <NUM> to run substantially parallel to the longitudinal axis A of a temple <NUM> when the module <NUM> is used in AR or VR glasses, yielding a slim design. And - with the above-mentioned ranges of angle of folding δ and angle of incidence φ - the expanded light beam <NUM> will exit towards the semi-transparent combiner <NUM> in a slightly oblique direction from the temple <NUM> to the eye glass <NUM>, as shown in <FIG>.

The rear side of the waveguide <NUM> where the static mirror <NUM> lies is that side <NUM> of the waveguide <NUM> that has an in-coupling area <NUM> for receiving the deflected light beam <NUM> from the MEMS mirror <NUM>. The out-coupling area <NUM> of the waveguide <NUM> is larger than the in-coupling area <NUM>, roughly by the enlargement ratio A<NUM> : A<NUM>, and lies on the opposite side <NUM> of the waveguide <NUM>. Due to the internal waveguiding structure of the waveguide <NUM> the in-coupling are <NUM> and the out-coupling area <NUM> do not overlap when seen in a direction R substantially perpendicular to the sides <NUM>, <NUM>. Therefore, there is installation space available beside the side <NUM> of the waveguide <NUM> in a region underneath the out-coupling area <NUM> for mounting the static mirror <NUM>.

The static mirror <NUM> therefore lies beside the side <NUM> in a region where it will overlap the out-coupling area <NUM> when seen in the direction R, for optimal compactness of the design. However, in other embodiments, the static mirror <NUM> may not overlap the out-coupling area <NUM> when seen in the direction R.

As shown in <FIG>, the static mirror <NUM> can be mounted substantially parallel to the side <NUM> of the waveguide <NUM>, although this is not obligatory. An air gap <NUM> should remain at the side <NUM> of the waveguide <NUM> so that its internal waveguiding function can work properly with total internal reflection at that side <NUM>. The air gap <NUM> may be established, e.g., by mounting the static mirror <NUM> at least at a distance D from the side <NUM>. The distance D may be in the range of several hundred microns or millimetres.

The in-coupling section <NUM> may be formed by a diffraction grating <NUM>' which diffracts the deflected light beam <NUM> mainly into one diffraction order, e.g., into the first diffraction order to couple the deflected light beam <NUM> under an angle above the critical angle of total internal reflection into the waveguide <NUM>. Instead of the diffraction grating <NUM>' any other optical element for coupling the deflected light beam <NUM> under said angle into the waveguide <NUM> section <NUM> may be employed such as a prism, a fibre, etc..

The waveguide internally guides the in-coupled deflected light beam <NUM> via a series of successive total internal reflections between its sides <NUM>, <NUM> towards the out-coupling section <NUM>, which may be in form of an out-coupling diffraction grating <NUM>'. Instead of the diffraction grating <NUM>' any other optical element for coupling the enlarged light beam <NUM> out from the waveguide <NUM> may be employed as out-coupling section <NUM> such as one or several semitransparent mirrors, a multitude of successive micro-mirrors, one or more prisms, fibres, etc..

Any or both of the diffraction gratings <NUM>', <NUM>' can be applied into or onto the waveguide <NUM> by e.g., etching, pressing or moulding surface structures like steps, grooves, ridges etc., or embedded into the waveguide <NUM>, e.g., in the form of structured voids or reflective films. Moreover, any or both of the diffraction gratings <NUM>', <NUM>' can be a reflection or a transmission grating. In optional embodiments, the waveguide <NUM> has a mirror <NUM>" behind the in-coupling diffraction grating <NUM>', i.e., at its far side with respect to the input light beam <NUM>, and/or reflectorised sides <NUM>, <NUM> between the in- and out-coupling areas <NUM>, <NUM>, and/or a mirror <NUM>" behind the out-coupling diffraction grating <NUM>', i.e., at its far side with respect to the output light beam <NUM>. In all these embodiments the mirrors <NUM>", <NUM>" could also be distanced from the corresponding side <NUM>, <NUM>, e.g., by an air gap of a few microns or more.

In some embodiments, the waveguide <NUM> will be provided with a transparent cover <NUM>, e.g. a thin glass plate, which is mounted at said distance D from the side <NUM> to protect the waveguide <NUM> while establishing the air gap <NUM>. The transparent cover <NUM> and the waveguide <NUM> can be attached to one another via a circumferential rim (not shown) or a set of spacers or studs <NUM>.

The transparent cover <NUM> can be mounted in the housing <NUM> and carry the waveguide <NUM>. The static mirror <NUM> could be mounted in the shown position directly on the support or housing <NUM>. In the shown embodiment the transparent cover <NUM> is co-used to mount the static mirror <NUM>, e.g., by covering one of the sides of the transparent cover <NUM> with a reflective coating as static mirror <NUM>. In particular, the side <NUM> of the transparent cover <NUM> that faces away from the side <NUM> of the waveguide <NUM> can be used to support the static mirror <NUM>, for example by coating that side <NUM> with a reflective coating.

The static mirror <NUM> should not cover the in-coupling area <NUM> of the waveguide <NUM>. In the region of the in-coupling area <NUM> the transparent cover <NUM> can be provided with an anti-reflective coating <NUM> to further the entering of the deflected light beam <NUM> into the waveguide <NUM>.

The housing <NUM> is configured to support and enclose the light source <NUM>, the static mirror <NUM>, the MEMS mirror <NUM>, the waveguide <NUM>, and the optional transparent cover <NUM>. The housing <NUM> may have a window <NUM> exposing the out-coupling area <NUM> of the waveguide <NUM> so that the enlarged light beam <NUM> can exit the housing <NUM>. The window <NUM> may be provided with a further transparent cover such as a glass plate.

Claim 1:
A light projector module for projecting an image, comprising:
a support (<NUM>);
a light source (<NUM>) supported by the support (<NUM>) and configured to emit in a beam path (P) a light beam (<NUM>) carrying said image (<NUM>);
a micro-electro-mechanical-system, MEMS, mirror (<NUM>) supported by the support (<NUM>) and configured to deflect the emitted light beam (<NUM>) received over the beam path (P) as a deflected light beam (<NUM>);
a waveguide (<NUM>) supported by the support (<NUM>) and having two parallel sides (<NUM>, <NUM>), the first side (<NUM>) having an in-coupling area (<NUM>) for receiving the deflected light beam (<NUM>) and the second side (<NUM>) having an out-coupling area (<NUM>) for projecting the deflected light beam (<NUM>) with enlarged cross section; and
a static mirror (<NUM>) in the beam path (P) from the light source (<NUM>) to the MEMS mirror (<NUM>) to fold the beam path (P) about an angle of folding (δ);
wherein the static mirror (<NUM>) lies beside the first side (<NUM>) of the waveguide (<NUM>),
characterised in that
the support (<NUM>) is a housing enclosing at least the light source (<NUM>), the MEMS mirror (<NUM>), the waveguide (<NUM>), and the static mirror (<NUM>), wherein the housing (<NUM>) has a window (<NUM>) exposing the out-coupling area (<NUM>) of the waveguide (<NUM>),
the housing (<NUM>) is attached to or integrated into a temple (<NUM>) of a spectacle frame (<NUM>), and
the beam path from the light source (<NUM>) to the static mirror (<NUM>) runs substantially parallel to the longitudinal axis (A) of the temple (<NUM>).