Patent Description:
Head-Mounted Displays (HMDs), which include near eye displays in a form resembling conventional eyeglasses or sunglasses, are being developed for a range of diverse uses, including military, commercial, industrial, fire-fighting, and entertainment applications. For many of these applications, there is particular value in forming a virtual image that can be visually superimposed over the real-world image that lies in the field of view of the HMD user.

In general, HMD optics must meet a number of basic requirements for viewer acceptance, including the following:.

Aspects (i) - (iii) relate to the eyebox. The eyebox relates to the volume within which the eye of the observer can comfortably view the image. The size of the eyebox depends in part on the length of the path of the light from the image source to where the image is viewed and image source size, and in part on the divergence of the image source and/or the collimation of the light after its emission by the image source. The desirable size of the eye box depends largely on the quality of viewing experience that is desired from the display.

In addition to optical requirements, HMD designs must also address practical factors such as variable facial geometry, acceptable form factor with expectations of reduced size for wearing comfort, weight, and cost, and ease of use.

A goal for most HMD systems is to make the imaging/relay system as compact as possible; however, when using conventional optics, there are basic limits. The output of the optic system must have a pupil that is large enough to support a reasonably sized virtual image and also allow for some movement of the eye. In a binocular system there is also the issue of varying intraocular distance (IOD) among different users and the need for the output pupil of the optical system to allow for this. Especially for the case of wide FOV of <NUM> degrees of greater, eye movement, user variations of IOD, and human pupil size may require horizontal output pupil size of <NUM> or greater. Although this may be achieved in very large immersive displays having long paths from the image source to where the image is viewed (e.g. <CIT>), compact HMD's having short optical paths impose significant challenges on the divergence of the collimated virtual image. As a result, wide FOV compact HMD's often require the use of fast optics which can be bulky, complex, costly, and exhibit spherical and other aberrations.

Wide FOV imaging systems having "monocentric" designs using ball lenses have been described in the prior art for very large systems (as in the previously mentioned <CIT>). Due to the long projection distances, large systems such as these can easily produce large output pupil sizes in spite of relatively narrow beam divergence. The image projected by the symmetric ball lens optics is in the path between the observer and collimating mirror, preventing the use of additional beam expanding elements. More compact HMD optics that use ball lenses and monocentric optical designs have also been proposed (see, for example, <CIT>). In those cases, however, ball lenses forming the image located at the focal plane of by a spherical mirror requires a large beam divergence to achieve the large output pupils given the short eye distances. Unfortunately, ball lenses or other monocentric optics used at high diverging angles exhibit significant spherical aberration which degrades the virtual image and ultimately compromises the required image resolution for high definition content.

Another HMD is described in <CIT>. The HMD includes a spatial light modulator which generates the image to be viewed. A projection lens projects the image to a rear projection screen, which diffuses the image to accommodate the spacing and size of the viewer's eyes. The image is reflected by a beam splitter to a curved reflector in front of the viewer's eyes. The reflector reflects the image through the beam splitter in parallel rays so that the image appears to be distant, and also magnifies the image to provide a wide field of view. <CIT> discloses a HMD comprising an image source with a projection lens, wherein the lens projects an intermediate image on a diffuser, forming an intermediate image on the diffuser matt surface, further comprising a first planar semi-transparent mirror and a semi-transparent collimating mirror.

Compact HMD's using concave mirrors and a "semitransmissive" elements for projecting an images have been described (see, for example, <CIT>, and <CIT>). In these cases, glass prismatic elements with planar or flat surfaces are able to project an image produced by a display (e.g. OLED, LCOS) coupled into one of the facets of the semitransmissive element. In order to achieve wide FOV's greater than <NUM> degrees, the image display must be of significant size, e.g. <NUM> or more. Since cost of semiconductor display technology increases dramatically with size, such displays can be very costly. This negatively impacts HMD price. Furthermore, the prism-like geometries of the semitransmissive elements complicate their use in augmented reality configurations, since their curved or angled surfaces refract light from the direct "see-through" or ambient environment. Therefore, in order for these systems to be used in augmented reality modes, either corrective optics or digital imagers must be used, increasing cost, size, and weight.

For these reasons, conventional HMD designs fail to provide economical solutions to simultaneously achieving high FOV, very large output pupils, means to simply achieve augmented reality and digital high definition content, with compact geometries for wearability.

It is an object of the present disclosure to advance the art of virtual image presentation using compact head-mounted devices. Advantageously, embodiments of the present disclosure provide an enlarged pupil size presenting high resolution wide FOV content to viewers having wide range of IOD's with minimal or no optical adjustment required. However, the invention is set out in the appended claims.

These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.

According to an aspect of the invention, there is provided a head-mounted imaging apparatus that comprises:.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings.

The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.

Where they are used herein, the terms "first", "second", and so on, do not necessarily denote any ordinal, sequential, or priority relation, but are simply used to more clearly distinguish one element or set of elements from another, unless specified otherwise. The terms "top" and "bottom" do not necessarily designate spatial position but provide relative information about a structure, such as to distinguish opposing surfaces of a planar (flat) waveguide.

In the context of the present disclosure, the terms "viewer", "operator", "observer", and "user" are considered to be equivalent and refer to the person who wears the HMD viewing device.

As used herein, the term "energizable" relates to a device or set of components that perform an indicated function upon receiving power and, optionally, upon receiving an enabling signal.

The term "actuable" has its conventional meaning, relating to a device or component that is capable of effecting an action in response to a stimulus, such as in response to an electrical signal, for example.

The term "set", as used herein, refers to a non-empty set, as the concept of a collection of elements or members of a set is widely understood in elementary mathematics. The term "subset", unless otherwise explicitly stated, is used herein to refer to a non-empty proper subset, that is, to a subset of the larger set, having one or more members. For a set S, a subset may comprise the complete set S. A "proper subset" of set S, however, is strictly contained in set S and excludes at least one member of set S.

In the context of the present disclosure, the term "oblique" means at an angle that is not an integer multiple of <NUM> degrees. Two lines, linear structures, or planes, for example, are considered to be oblique with respect to each other if they diverge from or converge toward each other at an angle that is at least about <NUM> degrees or more away from parallel, or at least about <NUM> degrees or more away from orthogonal.

In the context of the present disclosure, the term "coupled" is intended to indicate a physical association, connection, relation, or linking, between two or more components, such that the disposition of one component affects the spatial disposition of a component to which it is coupled. For mechanical coupling, two components need not be in direct contact, but can be linked through one or more intermediary components. A component for optical coupling allows light energy to be input to, or output from, an optical apparatus. The terms "beam expander" and "pupil expander" are considered to be synonymous, used interchangeably herein.

As an alternative to real image projection, an optical system can produce a virtual image display. In contrast to methods for forming a real image, a virtual image is not formed on a display surface. That is, if a display surface were positioned at the perceived location of a virtual image, no image would be formed on that surface. A virtual image display has a number of inherent advantages for an augmented reality display. For example, the apparent size of a virtual image is not limited by the size or location of a display surface. Additionally, the source object for a virtual image may be small; a magnifying glass, as a simple example, provides a virtual image of its object. In comparison with systems that project a real image, a more realistic viewing experience can be provided by forming a virtual image that appears to be some distance away. Providing a virtual image also obviates any need to compensate for screen artifacts, as may be necessary when projecting a real image.

Embodiments of the present disclosure provide an optical system for providing a virtual image with an enlarged view pupil.

The image space f/# of a projector depends on the distance to the image divided by the diameter of the system stop (approximated by the aperture size of the last optic). A projector at greater than f/<NUM> over a reasonable distance would be too bulky for an HMD system. A more compact projector with an exit aperture diameter of <NUM> or less projecting at a >f/<NUM> distance does not properly fill the aperture. The challenge for system optics is to provide an optical solution that provides an increased f/# without appreciably adding to the bulk of the optical system.

The schematic side view of <FIG> shows components of an imaging apparatus <NUM> for providing a virtual image to one eye at an exit pupil E that lies within an eyebox B that is defined between the two generally parallel lines shown and that defines where the image is visible to the observer. A projector <NUM> projects an image to a lenslet array <NUM> at an image plane, forming a real image that is at or near the lenslet array <NUM>. Lenslet array <NUM> acts as a diffuser in transmission of light, as opposed to solutions that utilize a reflective diffuser. The substrate that provides lenslet array <NUM> is curved to reduce Petzval curvature in subsequent light handling. Lenslet array <NUM> is disposed at one focal length from a spherical curved mirror <NUM>.

With respect to the view of <FIG>, lenslet array <NUM> curvature extends into the page; an edge <NUM> is substantially parallel to the page surface and lies outside the page surface. Lenslet array <NUM> expands the effective beam width, and thus the f/# of the projected beam from projector <NUM>. A beam splitter <NUM> directs the expanded beam toward spherical curved mirror <NUM> that forms a magnified virtual image V by redirecting the light toward exit pupil E. Virtual image V appears to lie beyond the outer edge of curved mirror <NUM>. Eye box B is sufficiently sized to allow multiple pupil positions. The surface of curved mirror <NUM> is substantially spherical. Curved mirror <NUM> is partly transmissive, such as to allow the observer to see at least a portion of the actual ambient environment for augmented reality applications. An optical shutter, such as a mechanical shutter or light valve such as an LCD (liquid crystal device), can optionally be provided to control light transmission through the curved mirror <NUM> and to block or transmit light from the ambient environment.

<FIG> shows, with dimensions exaggerated for emphasis, how lenslet array <NUM> expands the beam width to provide a larger f/# within the optical system. Of particular interest are the following:.

<FIG> is a schematic diagram that shows a principle of operation for the imaging system. Projector <NUM> forms a real image on lenslet array <NUM>, which operates to spread the light from each pixel. The magnification system, represented schematically by a lens L in <FIG> but performed by curved mirror <NUM> in the optical imaging apparatus <NUM> of the present disclosure, then provides the light that forms the virtual image for the observer's eye. Light at the exit pupil is at least at about f/<NUM>.

<FIG> is a view of a portion of a portion of the optical system that shows the relative position of the eye box B of the observer, in dashed outline.

<FIG> is a rear perspective view that shows optical components in a head-mounted device (HMD) <NUM>. <FIG>, <FIG>, and <FIG> show views from in front of the observer. Light-conditioning optics <NUM> include one or more optical elements that direct and shape the image-bearing light from the projector in order to form a real image plane. Light conditioning optics <NUM> guide light from projector <NUM> and through curved lenslet array <NUM> to beam splitter <NUM>. The front views of <FIG>, <FIG>, and <FIG> show curved mirror <NUM> that forms the virtual image from the real image that is projected onto curved lenslet array <NUM>. Headphones <NUM> provide audio signal output.

The lenslet array can be provided on a glass substrate or on a plastic substrate. Curvature of the lenslet array can be provided by permanently bending the array or by mounting the array in a frame that causes the array to bend to an appropriate shape. The projector can use a solid-state light source, such as a light-emitting diode (LED) coupled with one or more light modulating display panels such as liquid crystal on silicon (LCOS) or digital light processor (DLP), for example. Light-conditioning optics <NUM> can include lenses, mirrors, prism-based waveguides, or other devices to direct, shape, and modify the image-bearing light from the projector <NUM> to lenslet array <NUM>. Image field or Petzval curvature can be achieved with proper design of all elements of light conditioning optics <NUM>.

Embodiments of the present disclosure allow the use of a small projector device for displaying a virtual image to the observer with a large eye box.

Claim 1:
A head-mounted imaging apparatus (<NUM>), comprising:
a projector (<NUM>) that is energizable to project image-bearing light;
a light-conditioning element (<NUM>) configured to direct and shape the image-bearing light from the projector (<NUM>) to form a real image plane;
a curved mirror (<NUM>) that is partially transmissive to allow at least partial visibility of the ambient environment to one eye of an observer, wherein a surface of the curved mirror (<NUM>) is substantially spherical;
a lenslet array (<NUM>) positioned adjacent to the real image plane and optically disposed at substantially one focal length away from the curved mirror (<NUM>), wherein the lenslet array (<NUM>) is curved about a single axis (A); and
a beamsplitter (<NUM>) in a path of light from the real image at the lenslet array (<NUM>), wherein the beamsplitter (<NUM>) is arranged to direct at least a portion of the light from the real image toward the curved mirror (<NUM>);
wherein the curved mirror (<NUM>) is configured to direct light from the beamsplitter (<NUM>) to form a virtual image for the one eye of the observer who wears the head-mounted imaging apparatus (<NUM>);
wherein the lenslet array (<NUM>) includes a plurality of lenslets (<NUM>) facing the curved mirror (<NUM>) side of the path of light, the plurality of lenslets (<NUM>) having respective focal points (F1), and wherein the real image plane is located along or within a substrate of the lenslet array (<NUM>) and before the focal points (F1) of the plurality of lenslets (<NUM>) in the path of light.