Optics of a display using an optical light guide

A projection display having an optical light guide for a see-though display system using holographic optical element or diffractive optical element as in-coupling optics and combined with Fresnel mirrors as out-coupling optics is disclosed. A display using this light guide enables a wide angle (e.g., over 90 degrees field of view), high resolution, and a large eye-box with a compact size.

TECHNICAL FIELD

This disclosure relates to optics of a display using an optical light guide (e.g., a see-through display for projecting an image through a thin light guide using a hologram or diffractive optical element).

BACKGROUND

See-through displays have received attention in recent years (e.g., for head-up-displays and wearable displays) in the context of smart phones being well accepted by the market. See-through displays provide hands free operation and provide an ability to show an image in the distance same as regular sight. While see-through displays are useful, in the past, some see-through displays (e.g., using holograms) have not necessarily satisfied viewers, because they do not provide a large enough viewing angle. Optical systems enabling light, small, bright, high resolution, and/or see-through characteristics would be useful, but there are challenges associated with some systems achieving a relatively large viewing angle.

SUMMARY

In one aspect, in general, a display system for providing light rays toward an eye of a viewer, comprises: a display device; a set of one or more lenses configured to receive light rays from the display device; a light pipe; an in-coupling optical element configured to receive light rays from said set of one or more lenses and provide light into said light pipe having three or more sides and at least a portion of surfaces of the sides of said light pipe are reflective by coating or total internal reflection; an out-coupling light guide; and one or more mirrors configured to reflect light rays from an exit portion of said light pipe into said out-coupling light guide. The one or more mirrors include at least a first Fresnel reflector that comprises a plurality of saw-tooth shaped grating elements configured to reflect rays into said out-coupling light guide, and said out-coupling light guide includes a second Fresnel reflector that comprises plurality of saw-tooth shaped grating elements.

Aspects can include one or more of the following features.

The in-coupling optical element comprises a prism, and said set of one or more lenses is telecentric, where principal rays emitted from pixels of the display device are substantially perpendicular to the surface of said display device and the principal rays cross each other within or in proximity to said prism.

The in-coupling optical element comprises a prism, said prism has a reflective surface configured to receive light rays from said set of one or more lenses and to reflect the light rays into said light pipe, and a normal vector of the reflective surface is between 15 and 45 degrees with respect to an optical axis of said set of one or more lenses.

The in-coupling optical element comprises a prism, said prism has a reflective surface configured to receive light rays from said set of one or more lenses and to reflect the light rays into said light pipe, and a normal vector of the reflective surface is configured to rotate to form an angle between 30 and 60 degrees with respect to a long side of said light pipe.

The in-coupling optical element comprises a prism, and a refractive index of said prism is over 1.4, and a refractive index of said light pipe is over 1.4.

A size of the exit portion varies based at least in part on a location of the exit portion along a long side of said light pipe.

The grating elements of said first Fresnel reflector are curved on a surface of a triangular prism in saw-tooth shape and said surface is tilted between 15 and 45 degrees with respect to a top surface of said light pipe and a normal vector of the first Fresnel reflector is set so that a light ray parallel to an optical axis of said set of one or more lenses is substantially parallel to a normal vector of the top surface of said light pipe.

The in-coupling optical element comprises a prism, and a refractive index of said prism is over 1.3, a refractive index of said out-coupling light guide is over 1.3, and the refractive index of said prism is substantially equal to the refractive index of said light guide.

The out-coupling light guide has a slant side forming a prism where light rays are able to enter, and a normal vector of the slant side is between 15 and 45 degrees with respect to a top surface of said light pipe to enable light rays from said one or more mirrors to be reflected into said out-coupling light guide.

The out-coupling light guide has a slant side forming a prism where light rays enter and the normal vector of the slant side is between 15 and 45 degrees with respect to a top surface of said light pipe to enable light rays from said one or more mirrors to be reflected into said out-coupling light guide.

The second Fresnel reflector includes a grating region that includes multiple saw-tooth shaped grating elements configured to reflect light rays from said one or more mirrors toward the eye of the viewer and flat regions between the saw-tooth shaped grating elements, where no saw-tooth shaped grating elements exist in the flat regions, and the out-coupling light guide reflects the light rays by total internal reflection and is substantially transparent so that external light can reach the viewer's eye, and the grating region includes a reflective coating.

The second Fresnel reflector includes a grating region that includes multiple saw-tooth shaped grating elements configured to reflect light rays from said one or more mirrors toward the eye of the viewer and flat regions between the saw-tooth shaped grating elements, where dual reflections by both the flat regions and the saw-tooth shaped grating elements are prevented by light absorbing areas.

At least one of said light pipe and/or said out-coupling light guide includes one or more layers that are configured to at least partially reflect light to reduce unilluminated areas.

A surface of said out-coupling light guide includes a curved envelope of multiple flat surfaces so that internally reflected light beams have the same angles as those of a flat surface.

Angles of multiple saw-tooth shaped grating elements of said second Fresnel reflector of said out-coupling light guide vary by location so that an image is focused at a finite distance.

The display system further comprises: a plurality of sets of the display system overlaid so that images at multiple distances are viewable.

The display system further comprises: a plurality of waveguides integrated into a single waveguide using one or more dichroic filters and at least one dichroic cross prism.

At least one of said light guide and/or said light pipe is coated with a holographic optical element, a diffractive optical element, or a multi-layer thin film so that a total internal reflection angle is reduced to enlarge a field of view of the display system.

At least one of said light guide and/or said light pipe comprises a lower refractive index material adjacent to said in-coupling optical element and a higher refractive index material within or in proximity to the out-coupling light guide.

The light guide is laminated with higher refractive index material than a refractive index of said light guide, and said second Fresnel reflector is formed on the higher refractive index material.

The in-coupling optical element comprises a first optical element comprising at least one of a holographic optical element (HOE) and/or a diffractive optical element (DOE) aligned so that a subtending angle of diffracted light beams is lower than a subtending angle of incoming light beams, and a second optical element comprises at least one of a HOE and/or DOE placed in the light guide to receive light from the light pipe and the second optical element is aligned so that a subtending angle of outgoing light beams is larger than the subtending angle of the incoming light beams, and an angle of outgoing light from a normal direction of the second optical element is substantially equal to an angle of incoming light to a normal direction of the first optical element for a light beam from a same pixel.

In another aspect, in general, a display system comprises: a display device; a set of one or more lenses configured to receive light rays from the display device; an in-coupling optical element configured to receive light rays from said set of one or more lenses; an out-coupling light guide; and one or more mirrors configured to reflect light rays into said out-coupling light guide. A diffractive optical element (DOE) in at least one of the in-coupling optical element or the out-coupling light guide is configured to use 7th order diffraction for light including a blue spectral component, 6th order diffraction for light including a green spectral component, and 5th order diffraction for light including a red spectral component.

Aspects can include one or more of the following features.

The display system further comprises: a light pipe configured to couple light between a first DOE in the in-coupling optical element and a second DOE in the out-coupling light guide.

The display system further comprises: an optical element with an electronically controllable focal distance.

The optical element with an electronically controllable focal distance is configured to display multiple images at multiple respective distances sequentially synchronized with the display device.

Aspects can have one or more of the following advantages.

Some implementations of the display systems described herein work as a periscope with a thin light guide (e.g., a waveguide or light pipe) combined with prisms or Fresnel mirrors having reflective surfaces. The display systems may be suitable for see-through head-up-displays for automobiles, and can be configured as wearable displays (e.g., as compact as eyeglasses), and can be configured to achieve a wide viewing angle (also referred to as field of view, high resolution, and a large eye-box.

Some implementations are capable of increasing the field of view (FOV) of a display using a light guide, such as a waveguide or a light pipe, to pass light from a display to a viewer. In some examples, the angle subtending incoming light into a waveguide remains same if the optical system of the waveguide uses only specular reflection. This means, in such examples, the FOV is identical to the subtending angle of incoming light and the FOV can be increased using a larger angle of converging incoming light into a waveguide, which can be achieved by using more lenses or more complex optical system, for example. Generally, a holographic optical element (HOE) or diffractive optical element (DOE) will increase the subtending angle of diffracted light when it is used as in-coupling optics, and a HOE or DOE will decrease the subtending angle of diffracted light when it is used as out-coupling optics. Some implementations of the systems and devices described herein use HOE or DOE for in-coupling optics to increase the subtending angle of diffracted light and use specular reflection for out-coupling, which maintains the subtending angle (e.g., no-increase and no-decrease). Because of the combination of diffractive optics for in-coupling where the incident light beams are expanded, and geometrical optics, which do not change the subtending angles of light beams, the resulting combined optics configuration will expand the subtending angle (dθi to dθout) and increase the FOV. As in the example shown inFIG.4, dθout is larger than dθi. If the reflector (403inFIG.4) is an ordinary mirror, dθi=dθout and the subtending angle will not be increased, but if the reflector403is a HOE or DOE, for example, the angle will be increased.

Other features and advantages will become apparent from the following description, and from the figures and claims.

DETAILED DESCRIPTION

A characteristic of some implementations of a display system is a relatively large FOV of a see-through display (e.g., over 90 degrees (or +/−45 degrees) horizontally with an eye-box larger than 15 mm). An example implementation of a display system is shown inFIG.1. A display system104includes a display device108comprising an array of pixels and a set of projection lenses101. Light rays from the display device108are projected on a HOE102used as in-coupling optics and inputted to a light pipe103. The light rays propagate into the light pipe103and only the light rays having the same direction as the original rays from the HOE102are selected and lead to folding mirrors105. The light rays are reflected by the folding mirrors105about 90 degrees toward an out-coupling light guide107. In some implementations, the light guide107is a plate or slab waveguide that guides light between surfaces at interfaces with air or some lower index material by total internal reflection (TIR), and in some implementations the light guide107is a plate or slab that guides light reflected between reflective surfaces of the plate or slab. In this example, the light rays are reflected toward a viewer's eye by saw-tooth shaped Fresnel mirrors formed on a surface of the light guide107. Examples of each element will be explained in more detail below.

In one aspect, the display system acts as an exit pupil expander. For example, a viewer should be able to see an image even when he/she moves their eyeballs. Some systems only have a relatively small exit pupil (e.g., 0.5 mm diameter exit pupil, for example, on the order of a mobile phone's camera lens). But, a human eyeball can move +/−half an inch, or a 1-inch range. The exit pupil expander functionality of the display system described herein is able to expand the exit pupil from around a 0.5 mm diameter to around a 1 in (25.4 mm) diameter.

An in-coupling HOE or DOE can redirect light beams from an external projector lens system to the inside of a light pipe. Some of the light beams exit from the light pipe through one or more slits or other openings to a Fresnel reflector. The light beams are reflected by an out-coupling element, such as a Fresnel reflector composed of individual reflective grating elements (also referred to herein as “Fresnel mirrors”), toward the outside of the light pipe to enter a light guide. The beams hit a second in-coupling HOE or DOE attached to the light guide and are reflected toward the inside of the light guide. After propagating inside the light guide, beams are reflected by out-coupling Fresnel mirrors to an eyeball of a viewer. The light guide can be implemented, for example, as a light guide plate (flat rectangular plate with surfaces of around 50 mm×50 mm in size).

Examples of some of the features that contribute to the ability of the display system to act as an exit pupil expander include the following. The incoming beam angle can be compressed (e.g., 60 degrees to 20 degrees). The rays of the compressed beam propagate inside the light pipe to be redirected and distributed from multiple openings along the light pipe into the wide area of the light guide plate, allowing a much larger FOV. For example, the grating elements of a Fresnel folding mirror redirect the light to the light guide plate, which expands the exit pupil in a first dimension (e.g., an x-dimension). An out-coupling DOE, such as another Fresnel reflector, attached to the light guide plate also expands the angle of beams exiting to the viewer. The out-coupling Fresnel reflector expands the exit pupil in a second dimension (e.g., a y-dimension perpendicular to the x-dimension).

Additional features can be included in some implementations of the display system. For example, a single DOE is able to diffract three primary color beams into the same direction. This will reduce the number of needed layers of a light guide plate or other out-coupling optics (e.g., from 3 layers to a single layer). Also, images can be formed at each of multiple selected distances dynamically.

Example implementations of different kinds of in-coupling optics are shown inFIGS.2A,2B,2C, and2D. InFIG.2A, a display device203provides light to a set of projection lenses205, and the distance between the center of the display device203and a given pixel (marked as “X”201) is proportional to the projected angle θ202of light from that given pixel. For example, the projection lenses205are arranged into a lens assembly that ensures that the angle of an output light ray is proportional to the distance of pixel from the center of the pixel array (or0is proportional to X). The display device203emits light beams204from each of its pixels to the set of projection lenses205. This lens set205is designed as a telecentric optics arrangement meaning that the principal ray of light beams204from each pixel of the display device203entering the projection lens set205is substantially parallel to the optical axis of the projection lens set. This lens set205can be configured such that an image is focused at an infinite or substantially far distance so that all of the rays projected from a single pixel are substantially parallel to each other as shown for light rays207A,207B, and207C inFIG.2A. This feature is also called an F-Theta lens. This optics arrangement ensures that all the rays from a single pixel are parallel. This is a principle used in some implementations to focus an image after the rays are mixed in the light pipe, because this optics arrangement ensures that all rays having a same direction are from a single pixel.

FIG.2Bshows an example of a light guide214(e.g., a light pipe or a waveguide) having a prism213included as part of in-coupling optics receiving light from the display device211through a lens set212.FIG.2Cshows an example of a light guide225(e.g., a light pipe or a waveguide) having a hologram221included as part of in-coupling optics222receiving light from the display device220through a lens set223.FIG.2Dshows an example of a light guide233(e.g., a light pipe or a waveguide) with a projection lens set231providing light to in-coupling optics232(e.g., HOE or DOE or prism), showing a light beam234propagating inside the light guide233.

Another example implementation is shown inFIG.3, in which multiple waveguides301,302,303are integrated into a single waveguide (or layer)310using dichroic filters308and a cross prism311. A HOE allows multiple recording and superimposing multiple wavelength holograms into a single layer. However, some types of hologram materials have a limitation of the maximum number of recording, and a surface relief type DOE does not allow multiple wavelength diffractions. These can use a multi-layer waveguide and often one layer for one color of incoming rays304. For a color display, a 3-layer waveguide can be used. This example implementation enables a single layer waveguide for multi-color display integrating3color layers of a waveguide into a single layer with a dichroic filter308and a dichroic cross prism311as shown inFIG.3. Optical element303is a waveguide for blue and has a HOE or DOE layer at307. Light beams diffracted by the HOE or DOE contains 3 colors, but only blue is filtered by the dichroic filter marked as308. Optical elements302and301are waveguides for green and red respectively. All three color light beams after the filters are lead into the cross prism311and integrated into the middle layer310for output as reflected rays309. This integration of 3 layers into a single layer is not limited to waveguide, but also applicable to light pipe.

Another example implementation is shown inFIG.4. Incident light rays are shown as (e.g., between ray405and ray406) and their subtending angle is shown as dθi401. The ray Oi409is the middle ray between them. The incident rays are focused to a HOE or DOE403and diffracted by the HOE or DOE403to an output angle subtending between ray402and ray407. The angle dθout is the angle of the middle ray θout408between ray402and ray407. Because dθout=cos(θi)*dθi/cos(θout), θi is about zero, and θout is about 120 degrees, dθout=−2*dθi. This means that the diffracted beam has a divergence of angles among its constituent rays that is twice as large as the incident beam. Thus, the incoming beam ray angles can be enlarged using the HOE or DOE. If the HOE or DOE is used as out-coupling optics, where θi is about 60 degrees and θout is about 180 degrees, the relation becomes opposite and dθout=−0.5 dθi. This means the angle will be reduced by half. Therefore, if the system uses a HOE or DOE for in-coupling and an ordinary optical reflector such as a mirror, prism, or Fresnel reflector, the system can obtain enlarged output angles from smaller incident angles. This will enlarge the FOV.

Another example implementation is shown inFIG.5. A light pipe501receives light from an in-coupling optical element503(e.g., a HOE or DOE). A first expansion direction504indicates the direction of expansion of beam angles by the in-coupling optical element503. The diffracted beams have a distortion of parallelogram toward the expansion direction504. After the light beam exits the light pipe501through openings along the light pipe501, the light beam enters a slab light guide502. The openings along the light pipe501can be locations at which a reflective material is absent, or locations at which an angle of incidence is less than a total internal reflection angle for some of the rays that propagate along the light pipe501. At the out-coupling optical element505(e.g., folding mirrors including Fresnel mirrors of a Fresnel reflector), a second expansion direction506can be configured to be perpendicular to the expansion direction504to minimize the distortion of images.

An example implementation of a light guide is shown inFIG.6. In some cases, the light guide can be implemented as a waveguide with a high index core surrounded by a lower index cladding (e.g., which could be air in some cases). But, the FOV of a waveguide is limited by the minimum angle of total internal reflection (TIR) of the material used for waveguide core. To increase the FOV, for a light guide601configured as a light pipe, coatings605,608on either surface of a transparent slab of material reflect a light beam whose constituent rays are incident at an angle that is lower than a TIR angle that would be associated with the slab material. An in-coupling optic602is a HOE or a DOE at which the incoming beams603are directed. If there were no coatings, the reflected beam606would be transmitted as a beam607, but with the coating608the reflected beam606can be reflected as beam604.

Another example implementation of a light guide is shown inFIG.7. This light guide comprises two regions made up of two different materials (region701and region707), each having a different refractive index. This example enables a HOE702to diffract an incoming beam700to a beam705that is incident below a TIR angle associated with the material703and leads to the region707having a higher refractive index and smaller TIR angle to enlarge the FOV. The maximum angle of a diffracted beam that is diffracted by the HOE702is limited due to the refractive index of HOE's base material and it calls for a low or similar refractive index material for a waveguide where HOE is placed. A waveguide can be connected with a higher refractive index material toward an out-coupling region, and the FOV can be increased by this technique. A diffracted beam705would be transmitted (as beam704) if there was not a higher refractive index material in region707, but the diffracted beam705can be reflected as shown by reflected beams706and708.

Example implementations of out-coupling from a light guide are shown inFIGS.8A,8B, and8C. In these examples, unwanted double reflection (or “Ghost image”) of a waveguide having Fresnel mirrors can be avoided. Waveguide material808has a higher refractive index than the material on either side. The waveguide is designed for a single reflection at the flat area801or the Fresnel mirror area802as shown for incoming rays803A,803B and reflected rays804A,804B, and double reflection as shown for an incoming ray805to a reflected ray806is unwanted. Another limiting factor of the FOV is this unwanted reflection by the individual saw-tooth shaped mirrors of the Fresnel reflector of a waveguide as shown inFIG.8A. As the angle of a propagating light beam increases (larger angle from the normal vector of the surface of waveguide), beams may hit twice (as shown by beams806) before out-coupling from the waveguide, which may not be desired. The example ofFIG.8Bis to laminate a higher refractive index material808on waveguide and place Fresnel mirrors on the higher refractive index material808.FIG.8Cshows another example to avoid double reflection with external saw-tooth shaped protrusion of Fresnel mirror grating elements815.

An example shown inFIG.9illustrates how the maximum angle θmax of light beam905is limited by the refractive index of the base material of a HOE909,910. If the refractive index of the base material of the HOE is smaller than that of the waveguide908, the diffracted beam904that is diffracted by the HOE cannot exceed 90 degrees and θmax is limited by the TIR angle between the material of the waveguide908and the base material of the HOE909.

The subtending angle dθout of diffracted light beams402and407as shown inFIG.4is substantially larger than incoming beam angle dθi401due to diffraction and the light beams402and407must be within total internal reflection. This limits the angle of incoming light beams405and406. Another example implementation is shown inFIG.10to avoid this potential limitation. A structure is shown inFIG.10wherein the subtending angle θin (θin1+θin2) of incoming light beams1007is reduced by a high refractive index prism1008and diffracted by a DOE1003, which reduces the angle further, e.g., to θ′in (θ′θin1+θ′in2). An example is shown in which the original incoming beam angle θin is 60 degrees and is reduced to 40 degrees by a high refractive index prism1008and 20 degrees after the diffraction by the DOE1003. The angle of a light beam can be reduced to ⅓ in this example. After propagating the light beams with this reduced angle through a light pipe1001, this example shows widening the angle by the second DOE1002, then propagating inside a waveguide, then widening to the original angle to the human eye. The use of this technique (Compress=>propagate inside light pipe and waveguide=>Uncompress=>eyes) enables even a 90-degree FOV. This technique facilitates a distortion-free image, because two DOEs compensate distortions caused by each other so long as the two DOEs are parallel.

Another example implementation is shown inFIG.11. By tilting a DOE1102, the FOV can be enlarged even further than in the example inFIG.10, although a rotational distortion may take place. The rotational distortion can be compensated by rotating the display system.

Another example display system is shown inFIG.12, where half-mirrors are inserted in the light pipe to increase the number of beams keeping the same propagation angles. This will reduce the distance between two beams in the waveguide.

Light beam trajectories are shown inFIG.13Afor light diffracted from an in-coupling DOE1305where the location of images is at an infinite distance because the beams coming from a single pixel are parallel. An example is shown inFIG.13Bwherein the location of images can be electronically controllable using a variable focal length diffractive lens with liquid crystal material electro-statically driven by a driver1320to vary its refractive index.

An example of the variable focal length lens is shown inFIG.14. The refractive index of liquid crystal can be changed by applying electro-static voltage up to 0.2. However, this is not enough to control the image distance by a refractive lens. The diffractive lens shown inFIG.14can change its focal length enough, because diffraction bends light much more than refraction lens. Multiple images can be displayed sequentially at multiple distances by the single DOE lens with liquid crystal driven voltages. Each image can be synchronized with the focal distance adjusted by the DOE lens. For example, person-A is displayed at 3 m distance at time-1and person-B is displayed at 10 m distance at time-2and house-A is displayed at 100 m distance at time-3and a background scene is displayed at an infinite distance at time-4. If these images are switched fast enough, the viewer will not notice the changes and will recognize the resulting combined scene as a virtual and/or augmented reality 3D scene as vergence and accommodation coincide (e.g., not as stereoscopic, but as a so called “light field”).

An example of super wide FOV augmented reality (AR) display having 90° (horizontal)×90° (vertical) was successfully designed and simulated with a optical design tool and shown inFIGS.15A and15B.

An example of a DOE implementation is shown inFIG.16. Without being bound by theory, the relation between the angle of incoming light and the angle of outgoing light can be given by
sin(θout)−sin(θin)=m/λ*constant(function of pitch of grooves, such as the saw-tooth shaped mirrors)

where θout=the angle of outgoing light to the normal direction of DOE surface

θin=the angle of incoming light to the normal direction of DOE surface

λ=wavelength of light

If the first order of diffraction is used for a DOE, the outgoing angle will differ depending of the incoming light's wavelength. This is the reason why a single DOE may in some cases not be used for different colors, and multi-plate or multi-layer DOE may be used to provide a color display. This disclosure describes a way to use a single layer DOE for 3 primary colors. If different diffraction orders are used for different colors, in other words, m/λ=same for 3 primary colors, for example m=7 for λ=0.45μ(blue), m=6 for λ=0.525μ(green) and m=5 for λ=0.63μ(red), m/λ, =15.555 for all 3 colors. This means that all three colors will be diffracted to a same direction. The next question is whether there is cross contamination, meaning that different order diffraction may come into the field of view and causes cross contamination or so-called ghost image. The chart inFIG.17showing the spectrum of diffraction efficiency exemplify the possibility of no cross contamination or very little contamination. This indicates that it is possible to make a single layer DOE for color display.