WO2024223273A1 - Optical system comprising fresnel lens adapted to augmented reality applications - Google Patents

Abstract

The invention relates e.g. to a head-mounted optical system for augmented reality applications. A Fresnel lens (220) having multiple regions (91, 92) of different geometrical properties of the particular Fresnel zone or of different supported optical resolutions is used.

Description

Description

OPTICAL SYSTEM WITH FRESNEL LENS ADAPTED FOR AUGMENTED REALITY APPLICATIONS

Applications in the field of augmented reality (AR) display images (also known as AR images) in the line of sight of a viewer of the environment. The viewer can therefore perceive not only the environment but also other information in his field of vision.

Head-mounted optical systems (HMD) are often used for AR applications. Head-mounted optical systems for AR applications are often also referred to as AR glasses.

Lenses are regularly used in HMDs to shift a focal plane of an image superimposed on the viewer's line of sight. A corresponding head-mounted optical system is disclosed in WO 2022/245447 A1. The use of Fresnel lenses is proposed for some of the embodiments described in WO 2022/245447 A1.

The use of Fresnel lenses enables a reduction in volume and, as a result, a reduction in mass compared to volume lenses. On the other hand, Fresnel lenses typically have a lower image quality than volume lenses due to the step structure.

Based on this, the following invention is based on the object of specifying a head-mounted optical system which enables an appropriate imaging quality of the image displayed in the line of sight of a viewer while at the same time keeping the mass of the head-mounted optical system as low as possible. This object is achieved by a head-mounted display system according to the main claim. Advantageous embodiments are described in the subclaims.

An HMD with a waveguide is proposed as an example, wherein the waveguide is designed to generate an image (AR image) in a display region of the waveguide for an eye located on one side of the waveguide. The waveguide generates the image together with other optical components, such as a display and an optic that couples the image shown by the display into the waveguide. The optical properties of the image generated by the waveguide, such as its optical resolution, are given by the overall system of such optical components, i.e. typically by the display, the coupling optics and the waveguide.

The head-mounted optical system also includes a Fresnel lens. The Fresnel lens can form a spectacle lens on its own or together with one or more other lenses.

The Fresnel lens is divided into a first optical region and a second optical region. The Fresnel lens is therefore further segmented - in addition to the use of Fresnel zones.

The first region is arranged to at least partially image the display region of the waveguide.

The first region can therefore be arranged adjacent to the display region. The first region can extend at least partially along the display region. If the first region completely maps the display region, then the lateral extent of the display region is approximately equal to the lateral extent of the first region. The second region can also be arranged in the area of the display region or can be arranged at a distance from the display region. For example, the first region could be in a central area the display region, close to or at a main viewing point; the second region could be arranged in a peripheral area of the display region. The second region can be arranged adjacent to the first region. The second region can surround the first region. The second region can surround the first region in a ring shape. The first region can be circular or elliptical.

Typically, the display region is arranged in the area of a zero viewing direction or in the area of the main visual point (typically, the main visual point corresponds to the zero viewing direction). The zero viewing direction is defined with respect to the eye rotation point. The positioning of the zero viewing direction with respect to the Fresnel lens depends on an inclination angle of the head-mounted optical system, a frame disc angle of the head-mounted optical system, a corneal vertex distance and/or an eyebox of the head-mounted optical system.

The zero viewing direction is typically the viewing direction parallel to the ground up to an angle of 15° downwards from the horizontal with a normal head and body posture. See also DIN EN ISO 13666 (2013). The viewing lines are parallel to each other when looking into the distance. The display region is typically arranged in a center of the lens or in particular in a center of the Fresnel lens. Accordingly, the first region is also typically arranged in the center of the lens or in particular in the center of the Fresnel lens.

The first region provides a first optical resolution. The second region provides a second optical resolution. The first optical resolution can be better than the second optical resolution (a "better" optical resolution corresponds to a greater resolving power, i.e. small structures are imaged).

The optical resolution of an imaging system - here the Fresnel lens - refers to the minimum distance between two object points, which Image can still be perceived as separate points by the imaging system. In optics, resolution is often described by the Rayleigh criterion or the Sparrow criterion. The Rayleigh criterion defines the resolution limit as the point at which the intensity maxima of two neighboring points are in the first intensity minimum of the other point. The optical resolution can be specified in different units, such as line pairs per millimeter (Ip/mm) or arc seconds (arcsec) of arc minutes (arcmin). Typical resolutions of the eye and of Fresnel lenses are on the order of arc minutes. The optical resolution can vary as the position of the field position (ie perpendicular to the beam path or lateral). In the disclosed examples, particular reference is made to the minimum optical resolution in the corresponding area (eg the first region or the second region), even if this is not specifically mentioned every time. For other lateral positions, an even better optical resolution can be achieved locally.

By using different optical resolutions in the first region and in the second region, it is possible to better address a trade-off between the thickness and weight of the Fresnel lens on the one hand and the optical resolution on the other. The optical resolution can vary according to the regions and can thus be tailored to the performance of the eye or at least the optical resolution with which the AR image is generated. Only as much material with refractive power is used for the Fresnel lens as is necessary or required in view of the design specifications (for example, the thickness of the lens or the weight of the lens).

For example, a reduction in the number of Fresnel zones in the first region compared to the second region can lead to a reduction in the loss of resolution due to the division of the aperture of the Fresnel lens into the sub-apertures of the individual Fresnel zones. The first optical resolution of the first region is therefore better than the second optical resolution of the second region. Zone boundaries between the Fresnel zones can possibly be perceived as shadows in the viewer's field of view. Zone boundaries can also cause a A reduced number of zone boundaries can therefore contribute to improved image quality. The improved image quality can be provided specifically for the first region; in the second region, a lower optical resolution is sufficient.

A Fresnel zone is often referred to as a "slope facet". Two Fresnel zones are separated by a Fresnel zone boundary - also referred to as a "draft facet", see Davis, Arthur, and Frank Kühnlenz. "Optical design using Fresnel lenses: Basic principles and some practical examples." Optik & Photonik 2.4 (2007): 52-55. Fresnel zones can be ring-shaped or elliptical. The Fresnel zones can accordingly have a radial extension (in relation to the optical axis), which is characterized by the distance between two Fresnel zone boundaries.

Generally speaking, it is possible that the number of zone boundaries of Fresnel zones in the first region is smaller than the number of zone boundaries of Fresnel zones in the second region. In particular, the density of the zone boundaries in the first region can be smaller than in the second region, i.e. the number of zone boundaries (e.g. radial in the case of ring- or elliptical Fresnel zones) per length or area is smaller in the first region than in the second region. This means that better optical resolution can be achieved in the first region than in the second region. The minimum distance between Fresnel zone boundaries in the first region can be different from the minimum distance of the Fresnel zone boundaries in the second region. In particular, the minimum distance of the Fresnel zone boundaries in the first region can be greater than in the second region. The minimum distance is typically given in the radial direction starting from a center of the Fresnel lens.

Typically, the achievable resolution of a Fresnel lens is limited by the minimum width of the zones involved, ie the distance between the Fresnel zone boundaries, in the respective viewing direction. By specifying a minimum distance, the optical resolution in the corresponding region can be set. In one example, it would even be conceivable for the first region to have no zone boundaries at all. For example, alternatively or in addition to a variation of such geometric parameters of the zone boundaries of the Fresnel zones, the height or thickness of the Fresnel zones between the first and second regions can also vary. The height refers to the distance between the upper edge and the lower edge of a Fresnel zone or in particular a Fresnel zone boundary. For example, the Fresnel zones in the first region could be correspondingly thicker, at least on average, than the Fresnel zones in the second region. In this way, more refractive material can be present in the first region in accordance with a classic lens. Typically, higher Fresnel zones are also wider and thus provide a higher optical resolution, while lower Fresnel zones can achieve a reduction in mass.

In the various examples, it is possible for the Fresnel zones to have a constant height. This means that the height does not vary as a function of the lateral position within the Fresnel lens. In particular, it would be possible for the Fresnel zones within a region of the Fresnel lens to have a constant height. In some examples, however, it would also be conceivable for the height of the Fresnel zones to vary within a region. In such an example, it would be conceivable that the average height of the Fresnel zones in the first region is different from the average height of the Fresnel zones in the second region. For example, the average height of the Fresnel zones in the first region can be greater than the average height of the Fresnel zones in the second region.

In the different examples described, it is possible that the height of the Fresnel zones within the first region varies. At the same time, it would be conceivable that the height of the Fresnel zones within the second region is constant. This means that within the first region, different Fresnel zones have different heights; while in the second region, all Fresnel zones have the same height. However, this is only an example implementation and other scenarios are conceivable. By varying the height of the Fresnel zones within the first region, a relatively good optical resolution can be provided by the first region, whereby at the same time, the amount of optical material of the Fresnel lens is limited in the first region. For the second region, a constant height of the Fresnel zones may be possible due to the poorer optical resolution. In general, dividing the Fresnel lens into different regions enables a better compromise between the trade-off fundamentally required for Fresnel lenses between the greatest possible reduction in the mass of the lens or its center and edge thickness, which is made possible by a high number of Fresnel zones, and high image quality, which can be achieved by the smallest possible number of Fresnel zones. For example, the height of the Fresnel zones should be as low as possible from an aesthetic point of view. However, a low height results in a smaller width of the Fresnel zones and thus smaller sub-apertures, which impair the resolution, while the curvature of the surface remains the same.

For example, the first optical resolution of the first region can support the optical resolution of the AR image. In other words, this means that in examples, the first optical resolution is equal to or better than the optical resolution with which the waveguide generates that part of the AR image that is imaged by the first region. For example, the optical resolution of the AR image could be equal to or better than 1 arcmin. Accordingly, the first optical resolution of the first region can be equal to or better than 1 arcmin. Such an optical resolution of the AR image in the range of one arc minute is useful if the AR image is generated in the area of the central field of view. Aspects related to the optical resolution of the human eye are explained below.

The human eye has different resolving powers in the central and peripheral visual fields. In the central visual field, more precisely in the area of the fovea (typically the central visual field has an extension of +/-1° around the central optical axis - the zero viewing direction - of the eye, but +/-15° can be achieved by moving the eye), where the density of photoreceptors is highest, the resolving power of the eye is about 1 arc minute (arcmin). In this area the cones (photoreceptors for color vision and high visual acuity) are most densely concentrated, enabling the sharpest vision. In the peripheral field of vision, the resolving power of the eye decreases because the density of photoreceptors is lower and rods (photoreceptors for peripheral and low light conditions) dominate. The resolution in the peripheral field of vision depends on various factors, such as the distance from the center of the field of vision and the brightness of the light. A typical value for the resolution of the peripheral field of vision (about +40° upwards from the optical axis, -20° downwards from the optical axis and +/-35° horizontally) around the optical axis is 4 arcmin, i.e. a factor of 4 less than in the central field of vision.

For example, the lateral extent of the first region may correspond to viewing angles ranging from +/- 1° to +/- 15° (along the horizontal and/or the vertical) around the zero gaze direction; thus, the lateral extent of the first region may be adapted to the central field of view - either with or without eye rolling. This means that the first region is centered around the zero gaze direction (along the display region), and extends over an angular range that covers these viewing angles from -1° to +1° (or viewing angles from up to -15° to +15°).

As a general rule, the conversion of viewing angles into lateral extent and position on the Fresnel lens is carried out in a known manner depending on factors such as: an inclination angle of the head-mounted optical system, a frame lens angle of the head-mounted optical system, a corneal vertex distance and/or an eyebox of the head-mounted optical system.

As a general rule, the lateral extent of the first region can be smaller, equal to or larger than the lateral extent of the display region. The first region can display the entire AR image or only parts of it. If the AR image is smaller than the central field of view of the eye, the first region can be larger than the display region, so that surrounding objects in the central field of view can also be displayed with the high optical resolution of the first region that is perceived outside the AR image. The second region can then cover the surrounding area, i.e. be adapted to the peripheral field of vision. The second region can accordingly provide an optical resolution that is worse than 4 arcmin, optionally worse than 10 arcmin.

For example, the second region could be annular (ie, surrounding the first region). The second region could extend in a region of the Fresnel lens corresponding to viewing angles from -15° to +15° about the zero viewing direction up to +/-30 ^° (along the horizontal and/or the vertical) about the zero viewing direction.

The first region may have a lateral extent or area that is in the range of 10% to 50%, optionally 20% to 40% of the lateral extent or area of the second region. This means that the first region may have a significant extent in relation to the extent of the second region. For example, the first region may have a lateral extent that is adapted to a central field of view of the human eye.

It is not necessary in all variants that the AR image is generated with a particularly high optical resolution, in particular with the high optical resolution of the human eye (for example, 1 arc minute in the area of the central field of view). For example, it would be conceivable that the optical resolution with which the waveguide generates the AR image in the area of the display region is worse than 1 arc minute, optionally worse than 4 arc minutes. In such an example, there are different variants for the dimensioning of the optical resolution of the Fresnel lens in the area of the first region. For example, the first optical resolution can support the high optical resolution of the human eye in the area of the central field of view and thus be, for example, approximately 1 arcmin, or better. However, it would also be possible that the first optical resolution does not support the high optical resolution of the human eye in the area of the central field of view and instead adapts to the comparatively poor optical resolution with which the waveguide generates the AR image in the area of the display region. For example, it would be possible that the first optical resolution is worse than 1 arcmin, optionally worse than 4 arcmin. Such a scenario is particularly useful when a head-mounted optical system is optimized for the small form factor and low weight, but not for particularly high imaging quality of the environment.

In addition to at least the first and second regions, the Fresnel lens can also have further regions that differ from the first and second regions in terms of optical resolution and/or the number of zone boundaries. For example, the third region can take into account the fact that the human eye has a lower resolution in the peripheral field of vision.

It was recognized that by adapting the optical resolution in different regions of the Fresnel lens to the different resolution capabilities of the human eye and to the properties of the augmented image, such as field of view (FOV) or digital resolution in different viewing directions, a reduction in the mass and thickness of the lens through Fresnelization and thus of the optical system can be achieved with the same globally perceived image quality or an improved globally perceived image quality with the same mass of the head-mounted optical system.

The Fresnel lens can in particular be designed to shift, in particular pull, a focal plane of the image to a predefined distance from the waveguide (focal length). The definition of the focal length can depend on the type of application of the lens (a single lens can even have different focal lengths across its aperture). In general, however, the focal length is the distance between the lens and the point at which an idealized collimated input beam converges to a point. The Fresnel lens can therefore also be called a pull lens.

Accordingly, the effect of shifting the focal plane neutralizing second lens can be considered a push lens. The push lens can also be designed as a Fresnel lens.

It is conceivable that the Fresnel lens is designed to correct a visual impairment of a wearer of the head-mounted display. For example, the Fresnel lens can be designed to correct short-sightedness (myopia), long-sightedness (hyperopia) or astigmatism. In particular, if a lens with high refractive power is required to correct the visual impairment, a significant reduction in the center or edge thickness can be made possible.

The Fresnel zones in the first region can be adapted in particular to an inclination angle of the head-mounted optical system, a frame lens angle of the head-mounted optical system, a corneal vertex distance and/or an eyebox of the head-mounted optical system in order to optimally adapt the head-mounted optical system to the viewer. In particular, the Fresnel zones in the first region can be adapted to a spectacle frame geometry.

The Fresnel zones can be formed on the side of the Fresnel lens facing the waveguide, so that the structured surface is not exposed. It can therefore be less susceptible to contamination.

However, the Fresnel zones can also be formed on the side of the Fresnel lens facing away from the waveguide. This can make it possible to align the flanks of the Fresnel zones, for example in relation to the observer's eye rotation point, so that the observer perceives the zone boundaries as less disturbing. Other alignments of the flanks of the Fresnel zones (draft facet), e.g. towards the pupil (pupil center in rest position) or individually for each flank, are also conceivable.

The eye pivot point, also called the center of rotation or axis of rotation of the eye, is an imaginary point within the eye around which the eye rotates during Eye movements rotate. As a rule, three main eye movements are distinguished: horizontal rotation, vertical rotation and torsional rotation. These movements are controlled by the six external eye muscles that hold and move the eye in the eye socket.

The eye pivot point is located approximately near the front lens and is considered to be the geometric location where all axes of rotation of the eye movements meet. However, the exact position of the pivot point can vary from person to person and also depends on the direction of gaze and the individual eye anatomy.

In some embodiments, the Fresnel lens may have a wedge profile. The wedge profile may be arranged on the side of the Fresnel lens facing the waveguide and/or on the side of the Fresnel lens facing away from the waveguide.

It would be possible for a side of the Fresnel lens opposite the side on which the Fresnel zones are formed to have an optical effect. It is conceivable that the side opposite the Fresnel zones of the Fresnel lens has a convex, concave or toric shape. In particular, the Fresnel lens can be a hybrid lens. This can make it possible to reduce the required curvature on the side of the Fresnel lens facing the eye, which can lead to greater distances and/or lower heights of the Fresnel zones.

In embodiments, the head-mounted optical system comprises a second lens, wherein the second lens is arranged on a side of the waveguide opposite the Fresnel lens, and wherein the second lens compensates for a shift in the focus position caused by the Fresnel lens. This would be a push-pull lens system.

Embodiments of the invention will now be explained in more detail with reference to the drawings, in which: FIG. 1 shows a head-mounted optical system;

FIG. 2 shows a head-mounted optical system;

FIG. His Fresnel lens;

FIG. 4 a Fresnel lens;

FIG. His volume lens;

FIG. 6 a Fresnel lens;

FIG. 7 a Fresnel lens;

FIG. His Fresnel lens;

FIG. 9 a Fresnel lens;

FIG. 10 a Fresnel lens,

FIG. 11 a Fresnel lens,

FIG. 12 a Fresnel lens,

FIG. 13 a Fresnel lens, and

FIG. 14 a Fresnel lens.

Aspects related to the design of a Fresnel lens for a head-mounted optical system for AR applications are described below. An AR image is generated by a waveguide with a certain optical resolution. The AR image is imaged by the Fresnel lens; this imaging is also carried out with a certain optical resolution, which is typically not smaller than the optical resolution with which the AR image is generated.

For this purpose, the Fresnel lens has several regions, for example a first region and a second region. The first region and the second region provide different optical resolutions. This allows a targeted adjustment of geometric parameters of the first and second regions to be optimized, taking into account the required optical resolution on the one hand and the thickness or weight of the Fresnel lens on the other.

In TABLE 1 below, some scenarios for the implementation of such regions are described.

TAB. 1 : Various examples of the relative dimensioning of the size and optical resolution of the display region, the first region and the second region.

FIG. 1 shows a schematic view of a head-mounted optical system 100 with a waveguide 110, a first lens 120 and a second lens 130. The waveguide transports the light 140 of the AR image to be displayed, coupled in from a display (not shown), into a display region of the waveguide 110, where it is coupled out. For this purpose, for example, a coupling-out element (not shown) can be used. The coupling-out element can be a reflective, diffractive or holographic coupling-out element.

Typically, collimated light with a focal plane at infinity is used to transport the AR image, which is also called an augmented image.

For the viewer of the AR image, indicated by the eye 160 and the zero viewing direction 161, it can be rather strenuous if their focal plane is at infinity. Therefore, a first lens such as the first lens 120 is regularly used, with which the focal plane of the AR image is shifted. The first lens 120 can be used in particular to pull the focal plane into the close range. The first lens 120 is therefore also referred to as a "pull" lens.

For viewing the surroundings, the influence of the "pull" lens is compensated by the second lens 130, which is also called a "push" lens. The light rays 150 coming from the surroundings should therefore not be influenced as much as possible.

In addition to adjusting the focal plane of the AR image, the first lens 120 can also be used for individual refraction correction for the user.

The head-mounted optical system 100 can be part of AR glasses. Compared to conventional glasses for correcting visual impairments such as myopia, hyperopia or astigmatism, AR glasses place even greater emphasis on being as thin and light as possible. For this purpose, as shown in FIG. 2, a Fresnel lens 220 can be used as the first lens 220 of the head-mounted optical system 200. The second lens 230, the push lens, arranged on the side of the waveguide 210 opposite the Fresnel lens 220 can also be designed as a volume lens.

AR glasses are designed to allow a viewer to perceive both the environment and the displayed AR image. The resolution perceived by the viewer depends on the position of the incident light on the retina of the viewer's eye, which is determined by the angle at which the incident light hits the pupil. In other words, the perceivable resolution depends on the direction of gaze in relation to the resting eye.

By moving the eyes or head, the viewer can, within limits, choose the direction of view in relation to the resting eye so that the perceptible resolution corresponds to the desired perceptible resolution. However, moving the head cannot change the position of the displayed AR image in relation to the eye position or orientation.

Typically, the resolution of the human eye is about 1 arcmin in an area of about +/- 1° of the viewing direction around the central axis (zero viewing direction) - this area is often referred to as the central field of vision. The central field of vision is expanded by rolling the eye to about +/-15° around the zero viewing direction. Text recognition (without rolling) is normally possible in the range of about +/-10° around the zero viewing direction, whereby the resolution here is usually better than about 4 arcmin. For larger angles up to the area of peripheral vision, the resolution decreases.

A deflection (rolling) of the eye of approx. +- 15° vertically and/or horizontally from the resting position is usually not perceived as strenuous by the observer. If a larger deflection of the eye is required to view an object with the necessary resolution, the observer moves usually the head. The maximum deflection of the eye for most viewers is about 40° upwards, about 20° downwards and about +-35° horizontally.

In the deflection range of the eye, it is generally possible to perceive the environment or displayed AR images with an appropriate resolution, although it should be noted that the position of an AR image displayed using the AR glasses cannot be changed by turning the head.

With regard to the resolving power of the eye itself and when the eye is not deflected strenuously, it can be seen that in a range of approx. +- 15° in relation to the resting position of the eye or the zero viewing direction, the perceptible resolution is approx. 1 arcmin (this viewing angle range is referred to herein as the central visual field) and in a range of approx. +- 25° with a resolution of approx. 4 arcmin (this viewing angle range is referred to herein as the peripheral visual field).

These values are therefore usually used as the basis for the design of glasses that are used to correct vision problems. However, the requirements for AR glasses may differ.

Some AR glasses, for example, allow AR images to be displayed that require the eye to be deflected by more than +/- 15°. See TABLE 1: Scenario 1. Here, in a first region of the Fresnel lens - which is adapted to the central field of vision - a high optical resolution of the AR image can be supported, for example, while in a second region of the Fresnel lens, which adjoins the first region towards the outside, only a lower resolution that corresponds to the optical resolution of the human eye in the peripheral field of vision can be supported.

If AR glasses are only intended to display AR images of one size (i.e. the display region is dimensioned accordingly small) that require a maximum deflection of the eye of +- 15° or less (cf. TAB. 1 : Scenario 3B), the optical resolution of the Fresnel lens can be chosen to be correspondingly lower outside this range and greater weight savings can be achieved. However, it would also be possible to support a high optical resolution for viewing angles outside the AR image (see TAB. 1 : Scenario 3A).

Some AR glasses provide for parts of the AR image to be displayed with high resolution in a central area of the display region and parts of the AR image to be displayed with low resolution in peripheral areas of the display region. In such a scenario, the Fresnel lens can have a high optical resolution in a central first region and a lower optical resolution in a peripheral second region in order to meet the resolution requirements while keeping the weight as low as possible. See TABLE 1: Scenario 4).

If the resolution of the displayed AR image is lower than the perceptible resolution of the human eye, the resolution of the Fresnel lens can also be selected to be correspondingly lower, so that further weight savings are possible.

Among other parameters, the optical performance of a Fresnel lens can be adjusted in the following way.

Each Fresnel zone can be considered as a sub-aperture, which corresponds to the achievable angular resolution. As a good approximation, a Fresnel zone with the width w enables an angular resolution Aa of

A Acr = — nw where n is the refractive index in air. For each viewing direction, the Fresnel zones that contribute to the perceived AR image correspond to the Region of the Fresnel lens seen through the pupil. The smallest value of Ac indicates a maximum for the achievable resolution.

As indicated in FIG. 3, for the central region 301 of the field of view, in which high resolution is normally required, there should be as few Fresnel zone boundaries as possible, and ideally none at all. In the central region 301 of the field of view, in which high resolution is desired, the light rays perceived by the eye thus essentially only pass through a single Fresnel zone 302 with a continuous surface, as in a volume lens.

A first region 91 is therefore aligned with the central area 301 of the field of view. In the first region 91, only a comparatively small number of Fresnel zones are arranged - in the example shown in FIG. 3, only a single Fresnel zone 302, although more than just a single Fresnel zone can also be used. In more general terms, the Fresnel zone density in the first region 91 is lower than in the surrounding second region 92. In the example in FIG. 3, the second region 92 surrounds the first region 91 in a ring shape, i.e. extends radially outwards from the first region 91. Both regions 91, 92 are centered on the optical axis (dotted line) - defined by the resting position of the pupil.

At least the first region 91 is arranged in the area of the display region of the waveguide (not shown in FIG. 3). However, it would also be conceivable for the display region to have such a large lateral extent that it extends into the area of the second region 92 (i.e. the first region 91 has a lateral extent that is smaller than the lateral extent of the display region).

As a general rule, the Fresnel zone boundaries should be as imperceptible as possible in the different areas. This can be done, for example, by orienting the Fresnel zone boundaries (in FIG. 3 a Fresnel zone boundary 303 is highlighted) towards the pivot point of the eye or towards the pupil of the eye. so that they form a particularly small footprint. Furthermore, the height of the Fresnel zones (or Fresnel zone boundaries or "draft facets"), ie the step height at the Fresnel zone boundary, should be small enough to influence the perceived image as little as possible.

Nevertheless, the resolution decreases somewhat for larger viewing angles because, as indicated in FIG. 4, the resolution of the corresponding light rays 402 is limited by the sub-aperture 403 (in contrast to the central light rays 401 ), since they are only influenced by a smaller area 403 of the lens surface, so that they are perceived by the retina 406 of the observer after passing through the pupil 404 and the eye lens 405. In addition, the discontinuities at the Fresnel zone boundaries can be perceived as disturbing.

With a step height of 0.5 mm, with vertical Fresnel zone boundaries, with a beam height of 5 mm, a distance of the Fresnel lens from the pivot point of the eye of 25 mm and a viewing angle of approximately 11 °, the footprint of the Fresnel zone boundary corresponds to approximately 100 µm.

Variants for the design of the Fresnel zone widths and the position of the Fresnel zone boundaries are now explained in TAB. 2 for a given surface profile, which corresponds to a spherical lens with a refractive power of -6 diopters. It should be noted that the required refractive power influences the width of the Fresnel zones and the position of the Fresnel zone boundaries. The center thickness was neglected in the analysis in order to illustrate the influence of the division into Fresnel zones.

TAB. 2: Different variants for the dimensioning of the first and second region of the Fresnel lens

A locally larger height of the Fresnel zones usually leads to a wider Fresnel zone. In principle, the height of the Fresnel zone boundaries can be selected individually for each region or each Fresnel zone of the Fresnel lens.

Another variant for influencing the width of the Fresnel zones and the position of the Fresnel zone boundaries is shown in FIG. 9. Part of the refractive power of the Fresnel lens 900 can be generated from the back side 901, i.e. the non-structured side, of the Fresnel lens 900. This makes it possible to reduce the curvature on the front side 902 of the Fresnel lens 900, so that the distance between the Fresnel zones can be increased.

Saddle surfaces of such a lens geometry can be used for cylinder correction.

While FIG. 9 shows a scenario in which the rear side 901 of the Fresnel lens 900 is not Fresnel-coated, it would be conceivable that Fresnel zones are formed on both the front side 902 and the rear side 901.

It is also conceivable to provide the Fresnel lens with a wedge structure on the back in order to direct the AR image to be displayed into the viewer's pupil in a decentralized display region. In principle, it is also conceivable to use a Wedge structure on the front to shift the central Fresnel zone.

FIG. 10 shows a Fresnel lens 220 in which the flanks 1011 of the Fresnel zones are aligned with respect to the pivot point of the observer's eye 1020 (cf. zero viewing direction 161), so that the observer perceives the zone boundaries less.

In the Fresnel lens shown in FIG. 11, the Fresnel zones in the second region 92 have a constant height 1198. The Fresnel zones in the first region 91 have a variable height 1199; the average height in the first region 91 is greater than in the second region 92.

FIG. 12 illustrates the lateral extent of the display region 180 in relation to the first region 91 and the second region 92. The first region 91 extends along the display region 180. In this example, the first region 91 covers the entire display region 180 (cf. TAB. 1 , Scenario 1 and Scenario 3B). Other variants would also be conceivable, i.e. the first region 91 could have a smaller or larger lateral extent than the display region 180. This can be desirable, for example, if the optical resolution of the display region 180 varies or if the display region has such a large extent that the peripheral area of the field of view is also used for the AR image. Example scenarios were discussed in TAB. 1.

From a comparison of FIG. 13 with FIG. 14 it is clear how a specific lateral extent of the first region 91 can be determined based on the corneal vertex distance 82 (distance between the glass side of the spectacle lens and the cornea; varies between FIG. 13 and FIG. 14) and the viewing angle range 81 around the zero viewing direction 161 (the viewing angle range 81 is the same for FIG. 13 and FIG. 14). In the example of FIG. 14, the first region 91 is smaller than in FIG. 13 (cf. TABLE 1: Scenario 3A). Furthermore, an inclination angle of the head-mounted optical system, a mounting disc angle of the head-mounted optical system and/or the eyebox could also be taken into account. In summary, various aspects related to the geometric design of Fresnel lenses for an HMD for AR applications were disclosed above. These techniques were described in particular in connection with a pull lens. Alternatively or additionally, such techniques can also be used for a push lens.

Claims

patent claims 1 . Head-mounted optical system (200) for augmented reality applications, the head-mounted optical system (200) comprising: - a waveguide (210) arranged to generate an image in a display region (180) of the waveguide (210) for an eye located on one side of the waveguide (210), - a Fresnel lens (220), the Fresnel lens (220) comprising a first region (91) having a first optical resolution and at least a second region (92) having a second optical resolution, at least the first region (91) being arranged to at least partially image the display region (180), the first optical resolution being different from the second optical resolution. 2. Head-mounted optical system (200) according to claim 1, wherein the first region (91) has a first zone boundary number of Fresnel zones (302), wherein the second region (92) has a second zone boundary number (303) of Fresnel zones, wherein the second zone boundary number is greater than the first zone boundary number. 3. A head-mounted optical system (200) according to claim 1 or 2, wherein the first region (91) has a first zone boundary density of Fresnel zones (302), wherein the second region (92) has a second zone boundary density of Fresnel zones, wherein the second zone boundary density is greater than the first zone boundary density. 4. Head-mounted optical system (200) according to one of the preceding claims, wherein a first minimum distance from Fresnel zone boundaries in the first region (91) and a second minimum distance from Fresnel zone boundaries in the second region (92) differ. 5. Head-mounted optical system (200) according to one of the preceding claims, wherein the first region (91) has no Fresnel zone boundaries. 6. Head-mounted optical system (200) according to one of the preceding claims, wherein a first average height (1199) of Fresnel zones in the first region (91) differs from a second average height (1198) of Fresnel zones in the second region (92). 7. Head-mounted optical system (200) according to one of the preceding claims, wherein the first optical resolution is equal to or better than an optical resolution of that part of the image imaged by the first region. 8. Head-mounted optical system (200) according to one of the preceding claims, wherein the first optical resolution is better than the second optical resolution. 9. A head-mounted optical system (200) according to any preceding claim, wherein the first optical resolution is equal to or better than 1 arcmin. 10. Head-mounted optical system (200) according to one of claims 1 to 8, where the first optical resolution is worse than 1 arcmin, optionally worse than 4 arcmin. 11. Head-mounted optical system (200) according to one of the preceding claims, wherein the second optical resolution is worse than 4 arcmin, optionally worse than 10 arcmin. 12. Head-mounted optical system (200) according to one of the preceding claims, wherein an optical resolution of the image is worse than 1 arcmin, optionally worse than 4 arcmin. 13. Head-mounted optical system (200) according to one of the preceding claims, wherein the first region (91) and the display region (180) are arranged in the region of a zero viewing direction (161). 14. Head-mounted optical system (200) according to one of the preceding claims, wherein the first region (91) has a lateral extent corresponding to viewing angles in the range of +/-1° to +/-15° around a zero viewing direction (161), or wherein the first region has a lateral extent corresponding to viewing angles of more than +/-15° around the zero viewing direction (161). 15. Head-mounted optical system (200) according to one of the preceding claims, wherein the Fresnel lens (220) is configured to shift, in particular to draw, a focal plane of the image to a predefined distance from the waveguide (210). 16. Head-mounted optical system according to one of the preceding claims, wherein the Fresnel lens (220) is adapted to correct a visual impairment of a wearer of the head-mounted system. 17. Head-mounted optical system according to one of the preceding claims, wherein a lateral extent of the first region (91) and a position of the first region (91) are adapted to an inclination angle of the head-mounted optical system, a frame disc angle of the head-mounted optical system, a corneal vertex distance and/or an eyebox of the head-mounted optical system. 18. Head-mounted optical system (200) according to one of the preceding claims, wherein Fresnel zones are formed on the side of the Fresnel lens (220) facing the waveguide. 19. Head-mounted optical system (200) according to one of the preceding claims, wherein Fresnel zones are formed on the side of the Fresnel lens (220) facing away from the waveguide. 20. Head-mounted optical system (200) according to one of the preceding claims, wherein the Fresnel lens (220) has a wedge profile, in particular on the side of the Fresnel lens (220) facing the waveguide (210) and/or on the side facing away from the waveguide (210). 21. Head-mounted optical system (200) according to one of the preceding claims, wherein the side of the Fresnel lens (220) opposite the Fresnel zone has a further surface with optical effect. 22. Head-mounted optical system (200) according to one of the preceding claims, further comprising: - a second lens arranged on a side of the waveguide opposite the Fresnel lens, and wherein the second lens neutralizes or reduces a shift in the focus position caused by the Fresnel lens. 23. Head-mounted optical system (200) according to claim 22, wherein the Fresnel lens (220) and the second lens are designed as a push-pull lens system. 24. Head-mounted optical system (200) for augmented reality applications, the head-mounted optical system (200) comprising: - a waveguide (210) arranged to generate an image in a display region (180) of the waveguide (210) for an eye located on one side of the waveguide (210), - a Fresnel lens arranged on a side of the waveguide (210) opposite the eye side and comprising a first region (91) with a first optical resolution and at least one second region (92) with a second optical resolution, wherein the first optical resolution differs from the second optical resolution.