Patent ID: 12188840

PARTS LIST

2—optical system, e.g., periscope or periscope system4—housing or enclosure6—pupil or aperture8—camera or image capture device10—lens system12—front principal plane13—rear principal plane14—imaging plane16—central plane18—intersection angle of two optical axes20—device under test (DUT), e.g., exit pupil expansion (EPE) of waveguide22—light ray coupling-in optics such as holographic grating, metasurface, micro/nano structures or prisms24—controller26—first set of light rays28—second set of light rays30—distant object32—human eye34—optical rays with a specific viewing angle36—optical rays with another viewing angle38—lens of extended reality (XR) device40—reference port, e.g., reference exit port42—lens system center axis44—plane upon which apertures are disposed46—focal point

Particular Advantages of the Invention

The present periscope enables parallelism tests to be conducted on XR devices, and in particular Exit Pupil Expansion (EPE) devices. The use of a single refractive optical system with two or more entrance pupils ensures the high precision and absolute stability of parallelism measurements, by eliminating any alignment issue or motion errors, e.g., when using a beam splitter to align and combine two channels or when using a motion stage to scan eye boxes. In addition, the diffraction-limited angular resolution offers the highest angular accuracy capable of discerning a slight angular and boresight deviation.

The present periscope enables parallelism tests to be conducted on XR devices as well as enabling measurements to be collected and used for obtaining virtual imaging distances (VIDs) of the XR devices. The present periscope allows lateral offsets to be used for measuring VIDs. Longitudinal offsets are always used in conventional methods including refocusing methods, interference methods or wavefront measurements. VID measurements using conventional methods have been challenging due to the small pupil size in XR devices. However, the use of lateral offsets and axial focal measurements can enhance the accuracy of VID measurements.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The term “about” is used herein to mean approximately, roughly, around, or in the region of. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. In general, the term “about” is used herein to modify a numerical value above and below the stated value by a variance of 20 percent up or down (higher or lower).

Depending on the virtual imaging distance (VID) of a virtual image projected by an extended reality (XR) device, an observer may perceive optical rays differently via both eyes compared to a single eye. When the XR device projects an object30far away or at infinity, optical rays are parallel to each of the two eyes. In this case, one or both eyes perceive optical rays similarly. However, when a projected object is closer to the observer, the two eyes of the observer see optical rays from the object with a vergence angle. In other words, the optical rays are not parallel between the two eyes, as shown inFIG.1.

Due to the exit pupil expansion (EPE), however, optical rays from the same view angle are parallel within the single eye. For instance, all rays labelled34come from the same viewing direction or all rays labelled36also come from the same viewing direction as shown inFIG.2and each set34,36of rays is parallel to the single eye.FIG.2depicts the parallelism of optical rays being maintained from the same viewing angle due to the exit pupil expansion (EPE) of a light emitter, e.g., a lens/waveguide38, a pancake lens system, freeform optics of an XR device, light engines or micro-display modules, or any devices and systems that produce virtual images. It shall be noted that only one lens38is shown.

FIG.3depicts an eye of an observer which perceives the same virtual image even when the eye box position is slightly offset within the EPE. Due to the EPE, the human eye32perceives exactly the same object even when the eye box position is slightly offset within the EPE when the human eye32moves across the width of the lens38. The EPE feature is critical in XR technology and applications due to the frequent movements of human eyeballs and head as well as the interpupillary distance (IPD) differences between various people. An entrance port22of the waveguide receives light rays or images to be displayed on an EPE of a waveguide20.

FIG.4is a diagram depicting a present refractive periscope2useful for measuring and comparing optical rays from different areas of an EPE device, e.g., a waveguide20.FIG.5is a diagram depicting a present refractive periscope2useful for measuring and comparing optical rays between the EPE area and a reference exit port40. These are two example devices under test (DUTs) or light emitters the present refractive periscope2is configured to test, with the apertures6of the periscope2disposed at a suitable distance, sufficient to receive two sets of light rays26,28. To evaluate extended-pupil parallelism, the EPE device is configured to project a parallel beam, which is equivalent to a point image at infinity, or a target pattern such as a crosshair at any virtual imaging distances. A periscope or periscope-like optical system can be used to select the optical rays from the EPE device at different areas of interest or different eye box locations and the optical rays are then compared to discern any angular or boresight deviations. Two or more apertures are required to sample the optical rays with adjustable aperture sizes and variable distances between apertures. Ideally, all optical rays are aligned in parallel when the VID is disposed at infinity. In this case, only one spot or crosshair is to be observed on the imaging plane of the periscope sensor. The angular resolution of the periscope is required to be diffraction-limited to ensure the effective detection of any possible misaligned optical rays.

FIG.6is a diagram depicting a present refractive optical system based on a present optical system, e.g., a refractive periscope with two or more pupils when the incoming light rays are received from an object disposed at infinity.FIG.7is a diagram depicting a present refractive optical system based on a present optical system, e.g., a refractive periscope with two or more pupils when the incoming lights are received from a negative object.FIG.8is a diagram depicting a present refractive optical system based on a present optical system, e.g., a refractive periscope with two or more pupils when the incoming light rays are received from a positive object. An object-telecentric optical lens system is required as the two or more pupils collect parallel optical chief rays. It is possible for the present refractive optical system to have one or more additional apertures or pupils6disposed equidistant to the lens system center axis42as those other pupils6along the same plane, i.e., plane44, on which the pupils6are disposed, to allow the periscope to be set up for parallelism tests and to allow measurements to be taken for VID determination in cases where there is more than one DUT20, a DUT with more than two EPE devices or a DUT with more than two sets of rays that are emitted at a time. The periscope2or optical system essentially includes an enclosure4including a front end and a rear end, a pair of apertures6configured to be disposed on the front end of the enclosure4on a central plane16(shown inFIG.10), and a refractive lens system10disposed interior of the enclosure4. A first aperture6of the pair of apertures6is configured to allow a first set of light rays into the enclosure4at a location of the lens system10before being redirected to be cast as a first spot on an imaging plane14. A second aperture6of the pair of apertures6is configured to allow a second set of light rays into the enclosure4at another location of the lens system10before being redirected to be cast as a second spot on the imaging plane14. Each aperture6of the pair of apertures6is configured to be variable in size, e.g., about 2-5 mm, using, e.g., a diaphragm mechanism, etc. The intersection angle18or6, of two optical axes is defined as follows:
θ=2 arctan(D/(2f)).  Equation (1):

Here, D is the distance between two pupil centers and f is the focal length of the refractive periscope2. Since each pupil6is offset from the lens system center axis42, an object will be imaged along the pupil axis, i.e., a dot dash line, rather than the lens system center axis42. For an object at infinity, the optical rays are parallel and they will be focused to the same focal point46of the periscope2regardless of pupil location. For a positive or negative object with a certain object distance or a VID, the images are laterally separated when optical rays pass through different pupils. Assuming d is the separation distance between two imaging centers through different pupils for the same object distance, d can be defined as follows:
d=2Δ tan(θ/2).  Equation (2):

Here, Δ is the offset of an imaging plane14from the lens system focus position which intersects the focal point46when the object is not at infinity. In one embodiment, the imaging plane14is an imaging plane of an image capture device8, e.g., a camera. In one embodiment, a controller24is functionally connected to a linear stage or another suitable positioner configured to manage the positioning of the imaging plane14and to position the imaging plane14at a location along lens system center axis42either manually or automatically. Based on imaging principles, A or an offset of the imaging plane14from the focus position of the lens system10when the light emitter or DUT is not optically disposed at infinity, can be resolved as follows:
Δ=((VID*f)/(VID−f))−f=f{circumflex over ( )}2/(VID−f),  Equation (3):
where the term f is the focal length of the periscope. Therefore, in addition to parallelism measurements, a VID can be resolved using the present optical system based on either a lateral or axial offset of the image center or VID can be resolved as follows:
VID=(f*(f+Δ))/Δ.  Equation (4):

Based on Equation (4), it shall be seen that VID is a function of Δ and thus d.FIG.9is a diagram depicting one or more factors considered in determining a VID shorter than infinity and the parallelism between incoming sets of light rays. For a long VID such as infinity, the above equation yields an accurate result. For shorter VIDs, the distance from the pupil plane44to the front principal plane12or the quantity L, is required to be considered.

So, the VID is required to be corrected as follows where VID′ represents the correct VID:
VID′=VID−L.Equation (5):

FIG.10is a top perspective view of the lens system of a present refractive optical system with a pair of pupils located within a central plane16.FIG.11depicts a lens system10useful for a present refractive periscope in imaging a negative object.FIG.12depicts a lens system10useful for a present refractive periscope in imaging an object disposed at infinity.FIG.13depicts a lens system10useful for a present refractive periscope in imaging a positive object. There are two apertures or stops where optical rays are sampled for measurements. The lens system includes only two glass types arranged symmetrically about the middle of the lens system.

FIG.14is a diagram depicting the optical design data for the present optical system. Compared to a conventional lens system, the present optical system has two or more apertures with diffraction-limited performance across all apertures and FOV as well as covering broad VID ranges. In one embodiment, the total length of the lens system is less than about 120 mm with a working distance, i.e., the entrance pupil to the first front lens, of about 50 mm, only four lens elements, i.e., two doublets with surfaces B1, B2, B3, C1, C2 and C3 and two singlets with surfaces A1, A2, D1 and D2, only two glass types and the lens system is compatible with specific requirements of the XR, Head-Up Display (HUD) and Near-Eye Display (NED) systems, making the present optical system very compact which is an important requirement for XR applications. The optical stop is placed in the front of the lens system to match human eye pupil.

FIG.15is a diagram depicting a present optical system useful for determining the parallelism of incoming sets of light rays and the VID of the DUT from which the incoming sets of light rays are emitted. The EPE device is configured to project a point image or a collimating light through each aperture of the present optical system2.FIG.16is a diagram depicting a present optical system useful for determining the parallelism of incoming sets of light rays and the VID of the DUT from which the incoming sets of light rays are emitted. Here, again, the DUT is an EPE device and it is configured to project a crosshair object pattern for ease of angular measurements. Other patterns can also be used as long as the patterns indicate some form of orientation. An orientation-indicating pattern, e.g., a crosshair, can provide additional measurement information such as rotation misalignment where the crosshairs, each exhibited by a set of light rays, can be determined to be angularly rotated with respect the central plane16or the other crosshair. The crosshairs or another pattern, e.g., checkerboard pattern, can also be used to measure the point spread function (PSF) or modulation transfer function (MTF) as well as contrast ratio (CR), which are all important in determining the optical quality of the EPE devices. Referring back toFIG.6-9,11as well asFIGS.15and16, it shall be seen that various images may be cast on the imaging plane14. In one embodiment, the image capture device8includes a controller24configured to receive an image of the first spot and the second spot.FIG.15shows the use of circular spots. The top right image depicts two sets of light rays that are not parallel as one of the spots is disposed to the upper right of a plane corresponding to the central plane16and the spot appears obscured or not round. The bottom right image depicts two sets of light rays that are parallel to one another as both of the spots are disposed along a plane that corresponds to the central plane16and the spots appear round and spread apart a distance d.FIG.16shows the use of crosshairs. Again, the top right image depicts two sets of light rays that are not parallel as one of the spots is disposed to the upper right of a plane corresponding to the central plane16and the spot appears obscured or an image of a distorted crosshair. The bottom right image depicts two sets of light rays that are parallel to one another as both of the spots are disposed along a plane that corresponds to the central plane16and the spots appear as crosshairs and spread apart a distance d. If at least one of the first spot and the second spot is not disposed on the central plane16or a plane corresponding to the central plane16, the first set of light rays and the second set of light rays are determined to not be disposed parallel to one another. In a present optical system2, as the parameters of the components and their locations are known, for every d, i.e., the distance between the first spot and the second spot, there is a corresponding value for D, i.e., the distance between the first set of light rays and the second set of light rays. The parameters d and D are related through Equations (1) and (2). For a D value, there can be multiple d values. Given a d value, a D value can be computed. As the D value is fixed, i.e., the distance between the center of the first set of light rays and the center of the second set of light rays, a computed D value which deviates from the fixed value is deemed to occur when d does not correspond with D. Therefore, if d does not correspond or agree with D, the first set of light rays and the second set of light rays are said to not be disposed parallel to one another. In one embodiment, the controller24is configured to determine if the first spot and the second spot are disposed on a central plane16of the optical system2and if a distance between the first spot and the second spot or d, corresponds with a distance between the first set of light rays and the second set of light rays or D. A spot may be detected when an image captured by the image capture device8is scanned for a round feature as inFIG.15or a crosshair feature as inFIG.16. This determination may also be performed manually by a technician who adjusts, determines and selects the position of the imaging plane14at which the sharpest spots may be obtained. As disclosed elsewhere herein, the selection of the position along the lens system center axis42at which spot images are obtained for parallelism and/or VID determination may be performed manually or automatically. If automatic positioning of the imaging plane14is desired, the spots are scanned automatically along the lens system center axis42for when they form the clearest or sharpest spots on the imaging plane14before an image at this position is obtained for further determination of parallelism between the sets of rays and/or the VID.

The present optical system includes several critical features required for XR metrology, e.g., an effective focal length (EFL) of about 180 mm, angular FOV of about 5 degrees, RGB broad band spectra, entrance pupil sizes of about 2-5 mm, a single optical lens system with two apertures to ensure the absolute accuracy of measurements, a diffraction-limited angular resolution to provide the highest angular accuracy capable of discerning slight angular and boresight deviation.

FIG.17is a diagram depicting the modulation transfer function (MTF) and spot diagrams with Airy disc (black circles) indicating diffraction-limit performance at D=30 mm, VID=−20 D and aperture=3 mm in diameter, of the present optical system.FIG.18is a diagram depicting the MTF and spot diagrams with Airy disc (black circles) indicating diffraction-limit performance at D=30 mm, VID=−10 D and aperture=3 mm in diameter, of the present optical system.FIG.19is a diagram depicting the MTF and spot diagrams with Airy disc (black circles) indicating diffraction-limit performance at D=30 mm, VID=−6 D and aperture=3 mm in diameter, of a present optical system.FIG.20is a diagram depicting the MTF and spot diagrams with Airy disc (black circles) indicating diffraction-limit performance at D=30 mm, VID=0 D and aperture=3 mm in diameter, of a present optical system.FIG.21is a diagram depicting the MTF and spot diagrams with Airy disc (black circles) indicating diffraction-limit performance at D=30 mm, VID=+6 D and aperture=3 mm in diameter, of a present optical system. The present optical system is suitable for object-space telecentric imaging with a long VID range or an object distance ranging from at least −20 D to +6 D for 3-mm apertures.

FIG.22is a diagram depicting spot diagrams of the present refractive optical system with the object distance or VID at infinity under aperture sizes of 5 mm and 4 mm.FIG.23is a diagram depicting spot diagrams of the present refractive optical system with the object distance or VID at infinity under aperture sizes of 3 mm and 2 mm. It shall be noted that the diffraction-limited performance under the various aperture sizes is important for XR metrology, to match the variable pupil sizes of human eyes.

FIG.24is a table summarizing various parameters under which the performance of the present refractive optical system remains diffraction-limited. It shall be noted that the VID number sign in diopter is reversed compared to mm, since it is convenient to do so, for calculations associated with measurements of DUTs with additional prescriptions or corrective lenses. The EPE area or whole eye box of XR devices is normally less than about 30 mm in diameter, so the 0-30 mm aperture distance or IPD is sufficient for the EPE device measurements. At VID=−167 mm, the imaging plane is close to the rear lens element, so it limits the negative VID range which can be extended by re-optimization of the optical system. In addition to the diffraction-limited angular resolution, the optical system also has diffraction-limited MTF performance under various apertures, e.g., from about 2 mm to about 5 mm, a size which matches the human eye pupil size and across the long range of VIDs from at least −10 D to +6 D. The 3-mm aperture is the most used one for various measurements in VR metrology. Therefore, it shall be understood then that the present optical lens system can be used for MTF measurements in addition to parallelism measurements.

FIG.25depicts imaging results of a crosshair object at −167 mm (+6 D) VID after passing through the present refractive optical system.FIG.26depicts imaging results of a crosshair object at infinity (0 D) VID after passing through the present refractive optical system.FIG.27depicts imaging results of a crosshair object at +167 mm (−6 D) VID after passing through the present refractive optical system. It shall be noted that the size of the crosshair object corresponds to a 5-degree full field angle and the imaging results are very sharp across the FOV due to the diffraction-limit performance of the refractive optical system.

FIG.28depicts measurement results of object distances or VIDs compared to preset values. In addition to angular measurements for EPE devices, the proposed refractive optical system can also be used to measure the VID or object distances which are some of the most important parameters for XR devices and applications. Using Equation (1) defined elsewhere herein and f=179.882 mm, 8=9.533 degrees, L=158.44 mm (from the aperture to the front principal plane12), EFL=179.882, the offset (Δ) of the imaging plane from the lens system focus position can be calculated based on Equation (2) defined elsewhere herein and the measured d values, where the aperture diameter is 3 mm. The VIDs can be calculated using Equation (5) defined elsewhere herein, based on the above measurements. It can be seen that the calculated results based on the measurements are very accurate compared to the preset VID values.FIG.29depicts a curve plotted using measurement results of object distances or VIDs vs. preset values, as shown inFIG.28. The dash line is a linear trendline. It shall be noted that the preset VID values are aligned extremely well with the calculated results based on the measurements. The results demonstrate that the present optical system is useful for precisely measuring VIDs based on the lateral offset (d). By contrast, conventional methods always use longitudinal offsets including refocusing methods, interference methods or wavefront measurements.

The detailed description refers to the accompanying drawings that show, by way of illustration, specific aspects and embodiments in which the present disclosed embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice aspects of the present invention. Other embodiments may be utilized, and changes may be made without departing from the scope of the disclosed embodiments. The various embodiments can be combined with one or more other embodiments to form new embodiments. The detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, with the full scope of equivalents to which they may be entitled. It will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of embodiments of the present invention. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon studying the above description. The scope of the present disclosed embodiments includes any other applications in which embodiments of the above structures and fabrication methods are used. The scope of the embodiments should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.