Abstract: An electronic device such as a head-mounted device may have a user display for displaying an image to a user. The image may be displayed at an eye box after passing through a lens that is interposed between the display and the eye box. A forward-facing component such as a forward-facing camera and/or an externally viewable display may be supported in the housing. The forward-facing component may be overlapped by an optical system. The optical system may have a double-folded light path that virtually shifts the position of the forward-facing component. The optical system may have a reflective polarizer and a partially reflective mirror that are separated by a given distance. The optical system may virtually shift the forward-facing component toward the eye box by twice the given distance.

Abstract: An electronic device may include a display system with a pixel array and a catadioptric lens. The display system may include a linear polarizer through which image light from the pixel array passes and a first quarter wave plate through which the light passes after passing through the polarizer. The lens may include a partial mirror, a second quarter wave plate, and a reflective polarizer. A third quarter wave plate may be formed between the linear polarizer and the pixel array to mitigate ghost images. Control circuitry may predict potential ghost images based on the geometry of the lens and data from an image frame. Tone mapping circuitry may adjust contrast of the image frame within a region overlapping the predicted ghost image. The control circuitry may adjust luminance of the image frame outside of the region overlapping the predicted ghost image.

Abstract: Some implementations provide a multi-focus display system that renders images at multiple focus distances for display in conjunction with the use of appropriately powered lenses. For example, an HMD may include a fast switching lens element that allows quickly alternating between two or more focus distances. The displayed images are configured to correspond to the alternating focus distances by adjusting a high-frequency part of the images. This can provide a more natural user experience that will include near objects that require the user's eye to focus on a close focal depth plane and far objects that require the user's eye to focus on a far focal depth plane. Moreover, the user experience can be provided with little or no loss of brightness and without requiring processor and resource intensive computations.

Abstract: Some implementations disclosed herein include devices, systems, and methods that display images at multiple focus distances on HMDs. The systems can avoid or mitigate vergence-accommodation conflict (VAC) and depth-blur conflict (DBC) experienced by users on traditional HMDs. Some implementations use a geometric phase lens and a circular polarization switch. The geometric phase lens passively changes its power depending on the light's circular polarization. The circular polarization switch changes the polarization of the light at appropriate times. With the circular polarization switch and geometric phase lens positioned over an HMD screen, the system can quickly change the apparent distance to the screen, which in turn changes the accommodative state of the eye needed to focus on the rendered image.

Abstract: A scene camera system that includes two or more mirrors that reflect light from a respective portion of a field of view (FOV) in front of the system and two or more cameras that each capture the light reflected by a respective one of the two or more mirrors. By using the mirrors to reflect the light, the cameras' entrance pupils are imaged to a location closer to a user's eyes to thus achieve a more accurate representation of the perspective of the user.

Abstract: Redirected walking methods are described that may be implemented in virtual reality (VR) systems for guiding or redirecting users' movements within constrained physical environments or in unconstrained physical environments, while the users perceive that they are freely moving about within virtual worlds provided by the VR systems. The methods may involve rendering views of a VR world based at least in part on a user's movements in the real environment, adjusting the views according to the user's position and movements with respect to objects in and boundaries of the real environment, and displaying the adjusted views to the user. In response to visual cues introduced by the methods, the user may adjust their path in the real world environment. For example, a rotation of the VR environment introduced by a shift of one or more views may cause the user to turn in a direction opposite to the rotation.

Abstract: Systems and methods for measuring brain activity of a subject are disclosed, comprising: positioning a plurality of electric field sensors at multiple positions on the exterior of a skull of the subject; measuring one to three components of a plurality of instantaneous electric field vectors generated by a plurality of electric field sources, the electric field vectors being measured by the plurality of electric field sensors; and determining brain activity of the subject based on the measurement of the plurality of instantaneous electric field vectors.

Abstract: A head-mounted device may include a display system and an optical system in a housing. The display system may have displays that produce images. The optical system may have Fresnel lenses through which a user of the head-mounted device may view the images. The Fresnel lenses may have concentric rings with slope facets and draft facets angled parallel to the chief rays. Light scattering in the Fresnel lenses may be reduced by coating the draft facets with opaque masking material and/or by aligning concentric rings of the opaque masking material that are supported on a transparent substrate with the draft facets. A central portion of the Fresnel lens that is free of facets may be enlarged to reduce scattering. The Fresnel lenses may have wedge-shaped cross-sectional profiles and may have outer portions that are thicker than inner portions. Gradient-index material may be used in forming the Fresnel lenses.

Abstract: A sensor system and process for measuring electromagnetic activity of a brain are provided. The system and process employ a sensor assembly having a plurality of electrodes arranged in a closely spaced arrangement and a processor to determine a weighted average of the signals indicative of an electric field generated by electromagnetic activity of the brain. The system provides a medical body area network of a subject including one or more of the sensor assemblies and one or more additional sensors, which may be within a smartphone or other wearable device.

Abstract: An electronic device may include a display with a concave surface. A linear polarizer may be formed on the concave surface. A quarter wave plate may receive light from the linear polarizer. A catadioptric lens may have first and second lens elements. The first lens element may have first and second opposing surfaces. The second lens element may have opposing third and fourth surfaces. The first surface may be convex and may face the display. The fourth surface may be concave. The second surface may be concave. The third surface may be convex and may match the second surface. An additional quarter wave plate may be formed as a coating on the third surface. A partially reflective coating may be formed on the first surface. A reflective polarizer may be formed as a coating on the fourth surface. An additional polarizer may be formed on the reflective polarizer.

Abstract: A scene camera system that includes two or more mirrors that reflect light from a respective portion of a field of view (FOV) in front of the system and two or more cameras that each capture the light reflected by a respective one of the two or more mirrors. By using the mirrors to reflect the light, the cameras' entrance pupils are imaged to a location closer to a user's eyes to thus achieve a more accurate representation of the perspective of the user.

Abstract: An electronic device may include a display with a concave surface. A linear polarizer may be formed on the concave surface. A quarter wave plate may receive light from the linear polarizer. A catadioptric lens may have first and second lens elements. The first lens element may have first and second opposing surfaces. The second lens element may have opposing third and fourth surfaces. The first surface may be convex and may face the display. The fourth surface may be concave. The second surface may be concave. The third surface may be convex and may match the second surface. An additional quarter wave plate may be formed as a coating on the third surface. A partially reflective coating may be formed on the first surface. A reflective polarizer may be formed as a coating on the fourth surface. An additional polarizer may be formed on the reflective polarizer.

Abstract: An electronic device may include a display with a concave surface. A linear polarizer may be formed on the concave surface. A quarter wave plate may receive light from the linear polarizer. A catadioptric lens may have first and second lens elements. The first lens element may have first and second opposing surfaces. The second lens element may have opposing third and fourth surfaces. The first surface may be convex and may face the display. The fourth surface may be concave. The second surface may be concave. The third surface may be convex and may match the second surface. An additional quarter wave plate may be formed as a coating on the third surface. A partially reflective coating may be formed on the first surface. A reflective polarizer may be formed as a coating on the fourth surface. An additional polarizer may be formed on the reflective polarizer.

Abstract: A sensor system and process for measuring electromagnetic activity of a brain are provided. The system and process employ a sensor assembly having a plurality of electrodes arranged in a closely spaced arrangement and a processor to determine a weighted average of the signals indicative of an electric field generated by electromagnetic activity of the brain. The system provides a medical body area network of a subject including one or more of the sensor assemblies and one or more additional sensors, which may be within a smartphone or other wearable device.

Abstract: Systems and methods for measuring brain activity of a subject are disclosed, comprising: positioning a plurality of electric field sensors at multiple positions on the exterior of a skull of the subject; measuring one to three components of a plurality of instantaneous electric field vectors generated by a plurality of electric field sources, the electric field vectors being measured by the plurality of electric field sensors; and determining brain activity of the subject based on the measurement of the plurality of instantaneous electric field vectors.

Abstract: Fluorine generation systems are provided that can include, in exemplary embodiments, a reactor configured to decompose a fluorine-comprising material. The reactor can include a plurality of chambers with at least one of the chambers being configured to receive the fluorine-comprising material. The chamber includes sidewalls with the exterior of the sidewalls being at least partially encompassed by heating elements. The system can also include a fluorine reservoir coupled to the reactor with the reservoir configured to receive fluorine upon the decomposition of the fluorine-comprising materials. Fluorine-generation processes are provided that can include, in exemplary embodiments, decomposing pellets of a fluorine-comprising material with the pellets having an average size of from about 1.0 mm to about 3.0 mm. Processes can also include decomposing a composition comprising manganese-fluoride.