Abstract: A lens driving device is provided, including: a holder member; a bobbin disposed at an inner side of the holder member; a magnet disposed at the holder member; a first coil unit disposed at the bobbin, and facing the magnet; a first support member coupled to the holder member and the bobbin; and a detection sensor disposed at the bobbin, and configured to detect magnetic force of the magnet, wherein the magnet includes a facing surface and an opposite surface disposed at an opposite side of the facing surface, wherein a polarity of the facing surface and a polarity of the opposite surface are different from each other, wherein a polarity of an upper portion of the facing surface and a polarity of a lower portion of the facing surface are different from each other. According to an embodiment, Hall output detected by the detection sensor can be enhanced.

Abstract: A mobile device case for coupling around a mobile device that includes a miniature camera module has defined therein a lens attachment aperture shaped both to permit light from an object to be captured as a digital image to travel along the optical path of the miniature camera module to a built-in image sensor of the miniature camera module of the mobile device, and to facilitate stable coupling of an auxiliary lens in optical alignment with the miniature camera module. The case comprises electric circuit components positioned to balance an attached auxiliary lens approximately at a grip location.

Abstract: An optical imaging lens set includes an aperture stop, a first lens element to a sixth lens element from an object side toward an image side along an optical axis. The first lens element has an image-side surface with a convex portion in a vicinity of its periphery. The second lens element has an image-side surface with a concave portion in a vicinity of the optical axis and a convex portion in a vicinity of its periphery. The third lens element is made of plastic. The fourth lens element has an image-side surface with a concave portion in a vicinity of its periphery. The fifth lens element is made of plastic. The sixth lens element is made of plastic and has an object-side surface with a concave portion in a vicinity of the optical axis.

Abstract: An optical imaging lens includes: a first, second, third, fourth, fifth and sixth lens element. The first lens element has positive refracting power, an object-side surface with a convex part in a vicinity of the optical axis, and a convex part in a vicinity of its periphery, the second lens element has negative refracting power, an image-side surface with a convex part in a vicinity of its periphery, the third lens element has an image-side surface with a concave part in a vicinity of the optical axis, the fourth lens has an image-side surface with a convex part in a vicinity of the optical axis, the fifth lens element has object-side surface with a concave part in a vicinity of the optical axis, and the sixth lens element has an image-side surface with a concave part in a vicinity of the optical axis.

Abstract: In an optical imaging lens set a first lens of positive refractive power has an object-side surface with a convex portion around the optical axis and a convex portion around its periphery, an image-side surface with a convex portion around its periphery, a second lens of negative refractive power has an object-side surface with a concave portion around its periphery, a third lens has an image-side surface with a concave portion around its periphery, a fourth lens has an image-side surface with a convex portion around the optical axis, a fifth lens has an object-side surface with a concave portion around its periphery, a sixth lens has an image-side surface with a concave portion around the optical axis and a convex portion around its periphery. The sum of total five air gaps AAG and a second lens thickness T2 satisfy AAG/T2?3.6.

Abstract: An imaging lens includes first to sixth lens elements arranged from an object side to an image side in the given order. Through designs of surfaces of the lens elements and relevant optical parameters, a short system length of the imaging lens may be achieved while maintaining good optical performance.

Abstract: An imaging lens includes first to sixth lens elements arranged from an object side to an image side in the given order. Through designs of surfaces of the lens elements and relevant lens parameters, a short system length of the imaging lens may be achieved while maintaining good optical performance.

Abstract: Present embodiments provide for mobile devices and optical imaging lens thereof. Optical imaging lens may comprise an aperture stop and six lens elements positioned sequentially from an object side to an image side. Through controlling the convex or concave shape of the surfaces of the lens elements and designing parameters satisfying at least one inequality, the optical imaging lens may exhibit better optical characteristics and the total length of the optical imaging lens may be shortened.

Abstract: Present embodiments provide for a mobile device and an optical imaging lens thereof. The optical imaging lens may comprise an aperture stop and six lens elements positioned sequentially from an object side to an image side. Through controlling the convex or concave shape of the surfaces of the lens elements and designing parameters satisfying at least one inequality, the optical imaging lens may exhibit better optical characteristics and the total length of the optical imaging lens may be shortened.

Abstract: Present embodiments provide for a mobile device and an optical imaging lens thereof. The optical imaging lens may comprise an aperture stop and six lens elements positioned sequentially from an object side to an image side. Through controlling the convex or concave shape of the surfaces of the lens elements and designing parameters satisfying at least one inequality, the optical imaging lens may exhibit improved optical characteristics and the total length of the optical imaging lens may be shortened.

Abstract: An imaging lens is constituted essentially by six lenses, including, in order from the object side to the image side: a first lens having a positive refractive power and is of a meniscus shape with a concave surface toward the object side; a second lens having a positive refractive power and a convex surface toward the object side; a third lens having a negative refractive power; a fourth lens; a fifth lens; and a sixth lens having a negative refractive power. An aperture stop is positioned at the object side of the surface toward the object side of the third lens.

Abstract: An imaging lens includes a first lens; a second lens; a third lens; a fourth lens; and a fifth lens, arranged from an object side to an image plane side. The first lens is formed in a shape so that a curvature radius of a surface on the object side is positive. The third lens is formed such that a curvature radius of a surface on the object side is positive. The fourth lens is formed such that a surface on the object side and a surface on the image plane side are aspheric. The fifth lens is formed such that a surface on the object side and a surface on the image plane side are aspheric, and a curvature radius of the surface on the image plane side is positive near an optical axis thereof. The fourth lens and the fifth lens have specific Abbe's numbers.

Abstract: An imaging lens includes a first lens; a second lens; a third lens; a fourth lens; and a fifth lens, arranged from an object side to an image plane side. The first lens is formed such that a curvature radius of a surface on the object side is positive. The third lens is formed such that a curvature radius of a surface on the object side is positive. The fourth lens is formed such that a surface on the object side and a surface on the image plane side are aspheric. The fifth lens is formed such that a surface on the object side and a surface on the image plane side are aspheric, and a curvature radius of the surface thereof on the image plane side is positive near an optical axis thereof. The first lens and the third lens have specific focal lengths.

Abstract: An optical image capturing lenses includes, in order from an object side to an image side, a front lens group, a stop, and a rear lens group. The front lens group includes, in order from the object side to the image side, at least a first lens element and a second lens element. The first lens element has a convex object-side surface and a concave image-side surface. The rear lens group includes, in order from the object side to the image side, at least a third lens element, a fourth lens element, a fifth lens element, and a sixth lens element. The sixth lens element is made of plastic material. The object-side surface and the image-side surface of the sixth lens are aspheric. The sixth lens element has at least one inflection point formed on at least one of the object-side surface and the image-side surface thereof.

Abstract: The present disclosure provides an image capturing optical system comprising: a positive first lens element having a convex object-side surface; a negative second lens element having a concave object-side surface; a third lens element; a fourth lens element having a convex object-side surface and a concave image-side surface, the object-side surface and the image-side surface thereof being aspheric; a fifth lens element having a concave image-side surface concave, both of the object-side surface and the image-side surface being aspheric, at least one of the object-side surface and the image-side surface having at least one convex shape in an off-axis region thereof.

Abstract: An optical imaging lens includes first, second, third, fourth, fifth, and sixth lens elements, each having an object-side surface facing toward an object side and an image-side surface facing toward an image side. The image-side surface of the first lens element comprises a concave portion in a vicinity of a periphery of the first lens element and the image-side surface of the second lens element comprises a convex portion in a vicinity of the optical axis. The fourth lens element has positive refractive power. The image-side surface of the sixth lens element comprises a concave portion in a vicinity of the optical axis and the optical imaging lens as a whole has only the six lens elements having refractive power.

Abstract: Provided herein are an optical zoom lens capable of actively zooming in and/or out and an apparatus using the same. The optical zoom lens may include an electro-active polymer layer configured to be deformable in response to an electric signal; a lens structure formed over the electro-active polymer layer; a first electrode formed on an upper surface of the electro-active polymer layer; a second electrode formed on a lower surface of the electro-active polymer layer; and a control circuit unit configured to apply a voltage to the first electrode and the second electrode. The optical zoom lens, having a structure of a combination of the polymer lens structure and the electro-active polymer actuator, is able to embody active variation in position of the polymer lens structure body and control therefor using deformation of the electro-active polymer layer by an electric signal.

Abstract: A method for packaging applies to packaging a plurality of wafer-level lenses. Each wafer-level lens includes (a) a substrate with opposite facing first and second surfaces and (b) a respective lens element on at least one of the first and second surfaces. Each lens element has a lens surface facing away from the substrate. The method includes partially encasing the plurality of wafer-level lenses with a housing material to produce a wafer of packaged wafer-level lenses. In the wafer of packaged wafer-level lenses, the housing material supports each of the plurality of wafer-level lenses by contacting the respective substrate, and the housing is shaped to form a plurality of housings for the plurality of wafer-level lenses, respectively.

Abstract: A projection system includes a first lens group to an n-th lens group with n being 6 or 7 sequentially arranged from a enlargement side. The first lens group includes a first-first lens group and a first-second lens group sequentially arranged from the enlargement side. The two lens groups are separated from each other by a variable distance for image plane correction. The first lens group and the n-th lens group are fixed and the second lens group to the (n?1)-th group are moved when the magnification is changed. The following conditional expression is satisfied: 2<BF/fw<2.8, where fw represents the focal length of the projection system operating at the wide angle end, and BF represents the air conversion length of the back focal distance of the projection system.

Abstract: A projection lens includes a first lens group and a second lens group, all of which are arranged in sequence from a projection side to an image source side along an optical axis. The first lens group is with negative refractive power and includes a first lens with negative refractive power. The second lens group is with positive refractive power and includes a second lens, a third lens, a fourth lens and a fifth lens, wherein the second lens is with positive refractive power, the third lens is with negative refractive power, the fourth lens is a concave-convex lens with positive refractive power and the fifth lens is with positive refractive power. The first lens group and the second lens group satisfy ?1.8<f1/f2<?1.42, wherein f1 is an effective focal length of the first lens group and f2 is an effective focal length of the second lens group.

Abstract: Provided is a variable power optical system (ZL) used for an optical apparatus such as a camera (1). The variable power optical system (ZL) includes, in order from an object: a first lens group having positive refractive power; a second lens group having negative refractive power; a third lens group having negative refractive power; a fourth lens group having positive refractive power; and a fifth lens group. Upon zooming from a wide-angle end state to a telephoto end state, a distance between each of the lens groups changes, and upon focusing from an object at infinity to an object at a close distance, the third lens group moves in the optical axis direction.

Abstract: The 1-2nd lens group is divided into three lens groups which move when focusing is performed during the magnification change. Even in a case in which the second optical group is formed of one mirror, it is possible for a primary image to contain appropriate aberration and to hereby reduce aberration of an image which is finally projected onto a screen through the second optical group.

Abstract: The invention relates to a microscope having an objective that focuses illuminating light to an illuminating light focus, and having a light-guiding fiber which transports the illuminating light and at whose end is arranged a fiber coupler that couples the illuminating light out of the light-guiding fiber and generates a preferably collimated illuminating light bundle. An element for modifying the shape of the illuminating light focus, which is prealigned relative to the illuminating light bundle to be coupled out, is arranged in or on the fiber coupler.

Abstract: A plan apochromat corrected microscope objective including multiple subsystems with optical components and/or component groups, wherein an optical component or a component group can move axially in the interior of the microscope objective. The axially movable component or the component group includes a concave-convex lens or lens group oriented toward the object plane, with a form factor of: X = c 1 + c 2 c 1 - c 2 wherein X is the form factor, c1 is the curvature of the surface oriented in the direction of the object plane, and c2 is the curvature of the surface oriented in the direction of the image plane, of the axially movable lens or lens group, X lying within a range from ?8<X<?1.

Abstract: A microscope illumination apparatus includes: a dark-field illumination unit including a light source that is arrangeable in an outer periphery of an observation light path of a microscope, the dark-field illumination unit being removably provided in the outer periphery of the observation light path; a detector that detects that the dark-field illumination unit has been arranged in a prescribed position of the outer periphery of the observation light path, and outputs a turn-on signal; and a controller that controls the light source to be turned on in accordance with the detection of the detector.

Abstract: Disclosed are several technical approaches of using low coherence interferometry techniques to create an autofocus apparatus for optical microscopy. These approaches allow automatic focusing on thin structures that are positioned closely to reflective surfaces and behind refractive material like a cover slip, and automated adjustment of focus position into the sample region without disturbance from reflection off adjacent surfaces. The measurement offset induced by refraction of material that covers the sample is compensated for. Proposed are techniques of an instrument that allows the automatic interchange of imaging objectives in a low coherence interferometry autofocus system, which is of major interest in combination with TDI (time delay integration) imaging, confocal and two-photon fluorescence microscopy.

Abstract: The invention relates to a microscope having an illumination beam path with wide field illumination of a sample and a first detection beam path having a spatially resolved surface receiver, which is reached by a first part of the detection light coming from the sample via the first detection beam path, or an image divider assembly for a microscope. In order to lengthen the optical path length, at least a second part of the detection light coming from the sample is masked out of the detection beam path and, via deflection means belonging to the detection beam path, is led into a second detection beam path and, preferably via further deflection means, is deflected back in the direction of the detection in such a way that detection light is applied to at least two partial regions beside one another on the surface receiver.

Abstract: In one aspect an imaging system includes: an illumination system including an array of light sources; an optical system including one or more lens arrays, each of the lens arrays including an array of lenses, each of the lenses in each of the one or more lens arrays in alignment with a corresponding set of light sources of the array of light sources; an imaging system including an array of image sensors, each of the image sensors in alignment with a corresponding lens or set of lenses of the one or more lens arrays, each of the image sensors configured to acquire image data based on the light received from the corresponding lens or set of lenses; a plate receiver system capable of receiving a multi-well plate including an array of wells, the plate receiver system configured to align each of the wells with a corresponding one of the image sensors; and a controller configured to control the illumination of the light sources and the acquisition of image data by the image sensors, the controller further configure

Abstract: The invention is based on an autofocus method in which light from a light source is focused at a measurement light focus in a sample and is reflected from there and the reflected light is guided through an optical system in two light paths onto at least two detector elements. In order to achieve fast and accurate automatic focusing on the sample, it is proposed that the measurement light focus is moved in layers of the sample which reflect light to different extents, and the detector elements are arranged in such a way that, in this case, profiles of a radiation property registered by the detector elements are different and a focus position is set in a manner dependent on the profiles.

Abstract: A structured illumination microscope device includes acquisition unit that repeats a series of processing steps including controlling a combination of a wave vector and phase of fringes and acquiring N images; and computing unit in which processing for demodulating the image of the sample using a required M types of images from an image set of consecutive P images. The M types of images include: Q types (where Q?3) of modulated images; and one type of modulated image having the wave vector in common with and the phase differing from at least one among the Q types of modulated images, or one unmodulated image. An arrangement of the N images satisfies the uniformity condition, meaning “intensity distribution of the fringes is spatially uniform when accumulated between the N images” and the refresh condition, meaning “the M types of images are always included in the image set”.

Abstract: An optical module includes a circuit substrate that has a concave portion and a flat surface, an optical sensor that is disposed inside a space, and an optical filter device that has a base which accommodates a variable wavelength interference filter and has a light-through hole through which light emitted from the variable wavelength interference filter passes and a first glass member which is disposed in the light-through hole. The first glass member is positioned inside the space. The base is bonded to the flat surface. The distance between the first glass member and the optical sensor is set to a distance at which light emitted from the variable wavelength interference filter does not interfere between the first glass member and the optical sensor.

Abstract: An optical device with a light source and a detector is provided. A digital micromirror device positioned between the detector and the light source may deflect light beams projected from the light source. An aperture in front of the detector may block an incoming light beam from the detector when the incoming light beam is incident on the detector outside of a passable incident range and including an aperture opening configured to pass the incoming light beam to the detector when the incoming light beam is incident on the detector within a passable incident range. The digital micromirror device may rotate between a first position causing the light beam to pass through the aperture opening and a second position causing the light beam to be blocked by the aperture. The optical device may be configured to operate as a shutter, chopper, modulator and/or deflector.

Abstract: Provided is an actuator having a structure with which stress concentration on an elastic member of the actuator can be reduced in a manufacturing process thereof and breakage of the elastic member can be inhibited. The actuator includes a movable member, an elastic member configured to connect the movable member and a supporting member to each other, and an electrode pair having a comb electrode structure for displacing the movable member in a direction perpendicular to a reflective surface in which all movable comb electrodes extending from the movable member are substantially in parallel with one another, and a portion of the elastic member, which is located at a beginning of extension from the movable member, is substantially in parallel with the movable comb electrodes.

Abstract: A substrate, a plurality of mirrors disposed on one surface of the substrate to be separated from the substrate, a mirror support post disposed between the substrate and the mirror and connected to a part of the mirror to support the mirror, a first electrode disposed between a first portion of the mirror and the substrate on the substrate, a second electrode disposed between the substrate and a second portion facing the first portion are included. The mirror includes a thin portion in which a thickness of at least a part of an end portion of a rear surface of the mirror is thinner than a thickness of a middle of the mirror.

Abstract: An optical fiber scanner is provided with an illumination optical fiber that guides light and emits the light from a distal end thereof; a plurality of piezoelectric elements that are secured on a side surface of the illumination optical fiber, that have polarizations in radial directions of the illumination optical fiber, and that vibrate the illumination optical fiber when an alternating voltage is applied in the polarization directions; and a vibration suppressing part that suppresses vibrations in the radial directions generated at a position of the illumination optical fiber away from the piezoelectric elements toward a base end.

Abstract: A method determines an identification value of a rotatable element having a plurality of mirror facets. The identification value is determined based on a facet scan duration obtained of the mirror facets or based on a prismatic error of the mirror facets.

Abstract: A strap system and a method for wearing the strap system are disclosed. The strap system includes a first flexible segment comprising a first stretchable band; a first semi-rigid segment to conform to a portion of the user's head and comprising a first arc portion to extend from above a user's first ear to below the user's occipital lobe; and a first rigid guide segment connected to the first flexible segment and the first semi-rigid segment. The first flexible segment extends beyond a first end of the first rigid guide segment and the first semi-rigid segment extends from a second end of the first rigid guide segment, the first and second ends of the first rigid guide segment being opposite to each other in a lateral dimension. The first flexible segment is stretchable within the first rigid guide segment along the lateral dimension.

Abstract: A head-up display makes an observer visually recognize an image, which is optically reflected by a combiner, as a virtual image. The head-up display has a main body unit and an attaching unit. The main unit has a light source which is a projection unit for projecting the image. The attachment unit is configured such that the attachment unit can be attached to a sun visor that is provided to a vehicle. The joint member has a rotation shaft that rotatably joints the main unit to the attachment unit. The damper reduces vibration of the main unit in the rotational direction of the rotation shaft. When the rotation shaft of the joint member is attached to the sun visor by the attachment unit, the rotation shaft is substantially parallel to a sun visor supporting shaft so the sun visor can rotate in the direction in which the sun visor opens/closes.

Abstract: A pair of spectacles equipped with at least one lens and intended to be worn by a user, the spectacles comprising displaying means allowing the data to be projected into a field of view of the user, and an anti-glare screen provided with a variable transmission coefficient allowing the intensity of incident light intended to pass through the lens toward the user to be attenuated, the displaying means and the anti-glare screen being positioned so that some of the rays emitted by the displaying means, in order to display the data, strike the anti-glare screen, the spectacles being configured to adapt the transmission coefficient of the anti-glare screen depending on the intensity of the incident light, the displaying means furthermore being coupled to the anti-glare screen so that the displaying means display data when the anti-glare screen transmits the light.

Abstract: Methods and systems for image display with a head-mounted device, wherein customization of the display is achieved by taking into account wearer-specific visualization parameters. Such visualization parameters include parameters pertaining to the design of the device and/or wearer ophthalmic data such as wearer prescription data. Preferred images are computer-generated holographic images.

Abstract: An optical system for a head-worn computer may include a light source positioned within the head-worn computer and adapted to project non-polarized illuminating light towards a partially reflective partially transmissive surface such that the illuminating light reflects through a field lens and towards a reflective display and a polarizing film adjacent to a surface of the reflective display that polarizes the illuminating light after it passes through the field lens. The illuminating light reflects off a surface of the reflective display, forming image light which is then analyzed by the polarizing film prior to being transmitted through the field lens and then through the partially reflective partially transmissive surface to a non-polarizing lower display optical system adapted to present the image light to an eye of a user wearing the head-worn computer.

Abstract: A compact optical system with improved contrast for a head-worn computer includes a light source including a lens with positive optical power positioned within the head-worn computer and adapted to project converging illuminating light towards a partially reflective partially transmissive surface wherein the illuminating light forms a spot with an area smaller than the light source on the partially reflective partially transmissive surface prior to being reflected as diverging illuminating light that passes through a field lens and towards a reflective display. The illuminating light reflects off a surface of the reflective display, forming diverging image light which is transmitted through the field lens and then through the partially reflective partially transmissive surface to a lower display optical system adapted to present the image light to an eye of a user wearing the head-worn computer.

Abstract: A display method using a display operable to display an image to a viewer is provided. The method includes detecting an initial position and/or orientation of an image view. Foreground portions of an image are rendered according to the detected initial position and/or orientation. Background portions of the image are rendered according to the detected initial position and/or orientation, at an image rendering rate. The following are performed at an image display rate higher than the image rendering rate: (i) detecting a current position and/or orientation of the image view; (ii) re-projecting the background portions of a most recent image according to any differences between the initial position and/or orientation and the current position and/or orientation of the image view; and (iii) combining the foreground portions and the re-projected background portions to generate an image for display.

Abstract: A wearable display device that employs a sliding structure is disclosed. The device includes: a frame; a main unit coupled to the frame; a non-bending part, which includes a camera configured to capture an image of a forward direction, and which is coupled to the main unit and is configured to slide to a first position; a hinge coupled with the non-bending part; and a bending part, which is configured to provide image information to a user, and which is coupled with the hinge to rotate within a predefined angle. When the bending part is rotated, the camera is exposed to the exterior and moves from the first position to a second position. The device prevents malfunctioning and damage even when the device is worn for extended periods, and it is possible to resolve the problem of the wearable display device obstructing the field of vision when not in use.

Abstract: A transmission type head mounted display device includes an imaging unit that images an outside scene and an image display unit that is capable of transmitting the outside scene. An imaging range of the imaging unit is set based on a set distance from the image display unit set based on a focal length of a user and an estimated field of view estimated as a field of view of the user who wears the image display unit.

Abstract: Positional error (erroneous display) of a virtual image displayed in correspondence to a specific object in a scenery outside a vehicle is suppressed so as to enable a viewer to recognize information without a sense of discomfort. A forward information acquisition unit estimates the position of a specific object outside a vehicle, and a first display means generates an information picture about the specific object. A picture position adjustment means adjusts the position at which the information picture is projected in accordance with the position of the specific object estimated by the forward information acquisition unit. A behavior detection means detects a behavior of the vehicle, and the picture position adjustment means corrects the position at which the information picture is projected on the basis of the vehicle behavior detected by the behavior detection means.

Abstract: A laser apparatus includes a plurality of laser modules each generating a laser line in a working plane. The laser modules are juxtaposed so that the laser lines generated by the modules combine into a single laser line. Each of the laser modules includes at least one laser line generator and a laser line shaper. The laser line shaper includes an array of microlenses such that the final laser line generated by each laser module has, in the working plane, a power density profile with a width (L90) at 90% of the maximum power density and a width (L10) at 10% of the maximum power density. A ratio L90/L10 is between 1/15 and ?.

Abstract: An embodiment relates to a device comprising an optical pipe comprising a core and a cladding, the optical pipe being configured to separate wavelengths of an electromagnetic radiation beam incident on the optical pipe at a selective wavelength through the core and the cladding, wherein the core is configured to be both a channel to transmit the wavelengths up to the selective wavelength and an active element to detect the wavelengths up to the selective wavelength transmitted through the core. Other embodiments relate to a compound light detector.

Abstract: A lensed beam-splitter prism array includes a beam-splitter substrate with a plurality of planar and parallel thin-film coatings each spanning a top substrate surface and a bottom substrate surface, and making an oblique angle therebetween, and a lens form layer formed on the top surface and having a plurality of lens forms, each lens form being above one of the plurality of coatings. A method for fabricating a lensed beam-splitter prism includes bonding a plurality of substrates to form a substrate stack having a coating between each adjacent substrate pair. The method also includes forming a stack slice by applying a plurality of parallel cuts at an oblique angle with respect to each coating. Each coating spans a first stack-slice surface and a second stack-slice surface opposing the first stack-slice surface. The method also includes forming a lens form layer on the first stack-slice surface spanning one or more coatings.

Abstract: A spectral splitting component is provided, having two faces, a planar front face comprising a dichroic treatment and a back face. It is intended to be placed downstream of a convergent objective. The back face is convex and forms a cylindrical surface defined by a generatrix of fixed direction moving perpendicularly along a circular arc comprising two ends, the plane passing through these two ends and parallel to the generatrix of the cylindrical surface forming a dihedral with the plane of the front face, the generatrix of the cylindrical surface being parallel to the edge of the dihedral.