Abstract: A method and apparatus operable for displacing an image, which is transmitted by electromagnetic radiation, perpendicular to the direction of radiation by use of a rotatably disposed radiation-refracting body, located in the path of an imaging beam, having a refractive index that differs from the surrounding medium with regard to the radiation used. The method allows for precise image displacement. The rotation of the radiation-refracting body causing the displacement is measured, and the change of the optical path length caused by the angular change in a further electromagnetic beam utilized for the measurement is determined by evaluating the superimposition of the radiation reflected by a planar reflecting surface with the incident electromagnetic radiation. Part of the electromagnetic measurement beam, which is reflected on the surface of the radiation-refracting body, strikes a measurement unit, which supplies an electrical signal as a function of the incident position of the electromagnetic radiation.

Abstract: The present invention relates to alignment marks for use on substrates, the alignment marks consisting of periodic 2-dimensional arrays of structures, the spacing of the structures being smaller than an alignment beam but larger than an exposure beam and the width of the structures varying sinusoidally from one end of an array to the other.

Abstract: An optical measuring element measures forces in at least one direction. The measuring element has a single-piece structure. There is an outside wall with notches introduced therein. Each notch defines parallel edges, and the notches define more or less elastically flexible zones in the structure and constitute the only connection between a first region and a second region of the structure. For optical distance measurements between the two regions of the structure, one or more optical fibers are each attached with one end thereof to a region of the structure such that reflective surfaces are located close to the ends. The reflective surfaces are firmly connected to another region. The optical fibers are disposed on the outside wall.

Abstract: A measurement system configured to measure a position of an object in a lithographic apparatus, includes at least three position detectors configured to detect the position of the object, the at least three position detectors each including a single or multi-dimensional optical encoder to provide at least six position values, the optical encoders being coupled to the object at different locations within a three dimensional coordinate system, wherein at least one position value is provided for each dimension of the three dimensional coordinate system, and wherein the measurement system is configured to calculate the position of the object within the three dimensional coordinate system from a subset of at least three of the six position values and to calculate an orientation of the object with respect to the three dimensional coordinate system from another subset of at least three of the six position values.

Abstract: A method is disclosed whereby a laser-based spherical coordinate measurement system is dynamically calibrated. A mechanical oscillator, such as, but not limited to, a Foucault pendulum is used to generate periodic motions which can be fitted to Fourier series models. The residuals between the experimental measurements and the model can provide information which can be used to calibrate the instrument. The calibration information is used to augment the ASME B89.4.19-2006 standard to improve sensitivity to cyclic errors and include the servo systems.

Abstract: The time delay (and therefore the OPD) between object and reference beams in an interferometer is manipulated by changing the spectral properties of the source. The spectral distribution is tuned to produce a modulation peak at a value of OPD equal to the optical distance between the object and reference arms of a Fizeau interferometer, thereby enabling the use of its common-axis configuration to carry out white-light measurements free of coherence noise. Unwanted interferences from other reflections in the optical path are also removed by illuminating the object with appropriate spectral characteristics. OPD scanning is implemented without mechanical means by altering the source spectrum over time so as to shift the peak location by a predetermined scanning step between acquisition frames. Finally, the spectrum is controlled on a pixel-by-pixel basis to create a virtual surface that matches the profile of a particular sample surface.

Abstract: An electro-optical distance-measuring unit (10) includes a light source (1), the emitted light of which in the distance-measuring unit (10) is guided onto a measurement path (8) at least by one polarizing beam splitter (3), an electro-optical modulator (5) and a retarder (6). Light that is returned along the measurement path (8) is guided at least by the retarder (6), the electro-optical modulator (5) and the polarizing beam splitter (3) onto a detector (4). The distance-measuring unit (10) furthermore includes a control and evaluation unit (11) for determining a length of the measurement path (8) in accordance with a modulation frequency of the electro-optical modulator (5) and a signal of the detector (4). The light source (1) has a broad spectrum of the emitted light and is preferably a super-luminescent diode.

Abstract: An interference measuring device 1 comprises light sources 11, 12, lenses 21 to 25, an aperture 31, an optical multiplexer 41, an optical branching filter 42, a half mirror 43, an imaging unit 51, an analyzing unit 52, a light receiving unit 61, a displacement detecting unit 62, a piezoelectric actuator 71, a drive unit 72, a mirror 73, a stage 81, a drive unit 82, and a control unit 90. According to a result of optical path length difference detection by the displacement detecting unit 62, the control unit 90 controls optical path length difference adjusting operations by the piezoelectric actuator 71 and stage 81 through the drive units 72 and 82 such that the optical path length difference becomes a plurality of target values in sequence. The control unit 90 subjects the moving operation by the piezoelectric actuator 71 to a feedback control such that the optical path length difference becomes each of the target values upon the moving operation by the stage 81 as well.

Abstract: In a method for gauging surfaces (7?), in which a frequency-modulated laser beam is generated, the laser beam is emitted onto the surface as measuring radiation (MS), the measuring radiation (MS) backscattered from the surface (7?) is received and the distance between a reference point and the surface (7?) is measured interferometrically, wherein the measuring radiation (MS) is emitted and received while the surface to be gauged is being scanned, and a measuring arm and a reference interferometer arm with a partially common beam path are used, deviations from the essentially perpendicular impingement of the measuring radiation (MS) on the surface (7?) are taken into account algorithmically during distance measurement and/or are avoided or reduced during scanning by controlling the emission of the measuring radiation (MS).

Abstract: A technique is disclosed which offers an improvement in the performance of an atom interferometric (AI) sensor, such as one that is used in an accelerometer or a gyroscope. The improvement is based on the recognition that the AI-based device, which is associated with superior low-frequency performance, can be augmented with a conventional device having a superior high-frequency performance, as well as a wider frequency response, compared with that of the AI-based device. The disclosed technique combines acceleration measurements from the AI-based device, which is characterized by transfer function G(s), with acceleration measurements from the conventional device that have been adjusted by a complementary function, 1??(s), where ?(s) is an approximation of G(s). The conventional device has a considerably wider bandwidth than that of the AI-based device, and the quasi-unity transfer function of the conventional device makes possible the 1??(s) adjustment of the measurements provided by the conventional device.

Abstract: Positional information of a movable body in a Y-axis direction is measured using an interferometer and an encoder whose short-term stability of measurement values excels when compared with the interferometer, and based on the measurement results, a predetermined calibration operation for obtaining correction information for correcting measurement values of the encoder is performed. Accordingly, by using measurement values of the interferometer, correction information for correcting the measurement values of the encoder whose short-term stability of the measurement values excels the interferometer is obtained. Then, based on the measurement values of the encoder and the correction information, the movable body is driven in the Y-axis direction with good precision.

Abstract: A method for out-of-plane displacement detection is disclosed. The out-of-plane displacement is detected by analyzing all the fringe density indexes calculated using the frequency-domain information extracted from a series of interference images of the sample vibrating at different frequencies. The present invention further discloses a method and an apparatus for resonant frequency identification by detecting the peak value of all the fringe indexes calculated at different scanning frequencies. With the identified resonant frequency, the full-field vibratory surface profile of the sample in various resonance modes can be reconstructed.

Abstract: A microphone having an optical component for converting the sound-induced motion of the diaphragm into an electronic signal using a diffraction grating. The microphone with inter-digitated fingers is fabricated on a silicon substrate using a combination of surface and bulk micromachining techniques. A 1 mm×2 mm microphone diaphragm, made of polysilicon, has stiffeners and hinge supports to ensure that it responds like a rigid body on flexible hinges. The diaphragm is designed to respond to pressure gradients, giving it a first order directional response to incident sound. This mechanical structure is integrated with a compact optoelectronic readout system that displays results based on optical interferometry.

Abstract: The present invention relates to a device for detecting or generating and modulating optical signals, and having an angular dispersive element arranged to change angles of the optical signals or carrier and/or reference rays brought to interference.

Abstract: A detector element array for an optical position measuring instrument, by way of such array a fringe pattern resulting in a detector plane can be converted into electrical scanning signals. The detector element array includes a plurality of light-sensitive detector elements disposed in matrix-like fashion in rows and columns. The plurality of light-sensitive detector elements include a first detector element in a first column of the columns, a second detector element in the first column that is adjacent to the first detector element and a third detector element in a second column of the columns that is adjacent to the first column, wherein the third detector element is diagonally adjacent to the first detector element. The detector element array includes a first switch that selectively directly connects the first detector element with the second detector element and a second switch that selectively directly connects the first detector element with the third detector element.

Abstract: An absolute position measurement apparatus measures an absolute position of an object to be measured using a first light source and a second light source which has coherency lower than that of the first light source. The absolute position measurement apparatus includes a measurement part which measures a point where phases of interference signals from the first and the second light sources coincide with each other or a point where an intensity of the interference signal from the second light source is maximized, an origin defining part which defines the point measured by the measurement part as an origin position, a phase storing part which stores the phase of the interference signal from the first light source at the origin position, an origin redefining part which redefines the origin position, and a position calculating part which calculates the absolute position of the object to be measured.

Abstract: A position measurement system for measuring positional coordinates of a point under measurement includes a first noise removal unit, a parameter determination unit and a second noise removal unit. The first noise removal unit removes noise from the measured positional coordinates to acquire first positional coordinate values. The parameter determination unit determines a noise removal parameter on a basis of the first positional coordinate values. The second noise removal unit again removes noise from the first positional coordinate values with using the noise removal parameter, to acquire second positional coordinate values.

Abstract: There is provided a pointing device including a light source that provides incident light for illuminating a front surface of a substrate at a predetermined angle of incidence, the substrate having a rear surface as well as the front surface and being transparent to the wavelength of the incident light, a detector that detects the intensity of speckle light from the front and rear surfaces, the speckle light generated from the incident light, and legs that are in contact with the substrate and charge the front surface of the substrate through friction between the legs and the substrate.

Abstract: A method for calibrating an encoder in a lithographic apparatus, the encoder including a sensor and a grating, the encoder configured to measure a position of a moveable support of the lithographic apparatus, the method including measuring a position of the moveable support using an interferometer; and calibrating the encoder based on the position of the moveable support measured by the interferometer.

Abstract: A method for out-of-plane displacement detection is disclosed. The out-of-plane displacement is detected by analyzing all the fringe density indexes calculated using the frequency-domain information extracted from a series of interference images of the sample vibrating at different frequencies. The present invention further discloses a method and an apparatus for resonant frequency identification by detecting the peak value of all the fringe indexes calculated at different scanning frequencies. With the identified resonant frequency, the full-field vibratory surface profile of the sample in various resonance modes can be reconstructed.

Abstract: An apparatus includes an interferometer configured to generate an interference pattern by combining test light from a test object with reference light reflected from a reference object, the interferometer being further configured to direct at least a first part of a monitor test beam to the test object at a first incident angle and at least a second part of a monitor reference beam to the reference object at a second incident angle, and recombine the first part and the second part of the monitor beams after they reflect from the test and reference surfaces to interfere with one another and form a monitor pattern, where the first and second angles cause the monitor pattern to have spatial interference fringes, and wherein a change in the position of the interference fringes is indicative of a change in a relative position between the test and reference objects.

Abstract: In an optical tomographic imaging apparatus in which a tomographic image is acquired using an interference light between a return beam of a first measuring beam and a first reference beam traveling on a beam path having first beam path length changing unit thereon, an optical interferometer detects movement of the inspection object by using an interference light between a return beam of a second measuring beam from a movement detection position and a second reference beam traveling on a beam path having second beam path length changing unit thereon; and positional misplacement correcting unit changes the beam path length of the first reference beam based on the moving amount detected by the optical interferometer, wherein the beam path lengths of the two reference beams can be changed by a mechanism, thus further reducing deformation of an acquired image in an eye ball depth direction caused by a back and forth motion.

Abstract: A displacement measuring apparatus 100 which measures a displacement of an object to be measured 1 comprises a ranging sensor 5 configured to detect a first origin position based on a distance to a base 2, a ranging sensor 6 configured to detect a second origin position based on a distance to the object to be measured 1, a stage 4 mounting the ranging sensors 5 and 6 and configured to move in a ranging direction of the ranging sensors 5 and 6, and a controller 7 configured to measure a displacement of the object to be measured 1 with respect to the base 2 using the first and second origin positions detected while moving the ranging sensors 5 and 6.

Abstract: An optical pointing device is provided. The optical pointing device includes: a base plate; a circuit board disposed on one side of the base plate, and having a light source irradiating light and a sensor receiving a light reflected from a working surface; an optical structure disposed between the base plate and the circuit board and having a light path guiding the light irradiated from the light source to the sensor; and a light blocking mean disposed on the light path to block the light path when the base plate is separated from a predetermined position.

Abstract: The present invention discloses an optic system for providing illumination and imaging functions in an optical navigation system. Generally, the optic system includes a unitary optic component having an illumination lens and at least one prism to project a collimated beam of light from a light source in the optical navigation system onto a surface, and an imaging lens to image at least a portion of the illuminated surface to an array of photosensitive elements. In one embodiment, optic system further includes an aperture component having a precision aperture, the aperture component configured to locate the precision aperture between the imaging lens of the unitary optic component and the array of photosensitive elements in a path of light reflected from the portion of the illuminated surface to the array of photosensitive elements. Other embodiments are also described.

Abstract: In an exposure station, positional information of a stage holding a wafer is measured by a first fine movement stage position measurement system including a measurement arm, and in a measurement station, positional information of a stage holding a wafer is measured by a second fine movement stage position measurement system including another measurement arm. An exposure apparatus has a third fine movement stage position measurement system which can measure positional information of a stage when the stage is carried from the measurement station to the exposure station. The third fine movement stage measurement system includes an encoder system including a plurality of Y heads and a laser interferometer system including a laser interferometer.

Abstract: The invention relates to a photoacoustic detector, comprising at least a first chamber (V0) suppliable with a gas to be analyzed, a window for letting modulated and/or pulsed infrared radiation and/or light in the first chamber (V0), and means for detecting pressure variations created in the first chamber by absorbed infrared radiation and/or light. The means for detecting pressure variations created in the first chamber by absorbed infrared radiation and/or light comprise at least an aperture provided in the wall of the first chamber (V0), in communication with which is provided a door arranged to be movable in response to the movement of a gas, and means for a contactless measurement of the door movement. The invention relates also to a sensor for a photoacoustic detector and to a method in the optimization of a door used as a sensor for a photoacoustic detector.

Abstract: The present invention relates to a laser sensor for self-mixing interferometry. The laser sensor comprises at least one semiconductor laser light source emitting laser radiation and at least one photodetector (6) monitoring the laser radiation of the laser light source. The laser light source is a VECSEL having a gain medium (3) arranged in a layer structure (15) on a front side of a first end mirror (4), said first end mirror (4) forming an external cavity with an external second end mirror (5). The proposed laser sensor provides an increased detection range and can be manufactured in a low-cost production process.

Abstract: A method and system for performing measurements on a test sample with a metrology or inspection tool are disclosed. At least one of the test sample and the tool is moved with respect to the other from a first position to a second position. At the second position, the tool is aligned for measurement of a measurement target on the sample. A focus of the tool on the test sample is adjusted while moving from that first position to the second position.

Abstract: An optical displacement gage includes the phase determining unit includes a relative phase deciding unit to decide a relative phase of the frequency component within a range of 360 degrees, an absolute phase computing unit to compute an absolute phase by combining the relative phase based on the decision result by the relative phase deciding unit and the past decision result by the relative phase, and a phase reference updating unit to update a reference point of the absolute phase based on a reset instruction, and the displacement amount deciding unit decides the displacement amount based on the absolute phase.

Abstract: An image invariant optical speckle capturing device and method. A highly coherent light is used to illuminate a surface and is scattered by the surface, and is captured from the direction with a ±10° from the angle of specular reflection. A light restrictive module is designed to confine the angular field of view of the sensor, when the speckle capturing device has a relative motion with respect to the surface, the speckle only move on the image but the shape and the intensity are almost keep constant, that is favorable for high accuracy optical pattern recognition and positioning.

Abstract: The present invention provides a measurement apparatus which measures a distance between a reference surface fixed on a fiducial surface and a test surface located on a test object, the apparatus including an optical frequency comb generation unit configured to generate a light beam with a plurality of optical frequency components, which have equal optical frequency separations therebetween, a detection unit configured to, for at least two of the plurality of optical frequency components, detect an interference signal between a light beam reflected by the reference surface and a light beam reflected by the test surface to detect a phase corresponding to an optical path length between the reference surface and the test surface, and a calculation unit configured to calculate a geometric distance between the reference surface and the test surface based on the phases detected by the detection unit.

Abstract: An apparatus which includes a measurement device measuring a surface position of a substrate at each of a plurality of measurement points; which is configured to scan-expose the substrate using slit-shaped light while controlling the surface position based on the measurement result; and in which a width, in a non-scanning direction of the substrate, of a region where the plurality of measurement points are arranged is wider than a width of the slit-shaped light, comprises a controller configured to control the measurement device so as to measure the surface positions in at least two shot regions on the substrate at once at the measurement points, at each of which a portion whose distance from a measurement target position on the substrate falls within a tolerable distance can be measured, of the plurality of measurement points.

Abstract: A displacement sensor employs an electromagnetic radiation source that generates a beam of electromagnetic radiation for measuring a feature of an object. The displacement sensor includes a displacement probe, a multi-dimensional diffraction grating and a plurality of photon detectors. A reflection surface, which is changed when the probe interacts with the object, interacts with the beam from the electromagnetic radiation source and reflects a beam from the reflection surface. The multi-dimensional diffraction grating interacts with the reflected beam and generates a pattern of diffracted beams. Each photon detector senses a different diffracted beam, thereby providing information about the state of the probe.

Abstract: An apparatus for sensing the position of an object includes an irradiation portion for irradiating the object with continuous electromagnetic radiation, a detection portion for detecting electromagnetic radiation reflected by the object, and an output portion for supplying a change in an amplitude intensity or a phase of the electromagnetic radiation based on information obtained by the detection portion. The position of the object is detected based on information supplied from the output portion.

Abstract: A exemplary method for determining vibration displacement in interferometric scanning, in which two optical signals having a phase difference with each other of a high-coherence interferogram corresponding to a tested surface is detected for determining a shifting displacement between the reference plane of interferometric apparatus and the tested surface. In one embodiment, a series of the shifting displacements with respect to a time interval are measured for determining the vibrating frequency of the tested surface by spectrum analysis. Meanwhile, an exemplary interferometric apparatus is also disclosed for calculating the relative position between the tested surface and the reference plane of interferometric apparatus whereby the interferometric apparatus is capable of compensating influences of vibration caused by the environment or the tested surface itself so as to obtain the surface profile and vibration frequency of the tested surface.

Abstract: To reduce the probability of incorrect determination to detect an object reliably. A reflective photoelectric sensor is provided with: a light projecting device for emitting light; a light receiving device for receiving the optical feedback of the light that is emitted from the light projecting device; a determining portion for determining whether or not an object exists in the direction in which the light is emitted from the light projecting device, based on the optical feedback; and a reflection preventing plate of a moth-eye structure, disposed at a position that is on the optical path of the light that is emitted from the light projecting device at a position that is more distant than the location wherein the object is anticipated to appear.

Abstract: A measuring device for the optic measuring of an object 13a is provided, in particular for measuring a motion of the object. The device includes an interferometer 20 with a measuring beam exit 12, a reflection beam entry 14, an interfering beam exit 15, and a light source 1 for creating a light beam 8, an optic detector 16, which is arranged at the interfering beam exit 15 of the interferometer 20 such that a light beam exiting the interfering beam exit 15 impinges the detector and a signal processing unit 17 connected to the detector 16 being embodied such that they can measure measuring signals of the detector 16.

Abstract: A laser interferometer (10) comprises: a first beam splitter (22) which splits a laser beam emitted from a light source (3) into a first beam (L2) and a second beam; a second beam splitter (24) which splits the second beam into a third beam (L1) and a fourth beam (L3), and which causes reflected beams, produced by reflection of the split beams (L1, L3) and incident from reverse directions to the directions of the split beams, to exit in a reverse direction to the direction of the second beam; and a beam selecting unit (50, 51, 60, 66, 68) which, from among the reflected beams produced by reflection of the third and fourth beams (L1, L3) and caused to exit the second beam splitter (24) in the reverse direction to the direction of the second beam, selects a beam to be combined in the first beam splitter (22) with a reflected beam produced by reflection of the first beam (L2) and incident on the first beam splitter from a reverse direction to the direction of the first beam (L2).

Abstract: A system and method for calibrating the focal position of the imaging plane of a sequential lateral solidification (SLS) system. A test pattern is formed on a test substrate while varying the z-position of the focal position. Information concerning the z-position of the focal position is stored by a data processing system for various positions in the test pattern. An inspection light beam is directed onto the test pattern at a predetermined angle. The reflection of the inspection light beam is detected by an optical detector. The data processing system analyzes the reflection and determines whether the reflected light is substantially specular or substantially scattered. The data processing system uses the analysis of the reflected light and the information concerning the z-position of the focal position to select an optimal focal position for calibrating the SLS system.

Abstract: A method comprises determining a first processing center position to calculate a wavefront aberration of an optical system, determining a second processing center position to calculate a wavefront aberration, correcting the first processing center position in a first direction using the second processing center position in the first direction and correcting the second processing center position in a second direction using the first processing center position in the second direction.

Abstract: The invention relates to a system and to a corresponding method for optical coherence tomography having an interferometer (10) for emitting light with which a specimen (1) is irradiated, the interferometer (1) comprising a beam splitter (13) and at least one reflector (12) the optical distance (I) of which from the beam splitter (13) is changeable, a specimen objective by means of which light emitted by the interferometer (10) is focussed in a focus lying within the specimen (1), and a detector (30) for collecting light which is reflected by the specimen (1).

Abstract: An exemplary interferometer system includes an interferometer producing data from at least one interferometer beam. A source of gently flowing gas or gas mixture (atmosphere) produces a gas flow substantially normal to the beam pathway. A perturbation source (e.g., resistance heater) upstream of the beam pathway produces, in a repetitively pulsed manner, perturbed loci in the flowing atmosphere in selected locations upstream of the beam pathway. The perturbed loci flow to the interferometer beam(s). Data from the interferometer are received by a processor programmed with an algorithm that calculates, based on the data obtained during a perturbation pulse, the effect of the perturbed loci on the at least one interferometer beam as the loci pass through the interferometer beam. The processor also updates the algorithm based on data obtained from the interferometer during a subsequent perturbation pulse, compared to a previous perturbation pulse.

Abstract: In a distance/speed meter, first and second semiconductor lasers emit parallel laser light beams to a measurement target. A first laser driver drives the first semiconductor laser such that the oscillation interval in which at least the oscillation wavelength monotonically increases repeatedly exists. A second laser driver drives the second semiconductor laser such that the oscillation wavelength increases/decreases inversely to the oscillation wavelength of the first semiconductor laser. First and second light-receiving devices convert optical outputs from the first and second semiconductor lasers into electrical signals. A counting unit counts the numbers of interference waveforms generated by the first and second laser light beams and return light beams of the first and second laser light beams.

Abstract: An interferometric tracking device is disclosed. A first grating is optically coupled to a second grading such that the second grating is rotationally offset from the first grating. Imaging optics are adapted to image light passing through the first and second gratings onto a focal plane array. A plurality of wedge plates are optically disposed between the imaging optics and the second grating, such that the wedge plates generate a plurality of spots on the FPA when light from a point source is incident upon the first grating.

Abstract: A static/dynamic multi-function measuring device for linear unit, includes a foundation, a multi-direction sliding unit having eddy current detector, a linear motor, a linear unit with a sensing element and an optical measuring unit; the static/dynamic multi-function measuring device uses the linear motor to drive the multi-direction sliding unit with low friction to provide a non-contact measurement of a parallel alignment of the linear unit, a linear accuracy measurement of the linear unit, and a vibration measurement of the linear unit to detect the parallel alignment of the linear unit and the linear accuracy of a slide rail and so on, the static/dynamic multi-function measuring device also measures errors of the linear unit and a slide block moving in the vibrating and the rolling direction.

Abstract: An optical member is irradiated with light in order to measure position information in a first direction. The optical member has a first reflecting surface, onto which light propagating in a second direction intersecting the first direction is incident, and a second reflecting surface, onto which light propagating in the second direction is incident. The first reflecting surface and second reflecting surface are optically connected, and light reflected by one among the first reflecting surface and second reflecting surface is incident on the other reflecting surface.

Abstract: A position measurement system for measuring positional coordinates of a point under measurement includes a first noise removal unit, a parameter determination unit and a second noise removal unit. The first noise removal unit removes noise from the measured positional coordinates to acquire first positional coordinate values. The parameter determination unit determines a noise removal parameter on a basis of the first positional coordinate values. The second noise removal unit again removes noise from the first positional coordinate values with using the noise removal parameter, to acquire second positional coordinate values.

Abstract: An optical lens system includes a lens surface capable of forming concentric interference patterns on an object as if light emitted from a single light source were virtually emitted from two or more light sources within a plane containing an optical axis of the single light source.

Abstract: Multidirectional retroreflectors and methods of reflecting light beams from multiple directions are provided. The multidirectional retroreflectors utilize a four-mirror retroreflector with a common virtual reflection point.