Abstract: A point diffraction interference measuring method comprises forming a substantially ideal spherical wave by using a point light source-generating unit 101, 102, allowing a light flux composed of the spherical wave to pass through a test sample 109, thereafter dividing the light flux into two light fluxes by using an optical path-dividing element 105, allowing one light flux of the divided light fluxes to pass through a pinhole 129 to covert the one light flux into a reference light beam which is a substantially ideal spherical wave, and detecting interference fringes generated by causing interference between the reference light beam and a measuring light beam which is the other light flux of the divided light fluxes. The wavefront aberration of the test sample can be measured by observing the interference fringes without being affected by the disturbance which would be otherwise caused by any system vibration or the like.

Abstract: Conical surfaces (and other complex surface shapes) can be interferometrically characterized using a locally spherical measurement wavefront (e.g., spherical and aspherical wavefronts). In particular, complex surface shapes are measured relative to a measurement point datum. This is achieved by varying the radius of curvature of a virtual surface corresponding to a theoretical test surface that would reflect a measurement wavefront to produce a constant optical path length difference (e.g., zero OPD) between the measurement and reference wavefronts.

Abstract: A method and apparatus for wavefront analysis including obtaining a plurality of differently phase changed transformed wavefronts corresponding to a wavefront being analyzed which has an amplitude and a phase, obtaining a plurality of intensity maps of the plurality of phase changed transformed wavefronts and employing the plurality of intensity maps to obtain an output indicating the amplitude and phase of the wavefront being analyzed.

Abstract: An optical sphere of nominal radius is positioned and supported within an assembly so that is can be activated with a predetermined motion with respect to an incoming converging spherical wavefront from an interferometer to calibrate the interferometer by determining the differences between the converging spherical wavefront and the reflected wavefront from portions of the area of the optical sphere as sampled thereover. The nominal optical sphere supported and sampled in this way mimics a “perfect” comparison sphere from which the systematic error of the interferometer wavefront may be determined. The optical sphere is preferably hollow and composed of Zerodur®. Internal magnets in conjunction with external induction coils provide motion control while the optical sphere is mounted on an air bearing for freedom of rotation and safety in transportation.

Abstract: An optical arrangement of the Michelson interferometer type having a measuring arm for measuring optical properties of at least two mutually spaced regions in an optically transparent and/or diffusive object. A reference arm has a path length variation unit which generates a periodic change in path length for the radiation in the reference arm. Arranged in the measuring arm upstream of the object is a detour unit. The detour unit imparts a detour which is larger than at least one second measuring beam into at least one measuring beam. The detour unit selects a detour to be equal to a distance between regions to be measured in the object. Using the optical arrangement the object is irradiated with the aid of a number of measuring beams corresponding to the number of regions. In each case two measuring beams have an optical path difference which corresponds to a geometrical distance between two regions.

Abstract: Interferometeric apparatus for analyzing the wavefront and intensity distribution of collimated beams from fiber optic collimators, lasers, and the like. The collimated beams are directed through a plate beamsplitter and combined with a well defined diverging, spherical reference beam preferably generated by a single mode fiber. Reference and test beams are directed through an optical assembly for changing magnification to selectively control lateral resolution at a detector plane. Phase shifting analysis is performed on the resultant interference patterns to analyze the wavefront, and intensity profile is determined as a separate step.

Abstract: Interferometric apparatus and methods for reducing the effects of coherent artifacts in interferometers. Fringe contrast in interferograms is preserved while coherent artifacts that would otherwise be present in the interferogram because of coherent superposition of unwanted radiation generated in the interferometer are suppressed. Use is made of illumination and interferogrammetric imaging architectures that generate individual interferograms of the selected characteristics of a test surface from the perspective of different off-axis locations of illumination in an interferometer and then combine them to preserve fringe contrast while at the same time arranging for artifacts to exist at different field locations so that their contribution in the combined interferogram is diluted.

Abstract: A method including comparing information derivable from a scanning interferometry signal for a first surface location of a test object to information corresponding to multiple models of the test object, wherein the multiple models are parametrized by a series of characteristics for the test object. The information corresponding to the multiple models may include information about at least one amplitude component of a transform of a scanning interferometry signal corresponding to each of the models of the test object.

Abstract: A system for characterizing an aspheric surface with respect to an expected shape thereof, comprising a light source to emit light of desirable properties. A ferrofluid deformable mirror apparatus has a reflective surface being controllably deformable. The reflective surface is shaped as a function of the expected shape so as to reflect light in a known path toward a detector. Optical elements project light from the light source onto the aspheric surface such that light is reflected along an expected path if the aspheric surface has the expected shape. Light is reflected from the aspheric surface along an actual path. The optical elements project light on the reflective surface of the ferrofluid deformable apparatus such that light is reflected along the known path. A detector receives light reflected along at least one of the actual path and the known path to obtain an interference pattern as a function of the expected path.

Abstract: The invention relates to a method of calculating the three dimensional surface coordinates for a set of points on the surface of an object. The method comprises the following steps: using a projector for illuminating the object with a set of fringes, adjusting the fringes, capturing a plurality of images of the surface with a camera with different fringe phase settings, processing the images to produce an absolute fringe phase map of the parts of the surface which are both illuminated by the projector and visible to the camera, and processing the fringe phase map to give a set of coordinates for points on the surface of the object. The method provides surface profiling and ranging by using temporal phase measurement interferometry (TPMI) based on a modified Carré technique.

Abstract: An interferometer system comprises a reference surface, a support for an object providing an object surface, a radiation source for emitting radiation of an adjustable frequency onto the reference surface and the object surface, a position-sensitive radiation detector, a controller for adjusting a plurality of different frequencies of the radiation emitted by the radiation source, and an integrator for averaging the interference patterns superposed on the radiation detector at different frequencies. Moreover, there is provided a method for recording an interferogram, a method for providing an object with a target surface as well as a method for manufacturing an object with a target surface.

Abstract: The invention relates to an interferometric method for measuring a height of a first region on a first surface, the first surface having first areas having first optical properties and second areas having second optical properties, the method comprising the steps of generating of first and second coherent light beams, reflecting at least the first coherent light beam from the first region into a first return beam and reflecting the second coherent light beam from a second region into a second return beam, measuring at least a first reflectivity of the first region, determining a topography-dependent phase shift of the first and second return beams for the height measurement based on the first reflectivity.

Abstract: The present invention consists of a technique and device for measuring the thickness variation and shape of wafers or other polished opaque plates. A combination of two improved phase-shifting Fizeau interferometers is used to simultaneously measure the single-sided distance maps between each side of the wafer and the corresponding reference flat, with the thickness variation and shape being calculated from these data. Provisions are made to determine and eliminate the shape and tilt of the reference surfaces, and also to facilitate the correct overlay of the two single-sided measurements for the calculation of thickness variation and shape.

Abstract: In a speckle interferometer apparatus 2, an optical member mounting device 10 mounted with optical members including a pair of luminous flux outputting devices 9a, 9b, 9c, 9d is disposed on the object side of a main unit of an imaging device 3 while being separated from the imaging device 3. A light-transmitting area 14 is formed between the pair of luminous flux outputting devices 9a, 9b, 9c, 9d so as to transmit therethrough interference light from an object surface.

Abstract: A system for interferometric fit testing of a specimen having an aspherical surface in reflection, the specimen being a segment (2) (footprint) of a rotationally symmetrical basic body (1) (parent), comprises an interferometer (3) and a diffractive optical element (DOE) (5). An optical axis of the interferometer (3) in the beam direction behind the diffractive optical element (5) and an axis of rotation of the basic body (1) form an angle that differs from zero. The diffractive optical element (5) is designed in such a way that the rays produced by the interferometer (3) and falling into the diffractive optical element (5) strike the specimen (2) perpendicularly and from there run back in themselves.

Abstract: An aspheric microlens, particularly a conic constant of the microlens, may be evaluated and this evaluation may be used to determine an optimal process for creating the aspheric microlens. Such evaluation may include a curve fitting or a numerical expression of the wavefront.

Abstract: Techniques for performing phase-shifting interferometry are disclosed.

Abstract: Interferometric scanning method(s) and apparatus for measuring rotationally and non-rotationally symmetric test optics either having aspherical surfaces or that produce aspherical wavefronts by comparing known and unknown spherical and aspherical shapes. Preferably, a spherical or partial spherical wavefront or reflecting surface is defined with respect to a known origin along a scanning axis. The test optic is aligned with respect the scanning axis and selectively moved along it relative to the known origin so that the spherical shape intersects the test optic at the apex of the aspherical shape and at radial positions where the spherical shape and the aspheric shape intersect at points of common tangency.

Abstract: A surface inspection apparatus for inspecting a shape of an inspected surface of an object to be inspected includes: a supporting unit capable of supporting the object in a state in which the inspected surface is in a substantially vertical state; and a moving state which moves the object in a substantially vertical direction while the inspected surface is maintained in the substantially vertical state.

Abstract: A surface inspecting apparatus for inspecting a shape of an inspected surface of an object to be inspected, the surface inspecting apparatus includes: a mounting base for mounting the object; positioning means for positioning the object to an inspecting position on the mounting base; a memory for storing position specifying information for specifying a two-dimensional position of the object when the object is positioned; inputting means for inputting an outer shape data of the object; and edge position determining means for acquiring an edge position of the object based on the stored position specifying information and the inputted outer shape data.

Abstract: A phase shift fringe image analysis method comprises the steps of shifting an object to be observed and a reference relative to each other by using a phase shift device, obtaining fringe image data at three or more phase shift positions having a given phase gap therebetween, and determining a phase of the object by analyzing thus obtained plurality of fringe image data items. The positional data of at least three phase positions are specified, and the whole or part of the fringe image data on which carrier fringes at these phase positions are superposed is subjected to a predetermined arithmetic operation so as to carry out a phase analysis and determine the phase of the object.

Abstract: A scale (10) is illuminated by a light source (18). After interaction with an index grating (12), fringes (F) are formed in a Talbot plane and analyzed by an analyzer grating (14). To decrease the sensitivity to changes in the ride height of the index grating above the scale, the light source is restricted to a small but finite size, and positioned so that it subtends a small angle Ø at the analyzer grating (or, if a collimating lens (24) is used, positioned so that it subtends a small angle Ø at the lens). The size of the light source should preferably be smaller than a predetermined value such that the extent of a geometric fringe visibility envelope exceeds the extent of a Talbot fringe visibility envelope.

Abstract: Interferometric method(s) and apparatus for accurately measuring aspherical surfaces and transmitted wavefronts, particularly of the type having relatively large diameters and departure employed in lithographic applications used in the fabrication of integrated circuits and the like. An interferometer, preferably of the Fizeau type, is provided with at least one aspherical reference surface that is positioned adjacent the test optic. The test optic can be either rotationally or non-rotationally symmetric, a reflecting aspherical test surface, or a refracting system that is illuminated by an aspherical wavefront or produces a transmitted aspherical wavefront. In any case, the departure of the test optic from its intended performance is ultimately determined. The aspherical reference surface is illuminated by an aspherical wavefront provided by upstream optics structured so that the incident aspherical wavefront propagates normal to the aspherical reference surface across its entire surface.

Abstract: In a fringe analysis method comprising the step of subjecting fringe image data carrying wavefront information of an object to be observed to Fourier transform method so as to determine a wavefront of the object, the fringe image data is multiplied by a window function corresponding to an effective data area of the fringe image data before the Fourier transform method is applied to.

Abstract: A Fizeau interferometer apparatus is used for both low and high interference measurement. When irradiating a reference surface and a sample with a low coherent luminous flux, a path-matching passage divides the low coherent luminous flux into first and second paths, while the optical path length difference between the respective luminous fluxes passed through the two paths equals twice the optical distance between the reference surface and the sample. When irradiating the reference surface and the sample with a high coherent luminous flux, the luminous flux is made incident on the sample side of the path-matching passage at a position coaxial with the low coherent luminous flux.

Abstract: A support apparatus for an optical wave interferometer reference plate comprises a support member for supporting an outer peripheral face of the reference plate. The support member is bonded to the outer peripheral face of the reference plate at a plurality of positions spaced from each other along the circumferential direction of the outer peripheral face and adapted to deform elastically in a circumferential/diametric direction of the reference plate but less in the optical axis direction of the reference plate than in the circumferential/diametric direction.

Abstract: An interferometric measuring method and device for measuring the shape of, or the distance to, surfaces are provided, in which light is generated, modulated with respect to its frequency, conducted to both an object surface and a reference surface, brought to interference, and conducted to a photodetector, and, to detect the particular distance, a phase of the photodetector signal is evaluated. A simple, rugged configuration, with the capability of taking a high-resolution measurements in the context of a large unambiguity range, is achieved in that the phase is considered in at least two different instants with the wavelengths that correspond on the basis of the frequency modulation, and the results are fed to the evaluation.

Abstract: A system for wavefront aberration reduction of an incident optical beam. The system includes a spatial light modulator for receiving the incident optical beam and forming an output optical beam, the output beam being aberration-reduced compared to the incident beam; a common-path interferometer for receiving a portion of the output optical beam and for generating an interference fringe pattern by introducing a phase shift to one part of said portion of the output optical beam, the interference fringe pattern being applied to said spatial light modulator and comprising essentially all of the light available in said portion of the output optical beam; wherein the interference fringe pattern is representative of the wavefront error of the incident optical beam and the interference fringe pattern activates said spatial light modulator such that said spatial light modulator performs wavefront error correction on the output optical beam.

Abstract: Flatness and thickness variation information concerning transmissive plane-parallel test plates is obtained from a grazing incidence interferometer modified to distinguish between superimposed interference patterns containing both types of information. The grazing angle of the interferometer is varied, and unique modulation frequencies of local fringe intensities within the superimposed interference patterns are identified. The local fringe intensities attributable to the different interference patterns are distinguished by their respective modulation frequencies.

Abstract: An imaging optical system for an oblique incidence interferometer comprises first and second optical systems and an intermediate imaging surface therebetween. Each of the first and second imaging optical systems comprises two telecentric lenses, arranged afocal to each other, having respective focal lengths different from each other. A first image of a surface to be inspected having a deformed aspect ratio with respect to this surface is formed on the intermediate imaging surface by way of the first imaging optical system. The second imaging optical system is arranged such that the first image is focused onto the imaging surface of the interferometer as a second image corrected so as to have substantially the same aspect ratio as that of the surface to be inspected.

Abstract: An method for measuring an optical thickness of a test object, the method includes: interfering a first optical wave front from the test object and a second optical wave front from a reference surface to produce an interference signal; for a selected location on the test object, obtaining an interference pattern of the test object at a first wavelength &lgr;1; for the selected location, calculating a first estimate of the optical thickness from the interference pattern recorded at wavelength &lgr;1; for the selected location obtaining an interference pattern of the test object at a second wavelength &lgr;2; for the selected location, calculating a second estimate of the optical thickness from the interference pattern recorded at wavelength &lgr;2; and for the selected location, calculating a third estimate of the optical thickness by combining the first and second estimates of optical thickness.

Abstract: A system of frequency-scanning interferometry uses a computer system operating in accordance with a program for measuring distances or range, including measuring topographical information about test object surfaces. Interferometric data is detected using a single point detector or an array of such detectors and recorded (stored) in the computer system, and a series of measurements are taken over a range of illumination frequencies. The interferometric data varies in a sinusoidal manner with a change in illumination frequency at interference frequencies corresponding to particular measures of distance or range. A Fourier transform for locating peak interference frequencies is first limited in frequency space and second divided into stages to save processing time. A coarse spacing between Fourier frequency samples is used for a first approximation, and finer spacing between Fourier frequency samples are used in the vicinity of the first approximation to make a second more accurate approximation.

Abstract: One embodiment of the present invention is a scanner for a beam of scanning optical coherence tomography (“OCT”) radiation that includes: (a) a source of OCT radiation; (b) a scanner; and (c) scanning optics whose image surface has a negative field curvature.

Abstract: The invention concerns a method for identifying an object comprising the following steps: (a) selecting an intrinsic surface of an object to be identified; (b) illuminating at least an identification zone in the intrinsic surface of the object to be identified with a coherent light and intercepting at least part of the light reflected by the identification zone, so as to obtain an interference figure in specific illuminating and intercepting conditions; (c) preserving said interference figure as reference interference figure of the object to be identified; (d) placing a candidate object which could possibly be the object to be identified in similar illuminating and intercepting conditions as those used to obtain the reference interference figure and obtain from said candidate object an interference figure; (e) comparing the reference interference figure with the interference figure of the candidate object; and (f) assessing the probability of identity between the object to be identified and the candidate obje

Abstract: A method with a traceable calibration for reducing the uncertainty in the precision measurement of aspheric surfaces and wavefronts. A transmission sphere is mounted in an interferometer and its emergent wavefront is calibrated by comparing it to an optical sphere that is moved and sampled in a predetermined manner to make it act like a substantially “perfect” sphere mounted on a slideway. Afterwards, the calibrated wavefront from the transmission sphere is used to measure a spherical artifact mounted on the slideway to calibrate its surface. A transmission asphere is then mounted in the interferometer, and its emergent wavefront is calibrated by comparing it to the calibrated spherical artifact mounted on the slideway. The calibrated aspheric wavefront from the interferometer is then used to measure an aspherical artifact mounted on the slideway to determine is surface shape.

Abstract: A profile measurement apparatus of the present invention is characterized in that two flash light beams, having wavelengths slightly different from each other, are emitted to an object with a predetermined interval t1 therebetween, and when a camera picks up interference light formed by light beams reflected from the object and a light beam reflected from the reference mirror, while moving the object in a direction, in which the two flash light beams are directed, in units of intervals t2 at which each of the two flash light beams is cyclically emitted, a phase shift amount corresponding to a movement amount of the object at a time is set to a value falling within a range of 2n&pgr;±&pgr;/2±&pgr;/4.

Abstract: A beamsplitter (16) splits a source beam (14) into a sample path (17) and a reference path (19). The reflected beams recombine to form an interference signal (100). Reference surface (20) is horizontaly suspended on a loudspeaker membrane (30), and moved via a voice coil attached to the membrane (30). this arrangement isolates the reference surface (20) from external vibrations. The surface profile of the sample (18a) is analysed via the interference fringes, which are determined by computing maxima or minima in the statistical variance of digital data representing the interference signal (100). The apparatus may be used for calibrating laser ablation procedures in eye surgery.

Abstract: A system of frequency-scanning interterometry uses a computer system operating in accordance with a program for measuring distances or range, including measuring topographical information about test object surfaces. Interferometric data is detected using a single point detector or an array of such detectors and recorded (stored) in the computer system, and a series of measurements are taken over a range of illumination frequencies. The interferometric data varies in a sinusoidal manner with a change in illumination frequency at interference frequencies corresponding to particular measures of distance or range. A Fourier transform for locating peak interference frequencies is first limited in frequency space and second divided into stages to save processing time. A coarse spacing between Fourier frequency samples is used for a first approximation, and finer spacing between Fourier frequency samples are used in the vicinity of the first approximation to make a second more accurate approximation.

Abstract: Conical surfaces (and other complex surface shapes) can be interferometrically characterized using a locally spherical measurement wavefront (e.g., spherical and aspherical wavefronts). In particular, complex surface shapes are measured relative to a measurement point datum. This is achieved by varying the radius of curvature of a virtual surface corresponding to a theoretical test surface that would reflect a measurement wavefront to produce a constant optical path length difference (e.g., zero OPD) between the measurement and reference wavefronts.

Abstract: Interferometric apparatus and methods by which aspheric surfaces and wavefronts may be precisely measured. The apparatus is provided with two modes of operation. In one mode, the apparatus is configured generally as a Fizeau interferometer in which an aspheric reference surface is used to permit the rapid, robust measurement of the difference between the aspheric reference surface and an aspheric test optic or wavefront. In another mode of operation, the aspheric test surface itself is completely characterized through in-situ use of an interferometric scanning technique using a spherical reference surface.

Abstract: A method of detecting a posture of an object comprises the steps of acquiring fringe image data carrying phase information of the object; subjecting the whole or part of fringe image data to arithmetic processing using Fourier transform so as to determine a tilt frequency of a fringe corresponding to an inclination of the object in the fringe image data; and detecting the inclination of the object according to the tilt frequency.

Abstract: To measure a convex mirror, a reference beam and a measurement beam are both provided through a single optical fiber. A positive auxiliary lens is placed in the system to give a converging wavefront onto the convex mirror under test. A measurement is taken that includes the aberrations of the convex mirror as well as the errors due to two transmissions through the positive auxiliary lens. A second, measurement provides the information to eliminate this error. A negative lens can also be measured in a similar way. Again, there are two measurement set-ups. A reference beam is provided from a first optical fiber and a measurement beam is provided from a second optical fiber. A positive auxiliary lens is placed in the system to provide a converging wavefront from the reference beam onto the negative lens under test. The measurement beam is combined with the reference wavefront and is analyzed by standard methods.

Abstract: When analyzing the fringe pattern of an object to be observed by using Fourier transform method with respect to closed interference fringe image data, the original data expressed by an orthogonal coordinate system are once converted into fringe image data represented by a polar coordinate system, the converted data are analyzed by the conventional Fourier transform method, and then the resulted data are converted back into the original orthogonal coordinate system by inverse coordinate transformation, so as to obtain the wavefront corresponding to the original wavefront to be observed.

Abstract: Apparatus and methods of measuring three-dimensional position information of a point on the surface of an object. In one embodiment, the method includes the steps of providing two sources of radiation having a spectral distribution, illuminating the surface with each of the sources to produce a first fringe pattern, moving the first fringe pattern to a second position, generating a first wrapped cycle map, estimating fringe numbers in the first fringe pattern, changing the first fringe pattern, moving the second fringe pattern to a second position, generating a second wrapped cycle map, estimating fringe numbers in the second fringe pattern, and determining position information in response to the estimated fringe numbers in the second fringe pattern and the second wrapped cycle map.

Abstract: A measurement system is provided that facilitates the measuring of a shape of a flexible electromagnetic radiation structure. The measurement system includes a plurality of local sensors, a central sensor and a measurement processor. Each of the plurality of local sensors is configured to sense the position of a portion of the flexible electromagnetic radiation structure. The central sensor is configured to determine an overall shape of the flexible electromagnetic radiation structure. The measurement processor provides the ability to combine sensor data from the local sensors and sensor data from the central sensor to provide an accurate measurement of the shape of the flexible electromagnetic radiation structure.

Abstract: In a method for calibrating a radius test bench for measuring radii of optical elements, in particular of lenses and spherical mirrors, there are provided an illuminating system 1 that generates a spherical wave and a diffractive optical element 3 that retroreflects a spherical wave of a specific radius into itself. The diffractive optical element 3 is introduced into the radius test bench in at least two positions, a first position thereof being a cat's eye position 1 and another position being an autocollimation position, as a result of which it is possible to use the radius of curvature simulated by the diffractive optical element 3 to detect deviations of the radius test bench from this radius of curvature as errors of the radius test bench, and thus to take them into account in the measurements of optical elements to be tested.

Abstract: A dispersed Hartmann sensor includes a Hartmann lenslet in combination with a dispersive element, whereby a Hartmann spot formed by light passing through the Hartman lenslet is dispersed parallel to the phase step of the light. The shape of the blur spot can then be examined at many wavelengths. Measuring the size of a discontinuity in the wavefront of light is then performed by forming a single image of the wavefront, dispersing the image in wavelength using a combination of a Hartman lenslet and a dispersive element, and analyzing the dispersed image along a dispersion direction of the dispersed image to measure the size of the discontinuity.

Abstract: In an optical system for an oblique incidence interferometer, first and second prisms are used for luminous flux dividing and for luminous flux combining, respectively. Reference light and measurement light are separated from each other at a surface where collimated coherent light enters or exits from the first prism, whereas the reference light and measurement light are combined together at a surface where the measurement light enters or exits from the second prism.

Abstract: A method and device is capable of amplitude, phase, polarization shear, wavelength, and diversity measurement along a common path by using a sampling device to get a sparse sampled optical field. The sampled field is fed to a diversity generator device. The diversity generator device generates the diversity information associated with each sample/element of the sparse input in empty space provided by sampling. The diversity information may consist of phase-shifted copies of the input sample having known/device dependent relative phases, amplitudes, polarization, shear, or wavelength with respect to their originating sample. The geometry of the fan out allows overlap between part or all different diversity copies (originating from the same or from different samples) thus making the method interferometric.

Abstract: An improved wavefront sensor for characterizing phase distortions in incident light including optical elements that spatially sample the incident light and form a dispersed spot with a fringe pattern corresponding to samples of the incident light. An imaging device captures an image of the dispersed spot with said fringe pattern formed by said optical elements. And an image processor that analyzes the spectral components of the fringe pattern of a given dispersed spot to derive a measure of the local phase distortion without ambiguity in the corresponding sample of incident light. The optical elements may comprise refractive elements, diffractive elements or a combination thereof (such as a grism). The wavefront sensor may be part of an adaptive optic system (such as a large-aperture space telescope) to enable the measurement and correction of large phase steps across adjacent mirror segments of a deformable mirror.