Abstract: An aspheric surface measuring apparatus includes an interferometer which generates incident light; a test piece having an aspheric surface from which the incident light is reflected as test light; a first optical element disposed on an optical path of the incident light, having at least one surface with a hologram for diffracting the incident light toward the test piece; and a second optical element disposed after the first optical element, which transmits the incident light toward the aspheric surface and has a concave surface to reduce an incident angle of the test light entering the hologram after having been reflected from the aspheric surface. Alternatively, a single optical element with a hologram and a concave surface can be used instead of the separate first and second optical elements. An extremely aspheric lens can be precisely measured using the apparatus.

Abstract: Respective regional form information items obtained from regional interference fringe images corresponding to partial regions of a spherical surface to be inspected are transformed into regional synthesis form information items corresponding to a common coordinate system set for aperture synthesis by using a relationship among a polar coordinate system of the spherical surface, a plane coordinate system of an imaging plane, and the common coordinate system. Thus obtained regional synthesis form information items are subjected to aperture synthesis processing, so as to determine the overall form information of the spherical surface.

Abstract: Both the optical distance of the detected-light light path and the optical distance of the reference-light light path are simultaneously varied according to respective specified patterns, so that differences are created between the period of variation in the intensity of the required signal component (in the interference signals) and the periods of variation in the intensities of the coherent noise components. When the modulation scanning procedure is performed, the phase difference between the detected light and reference light in a specified state is determined as shape information for the above-mentioned detected surface on the basis of the interference signals output from the light-receiving element. As a result, it is possible to securely reduce the effects of coherent noise components generated as a result of the interference of specified noise light that has passed through at least portions of the detected-light light path and reference-light light path with the detected light or reference light.

Abstract: In a fringe analysis method using Fourier transform, fringe image data is determined in a state where a wavefront from an object and a wavefront from a reference are relatively inclined with respect to each other by a minute amount, and a carrier fringe occurring due to this inclination is superposed on a fringe occurring due to wavefront information of the object. The inclination is set such that the carrier frequency occurring due to the inclination is a predetermined multiple of the basic frequency determined by the wavefront information of the object and observing means.

Abstract: There is provided a shape measuring apparatus using an interferometer comprising a lens for condensing temporarily light waves from a light source, and a light wave shaping plate having a pinhole with suitable size adapted to convert the condensed light waves into an ideal spherical wave and a window provided in the vicinity of the pinhole and having enough size to pass therethrough light wave surface information, in which at least one lens having a reference surface and a surface to be measured the optical axes of which are slightly decentered in an optical path of the light waves passed through the pinhole is arranged in a position where the light waves which are made incident perpendicularly to the reference surface to be reflected therefrom pass through the pinhole again, and the light reflected from the surface to be measured pass through the window, and the reflected light reflected by the reference surface to pass through the pinhole again and the reflected light reflected by the surface to be measured

Abstract: A surface comparison apparatus for measuring surface characteristics of a specimen through which visible ray can not be transmitted comprises: a light source generating light having a predetermined wavelength; an optical device changing the light into collimated light; an irradiating unit for irradiating the collimated rays on both surfaces of a specimen which needs to be measured after dividing them into two paths and making the lights, which are reflected on both surfaces, be focused on opposite direction of the incident light after passing through the paths and be interfered with each other; and a display means for displaying the interfered lights which are made by interfering the reflected lights, and thereby, a parallelism or surface characteristics for both surfaces of the specimen can be measured simultaneously through an interference pattern which is obtained by dividing the light into two paths, irradiating the light on both surfaces of the specimen, and reflecting to be interfered with each other, a

Abstract: An interferometry method including: i) forming an optical interference image by combining different portions of an optical wave front reflected from a pair of surfaces; ii) recording an interference signal at different locations of the optical interference image in response to varying the relative position of the two surfaces over a range of positions; iii) transforming the interference signal for at least one of the locations to produce a spectrum having a peak at a spectral coordinate corresponding to the variation in the relative position of the two surfaces over a range of positions; iv) identifying the spectral coordinate of the peak; and v) for each location, extracting the spectral phase of the interference signal at the coordinate of the peak. For example, the method may further include, for each of the different locations, determining a surface profile of one of the surfaces based on the spectral phase of the interference signal at each of the multiple locations.

Abstract: The invention features a surface profiling method including: collecting interferometric data related to a surface profile of a measurement object; and calculating the surface profile based on the collected interferometric data and at least one value indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object. The invention also features a surface profiling system including: an interferometry system which during operation provides interferometric data related to a surface profile of a measurement object; and an electronic processor coupled the interferometry system, wherein during operation the electronic processor calculates the surface profile based on the interferometric data and at least one parameter indicative of dispersion in the phase change on reflection (PCOR) of the profiled surface of the measurement object.

Abstract: Interferometric method and apparatus for measuring aspheric surfaces and wavefronts by directing a spherical wavefront of known design at a wavelength &lgr;1 at a reference sphere with known measured surface properties to generate a first electronic signal containing information about the optical path differences between the reference and measurement wavefronts generated an interferometer and directing an aspherical wavefront of known design at a wavelength &lgr;2 at an aspherical surface or wavefront to be tested to generate a second electronic signal containing information about the optical path differences between the reference and measurement wavefronts generated by the interferometer.

Abstract: A lateral shearing interferometer wavefront sensor system (10) that employs a double-shear/full aperture approach to correct for branch points in the wavefront of an optical beam (24) that has been aberated. The wavefront sensor system (10) includes a lateral shearing interferometer (12) having a beam splitter (14) that splits the beam (24) into a first split beam (28) and a second split beam (26), a beam shifter (22) that shifts the first split beam (28) relative to the second split beam (26), and a beam combiner (16) that combines first shifted split beam (28) and the second split beam (26) into a combined beam (30). The combined beam (30) provides an interference pattern that includes a plurality of interfered beam portions (76, 78, 82, 84). A deformable mirror (72) includes a plurality of actuators (74) which deform the mirror (72) to correct the beam wavefront.

Abstract: A method for accurately synthesizing a full-aperture data map from a series of overlapped sub-aperture data maps. In addition to conventional alignment uncertainties, a generalized compensation framework corrects a variety of errors, including compensators that are independent in each sub-aperture. Another class of compensators (interlocked) include coefficients that are the same across all the sub-apertures. A constrained least-squares optimization routine maximizes data consistency in sub-aperture overlap regions. The stitching algorithm includes constraints representative of the accuracies of the hardware to ensure that the results are within meaningful bounds. The constraints also enable the computation of estimates of uncertainties in the final results. The method therefore automatically calibrates the system, provides a full-aperture surface map, and an estimate of residual uncertainties.

Abstract: An evaluation light 100 from a light source 1 is split to obtain a divided evaluation signal 120 and a reference signal 110, the divided evaluation signal 120 and the reference signal 110 being shone, respectively, onto a sample surface 32 and a reference surface 11 that is made of the same material as the sample, after which a reflected divided evaluation signal 121 and a reflected reference signal 111, which are reflected respectively on the sample surface 32 and the reference surface 11, are caused to interfere for analysis of the sample surface 32.

Abstract: In an interferometer optical system comprising a reference standard having an entrance surface on which a luminous flux from a light source is incident and a reference plane on which the luminous flux entering from the entrance surface is obliquely incident, an angle within the range of 10° to 30° is formed between the entrance surface and reference plane of the reference standard.

Abstract: Apparatus and methodology by which the radius of curvature of individual optics may be determined through the interferometric measurement of the optical length of a spherical cavity established from null tests of combinations of the individual optics and an algorithm that mutually intercompares the measured cavity lengths and radii of curvature of the individual optics.

Abstract: A pulsed light source in used conjunction with a ramping scanning mechanism for phase-shift and vertical-scanning interferometry. The pulse length and the scanning velocity are selected such that a minimal change in OPD occurs during the pulse. As long as the duration of the pulse is shorter than the detector's integration time, the effective integration time and the corresponding phase shift are determined by the length of the pulse, rather than the detector's characteristics. The resulting minimal phase shift produces negligible loss of fringe modulation, thereby greatly improving signal utilization during phase-shifting and vertical-scanning interferometry.

Abstract: Systems and methods for near-field, interferometric microscopy are disclosed in which a mask having an array of sub-wavelength apertures is used to couple near-field probe beams to a sample. The periphery of the mask further includes one or more larger apertures to couple light to the sample that forms the basis of an interferometric signal indicative of the relative distance between the mask and the sample. The interferometric signal can be the basis of a control signal in a servo system that dynamically positions the mask relative to the sample.

Abstract: A phase shift interferometer (4) images a plurality of interference fringe images each generated from interference between a test light reflected at a test surface (3a) and a reference light reflected at an reference surface (5) by emitting a coherent light to both surfaces while varying a phase difference between the test light and the reference light from image to image. A surface shape analyzer (12) analyzes the plurality of interference fringe images taken by the phase shift interferometer (4) to obtain surface shape information of the test surface (3a). A typical plane computer (13) computes a typical plane of the test surface (3a) on the basis of the surface shape information obtained from the surface shape analyzer (12). An oblique angle computer (14) computes an oblique angle of the typical plane computed by the typical plane computer (13) to the reference surface (5).

Abstract: Average fringe contrast is determined by PSI measurements at each step of relatively large translations of the reference optics and mirror of a white-light Linnik interference objective and the resulting data are used to minimize the OPD between the reference and test beam paths of the instrument. Utilizing the same algorithms used in the art to perform conventional vertical scanning interferometry, the position of minimum OPD is determined by finding the position of maximum average fringe contrast. Furthermore, by automating the system with a precise translation mechanism, operator-to-operator variations are completely eliminated and the precision of the process of OPD minimization is greatly improved. In another embodiment of the invention, the optimal focal position of the reference mirror is found at the same time by calculating the average lateral variation of fringe contrast as a function of OPD and focal position.

Abstract: An apparatus for measuring an object's surface shape that obtains complex amplitude at a plurality of positions substantially in the directions of the optical axis of the light reflected by the object and calculates the first surface shape of the object by the depth-from-focus principle. On the other hand, using the phase data of the complex amplitude, the apparatus can measure a micro shape in the sub-fringe order by ordinary laser interference measurement. The first surface shape data are applied to a region including a step of a height greater than ½ of the light's wavelength and second surface shape data are applied to the other region.

Abstract: An adapter for mounting on an interferometer to provide a phase shift of the reference transmission sphere or flat used on the output of the interferometer is made of a monolithic body separated into two portions. The first portion is a fixed end support portion that mounts on the interferometer, and the second portion is a movable end support portion that mounts the sphere or flat that is used for transmission of the optic beam. The movable end support portion is mounted and the fixed end support portion are connected through flexure hinges that permit axial movement between the support portions, and a single actuator can be used between the support portions for causing axial movement. The flexure hinges are located so the adapter is very stiff against all motions except axial motions.

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: Fringe image data obtained by use of the phase shift method is Fourier-transformed, so as to determine the carrier frequency and the complex amplitude which occur due to the deviation between the respective wavefronts from the object to be observed and a reference. According to the carrier frequency and the complex amplitude, the amount of translational displacement of phase shift and the amount of inclination are detected, so as to correct the results determined by the phase shift method. Thus, the influences of errors in the amount of inclination/amount of displacement in the phase shift element can be eliminated easily.

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: In the case of a method for absolute calibration of an interferometer with the aid of a spherical output wave, comprising an optical element which retroflects the incident spherical wave itself or via a mirror, at least four measuring procedures are undertaken to determine a wave aberration W. The optical element is measured in the at least four measuring positions intrafocally and extrafocally in at least two different rotational angle positions. It is also possible, in addition, to undertake a measurement via a mirror in the cat's eye position (focus 3).

Abstract: Three aspect of a interferometer system for testing and characterizing micro-optical components an automatic system for testing a plurality of micro-optical components in sequence, a special holding device including a vacuum chuck arrangement allows for individual micro-optical components to be picked up and held during testing, and a modified Linnik objective is used with short coherent light and preferably a opaque reference sphere to carry out reflection tests on micro-optical component having at least one curved surface.

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: There is provided an interferometer for measuring a surface shape of an optical element using interference, including a reference wave-front generating unit for generating a reference wave front for measuring the surface shape, which is provided in a target optical path, and includes an Alvarez lens.

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: An oblique incidence interferometer in which laser light from a laser light source is transmitted through a reference plane and is made incident upon a measurement surface from an oblique direction. The shape of the measurement surface is measured on the basis of interference fringes formed by reflected light reflected from the reference plane and reflected light reflected from the measurement surface. A deflecting element for deflecting the laser light emitted from the laser light source in order to decrease the coherence of the laser light is disposed in an optical path leading to the reference plane, the deflecting element being adapted to continuously change the direction of deflection.

Abstract: A surface profile measuring apparatus for measuring rugged profiles of object surfaces includes a white light source, a varying device for varying a relative distance between an object surface and a reference surface, a frequency band limiting device for limiting the frequency band of white light to a particular frequency band, an image pickup device for picking up an image of the object surface, a sampling device for successively acquiring intensity values of interference light corresponding to varying interference fringes, from a particular location at sampling intervals corresponding to a bandwidth of the particular frequency band, a storage device for storing a group of interference light intensity values acquired, and a computing device for estimating characteristic functions from the group of interference light intensity values stored, and determining a height in the particular location based on a peak position of the characteristic functions.

Abstract: A shape measuring apparatus is provided, which is capable of accurately measuring the shape of an observed surface of an object to be measured and which can be designed to be compact in size and light in weight. An optical reflected image from an observed surface of an object to be measured and an optical reflected image from a reference surface are interfered with each other to generate interference light. The interference light is dispersed into at least three beams, which are shifted in phase by respective predetermined phase shifting amounts. At least three interference fringe images are simultaneously captured by at least one image pickup device which is smaller in number than the interference fringe images.

Abstract: A method for interferometric measurement comprises emitting waves onto a reference surface (1) and onto a measured object (2), which each reflects a part of the emitted waves, and receiving both the reflected parts of the waves by the same receiver (4). In the receiver the parts of the waves generate a representation of the measured object (2) in the form of an interferogram, from which the form of the measured object (2) is determined. The emitted waves comprise waves of three well-defined wavelengths &ngr;1, &ngr;2 and &ngr;3, where &ngr;1≠&ngr;2≠&ngr;3, which wavelengths are chosen to substantially satisfy the mutual relation &ngr;1=(&ngr;2.&ngr;3)/(2.&ngr;3−&ngr;2) and generate an interferogram each in the receiver (4).

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: Techniques for performing phase-shifting interferometry are disclosed.

Abstract: There is provided an interferometer for measuring a surface shape of an optical element using interference, including a reference wave-front deformation system for deforming a wave front of reference light.

Abstract: A scanning wide-area surface shape analyzer drastically shortens a time period required for measuring the surface shape of a surface to be inspected. A scanner moves the measuring head in parallel with a flat surface for the measuring head to acquire the measured image data. An attitude-varying mechanism varies the acquisition attitude of the measuring head for acquiring the measured image data. An image processing control device analyzes and calculates the surface shape of an acquisition area corresponding to the acquisition position and in the acquisition attitude. The image processing control device controls driving of the scanner and the attitude-varying mechanism, calculates the next acquisition position by using measured image data acquired at the present acquisition position and in the present acquisition attitude, and calculates an acquisition attitude to be taken at the calculated next acquisition position.

Abstract: An improved method and apparatus for profiling surfaces is provided. The subject apparatus avoids the earlier used constructions of such apparatus in the normally used Fizeau interferometer form, and instead uses a retroreflector (200) located at the end of the optical path (128) of the beam reflecting off of the surface under test (150) in order to achieve alignment-free surface profiling. In addition, in a second embodiment, a second retroreflector (140′) is used to assure a more accurate result. While in yet another improvement, the retroreflector (200) is selectively moveable in relation to the optical path of the beam, to compensate and correct for a shearing effect resulting from the use of variously sized (thicknesses) flats under test.

Abstract: The present invention provides a mask for measuring an optical aberration, the mask including at least a measuring pattern comprising plural pattern parts being separated from each other, wherein the plural pattern parts provide individual widths which are simply increased on first and second directional axes non-parallel to each other and vertical to a plane of the mask, provided that the width of each of the plural pattern parts is unchanged at least on the first and second directional axes.

Abstract: The present invention disclose a method for measuring a thermal expansion coefficient of a thin film, in which the thin film is first deposited on two substrates having different thermal expansion coefficients under the same conditions. For each of the two deposited substrates, a relationship between the thin film stresses and the measuring temperatures is established by using a phase shifting interferometry technique, in which the stresses in the thin films are derived by comparing the deflections of the substrates prior to and after the deposition. Based on the two relationships the thermal expansion coefficient, and elastic modulus, E f ( 1 - v f ) , can be calculated, wherein Ef and &ngr;f are the Young's modulus and Poisson's ratio of the thin film, respectively.

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 electronic template representing the topography of a magnetic head being tested and delineating distinct patterns corresponding to particular regions of interest, such as ABS surfaces, is provided in order to limit analysis of the sample's surface to such regions. The outline of the slider positioned within the field of view of an interferometric microscope is identified by measuring the modulation of incident light at each pixel inside and outside the contour of the sample using the loci of perceivable fringe contrast as the criterion for establishing the location of such edges. Once the outline of the slider within the plastic tray is so established, the slider is shifted within the field of view to match the position of the template, thereby automatically achieving a precise alignment of the template with the boundary of the slider.

Abstract: A measurement system (10) for accurately measuring the surface geometry of a part (32) in three dimensions includes a laser (12) for emitting a laser beam (14). The laser beam (14) is transmitted to a birefringent crystal (16) which splits the laser beam (14) into a pair of beams (18,20). The pair of beams (18,20) are then subjected to a phase shift by a liquid crystal system (24) as controlled by a computer (26). The pair of beams (18,20)are then expended in order to form a fringe pattern (30) on the surface of the part (32) to be measured.

Abstract: An interferometric system for measuring the radius of curvature of a measurement object that includes a tunable coherent radiation source capable of emitting a radiant energy beam having a characteristic wavelength and of scanning the characteristic wavelength over a range of wavelengths; an unequal path interferometer which during operation includes a reference object and the measurement object and which receives a portion of the radiant energy beam from the tunable radiant energy source and generates an optical interference pattern; a detecting system including a detector for receiving the optical interference pattern; and a system controller connected to the tunable radiant energy source and the detecting system.

Abstract: An improved method and apparatus for profiling surfaces is provided. The subject apparatus avoids the earlier used constructions of such apparatus in the normally used Fizeau interferometer form, and instead uses a retroreflector (200) located at the end of the optical path (128) of the beam reflecting off of the surface under test (150) in order to achieve alignment-free surface profiling. In addition, in a second embodiment, a second retroreflector (140′) is used to assure a more accurate result. While in yet another improvement, the retroreflector (200) is selectively moveable in relation to the optical path of the beam, to compensate and correct for a shearing effect resulting from the use of variously sized (thicknesses) flats under test.

Abstract: An interferometer scans the sample surface laterally with respect to the optical axis of the interferometric objective. The objective is tilted, so that the sample surface is placed at an angle with respect to the maximum coherence plane of the instrument. By moving the sample stage laterally, at an angle, through a point at a set distance from the objective on the objective's optical axis, rather than vertically along the optical axis, different parts of the object intersect the maximum coherence plane at different times as the surface passes through the coherence plane, the precise time depending on the profile of the surface. When the OPD of a point on the object's surface is greater than the coherence length of the light source, the intensity of light reflected from this point does not produce interference fringes. Therefore, the intensity registered by the detector is approximately constant.

Abstract: The invention features methods and systems in which wavelength-tune PSI data is analyzed in the frequency domain to produce spectrally separated frequency peaks each corresponding to a particular pair of surfaces in an interferometric cavity defined by multiple pairs of surfaces. Each frequency peak provides optical path length information about a corresponding pair of surfaces in the cavity. As a result, the interferometric data from such cavities provides simultaneous information about multiple surfaces.

Abstract: A measurement system is used in particular to detect defects in test objects. To that end, the test object is lighted from a lighting unit that includes a plurality of laser diodes. The beam cones of the individual laser diodes generate a common lighting spot on the test object. The object being measured is observed interferometrically, where the interferometer is part of a measurement head. An electronic pattern sensor detects interference patterns generated by the interferometer.

Abstract: A surface shape measuring apparatus that can measure surface shapes of optical elements such as lenses and mirrors with high degree of accuracy, and a method for calibrating a lateral coordinate of the surface shape measuring apparatus, are disclosed.

Abstract: The method is used for the interferometric measurement, in particular for the interferometric absolute measurement, of non-rotationally symmetric wavefront errors on a specimen. The specimen can in this case be brought into a plurality of rotational positions, at least one measurement result being determined in each of the rotational positions. A mathematical evaluation of all measurement results is carried out in conclusion. The measurement is carried out in at least two measurement series (M, N). The measurement results (M1 . . . Mm, N1 . . . Nn) of each of the measurement series (M, N) are respectively determined in mutually equidistant rotational positions of the specimen. Each of the measurement series (M, N) comprises a specific number m, n of measurements. The individual numbers m, n are natural and mutually coprime numbers.

Abstract: A hybrid curvature/tilt wave front sensor (50) that employs both tilt measurements and curvature measurements of the wave front of a light beam (16). The light beam (16) is split into a first path and a second path. The light beam (16) on the first path is directed to a tilt sensor (12) employing a lenslet array (24) having a plurality of lenses (26). The lenses (26) focus separate portions (32) of the wave front onto a CCD (28) that provides local intensity measurements. A computer (30) receives electrical information of the intensity of the various beam portions (32) and computes a tilt measurement based on this information. The light beam (16) on the second path is directed to a curvature sensor (14) that includes a pair of CCDs (40, 42) positioned at the same distance before and after the focal plane of a lens (52). The intensity measurement of the beam (16) at these locations is sent to the computer (30) which performs curvature measurements on the beam wave front.