WO2016067268A1

WO2016067268A1 - Method, system and subsystem for interferometrically determining radius of curvature

Info

Publication number: WO2016067268A1
Application number: PCT/IB2015/058415
Authority: WO
Inventors: Arkadiusz SWAT
Original assignee: Swat Arkadiusz
Priority date: 2014-10-30
Filing date: 2015-10-30
Publication date: 2016-05-06

Abstract

Invention concerns a method and corresponding apparatus for interferometrically determining a radius of curvature r_x of a curved surface (4a) of a test surface object. The method comprises known in the art steps: step of positioning the test surface relative to a reference surface of reference surface object, along the optical path, step of illuminating reference surface object and test surface object with a substantially monochromatic light beam from the first light source, along the optical path, step of moving the reference surface object and test surface object until confocal position of reference surface and test surface is detected, step of determining radius of curvature of the test surface from position of the test surface object. In the method according to the invention, as opposed to the state of the art, the step of determining radius of curvature of the test surface from position of the test surface object consists in determining the radius of curvature of the test surface from the radius of curvature r_rof the reference surface and the distance d between the reference surface object and test surface object. The distance d between the test surface object and reference surface object is determined in following steps: illuminating the optical system with a broadband measuring- light ray along the measurement optical path, while delivering the portion of this broadband measuring-light to the reference optical path having tunable length, superimposing light beams reflected from the reference surface and test surface with the light propagating along the reference optical path, tuning the length of the reference optical path and observing superimposition of the light to detect at least two interference patterns each corresponding either to condition that length of optical path of light reflected from the reference surface is equal to the length of reference optical path or to condition that length of optical path of light reflected from the reference surface is equal to the length of reference optical path, determining the distance d from the difference of length of tuned reference optical path corresponding to both conditions.

Description

Method, system and subsystem for interferometrically determining radius of curvature

[ 0001 ] Invention relates to the interferometric measurement system. More specifically invention relates to the radius of curvature measurements.

[ 0002 ] Differential techniques for measuring radii of curvature are discussed, for example, in "Differential technique for accurately measuring the radius of curvature of long radius concave optical surfaces" published in SPIE, 192, 75-84, 1979. r

[ 0003 ] A general method for determining the radius of curvature of an object's surface known in the art is based on a differential measuring technique it was disclosed for example with respect to the Fizeau interferometer, by Lars A. Selberg in "Radius measurement by interferometry" , Optical Engineering, Vol. 31, No. 9/ 1961, September 1992 incorporated herein by reference. This paper discloses a number of errors related to required movement of the test object.

[ 0004 ] In general, the measurement setup includes a monochromatic light source, an unequal path interferometer cavity, and a detection system. The interferometric cavity is formed between a reference sphere and a curved test object. The curved test object is placed relative to the reference sphere in the interferometric cavity to form a confocal cavity. In a confocal cavity, an optical wave front returning from the cavity after one or more reflections from the test object surface, is substantially identical to an optical wave front reflecting from a reference surface. Confocal position of two reflecting elements is sometimes also called concentric position - Optical Shop Testing Third Edition by DANIEL MALACARA John Wiley & Sons, Inc.

[ 0005 ] Such situation does take place when the reference object and object under test are in such relation with respect to each other that the centers of curvature of their surfaces are located in the same point.

[ 0006 ] The test object having a curved surface is usually mounted on a rail, that allows movement along the axis of the confocal cavity. The rail is equipped with distance measurement system. The position of the test object is read, and noted it with dl . [0007] Then the test object is translated to so called cat's eye position. The cat's eye position is when the vertex of the test object is at the centre of curvature of the reference sphere. Then again, optical wave front returning from the cavity after one or more reflections from the test surface of test object, is substantially identical to an optical wave front reflecting from a reference surface. The position of the test object is read again, and noted with d2.

[0008] The two positions of the test object corresponding to distance dl and d2 are distinct as only for these positions two interfering wavefronts are substantially identical.

[0009] The test object radius of curvature r_x is calculated by simple subtraction r_x=|dl-d2| .

[0010] That method, however, requires test surface to be translated from confocal to cat's eye position which takes time, requires rail equipped with a distance measurement system and more importantly is prone to operator errors. This problem have been indicated in US patent document US20020126293 a solution is provided. The same document provides disclosure of the automated system providing solution to this problem. Said system is however overly complex and did not find commercial use. Modification of the existing systems for benefit from the features of the solution proposed therein is not possible.

[0011] The purpose of the invention is therefore to overcome the problems with the state of the art measurements indicated above.

[0012] Invention provides a method for interferometrically determining a radius of curvature r_x of a curved surface 4a of a test surface object. The method comprises known in the art steps: step of positioning the test surface relative to a reference surface of reference surface object, along the optical path, step of illuminating reference surface object and test surface object with a substantially monochromatic light beam from the first light source, along the optical path, step of moving the reference surface object and test surface object until confocal position of reference surface and test surface is detected, step of determining radius of curvature of the test surface from position of the test surface object. In the method according to the invention, as opposed to the state of the art, step of determining radius of curvature of the test surface from position of the test surface object consists in determining the radius of curvature of the test surface from the radius of curvature r_r of the reference surface and the distance d between the reference surface object and test surface object. The distance d between the test surface object and reference surface object is determined in following steps: step of illuminating the optical system with a broadband measuring-light ray along the measurement optical path, while delivering the portion of this broadband measuring-light to the reference optical path having tunable length, step of superimposing light beams reflected from the reference surface and test surface with the light propagating along the reference optical path, step of tuning the length of the reference optical path and observing superimposition of the light to detect at least two interference patterns each corresponding either to condition that length of optical path of the light reflected from the reference surface is equal to the length of reference optical path or to condition that length of optical path of the light reflected from the reference surface is equal to the length of the path of the light propagating along reference optical path, step of determining the distance d from the difference of length of tuned reference optical path corresponding to both conditions .

[0013] Advantageously distance d is determined as a difference of the lengths of reference optical path corresponding to two occurrences of at least two local maxima in the function of contrast of the interference pattern versus the length of optical path.

[0014 ] Advantageously the tuning of the length of the reference optical path is done by moving reflecting element located at its end .

[0015] A method according to the invention advantageously comprises additional step of applying residual power correction.

[0016] Advantageously the step of determining radius of curvature of the test surface from the position of the test surface object using distance between test surface and the reference surface involves subtraction of the reference radius of curvature of the reference surface (3a) from the distance d between the reference surface and the test surface. [0017] Advantageously steps of the method are performed for at least three objects that are measured when forming confocal cavity one with each other to give at least three values of the distance between each pair, where the step of determining radius of curvature of the test surface from the position of the test surface object using distance d between test surface and the reference surface

(3a) involves solving a set of at least three equations with respect to radii of curvature.

[0018] The invention further provides a distance determination subsystem for interferometric measurement system having an illuminating device, a reference surface object located in the first path of the light produced by the illuminating device and test surface object movable with respect to the reference surface object along the first path. The distance determination subsystem comprises: the second light source producing broadband light beam, the second beam splitting element dividing the light beam into measurement optical path and the reference optical path, so that reference surface object and test surface object are part of the measurement optical path. Reference optical path has tunable length and is ended with reflecting element such that the range in which length reference optical path is tuned corresponds to the range in which reference object and test object can be spaced from each other. The subsystem further comprises means for superimposing light reflected form the elements located in the measurement optical path with light reflected from the reflecting element located in reference optical path in the detecting device adapted to detect interference pattern.

[0019] Advantageously the distance determination subsystem is provided with a beam forming element within the reference path (22a) . That element forms a light beam before it reaches test surface element or reference surface element. Preferably it forms the light beam in such a manner that beams reflected from both test and reference surface are focused in the same point - center of the reference surface curvature. When this is obtained the level of the signal returning to the detector reaches the highest value. Particularly, when light is fed through the reference element then the formed beam shape typically is a shape of parallel beam. [ 0020 ] The beam splitting element and means for superimposing the light advantageously are the same element. It is possible to achieve this when light returning along the measurement path is combined with the light returning along reference path in the beam splitting element that further split the light into those two paths. However, it is also possible to use optical couplers in to obtain a measurement setup in which light travels along reference path only in one direction. Then separate beam splitting element and means for superimposing the light are required.

[ 0021 ] The length of reference optical path is advantageously tunable by moving of the reflecting element located on its end.

[ 0022 ] Optical path is advantageously formed by an optical fiber and its length is tunable by mechanical stretching.

[ 0023 ] Advantageously distance measurement subsystem contains processing unit provided with means to determine said difference in the length of the reference optical path (22b) and thus determine distance d , means to calculate difference between distance d and radius r of curvature of the reference surface (3a) . Optionally it further comprises displaying means connected to the processing unit and adapted to display resulting radius of curvature of the test surface. Optionally it further comprises memory device connected to the processing unit and adapted to store a number of radii of curvature of the test surface.

[ 0024 ] The invention further provides an interferometric measurement system having an illuminating device comprising the first light source producing a substantially monochromatic light beam, the first beam splitting element located in the light beam produced by the first light source and directing the light beam from the first light source into the interferometric cavity formed by reference surface and test surface, and detecting device adapted to detect interference pattern, where the test surface object is movable with respect to the reference surface object along the optical path. The system comprises additional distance determination subsystem comprising the second light source producing broadband light beam fed to the second beam splitting element dividing the light beam into the measurement optical path and the reference optical path, so that reference surface object and the test surface object are located in the measurement optical path. The reference optical path has tunable length and a reflecting element. The subsystem further comprises means for superimposing light reflected form elements located in the measurement optical path with light reflected from the reflecting element located in the reference optical path, and a detecting device adapted to detect an interference pattern.

[0025] The beam splitting element and means for superimposing the light advantageously are the same element.

[0026] The length of reference optical path is advantageously tunable by movement of the reflecting element located on its end.

[0027] The reference optical path is advantageously formed by an optical fiber and its length is tunable by mechanical stretching.

[0028] The beam splitting element advantageously is a fiber optic coupler .

[0029] Advantageously the distance measurement subsystem contains a central processing unit capable of controlling the length of optical path and identifying the interference pattern detected by a detecting device.

[0030] The interferometric system according to the invention advantageously is provided with means to detect and/or measure the residual power. Detecting device can be adapted to this purpose. Detailed description of embodiments

[0031] The invention will now be explained with reference to the drawings attached, wherein

Fig. la presents an optical system suitable for use with a method of measurement of the radius of curvature according to the invention,

Fig. lb presents a test element and a reference element forming a cavity,

Fig. lc presents a test surface and a reference surface in concentric position, forming confocal cavity, with indication of radii of curvature and the distance between the surfaces,

Fig. 2 presents a distance determination subsystem according to the invention, Fig. 3 presents light intensity variation as the optical path is changed, for position 0 the modulation is the maximum, as the fringe contrast is maximal for 0 optical path difference,

Fig. 4a presents an embodiment of optical system for measurement of the radius of curvature according to the invention,

Fig. 4b presents the cavity formed by test surface element and reference surface element in line with tunable reference path of the distance measurement subsystem,

Fig. 5a presents an alternative configuration in which light is fed when light is fed directly into the interior of the confocal cavity, formed by test surface and reference surface,

Fig. 5b presents alternative embodiment of the system according to the invention, when light is fed directly into confocal cavity formed by test surface and reference surface,

Fig. 6a presents a confocal cavity formed by a test surface and a reference surface when light is fed from the side of the test surface,

Fig. 6b presents alternative embodiment of the system according to the invention, with light fed into confocal cavity formed by a test surface and a reference surface from the side of the test surface,

[ 0032 ] A measurement system compatible with the method and distance measurement subsystem according to the present invention is presented in Fig. la. It comprises a monochromatic laser as first light source 1 providing light beam to the beam expander 7 which gives it desired size and shape. The beam then protrudes through beam splitting element 2 preferably a beam splitting plate, protrudes through reference surface element 3 to reach test object 4. Reference surface element 3 and test surface object 4 form a cavity presented in Fig. lb. Reference surface 3a of a reference surface element 3 and test surface 4b of a test element have spherical shape. The test surface element is mounted on the rail allowing one dimensional adjusting along the light propagation path. Light reflected from the reference surface 3a and test surface 4a returns to the beam splitting element 2 and is directed to the detecting means 5. That configuration forms typical Fizeau interferometer. A device formed by the first light source 1, beam splitter 2 and detecting device 5, with possible additional elements form available of the shelf illuminating device denoted by 9. The beam splitter can be in any other beam splitting element known in the art, for example beam splitting prism, plate, pellicale etc.

[ 0033 ] According to the invention the test object is not moved between catseye position and confocal position, as opposed to the state of the art methods. Instead test surface object 4 is moved along the rail as long as confocal cavity is formed by reference surface 3a and test surface 4a, as presented in Fig. lc. The confocal position corresponds to one of two maximal values of the interference pattern contrasts are obtained. The other maximal value corresponds to the catseye position. In contrast to the state of the art methods and measurement systems, the test surface object 4a is thereafter moved no more. Methods and devices for setting up surface in confocal relation to each other are well known in the art. Optical Shop Testing Third Edition by DANIEL MALACARA John Wiley & Sons, Inc., provides comprehensive study of the matter.

[ 0034 ] In a perfect confocal position the distance between the test surface 4a and reference surface object 3a is equal exactly to sum of the known curvature radius r₀ of the reference surface 3a and unknown curvature radius r_x of the test surface object 4a.

[ 0035 ] In fact the perfect confocal position is impossible to achieve, also it is impractical to make substantial effort to approach the perfect position. Practically once the near confocal position is set, as long as the residual power ΔΡ is a fraction of a fringe i.e 0.5-0.1. The numerical value of the residual power is important though to perform most precise radius of curvature calculation. Further, the Fizeau system shall be capable of indicating both magnitude and the sign of the residual power term , commercially available systems usually have such a capability built in, especially phase shifting/changing systems.

[ 0036 ] Hence once the distance d between test surface object and reference surface object is determined, the residual power term ΔΡ in Fizeau testing is extracted and the sign is established, the unknown curvature radius of the test surface object can be calculated by simple equation:

r_x=d-r₀+AP [ 0037 ] where ΔΡ is the residual power converted to correction in radius of curvature units. Calculation from fringes to radius is commonly known, i.e. ISO 10110-5:2007, Annex A. Hence further movement of the test object and further actions of the operator of the system are not necessary. Negative result of r_x indicates convex test surface. Therefore, consistently, should r₀ be concave, the sign is positive.

[ 0038 ] Application of the method according to the invention requires interferometer with additional distance measurement subsystem. Measurement system according to the invention with state of the art interferometer and distance measurement subsystem according to the invention is presented in Fig. 2.

[ 0039 ] The distance measurement subsystem according to the invention is provided with the second light source 21 producing broadband light. Light beams produced by the same broadband source produce visible interference only when certain condition is met. This condition is that two superimposed light beams originating from the same source are travelling through optical paths that have nearly exactly the same length. Otherwise overlapping interference patterns generated at different wavelengths add up to form non- detectable pattern. More precisely, for the second light source of limited but still broad bandwidth, the contrast of interference fringes observed decreases as optical path lengths are tuned out of the perfect equality. A plot of light intensity versus the difference of o lengths of two optical beams originating from the same broadband source is presented in Fig. 3. It was obtained for the wavelength 2 = 1310nm and bandwidth of 52 = 30nm and published in "Non-contact in-process metrology using a high accuracy low- coherence interferometer" by A. Courteville, R. Wilhelm, M. Delaveau, F. Garcia in Laser Metrology and Machine Performance VII, 7th International Conference and Exhibition on Laser Metrology, Machine Tool, CMM and Robotic Performance Lamdamap 2005, pp. 534 — 544, 2005. The difference Ad is in μνα. The changes observed in the plot evidence fringe scan through a point detector while fringe contrast is changing. The contrast measured as ratio between the maximal and minimal light intensity in certain range is highest when optical paths are exactly the same and is gradually decreasing as optical path difference is increasing. The broader the bandwidth of the second light source is the steeper the decrease of contrast with the difference of optical path Ad is. That phenomenon is utilized for distance measurements. As disclosed by A. Courteville et . All, if layered object is illuminated by a broadband beam and reflected light is superimposed with reference beam originating from the same source and propagating tunable reference optical path then as the length of reference optical path is tuned subsequent contrast maxima corresponding to subsequent layer can be detected. Each of the maxima resembles what is observed for Ad = 0 in Fig. 3. Application of this method to the state of the art interferometer makes it possible to measure the distance d between the test surface 4a and the reference surface object 3a when they are in confocal position, without actually touching them or moving them and then determine the unknown radius of curvature of test surface 4a as simple subtraction.

[0040 ] This is implemented in distance measurement subsystem according to the invention. It comprises the second light source 21, providing a beam of broadband light to the second beam splitting element 22 which divides the light beam into measurement optical path 22a and tunable reference optical path 22b. Optical path 22a provides light to the cavity formed by the test surface 4a and reference surface 3a. Desired shape and size of the beam can be obtained with a collimator at the end of the optical path 22a (not shown in Figures) . As described in Fig. 2 in the first embodiment light enters the cavity propagating first through the reference surface object 3 comprising state of the art beam shaping element attached thereto. Firstly light is partly reflected from reference surface 3a and propagates back to the distance measurement subsystem. Remaining portion of light travels towards the test surface object 4 and is again partly reflected from test surface 4a to finally propagate back. As light travelling back along the path 22a reaches back of the second beam splitting element 22 it is superimposed with reflected light propagating back along reference optical path 22b.

[0041 ] The optical path 22b is a reference optical path of known and tunable optical length. It can be created by a movable mirror 29 located on a rail reflecting the light back. The range of tenability of the reference optical path has to be broader than the measured distance Ad .

[0042 ] Light propagating back along the measurement optical path 22a is superimposed with light propagating back along the path 22b as they reach the second detecting device 25. It can be a photodiode capable of detecting the light intensity.

[0043 ] It is pointed out that there are two portions of light travelling through measurement optical path 22a. The first is reflected from the reference surface 3a and the second reflected from the test surface 4a. The optical lengths of propagation of these two portions of light differ exactly by double distance between test surface 3a and reference surface 4a. Therefore, there are two positions of mirror 29 for which maximum fringe contrast can be observed as shown in Fig. 3.

[ 0044 ] The difference between the lengths of the optical paths for which two adjacent maxima of the contrasts were detected corresponds to the double distance 2Ad between the test surface 4a and reference surface 3a. As in this embodiment the length of reference optical path 22b is tuned by relocation of the reflecting element 29 namely a mirror, as presented in Fig. 4b. Maximal values of contrast are obtained for two positions of the mirror Zl and Z2. Position Zl corresponds to the situation when optical path of light reflected back from the reference surface 3a is equal to the length of reference path. Position Z2 corresponds to the situation when optical path of light reflected back from the test surface 4a is equal to the length of reference path. The distance Ad between test surface 4a and reference surface 3a corresponds to the difference of positions Zl and Z2 of the mirror 29.

[ 0045 ] Optical lenses that are typically applied in interferometer before reference surface element 3 are construed so that when parallel beam reaches reference surface element from the light source 1 i.e. light is coming from left to right side of Fig. 4a and Fig. 4b, then the light is focused in the center of curvature of reference surface 3a. This point is denoted by 0 in Fig lc. When in distance determination subsystem a beam forming element 28 that forms a parallel beam of broadband light, then this light also is focused in point 0. That situation corresponds to maximal returning signal . [ 0046 ] If reference object 3 is prepared so that only reference surface 3a reflects light the said two maxima are the only ones. Otherwise there might appear more. The two relevant ones can be selected based on optional initial rough estimation of the Ad value or any other obvious conditions they must meet.

[ 0047 ] As the test surface element 4 and reference surface element are known to be in confocal position at this point and the radius of curvature of the reference surface element 3 is known, determination of the radius of curvature of the test surface element 4 is merely a matter of subtraction.

[0048 ] Naturally the question how the first reference radius r₀ was obtained is here in place. There are many possibilities available, here 2 methods are briefly described. These methods can be performed on the same equipment as the test surface measurement.

[ 0049 ] First, preferred method of determination of radius of curvature of reference surface requires three measurements of three future reference spheres is actually a variant of the method according to the invention. It is common in optical workshops and laboratories to have a set of so called Transmission Spheres TS, as accessories for Fizeau interferometer measurements, therefore there is no need to have a dedicated set of reference spheres for the purpose of determination of the value of radius r₀ reference surface

3a.

[ 0050 ] There are analogies in the state of art for this type of calibration, so called "three flat method" - D. Malacara, chapter 1.5 "ABSOLUTE TESTING OF FLATS" or "two sphere method" - Zygo OMP- 0388B, "Two Sphere Application". Yet these are not as advantageous.

[ 0051 ] Three transmission Spheres SI, S2 and S3 having radii of curvature ?oi<^r02<^r03' respectively are set in 3 measurements sequence. Namely each pair of them (S1,S2), (SI, S3) and (S2, S3) is set up to form confocal cavity. During the three measurements A, B, C, three respective distances between the surfaces forming the cavity are measured: d_A12, d_B13, d_C23. Therefore a set of three equations with three unknowns can be written:

d B,13 = r₀₁ + r₀₃ + APi B <^C23 ^{= r}02 + ^r03 + AP_C

Solving this set of equations with respect to the ₀₁,r₀₂,r₀₃ results in three accurate values of radius of curvature. Each of SI, S2, S3 can be a reference surface 3 thereafter.

[ 0052 ] The second method is simpler, the measurement of distance d for the case when a well polished surface is positioned at cat's eye position, then the following equation applies d=r_s+AP . Although this is very appealing, assessment of residual power ΔΡ is less certain than in confocal cavity.

[ 0053 ] Naturally the identification of maximal contrast is only one of the methods to locate the point at which optical distances travelled by light propagating along measurement optical path 22a and reference optical path 22b are equal. As it is the light intensity what is detected it produces a signal as length of path 22b is tuned. Extraction of the distance form that signal can be done in a number of signal processing techniques. One of them is calculation of autocorrelation function of that signal to measure the its value instead of calculating the contrast. There are numerous other techniques known to those skilled in the art. It is clear that they all lay in the scope of protection as defined in claims .

[ 0054 ] In another embodiment of the invention broadband light from the distance measurement subsystem is delivered into the middle of the space between test surface object 4 and reference surface element 3 as presented in Fig. 5a and Fig. 5b. The light is delivered in-between reference object 3 and test object 4 in conventional way and then split by means of the beam splitter 26 into two beams one of which is directed towards reference surface 4a and the second towards test surface 3a. Light reflected from test surface 3a and reference surface 4a is combined when reaches again beam splitter 26 and propagates back along optical path 22a. This embodiment is advantageous as it allows using the distance measurement subsystem without any other modification to or interaction with the classical off-the shelf Fizeau interferometer. Beam splitter 26 is advantageously construed in such a manner that it forms a beam of light that when reflected from test surface 3a is focused in point 0 corresponding to its center of curvature. [ 0055 ] However it is not always possible to introduce any object inbetween surfaces 3a and 4a. For example when test surface 4a is convex then in a confocal position both reference surface 3a and test surface 4a are located on the same side of the center of curvature 0 of the reference surface 3a and the space between them is severely confined. In such a situation it is possible to either apply the first embodiment of the invention and introduce broadband light through the reference object 3 or to introduce the light into the cavity from the opposite side, through the test surface object 4, as presented in Fig. 6a and 6b. Such approach is applicable only when test element 4 is substantially transparent. It also may require additional beam forming element 28 to form the broadband light before it reaches test surface element. The test object 4 as opposed to reference object 3 is not equipped with lenses that guarantee that parallel beam of approaching light after coming through the test element is going to be focused in the point 0 corresponding to the center of curvature of reference surface. Therefore in this embodiment the shape of the beam desired after beam shaping element 28 is not necessarily parallel. Beam shaping element should be selected appropriately to provide a light beam shape that meets above requirement. If that requirement is not met, namely if beam of broadband light used for distance determination is not focused exactly in point 0 then the system still operates but the level of the received signal is lower and detection is more difficult.

[ 0056 ] There is a number of methods of tuning the optical path known in the art. Many of them may be applied for the present invention.

[ 0057 ] In another embodiment optical path 22b is at least partly made of the fibre optic wound on the reel. The Fibre optic is then stretched and released to obtain reliable tuning of the length. The tension angle of rotation of the reel can be then recalculated to the change of reference optical path 22b.

[ 0058 ] In certainly advantageous embodiment the distance measurement subsystem is provided with a processing unit and electronically steerable mechanisms for tuning the reference optical path 22b, such as rotors or motorized rail. The signal from the detecting device 25 is fed to the processing unit. Such configuration makes it possible to implement automatic measurement with a signal processing method for precise detection of the relative distance between optical path lengths for which conditions of optical paths equality are met. Application of the detector allows observing an interference pattern in real time to speed up positioning. Further providing the system with a memory makes it possible to store signals as presented in Fig 3 specific for certain reference object and even further enhance precision by filtering and calibration.

[ 0059 ] There are multiple ways to split the light to form the light and to direct light. Invention can be embodied with the use of lens and mirror system, fibre optics or other means known in the art including their combination. Therefore it is stressed that the scope of protection is not limited to the embodiments but defined by the claims attached.

Claims

1. A method for interferometrically determining the radius of curvature r_x of a test surface (4a) of a test surface object (4), the method comprising steps of:

positioning the test surface (4a) relative to a reference surface (3a) of the reference surface object (3) , along the optical path (2a) ,

illuminating the reference surface object (3) and the test surface object (4) with a substantially monochromatic light beam from the first light source (1), along the optical path (2a) ,

moving the reference surface object (3) or the test surface object (4) until confocal position of the reference surface (3a) and the test surface (4a) is detected, and the test surface (4a) forms confocal cavity with the reference surface (3a) ,

determining the radius of curvature of the test surface (4a) using position of the test surface object (4), characterized in that

the step of determining the radius of curvature of the test surface (4a) from the position of the test surface object (4) consists in determining the radius of curvature of the test surface (4a) using a distance d between the reference surface object (3) and the test surface object (4), where the distance d between the test surface (4a) and the reference surface (3a) is determined in following steps: illuminating the confocal cavity formed by the reference surface (3a) and the test surface (4a) with a broadband measurement light along the measurement optical path (22a) , while delivering the portion of this broadband light to the reference optical path (22b) having tunable length,

superimposing light beams of broadband light reflected from the reference surface (3a) and test surface (4a) with the light propagating along the reference optical path (22b) ; tuning the length of the reference optical path (22b) and observing, with the second detecting device (25), said superimposition of the light to detect at least two interference patterns, each corresponding

the first condition according to which the length of the optical path of the light reflected from the reference surface (3a) is equal to the total length of the path of the light propagating along the reference optical path (22b) or

the second condition, according to which the length of optical path of the light reflected from the test surface (4a) is equal to the total length of the path of the light propagating along the reference optical path (22b) ,

determining the distance d from the difference of lengths of tuned reference optical path (22b) for which the first and the second conditions are met.

2. The method according to the claim 1, characterized in that the distance d is determined as a difference of the lengths of reference optical path (22b) corresponding to two occurrences of local maxima in the signal of contrast of the interference pattern versus length of the optical path (22b) .

3. The method according to the claim 1 or 2, characterized in that tuning of the length of the reference optical path (22b) is done by moving reflecting element (29) located at the end of optical path (22b) .

4. The method according to any of the claims 1-3, characterized in that it comprises a step of applying a residual power correction.

5. The method according to any of the claims 1-4, characterized in that the step of determining radius of curvature of the test surface (4a) from the position of the test surface object (4) using distance d between test surface (4a) and the reference surface (3a) involves subtraction of the radius of curvature r_r of the reference surface (3a) from the distance d between the reference surface (3a) and the test surface (4a) .

6. The method according to any of the claims 1 to 4, characterized in that a step of determining the distance d from the difference of lengths of tuned reference optical path (22b) for which the first and the second conditions are met is repeated for at least three confocal cavities formed by at least three pairs of objects, and step of determining radius of curvature of the test surface (4a) from the position of the test surface object (4) using distance d between test surface (4a) and the reference surface (3a) involves solving a set of at least three equations with respect to radii of curvature.

7. A distance determination subsystem (20) for interferometric measurement system having an illuminating device (9), a reference surface object (3) located in the first path (2a) of the light produced by the illuminating device (9) and the test surface object (4) movable with respect to the reference surface object (3) along the first optical path (2a) ,

characterized in that said distance determination subsystem (20) comprises :

the second light source (21) producing broadband light beam, the second beam splitting element (22) dividing the light beam into the measurement optical path (22a) and the reference optical path (22b) , so that the reference surface object (3) and the test surface object (4) are part of the measurement optical path (22a) while the reference optical path (22b) has tunable length and is ended with reflecting element (29) where reflecting element (29) is arranged such that the range in which the length of reference optical path (22b) is tunable corresponds to the range in which the distance between reference object (3) and the test object (4) is tunable,

means for superimposing light reflected form the elements that are part of the measurement optical path (2a) with light reflected from the reflecting element (29) located in the reference optical path (22b) in the detecting device (25) adapted to detect interference pattern.

8. The distance determination subsystem according to the claim 7, characterized in that it has a beam forming element (28) within the reference path (22a) , forming a light beam before it reaches test surface element (4) or reference surface element (3) .

9. The distance determination subsystem according to the claim 8, characterized in that beam forming element (28) forms parallel beam.

10. The distance determination subsystem according to the claim 7 or 8 or 9, characterized in that the second beam splitting element (22) and means for superimposing the light are the same element.

11. The distance determination subsystem according to any of the claims 7 to 10, characterized in that the second beam splitting element (22) is a fiber optic coupler.

12. The distance determination subsystem according to any of the claims from 7 to 11 characterized in that the length of the reference optical path (22b) is tunable by movement of the reflecting element (29) located on its end.

13. The distance determination subsystem according to any of the claims from 7 to 12, characterized in that the reference optical path (22b) is formed by an optical fiber and its length is tunable by a mechanical stretching.

14. The distance determination subsystem according to any of the claims from 7 to 13, characterized in that it contains a processing unit provided with means for determination said difference in the length of the reference optical path (22b) and thus to determine the distance d , means for calculation of the difference between the distance d and the radius r of curvature of the reference surface (3a)

15. The distance determination subsystem according to the claim 14, characterized in that it further comprises displaying means connected to the processing unit and adapted to display resulting radius of curvature of the test surface (4a) .

16. The distance determination subsystem according to the claim 14 or 15, characterized in that it further comprises a memory device connected to the processing unit and adapted to store a number of values of radius of curvature of the test surface (4a) .

17. An Interferometric measurement system having

an illuminating device (9) comprising

the first light source (1) producing a substantially monochromatic light beam, the first beam splitting element (2) located in the light beam produced by the first light source (1) and directing

the light beam from the first light source (1) into the cavity formed by the reference surface (3a) and the test surface (4a) ,

and a detecting device (5) adapted to detect interference pattern where test surface object is movable with respect to the reference surface object (3) along the first optical path (2a) ,

characterized in that

the system comprises additional distance determination subsystem

(20) comprising:

the second light source (21) producing broadband light beam, the second beam splitting element (22) dividing the light beam into

measurement optical path (22a) and

reference optical path (22b) ,

whereas the reference surface object and the test surface object are located in measurement optical path (22a) while the reference optical path (22b) has tunable length and is ended with the reflecting element (29),

wherein

the subsystem (20) further comprises means for superimposing the light reflected form the elements located in the measurement optical path (22a) with the light reflected from the reflecting element (29) located in the reference optical path (22b) in the detecting device (25) adapted to detect interference pattern.

18. The interferometric measurement system according to the claim 17, characterized in that the second beam splitting element (22) and means for superimposing the light are the same element.

19. The interferometric measurement system according to the claim 17 or 18, characterized in that the second beam splitting element (22) is a fiber optic coupler.

20. The interferometric system according to any of the claims from 17 to 19, characterized in that the length of reference optical path (22b) is tunable by movement of the reflecting element (29) located on its end.

21. The interferometric system according to any of the claims from 17 to 19, characterized in that the reference optical path (22b) is formed by an optical fiber and its length is tunable by a mechanical stretching .

22. The interferometric system according to any of the claims from 17 to 21, characterized in that detecting device (5) is adapted to measure residual power.