DEVICE, MEASURING METHOD AND MANUFACTURING METHOD

Abstract

An apparatus includes a first light source for illuminating a panel including an index surface on which an index is provided, an objective lens for guiding index light emitted from the panel to an optical system, a first element for receiving the index light reflected on an optical surface in the optical system, an interferometer configured to include a second light source and a second element and to detect a wavefront of measurement light, the measurement light being test light reflected on the target surface, and a reduction unit for reducing an intensity of a signal of light reflected on an optical surface other than the target surface of the test light reflected on the plurality of optical surfaces and received by the second element.

Description

BACKGROUNDTechnical FieldThe aspect of the embodiments relates to a measurement technique for measuring an eccentricity of an optical surface in an optical system.Description of the Prior ArtAn optical performance of an optical system including a plurality of optical elements is influenced by an arrangement accuracy of each optical element. In order to measure whether each optical element is disposed at a desired position, a technique for measuring an eccentricity of an optical surface of each optical element is known.Japanese Patent Application No. 2005-164267 describes an apparatus that measures eccentricity of an optical surface in a target optical system including a plurality of optical surfaces. The apparatus described in Japanese Patent Application No. 2005-164267 projects index light to a position of an apparent spherical center of each target surface among a plurality of optical surfaces. Next, the apparatus moves the target optical system and a projection optical system relative to each other according to a position of an image formed by light which is the index projected light reflected on the target surface. Then, eccentricity of the optical surface is measured based on an amount of relative movement between the target optical system and the projection optical system.SUMMARY OF THE INVENTIONAn apparatus includes a first light source configured to illuminate a panel including an index surface on which an index is provided, an objective lens configured to guide index light emitted from the panel to an optical system, a first element configured to receive the index light reflected on an optical surface in the optical system, an interferometer configured to include a second light source and a second element, divide light from the second light source into test light and reference light, receive measurement light via the objective lens and the reference light with the second element, and detect a wavefront of the measurement light, the measurement light being the test light reflected on a target surface, and a reduction unit configured to:, reducing an intensity of a signal of light reflected on an optical surface other than the target surface of the test light reflected on the plurality of optical surfaces and received by the second element.Other features of the embodiment will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 illustrates a configuration of a measurement device according to a first exemplary embodiment. FIG. 2 schematically illustrates a panel, an image forming surface panel, and images captured by an imaging element according to the first exemplary embodiment. FIG. 3 is a flowchart illustrating a measurement method according to the first exemplary embodiment. FIG. 4 illustrates a configuration of a measurement device according to a second exemplary embodiment. FIG. 5 is a flowchart illustrating a measurement method according to the second exemplary embodiment. FIG. 6 illustrates a configuration of a measurement device according to a third exemplary embodiment. FIG. 7 illustrates a configuration of a measurement device according to a fourth exemplary embodiment. FIG. 8 is a flowchart illustrating a manufacturing method of an optical system using a measurement device according to each exemplary embodiment.DESCRIPTION OF THE EMBODIMENTSExemplary embodiments according to the present disclosure will be described below with reference to the accompanying drawings. Each drawing may be drawn on a scale other than an actual scale for convenience. Further, the same reference numerals are used for the similar elements in each drawing to avoid repetition in descriptions.FIG. 1 illustrates a configuration of a measurement device 1 according to a first exemplary embodiment. FIG. 2 illustrates schematic diagrams of a panel, an image forming surface panel, and images captured by an imaging element according to the first exemplary embodiment.The measurement device 1 includes an illumination light source (first light source) 11, a board 40, an objective lens 55, a first imaging element 90, a stage (setting unit) 150, a low coherence interferometer 400, and a computer (calculation unit) 100. A target optical system 60 is an optical system configured by combining a plurality of lenses, and the measurement device 1 measures eccentricity of a plurality of optical surfaces (the number of surfaces is N) in the target optical system 60.The first light source 11 emits illumination light 290 to illuminate the transmissive panel 40. The first light source 11 according to the present exemplary embodiment includes, for example, a halogen lamp or a light emitting diode (LED). A processor in the measurement device 1, an external device, or another storage medium may be used as the calculation unit. The first imaging element 90 (and an imaging element 95 described below) are, for example, a charge coupled device (CCD) sensor or a complementary metal oxide semiconductor (CMOS) sensor.An upper left diagram in FIG. 2 is a schematic diagram of the panel 40 according to the present exemplary embodiment. The panel 40 includes an index surface on which an index is provided. White portions in the upper left diagram in FIG. 2 indicate regions through which light passes. If a position of a white circle is the origin of the panel 40, patterns (white triangles) formed on the panel 40 according to the present exemplary embodiment are asymmetric with respect to the origin.In one embodiment, the panel 40 containing the patterns asymmetric with respect to the origin as illustrated in the upper left diagram in FIG. 2 is used because it makes it easy to determine whether a position of the origin of the panel image formed by the index light 250 is located at the center of curvature of the optical surface or on a surface of the optical surface. For example, in a case where the panel 40 is used in the upper left diagram in FIG. 2, an image as illustrated in a lower right diagram in FIG. 2 (an image in which the panel image is inverted with respect to the origin as compared with an image in a lower middle diagram in FIG. 2 ) is observed if the panel image is generated at the surface position of the optical surface. Further, in a case where the objective lens 55 is disposed near a design position in step S 30 (described below), the index light may be converged on the optical surface other than a target surface, and a plurality of panel images may be observed on the first imaging element 90. Even in such cases, the panel 40 containing the asymmetric patterns is used because it makes it easy to distinguish the reflected light from the target surface.According to the present exemplary embodiment, a panel having a shape illustrated in the upper left diagram in FIG. 2 is used as the panel 40, but the shape is not limited thereto. The shape of a region through which light passes may also be any shape, and for example, a cross, a circle, and a square may be arranged instead of a triangle.The panel 40 emits divergent light (index light) 250. The patterns formed on the panel 40 (the white circle and the white triangles in the upper left diagram in FIG. 2 ) are close to each other (e.g., at a distance of 1 mm or less), so that they are drawn in FIG. 1 as if they were light diverging from a single point.The index light 250 having passed through a beam splitter 35 is collimated by a collimator lens 50, passes through beam splitters 36 and 30, and is converged by the objective lens 55 to enter the target optical system 60. The objective lens 55 is placed on the table 150 moving in respective X, Y, and Z directions as illustrated in FIG. 1, thereby making it possible to adjust a position of a panel image (real image or virtual image) generated by the index light 250. A position of the table 150 is managed by the computer 100.A method of adjusting a relative position between the objective lens 55 and the target optical system 60 is not limited thereto. For example, the objective lens 55 may be fixed, the target optical system 60 may be placed on a table, and the position of the target optical system 60 may be adjusted. Each of the objective lens 55 and the target optical system 60 may also include an adjustment unit. Further, all the optical elements including the objective lens 55, which is not the target optical system 60, may be placed on the table, and the relative position between the objective lens 55 and the target optical system 60 may be adjusted by moving positions of these optical elements together.In the measurement, the position (X, Y, Z) of the origin of the table image formed by the index light 250 is brought close to a position (Xc, Yc, Zc) of an apparent center of curvature of a k-th (k=1, 2,..., N) optical surface (a target surface 60 k) in the target optical system 60. The index light 250 is thereby reflected on the target surface 60 k, and moves backward along a light path similar to an incident light path. In other words, the index light 250 reflected on the target surface 60 kpass through the objective lens 55, the beam splitters 30 and 36, and the collimator lens 50 and reaches the beam splitter 35.At this time, the image of the panel 40 is formed on the image forming surface panel 80. Further, an imaging lens 52 forms the image on the imaging surface panel 80 on the first imaging element 90. The image captured by the first imaging element 90 is transmitted to the computer 100. The index light 250 reflected on the optical surface other than the target surface 60 kmoves backward as light having a different degree of convergence / divergence with respect to the incident light, so that it does not generate an image on the first imaging element 90.The image forming surface panel 80 according to the present exemplary embodiment has reference lines (broken lines) drawn thereon as illustrated in an upper right diagram in FIG. 2. Thus, the first imaging element 90 simultaneously captures an image of the panel and an image of the reference lines. The image forming surface sheet 80 is not limited thereto, and a similar effect can be obtained with a sheet having an arbitrary shape. The reference lines may be drawn directly on a display that displays an image captured by the first imaging element 90, or may be displayed by being superimposed on image data.A lower left diagram in FIG. 2 is a schematic diagram of an image captured by the first imaging element 90 in a case where the position (X, Y, Z) of the origin of the table image and the position (Xc, Yc, Zc) of the apparent center of curvature of the target surface 60 khave a relationship of X≠Xc, Y≠Yc, and Z=Zc. The lower middle diagram in FIG. 2 is a schematic diagram of an image captured by the first imaging element 90 in a case where X=Xc, Y=Yc, and Z=Zc. The lower middle diagram in FIG. 2 is similar to an image observed when a plane mirror is placed between the collimator lens 50 and the objective lens 55 to be parallel to an XY plane and the index light 250 is reflected from the plane mirror. A position of the image at this time is set as a reference position according to the present exemplary embodiment. To simplify the description, the lower left and lower middle diagrams in FIG. 2 illustrate examples in which the arrangement and the vertical and horizontal directions of the patterns coincide with those in the upper left diagram in FIG. 2. The arrangement and the vertical and horizontal directions of the patterns are not limited to these, and an image inverted vertically and horizontally may be acquired depending on the vertical and horizontal directions of the first imaging element 90 and signal processing by the computer 100.The low coherence interferometer 400 according to the present exemplary embodiment is a Twyman-Green type interferometer including a low coherence light source (second light source) 10, a fiber 20, a collimator lens 51, a collimator lens 54, a diaphragm 110, the beam splitter 30, the beam splitter 36, the objective lens 55, a reference mirror 70, a reference table (changing unit) 160, a condenser lens 53, a spatial filter 300, and the second imaging element 95. the spatial filter 300 is, for example, a pinhole or a diaphragm, and corresponds to a reducing unit according to the present exemplary embodiment.The measurement apparatus 1 according to the present exemplary embodiment includes the aperture 110 between the collimator lens 51 and the beam splitter 36. the position of the objective lens 55 can be adjusted with high accuracy by narrowing a light flux of low coherence light 200 with the aperture 110 to reduce an irradiated area on the target surface 60 k, even in a case where the target surface 60 kis an aspherical surface having multiple positions of apparent spherical centers. This is because the aspherical surface can be regarded as a spherical surface by reducing the irradiated area with the aperture 110. In an embodiment, the index light 250 requires a light beam having a high spatial frequency to some extent (a large light flux including peripheral light beams) to generate a table image, but the low coherence light 200 may be a light beam having a low spatial frequency (a light flux less than the light flux of the index light) as long as it can form interference fringes.According to the present exemplary embodiment, the aperture 110 is placed between the collimator lens 51 and the beam splitter 36, but is not limited thereto. The aperture 110 may be placed anywhere before irradiation on the target surface 60 k. For example, the aperture 110 may be placed between the beam splitter 30 and the objective lens 55. Alternatively to a method of placing the aperture, the irradiated area may also be reduced by changing a numerical aperture (NA) of the collimator lens 51 to be larger than an NA of the fiber 20. The same effect is also obtained with this configuration.The second light source 10 is a wide-band light source such as an LED or a superluminescent diode (SLD). As the second light source 10, a semiconductor laser having a relatively short coherence length may also be used. The low coherence light 200 emitted from the second light source 10 is emitted through the fiber 20, collimated by the collimator lens 51, passes through the aperture 110, and then reflected by the beam splitter 36. The low coherence light 200 reflected by the beam splitter 36 propagates in the same direction as the index light 250, and is divided into reflected light (reference light) 200 rand transmitted light (test light) 200 sin the beam splitter 30.The reference light 200 rwhich is reflected by the beam splitter 30 is reflected by the reference mirror 70 placed on the reference table 160 and returns to the beam splitter 30. The reference table 160 may move in an arrow direction (Y direction) as illustrated in FIG. 1, and change a light path length of the reference light 200 r(reference light path length). A position of the reference table 160 according to the present exemplary embodiment is managed by the computer 100.Between the beam splitter 30 and the reference mirror 70, a neutral density (ND) filter for adjusting an intensity of the reference light 200 r, and the like may be placed. The reference mirror 70 may be a metal mirror having high reflectance, but may be a glass surface having reflectance similar to that of the optical surface. A part of the reference light 200 rpass through the beam splitter 30, is condensed by the condenser lens 53, diverges after passing through the spatial filter 300, is collimated by the collimator lens 54, and enters the second imaging element 95.Test light 200 s, which has passed through the beam splitter 30, is converged by the objective lens 55 and enters the target optical system 60. Similar to the index light 250, the test light 200 sis reflected on the target surface 60 k, and moves backward along a light path similar to the incident light path in a case where a position of an apparent condensing point of the test light 200 scoincides with the position of the apparent center of curvature of the target surface 60 k. The test light (measurement light) 200 swhich is reflected on the target surface 60 kis collimated by the objective lens 55 to reach the beam splitter 30. A part of the test light (measurement light) 200 sis reflected by the beam splitter 30, condensed by the condenser lens 53, diverged after passing through the spatial filter 300, collimated by the collimator lens 54, and enters the second imaging element 95.In contrast, unnecessary light, which is the test light reflected on the optical surface other than the target surface 60 k, moves backward as light having a different degree of convergence / divergence with respect to the incident light. The unnecessary light passes through the objective lens 55, is partially reflected by the beam splitter 30, and is guided to the spatial filter 300 via the condenser lens 53. However, the unnecessary light does not generate an image at a position of the spatial filter 300 because the degree of convergence / divergence thereof is different from that of the measurement light 200 s. As a result, the spatial filter 300 can reduce the unnecessary light incident on the second imaging element 95.The second imaging element 95 receives the measurement light 200 sand the reference light 200 r. The measurement light 200 sand the reference light 200 rinterfer with each other and enter the second imaging element 95 while forming interference fringes. A signal detected by the light received from the second imaging element 95 is transmitted to the computer 100. At this time, the second imaging element 95 converts the received light into a signal by photoelectric conversion processing or the like. The computer (calculation unit) 100 calculates a wavefront based on the signal (interference signal) of the measurement light 200 sand the reference light 200 rdetected by the second imaging element 95. The computer 100 then calculates eccentricity based on the position of the objective lens 55 (the table 150) and the calculated wavefront.The interferometer according to the present exemplary embodiment generates interference fringes using the reference table 160 by making a light path length of the measurement light and the light path length of the reference light substantially the same. In contrast, a light path length of the test light reflected on the optical surface other than the target surface 60 kis not substantially the same as the light path length of the reference light, so that the interference fringes are not generated even when the test light is combined.A conventional measurement device using an autocollimation method detects a position of an image generated by reflected light (measurement light) from the target surface to calculate eccentricity of the target surface from the position. However, in a case where the number of optical surfaces is large, or in a case where a lens having large eccentricity is present in the target optical system, an image may become ambiguous due to an influence of aberration or vignetting.Since reflected light (unnecessary light) from the optical surface other than the target surface also enters the imaging element, an image generated by the measurement light cannot be distinguished due to background noise caused by the unnecessary light in some cases. These factors may be causes of a decrease in the accuracy of eccentricity measurement.Therefore, the measurement device 1 according to the present exemplary embodiment reduces reflected light from the optical surface other than the target surface entering the second imaging element by the reduction unit (the spatial filter 300). With this configuration, the measurement device 1 can measure eccentricity of the optical surface in the target optical system with high accuracy.Next, a measurement method according to the present exemplary embodiment will be described with reference to FIG. 3. FIG. 3 is a flowchart illustrating a method for measuring eccentricity of the target optical system according to the first exemplary embodiment.In step S 10, the measurement device 1 selects the target surface 60 k(the k-th optical surface) from among the plurality of optical surfaces (the number of surfaces is N). The target surface may be selected in the order from the first surface to the N-th surface of the target optical system (in the order of the optical path length), or may be selected in the order from far to near the position of the apparent center of curvature. The target surface may also be selected in random order.In step S 20, the measurement device 1 detects the design position of the objective lens 55 and the design position of the reference table 160 on the target surface. The design position of the objective lens 55 is a position that has X, Y, and Z (calculated values) of the objective lens 55 in a state in which the position of the origin on the table image coincides with the position of the apparent center of curvature of the target surface 60 k. The values are calculated by multiaxial calculation or ray tracing using design values of the target optical system 60. The design position of the reference table 160 is a position of the reference table 160 in a case where the light path length of the test light reflected on the target surface 60 kis equal to the light path length of the reference light, and the design position is calculated based on the design value of the target optical system 60.In step S 30, the measurement device 1 adjusts the position of the objective lens 55 with the table 150 such that the image of the panel 40 is formed at the reference position on the first imaging element 90. In an embodiment, a setting amount (or position information) of the objective lens 55 is recorded with a first setting unit at this time as a first setting amount. In step S 30, the objective lens 55 is first placed near the design position (initial position), and the position of the objective lens 55 in the Z direction is adjusted so that the image of the panel 40 is formed on the first imaging element 90.In a case where the target surface 60 kis eccentric, the table image deviating from the reference position is observed as illustrated in the lower left diagram in FIG. 2. Therefore, the X and Y positions of the objective lens 55 are adjusted by the adjusting unit (the stage 150) such that the image observed on the first imaging element 90 is generated at the reference position (as illustrated in the lower middle diagram in FIG. 2 ). The adjustment performed in step S 30 corresponds to the first adjustment (coarse adjustment) of the objective lens 55.In step S 40, the low coherence interferometer 400 detects interference fringes caused by interference between the measurement light (test light) 200 sand the reference light 200 rwhile scanning the light path length of the reference light with the reference stage 160.In step S 50, the measurement device 1 calculates a wavefront based on the interference fringes (interference signal) and adjusts the position of the objective lens 55 based on the wavefront. The measurement device 1 records a setting amount (or position information) of the objective lens 55 with a second setting unit at this time as a second setting amount. The wavefront may be calculated using a phase shift method, for example. The X and Y positions of the objective lens 55 may be adjusted based on a tilt component of the wavefront. The Z position of the objective lens 55 may be adjusted based on a focus error component of the wavefront.According to the present exemplary embodiment, as an example, the inclination component and the focus error component of the wavefront are respectively set to be an inclination component (reference amount) and a focus error component (reference amount) of a wavefront detected when a mirror parallel to the XY plane is inserted between the beam splitter 30 and the objective lens 55. However, it is not necessary to set the components until they have exactly the same values as the reference amounts, and it is sufficient to set the components to fall within a range of the reference amount±λ / 2. The setting in step S 50 corresponds to fine adjustment (second setting) of the objective lens 55.In step S 60, the measurement device 1 acquires the adjustment amount of the position of the objective lens 55. The adjustment amount of the position of the objective lens 55 is the first adjustment amount acquired in step S 30, the second adjustment amount acquired in step S 50, an amount corresponding to a sum of the first adjustment amount and the second adjustment amount, or information on an end position of the objective lens 55.In step S 70, the measurement device 1 determines whether the adjustment amount of the position of the objective lens 55 has been detected for each optical surface. If the detection is completed (YES in step S 70), the processing proceeds to step S 80, whereas if the detection is not yet completed (NO in step S 70), the processing returns to step S 10.In step S 80, the measurement device 1 calculates eccentricity of each optical surface based on the setting amount of the objective lens 55 for each optical surface. According to the present exemplary embodiment, the measurement device 1 calculates the eccentricity from the first surface to the N-th surface in the target optical system in order (in the order of the optical path length). The relative position between the objective lens 55 and the target optical system 60 is determined from the adjustment amount of the objective lens 55, and the eccentricity of the target surface 60 kmay be calculated using triaxial calculation or tracking from the objective lens 55 to the target surface 60 k. In an embodiment, in a case where the eccentricity of the k-th surface is calculated, the eccentricity of each of the first to (k-1)-th surfaces is required, and thus the eccentricity is calculated in the order from the first surface.According to the present exemplary embodiment, the illumination of the first light source 11 and the second light source 10 is divided in time. With this configuration, it is not necessary to perform processing such as image processing to share a signal from the received reflected light.According to the present exemplary embodiment, the first light source 11, the objective lens 55, and the first imaging element 90 are not essential for acquiring a panel image, and the effect of the present embodiment can be achieved by the interferometer and the reduction unit. Similarly, the reduction unit is not essential, and the eccentricity of the target optical system can be measured with high accuracy using the configuration for acquiring a table image and the interferometer.In a case where a conventional measurement device that calculates eccentricity of a target surface from a wavefront detected by a low coherence interferometer measures an optical system including an optical surface having large eccentricity, a light path of reflected light is significantly tilted, and interference fringes become dense, so that a wavefront cannot be detected with high accuracy. In addition, an influence of background noise due to reflected light from the optical surface other than the target surface is also a factor in reducing the accuracy. Thus, in step S 30, in the measurement device 1 according to the present exemplary embodiment, the setting unit sets the relative position between the objective lens 55 and the target optical system 60 such that a table image is generated at the reference position on the first imaging element 90. Adjustment using the adjustment unit is performed so that the inclination of the light path of the reflected light is reduced, and non-densely arranged interference fringes are detected. Further, the adjustment using the adjustment unit allows the measurement light (low coherence light) 200 to pass through the spatial filter 300, and the unnecessary light can be separated from the measurement light 200. The background noise is sufficiently reduced so that the position of the objective lens 55 can be adjusted based on the wavefront with high accuracy. As a result, the accuracy of eccentricity measurement can be improved.FIG. 4 illustrates a configuration of a measurement device 2 according to a second exemplary embodiment.The measurement device 2 includes a light source 12, a panel 41, the objective lens 55, the first imaging element 90, a table (adjustment unit) 151, the low coherence interferometer 400, and the computer (calculation unit) 100. The light source 12 according to the present exemplary embodiment is different from that according to the first exemplary embodiment in that the light source 12 functions as the first light source and the second light source.The light source 12 emits the illumination light 290 and illuminates the transmissive panel 41 via a condenser lens 56. the panel 41 according to the present exemplary embodiment is a panel (pinhole) including only the white circle (the origin) without the white triangles in the upper left diagram in FIG. 2. The panel 41 emits divergent light (the index light 250 and the low coherence light 200).The index light 250 passes through the beam splitter 35, is collimated by the collimator lens 50, passes through beam splitters 31 and 30, is converged by the objective lens 55, and enters the target optical system 60.The table 151 is driven in the X, Y and Z directions, respectively. The target optical system 60 is placed on the table 151. The table 151 adjusts the position of the target optical system 60 with respect to a position of a table image generated by the index light 250. A position of the table 151 is managed by the computer 100.In the measurement, the position (X, Y, Z) of the origin of the table image generated by the index light 250 is brought close to the position (Xc, Yc, Zc) of the apparent center of curvature of the kth (k=1, 2,..., N) optical surface (the target surface 60 k) in the target optical system 60. Accordingly, the index light 250 is reflected on the target surface 60 k, and moves backward along a light path similar to the incident light path. The index light 250 reflected on the target surface 60 kpass through the objective lens 55, the beam splitters 30 and 31, the collimator lens 50, the beam splitter 35, the image forming surface board 80, and the imaging lens 52, and then enters the first imaging element 90.The low coherence interferometer 400 according to the present exemplary embodiment is a Twyman-Green type interferometer including the light source 12, the collimator lens 50, the beam splitters 30 and 31, the objective lens 55, the reference mirror 70, the reference table (changing unit) 160, and the second imaging elements 95 and 96. According to the present exemplary embodiment, the above-described configuration is adopted such that the shapes of the wavefronts at the second imaging elements 95 and 96 are substantially the same.The low coherence light 200 passes through the beam splitter 35, is collimated by the collimator lens 50, passes through the beam splitter 31, and is divided into reflected light (reference light) 200 rand transmitted light (test light) 200 sby the beam splitter 30.The reference light 200 rwhich is reflected by the beam splitter 30 is reflected by the reference mirror 70 placed on the reference table 160 and returns to the beam splitter 30. Light 200 r 1, which is a part of the reference light 200 r, passes through the beam splitter 30 and enters the second imaging element 95. Light 200 r 2, which is a part of the reference light 200 r, is reflected by the beam splitters 30 and 31 and enters the other second imaging element 96. The position of the reference table 160 is managed by the computer 100.The test light 200 s, which has passed through the beam splitter 30, is converged by the objective lens 55 and enters the target optical system 60. Similar to the index light 250, the test light 200 sis reflected on the target surface 60 k, and moves backward along a light path similar to the incident light path in a case where the position of the apparent condensing point of the test light 200 scoincides with the position of the apparent center of curvature of the target surface 60 k. The test light (measurement light) 200 s, which is reflected on the target surface 60 k, is collimated by the objective lens 55, and reaches the beam splitter 30. light 200 s 1, which is a part of the measurement light 200 s, is reflected by the beam splitter 30, and enters the second imaging element 95. Light 200 s 2, which is a part of the measurement light 200 s, passes through the beam splitter 30, is reflected by the beam splitter 31, and enters the other second imaging element 96.In an embodiment, a sum of intensities of the test light 200 s 1 and the reference light 200 r 1 that have entered the second imaging element 95 and a sum of intensities of the test light 200 s 2 and the reference light 200 r 2 that have entered the second imaging element 96 are set in advance to have the same signal intensity A(x, y). The signal intensity may be adjusted by changing an exposure time or a gain of each imaging element or by inserting an ND filter before the imaging element.The measurement light 200 s 1 and the reference light 200 r 1 enter the second imaging element 95 while forming interference fringes (first interference fringes). A first signal |1(x, y) detected based on the light received from the second imaging element 95 is transmitted to the computer 100. The first signal |1(x, y) is expressed by the following formula (1) by using an amplitude B(x, y) of the interference fringes, a wavefront Φ(x, y), and an intensity N(x, y) of the test light (unnecessary light) reflected on the optical surface other than the target surface 60 k.Similarly, measurement light 200 s 2 and the reference light 200 r 2 enter the second imaging element 95 while interference fringes (second interference fringes) are formed. A second signal I 2( x, y) detected based on the light received from the second imaging element 95 is transmitted to the computer 100. The second signal I 2( x, y) is expressed by the following formula (2) using the amplitude B(x, y) of the interference fringes, the wavefront Φ(x, y), and the intensity N(x, y) of the test light (unnecessary light) reflected on the optical surface other than the target surface 60 k. A phase of the second signal I 2( x, y) is inverted with respect to that of the first signal | 1( x, y).The reducing unit according to the present exemplary embodiment reduces an influence of the unnecessary light by performing calculation processing to calculate a differential interference signal D(x, y) between the first signal |1(x, y) expressed by Formula (1) and the second signal I2(x, y) expressed by Formula (2). The differential interference signal D(x, y) is expressed by the following formula (3).With this configuration, background noise of interference fringes can be reduced, and the accuracy of eccentricity measurement can thereby be improved. The reducing unit according to the present exemplary embodiment is executed by calculation processing by the computer 100.The reducer according to the present exemplary embodiment is not limited thereto, and may be configured with a circuit (not illustrated) for communicating a signal between the second imaging elements 95 and 96 and the computer 100, a central processing unit (CPU) different from the computer 100, or a computing device.The reducing unit according to the present exemplary embodiment and the reducing unit (spatial filter) according to the first exemplary embodiment may also be combined. The reduction unit according to the first exemplary embodiment may be combined by disposing the condenser lens, the spatial filter, and the collimator lens in front of each of the second imaging elements 95 and 96 of the measurement device 2 according to the present exemplary embodiment.In one embodiment, the reducing unit using calculation processing according to the present exemplary embodiment is combined with unnecessary light separation with the spatial filter, because this can further reduce the influence of unnecessary light.FIG. 5 is a flowchart illustrating a method for measuring eccentricity of the target optical system according to the second exemplary embodiment.In step S 11, the measurement device 2 acquires the design position of the target optical system 60 and the design position of the reference table 160 for each optical surface.In step S 21, the measurement device 2 selects the target surface 60 k(the k-th optical surface) from among the plurality of optical surfaces (the number of surfaces is N).In step S 31, the measurement device 2 adjusts the position of the target optical system 60 with the table 151 such that the image of the panel 40 is generated at the reference position on the first imaging element 90. In an embodiment, a setting amount (or position information) of the target optical system 60 by the first setting unit at this time is recorded as the first setting amount.In step S 41, the low coherence interferometer 400 detects the interference fringes by scanning the light path length of the reference light using the reference stage 160.In step S 51, the measurement device 2 calculates the wavefront from the interference fringes. According to the present exemplary embodiment, a second adjustment (fine adjustment) of the target optical system 60 performed by the second adjustment unit based on the wavefront is not performed, unlike the first exemplary embodiment, and the wavefront is detected from the interference fringes.In step S 61, the measurement device 2 determines whether the first adjustment amount and the wavefront have been detected with respect to each optical surface. If the detection is completed (YES in step S 61), the processing proceeds to step S 71, whereas if the detection is not yet completed (NO in step S 61), the processing returns to step S 21.In step S 71, the measurement device 2 calculates eccentricity of each optical surface from the first adjustment amount and the wavefront of each optical surface. In step S 71, according to the present exemplary embodiment, an amount corresponding to the second adjustment amount according to the first exemplary embodiment is calculated by multiaxial calculation or ray tracing using the relative position between the objective lens 55 and the target optical system 60 at the time of detecting the wavefront of the target surface 60 kand the wavefront at that time. The eccentricity of the target surface 60 kis then calculated based on the first adjustment amount and the second adjustment amount.In the measurement apparatus 2 according to the present exemplary embodiment, the reduction unit performs processing for calculating a difference between interference fringes I0(x, y) and I1(x, y) generated by reflection or transmission at the beam splitter 30. With this configuration, background noise of the interference fringes can be reduced, and eccentricity of the optical surface in the target optical system can be measured with high accuracy.In the measurement method according to the present exemplary embodiment, the relative position of the target optical system 60 with respect to the objective lens 55 is adjusted (roughly adjusted) so that the table image is generated at the reference position on the first imaging element 90, and then the low coherence interferometer 400 detects the interference fringes. This configuration makes it possible to detect interference fringes that are not densely arranged. As a result, the wavefront can be calculated with high accuracy, and the measurement accuracy of eccentricity detected based on the wavefront can be improved.FIG. 6 illustrates a configuration of a measurement device 3 according to a third exemplary embodiment.The measurement device 3 includes the light source 12, the panel 40, the objective lens 55, an imaging element 99, the stage (setting unit) 150, the low coherence interferometer 400, and the computer (calculation unit) 100. The light source 12 according to the present exemplary embodiment functions as the first light source and the second light source as in the second exemplary embodiment. The imaging element 99 according to the present exemplary embodiment is different from that in the first exemplary embodiment in that the imaging element 99 functions as the first imaging element and the second imaging element depending on the optical system inserted by a replacement unit 550, as described below.The measuring apparatus 3 also includes the image forming surface board 80, the imaging lens 52, an optical system 500A for detecting a board image, the spatial filter 300, the collimator lens 54, an optical system 500B for detecting interference fringes, and the exchanging unit 550. The optical system (optical system for detecting a panel image) 500A and the optical system (optical system for detecting interference fringes) 500B are configured to be interchangeable. The exchange unit 550 is configured with a table and the like, inserts the optical system 500A into the light path when a panel image is detected, and inserts the optical system 500B into the light path when interference fringes are detected.The index light 250 passes through the beam splitter 35, is collimated by the collimator lens 50, passes through the beam splitter 30, is converged by the objective lens 55, and enters the target optical system 60.The position (X, Y, Z) of the origin of the table image generated by the index light 250 is brought close to the position (Xc, Yc, Zc) of the apparent center of curvature of the kth (k=1, 2,..., N) optical surface (the target surface 60 k) in the target optical system 60. Accordingly, the index light 250 is reflected on the target surface 60 k, and moves backward along a light path similar to the incident light path. The index light 250 reflected on the target surface 60 kpass through the objective lens 55, the beam splitter 30, the collimator lens 50, the beam splitter 35, and the optical system 500A and enters the imaging element 99 (first imaging element).The low coherence interferometer 400 according to the present exemplary embodiment is a Twyman-Green type interferometer including the light source 12, the collimator lens 50, the beam splitter 30, the objective lens 55, the reference mirror 70, the reference table (changing unit) 160, the optical system 500B, and the imaging element 99 (second imaging element).The low coherence light 200 passes through the beam splitter 35, is collimated by the collimator lens 50, and is divided into the reflected light (reference light) 200 sand the transmitted light (test light) 200 rby the beam splitter 30.The reference light 200 rwhich is reflected by the beam splitter 30 is reflected by the reference mirror 70 placed on the reference table 160 and returns to the beam splitter 30. A part of the reference light 200 ris reflected by the beam splitter 30, collimated by the collimator lens 50, reflected by the beam splitter 35, and guided to the spatial filter (reduction unit) 300 in the optical system 500B. The reference light 200 rthat has passed through the spatial filter 300 diverges, is collimated by the collimator lens 54, and enters the imaging element 99.The test light 200 s, which has passed through the beam splitter 30, is converged by the objective lens 55 and enters the target optical system 60. Similar to the index light 250, the test light 200 sis reflected on the target surface 60 k, and moves backward along a light path similar to the incident light path in a case where the position of the apparent condensing point of the test light 200 scoincides with the position of the apparent center of curvature of the target surface 60 k. The test light (measurement light) 200 swhich is reflected on the target surface 60 kis collimated by the objective lens 55 and reaches the beam splitter 30, and a part of the measurement light 200 spass through the beam splitter 30, is collimated by the collimator lens 50, is reflected by the beam splitter 35, and is collimated by the spatial filter 300 in the optical system 500B. A part of the measurement light 200 s, which has passed through the spatial filter 300, diverges, is collimated by the collimator lens 54, and enters the imaging element 99. The reducing unit according to the present exemplary embodiment is the spatial filter 300 that reduces the unnecessary light entering the second imaging element 95 using a difference in the degree of convergence / divergence between the unnecessary light and the measurement light 200 s.The measurement light 200 sand the reference light 200 rform the interference fringes and the interference fringes are received by the imaging element 99. A signal detected based on the light received from the imaging element 99 is transmitted to the computer 100.In the measurement device 3 according to the present exemplary embodiment, the unnecessary light is blocked by the reduction unit (spatial filter 300). With this configuration, the measurement device 3 can measure the eccentricity of the optical surface in the target optical system with high accuracy.The objective lens 55 is set (roughly set) such that the table image is generated at the reference position on the first imaging element 90, and then the low coherence interferometer 400 detects the interference fringes. It is thus easier to detect interference fringes which are not arranged in a sealed manner. As a result, eccentricity is calculated with the wavefront detected based on the non-densely arranged interference fringes, and thus eccentricity measurement can be realized with high accuracy.FIG. 7 illustrates a configuration of a measurement device 4 according to a fourth exemplary embodiment.The measurement device 4 includes the light source 12, the panel 41, the objective lens 55, the first imaging element 90, the table (setting unit) 150, the low coherence interferometer 400, and the computer (calculation unit) 100. The light source 12 according to the present exemplary embodiment functions as the first light source and the second light source as in the second and third exemplary embodiments.The index light 250 is collimated by the collimator lens 50, passes through the beam splitter 30, is converged by the objective lens 55, passes through a beam splitter 32, and enters the target optical system 60. The target optical system 60 is placed on the table (adjusting unit) 151 which moves in the X, Y, or Z direction and can adjust the relative position between the position of the panel image generated by the index light 250 and the position of the target optical system 60. The position of the table 151 is managed by the computer 100.In a case where a virtual image of the panel 41 is generated at a position at negative infinity on the Z axis by the index light 250 reflected on the target surface 60 k, the index light 250 is emitted from the target optical system 60 as collimated light. The index light 250 reflected on the target surface 60 kis reflected by the beam splitter 32, passes through a beam splitter 33, is converged by the imaging lens 52, and enters the first imaging element 90. An image captured by the first imaging element 90 is transmitted to the computer 100.The low coherence interferometer 400 according to the present exemplary embodiment is a Mach-Zehnder type interferometer including the light source 12, the collimator lens 50, the beam splitters 30 to 33, the objective lens 55, the reference mirror 70, the reference table (changing unit) 160, and the second imaging element 95. the second imaging element 95 is placed on a table 170 driven in the Z direction, so that a position at which interference light of the measurement light 200 sand the reference light 200 rengages can be adjusted. A position of the table 170 is managed by the computer 100.The low coherence light 200 is collimated by the collimator lens 50, and is divided into the reflected light (reference light) 200 rand the transmitted light (test light) 200 sin the beam splitter 30.The reference light 200 rwhich is reflected by the beam splitter 30 passes through the beam splitter 31, is reflected by the reference mirror 70 placed on the reference table 160, and returns to the beam splitter 31. Thereafter, the reference light 200 ris reflected by the beam splitter 31, passes through the beam splitter 33, and enters the second imaging element 95.The test light 200 s, which has passed through the beam splitter 30, is converged by the objective lens 55, passes through the beam splitter 32, and enters the target optical system 60 (is guided to the target surface 60 k). The test light (measurement light) 200 swhich is reflected on the target surface 60 kis emitted from the target optical system 60 as collimated light as in the case of the index light 250, is reflected by the beam splitters 32 and 33, and enters the second imaging element 95.In contrast, the test light (unnecessary light) reflected on the optical surface other than the target surface 60 kconverges or diverges is emitted from the target optical system 60 and also enters the second imaging element 95 in the form of convergent or divergent light. According to the present exemplary embodiment, the position of the second imaging element 95 is adjusted by the table (reducing unit) 170 to minimize the influence of the unnecessary light, and the interference light between the measurement light 200 sand the reference light 200 ris received by the second imaging element 95 to detect interference fringes. A signal detected based on the light received from the second imaging element 95 is transmitted to the computer 100.With this configuration, the position of the second imaging element 95 is adjusted by the reducing unit (the table 170), and the influence of background noise can be reduced. As a result, a highly accurate wavefront can be detected, and the measurement accuracy of eccentricity can be improved.In the measurement method according to the present exemplary embodiment, the relative position between the objective lens 55 and the target optical system 60 is also set (roughly set) such that the table image is generated at the reference position on the first imaging element 90, and then the low coherence interferometer 400 detects the interference fringes. With this configuration, non-densely arranged interference fringes can be detected. Consequently, the wavefront can be calculated with high accuracy, and the measurement accuracy of eccentricity detected based on the wavefront can be improved.FIG. 8 is a flowchart illustrating a manufacturing method of an optical system according to the present exemplary embodiment.A result of eccentricity measured using any one of the measurement devices 1 to 4 described in the first to fourth exemplary embodiments can be fed back to a manufacturing method of the optical system (target optical system 60).In step S 101, a manufacturer constructs an optical system using a plurality of optical elements (lenses and the like), and sets a position of each optical element.In step S102, the manufacturer evaluates accuracy and performance of the assembled and set optical system. In a case where a judgment result satisfying a criterion is not acquired (NG in step S 102), the processing proceeds to step S 103. In step S 103, analysis is performed to determine a factor why the criterion is not satisfied. One of the items of analysis is the eccentricity of the optical element. Any one of the measurement devices 1 to 4 may be used to measure eccentricity. In contrast, in a case where an evaluation result satisfying the criterion is acquired (OK in step S 102), the manufacturing of the optical system according to the present manufacturing method is finished.The result of measurement of the eccentricity can be used not only for the analysis of the unsatisfactory factor in step S103 but also for the position adjustment of the optical element in step S101. In other words, it is possible to measure eccentricity of a plurality of optical elements in the optical system using any one of the measurement devices 1 to 4 and adjust positions of the optical elements using the result.Although the embodiments of the disclosure have been described with reference to the exemplary embodiments, it is to be understood that the disclosure is not limited to these exemplary embodiments and may be combined, modified, and changed in various ways within a range that does not depart from the scope of the disclosure.Further EmbodimentsEmbodiment(s) of the present disclosure may also be implemented by a computer of a system or apparatus that reads out and executes computer-executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be more fully referred to as a "non-transitory computer-readable storage medium") to perform the functions of one or more of the above-described embodiment(s), and / or that includes one or more circuits (e.g., an application specific integrated circuit (ASIC)) to perform the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus, for example, by reading out and executing the computer-executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s), and / or the one or more circuits are controlled to perform the functions of one or more of the embodiment(s) described above. The computer may include one or more processors (e.g., a central processing unit (CPU), a micro processing unit (MPU)), and may include a network of separate computers or separate processors to read out and execute the computer-executable instructions. The computer-executable instructions may be provided to the computer from, for example, a network or the storage medium. The storage medium may include, for example, a hard disk and / or a random access memory (RAM) and / or a read only memory (ROM) and / or a distributed computing system memory and / or an optical disk (such as a compact disc (CD), a digital versatile disc (DVD) or a blu-ray disc (BD)TM), and / or a flash memory device and / or a memory card and the like.Although the embodiments of the disclosure have been described above, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedJP 2005-164267

[0003]

Claims

An apparatus configured to measure eccentricity of an optical system based on a wavefront detected for a respective target surface in a plurality of optical surfaces in the optical system, the apparatus comprising: a first light source configured to illuminate a panel including an index surface on which an index is provided; an objective lens configured to direct index light emitted from the panel to the optical system; a first element configured to receive the index light reflected on an optical surface in the optical system; an interferometer including a second light source and a second element and configured to split light from the second light source into test light and reference light and receive measurement light via the objective lens and the reference light with the second element to detect a wavefront of the measurement light, the measurement light being the test light reflected on the target surface; and a reduction unit configured to reduce an intensity of a signal of light reflected on an optical surface other than the target surface of the test light reflected on the plurality of optical surfaces and received by the second element.The apparatus of claim 1, wherein the reducing unit is a spatial filter.The apparatus of claim 2, wherein the spatial filter is a pinhole.The apparatus of claim 1, wherein the reducing unit is configured to adjust a position of the second member.The apparatus according to any one of claims 1 to 4, wherein the interferometer includes a beam splitter, and wherein the reducing unit is configured to acquire a first signal obtained by receiving the reference light and the measurement light reflected by the beam splitter and a second signal obtained by receiving the reference light and the measurement light having passed through the beam splitter, and acquire the wavefront based on a difference between the first signal and the second signal.The apparatus according to any one of claims 1 to 5, wherein the reducing unit is configured to reduce the intensity of the signal of light reflected on the optical surface other than the target surface to be smaller than an intensity of a signal of the measurement light.The apparatus according to any one of claims 1 to 6, further comprising a first setting unit configured to set a relative position between the objective lens and the optical system based on a position of an image formed by index light received from the first element.The apparatus according to claim 7, wherein the eccentricity of the target surface is measured based on a first adjustment amount and the wavefront, the first adjustment amount being detected in a case where the first adjustment unit adjusts the relative position.The apparatus according to claim 8, further comprising a second adjustment unit configured to adjust an inclination component of the wavefront, wherein the eccentricity of the target surface is measured based on the first adjustment amount and a second adjustment amount detected in a case where the second adjustment unit adjusts the relative position.The apparatus according to any one of claims 1 to 9, wherein the interferometer includes a changing unit configured to change a light path length of the reference light.The apparatus of any one of claims 1 to 10, further comprising a shutter in a light path from the second light source to the objective lens.A method for measuring eccentricity of an optical system based on a wavefront detected for a respective target surface in a plurality of optical surfaces in the optical system, the method comprising: illuminating a panel including an index surface on which an index is provided; detecting an image by causing index light emitted from the panel to enter the optical system via an objective lens, and receiving the index light reflected on the plurality of optical surfaces in the optical system with a first element; Splitting light emitted from a second light source into test light and reference light, receiving, with a second element, the reference light and measurement light that is the test light that has passed through the objective lens and is reflected on the target surface, and detecting a wavefront of the measurement light; and reducing an intensity of a signal of light reflected on an optical surface other than the target surface of the test light reflected on the plurality of optical surfaces and is received by the second element in the detecting.The method according to claim 12, further comprising: detecting a first signal obtained by receiving the reference light and the measurement light reflected by the beam splitter and a second signal obtained by receiving the reference light and the measurement light having passed through the beam splitter, and detecting the wavefront based on a difference between the first signal and the second signal.The method of claim 12, wherein the decreasing adjusts a position of the second element.The method of claim 12, wherein the reducing reduces the intensity of the signal of light reflected on the optical surface other than the target surface to be smaller than an intensity of a signal of the measurement light.The method of claim 12, further comprising adjusting a relative position between the objective lens and the optical system based on a position of an image formed by index light received from the first element.The method of claim 12, further comprising changing a light path length of the reference light.A method of manufacturing an optical system, comprising: measuring an eccentricity of the optical system using the method of claim 12; and adjusting the optical system using a measured result of the eccentricity of the optical system.

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