JP2005024505A - Device for measuring eccentricity - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device for measuring eccentricity, capable of measuring interference fringes with high accuracy and capable of measuring an eccentricity with high accuracy, even with respect to an object to be measured having a large amount of aspherical surfaces. <P>SOLUTION: This eccentricity measuring device includes a light source 3'; a reference mirror 6 movable on an optical axis; a beam splitter 5; and an imaging element 10 for imaging the light composed by the beam splitter 5. The device has a processing means for determining the eccentricity on the i surface of a lens system 1 to be inspected, from interference fringes due to the reflected light which includes the i surface of the lens system 1 obtained that is imaged by the imaging element 10 (where the i is in the range of 1-n and the surface position of the lens system 1 that the wave surface of the reference mirror 6 and the wave surface of an object to be inspected intersect, when the position of the reference mirror 6 on the optical axis) is moved gradually by passing beams through the lens system 1 placed on an optical path, and the reflection light of the reference mirror 6; and the data related to the eccentricity determined by a calculation. The device, also, is composed of a ultra-low coherent light source having the light source 3' with a coherence distance of 10 μm or smaller. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an eccentricity measuring apparatus for measuring an assembled eccentricity of an optical system including an aspherical surface.

  Optical systems used for cameras, digital cameras, endoscopes, etc. are required to be compact and reduce costs while ensuring high performance. Therefore, in these optical systems, there is a tendency to use many aspheric surfaces to reduce the number of lenses. Further, an optical system in which an aspheric surface is used for more than half of the total number of lens surfaces and the aspheric amount of each aspheric surface is increased is becoming common. Therefore, if there is an aspheric decentration in the assembled optical system, the optical performance is greatly affected.

  Thus, in recent years, the importance of assembly decentration measurement for optical systems including aspheric surfaces has increased. Here, the assembled decentering measurement is to measure in what posture each surface of the optical system is held after the optical system is assembled. In addition, examples of the optical system that is an object of the assembled decentration measurement include a photographing lens unit set for a portable device and a lens unit set for a digital camera.

By the way, as a conventional assembly eccentricity measuring apparatus for an optical system, for example, an assembly eccentricity measuring machine as shown in the following Non-Patent Document 1 has been proposed.
OPTRONICS (1995) No. 3, p. 120 to 121 This eccentricity measuring machine will be briefly described with reference to FIG. Reference numeral 201 denotes a light source, which is a semiconductor laser here. Reference numeral 203 denotes a test object, which is a lens to be measured here. In the eccentricity measuring machine, the laser light from the semiconductor laser 201 is sequentially projected onto the center of curvature of each surface of the lens to be measured 203 via the measuring optical system 202 and the beam splitter 204. Then, the spot image by this reflected light is captured by the CCD 207. Then, the arithmetic processing unit 210 performs image processing to measure the eccentricity by detecting the position of the spot image.

  At this time, in order not to rotate the lens 203 to be measured, a reference axis setting optical system 206 using an image rotator 205 is provided on the extension of the optical axis of the measurement optical system 202 divided by the beam splitter 204. . When the image rotator 205 is rotated, the spot image of the light beam reciprocating the reference axis setting optical system 206 is rotated on the image plane of the CCD 207. This center of rotation is a reference for eccentricity. Therefore, the spot image rotating on the image plane is captured at four points on the rotation locus to obtain these positions. Then, the rotation reference position is obtained from this. Subsequently, the shake amount of the spherical image of each surface of the lens 203 to be measured with respect to the reference position is obtained. By doing so, assembled eccentricity data is obtained.

  However, the assembled decentration measuring machine as shown in FIG. 4 is slightly inaccurate and the decentering of each lens of the lens system including the aspherical surface cannot be obtained by this measuring machine alone.

Therefore, conventionally, for example, an interference measuring machine as shown in the following Patent Documents 1 and 2 has been proposed as an interference device that can measure an assembled eccentricity including an aspherical surface.
JP 2001-147174 A JP, 2002-5619, A The interference measuring machine of patent documents 1 and 2 is explained using FIG. In this type of interferometer, light emitted from a low-coherence light source 3 such as SLD (Super Luminescent Diode) becomes a substantially parallel light beam through lenses 14 and 4 and is incident on a beam splitter 5 and divided. One light beam l divided by the beam splitter 5 enters the reference mirror 6. The reference mirror 6 can be moved in the x direction of FIG. 5 via a driving unit (not shown).

  The other light beam m split by the beam splitter 5 is incident on the lens system 1 to be tested via the lenses 7 and 8 and the variable aperture stop 9. At least one of the lenses 7 and 8 is movable on the optical axis. Therefore, by changing these positions, the light beam enters the substantially spherical center of the desired test surface i in the test lens system 1. The light beam reflected by the surface i is superimposed on the light beam 1 by the beam splitter 5 via the aperture variable stop 9 and the lenses 8 and 7. Then, the light enters the image sensor 10 and is imaged. Information picked up by the image pickup device 10 is processed by a data processing device including an electronic circuit 11, a personal computer 12, and a display 13.

In the data processing apparatus, the air-converted lengths of the light beams 1 and m measured from the center of the beam splitter 5 are L ₁ and L _m, and the coherence length from the light source 3 is S. Here, S is measured by the full width at half maximum. Then
L _l −2S ≦ L _m ≦ L _l + 2S (1)
In this case, the two light beams interfere with each other, and interference fringes are formed on the image sensor 10.

Therefore, in the interferometer of FIG. 5, the interference fringes due to the reflected light from the surface i are received by the image sensor 10 by moving the reference mirror 6 in the x direction so as to satisfy the conditional expression (1). To do. When L _m = L _l , the interference fringe has a peak intensity. At this time, it is possible to know from which position of the combination lens system 11 the light is reflected from the position of the reference mirror 6. By knowing the position of the reference mirror 6 at which the interference fringe intensity on each surface reaches a peak, it is possible to know the surface spacing of the optical system such as the lens surface spacing and the lens thickness.

  Here, it is assumed that the test lens system is a combination lens system. In order to obtain the decentering of each surface in the combination lens system 1, first, the reference mirror 6 is moved in the x direction to form the interference fringes of the first surface on the image sensor 10. Then, the eccentricity of the first surface is obtained from the tilt component of the interference fringes on the first surface. Next, the reference mirror 6 is further moved in the x direction, and the apparent eccentricity of the second surface is obtained from the tilt component of the interference fringes on the second surface. In addition, considering the decentering of the first surface, the decentering of the second surface is obtained by performing ray tracing or the like for the combination lens system 1. When the i-plane is an aspherical surface, the shape of the interference fringe is different from that of the spherical surface. Therefore, the eccentricity of the aspherical surface is obtained by comparison with the shape of the interference fringe obtained by ray tracing or the like.

  Alternatively, the interference fringes on the pupil plane (in the case of a Fourier transform hologram, the interference fringes themselves formed on the image sensor 10) are reproduced. Then, the decentering of each surface can be obtained by expanding the phase of the fringes into a Zernike function or the like and comparing the expansion coefficient of the asymmetric term such as the tilt component and the coma component with a predetermined expansion coefficient. Here, the predetermined expansion coefficient is the expansion coefficient of the phase of the wavefront obtained by the ray tracing simulation for the position of the reference mirror 6, the image sensor 10, and the combination lens system 1.

Moreover, in the interferometers described in Patent Documents 1 and 2, a procedure for obtaining the eccentricity of each surface of an optical system composed of a plurality of surfaces is disclosed. In this procedure, as shown in FIG. 6, first, a hologram including an i-plane (a surface for obtaining eccentricity) is imaged to create a reproduced image (step S1). In parallel with this, using the known eccentricity data from the 1st plane to the i-1th plane, the interference fringes are simulated using the tilt ε _i and δ _i in the tilt direction and the shift direction of the i plane as variables, and a reproduced image is obtained. Create (step S2). Next, the two reproduced images are compared (step S3). If they do not coincide, a simulation is performed by changing the values of the eccentricity ε _i and δ _i to create a reproduced image (step S2), and the two reproduced images are compared (step S2). S3) Repeat until they match. Then, the eccentricity ε _i and δ _i when the two reproduced images coincide with each other is determined as the eccentricity of the i-plane (step S4). It is checked whether or not the eccentricity has been obtained for all the surfaces (step S5). If all the surfaces have not been obtained, the process of steps S1 to S5 is repeated again with the next surface as the i surface. As a result, the eccentricity of all surfaces is obtained.

  However, in the interference measuring apparatus using the low-coherence light source described in Patent Documents 1 and 2, when the aspherical amount of the object to be measured is increased, the interval between the interference fringes becomes dense. Therefore, there is a problem that the measurement accuracy is lowered.

  The SLD used as the light source of these interference measuring apparatuses has a wavelength band of several tens of nm. An SLD in such a wavelength band has a coherence distance of several tens of μm. For example, assume that the wavelength of the light source is 830 nm and the coherence distance is 50 μm. Then, in order to measure the surface shape and eccentricity using this light source, the light is incident on the sphere center of the i-th surface in the assembled lens, and the reflected light from the i-th surface is interfered with the reference light to cause interference. The stripes shall be measured. At this time, when the wavefront shift due to the surface shape of the first surface to the i-th surface and the eccentricity is within 50 μm, a maximum of 60 interference fringes are generated. Further, the fringe density is high in the portion where the inclination of the wavefront deviation is large. For this reason, it becomes difficult or impossible to separate the adjacent stripes, and the measurement accuracy is lowered.

  The present invention has been made in view of the above problems, and provides an eccentricity measuring apparatus capable of measuring interference fringes with high accuracy and measuring eccentricity with high accuracy even for an object to be measured having a large aspheric amount. The purpose is to do.

  In order to achieve the above object, an eccentricity measuring apparatus according to the present invention includes a light source unit, a reference mirror configured to be movable on an optical axis, and light from the light source unit on a test object side and the reference mirror side. An optical path dividing means for dividing; an optical path synthesizing means for synthesizing the reflected light from the test object and the reflected light from the reference mirror; an imaging element for imaging the light synthesized by the optical path synthesizing means; and the imaging element The i-plane of the test object obtained by passing the light beam through the test object placed in the optical path (where i is a range of 1 to n on the optical axis) The interference fringes due to the reflected light including the reflected light including the wavefront of the reference mirror and the wavefront of the test object that interfere with the wavefront of the test object and the reflected light of the reference mirror when moved, and the eccentricity obtained by calculation Processing means for obtaining the eccentricity of the i-plane of the test object from the data, and The source unit is characterized by being configured with ultra low coherence light source.

  Further, the present invention is characterized in that the light source section is constituted by a broadband light source whose coherence distance does not exceed 10 μm.

  According to the present invention, it is possible to provide an eccentricity measuring apparatus capable of measuring interference fringes with high accuracy and measuring eccentricity with high accuracy even for a measurement object having a large aspheric amount.

  In the eccentricity measuring apparatus of the present invention, the light source unit is configured by a broadband super-coherence light source whose coherence distance does not exceed 10 μm, and other configurations are configured in substantially the same manner as the interferometer shown in FIGS. To perform interference measurement.

  However, the drive of the reference mirror is precisely controlled using a piezo element or the like, and the analysis is performed using both the drive amount and interference fringe data.

  When an ultra-low coherence light source whose coherence distance does not exceed 10 μm is used as the light source unit as in the present invention, interference occurs with the object to be measured having a large aspheric amount in the screen displayed on the display device. It becomes a part of. Therefore, in the eccentricity measuring apparatus of the present invention, the portion where interference occurs due to image processing, that is, the portion where the light intensity changes is extracted while driving the reference mirror.

  Then, the wavefront shape is reconstructed by measuring the portion where the interference occurs at a plurality of positions of the reference mirror.

For example, when interference fringes are measured at the position z of the reference mirror and each coordinate (x, y) on the image sensor,
P (x, y, z) = 1
age,
If no interference fringes are measured,
P (x, y, z) = 0
As
The wavefront shape of the reflected light from the i-th surface on the image sensor can be configured from the set of point sequences where P (x, y, z) = 1.

  The surface shape and eccentricity of each surface can be obtained from the shape of the wavefront on the image sensor. For this purpose, the wavefront changes due to surface shape errors and decentering of each surface and the change in wavefront tilt, coma, astigmatism, or zenike coefficient are obtained from lens data by using lens design software in advance. Like that.

  Further, by using the surface shape and eccentricity as variables, the actual light ray tracing is performed using the wavefront shape obtained by measurement as a target, and the surface shape and eccentricity can be obtained by optimization.

  As a broadband light source whose coherence distance does not exceed 10 μm used in the present invention, a halogen lamp, a femtosecond laser, a broadband with a photonic crystal fiber, a supercontinuum, or the like can be used.

  FIG. 1 is a schematic configuration diagram showing an embodiment of an eccentricity measuring apparatus according to the present invention.

  The eccentricity measuring apparatus of the present embodiment includes an ultra-low coherence light source 3 ′, a reference mirror 6, a beam splitter 5, an image sensor 10, and a data processing device 2.

  Here, the reference mirror 6 is configured to be movable on the optical axis. Further, the beam splitter 5 divides the light from the light source 3 ′ into the test lens 1 side and the reference mirror 6 side, and combines the reflected light from the test lens 1 and the reflected light from the reference mirror 6. . The image sensor 10 is a CCD, for example, and images the light combined by the beam splitter 5.

  The data processing device 2 is composed of a personal computer equipped with an electronic circuit and a display, for example. The electronic circuit is processing means for obtaining the decentering of the i-plane of the lens system 1 to be tested from the interference fringes obtained by measurement and the data relating to the decentration obtained by calculation. Here, the interference fringes obtained by the measurement are captured by the image sensor 10. Further, the interference fringes are generated in a stepwise manner in the test surface i of the test lens system 1 obtained by passing the light beam through the test lens 1 placed in the optical path (where i is in the range of 1 to n). The reflected light including the wavefront of the reference mirror 6 and the wavefront of the lens 1 to be tested interferes with the reflected light of the reference mirror 6 and the reference mirror 6 is reflected. Interference fringes due to light.

  The ultra-low coherence light source 3 ′ is composed of a broadband light source whose coherence distance does not exceed 10 μm.

  A collimator lens 14 is provided in the vicinity of the light source 3 ′. The collimator lens 14 converts the divergent light emitted from the light source 3 ′ into a parallel light beam and guides it to the beam splitter 5. A collimator lens 15 is movably provided in the z direction between the lens system 1 and the beam splitter 5. Then, the parallel light flux from the beam splitter 5 is converted into convergent light and guided to the lens 1 to be examined. At the same time, by changing the position, the light beam is incident on a substantially spherical center of a desired test surface i in the test lens system 1.

  The reference mirror 6 is movable with high accuracy in the x direction via a driving device 16 using a piezoelectric element. The driving device 16 is connected to the data processing device 2, and position information of the reference mirror 6 is detected and controlled in the data processing device 2.

  In the eccentricity measuring apparatus of the present embodiment, a broadband ultra-low coherence light source 3 ′ whose coherence distance does not exceed 10 μm is used. In this case, the interference with the test lens system 1 having a large aspheric amount is a part of the screen displayed on the display of the data processing device 2. Therefore, in the eccentricity measuring apparatus of the present embodiment, the reference mirror 6 is moved via the driving device 16 using a piezo element. At that time, it is driven with high accuracy at a pitch subdivided in the x direction. In this manner, the part where the interference occurs, that is, the part where the light intensity changes is extracted by the image processing in the data processing device 2.

  Then, by measuring the portion where the interference occurs at a plurality of positions of the reference mirror 6, the shape of the wavefront can be reconstructed.

For example, as described above, when interference fringes are measured at each coordinate (x, y) on the image sensor 10 with the wavefront z corresponding to the position of the reference mirror and the wavefront from the test surface,
P (x, y, z) = 1
age,
If no interference fringes are measured,
P (x, y, z) = 0
As
The wavefront shape of the reflected light from the i-th surface on the image sensor 10 can be configured from the set of point sequences where P (x, y, z) = 1.

  Then, the surface shape and eccentricity of each surface are obtained from the shape of the wavefront on the image sensor 10. Therefore, from the data of the lens system 1 to be tested, the wavefront change due to surface shape error and decentration of each surface, the wavefront tilt, coma, astigmatism, or the change of the zenike coefficient is obtained by lens design software or the like. Keep it.

  Further, by using the surface shape and eccentricity as variables, the actual light ray tracing is performed using the wavefront shape obtained by measurement as a target, and the surface shape and eccentricity can be obtained by optimization.

  As the broadband ultra-low coherence light source 3 'whose coherence distance does not exceed 10 [mu] m used in this embodiment, a broadband using a halogen lamp, femtosecond laser, photonic crystal fiber, supercontinuum, or the like can be used.

  2 (a) to 2 (e) are explanatory views showing the detection positions of interference fringes when scanning an aspherical surface in the eccentricity measuring apparatus using the ultra-low coherence light source according to the present embodiment, and FIG. It is explanatory drawing which shows the detection location of an interference fringe when scanning the eccentricity measuring apparatus using a low-coherence light source for comparison.

  For example, in the lens system 1 shown in FIG.

  In the eccentricity measuring apparatus of the present embodiment, an ultra-low coherence light source 3 ′ whose coherence distance does not exceed 10 μm is used. In this case, the interference with the test surface i of the test lens system 1 having a large aspheric amount is part of the screen displayed on the display device.

Accordingly, as shown in FIG. 2A, the reference mirror 6 is moved to a plurality of positions at a predetermined pitch subdivided into N pieces along the x direction. At this time, wavefronts from the reference mirror 6 that interfere with the test surface i are denoted by Z _{1 to} Z _N.

Then, for example, an interference image due to the wavefront Z ₂ from the reference mirror 6 and the wavefront from the test surface i of the test lens system 1 is as shown in FIG. Further, an interference image by the wavefront Z ₄ from the reference mirror 6 and the wavefront from the test surface i of the test lens system 1 is as shown in FIG. An interference image between the wavefront Z ₇ from the reference mirror 6 and the wavefront from the test surface i of the test lens system 1 is as shown in FIG.

In this way, by moving the reference mirror 6 at a predetermined interval along the x direction, the wavefronts Z _{1 to} Z _N from the reference mirror 6 that interferes with the test surface i at each of the subdivided positions. And an interference image by the wavefront from the test surface i of the test lens system 1 is obtained. Therefore, when these interference images are picked up by the image sensor 10 and the picked-up interference images are combined by the data processing device 2, an interference image of the entire test surface i is obtained as shown in FIG. .

  By using the method described with reference to FIGS. 5 and 6 from the obtained wavefront shape on the image sensor 10, the surface shape and eccentricity of each surface can be obtained.

  At this time, according to the eccentricity measuring apparatus of the present embodiment, the interference fringes between the wavefront from the reference mirror 6 and the wavefront from the surface i to be measured are accurately subdivided via the driving device 16 such as piezo. Is detected. Since the interference fringes for each position are synthesized, the interference image of the entire test surface can be detected with high accuracy. For this reason, even for a test object having a large amount of spherical surface, the eccentricity of the test object can be measured with high accuracy.

  On the other hand, when a low coherence light source is used as in a conventional interferometer, the coherence distance is longer than that of the ultra-low coherence light source of this embodiment. Therefore, for example, as shown in FIG. 3, the wavefronts Z′1 to Z′2 from the reference mirror corresponding to the coherent distance and the wavefront from the surface i to be inspected interfere with each other over a wide range. The interference image is picked up by the image pickup device 10. In this case, the density of the interference fringes becomes high at a portion where the inclination of the wavefront deviation is large, so that separation from the adjacent interference fringes becomes difficult or impossible, and the measurement accuracy of the interference fringes is lowered.

It is a schematic block diagram which shows one Example of the eccentricity measuring apparatus by this invention. (a)-(e) is explanatory drawing which shows the detection position of an interference fringe when scanning an aspheric surface with the eccentricity measuring apparatus using the ultra-low-coherence light source by a present Example. It is explanatory drawing which shows the detection position of an interference fringe when scanning the eccentricity measuring apparatus using a low coherence light source for the comparison with a present Example. It is a schematic block diagram which shows an example of the assembly decentration measuring apparatus of the conventional optical system. It is a schematic block diagram which shows the other example of the conventional interference measuring apparatus. It is a flowchart which shows the procedure which calculates | requires eccentricity of each surface of the optical system which consists of several surfaces in the interference measuring machine of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Test lens system 2 Data processing apparatus 3 Light source part 3 'Super-coherence light source 4,7,8,14 Lens 5 Beam splitter 6 Reference mirror 9 Aperture variable aperture 10 Imaging element 11 Combination lens system 12 Personal computer 13 Display 14, 15 Collimator Lens 16 Reference mirror drive device using piezo element 201 Semiconductor laser 202 Measurement optical system 204 Beam splitter 205 Image rotator 206 Reference axis setting optical system 207 CCD
208 Monitor TV 209 CRT
210 Arithmetic processing part

Claims (2)

A light source unit, a reference mirror configured to be movable on the optical axis, an optical path dividing unit that divides light from the light source unit into a test object side and the reference mirror side, and reflected light from the test object; An optical path synthesis unit that synthesizes the reflected light from the reference mirror, an image sensor that images the light synthesized by the optical path synthesis unit, and a test object that is imaged by the image sensor and placed in the optical path The i-plane of the test object obtained through the luminous flux (where i is a range of 1 to n, and when the position of the reference mirror is gradually moved on the optical axis, the wavefront of the reference mirror and the test object Processing means for determining the eccentricity of the i-plane of the test object from interference fringes due to the reflected light including the surface position of the test object that interferes with the wavefront) and the reflected light of the reference mirror, and data relating to the eccentricity obtained by calculation And having
An eccentricity measuring apparatus, wherein the light source unit is composed of an ultra-low coherence light source. The eccentricity measuring apparatus according to claim 1, wherein the light source unit includes a broadband light source whose coherence distance does not exceed 10 μm.

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