JPH08304229A - Method and apparatus for measuring refractive index distribution of optical element - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a measuring method and an apparatus which are less susceptible to environmental changes such as temperature. [Structure] Coherent light from the same light source 1 is divided into a reference wave a serving as a reference and a test wave b passing through a test object A formed of an optical element to be measured, which is immersed in a test solution having substantially the same refractive index. Then
An interference fringe image is formed by superimposing the reference wave a and the test wave b. The translucent solid 73 whose refractive index is almost the same as that of the optical element is replaced with a part of the test solution B, the interference fringe image detector 15 is arranged on the image plane of the interference fringe image, and the test object A is placed in the test solution. While rotating around the axis orthogonal to the optical axis of the detected wave b, the interference fringe image detected by the interference fringe image detector 15 is analyzed to measure the transmitted wavefront.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring a refractive index distribution of an optical element such as an optical lens, and more particularly to a method and apparatus for measuring a refractive index distribution of an optical element by analyzing an interference fringe image.

[0002]

2. Description of the Related Art In recent years, molded lenses made of plastics materials have become widespread as optical lenses used in optical devices such as laser printers and cameras. This plastics molded lens is superior in terms of manufacturability of an aspherical lens to a glass-polished lens and is inexpensive, but the distribution of the refractive index in manufacturing is unstable, and unevenness may occur inside the lens. Many. The non-uniformity of the refractive index inside the lens has a great influence on the optical characteristics and causes deterioration of image quality and resolution. For this reason, it is necessary to measure the distribution of the refractive index inside the optical lens with high accuracy and evaluate the homogeneity of the optical lens.

As a method of measuring the refractive index of an optical lens, there are two methods such as a method of measuring a refraction angle by a V-block method using a precision differential refractometer or the like to obtain the refractive index, and a method of Twyman-Green interferometer. There is a method of measuring the refractive index from interference fringes using a light flux interferometer. Also, as a method of measuring optical homogeneity, a two-beam interferometer such as a Fizeau interferometer or a Mach-Zehnder interferometer is used to measure the transmitted wavefront by analyzing the interference fringe image, and obtain the optical homogeneity from the refractive index distribution. The method is known.

If a more detailed explanation of these refractive index measuring methods and optical homogeneity measuring methods is required, see pages 63-68 of Optics Vol. 20, No. 2 (February 1991). See Refractive Index and Optical Homogeneity Measurements of Optical Materials.

In such a method, it is necessary to process a test object, that is, a sample into a predetermined shape with high precision, and it is necessary to destroy an optical element to be measured. Further, the refractive index distribution obtained from the transmitted wavefront is an average value integrated in the traveling direction of the optical path, and the three-dimensional spatial refractive index distribution cannot be measured. This makes it impossible to specify the non-uniform refractive index in three-dimensional space.

Therefore, the present inventor has previously proposed Japanese Patent Application No. 6-203502 as a method for measuring the refractive index distribution of an optical element. With this measuring method, the refractive index distribution of the optical element can be measured non-destructively and efficiently as a three-dimensional spatial refractive index distribution regardless of the shape, and the uneven refractive index portion Since it can be specified dimensionally and spatially, it is a very effective method for measuring the refractive index distribution.

[0007]

However, the test solution, which should be the standard, is sensitive to environmental changes, especially temperature.
When the temperature changes by ° C, the refractive index may change in the order of 0.0001 to 0.001. In order to prevent the environmental fluctuation from affecting the measurement accuracy, there is a problem that the measurement must be performed in a stable environment where the environmental fluctuation is small or the temperature must be compensated. Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a measuring method and apparatus that are less susceptible to environmental changes such as temperature.

[0008]

In order to achieve the above-mentioned object, the method of measuring the refractive index distribution of an optical element according to the present invention is such that the object to be measured is an optical element and the refractive index of this optical element is Substantially the same translucent solid is immersed in a test solution having a refractive index almost the same as that of the optical element, and the coherent light from the same light source is divided into a reference wave serving as a reference and a test wave passing through the optical element. , An interference fringe image is formed by superimposing the reference wave and the test wave, and the test object is immersed in the test solution and rotated around an axis orthogonal to the optical axis of the test wave, and at least two rotational angle positions In each of the above, the transmitted wavefront is measured by analyzing the interference fringe image, and the refractive index distribution of the optical element is measured from the measurement results of the transmitted wavefront from a plurality of directions.

In order to achieve the above object, the apparatus for measuring the refractive index distribution of an optical element according to the present invention uses a coherent light from the same light source as a reference in a sample solution whose refractive index is almost the same as that of a reference wave. A two-beam interferometer that forms an interference fringe image by superimposing a reference wave and a test wave, and a part of the test solution. The translucent solid having the same refractive index as that of the optical element to be replaced, the interference fringe image detector disposed on the image plane of the interference fringe image, and the interference fringe detected by the interference fringe image detector. A transmitted wavefront measuring means for analyzing the image and measuring the transmitted wavefront,
Rotating means for rotating the test object in the test solution around an axis orthogonal to the optical axis of the test wave.

Further, in the measuring device according to the present invention,
The translucent solid may include a cylindrical hollow portion that houses the optical element. Further, in the measuring apparatus according to the present invention, the translucent solid may be formed in a shape such that the shape combined with the test object is a substantially cylindrical shape. Further, in the measuring device according to the present invention, the container containing the reagent solution may be made of an optical material having a refractive index substantially the same as the refractive index of the measurement target. Also,
In the measuring device according to the present invention, the translucent solid is
It may be made of a powdery optical material mixed in the reagent solution.

[0011]

According to the above-mentioned structure, the light-transmissive solid having a refractive index substantially the same as that of the test object is put together with the test object into a reagent solution having a refractive index substantially the same as that of the test object for measurement. Therefore, it is possible to replace the reagent solution, which is easily affected by environmental changes, with a translucent solid, which has less environmental changes, and reduce the effect of environmental changes on measurement.

Further, when the light-transmissive solid has a cylindrical hollow portion for accommodating the optical element, the space required for rotating the test object can be minimized and the camera lens It is possible to minimize the area into which the test solution penetrates when measuring a rotationally symmetrical test object.

Further, when the transparent solid is combined with the object to be inspected so as to have a substantially cylindrical shape, the influence of environmental fluctuations should be reduced even in the measurement of a rotationally asymmetric lens. You can

Further, when the container containing the reagent solution is made of an optical material having a refractive index almost the same as the refractive index of the object to be measured, the number of parts can be reduced. Further, when the translucent solid is mixed into the test solution in the form of powder, the influence of environmental changes on the measurement can be reduced without changing the other components.

[0015]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 shows one embodiment of a refractive index distribution measuring device for an optical element according to the present invention. In this embodiment, an object to be tested is an object to be rotated and an axis is arranged perpendicular to the paper surface. ing. This measuring apparatus has a Mach-Zehnder interferometer as a basic configuration, and includes a laser light source 1 and
Beam expander 3, beam splitter 5 for splitting light flux, two high-reflection mirrors 7 and 9, beam splitter 11 for superimposing light flux, imaging lens 13, and interference fringe detection by area image sensor such as CCD Bowl 15
And an arithmetic processing unit 17 including a high-speed image processing device and a microcomputer.

A laser light beam emitted from the laser light source 1 as a coherent light beam has a beam diameter expanded by a beam expander 3 and travels straight by a beam splitter 5 to become a reference wave a, and FIG. Then, it is refracted downward at a right angle to proceed and is split into another laser beam which becomes the detected wave b.

The reference wave a is reflected by the high-reflecting mirror 7 and does not pass through the object A to be described later, and the beam splitter 11
The incident wave b is reflected by the high-reflection mirror 9, passes through the object A, and enters the beam splitter 11.

Reference wave a incident on the beam splitter 11
And the wave b to be detected are mutually superposed by the beam splitter 11, and the imaging fringe 13 forms an interference fringe image on the imaging surface of the interference fringe detector 15.

The high-reflecting mirror 9 is supported by an electric-displacement conversion element 19 such as a piezo element, and in order to perform interference fringe analysis by the phase shift method, an optical path for variably setting the optical path length of the test wave b in the wavelength order. It is arranged so that it can be finely displaced in any direction.

A container-shaped cell 21 for accommodating the object A to be inspected is arranged in the optical path of the wave b to be inspected. An object A, which is an optical element to be measured, is set in a fixed state in the cell 21, and as shown by an arrow in the figure, the axis A orthogonal to the optical axis of the wave b to be measured (perpendicular to the paper surface A rotatable test table 23 is arranged so as to be rotatable about its axis. The rotary inspection table 23 is drivingly connected to a servo motor (not shown), and is rotationally driven to a predetermined rotation angle position by the servo motor.

As shown in FIG. 2, both end faces of the cell 21 which cross the optical path of the wave b to be detected are composed of an entrance window 25 and an exit window 27 for the light beam. The entrance window 25 and the exit window 27 are optical flats 29 and 3 having high surface accuracy.
Liquid-tightly shielded by 1.

The cell 21 is filled with a test solution (contact solution) B, which is a matching solution having a refractive index almost equal to that of the object A to be inspected, and is placed on the rotating object table 23. The sample A is immersed in the test solution B. Further, in the cell 21, a first matching glass material 73 made of a known solid optical material whose refractive index is substantially the same as that of the test object A is provided. Since the temperature coefficient of refractive index of glass is 10 ⁻⁶ / ° C., it is much more excellent in environmental resistance than the reagent solution B of the order of 10 ⁻⁴ to 10 ⁻³ / ° C. For example, the inspection object A is PC (polycarbonate), the wavelength of the laser light is 633 nm, and the room temperature is 23.
When the measurement is performed under the condition of about 25 ° C., BaF3 (shot name) may have the smallest difference in refractive index as the first matching glass material 73.

The shape of the first matching glass material 73 is a rectangular parallelepiped, and the vicinity of the center of one surface of the rectangular parallelepiped is hollowed out into a cylindrical shape to form a hole 83 having a cylindrical hollow portion. Then, the structure is such that the object A can be inserted into the hole 83. As described above, the test solution B is filled in the cell 21, and the test object A on the rotating test object table 23 is the test solution B.
It is soaked in. The cell 21 and the first matching glass material 73 are fixed to the apparatus main body side, and only the inspection object A is rotatably supported on the rotating inspection object table 23.

The interference fringe image formed on the image pickup surface of the interference fringe detector 15 by the imaging lens 13 as described above is photoelectrically converted by the interference fringe detector 15 into an electric image signal, and the A / D After being A / D converted by the converter 33, it is input to the arithmetic unit 17. The arithmetic unit 17
Includes a transmitted wavefront measuring unit 35 that performs a calculation calculation of a transmitted wavefront by analyzing an interference fringe image by a phase shift method or the like.

Next, a method of measuring the refractive index distribution of the object A to be measured by using the measuring device having the above-mentioned structure will be described. First, before setting the inspection object A on the rotating inspection table 23, the image signal of the interference fringe image output from the interference fringe detector 15 is taken into the arithmetic unit 17 and the transmission wavefront measuring unit 35 causes the interference fringes. Analyze the image and measure the transmitted wavefront in the initial state.
Based on this measurement result, initial processing is performed to eliminate the steady error component of the measuring device itself.

Next, the inspection object A is set on the rotating inspection object table 23, and the interference fringe image output from the interference fringe detector 15 in a state where the rotating inspection object table 23 is positioned at the initial rotation position. The image signal of 1 is taken into the arithmetic unit 17, the interference fringe image is analyzed by the transmitted wavefront measuring unit 35, and the transmitted wavefront is measured.

Here, when the refractive index of the test object A is completely uniform and this refractive index is equal to the refractive index of the reagent solution B filled in the cell 21, an interference fringe image by the phase shift method is obtained. The analysis should be zero. On the other hand, when the refractive index of the test object A is slightly different from the refractive index of the test solution B, the following relational expression holds.

Φ (y) = (2π / λ) ∫Δn (x, y) dx where φ (y) is the transmitted wavefront (rad) Δn (x, y) is the refraction between the sample A and the test solution B. Index difference λ: wavelength of laser light

In the measurement of the transmitted wavefront only under the condition that the rotating object table 23 is located at the initial rotation position, the analysis result of the interference fringe image is integrated in the x direction (optical path traveling direction), and this is enough. It is not possible to specify the spatial position of the nonuniform refractive index portion.

Therefore, the rotary inspection table 23 is rotated by a predetermined angle, for example, 90 degrees from the initial rotation position, and the direction of the inspection wave b of the inspection object A on the rotational inspection table 23 is rotated with respect to the optical axis. The rotation angle from the initial rotation position of the inspection table 23 is changed. Even when the test object A is rotationally displaced in this way, the test object A is immersed in the test solution B, and therefore, as before the rotational displacement,
An interference fringe image is formed on the imaging surface of the interference fringe detector 15 by superimposing the reference wave a and the test wave b. Under this condition, the image signal of the interference fringe image output from the interference fringe detector 15 is used as the arithmetic unit 1
7, and the transmitted wavefront is measured by the transmitted wavefront measuring unit 35.

As a result, in a plurality of directions with respect to the inspection object A,
In this case, the transmitted wavefronts are respectively measured by the test wave b incident from two directions, and the spatial position of the non-uniform refractive index portion of the test object A is specified by the combination of the plurality of transmitted wavefront data. Will be possible.

When the refractive index distribution of the test object A is measured as a complete three-dimensional spatial distribution, the test object A is rotated around an axis orthogonal to the optical axis of the test wave b. The incident direction of the test wave b with respect to A is changed within a range of 180 degrees or 360 degrees, measurement data of the transmitted wave front at each rotation angle position is collected, and an image is reconstructed by a computer. Reconstruction of this image is performed by a known CT (Computer Tomography) method.

FIG. 3 shows the principle of the CT method, which should be obtained by performing a one-dimensional Fourier transform on the variable X with respect to the transmitted wavefront data p (X, φ) of the test wave incident from the direction of the angle φ. The φ direction component in the polar coordinate representation of the two-dimensional Fourier transform of the refractive index distribution Δn (x, y) can be obtained.

That is, 0 ≦ φ ≦ 2π or 0 ≦ φ ≦ π
After measuring the transmitted wavefront over the angle range of, the transmitted wavefront data is subjected to one-dimensional Fourier transform, and the Fourier-transformed polar coordinate data of each cross section is converted into rectangular coordinate data, and then 2
By performing the three-dimensional inverse Fourier transform, the three-dimensional refractive index distribution of the test object A can be reconstructed.

This can be expressed by the following equation. The relationship between the Cartesian coordinate system (ξ, η) and the polar coordinate system (r, φ) is expressed as follows: ξ = r cos θ η = r sin θ The transmitted wavefront is p (X, φ), and the refractive index of the sample A and the test solution B is If the difference is Δn (x, y), the two-dimensional Fourier transform F
(Ξ, η) is expressed as follows.

[0036]

[Equation 1]

Δn (x, y) = (1 / 4π ² ) ∫∫F
(Ξ, η) exp [i (ξx + ηy)] dξdη

FIG. 4 shows the configuration of the arithmetic unit 17 when the image is reconstructed by the CT method to obtain the three-dimensional refractive index distribution of the object A to be examined. In this case, the arithmetic unit 17 inputs the image signal from the interference fringe detector 15 for each rotation angle φ and calculates the amount of aberration of the transmitted wavefront, and the wavefront aberration amount for each rotation angle φ. A one-dimensional Fourier transform unit 39 for performing a one-dimensional Fourier transform, a polar coordinate-orthogonal coordinate transform unit 41 for transforming the Fourier-transformed polar coordinate data of each section into rectangular coordinate data, and a two-dimensional inverse Fourier transform of the orthogonal coordinate data 2 Dimensional inverse Fourier transform unit 43, refractive index conversion unit 45 that converts the result of two-dimensional inverse Fourier transform into a refractive index, and refractive index that outputs a refractive index distribution based on the refractive index converted by the refractive index conversion unit 45. And a distribution output unit 47.

In the case of the CT method, the refractive index distribution has no regularity, and even for an object whose distribution is unknown, the refractive index distribution is determined by the non-uniform portion of the refractive index of the object. It becomes possible to specify the spatial position.

FIG. 5 shows an embodiment in which the object to be inspected is not an object to be rotated. The parts other than the object to be inspected and the matching glass material are the same as those in the embodiment in FIG. Only the inside of the cell including is shown. Examples of the object A ₁ which is not rotationally symmetric include rotationally asymmetric lenses such as fθ lens, toroidal lens and toric lens used in laser printers and the like.

Even the test object A ₁ that is not the object of rotation can be measured by inserting it into the cylindrical space of the hole 83 of the first matching glass material 73 of FIG. 1 (FIG. 2). In this case, the gap between the test object A ₁ and the matching glass material 73 becomes large, so that the amount of the test solution B filling the clearance becomes large and the test object rotates due to the influence of the change in the refractive index due to the temperature change. It is easier to receive than the symmetrical case. In order to prevent this, in the present embodiment, the second matching in which the shape combined with the test object A ₁ is substantially cylindrical so as to fill the space between the test object A ₁ and the first matching glass material 73. Glass material 7
5 is provided on the rotary inspection table 23. The second matching glass material 75 has a refractive index almost the same as that of the test object A ₁ ,
At the time of measurement, the second matching glass material 75 and the object to be inspected A ₁ rotate integrally on the rotating object table 23.

FIG. 6 shows an embodiment in which the cell 21, the optical flats 9 and 31 and the first matching glass material 73 of FIG. 2 are integrated, and the other parts are similar to those of the embodiment of FIG. Not shown. In the figure, only the integrated third matching glass material 77 is shown. That is, the third matching glass material 77 is made of the same solid optical material as the first matching glass material 73 shown in FIG. 2, that is, has the same refractive index as that of the object to be inspected, and its outer shape is shown in FIG. As shown in, it is formed in a rectangular parallelepiped shape. Further, the third matching glass material 77 has an incident surface 79 on which the incident light Li is incident.
The emission surface 81 from which the emitted light Lo is emitted is optically polished to the same level of surface precision as the optical flats 9 and 31, and a hole 83 forming a cylindrical space is provided in the direction orthogonal to the optical axis. One end of the hole 83 is closed as shown in FIG. 6B, and the test substance A and the test solution B are inserted into the cylindrical space of the hole 83.

By thus integrating the cell 21, the optical flats 9 and 31, and the first matching glass material 73 into a third matching glass material 77 having the functions of the cell, the optical flat and the matching glass material, the number of parts is reduced. be able to. This reduction in the number of parts facilitates assembly adjustment and achieves cost reduction.

Moreover, since the optical flats 9 and 31 and the first matching glass material 73 are integrated, the reagent solution B does not exist between the optical flats 9 and 31 and the first matching glass material 73. Therefore, the environmental fluctuation of the refractive index due to the sample solution B in the test wave transmitting portion is eliminated, so that the influence of the wavefront aberration in this portion can be eliminated.

FIG. 7 shows an embodiment in which a matching glass material made of powder is used. Since parts other than the matching glass material are the same as those in the embodiment of FIG. 1, a cell containing the test object and the matching glass material is shown. Only the inside of is shown. In this embodiment, instead of the first matching glass material 73 of FIG.
A powdery matching glass material 85 having a particle diameter of several tens of μm is used, and the matching glass material 85 is mixed in the test solution B. By using the powdery matching glass material 85 in this manner, conventional materials other than the matching glass material 85 can be used, and it is not necessary to manufacture a dedicated jig or optical component. Further, it can be applied to measurement of various test objects A having different shapes.

The reagent solution B mixed with the above-mentioned powdery matching glass material 85 can be used not only in the cell 21 of FIG. 2 but also in the cell 21 of FIG. 5 and the cell 77 of FIG. 6, and is combined in this way. As a result, it is possible to further reduce the influence of environmental changes.

FIG. 8 shows another embodiment of the refractive index distribution measuring apparatus for an optical element according to the present invention. In addition, in FIG. 8, portions corresponding to those in FIG. 1 are denoted by the same reference numerals as those in FIG. In this embodiment, the polarization direction of the laser light emitted from the laser light source 1 is set to be about 45 degrees with respect to the paper surface, and the beam splitters 5 and 1 are used.
A polarization beam splitter is used as each 1. The reference wave a and the test wave b are overlapped by passing through the beam splitter 11, but they do not interfere with each other because their polarization planes are orthogonal to each other.

Further, in this embodiment, another beam splitter 49 is provided, and the beam splitter 4
Reference numeral 9 divides the superposed light flux of the reference wave a and the test wave b from the beam splitter 11 into two. One of the superimposed light beams split into two by the beam splitter 49 passes through the polarizer 51 to cause interference, and the interference fringes form the imaging lens 13
An image is formed on the imaging surface of the first interference fringe detector 15 by.
The other superimposed light flux passes through the λ / 4 plate 53, which is an optically anisotropic member, so that a phase difference of π / 2 is generated with respect to the one superimposed light flux. The other superposed light flux is then transmitted to the polarizer 5
As a result, the interference fringes are imaged on the image pickup surface of the second interference fringe detector 59 by the imaging lens 57.

The interference fringe images formed on the first interference fringe detector 15 and the second interference fringe detector 59 are for the same object A, but the second interference fringe detector is the same. The presence of the λ / 4 plate 53 in the optical path leading to
It is 0 degrees out of phase. For example, assuming that the interference fringe image formed on the first interference fringe detector 15 is in the state shown in FIG. 9A, the interference fringe image formed on the second interference fringe detector 59 is shown in FIG. ), And the interference fringe image is shifted by half.

In this case, the first interference fringe detector 15 and the second interference fringe detector 59 are arranged so as to receive optical information from the same point on the object A to be inspected. The deviation of the interference fringe image is not limited to 90 degrees = π / 2, and may be nπ / 2 in general, where n is a natural number.

FIG. 10 shows an apparatus for calculating the phase displacement amount of the interference fringes from the output signals of the first interference fringe detector 15 and the second interference fringe detector 59. This calculation device obtains a Lissajous waveform from the output signals of the first interference fringe detector 15 and the second interference fringe detector 59 and counts the change in brightness of the interference fringes.

Amplifier circuits 61 and 63 and square wave conversion circuits 65 and 67 are connected to the first interference fringe detector 15 and the second interference fringe detector 59, respectively. First interference fringe detector 1
5, the output signals output from the second interference fringe detector 59 are amplified by amplifier circuits 61 and 63, respectively.

FIG. 11A shows the output signal waveform of the first interference fringe detector 15, and FIG. 11B shows the second interference fringe detector 5.
The output signal waveforms of 9 are shown respectively. These are the objects to be inspected A
Is an output signal waveform in the case where the interference fringes change at a constant speed by rotating about the axis orthogonal to the optical axis, and the output signal waveform of the first interference fringe detector 15 is the first interference fringes. This is because the light intensity change at the point P1 (see FIG. 9) of the detector 15 is observed, and the output signal waveform of the second interference fringe detector 59 corresponds to the same observation point as the point P1. This is because the change in the light intensity due to the change in the light intensity at the point P2 (see FIG. 9) of the interference fringe detector 15 is observed, and both of them have a phase shift of 90 degrees.

These output signals are converted into a square wave conversion circuit 65,
According to the predetermined threshold level by 67, FIG.
It is converted into a square wave signal as shown in (c) and (d). These two square wave signals are used for the pulse counting circuit 6
9, the pulse count circuit 69 detects the moving direction and the number of moving interference fringes by detecting the input timing of the two square wave signals and counting the up pulse and the down pulse.

The data of the movement direction and the number of movements of the interference fringes are input to the phase displacement amount calculating device 71, and the phase displacement amount calculating device 71 calculates the phase displacement amount in the optical axis direction from these data, and the phase displacement amount is calculated. The amount of aberration of the transmitted wavefront is calculated by adding the initial value previously measured by the phase shift method to the amount. By performing this operation for each pixel of each interference fringe detector, the amount of phase displacement and the amount of transmitted wave front of the entire test object can be obtained.

A method of measuring the refractive index distribution of the object A to be measured using the measuring device having the above structure will be described. First,
The electric-displacement conversion element 19 is driven and the initial value of the transmitted wavefront distribution is measured using an analysis method such as a phase shift method. Next, the rotary inspection table 23 is rotationally driven in the direction of the arrow, and the inspection object A is rotated about an axis line orthogonal to the optical axis (about an axis line perpendicular to the paper surface). The rotation of the object A moves the interference fringe images formed on the first interference fringe detector 15 and the second interference fringe detector 59, and the movement direction and movement amount (number of movements) of the interference fringes are determined. As described above, the pulse counting circuit 69
Detect by.

Then, the phase displacement amount calculating device 71 calculates the phase displacement amount in the optical axis direction from the movement direction and the number of movements of the interference fringes, and adds the initial value previously measured by the phase shift method to this phase displacement amount. , The aberration amount p (x, θ) of the transmitted wavefront at a certain rotation angle φ is calculated. This operation is performed within a range in which the incident direction extends over 180 degrees or 360 degrees, the measurement data is analyzed using the CT method, and the refractive index distribution is calculated.

In the measuring device shown in FIG.
The first interference fringe detector 15 and the second interference fringe detector 59 are 1
A linear image sensor such as a three-dimensional CCD may be used. A linear image sensor such as a one-dimensional CCD can serially output an image signal at high speed, and thus real-time measurement of interference fringes becomes possible.

For example, when a high-resolution one-dimensional CCD of 7 μm × 5000 pixels is used, the resolution is 70 μm even if the number of images required for one fringe is 10 pixels.
Then, a maximum of 500 fringes, that is, a wavefront aberration amount of 500
It can handle up to λ (approximately 300 μm), the size of the test object d = 50 mm, and the refractive index difference Δn = 0.00 with the reagent solution B.
Even if it is 1, Δnd is approximately 79λ, which is sufficiently acceptable.

If the refractive index difference from the sample solution B is large, the interference fringes change rapidly due to the rotation of the test object, but the one-dimensional C
Since the CD can serially output the image signal at a high speed with a response speed of several milliseconds or less, the transmitted wavefront amount can be calculated in real time while rotating.

In the above embodiments, the transmitted wavefronts of the test waves incident on the test object A in a plurality of directions are measured while the test object A is immersed in the test solution B, and the plurality of transmitted waves are transmitted. When the refractive index distribution in the depth direction is measured for the object of any shape by combining the wavefront data and the non-uniform part of the refractive index is specified three-dimensionally, it is affected by environmental changes. Since a part of the easy test solution B is replaced with the light-transmissive solid 73 (75, 77, 85) that is not easily affected by environmental fluctuations, it is possible to reduce the refractive index fluctuations caused by environmental fluctuations and to perform highly accurate measurement. It will be possible to do.

Further, the translucent solid 73 (77) is the object A to be inspected.
When the hole 83 (cylindrical hollow portion) for accommodating the liquid is provided, it is possible to minimize the area where the test solution B enters when measuring the test object A of a rotationally symmetric body such as a camera lens. Therefore, the influence of environmental changes can be reduced.

When the transparent solid 75 and the object A ₁ are combined into a substantially cylindrical shape, the object A ₁ such as a rotationally asymmetric lens is measured. Also in this case, the influence of environmental changes can be reduced, so that, for example, a rotationally asymmetric test object A ₁ such as a scanning lens of a laser printer can be measured with high accuracy.

Further, when the container containing the reagent solution B is made of an optical material having a refractive index substantially the same as the refractive index of the object to be measured, the container and the light-transmissive solid are integrated to reduce the number of parts. Therefore, it is possible to reduce the amount of the reagent solution B in the region through which the test wave is transmitted, and thus it is possible to eliminate the influence of the wavefront aberration, and it is possible to perform more accurate measurement.

When the light-transmissive solid 85 is mixed in the sample solution B in the form of powder, the influence of environmental changes on the measurement can be reduced without changing other components, and higher accuracy can be obtained. You can make various measurements.

[0066]

As described above, according to the method and apparatus for measuring the refractive index distribution of the optical element according to the present invention, the test object is immersed in the test solution from a plurality of directions with respect to the test object. The transmitted wavefront by the incident test wave is measured, and the refractive index distribution in the depth direction is measured for the test object of any shape by combining the transmitted wavefront data, and the part where the refractive index is not uniform is measured. When three-dimensionally identifying a sample, a part of the reagent solution that is easily affected by environmental changes was replaced with a translucent solid that is not easily affected by environmental changes. Therefore, it becomes possible to perform highly accurate measurement.

Further, when the translucent solid has a cylindrical hollow portion for accommodating the optical element, a region where the test solution enters when measuring a rotationally symmetrical object such as a camera lens is measured. It can be minimized and the influence of environmental changes can be made smaller.

Further, when the transparent solid is combined with the object to be inspected into a substantially cylindrical shape, the influence of environmental fluctuations should be reduced even in the measurement of a rotationally asymmetric lens. Therefore, a rotationally asymmetric lens such as a scanning lens of a laser printer can be measured with high accuracy.

When the container containing the reagent solution is made of an optical material having a refractive index almost the same as the refractive index of the object to be measured, the container and the translucent solid should be integrated to reduce the number of parts. Therefore, it is possible to reduce the amount of reagent solution in the region through which the test wave is transmitted, and thus it is possible to eliminate the influence of wavefront aberration and perform more accurate measurement.

When the translucent solid is mixed into the test solution in the form of powder, the other components are not changed,
The influence of environmental changes on the measurement can be reduced,
More accurate measurement can be performed.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a refractive index distribution measuring apparatus for an optical element according to the present invention.

FIG. 2 is an enlarged view of a cell portion.

FIG. 3 is a diagram illustrating the principle of the CT method.

FIG. 4 is a block diagram showing an embodiment of an arithmetic unit based on the CT method.

FIG. 5 is a diagram showing an example of a cell portion suitable when a test object is not a rotation target.

FIG. 6 shows an embodiment in which a cell, an optical flat and a matching glass material are integrated, (a) is a perspective view,
(B) is a side view.

FIG. 7 is a diagram showing an example of a cell portion when a powder matching glass material is used.

FIG. 8 is a configuration diagram showing another embodiment of the measuring apparatus of the refractive index distribution of the optical element according to the present invention.

FIG. 9 is an explanatory diagram showing an example of an interference fringe image.

FIG. 10 is a block diagram showing an embodiment of an apparatus for calculating a phase displacement amount of interference fringes.

11A and 11B are explanatory views showing waveform examples of output signals of the interference fringe detector, and FIGS. 11C and 11D are waveform diagrams of square waves.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 laser light source 11 beam splitter 15 interference fringe detector (first interference fringe detector) 17 arithmetic processing unit 21 cell 23 rotating object table 29 optical flat 31 optical flat 35 transmitted wavefront measurement unit 37 transmitted wavefront amount calculation unit 39 One-dimensional Fourier transform unit 41 Polar coordinate-Cartesian coordinate transform unit 43 Two-dimensional inverse Fourier transform unit 45 Refractive index converter 47 Refractive index distribution output unit 49 Beam splitter 53 λ / 4 plate 59 Second interference fringe detector 69 Pulse counting circuit 71 Phase Displacement Calculator 73 Translucent Solid 75 Second Matching Glass Material 77 Cell (Matching Glass Material) 83 Hole 85 Matching Glass Material A Specimen B Test Solution

Claims (6)

[Claims] 1. A test object consisting of an optical element to be measured and a translucent solid having a refractive index almost the same as that of the optical element are dipped in a test solution having a refractive index almost the same as that of the optical element, Coherent light is divided into a reference wave that serves as a reference and the test wave that passes through the optical element, and an interference fringe image is formed by superimposing the reference wave and the test wave, and the test object is immersed in the test solution. Measure the transmitted wavefront by analyzing the interference fringe image at each of at least two rotation angle positions by rotating it around an axis orthogonal to the optical axis of the test wave, and measure the transmitted wavefront from multiple directions. A method for measuring the refractive index distribution of an optical element, which comprises measuring the refractive index distribution of the optical element from 2. Coherent light from the same light source is divided into a reference wave serving as a reference and a test wave passing through a test object formed of an optical element to be measured, which is immersed in a test solution having a refractive index of approximately the same. A two-beam interferometer that forms an interference fringe image by superimposing a reference wave and a test wave, a translucent solid having a refractive index substantially the same as that of an optical element, which is replaced with a part of the reagent solution, and the interference fringe image An interference fringe image detector disposed on the image plane of the, a transmitted wavefront measuring means for measuring the transmitted wavefront by analyzing the interference fringe image detected by the interference fringe image detector, and the test sample in the test solution. And a rotating means for rotating the optical axis of the test wave around an axis orthogonal to the optical axis of the wave to be detected. 3. The apparatus for measuring the refractive index distribution of an optical element according to claim 2, wherein the light-transmissive solid has a cylindrical hollow portion that houses the optical element. 4. The refractive index of the optical element according to claim 3, wherein the translucent solid has a shape such that a shape combined with the object to be inspected becomes a substantially cylindrical shape. Distribution measuring device. 5. The apparatus for measuring the refractive index distribution of an optical element according to claim 2, wherein the container containing the reagent solution is made of an optical material having a refractive index substantially the same as the refractive index of the object to be measured. . 6. The apparatus for measuring the refractive index distribution of an optical element according to claim 2, wherein the translucent solid is made of a powdery optical material mixed in the sample solution.