JPS63184029A - Method of measuring shape of wave front - Google Patents

Abstract

PURPOSE:To measure the shape of a wave front with high accuracy, on the basis of the subtraction data obtained by subtracting the reference data obtained by applying Fourier transform processing to the reference interference fringe data from a shearing interferometer from the data to be inspected due to an object to be measured. CONSTITUTION:The incidence of the fronts of the reflected waves due to both of a surface 8 to be inspected and a plane mirror 4 is performed by the opening and closing of shutters 3, 6 and the parallel beam passing through a half mirror 5 is incident to a shearing interferometer to be divided into two beams by a half mirror 9. One of the beams receives lateral shift by a parallel flat plate 13 having the angle phiof inclination to the beam so as to become preset shearing quantity and is reflected by a mirror 11 to reach a mirror 2. The other beam is reflected by a mirror 10 inclined by an angle theta to reach the mirror 12 and the shearing interference fringe corresponding to the angle theta is generated from two beams superposed each other on the mirror 12. This interference fringe is taken in a memory 16 by an area sensor 15 and subjected to Fourier transform processing to obtain reference data. Next, the reference data is subtracted from the data from the surface 18 to be inspected as correction data by an operation means 17 and the shape of a wave front can be measured with good accuracy.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for measuring the shape of a wavefront, and in particular, by measuring changes in the incident wavefront incident on the object to be measured, it is possible to measure the surface shape of the object to be measured and the medium. This invention relates to a wavefront shape measurement method suitable for determining uniformity and the like.

(Prior Art) Various types of interferometers have been proposed for measuring optical properties such as the surface shape of an object to be measured and the homogeneity of a medium. Among these, a shearing interferometer is an interferometer that can measure optical characteristics with relatively high accuracy.

The shearing interferometer divides the wavefront from the object to be measured into two parts.
The basic principle is to cause both wavefronts to interfere with each other by laterally shifting them in a direction perpendicular to the direction of travel.

FIG. 2 is a schematic diagram of the optical system of a conventional shearing interferometer. In the figure, L is a wavefront from the object to be measured. 21
．． 24 is a half mirror 122. 23 is a mirror. The wave front is first divided into two by a half mirror 21, then reflected by mirrors 22 and 23, and combined by a half mirror 24. Here, by tilting the transparent plane-parallel plate 25 (26) placed between the mirror 22 (23) and the half mirror 24 by an angle θ with respect to the direction of propagation of the wave surface, the wave surface can be adjusted to a thickness d of the plane-parallel plate. The wavefront is shifted laterally in a direction perpendicular to the direction of travel by an amount proportional to . And half mirror
At 24, interference fringes are formed by superimposing both wavefronts on each other.

Hereinafter, the amount of relative lateral shift of the wavefront from the object to be measured at this time will be referred to as the amount of shear. When the shear amount is sufficiently small relative to the diameter of the light beam, the phase of interference fringes obtained by mutual interference of two laterally shifted wavefronts is approximately proportional to the phase of the differential wavefront of the wavefront from the object to be measured.

For simplicity, if we consider it one-dimensionally, let S be the shear amount, W(x) be the wavefront from the surface to be measured, and T(x) be the shearing wavefront, then T(x) = W(x) −W(x − S) =
S (stomach (x) - W (x - S)) / S. That is, the phase of interference fringes obtained by a shearing interferometer can be approximated by multiplying the shear amount S by the differential of the wavefront from the object to be measured in the shear direction.

It is necessary to read the phase of the interference fringes at this time with high precision in order to measure the optical properties of the object to be measured, such as the surface shape, with high precision.

Conventionally, there is a Fourier transform method as a method for reading interference fringes at this time with high precision. Next, a case will be described in which, for example, the surface shape of the object to be measured is measured using the Fourier transform method.

The Fourier transform method generates a signal with a predetermined spatial frequency as a carrier in the interference fringes to be measured, performs a Fourier transform on the image data obtained from the light intensity distribution of this signal, and calculates the DC component and light intensity distribution on the spatial frequency coordinate axis. After removing noise components based on non-uniformity of the coordinates, the phase is determined by returning the coordinates to the real coordinate axes.

Figure 3 (A) shows interference fringes obtained by further tilting the interference fringes produced by shearing to generate carriers. Applying the plane of this interference fringe to the (x, y) coordinate system, we get
The light intensity distribution g (x, y) of the interference fringe is g (x, y) = a (x, y) ◆ b (x, y) cos
[2πfox+Φ (x, y) ]------(
]) It can be expressed as Here, FIG. 3(B) shows the light intensity distribution on one line with interference fringes, that is, for example, g (x,
h) (h is a constant). The light intensity distribution shown by equation (1) has a spatial frequency f. This figure shows the result of spatial phase modulation of a fine draft, which is a carrier signal of (x,y)l
p(2π1fox)+C”(x,y)e X p(-
2π1fox) --------(2). Here C(x,y)-1/2b(x,y)e x p[i
Φ(x, y) ] = (3), and C” (x
*y) indicates complex conjugation.

When the above equation (2) is subjected to one-dimensional Fourier transformation only in the X direction, G (f, y) - eight (f, y) + C (f - fo, y) + C
"(f+fo, y) - (4). Here, the capital letters G, A, C, C" represent the spatial frequency spectrum. Usually a (x.

y), b (x, y), and Φ(x, y) are carrier frequency f. Because it is smaller, the carrier frequency f on the frequency axis f is reduced by fast Fourier transform as shown in Fig.
. A frequency-modulated component C (f-fo, y) corresponding to the surface shape of the surface to be inspected is centered at .

It is separated into a low frequency component corresponding to C (f, y) and A (f, y) in equation (4).

C(f-fo, y) and C-response f"fo, y) are complex conjugates of each other, and only one of them needs to be used in the processing using the Fourier transform method below. Therefore, from the frequency spectrum, c(t -to, y) and obtains the cross-sectional shape of the surface to be inspected.

In Fig. 3(D), C(f
-fo, y) is extracted and further shifted by fo to the origin on the frequency axis. As a result, C(f,y) is obtained from the modulated component C(f-fo, 3/) of the carrier f in accordance with the shape of the surface to be inspected. C(f, y) can be thought of as containing only the differential information of the wavefront reflected from the test surface and excluding the carrier, and when it is inversely Fourier transformed, the wavefront from the test surface, That is, the wavefront expressed by equation (3) is found. (3
Φ(x, y) in the equation ) is obtained when the wavefronts from the surface to be inspected are laterally shifted and overlapped in an optical system as shown in FIG. It shows the phase of the intensity distribution corresponding to the differential wavefront of the wavefront from the test surface. To find this phase Φ, we take the complex logarithm of the data corresponding to equation (3) obtained by inverse Fourier transform.
y)] + i Φ (x, y)・・・・−
(5) Then, Φ(x, y) is found in the imaginary part. At this time, the imaginary part is calculated on the computer as Φ(x, y
) is given in the main region from -π to π, so it is found as discontinuous data, but by correcting the phase jump, Φ(x, y) is found as shown in FIG. 3(F).

Next, in order to obtain the wavefront from the test surface, Φ(x, y) is integrated according to equation (6) W(x, y)-1/S SΦ(x, y) dx
...-(6) Here, S is the shear amount.

As a result, the shape of the wavefront from the test surface as shown in FIG. 3(G) is obtained. This represents the amount of deviation from the ideal reference sphere.

Data processing using the Fourier transform method as described above has the following problems.

Interference fringes produced by a shearing interferometer are obtained by modulating the spatial frequency or phase by carrying information from the object to be measured on a carrier having a fixed spatial frequency.

Here, if the effective number of carriers including the phase in the sampling region is not an integer, Fourier transform will result in an integer number (or 20) of interference fringes on the frequency spectrum shown in Figure 3 (C). Because the calculation is performed as follows, the carrier frequency cannot be determined correctly.

For this reason, it is no longer possible to accurately remove the carrier frequency component according to Fig. 3 (C) to (D), and as a result, the processing from Fig. 3 (D) onward is performed with some carrier components remaining.
In the phase data shown in Figure 3 (F), the measurement error increases linearly on the X coordinate, and in the final surface shape, the measurement error increases quadratically from the origin of integration toward the periphery. It's coming.

In addition, the conventional surface shape measurement using a shearing interferometer does not have a means for removing the influence of surface shape measurement errors caused by inherent aberrations of the optical system.

In addition, in conventional surface shape measurement using the Fourier transform method, if the carrier spatial frequency of the interference fringes and the spatial frequency of the noise pattern are significantly different from each other for a spatially stationary noise pattern, it is difficult to detect noise on the frequency spectrum. It is possible to separate the components of interference fringes and suppress the influence of measurement errors caused by noise, but when the carrier spatial frequency and the spatial frequency of the noise pattern are close, it is difficult to suppress measurement errors caused by noise, etc. There was a problem.

(Problems to be Solved by the Invention) The present invention uses a Fourier transform method to process intensity distribution data of interference fringes and measure optical properties such as the surface shape of a measured object, when the effective number of carriers is an integer. The purpose of the present invention is to provide a wavefront shape measuring method that can perform highly accurate measurements even when the wavefront shape is not the same.

A further object of the present invention is to be able to eliminate measurement errors due to inherent aberrations of the optical system, and to ensure that the noise is spatially stationary even when the spatial frequency of the carrier and the spatial frequency of the noise are close to each other. An object of the present invention is to provide a highly accurate wavefront shape measurement method that can reduce measurement errors caused by noise in such cases.

(Means for solving the problem) Obtain test interference fringe data by making the reflected or transmitted wavefront from the object to be measured enter a shearing interferometer set in advance to have a predetermined amount of shear. The test interference fringe data is subjected to Fourier transformation processing to obtain test data, and then or first a plane wave for reference is made incident on the shearing interferometer to obtain reference interference fringe data, and the reference interference fringe data is subjected to Fourier transformation. Reference data is obtained through conversion processing, subtraction data is obtained by subtracting the reference data from the test data, and optical characteristics such as the surface shape of the object to be measured are determined using the subtraction data.

Other features of the invention are described in the examples.

(Example) FIG. 1 is a schematic diagram of an optical system according to an example when the present invention is applied to measuring the surface shape of an optical component.

In the figure, 1 is a coherent light source such as a laser, and 2 is a collimator lens, which magnifies the light beam from the light source 1 and converts it into a parallel light beam. A half mirror 5 divides the wavefront from the collimator lens 2 into two. 8 is the test surface, 7
4 is a so-called null lens (hereinafter referred to as "TSS lens") whose transmitted wavefront aberration is well corrected, and forms a spherical wave to the test surface 8. 4 is a plane mirror, and the reflection from the half mirror 5 The wavefront is returned to its original optical path.

3 and 6 are shutters that control the passing wavefront. For example, when the wavefront from the test surface 8 is made to enter a shearing interferometer, which will be described later, the shutter 6 is opened and the shutter 3 is closed.

9.12 is a half mirror; 110 and 11 are plane mirrors; each of these elements constitutes a part of a shearing interferometer. Reference numeral 13 denotes a parallel plane plate which laterally shifts the wavefront by tilting it with respect to the traveling direction of the wavefront in one optical path of the wavefront divided into two by the half mirror 9, thereby causing so-called shearing.

Then, the horizontally shifted wavefront and the other wavefront are combined into a half mirror 12.
They overlap to form interference fringes.

Also, in this embodiment, mirrors 10, 11 or half mirrors are used.
By tilting either one of 9 and 12 at a predetermined angle with respect to the wavefront, carrier interference fringes of a predetermined spatial frequency are formed.

15 is an area sensor such as a TV camera, and 14 is an imaging lens that forms an image of interference fringes on the surface of the area sensor 15. Reference numeral 16 denotes a frame memory which takes in the video signal from the area sensor 15. 17 is a calculation means that processes the video signal taken into the frame memory 16.

FIG. 4 is an explanatory diagram of shearing interference fringes obtained in the embodiment of FIG. 1. The figure shows interference fringes that result from aligning the wavefronts 41 and 42 after horizontally shifting them by S.

Here, if the original wavefront is W(r), (r is an arbitrary position in space), the phase of the interference fringe is ΔΦ, and the amount of lateral shift is S, it can be expressed as W(r) = l/SiΔΦdr. .

Next, a procedure for measuring the surface shape of the surface to be inspected 8 in this embodiment will be explained.

In this embodiment, either the reflected wavefront from the test surface 8 or the reflected wavefront from the plane mirror 4 may be first made incident on the shearing interferometer via the half mirror 5, so here A case will be described in which the reflected wavefront from the detection surface 8 is first made incident on the shearing interferometer.

First, shutter 3 is closed and shutter 6 is left open.

The light beam from the light source 1 is expanded by the collimator lens 2, becomes a parallel light beam, is reflected by the half mirror 5, is totally reflected by the mirror 4, passes through the half mirror 5 again, and enters the shearing interferometer. Here, the parallel light beam is split into two light beams by the half mirror 9, one parallel light beam is reflected by the mirror 10 and reaches the half mirror 12, and the other parallel light beam is laterally shifted by the parallel plane plate 13, and then reflected by the mirror 11. Half mirror 12
reach.

The parallel plane plate 13 has an inclination angle φ with respect to the luminous flux determined in advance so as to have a set shear amount (which may be a finite amount or approximately O). Thereafter, this inclination angle φ, that is, the shear amount is fixed. Also, the mirror 10 or the mirror 11,
Alternatively, either the half mirror 9 or the half mirror 12 is tilted by a predetermined angle. For example, the mirror 10 is tilted by an angle θ0. When θ=0° and φ=0°, the shearing interferometer is adjusted in advance so that the two beams passing through the shearing interferometer completely overlap as wavefronts traveling in the same direction at the half mirror 12.

By tilting the mirror 10 at an angle θ, the half mirror 1
Interference fringes with a constant spatial frequency corresponding to the angle θ are generated from the two overlapping beams at the angle θ.

Figure 5 shows shearing interference fringes of plane waves caused by parallel light beams (parallel light beams that are laterally shifted from each other by a predetermined shear amount at θ=θ° can obtain a uniform intensity distribution even if they overlap at the half mirror 12. ) The wavefront from the surface to be measured 8, especially when the surface to be measured is aspherical, shearing interference fringes in which the spatial frequency of the carrier is modulated can be obtained using a shearing interferometer. In this embodiment, the shearing interference fringes caused by the plane waves are once taken into the frame memory 16 as an image by the area sensor 15 of the TV camera, and Fourier transform processing is performed as interference fringe data of a predetermined shear amount according to the processing procedure shown in FIG. 3. . The Fourier transform process uses fast Fourier transform (FFT), and the resulting frequency spectrum is data of N different frequencies.

Here, the number of sampling points of the interference fringe is 2N. Therefore, there exists a case where the spatial frequency of the carrier is not effectively equal to any of the N divided frequencies, for example, a case where there are effectively X (non-integer) carrier interference fringes.

Even in this case, it can be seen that the carrier frequency measurement error can be reduced by increasing the number of sampling points. Figure 6 (A) shows a case where the effective number of carrier interference fringes is an integer and a case where the effective number of carrier interference fringes is an integer.
B) shows the case where it is not an integer.

In the case shown in FIG. 6(B), it is possible to restore the state shown in FIG. 6(A) by adjusting the angle θ, but the degree of adjustment is delicate and difficult. In such a case, even if the plane wave is input to the shearing interferometer and the processing shown in Figure 3 is performed, the desired plane shape cannot be obtained, and the wavefront obtained by linear integration using the phase of the shearing wavefront according to equation (8) with a second-order measurement error.

In this example, inaccurate phase data or surface shape data obtained by inputting a plane wave are used as reference data as correction data in the Fourier transform processing of shearing interference fringes obtained by inputting the wavefront from the test surface. The basic principle of the measurement algorithm is to subtract from the data of the test surface at the stage of Figure 3 (F) or G (H). The flowchart in FIG. 7 shows the sequence from the acquisition of carrier interference fringe data to the determination of the surface shape of the surface to be inspected.

In FIG. 7, the inclination angle θ of the mirror 12 for carrier generation is 1. OX 10-' radian, shear amount is 48 μm, maximum aspheric amount is 42 μm, diameter is 20Φ reference radius 5
The surface shape of a 0+m aspherical lens was measured.

FIG. 7(A) shows the one-dimensional intensity distribution of carrier interference fringes obtained according to the procedure described above and captured in the frame memory. FIG. 7(B) shows the intensity distribution obtained by applying a window function to the intensity distribution obtained in FIG. 7(A). A panning window is used here, but other known windows include the panning window, Akaike window, Bartlett window, and Kaiser window. The purpose of using the window function is to suppress errors in the frequency spectrum due to Fourier transformation, which is caused by truncating the carrier interference fringe data shown in FIG. 7(A) as data having a finite width. FIG. 7(C) shows a frequency spectrum obtained by performing fast Fourier transform on the intensity distribution in FIG. 7(B). The fast Fourier transform has a sampling point of 512 points and the product name is VA.
As a result, the processing time was 420 m5ec on X-780 (manufactured by DEC Corporation). To further speed up the processing time, use the array processor product name FPS-164/MAX.
(manufactured by FPS) etc. may be used. In this case, the processing time is about 2.9 m5ec.

The spatial frequency fc of the carrier is determined from the frequency spectrum determined in Figure 7 (C).In the case of the interference fringe intensity distribution shown in Figure 6(B), the spatial frequency fc of the carrier may not be determined correctly or later. Continuing from FIG. 7(C) used when processing the shearing interference fringe data of the wavefront from the test surface, FIG. 7(D) shows the spectrum corresponding to the carrier frequency of FIG. 7(C) with a predetermined width. This shows the result of picking up only Δf and shifting it by fc on the frequency axis toward the origin.Hereinafter, this kind of processing is called carrier reduction.The spectral data with a certain width on the frequency axis that is subjected to carrier reduction is the surface to be obtained. It contains differential phase information of the shape, and when the carrier-reduced spectrum is further subjected to inverse Fourier transform, the phase of the shearing wavefront shown in FIG. 7(E) can be found.

Normally, if the frequency spectrum is found correctly and the carrier spatial frequency is found correctly, the phase data of the shearing wavefront obtained by processing the carrier interference fringe data using the procedure described above should be found as data with completely zero phase everywhere. . However, this is a special case, and the shearing phase of a plane wave obtained by manually applying a plane wave with zero wavefront aberration to a shearing interferometer and Fourier transform processing is obtained as data with a finite error that is not zero.

The data containing errors thus determined is stored in a memory as correction data for later Fourier transform processing of shearing interference fringe data of the wavefront from the test surface. Here, the origin position of the carrier shearing phase data is the 8th
Wavefronts 81.8 laterally shifted relative to each other as shown in the figure
2 is the point that bisects the line segment connecting each of the optical axes of 2,
So that the origin of the shearing phase data obtained by inverse Fourier transformation after carrier reduction is the above bisecting point,
It is necessary to correct the zero point position of the phase data. Therefore, the phase is set to 0 at the origin of the shearing phase data, and the value integrated according to equation (8) is divided by the shear amount S to find the wavefront to be found, that is, the phase of the wavefront of the parallel light beam. The phase of the wavefront of the parallel light beam determined according to equation (8) as described above may be used as correction data when the shearing interference fringes of the wavefront from the test surface are processed later. Here, since the shearing interferometer manual wavefront is obtained by reflection from the test surface or mirror surface, the surface shape can be determined by dividing the phase W (r ) found by equation (8) by 2π radians, and dividing the light source wavelength λ (μ
It can be determined by multiplying by 1/2 of m).

Next, in the optical system shown in FIG. 1, the shutter 3 is closed and the shutter 6 on the side to be inspected is opened. Here TS lens 7
The distance between the TS lens 7 and the test surface 8 is adjusted in advance so that when the wavefront passing through the TS lens 7 reaches the test surface 8, a spherical wave is generated that becomes the reference spherical surface of the test surface. shall be taken as a thing. The reflected wavefront from the test surface passes through the TS lens again and reaches the half mirror 5. If the surface to be inspected is a spherical surface, the wavefront that has passed through the TS lens 7 again is a plane wave.

Furthermore, if the surface to be tested is aspherical, it has wavefront aberration corresponding to the amount of aspherical surface. Figure 7 (F) shows one step of the shearing interference pattern obtained by manually inputting a wavefront with a wavefront aberration corresponding to the amount of asphericity of the surface under test into a shearing interferometer and modulating the carrier interference pattern. shows the intensity distribution of Figure 7 (
In the same way as B), apply a window function to Fig. 7 (F) to obtain Fig. 7 (G
). Furthermore, the distribution in FIG. 7(G) is subjected to fast Fourier transform and is shown in FIG. 7(H). Obtain the frequency spectrum of shearing interference fringes of the wavefront from the test surface. FIG. 7(I) shows the result of carrier reduction performed using the previously determined carrier frequency fC. Here, the carrier is a 32-tree frame, and the shearing interference pattern of the wavefront from the test surface is approximately 34
It appears as stripes on the book. The pick-up width of the spectrum centered around the carrier frequency fc needs to be appropriately set according to the amount of shear. Since the number of sampling points is 512, the frequency spectrum is obtained as 256 consecutive losing data, but when the carrier frequency is set to 32, the spectrum pickup width Δf is actually set to a lower limit frequency of 4 and an upper limit frequency of 60.

After carrier reduction, inverse Fourier transform is performed to determine the phase of the shearing wavefront (differential wavefront) of the wavefront from the test surface as shown in FIG. 7(T).

Here, since carrier reduction generally includes errors as described above, the phase data obtained in FIG. 7(J) has errors due to carrier reduction. Therefore, the phase data of the shearing wavefront obtained from the Fourier transform processing of the carrier interference fringes obtained previously using the plane wave manually is subtracted from the phase data of the shearing wavefront of the wavefront from the test surface obtained in Fig. 7 (J). By doing so, the phase measurement error of the shearing wavefront is corrected. However, what is mainly corrected is the error associated with carrier reduction, which is an error in FFT processing due to the limitation of the number of sampling points, but other causes of surface shape measurement error include shear amount measurement error, shear amount origin detection Examples include errors, noise in spatially stationary carrier frequencies, quantization errors, errors associated with numerical integration according to equation (8), and in particular, samurai error (8) due to treating the shearing wavefront as a differential wavefront. Figure 7 (K) is based on the phase of the shearing wavefront obtained by correction processing (8)
The deviatio from the spherical surface of the test surface obtained according to the formula
n, that is, the measurement result of the aspherical amount (-cross section).

Figure 7 (L) is the same figure (L) obtained from the known surface shape of the surface to be tested.
) shows the error in the measurement results. It can be seen that the surface can be measured with an accuracy of 0.1 μm or less in this way. Examples have been described above based on the optical system shown in FIG. 1 with reference to FIGS. 7(A) to 7(L).

In the embodiment shown in FIG. 1, the parallel plane plate 13 may not be used, and the two wavefronts may be shifted laterally relative to each other by simply moving the mirror 11 or 10 in parallel using, for example, a piezo element.

In this example, in order to obtain the three-dimensional surface shape of the surface to be tested, the shear direction must be determined not only in the in-plane direction of the paper of FIG. 1, for example, in the Y-axis direction, but also in the Z-axis perpendicular to Fourier transform processing of the shearing interference fringes obtained by directional shearing is also required. In Fig. 1, the wavefront in the X-Y plane is laterally shifted by the parallel plate 13 to obtain the surface shape in the shear direction (aspherical shape), and then the inclination angle φ is set to zero and the wavefront in the X-Y plane is shifted laterally. Return the lateral shift to zero, and then tilt the parallel plate with the Y axis as the rotation axis to obtain the lateral shift in the Z-axis direction. Further, the tilt angle θ of the mirror 10 is returned to 0, and carrier interference fringes approximately parallel to the X axis are similarly generated. As a result, shearing interference fringes due to shearing in the Z-axis direction are generated. This interference pattern is shown in FIG.

In the optical system shown in Fig. 1, the shear direction can only be obtained within the X-Y plane, so in order to obtain the shear in the Z-axis direction, the 10th
As shown in the figure, a shearing interferometer 100 for shearing in the Z-axis direction is provided, in which the optical path is divided by a half mirror 19 and one of the divided beams passes through.

Further, a shearing interferometer for obtaining shear in the X-Y plane through which the other light beam passes is indicated by 101. In each shearing interferometer, an imaging lens +4-A, -B, T-camera or area sensor 15-A, 15-8 is set as shown in FIG. As described above, in the optical system shown in FIG. 10, information regarding the three-dimensional surface shape of the test surface is simultaneously captured into separate frame memories by two shearing interferometers 100 and 101, and the surface shape is measured by Fourier transformation processing. Is possible.

In addition to the Matsuhatsu-Zender type interferometer to which the shearing interference method is applied in the present invention, various other interferometers such as the Twyman-Green interferometer and the Fizeau interferometer can be used.

In this embodiment, the reflected wavefront from the test surface 8 is first made incident on the shearing interferometer to obtain test data, and then the reflected wavefront from the plane mirror 4 is made incident on the shearing interferometer to obtain reference data. Although the case where the reference data is subtracted from the test data is shown, the purpose of the present invention can also be achieved in the same way even if the order is reversed so that the reference data is obtained first and then the test data is obtained. can.

(Effects of the Invention) According to the present invention, the number of sampling points is limited when measuring the surface shape of an object using the Fourier transform processing method (usually due to the limited number of pixels of a TV camera or area sensor). carriers caused by measurement errors in the frequency spectrum of shearing interference fringes (approximately 512 to 1024), especially spectrum measurement errors when interference fringes as shown in Figure 6 (B) are taken in as data with a finite width. Frequency errors can be corrected, making highly accurate measurements possible. For example, the surface shape of a lens with an aspherical amount of about 40 μm and an aperture of 20φ can be measured with an accuracy of 0.1 μm or less.

Furthermore, the surface shape of a lens with an aspherical amount of about 200 μm and an F number of 1.4 or more can be measured with an accuracy of about 0.2 μm.

Further, according to the present invention, it is possible to reduce phase measurement errors caused by spatially standing noise patterns caused by dust, scratches, etc. generated in the measurement optical system. In other words, by using data including noise patterns on the shearing interference fringes obtained by manually applying a plane wave to a shearing interferometer as correction data, shape measurement errors caused by relatively low frequency noise that is spatially stationary can be corrected. can do. This means that shutter 3.6 is turned on when a plane wave is input to the shearing interferometer, and when the wave surface is manually applied from the surface to be measured.
It is a prerequisite that the setting of the optical system is not adjusted other than to turn it off, and such measurements are actually performed in the optical system according to the present invention.

Furthermore, according to the present invention, measurement errors due to aberrations inherent in the optical system can be reduced by using correction data manually generated by plane waves. Especially the distortion of the imaging lens installed in front of the TV camera or area sensor.
It is possible to satisfactorily correct phase measurement errors of interference fringes caused by

In addition, it is also possible to eliminate the influence on the measurement accuracy of the surface roughness of the mirror surface constituting the interferometer.

Further, in the present invention, even if the origin position of the shear amount is not detected with high precision, by reading the shear amount from near the zero point of the shear amount with high precision, highly accurate measurement is possible. In other words, of the conventionally required high-precision detection of the shear origin position and high-precision measurement of the shear amount in surface shape measurement using a shearing interferometer, highly accurate detection of the shear origin position is no longer required, and the shear amount can be measured with high precision. It is possible to measure surface shapes with high precision using only high-precision measurements.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an optical system according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of an optical system of a conventional Matsuhatsu Ender interferometer, and FIG.
Figures (A) to (G) are explanatory diagrams of the process of obtaining a surface shape from conventional shearing interference fringes, and Figures 4 and 5 are explanatory diagrams of forming interference fringes by shearing two wavefronts. ,
Figures 6 (A) and (B) are explanatory diagrams of the intensity distribution of one cross section of interference fringes imaged by the area sensor, and Figures 7 (A) to (L
) is an explanatory diagram of the process to obtain the surface shape according to the present invention, Fig. 8 is an explanatory diagram showing the phase origin when two wavefronts are sheared, and Fig. 9 is an explanatory diagram showing the phase origin obtained when shearing two wavefronts in the Z-axis direction. FIG. 10 is a schematic diagram of an optical system according to another embodiment of the present invention. In the figure, 1 is the light source, 2 is the collimator lens, 5.9°12
．． 19 is a half mirror - 14, 10, 11 are mirrors, 3.
6 is a shutter, 8 is a test surface, 13 is a parallel plane plate, 7 is a TS lens, 14 is an imaging lens, 15 is an area sensor,
16 is a frame memory, and 17 is a calculation means. Patent Applicant: Canon Co., Ltd. No. 1 → J4 Patient 3 Diagram (B) Little man 3 Ta^ Akira 4 Kaiaki 6 Kun (A') (B) Child 7 Kuchi (A') Yi 7 [W (B) also 7 Mouth (C) Death 7 Mouth (F) Yi 7 times (G) 11ri bo') I sumo 7 times (H) See 7 mouth (1) Kazuki u-5ufu IU Yi 7 Figure (J) Mo 7th (K) Figure 7 (lj Ii 10th procedural amendment (acid) July 9, 1988 Patent Application No. 16485 2, title of invention Wavefront shape measurement method 3, person making amendment case Relationship with Patent applicant address 3-30-2 Shimomaruko, Ota-ku, Tokyo Name (100
) Canon Co., Ltd. Representative Ryuzo Kaku
Part 4, Agent's residence: 2-17-3 (1) Okusawa, Setagaya-ku, Tokyo 158
Column 5 of detailed explanation of the invention in the specification, contents of amendment (1) Change “close” to “open” in page 13, line 14 of the specification
and correct "open" to "closed".

Claims (1)

[Claims] The reflected or transmitted wavefront from the object to be measured is made incident on a shearing interferometer set in advance to have a predetermined shear amount to obtain test interference fringe data, and the test interference fringe data is subjected to Fourier transform processing. Obtain test data, then or first make a plane wave for reference enter the shearing interferometer to obtain reference interference fringe data, perform Fourier transform processing on the reference interference fringe data to obtain reference data, and A method for measuring a wavefront shape, characterized in that the reference data is subtracted from the test data to obtain subtraction data, and the subtraction data is used to determine optical characteristics such as the surface shape of the object to be measured.