JP3327998B2 - Shape measuring method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring an aspherical shape.

[0002]

2. Description of the Related Art As a means for measuring an aspherical shape, a method using an interferometer has been conventionally known. FIG. 5 shows a conventional aspherical shape measuring device disclosed in JP-A-63-138204. Laser beam a emitted from light source
Is split into two by a beam splitter b in the middle of the optical path, and one laser beam a is reflected by the measured surface e by changing the objective lens d and returns to the optical path again as object light. The other laser beam is reflected by a reference mirror c that has been accurately formed and becomes reference light. Then, the object light and the reference light are converted into beam splitters b.
Interfere with each other.

Next, in order to control the influence of diffraction, after passing these lights through an imaging optical system f, the image sensor g
To project interference fringes. The image sensor g is moved by the moving table h under the control of the sensor movement control unit i, and is placed at a position where an interference fringe image is to be detected. At this time, the intensity information of the interference fringe image is sent to the arithmetic processing unit. This intensity information is obtained by changing the position of the reference mirror c by the phase modulation element m driven by the control of the main control unit k. This is information in a state where the optical path length is changed (fringe scan method). Then, based on this information, the arithmetic processing unit j calculates the phase together with the position of the image sensor g, and calculates the shape of the measured surface e. Then, the calculation result is displayed on the display unit n.

[0004]

However, the above-mentioned prior art has the following problems, and has not been satisfactory as means for measuring an aspherical shape. That is, when the laser beam a is replaced with the beam splitter b and the objective lens d, the transmitted wavefront aberration (hereinafter referred to as internal aberration) by these optical systems is affected.
Adds as a measurement error. For this reason, there has been a problem that an accurate shape of the surface to be measured e cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and is intended to reduce the influence of internal aberrations caused by a beam splitter and an objective lens, and to improve the shape measurement accuracy of a surface to be measured. It is an object of the present invention to provide a shape measuring method and an apparatus capable of measuring the shape.

[0006]

In order to achieve the above object, the shape measuring method according to the present invention, when measuring the shape of the surface 4 to be measured, as shown in FIG. The light is irradiated on the measured surface and the reference surface by dividing the light into two by the irradiation unit 3. At this time, in order to measure the internal aberration of the measurement optical system and the reference surface 5 having the same design value as the surface 4 to be measured, the design value of the surface shape is similar to the surface 4 to be measured. Attach the prototype 10. At this time, the prototype 10
An arbitrary reference position with the optical axis as the rotation axis is determined for the reference surface 5 and the wavefront aberration W0 measured by the wavefront aberration measurement unit 7 at both reference positions. replace the wavefront measured by the reference position in both aberration W0 ', also by reference surface rotating mechanism 11 the reference surface 5 to the prototype 10 of the measurement surface side of the reference position, relative to the reference position Wavefront aberration W measured at 90 ° rotated position
S90, the reference plane 5 is moved 180 degrees with respect to the reference position.
The wavefront aberration WS180 measured at the rotated position ,
With the reference surface 5 as the reference position, the prototype 10 on the measured surface side is rotated 90 degrees with respect to the reference position by the measured surface rotating mechanism 6.
° wavefront aberration WR90 rotated to measured, also obtains the wavefront aberration WR180 was measured by rotating 180 ° the prototype 10 of the measurement surface side with respect to the reference position, the wavefront aberration W0 measured respectively, w0 ' , WS90, WS180, WR9
0, WR180 in the arithmetic unit 8, WSA = (W0 +
W0 ') / 2, WAS = (WS90 + WR90) / 2,
The calculation of WCM = (WS180 + WR180) / 2 is performed. At this time, the WSA is for the spherical aberration which is the wavefront aberration of the symmetric component, and utilizes the property that the internal aberration of the symmetric component is equivalent even if the prototype 10 and the reference surface 5 are exchanged with each other. Can be obtained by the above equation. Also, by using the property that the aberration of the ass component can be separated by rotating the WAS by 90 ° and the aberration of the coma component can be separated by rotating the WCM by 180 °, the internal aberration is calculated by the above-described equation. Can be.

Next, WSA, WAS, and WCM are respectively developed into Zernike, and a spherical aberration component is obtained from the expanded Zernike coefficient WSA, an assembler component is obtained from WAS, and WCM is obtained.
From the coma component, the wavefront aberration is created from the extracted Zernike coefficients to determine the internal aberration of the measuring optical system. Thereafter, when measuring the surface shape of the surface 4 to be measured, a calculation is performed by subtracting the internal aberration of the measurement optical system from the calculation unit 8 from the value obtained by measuring the surface shape of the surface 4 to be measured by the wavefront aberration measurement unit 7. Thus, an accurate surface shape of the measured surface 4 is obtained. Then, this value is displayed on the display unit 9.

Further, as shown in the conceptual diagram of FIG. 1, the shape measuring apparatus 1 of the present invention includes a light source unit 2 for irradiating light to a surface to be measured and a reference surface, and a light source 2 for irradiating light emitted from the light source unit 2. The wavefront aberration is measured from the irradiating unit 3 that irradiates the measured surface and the reference surface by dividing the light into the measurement surface side and the reference surface side, and interference fringes generated by reflected light reflected from the measured surface and the reference surface. Aberration measurement unit 7, the reference surface 5 having the same design value as the surface 4 to be measured, and the design value of the surface shape similar to the surface 4 to measure the internal aberration of the measuring optical system. Prototype 1
0, the measured surface rotating mechanism 6 and the reference surface rotating mechanism 11 for rotating the prototype 10 and the reference surface 5 on the measured surface side around the optical axis.
And the prototype 10 is placed on the surface to be measured.
The reference surface 5 and the original 10 on the side to be measured were replaced, and both were rotated by 90 ° and 180 ° with respect to the reference position and the original 10 on the side to be measured as the reference position. position,
Wavefront aberrations W0, W measured by the wavefront aberration measurement unit 7 at positions where the original 10 on the surface to be measured is rotated 90 ° and 180 ° with respect to the reference position with the reference surface 5 as the reference position.
0 ′, WS90, WS180, WR90, WR180, WSA = (W0 + W0 ′) / 2, WAS = (WS9
0 + WR90) / 2, WCM = (WS180 + WR18)
0) / 2, and Zernike expansion of WSA, WAS, and WCM, respectively.
The coma component is extracted from M, and the wavefront aberration is calculated from the extracted Zernike coefficients to determine the internal aberration of the measurement optical system. An arithmetic unit 8 for performing an arithmetic operation for subtracting the internal aberration obtained by the arithmetic operation from the measured value, and a display unit 9 for displaying the arithmetic result.

[0009]

Embodiment 1 FIG. 2 is a structural explanatory view showing Embodiment 1 of a shape measuring apparatus 1 according to the present invention. In the following description, components corresponding to the components shown in FIG.
The same reference numerals are given to facilitate understanding of the configuration.

In FIG. 2, reference numeral 2 denotes a light source unit for referencing light to the surface to be measured and the reference surface, and 3 denotes a surface to be measured by dividing light emitted from the light source unit 2 into the surface to be measured and the reference surface. And an irradiating unit for irradiating the reference surface, 4 is a measured surface, 5 is a reference surface having the same surface shape design value as the measured surface 4, and 10 is a measured surface 4 for measuring internal aberration of the measuring optical system. The reference numeral 6 designates a surface to be measured having the same graduation for rotating the prototype 10 on the surface to be measured at an angle of 90 ° and 180 ° about the optical axis. A rotating jig 11 is a reference surface rotating jig provided with an angle scale for rotating the reference surface accurately at an angle of 90 ° and 180 ° about the optical axis, and 7 is a reflection from the measured surface and the reference surface 5. Wavefront aberration measuring units 7 and 8 for measuring wavefront aberration from interference fringes generated by reflected light On the side place the prototype 10, any reference position both, the reference surface 5 Replace the prototype 10 of the measurement surface side, the reference positions in both, standard 10 of the measurement surface side
And the reference plane 5 is set at 9 to the reference position.
With the reference plane 5 being the reference position and the reference plane 5 being rotated by 0 ° and 180 °, the prototype 10 on the side to be measured is moved 9
The wavefront aberrations W0, W0 ', and WS9 measured by the wavefront aberration measurement unit 7 at positions rotated by 0 ° and 180 °, respectively.
0, WS180, WR90, WR180, WSA =
(W0 + W0 ') / 2, WAS = (WS90 + WR9
0) / 2, WCM = (WS180 + WR180) / 2, and Zernike expansion of WSA, WAS, and WCM, respectively, a spherical aberration component from the expanded Zernike coefficient WSA, an ass component from WAS, and a coma component from WCM By calculating the wavefront aberration from the extracted Zernike coefficients, the internal aberration of the measuring optical system is obtained. Thereafter, when measuring the surface shape of the surface to be measured, the measurement value of the surface to be measured 4 is used. A computer as a calculation unit for performing a calculation for subtracting the internal aberration obtained by the calculation,
Reference numeral 9 denotes a monitor as a display unit for displaying a calculation result.

The light source unit 2 includes a laser light source 12 and a beam expander 13 that spreads the laser light of the laser light source 12 in parallel and projects the laser light on the irradiation unit 3. Further, the irradiating unit 3 includes a beam splitter 14 that divides the laser light emitted from the light source unit 2 into two, an objective lens 15 that irradiates one of the divided light onto the surface to be measured, and the other light. Face 5
Is constituted by an objective lens 16 for reference. When the reflected light from the measured surface is the object light and the reflected light from the reference surface 5 is the reference light, the wavefront aberration measuring unit 7 returns the object light and the reference light to the objective lenses 15 and 16, and the beam splitter 14 The superimposed light is projected onto an image sensor 18 to drive an imaging lens 17 for suppressing diffraction, a phase modulator 19 using a piezo element for changing the position of the reference surface 5 in the optical axis direction, and a phase modulator 19. And a two-dimensional image sensor 18 for detecting the intensity of interference fringes generated by the object light and the reference light.

Next, a method for measuring the shape of an aspherical surface using the apparatus having the above-described configuration will be described. First, the prototype 10 is attached to the non-measurement surface rotating jig 6. And the laser light source 1
2 irradiates the beam splitter 14 with laser light. The laser light is spread in parallel by the beam splitter 13,
The beam is split into two by a beam splitter 14 in the middle of the optical path. One of the two laser beams is split by the objective lens 15 into the prototype 1
It is irradiated to 0. Then, the light is reflected by the prototype 10 and returns to the optical path again as object light. The other laser beam is applied to the reference surface 5 by the objective lens 16. Then, the light is reflected by the reference surface 5 and becomes reference light. This object light and reference light are
The beams are superposed and interfere with each other by the beam splitter 14, and are projected onto a two-dimensional image pickup device 18 via the imaging lens 17.

Here, the computer 8 and the controller 2
The phase is obtained by performing a fringe scan method using 0, the phase modulation element 19, the reference surface 5, and the imaging element 18, and the wavefront aberration is calculated. At this time, an arbitrary reference position is determined with respect to the prototype 10 and the reference surface 5 by the measured surface rotation jig 6 and the reference surface rotation jig 11, and measured at the reference position, and the wavefront aberration at this time is defined as W0. , The reference surface 5 is attached to the surface to be inspected, the prototype 10 is attached to the reference surface, and the wavefront aberration obtained by measuring both at the reference position is W0 ′.
Is returned to the reference surface side, and the prototype 10 is returned to the measured surface side.
Is the reference surface 5 with the measured surface rotating jig 6 as the reference position.
Is 9 with respect to the reference position by the reference plane rotating jig 11.
The wavefront aberration obtained by measuring at a position rotated by 0 ° is represented by WS9
0, and the wavefront aberration obtained by measuring the reference surface 5 at a position rotated by 180 ° with respect to the reference position by the reference surface rotating jig 11 is referred to as WS180. The wavefront aberration obtained by measuring the prototype 10 at the position rotated by 90 ° with respect to the reference position with the measured surface rotating jig 6 using the fixture 11 at the reference position and the WR90, It is assumed that the wavefront aberration WR180 is obtained by measuring at a position rotated by 180 ° with respect to the reference position by the measured surface rotating jig 6. Then, the computer 8 calculates the internal aberration of the measurement optical system using the measured value,
When measuring the wavefront aberration of the measured surface 4 and calculating the surface shape, the internal aberration is subtracted to obtain an accurate surface shape of the measured surface 4. This value is displayed on the monitor 9. Next, the arithmetic processing by the computer 8 at this time will be described.

The wavefront aberrations (W0, W0 ', WS90, WS180, WR90, W
R180), WSA = (W0 + W0 ') / 2, WA
S = WS90 + WR90) / 2, WCM = (WS180)
+ WR180) / 2 to perform WSA, WA
Zernike expansion is performed on each of S and WCM to calculate Zernike coefficients. And the Zernike coefficient WS
Coefficient of spherical aberration from A, coefficient of ass component from WAS, W
The coefficient of the coma component is extracted from the CM, the wavefront aberration is calculated from the extracted coefficient, and this is the internal aberration. Thereafter, when measuring the surface 4 to be measured, the surface aberration of the surface 4 to be measured is calculated by subtracting the internal aberration from the value obtained by measuring the surface shape by the wavefront aberration measuring unit 7 to obtain the accurate surface shape of the surface 4 to be measured. Ask. According to this embodiment, the accuracy can be improved by using the fringe scan method.

[0015]

Second Embodiment FIG. 3 shows a second embodiment of the shape measuring apparatus according to the present invention.
FIG. The features of the device 1 of this embodiment are as follows.
Beam expander 1 of light source unit 2 in the first embodiment
Optical system 21 for converting 3 into divergent light corresponding to surface 4 to be measured
Alternatively, the objective lens 1 may be used as the optical system 22 for converting convergent light.
5 and 16 are removed. Other configurations are described in Example 1.
Therefore, the same components are denoted by the same reference numerals and description thereof will be omitted.

Next, a method of measuring the shape of an aspheric surface using this embodiment will be described. First, the prototype 10 is attached to the surface rotation jig 6 to be measured. Then, laser light is emitted from the laser light source 12 to the beam splitter 14. The laser light becomes divergent light or convergent light via an optical system 21 for converting to divergent light or an optical system 22 for converting to convergent light corresponding to the surface 4 to be measured, and is split into two by a beam splitter 14 in the optical path. 2
One of the divided laser beams is applied to the prototype 10. Then, the light is reflected from the prototype 10 and returns to the optical path again as object light. The other laser beam is applied to the reference surface 5. Then, the light is reflected by the reference surface 5 and becomes reference light. The object light and the reference light are superimposed on each other by the beam splitter 14 and interfere with each other, and are projected onto a two-dimensional image sensor 18 via the imaging lens 17.

Here, the computer 8 and the controller 2
The phase is obtained by performing a fringe scan method using 0, the phase modulation element 19, the reference surface 5, and the imaging element 18, and the wavefront aberration is calculated. At this time, with respect to the prototype 10 and the reference surface 5, an arbitrary reference position is determined by the surface rotation jig 6 to be measured and the reference surface rotation jig 11, and measurement is performed at the reference position. The reference surface 5 is attached to the surface to be measured, the prototype 10 is attached to the reference surface side, the wavefront aberration obtained by measuring both at the reference position is W0 ', and the reference surface 5 is attached to the reference surface side. The instrument 10 is returned to the surface to be measured,
0 is the reference surface rotating jig 6 by the reference surface rotating jig 11 with respect to the reference position.
The wavefront aberration obtained by measuring at a position rotated by 0 ° is represented by WS9
0, and the wavefront aberration obtained by measuring the reference surface 5 at a position rotated by 180 ° with respect to the reference position by the reference surface rotating jig 11 is referred to as WS180. The wavefront aberration obtained by measuring the prototype 10 at the position rotated by 90 ° with respect to the reference position with the measured surface rotating jig 6 using the fixture 11 at the reference position and the WR90, It is assumed that the wavefront aberration WR180 is obtained by measuring at a position rotated by 180 ° with respect to the reference position by the measured surface rotating jig 6. Then, the measured values are subjected to arithmetic processing by the computer 8 in the same manner as in the first embodiment to calculate the internal aberration of the measuring optical system, to measure the wavefront aberration of the surface 4 to be measured, and to calculate the surface shape. The internal aberration is subtracted to obtain an accurate surface shape of the surface 4 to be measured. This value is displayed on the monitor 9.

According to the present embodiment, the objective lenses 15 and 16 are not required as compared with the first embodiment, and the internal aberration can be reduced.

[0019]

Third Embodiment FIG. 4 is a structural explanatory view showing a third embodiment of the shape measuring apparatus according to the present invention. The feature of the device 1 of the present embodiment is that the wavefront aberration measuring method of the wavefront aberration measuring unit 7 in the first embodiment is a fringe scan method, whereas the wavefront aberration measuring method of the wavefront aberration measuring unit 7 is optical heterodyne. It is the point that we made the law. Based on this method, the configuration of other components was changed as follows.

The light source unit 2 includes a laser light source 12 that emits light having slightly different frequencies for the respective polarization methods, and a beam splitter 30 for dividing the laser light of the laser light source 12 into two directions. Are arranged in the laser light source 12 and the beam expander 13. In addition, the irradiation unit 3
A polarization beam splitter 23 for dividing the laser light projected from the light source unit 2 into polarizations P and S, a λ / 4 wavelength plate 24 for changing the polarization of light reflected from the surface to be measured from S to P, and a reference surface Λ for changing the polarization of light reflected from the side from P to S
The 波長 wavelength plate 25 is disposed between the objective lenses 15 and 16 and the polarizing beam splitter 23. Further, when the reflected light from the measured surface side is the object light and the reflected light from the reference surface 5 is the reference light, the wavefront aberration measuring unit 7 polarizes the polarizing plate 28 for causing the object light and the reference light to interfere with each other. It is arranged between the beam splitter 23 and the image sensor 18. The other light split by the beam splitter 30 interferes with the polarizing plate 26 and becomes a reference signal by the photodetector 27. Image sensor 1
The signal detected in step 8 is used as a measurement signal, and a phase detector 29 for determining the phase of the reference signal and the phase of the measurement signal is arranged.
Other configurations are the same as those of the first embodiment, and thus the same components are denoted by the same reference numerals and description thereof will be omitted.

Next, a method of measuring an aspherical shape using this embodiment will be described. First, the prototype 10 is moved to the non-measurement surface rotating jig 6.
Attach to Then, laser light is emitted from the laser light source 12 to the polarization beam splitter 23. The laser light is split into two by a beam splitter 30, and one is spread in parallel by a beam expander 13, and split into two P and S polarization components by a polarization beam splitter 23 in the optical path. 2
One of the divided laser beams is applied to the prototype 10 by the objective lens 15. Then, it is reflected from the prototype 10,
It returns to the optical path again as object light. At this time, the light passes through the λ / 4 plate and becomes P-polarized light. The other laser beam is applied to the reference surface 5 by the objective lens 16. Then, the light is reflected by the reference surface 5 and becomes reference light. At this time, the light passes through the λ / 4 plate and becomes S-polarized light. The object light and the reference light are supplied to the polarization beam splitter 23.
And interfere with each other via the imaging lens 17 and the polarizing plate 28 to project the image on the two-dimensional image pickup device 18. At this time, the interference fringes cause a beat due to a slight difference in frequency accompanying the polarization. The beat signal of this interference fringe is detected by the image sensor 18 as a measurement signal.

The other light split by the beam splitter 30 passes through the polarizing plate 26, causes interference, and is projected on the photodetector 27. At this time, the interference fringes cause a beat due to a slight difference in frequency accompanying the polarization. The beat signal of this interference fringe is detected by the photodetector 27 as a reference signal.

Here, the phase of the measurement signal and the reference signal is obtained by the phase detector 29, the difference between the two phases is obtained by the computer 8, and the wavefront aberration is calculated. At this time, with respect to the prototype 10 and the reference surface 5, an arbitrary reference position is determined by the non-measuring surface rotation jig 6 and the reference surface rotation jig 11, and measurement is performed at the reference position. The wavefront aberration at this time is W0. The reference surface 5 is attached to the surface to be measured, the prototype 10 is attached to the reference surface, and the wavefront aberration obtained by measuring both at the reference position is W0 ′. Further, the reference surface 5 is attached to the reference surface. The prototype 10 was returned to the measurement surface side, and the reference surface 5 was rotated 90 ° with respect to the reference position by the reference surface rotation jig 11 using the measurement surface rotation jig 6 as the reference position. The wavefront aberration obtained by measuring the position is measured by WS90, and the reference surface 5 is moved 180 degrees from the reference position by the reference surface rotating jig 11.
The wavefront aberration obtained by measuring at the rotated position
Set to 0. Next, the reference surface 5 is set with the reference surface rotating jig 11 as the reference position, and the prototype 10 is rotated by the measured surface rotating jig 6.
The wavefront aberration obtained by measuring at a position rotated by 90 ° with respect to the reference position is WR90, and further, the prototype 10 is rotated by 180 ° with respect to the reference position by the surface rotating jig 6 to be measured. The wavefront aberration obtained by the measurement is defined as WR180. Then, the measured values are subjected to arithmetic processing by the computer 8 in the same manner as in the second embodiment to calculate the internal aberration of the measuring optical system, to measure the wavefront aberration of the surface 4 to be measured, and to calculate the surface shape. The internal aberration is subtracted to obtain an accurate surface shape of the surface 4 to be measured. This value is displayed on the monitor 9.

According to the present embodiment, as compared with the first embodiment, the measurement accuracy is further improved because it is not affected by the measurement error caused when the reference light path is changed by the fringe scan method.

[0025]

As described above, according to the present invention, it is possible to reduce the deterioration of the measurement accuracy due to the internal aberration of the measurement optical system, and to improve the measurement accuracy of the aspherical shape.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a basic configuration of the present invention.

FIG. 2 is a block diagram illustrating a basic configuration of the first embodiment.

FIG. 3 is a block diagram illustrating a basic configuration of a second embodiment.

FIG. 4 is a block diagram illustrating a basic configuration of a third embodiment.

FIG. 5 is a block diagram showing a basic configuration of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Shape measuring apparatus 2 Light source part 3 Irradiation part 4 Surface to be measured 5 Reference surface 6 Rotation mechanism 7 Wavefront aberration measurement part 8 Calculation part 9 Display part

Claims (2)

(57) [Claims] 1. A surface to be measured is irradiated with one light obtained by dividing light radiated from a light source into two parts, and the other light is irradiated on a reference surface, and reflected light from the measured surface and reflected light from the reference surface. In the shape measurement method for measuring the shape of the surface to be measured from the interference fringes obtained from the design surface of the surface to be measured and the design value of the surface shape is placed the same reference surface, the surface to be measured and the surface shape of the surface shape Waveform aberration measured at the reference position for both reference positions with the same reference values as the design values, determined an arbitrary reference position with the optical axis as the rotation axis.
W0, the reference plane and the original on the measured surface side are exchanged, the wavefront aberration W0 ' both measured at the reference position, and the original on the measured surface side as the reference position, and the reference surface is set at the reference position. , The measured wavefront aberration WS90, the reference surface rotated by 180 ° with respect to the reference position, the measured wavefront aberration WS180, and the original on the surface to be measured using the reference surface as the reference position. The wavefront aberration WR90 measured by rotating the measuring device 90 ° with respect to the reference position, and the measured wavefront aberration WR measured by rotating the prototype on the surface to be measured 180 ° with respect to the reference position.
180, and the obtained wavefront aberration is represented by WSA = (W
0 + W0 ') / 2, WAS = (WS90 + WR90) /
2. While calculating WCM = (WS180 + WR180) / 2, the WSA, WAS, and WCM are each subjected to Zernike expansion.
The coma component is extracted from M, and the wavefront aberration is calculated from the extracted Zernike coefficients to determine the internal aberration of the measuring optical system. Thereafter, when measuring the surface shape of the surface to be measured, the measurement of the surface to be measured is performed. A shape measuring method characterized in that the surface shape of the surface to be measured is measured by subtracting the internal aberration obtained by the calculation from the value. 2. A light radiated from a light source is divided into two parts, and one light is radiated to a surface to be measured and the other light is radiated to a reference surface to reflect light from the surface to be measured and reflection from the reference surface. In a shape measuring device that measures the shape of a surface to be measured from interference fringes obtained from light, a light source unit for irradiating the surface to be measured and the reference surface side, and light irradiated from the light source unit to the surface to be measured An irradiating unit that divides the light into the measured surface and the reference surface by dividing it into two parts on the reference surface side, and a wavefront aberration for measuring the wavefront aberration from interference fringes generated by reflected light reflected from the measured surface and the reference surface A measuring unit, a reference surface having the same design values as the surface to be measured and the surface shape, a prototype having the same design values as the surface to be measured for measuring the internal aberration of the measuring optical system, and a surface to be measured; A rotation mechanism for rotating the reference surface around the optical axis and a rotation mechanism for the reference surface, Place a prototype on the measurement surface side, replace both reference positions with the reference surface, the reference surface and the prototype on the measurement surface side, and refer to both reference positions and the reference surface on the measurement surface side as the reference position. 90 °, 180 ° with respect to the reference position
Wavefront aberrations W0 and W0 'measured by the wavefront aberration measurement unit at positions where the original on the surface to be measured is rotated 90 ° and 180 ° with respect to the reference position with the rotated position and the reference surface being the reference position. , WS90, WS180, WR9
0, WR180, WSA = (W0 + W0 ′) / 2,
WAS = (WS90 + WR90) / 2, WCM = (WS
180 + WR180) / 2 and WSA, WAS,
Each WCM is Zernike-developed, the spherical aberration component is extracted from the Zernike's expanded coefficient WSA, the ass component and the coma component from WAS are extracted, and the wavefront aberration is calculated from the extracted Zernike's coefficient. Calculating the internal aberration of the optical system, and then, when measuring the surface shape of the surface to be measured, from a measurement value of the surface to be measured, a calculation unit for performing a calculation of subtracting the calculated internal aberration from the calculation, A display unit for displaying a calculation result.