JPH10253331A - Method and device for measuring surface shape - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a surface shape measuring device wherein a radius of curvature and surface power is measured with precision and a surface accuracy is accurately measured even when surface shape is complex. SOLUTION: A measurement optical system 10 which uses an interferometer, an opening member 20 to which an inspection surface La which is to be measured contacts, a movement means 101 which changes relative interval, along optical axis direction, between the first lens group 16 and the second lens group 17 in the measurement optical system 10, a movement amount measuring means 102 which measures a relative movement amount due to the movement means 101, an image input means 103 which takes in such interference fringe generated by interference in the measurement optical system as a picture information, and a calculation means 110 which calculates a shape of the inspection surface La based on the outputs of the movement amount measuring means 102 and the image input means 103, are provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface shape measuring device for optically measuring the surface power and distribution of optical elements such as lenses and mirrors, or the surface performance such as surface astigmatism.

[0002]

2. Description of the Related Art As a conventional surface shape measuring apparatus, for example, an apparatus disclosed in JP-A-63-47607 is known. In the apparatus disclosed in this publication, as shown in FIG. 9, a laser beam transmitted through a half mirror 1 is once condensed by a condenser lens 2, and this light beam is made incident on a test surface 5 by an objective lens 3. The reflected light from the surface to be measured passes through the objective lens 3 and the condenser lens 2, is reflected by the half mirror 1, and reaches the screen 6. In the drawing, reference numeral 7 denotes a member for determining the aperture diameter, 8 denotes a polarizing plate, and 9 denotes a λ / 4 plate.

By changing the distance between the condenser lens 2 and the objective lens 3 while keeping the object-side focal point F21 of the objective lens 3 coincident with the surface 5 to be inspected, the luminous flux is automatically adjusted with respect to the surface to be inspected. Collimation is performed, and the radius of curvature rt of the surface to be measured is obtained from the distance Δx between the image forming position P0 of the condenser lens 2 and the image-side focal point F22 of the objective lens in the auto-collimation state. Further, the accuracy of the surface can be confirmed from the image formed on the screen 6. The object-side focus is
The focal point and the image-side focal point of each lens located on the test surface side refer to the focal point of each lens located on the light source side or the observation surface side, respectively.

[0004]

However, in the above-described conventional surface shape measuring apparatus, since the surface accuracy is measured by visually observing an image on a screen, the surface accuracy is objectively evaluated numerically. In particular, there is a problem that an error is likely to occur when measuring a complicated surface shape such as a progressive multifocal lens.

[0005] The present invention has been made in view of the above-mentioned problems of the prior art, and can objectively evaluate surface accuracy and accurately measure surface accuracy even when the surface shape is complicated. It is an object of the present invention to provide a surface shape measuring device which can perform the measurement.

[0006]

In order to achieve the above object, a surface shape measuring apparatus according to the present invention has a first lens group for temporarily condensing a light beam emitted from a coherent light source, and a light beam from the first lens group. A second lens group that causes a light beam to enter the surface to be inspected, and a measurement that causes reflected light from the surface to interfere with light reflected by the reference surface without being incident on the first and second lens groups. An optical system; moving means for changing a relative distance between the first lens group and the second lens group along the optical axis direction; moving amount measuring means for measuring a relative moving amount by the moving means; Image input means for capturing the interference fringes generated by the interference as image information, and arithmetic means for calculating the shape of the surface to be inspected based on the outputs of the moving amount measuring means and the image input means.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a surface shape measuring apparatus according to the present invention will be described below. FIG. 1 is an explanatory view conceptually showing an embodiment of a surface shape measuring apparatus according to the present invention.

The apparatus according to the embodiment includes a measurement optical system 10 using a Fizeau interferometer, an opening member 20 to which an object L to be measured is applied, and a first optical system 10 in the measurement optical system 10.
Moving means 101 for changing the relative distance between the lens group 16 and the second lens group 17 along the optical axis direction;
Moving amount measuring means 102 for measuring the relative moving amount by
An image input unit 103 for capturing, as image information, interference fringes generated by interference in the measurement optical system; and a calculation for calculating the shape of the surface La to be detected based on the outputs of the movement amount measurement unit 102 and the image input unit 103 Means 110.

The measuring optical system 10 firstly collects a parallel light beam emitted from a coherent light source 11 such as a He-Ne laser, and makes the light beam from the first lens group incident on the surface to be measured La. A second lens group 17 that causes the reflected light from the test surface La to interfere with the reflected light reflected by the reference plane plate 15 that is the reference surface without entering the first and second lens groups; CCD camera 3 via lens 19
An interference fringe is formed on the 0 imaging plane 30a.

The opening member 20 is a cylindrical member having an opening at the center along the optical axis, and the second lens group is set so that the surface to be inspected La coincides with the object-side focal position of the second lens group 17. Is always provided to be constant.

The output of the CCD camera 30 is input to image input means 103 which is an image processing device, and the output is input to arithmetic means 110 which is a computer. In this example, the calculating means 110 includes a surface accuracy calculating means 111 for calculating the surface accuracy, astigmatism, etc. of the test surface based on the output of the image input means 103, and a movement amount measured by the movement amount measuring means 102 A curvature radius calculator 113 for calculating the curvature or radius of curvature of the surface La to be detected based on the focal length of the second lens group 17 and the calculated curvature or radius of curvature in accordance with the output of the surface accuracy calculator 111 Correction means 1 for correction
12 are provided. If the surface to be measured is not a perfect spherical surface, the curvature or radius of curvature here means an average value of the surface to be measured in the measurement area.

When the measured position does not exactly match the position where auto-collimation is performed, the surface accuracy calculating means 111 detects this error as defocus based on the output of the image input means 103, and the correcting means 112 And a function of correcting the curvature or the radius of curvature calculated by the radius of curvature calculation means based on the obtained defocus.

The display means 120 displays various data obtained by the arithmetic means as numerical information or in a display format such as a graph.

In this example, the moving means 101 includes an optical element from the light source 11 to the first lens group 16 and a CCD.
The frame 10a provided with the camera 30 is movable along the optical axis direction, and the movement amount measuring means 102 detects the movement amount. However, in order to change the relative distance between the first and second lens groups, the first lens group 16 is fixed, and the second lens group, the aperture member 20, and the test object L are integrally moved. Is also good. The radius of curvature of the wavefront of the light beam emitted from the second lens group 17 changes according to the distance from the object-side focal point of the first lens group 16 to the second lens group. When the light source 11 emits a parallel light beam as in this example, the light source 11 may be fixed outside the moving frame 10a. On the other hand, when the light beam emitted from the light source is not a parallel light beam, the distance between the light source and the first lens group needs to be constant, the gathering surface has a curvature that is in an auto-collimated state with respect to the light source, and At the reference position x0, it must be arranged so that the light-converging position of the light beam emitted from the first lens group coincides with the object-side focal point of the second lens group.

Next, a method of measuring the shape of the surface to be inspected using the above-described apparatus will be described. The surface accuracy required by the analysis of interference fringes includes tilt, defocus, astigmatism, coma, spherical aberration, etc. These are fringe analysis of interference fringe images using Zernike, Seidel polynomial approximation, etc. Required by Thus, the surface accuracy can be objectively detected as numerical information, and more accurate measurement can be performed as compared with the related art. Here, a method of measuring the radius of curvature and the curvature of the test surface La will be described.

FIG. 2 is a flowchart showing a basic method of measuring the radius of curvature when there is no error from the position where the auto-collimation state occurs at the measurement position. This method is almost the same as the method disclosed in Japanese Patent Application Laid-Open No. 63-47607 as a prior art, and the function of the correction means 112 in FIG. 1 becomes unnecessary.

As a first step, a plane mirror, which is a prototype, is arranged at a position substantially at the object side focal point F21 of the second lens group 17,
By moving the frame 10a, the position where the light beam enters the autocollimation state with respect to the prototype is detected as the reference position x0 of the moving means 101.

At the reference position x0, the image-side focal point of the second lens group 17 coincides with the focal point (in this example, coincident with the focal point) of the light beam by the first lens group, and the wavefront exiting from the second lens group 17 is shifted. This is a position where the light becomes a plane, that is, a position where the emitted light becomes a parallel light flux. The reference position x0 moves the frame 10a by placing a plane mirror as a prototype at the position of the opening member 20,
It can be detected as a position where the interference fringes on the imaging surface 30a of the CCD camera 30 are linear or uniform.

After the reference position x0 is determined, the test object L is disposed in place of the prototype, and the frame 10a is moved to determine the position where the light beam is in an auto-collimated state with respect to the test surface La by the moving means 101. Is detected as the measurement position x1.

The focal length f of the second lens group 17
And the reference position x detected by the movement amount measuring means 102
Based on the movement amount Δx from 0 to the measurement position x1 = (x1−x0), the curvature Ct or the curvature radius rt of the surface to be measured is obtained by the following equation (1). Ct = 1 / rt = Δx / f ² (1)

However, the sign of Δx is the first lens group 16
The movement in the direction in which the distance between the lens and the second lens group 17 increases is defined as positive, and the sign of the radius of curvature rt of the test surface La is defined as negative if it is concave on the left side (measuring optical system side) as shown in FIG. I do.

FIG. 3 is an explanatory view showing a state in which the light beam is autocollimated with respect to the surface La to be detected. Assuming that the virtual object point of the second lens group 17 obtained by developing the reflected light path on the test surface La is O, the interval between the object side focal point F21 and the object point O matches the curvature radius rt of the test surface. . Further, the image-side focal point F22
The distance between the first lens group 16 and the object-side focal point F11 is equal to the moving amount Δx. Therefore, assuming that the focal length of the second lens group 17 is f, Newton's imaging formula rt · Δx = f ²
Can be applied, and the above equation (1) is derived.

In the above, a plane mirror is used as a prototype, and
(1) The method of obtaining the curvature Ct or the curvature radius rt has been described. However, when the curvature radius of the surface to be measured can be predicted to some extent, it is preferable to use a prototype having a curvature radius close to the curvature radius of the surface to be measured. More preferred. Such a case will be described below.

First, the frame 10a is moved so as to perform auto-collimation with respect to the prototype having the curvature C0, and the reference position x0 is detected. Next, the measurement position x1 is detected by moving the frame 10a so as to perform auto-collimation with respect to the test surface. Based on the focal length f of the second lens group 17 and the moving amount Δx = x1−x0, the curvature Ct or the curvature radius rt of the surface to be measured is obtained by the following equation (2). Ct = 1 / rt = C0 + (Δx / f ² ) (2)

As described above, by using a prototype having a radius of curvature substantially equal to that of the surface to be measured, the amount of movement Δx
Can be reduced, and the measurement can be performed accurately and in a short time. Since the object side focal point F21 of the second lens group 17 always coincides with the surface to be measured La, the surface to be measured La and the image side focal point F12 of the first lens group 16 are always conjugate. The image side focal point F12 of the first lens group 16 and the imaging surface 30a of the CCD camera 30 are conjugated by the lens 19, and unless the positional relationship is changed, the distance between the first lens group 16 and the second lens group 17 is increased. Not in other words,
Regardless of the shape of the test surface La, the test surface La and the imaging surface 30a
The conjugate relationship with is guaranteed. The observed interference fringe region is also constant.

When the measurement position exactly matches the autocollimation position, the radius of curvature or the curvature can be measured without using the correcting means 112 according to the flowchart of FIG. On the other hand, when the measurement position does not exactly coincide with the position where auto-collimation occurs, and there is a defocus, the measurement is performed according to the flowchart shown in FIG. can do.
In this case, the function of the correction unit 112 in FIG. 1 is required.

In the method shown in FIG. 4, a plane mirror, which is a prototype, is disposed at a position substantially coincident with the object-side focal point F21 of the second lens group 17, and the frame 10a is moved so that a light beam is automatically collimated with respect to the prototype. Movement position 10
It is detected as one reference position x0.

Next, the test object L is disposed in place of the prototype, and the frame 10a is moved to set the position where the light beam is substantially in an autocollimation state with respect to the test surface La by the moving means 10.
It is detected as one measurement position x1. At this time, accurate positioning is unnecessary, and it is sufficient to perform positioning so that fringe analysis can be performed.

Then, by analyzing the image information of the interference fringes by the surface accuracy calculating means 111, the light flux is changed to the surface La to be detected.
Is detected at a position where the auto-collimation state is substantially reached, and based on the detected defocus, the movement amount measured by the movement amount measuring means and the focal length f of the second lens group 17 are calculated. The curvature or radius of curvature of the test surface La calculated based on the correction is corrected.

A method for analyzing the image information of the interference fringes and correcting the curvature or radius of curvature of the surface La to be inspected will be described below. FIG. 5 is a diagram showing a state in which a spherical wave S having a radius of curvature rs is incident on a surface La to be inspected having a radius of curvature rt.

The radius of curvature rs of the spherical wave is determined by the first lens group 1
1 / rs = Δx / f ² (1 ′) is given as a result of the relative positional relationship Δx between the second lens group 6 and the second lens group 17.

The sag amount difference ΔS at the height h from the optical axis of the test surface La and the incident spherical wave S is: Given by

On the other hand, between the defocus term W (df) obtained as a result of analyzing the image information of interference fringes and the sag amount difference ΔS, as shown in FIG. W (df) λ = ΔS · cos θ (6)

From the equations (1 ′), (5) and (6), Ct = 1 / rt = (Δx / f ² ) + (2W (df) λ / (h ² · cos θ)) (3) Be guided.

Here, h is the opening radius of the opening member 20, and the half opening angle θ of the light beam is obtained from sin θ = h / rt ≒ hΔx / f ² (7).

When a spherical surface having a curvature C0 is used as a prototype, the displacement Δx from the autocollimation state of the prototype is used.
And Ct = 1 / rt = C0 + (Δx / f ² ) + (2W (df) λ / h ² · cos θ) (4)

According to the apparatus of the embodiment, not only the surface accuracy can be numerically evaluated by using the interferometer, but also the defocus can be obtained by analyzing the interference fringes. As described above, it is possible to accurately measure the radius of curvature and the like without accurately setting the position of the auto-collimation. Therefore, the accuracy of lens positioning is reduced, and the time required for measurement can be reduced.

FIG. 6 is an explanatory diagram showing a more detailed configuration example of the measuring optical system 10 of the surface shape measuring apparatus of the embodiment. The parallel luminous flux emitted from the light source 11 is transmitted to the beam expander 1
2, the beam is enlarged to a predetermined beam diameter, reflected by the first total reflection mirror 13 and the half mirror 14, a part of which is reflected by the reference plane plate 15, and a light beam transmitted through the reference plane plate 15 The light reaches the test object L via the second lens groups 16 and 17.

The convergent light from the first lens group 16 is transmitted to the second lens group 1 disposed on the object L side from the object side focal point F11.
7 as divergent light. The second lens group 17 is fixed with the object side focal point F22 substantially coincident with the surface to be inspected La abutted on the opening member 20, and the frame 10a moves integrally in the optical axis direction. It is arranged as possible.

The light reflected from the test surface La is reflected by the first and second lens groups 16 and 17, the reference flat plate 15, and the half mirror 14.
And is reflected by the second total reflection mirror 18, and then reaches the imaging surface 30 a of the CCD camera 30 by the imaging lens 19. On the other hand, since the reference light reflected by the reference plane plate 15 also reaches the imaging surface 30a by the imaging lens 19,
These two wavefronts interfere on the imaging surface 30a.

By moving the frame 10a integrally in the optical axis direction, the light flux emitted to the surface La to be inspected in accordance with the distance from the object side focal point F11 of the first lens group 16 to the second lens group 17 is determined. The curvature of the wavefront can be changed.
Since the second lens group 17 used here is a spherical lens, the wavefront is a plane wave or a spherical wave.

When the surface La to be inspected is spherical and the radius of curvature of the wavefront substantially coincides with the radius of curvature of the surface La to be inspected, a substantially auto-collimating state is established, and the reflected light transmitted through the first lens group 16 and returned. Becomes almost parallel light. In this state,
By processing the image captured by the CCD camera by a computer, it is possible to detect shape data of an area to be measured on the surface to be measured La, for example, an aspherical amount, an astigmatism amount, and the like.

Further, the radius of curvature and the average curvature of the surface La to be inspected can be detected by the methods shown in FIGS. By adjusting the distance between the first lens group 16 and the second lens group 17 to set the state of autocollimation, a wide range of the radius of curvature of the surface to be measured that can be measured can be secured. When measuring a multifocal lens for spectacles as a test object, since the radius of curvature of the multifocal lens can range from infinity to several tens of mm in the distance portion and the near portion,
The movable range of the second lens group 17 needs to be set so that the auto-collimation state can be almost secured for any radius of curvature within the above range.

FIG. 7 shows a second embodiment of the present invention, and is a system diagram of a surface shape measuring apparatus using a Fizeau interferometer similar to the optical system shown in FIG. The operation of the measuring optical system is the same as that of FIG. In this example, the second lens group 17 and the test object L move along the optical axis direction with respect to the first lens group.

The image data of the interference fringe captured by the CCD camera 30 is transmitted to an image processing device 10 serving as image input means.
The movement amount Δx of the second lens group 17 is input to the computer 110 from the length measuring device 102 as the movement amount measurement means. The computer 110 analyzes the image of the interference fringe input from the image processing device 103 by fringe analysis using Zernike, Seidel's polynomial approximation or the like to measure the shape of the surface to be inspected, and inputs the image from the length measuring device 102. The radius of curvature, the curvature, and the refractive power of the test surface La are measured based on the movement amount Δx of the second lens group 17, and the measurement results are displayed as numerical data on the CRT 120 as display means.

As the type of interferometer, not only the Fizeau-type interferometer as described above, but also a Twyman-Green interferometer as shown in FIG. 8 can be used.
FIG. 8 shows a third embodiment of the present invention.

In the optical system shown in FIG. 8, the light beam emitted from the laser light source 11 and expanded by the beam expander 12 is
The light is split into two by the half mirror 14. The reference light reflected by the half mirror 14 is reflected by the reference mirror 15a. The component of the reflected reference light that has passed through the half mirror 14 reaches the CCD camera 30 by the imaging lens 19.

The detection light transmitted through the half mirror 14 reaches the surface to be detected La via the first and second lens groups 16 and 17. The component reflected by the half mirror 14 of the detection light reflected by the test surface La reaches the imaging surface 30a of the CCD camera 30 and interferes with the reference light. Reference mirror 15
a can be moved in the optical axis direction by the piezo element 15b, and can scan the interference fringes.

Data photographed by the CCD camera 30 is processed by a computer in the same manner as in the above example, and the measurement results are displayed on a CRT (not shown).

[0050]

As described above, according to the present invention, the shape of the test surface can be detected as numerical information by using the interferometer. Further, by changing the distance between the first and second lens groups, it is possible to detect information such as the radius of curvature and the curvature of the surface to be inspected regardless of the radius of curvature of the surface to be inspected.

Further, by obtaining the defocus based on the analysis result of the interference fringes, even when an error from the auto-collimation position occurs at the curvature radius or the set position at the time of the curvature measurement, the influence of the error is corrected. And accurate measurement becomes possible.

[Brief description of the drawings]

FIG. 1 is a system diagram showing an outline of an optical system and a control system block of a first embodiment of a surface shape measuring apparatus according to the present invention.

FIG. 2 is a flowchart showing a procedure for measuring a radius of curvature of a surface to be measured by the surface shape measuring method according to the present invention, and shows a procedure when there is no error in a measurement position of the surface to be measured.

FIG. 3 is an explanatory diagram showing an auto-collimation state with respect to a test surface in the apparatus of FIG.

FIG. 4 is a flowchart showing a procedure for measuring a radius of curvature of a surface to be measured by the surface shape measuring method according to the present invention, and shows a procedure when there is an error in a measurement position of the surface to be measured.

FIG. 5 is an explanatory diagram showing a state in which a wavefront with defocus is incident on a surface to be measured in the apparatus of FIG. 1;

FIG. 6 is an explanatory diagram showing a specific configuration example of an optical system of the device according to the first embodiment.

FIG. 7 is a system diagram showing a surface shape measuring apparatus according to a second embodiment of the present invention.

FIG. 8 is an explanatory diagram showing an optical system of a surface shape measuring device according to a third embodiment of the present invention.

FIG. 9 is an explanatory view showing an optical system of a surface shape measuring device disclosed in Japanese Patent Application Laid-Open No. 63-47607.

[Description of Signs] 10 Measurement optical system 10a Frame 11 Light source 14 Half mirror 15 Reference plane plate 16 First lens group 17 Second lens group 19 Imaging lens 20 Opening member 30 CCD camera 30a Imaging surface 101 Moving means 102 Movement amount measurement Means 103 Image input means 110 Calculation means 111 Surface accuracy calculation means 112 Correction means 113 Curvature radius calculation means 120 Display means

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Moriyasu Shirayanagi Asahi Gaku Kogyo Co., Ltd. 2-36-9 Maenocho, Itabashi-ku, Tokyo

Claims (13)

[Claims] A first lens group that temporarily collects a light beam emitted from a coherent light source; and a second lens group that causes a light beam from the first lens group to enter a surface to be measured. And a measuring optical system that causes the reflected light not to enter the first and second lens groups to be reflected by the reference surface, and an optical axis of the first lens group and the second lens group Moving means for changing the relative distance along the direction; moving amount measuring means for measuring a relative moving amount by the moving means; and image input means for capturing interference fringes generated by interference in the measuring optical system as image information. And a calculating means for calculating the shape of the surface to be inspected based on outputs of the moving amount measuring means and the image input means. 2. The moving means includes: an optical system from the light source in the measuring optical system to the first lens group;
The surface shape measuring apparatus according to claim 1, wherein the surface shape measuring device is moved with respect to a lens group and the surface to be inspected. 3. An opening member for positioning the test surface such that the test surface substantially coincides with the front focal point of the second lens group is provided so that the distance from the second lens group is always constant. The surface shape measuring device according to claim 1, wherein: 4. An arithmetic unit analyzes the image information of the interference fringes output from the image input unit, and calculates a light beam at a position where the light beam is substantially in an auto-collimation state with respect to the surface to be detected. Surface accuracy calculation means for obtaining focus; curvature radius calculation means for calculating the curvature or radius of curvature of the surface to be measured based on the movement amount output from the movement amount measurement means and the focal length of the second lens group; 2. The surface shape measurement according to claim 1, further comprising: a correction unit that corrects the curvature or the radius of curvature calculated by the curvature radius calculation unit based on the defocus obtained by the surface accuracy calculation unit. apparatus. 5. A surface shape measuring method using the surface shape measuring device according to claim 1, wherein a prototype is arranged so as to substantially coincide with a front focal point of the second lens group, and the luminous flux is controlled by the prototype. Is detected as the reference position x0 of the moving means with respect to the moving means, and the test surface is disposed in place of the prototype, and the light flux is substantially in an autocollimation state with respect to the test surface. A position is detected as a measurement position x1 of the moving means, and the reference position x0 detected by the movement amount measuring means is detected.
From the distance x to the measurement position x1 (= x1-x0)
And measuring a curvature Ct or a radius of curvature rt of the surface to be inspected based on the focal length f of the second lens group. 6. In a state where the surface to be detected is positioned so as to substantially coincide with the front focal point of the second lens group, a plane is used as the prototype, and the focal length of the second lens group is f,
The surface shape measuring method according to claim 5, wherein a curvature Ct or a curvature radius rt of the surface to be measured is obtained based on the following equation (1), where Δx is the amount of movement. Ct = 1 / rt = Δx / f ² (1) 7. In a state where the surface to be detected is positioned so as to substantially coincide with the front focal point of the second lens group, a spherical surface having a curvature C0 is used as the prototype, and the focal length of the second lens group is set to f. 6. The surface shape measuring method according to claim 5, wherein a curvature Ct or a radius of curvature rt of the surface to be measured is obtained based on the following equation (2), with the movement amount as Δx. Ct = 1 / rt = C0 + (Δx / f ² ) (2) 8. The method according to claim 7, wherein the prototype has a radius of curvature substantially equal to that of the surface to be inspected. 9. The method according to claim 1, wherein the calculating unit obtains a defocus at a position where the light flux is substantially in an auto-collimation state with respect to the surface to be detected, based on image information of interference fringes output from the image input unit. 6. The apparatus according to claim 5, wherein a curvature or a radius of curvature of the surface to be detected, which is obtained based on the amount of movement and a focal length of the second lens group based on the detected defocus, is corrected. Surface shape measurement method. 10. A light beam emitted from a coherent light source is once collected by a first lens group, and a light beam from the first lens group is incident on a surface to be measured via a second lens group. And the reflected light reflected by the reference surface without being incident on the first and second lens groups, causing interference between the first lens group and the second lens group along the optical axis direction. A surface shape measuring method for measuring a shape of the surface to be measured based on an analysis result of interference fringes and a change in the relative interval by changing a relative interval, wherein the shape substantially matches a front focal point of the second lens group. A position where the light beam enters an auto-collimation state with respect to the prototype device is detected as a reference position of the moving means. Set the position where the autocollimation state is almost Detecting as a measurement position of the movement means, detecting a movement amount from the reference position detected by the movement amount measurement means to the measurement position, and analyzing the previous image information to defocus at the measurement position. Detecting and correcting a curvature or a radius of curvature of the surface to be detected, which is obtained based on the amount of movement and a focal length of the second lens group based on the detected defocus. Measuring method. 11. The method according to claim 11, wherein a plane is used as the prototype, and the second
Assuming that the focal length of the lens group is f, the amount of movement is Δx, the amount of defocus at the measurement position is W (df) λ, the half-open angle of the light beam after exiting the second lens group is θ, and the opening radius is h, 11. The curvature Ct or the curvature radius rt of the surface to be measured is determined based on the following equation (3).
Surface shape measurement method according to 1. Ct = 1 / rt = (Δx / f ² ) + (2W (df) λ / h ² cos θ) (3) 12. A spherical surface having a curvature C0 is used as the prototype while the surface to be measured is positioned so as to substantially coincide with the front focal point of the second lens group, and the focal length of the second lens group is set to f. , The amount of movement is Δx, the amount of defocus at the measurement position is W (df) λ, the half-opening angle of the light beam after the second lens group is θ, and the opening radius is h.
11. The surface shape measuring method according to claim 10, wherein the curvature Ct or the curvature radius rt of the surface to be inspected is obtained based on (4). Ct = 1 / rt = C0 + (Δx / f ² ) + (2W (df) λ / h ² cos θ) (4) 13. The surface shape measuring apparatus according to claim 3, wherein the image-side focal point of the first lens group and the interference fringe observation surface of the image input unit are conjugate.