JP2000097654A - Interferometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To permit both the matching of the spherical center of a test surface with a point light source and the shortening of the overall length of a device by arranging a reflecting mirror at a location where the point light source arranged between a light source and the test surface is approximately conjugate with the spherical center of the test surface. SOLUTION: A spherical surface 2 emitted as measuring light from the point light source 1a of a fiber 1 is reflected by a plane reflecting mirror 3 accurately calibrated separately and becomes incident onto the reflector surface 4a of a reflector primary standard 4 equivalent to a test surface. In the case that the test surface is a spherical surface, the reflecting mirror 3 is arranged in such a way that the spherical center 1b of the reflector surface 4a is conjugate with the point light source 1a. By this, as the measuring light is vertically reflected, it becomes possible to reform an image on the face of the fiber 1. As the measuring light reflected at the face is guided to a CCD 6 together with the spherical surface 2 as reference waves via a condensing lends 5, it becomes possible to form desired interference fringes. By using this interferometer, it becomes possible to shorten the overall length of a device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interferometer for measuring spherical and aspherical surface shapes with high accuracy.

[0002]

2. Description of the Related Art Conventionally, a Fizeau interferometer or a Twyman-Green interferometer has been used for measuring a spherical shape.
These interferometers require a reference surface, and measure the spherical shape by comparison with the reference surface. Therefore, the measurement accuracy cannot exceed the surface accuracy of the reference surface.

As an interferometer that does not require a reference plane, a point-diffraction based on a wavefront diffracted by a pinhole is used.
Interforometer is known. (JP-A-2-228505)
This interferometer realizes highly accurate measurement of a spherical shape using an ideal spherical wave generated by diffraction of a pinhole as a reference wavefront. Further, with this method, it was possible to measure the shape of the spherical surface with high accuracy, but it was not possible to measure the shape of the aspherical surface. Point-Diffraction-Inter
When measuring an aspheric surface using a forometer, if the radius of curvature of the spherical wave diffracted by the pinhole and the radius of curvature of the aspheric surface to be measured match, interference fringes with coarse intervals will occur. Since the radius of curvature is different, the region where the interference fringe interval is coarse is only a part, and in the remaining region, the interference fringe interval is dense and measurement is impossible.

In order to solve this problem, only the portion where the interference fringe interval is coarse is measured, and then the position where the interference fringe interval is coarse is changed by changing the positional relationship between the aspheric surface to be measured and the pinhole. It is known to measure the shape of an aspheric surface by repeating the process of performing measurement again later and combining the measurement results.

[0005]

However, the spherical surface,
Regardless of the type of aspherical surface, when the radius of curvature of the approximate curved surface of the surface to be measured is long, the distance between the point light source and the surface to be measured must necessarily be equal to the radius of curvature, so the device becomes longer. It was inevitable to do. This will be described with reference to FIG. 7 (reprinted in Japanese Patent Application Laid-Open No. 2-228505).

In FIG. 7, the surface to be measured is a spherical surface. By making the spherical center of the spherical surface coincide with a point light source, the measuring light is incident and reflected substantially perpendicularly to the annular region of the surface to be measured. It becomes possible. However, since the distance between the vertex of the surface to be measured and the spherical center is also the same as the radius of curvature of the surface to be measured, the device is inevitably lengthened. SUMMARY OF THE INVENTION The present invention has been made in view of the above-described drawbacks of the related art, and has as its object to prevent an increase in the length of a device in a PDI interferometer or the like and to increase the accuracy of the device.

[0007]

According to the present invention, in order to achieve the above object, a first means is to irradiate a part of a light beam emitted from a light source as a measurement light beam onto a surface to be measured, The measurement light beam reflected by the test surface and the reference light beam having a predetermined wavefront, which is a part of the light beam emitted from the light source, interfere with each other, and detect the state of interference fringes caused by the interference. The interferometer for measuring the surface shape of the test surface, wherein the measurement light beam and the reference light beam are arranged on a reflection surface disposed between the light source and the test surface. Interferometer that is a light beam emitted from a point light source having a predetermined size, wherein a reflecting mirror is arranged at a position where the point light source and the spherical center of the approximate spherical surface of the test surface are substantially conjugate. Is used. This makes it possible to reduce the overall length of the device.

As a second means, a part of the light beam emitted from the light source is irradiated to the surface to be measured as a light beam for measurement, and the light beam for measurement reflected on the surface to be measured and the light beam emitted from the light source are emitted. A reference light beam having a predetermined wavefront, which is a part of the detected light beam, interferes with each other, and detects a state of an interference fringe caused by the interference, thereby measuring a surface shape of the surface to be detected. The interference in which the measurement light beam and the reference light beam are light beams emitted from a point light source having a predetermined size arranged on a reflection surface arranged between the light source and the test surface. In the instrument, an interferometer that arranges the test surface at a position where the point light source and the spherical center of the approximate spherical surface of the reflecting mirror are substantially conjugated is used. This allows
Not only the concave surface and the convex surface but also the interference measurement of the test surface can be performed based on the substantially hyperboloid wavefront.

The third means is that the surface of the test surface is an aspherical surface and the interference fringes represent a part of the shape of the test aspherical surface. A plurality of interference fringes representing the shape of another partial region of the test aspheric surface obtained by scanning at least one of the reflecting mirror and the point light source and equivalently changing the positional relationship are obtained. The interferometer which measures the shape of the desired aspherical surface in the desired range by analyzing the aspherical surface is used. Thereby, it is possible to facilitate the positioning of the positional relationship.

The fourth means is that “a part of the light beam emitted from the light source is irradiated on the surface to be measured as a measurement light beam,
The measurement light beam reflected by the test surface and the reference light beam having a predetermined wavefront, which is a part of the light beam emitted from the light source, interfere with each other, and detect the state of interference fringes caused by the interference. The interferometer for measuring the surface shape of the test surface, wherein the measurement light beam and the reference light beam are arranged on a reflection surface disposed between the light source and the test surface. In the interferometer, which is a light beam emitted from a point light source having a predetermined size, so that the surface to be inspected is in an actual use state (in the same direction and with the same holding method with respect to gravity). An interferometer having a point light source is used. This makes it possible to calibrate the surface accuracy of the surface to be inspected (corresponding to the latter when the deviation from the desired design shape is considered as a shape error and a waviness error) with high accuracy.

In the section of the means for solving the above-mentioned problems, which explains the configuration of the present invention, the drawings of the embodiments of the present invention are used to make the present invention easier to understand. However, the present invention is not limited to this.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An interferometer according to the present invention will be described below with reference to FIG. In FIG. 1, point light source 1 of fiber 1
The ideal spherical wave 2 emitted from a is measured light as
Reflected by a separately accurately calibrated flat reflecting mirror 3,
The light is incident on a ref surface 4a of the ref prototype 4 equivalent to the test surface. When the test surface is spherical, the spherical center 1b of the reflex surface 4a
By arranging the reflecting mirror 3 so that the point light source 1a and the point light source 1a are conjugate, the measuring light is reflected vertically and can be re-imaged on the end face of the fiber 1. The measurement light reflected by the end face is guided to the CCD 6 via the condenser lens 5 together with the spherical wave 2 as a reference wave, so that a desired interference fringe can be formed.

By using this interferometer, it is possible to shorten the overall length of the apparatus. In particular, when the surface to be inspected of the workpiece has a posture against gravity as shown in FIG. 1, the height of the apparatus can be reduced, which is advantageous in terms of installation space for suppressing vibration. Furthermore, it is possible to flexibly cope with the upward and downward postures of the test surface with respect to gravity. Even if the surface to be measured is an aspheric surface, as long as the amount of the aspheric surface is small and the fringe density of the observed interference fringes can be sufficiently resolved by the CCD 6, a reflex surface having an aspherical shape equivalent to the surface to be measured is provided. Is prepared, the entire surface of the reflex surface can be similarly measured.

In the measurement arrangement shown in FIG. 1, an opening is provided in the center of the reflex surface, and a point light source 1a is provided in the opening. This means that the central opening has no significant effect as a ref prototype for certifying the inspection surface of the workpiece of the product. In addition, since the surface accuracy can be transferred to a separately prepared reflex device without an opening by comparison and subtraction using wavefront synthesis, there is no problem when performing high-accuracy measurement.

When the surface to be inspected is an aspheric surface and the amount of the aspheric surface is so large that the observed fringe density of the interference fringes cannot be resolved by the CCD 6, a so-called "wavefront synthesis" (Japanese Unexamined Patent Publication No. Hei 5-40024). ), The entire surface of the reflex surface can be measured.
When performing this wavefront synthesis, a plurality of partial measurement data can be measured by moving the reflecting mirror as shown by the arrow in FIG. 1 instead of moving the reflex surface in the optical axis direction.
When performing wavefront synthesis, it is necessary to grasp the amount of movement of the moving member. In this case, it is easy to use the spherical mirror 7 as shown in FIG. This will be described below.

As shown in FIG. 1, if a spherical mirror is provided on, for example, the outer peripheral portion of the reflex prototype, the plane mirror is uniquely determined unless the positional relationship is changed as a set of a point light source and a spherical mirror. This is similar to the case where the position of the point light source is uniquely determined as the spherical center of the spherical mirror when applied to FIG. 7 of the conventional example. Therefore, irrespective of the use of the folding measurement with a plane mirror, if the refractor is set with good reproducibility with respect to the spherical mirror, the point light source is aligned with the spherical mirror reference, and the reference for the reflector is set. Can be achieved with the same positioning accuracy.

The setting of the ref prototype with respect to the spherical mirror may be based on metal hardware, or may be based on the flat surface of the back surfaces (opposite surfaces) of the two. In any case, the accuracy may be improved by calibrating the positional relationship between the two using a coordinate measuring device or the like in that state. Further, the outside of the effective portion of the reflex prototype itself may be processed into a spherical surface. Observation of interference fringes may be performed by appropriately arranging a masking mechanism and switching the measurement light between a spherical mirror and a reflex surface, or separately analyzing the interference fringes obtained at the same time. If the aliasing measurement by the plane mirror is not adopted, if the spherical mirror and the ref prototype are moved integrally, the movement amount of the ref prototype can be easily grasped by analyzing the interference fringes of the spherical mirror. When a plane mirror is used, the same movement amount can be grasped by moving only the plane mirror. The masking may be provided on the plane mirror or on the test surface side.

It goes without saying that the optical member may be moved on the point light source side. Hereinafter, a first embodiment of the interferometer according to the present invention will be described. The measurement arrangement of FIG. 2 is an embodiment in which the reflecting mirror of FIG. 1 is inclined by 45 degrees to eliminate the central opening of the ref prototype, and the overall length of the apparatus can be shortened similarly to FIG. As shown in FIG. 5 described later, an aspherical axis when the surface to be inspected is an aspherical surface (the rotational center axis of the designed aspherical surface when a rotationally symmetrical designed aspherical surface is fitted to the measured shape of the workpiece). In the case of a work used in a state where coincides with the gravity g, absolute calibration of the plane reflecting mirror in the state of FIG. 2 is possible in FIG.

Referring to FIG. 8, a plane mirror 31 is disclosed in
And the plane mirror 32 is disclosed in Japanese Patent Application No. 10-1.
34848 allows absolute calibration. Therefore,
The plane mirror 30 can be calibrated using these or directly according to Japanese Patent Application No. 10-134848. further,
It goes without saying that this calibration method can also be applied to the plane mirror 3 of FIG.

In the case where the surface to be measured is a spherical surface and its radius of curvature is short, it is possible to directly calibrate the reflex surface with an ideal spherical wave from a point light source without using a plane mirror in the measurement arrangement of FIG. After the calibration, the plane mirror 30 can be calibrated by measuring the same reflex surface in the measurement arrangement of FIG. Once the plane mirror 30 is calibrated, the same plane mirror can be used to calibrate a spherical surface having a long radius of curvature or an aspheric surface having a long radius of curvature of an approximate spherical surface without receiving errors of the plane mirror. . Further, if this procedure is taken, there is an advantage that it is not necessary to consider the attitude of the plane mirror with respect to gravity as described in FIG.

Incidentally, when actually used as a reflecting mirror, a coating is applied in order to obtain a highly reflecting surface, but it is necessary to calibrate it including an error caused by the coating. Further, since the mirror is a plane mirror used in a converging system, an error occurs when reflected obliquely as compared with vertical reflection, and it is important to correct the error. Also, as mentioned above,
By comparing the surface accuracy data of the plane mirror with the two measurement arrangements of the vertical reflection and the oblique reflection, it is also possible to measure the thickness unevenness of the coated surface of the plane mirror.

Hereinafter, a modification of the first embodiment of the interferometer according to the present invention will be described. While the test surface in FIG. 1 is in a so-called “base material” state, that is, a case where the test surface has a rotationally symmetric outer diameter, the measurement arrangement in FIG. In this case, the so-called “off-axis” test surface whose outer diameter is not symmetrical with respect to the aspherical axis, and the aspherical axis deviates from the effective area of the test surface.

Hereinafter, a second embodiment of the interferometer according to the present invention will be described. The measuring arrangement of FIG. 4 differs from the measuring arrangement of FIG.
0 is replaced by a convex aspherical surface
Are respectively arranged. This is a point light source 1a
And a convex aspheric surface to be measured by a hyperboloid wavefront uniquely determined by the spherical center 1b of the concave spherical surface. When the deviation from the hyperboloid of the test aspheric surface is large, the annular wavefront synthesis is performed by moving the test aspheric surface in the optical axis direction of the arrow.
This is the same as in FIG.

Hereinafter, a third embodiment of the interferometer according to the present invention will be described. The measurement arrangement of FIG. 5 is based on the so-called “base material” (rotationally symmetric material before off-axis processing and having an aspherical axis at the center of the outer diameter) as shown in FIGS. As the point light source, a pinhole 10a formed in the member 10 is used instead of the fiber. Further, a reflecting mirror for turning back is not used together (it goes without saying that it may be used together). Then, as described in FIG. 8, this is an arrangement in which the attitude of the reflex surface (test surface) with respect to gravity is considered.

Specifically, when the reflex surface is a spherical surface, its effective center axis is matched with the gravitational axis g, and when the reflex surface is a rotationally symmetric aspherical surface, its rotationally symmetric axis is matched with the gravity axis g. Is the case. By adopting this arrangement, it is possible to perform rotation averaging without changing the attitude of the reflex surface with respect to gravity, so that even if there is an error in the wavefront from the point light source 10a, it is possible to check it. Has advantages.

A modification of the third embodiment of the interferometer according to the present invention will be described below. FIG. 6 shows the point light source of FIG. 5 as a fiber. Compared with FIG. 5, there is an advantage that the lateral displacement of the measuring light 2 is facilitated. This lateral shift of the measurement light is particularly effective in the case of a spherical surface.

[0027]

As described above, the use of the interferometer according to the present invention enables highly accurate measurement of spherical and aspherical surfaces. In addition, a folded spherical surface is placed at the position of the reflex surface,
If the test surface is provided at the position of the plane mirror, it is possible to measure the test surface with reference to the wavefront of the hyperboloid, and it is useful for measurement, particularly when the test surface is convex, regardless of spherical or aspheric surface. is there.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of an interferometer according to the present invention.

FIG. 2 is an explanatory diagram of a first embodiment of the interferometer according to the present invention.

FIG. 3 is an explanatory view of a modified example of the first embodiment of the interferometer according to the present invention.

FIG. 4 is an explanatory diagram of a modification of the second embodiment of the interferometer according to the present invention.

FIG. 5 is an explanatory diagram of a third embodiment of the interferometer according to the present invention.

FIG. 6 is an explanatory view of a modification of the third embodiment of the interferometer according to the present invention.

FIG. 7 is an explanatory diagram of a conventional example.

FIG. 8 is a view for explaining absolute calibration of the reflecting mirror 3 shown in FIG. 2;

[Explanation of symbols]

 1 ··· Fiber 1a ··· Point light source 2 ··· Measuring light 3 ··· Reflector 4 ··· Reflex prototype 4a ··· Reflex surface 5 ··· Condenser lens 6 ··· CCD 7 ··· Positioning mirror

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F065 AA30 AA51 AA54 BB05 CC21 CC31 DD02 DD11 DD14 EE00 FF52 FF61 FF67 GG12 HH18 JJ03 JJ26 LL02 LL12 LL19 LL30 PP12 PP13 QQ00 QQ25 TT02 UU07

Claims (10)

[Claims] A light source for irradiating a part of the light beam emitted from the light source to the surface to be measured as a light beam for measurement, and a part of the light beam for measurement reflected by the surface to be measured and the light beam emitted from the light source; An interferometer for measuring a surface shape of the test surface by causing a reference light beam having a predetermined wavefront to interfere with each other and detecting a state of an interference fringe generated by the interference. The interferometer, wherein the light beam and the reference light beam are light beams emitted from a point light source having a predetermined size disposed on a reflection surface disposed between the light source and the test surface, An interferometer, wherein a reflecting mirror is arranged at a position where a light source and a spherical center of an approximate spherical surface of the test surface are substantially conjugate. 2. An interferometer according to claim 1, wherein said reflecting mirror is a flat surface or a hyperboloid. 3. A part of a light beam emitted from a light source is radiated to a surface to be measured as a measurement light beam, and a part of the measurement light beam reflected by the surface to be measured and one of the light beams emitted from the light source are reflected. An interferometer for measuring a surface shape of the test surface by causing a reference light beam having a predetermined wavefront to interfere with each other and detecting a state of an interference fringe generated by the interference. The interferometer, wherein the light beam and the reference light beam are light beams emitted from a point light source having a predetermined size disposed on a reflection surface disposed between the light source and the test surface, An interferometer, wherein the test surface is arranged at a position where a light source and a spherical center of an approximate spherical surface of a reflecting mirror are substantially conjugate. 4. The interferometer according to claim 3, wherein said reflecting mirror is spherical. 5. The interferometer according to claim 1, wherein the surface of the test surface is an aspherical surface. 6. The surface of the test surface is an aspherical surface, and the interference fringes represent a part of the shape of the test aspherical surface. By performing a comprehensive analysis of a plurality of interference fringes representing the shape of another partial region of the test aspheric surface obtained by scanning at least one of the point light sources to change the positional relationship, 6. The interferometer according to claim 1, wherein a shape of a desired range of the inspection aspheric surface is measured. 7. A part of a light beam emitted from a light source is radiated to a surface to be measured as a light beam for measurement, and a part of the light beam for measurement reflected by the surface to be measured and a light beam emitted from the light source. An interferometer for measuring a surface shape of the test surface by causing a reference light beam having a predetermined wavefront to interfere with each other and detecting a state of an interference fringe generated by the interference. In an interferometer, the light beam and the reference light beam are light beams emitted from a point light source having a predetermined size disposed on a reflection surface disposed between the light source and the surface to be measured. An interferometer comprising: 8. A part of a light beam emitted from a light source is radiated to a surface to be measured as a measurement light beam, and a part of the measurement light beam reflected by the surface to be measured and one of the light beams emitted from the light source are emitted. An interferometer for measuring a surface shape of the test surface by causing a reference light beam having a predetermined wavefront to interfere with each other and detecting a state of an interference fringe generated by the interference. In the interferometer, the light beam and the reference light beam are light beams emitted from a point light source having a predetermined size and disposed on a reflection surface disposed between the light source and the surface to be measured. An interferometer, wherein the point light source is arranged so that a test surface is in an actual use state. 9. A rotating means for rotating and averaging the test surface when the test surface is an aspheric surface and the actual use state is such that the aspheric axis of the aspheric surface coincides with the direction of gravity. The interferometer according to claim 8, wherein: 10. A part of a light beam emitted from a light source is radiated to a surface to be measured as a light beam for measurement, and a part of the light beam for measurement reflected by the surface to be measured and one light beam emitted from the light source are emitted. An interferometer for measuring a surface shape of the test surface by causing a reference light beam having a predetermined wavefront to interfere with each other and detecting a state of an interference fringe generated by the interference. The interferometer, wherein the light beam and the reference light beam are light beams emitted from a point light source having a predetermined size disposed on a reflection surface disposed between the light source and the test surface, An interferometer characterized in that the light source can be tilted.