JP3336666B2 - Surface shape measuring method and its measuring 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 apparatus for measuring a surface shape by "three-plane alignment" for absolutely calibrating an optical surface.

[0002]

2. Description of the Related Art Conventionally, as a method for absolutely measuring the surface shape of an optical surface, a method using "three-plane alignment" has been used.
FIG. 4 shows a measurement method in the case of a plane. The measurement principle is represented by the following equation. Note that the three surface shapes are A
(Work 1), B (Work 2), and F (Fizeau). -[Measurement data 1] = A + F-[Measurement data 2] = B + F-[Measurement data 3] = B + A (The underline means the case of reversing the work) From the above equation, for example, A is as follows Is determined.・ (A + A ) / 2 = ([measured data 1] − [measured data 2] + [measured data 3]) / 2

[0003]

However, in the conventional "three-plane alignment", the measurement data 1 and the measurement data 2 can be considered on the same coordinates because A and B face F directly. However, in the measurement data 3, since A and B face each other, the positional relationship on the coordinate axis orthogonal to the inversion axis for facing the faces is reversed. In other words, only three surface shapes can be measured only on the inversion axis (on one line).

Since the underline of A means the reversal of the work, "(A + A ) / 2" indicates the surface shape of A on the reversal axis. It is also possible to measure the surface shape of the entire surface A by connecting measurement data obtained by finely rotating the inversion axis around the optical axis of the interferometer measurement. However, in this case, since the averaging of the data described later is not performed during the rotation of A, there is a problem that the number of measurements for obtaining the entire surface data increases.

[0005] On the other hand, even when this "three-plane alignment" is made to correspond to a spherical surface, the same coordinates are inverted, so that only the surface shape on one line can be measured. However,
In the case of a spherical surface, as a method of obtaining the surface shape of the entire A surface,
In addition to the above-mentioned method, there is disclosed an invention utilizing reflection by a mirror at an image forming point of a Fizeau lens (Japanese Patent Publication No. Hei 1-2).
No. 44306), however, it is impossible to deal with a plane.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and has a surface shape measuring method and apparatus capable of measuring a two-dimensional surface shape of an optical plane with a minimum number of measurements with high accuracy. The purpose is to provide.

[0007]

For this purpose, the present invention firstly proposes "two or three of the three optical surfaces during the light beam of the Fizeau interferometer."
In a method of measuring the surface shape of each optical surface from three or more interference fringes observed in a state where the sheets are arranged so as to face each other, so-called “three-plane alignment”, The above-mentioned 2 which forms one interference fringe
The shape measurement data obtained by rotating one of the optical surfaces little by little around the measurement optical axis of the interferometer is averaged, and the non-rotational symmetry of the shape measurement data unique to the one optical surface is obtained. A surface shape measuring method (claim 1), characterized in that the two-dimensional surface shape of each optical surface is measured by relaxing the components.

[0008] The present invention also provides a second aspect of the present invention in which when a certain surface shape change due to an external force or the like is applied to the three optical surfaces,
The surface shape change is maintained to be line-symmetric with respect to at least one optical surface, and the respective optics after relaxation of the non-rotationally symmetric component in the “three-plane alignment” with respect to the one optical surface. A surface shape measuring method according to claim 1 (claim 2), characterized in that the surface is inverted around the axis of symmetry of the line symmetry.

The present invention also relates to a third aspect of the present invention in which "three or more interference fringes observed in a light beam of a Fizeau interferometer with two of the three optical surfaces arranged so as to face each other". In the apparatus for measuring the surface shape of each optical surface from the so-called "three-plane alignment", one of the two optical surfaces that form one of the interference fringes, A surface shape measuring apparatus (claim 3) comprising a rotating means for rotating the interferometer about a measurement optical axis and an arithmetic processing means for averaging the obtained shape measurement data.

[0010]

The symbols A and A, which have three surface shapes in the prior art section,
It is represented by B and F. The non-rotationally symmetric component is relaxed on one of the three surfaces (for example, A). This is because "[Measurement data 1] = A + F"
By rotating A once around the optical axis of the interferometer about the point by n equal divisions and averaging the measurement data A _i (i = 1 to n) for every n equal divisions, “ΣA _i → A _RS ”. Here, A _RS represents a rotationally symmetric component, and A _AS represents a non-rotationally symmetric component.

At this time, from the definition, "A = A _AS + A _RS ",
“ΣA _AS (i) → 0”. Therefore, the equations described in the prior art section are as follows. [Measurement data 1] = A _RS + F • [Measurement data 2] = B + F • [Measurement data 3] = B + A _RS Here, A _RS is a rotationally symmetric shape, and "A _RS = A _RS " holds. Therefore, _ARS = ([measurement data 1]-[measurement data 2] + [measurement data 3]) / 2, which is not affected by the inversion of the coordinates.

As an actual measurement procedure, it is unnecessary to perform rotation averaging of the inverted data. That is, only the non-rotationally symmetric element A _AS is reversed on the calculator, it may be added to the correction to the measured data. The effect of the number of additions for data averaging (corresponding to the number of equal parts n) on the obtained surface shape error of A can be determined from the data in FIG.

FIGS. 1 and 2 show measurement data of FIG.
(A) and this measurement data is converted into n
1 (b), which is obtained by equally dividing by n = 4, and FIG. 2 (a), which is similarly obtained by equally dividing by n = 8, similarly, by n = 16 FIG. 2B shows the averaged values. In terms of RMS,
At the stage of n = 4, it has a convergence of 4 × 10 ⁻⁴ λ or less in numerical value. Needless to say, this convergence depends on the surface accuracy of the used original data (a).

The original data (a) is referred to as “Japanese Patent Application No. 4-2.
The absolute measurement result on the simulation by the wavefront creation extraction method disclosed in “No.
At the stage of = 4, a shape reproducibility of 6 × 10 ⁻⁴ λ is realized numerically. In this case, the rotationally symmetric component needs point symmetry to extract it, but in the present invention, since only line symmetry is sufficient, more excellent shape reproducibility can be expected. In addition, since the error in principle of this degree is a negligible amount as compared with a normal measurement error, it is a sufficient number of times of measurement that can be practically used. If n is increased to infinity (more precisely, when it is averaged below the resolution of the measuring device), the error in principle converges to zero.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The measurement example described in the operation is the first embodiment of the present invention. In the measurement after the inversion of A in [Measurement Data 3], the rotationally symmetric components are added to each other. Therefore, even if the axis deviation of the inverted workpiece occurs, the method of extracting the rotationally symmetric components in the wavefront creation extraction method must be used. If used, there is an advantage that can be dealt with.

FIG. 3 shows a second embodiment of the present invention.
This is a case where the deformation that the gravity exerts on the workpiece W when performing the “three-plane alignment” is also considered. Incidentally, FIG.
It is the figure seen from the direction of M or M 'of (a) and (b). The deformation of each surface is represented by D _A, D _B, D _F. At this time, [Measurement data 1 to 3] are as follows. · Measurement Data _{1] = (A RS + D} A) + (F + D F) · [ Measurement Data _{2] = (B + D B} ) + (F + D F) · [ Measurement Data _{3] = (B + D B} ) + ( A _RS + D _A ) Here, since A _RS has a rotationally symmetric shape, there is necessarily a line symmetry. Therefore, if the line symmetry of D _A is ensured, A
_RS + D _A will have a line symmetry. At this time, "A
Since _{RS + D A = A RS +} D A "is _{satisfied, · A RS + D A =} ([ Measurement Data 1] - [Measurement Data 2]
+ [Measurement data 3]) / 2, which is not affected by the inversion of the coordinates. However, it is necessary to always match the inversion axis line symmetry axis of D _A.

More specifically, for a round work W, for example, a belt BL suspension is adopted so that the distortion generated by the work support is axisymmetric, and the work W is turned around the axis of symmetry of gravity in the direction of gravity G of the work W. I just need. Further, when the average of the rotational data, as D _A come riding as an offset,
Needless to say, it is necessary to keep the suspension constant.

FIG. 4 shows a third embodiment of the present invention.
With respect to one high-reflection mirror (work) W, the work surface shape is measured with high accuracy by using another dummy flat plate D. When the dummy plane plate D is measured in each of the states shown in FIGS. 4A and 4B, the dummy plane plate D is turned upside down based on the point P. What is necessary is just to perform rotation averaging around and form only the rotationally symmetric component.

As in the second embodiment, the deformation due to the suspension of the dummy flat plate D can be completely eliminated. Further, there is an advantage that the deformation of the work itself due to the suspension of the belt does not need to have line symmetry. Next, the necessity of aspect ratio correction, power, and tilt correction will be described. Correction of aspect ratio When Fizeau F, work W, and dummy D are all φa, even if the effective diameter of the interferometer is φa, the work W is set at an angle (the angle θ with the optical axis). The result is that the major axis (gravity direction) is a and the minor axis (horizontal direction) is a × sin θ
Ellipse.

Therefore, it is necessary to enlarge the horizontal scale by [× 1 / sin θ] with respect to the measurement result. Power and tilt correction Normally, when measuring a plane, power correction is not performed and measurement is performed only with tilt correction. By subtracting the data obtained in the term (if necessary, perform further tilt correction)
The shape of the work W can be determined. Here, the so-called habit of the work W (shape error obtained by removing the power component from the surface accuracy)
Is obtained by power-correcting the data obtained in the term. If power correction is performed before correcting the term, an accurate habit cannot be obtained. Others When the work W is not a mirror surface, it is necessary to form a reflection film on the surface to be measured of the work W. To prevent deformation when the reflection film is peeled off, increase the thickness of the work W or reversely. It is necessary to simultaneously form a reflection film on the surface.

[0021]

As described above, the surface shape measuring method and apparatus according to the present invention can measure the entire surface of the optical surface by "three-plane alignment" with a small number of measurements, and can also suspend an optical lens or the like. Measurement including surface shape errors due to surface deformation that occurs when lowered is also possible. Further, the optical surface to be measured is not limited to a flat surface, but may be similarly applied to a spherical surface.

[Brief description of the drawings]

FIG. 1 is a data diagram of measured data (a) and data (b) obtained by dividing the measured data into four equal parts and averaging them.

FIG. 2 is a data diagram of data (a) obtained by dividing measured data into eight equal parts on a simulation and averaged, and data (b) obtained by dividing sixteen equal parts and averaged.

FIG. 3 is a side view showing a second embodiment of the present invention.

FIG. 4 is a side view showing a third embodiment of the present invention.

FIG. 5 is a side view showing a conventional example.

[Explanation of symbols]

 W ... Work W1 ... Work 1 W2 ... Work 2 F ... Fizeau BL ... Belt G ... Gravity direction D ... Dummy plane plate

Claims (3)

(57) [Claims] 1. A surface shape of each optical surface from three or more interference fringes observed in a state where two of three optical surfaces are arranged to face each other in a light beam of a Fizeau interferometer. A method for performing surface shape measurement by so-called “three-plane alignment”, wherein one of the two optical surfaces forming one of the interference fringes is placed around the measurement optical axis of the interferometer. By averaging the respective shape measurement data obtained by rotating the optical surface little by little, and relaxing the non-rotationally symmetric component of the shape measurement data unique to the one optical surface, the two-dimensional surface shape of each of the optical surfaces is reduced. A method for measuring a surface shape, comprising measuring. 2. When a constant surface shape change due to an external force or the like is applied to the three optical surfaces, the surface shape changes are kept line-symmetric with respect to at least one optical surface, and 2. The optical system according to claim 1, wherein, in the "three-plane alignment" of the one optical surface, inversion of each of the optical surfaces after relaxation of the non-rotationally symmetric component is performed around the axis of symmetry of the line symmetry. Surface shape measurement method. 3. The surface shape of each optical surface from three or more interference fringes observed in a state where two of the three optical surfaces are arranged to face each other in the light beam of the Fizeau interferometer. In a device for performing surface shape measurement by so-called “three-plane alignment”, one of the two optical surfaces forming one of the interference fringes is rotated around the measurement optical axis of the interferometer. A surface shape measuring device, comprising: a rotating means for rotating the shape measuring means; and an arithmetic processing means for averaging the obtained shape measurement data.