JPH116784A - Device and method for measuring shape of aspherical surface - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the measuring device for an aspherical shape and the method for measurement, which can measure the aspherical shape in high accuracy. SOLUTION: The light from a light source 1 is split. On light is reflected from a reference surface 5a and made to be the reference light, and the other light is reflected from an aspherical tested surface 7 through a wave-front forming means 6 and made to be the measuring light. This measuring light is made to interfere with the reference light, and interference fringes are formed. The surface shape of the tested surface 7 is measured, based on this interference fringes. In this case, the wave-front forming means 6 includes at least a diffraction optical element, the diffraction light at a given nth order of the above diffraction optical element 6 is made to be the aspherical wave in correspondence with the surface shape of the above tested surface 7 and the O-degree light of the above diffraction optical element is made to be the spherical surface wave.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aspherical shape measuring apparatus and a measuring method for measuring a surface shape of an optical element such as a lens or a mirror constituted by an aspherical surface with high accuracy.

[0002]

2. Description of the Related Art Conventionally, in a surface shape measurement using light interference, a measurement light having a wavefront corresponding to the design shape of a surface to be measured is formed using a Fizeau interferometer or the like, and the measurement light is measured. By making the reflected light from the surface interfere with the reference light,
The difference between the test surface and the wavefront of the measurement light is measured. In particular, when the surface to be measured has an aspherical shape, a diffractive optical element such as a zone plate or the like, which can easily obtain an aspherical wave, or a combination of a diffractive optical element and an optical element having a lens function is used as a wavefront forming means. ing.

[0003]

However, the wavefront generated by the wavefront forming means has an error due to the manufacturing errors such as the radius of curvature and the surface interval of the substrate and lens member of the diffractive optical element of the wavefront forming means and the influence of the refractive index unevenness. , It was difficult and problematic to measure the shape with high accuracy.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an aspherical shape measuring apparatus and a measuring method capable of measuring an aspherical shape with high accuracy.

[0005]

An aspherical shape measuring apparatus according to the present invention divides light from a light source, reflects one light on a reference surface as reference light, and converts the other light through a wavefront forming means. An aspherical shape measurement for reflecting the light on a test aspheric surface to form a measurement light, causing the measurement light to interfere with the reference light to form an interference fringe, and measuring a surface shape of the test surface based on the interference fringe In the apparatus, the wavefront forming means includes at least a diffractive optical element, and the diffracted light of a predetermined order of the diffractive optical element is an aspherical wave corresponding to the surface shape of the surface to be measured, and the zeroth order of the diffractive optical element. Light is characterized by being a spherical wave.

[0006] With this configuration, it is possible to perform an interference measurement, that is, a so-called null test, on the aspheric surface to be measured using the aspherical wavefront of a predetermined order of the diffractive optical element. In addition, by calibrating the result of the null test based on a further measurement result with respect to the reference spherical surface using the spherical wave that is the 0th-order light of the diffractive optical element, it is possible to eliminate a manufacturing error of the diffractive optical element and the like and to achieve high precision. The measured aspheric surface can be measured.

In the aspherical surface shape measuring method according to the present invention, light from a light source is divided, one light is reflected by a reference surface to be a reference light, and the other light is diffracted by a predetermined order. The 0th-order light is an aspherical wave corresponding to the surface shape of the test surface, and the 0th-order light is reflected by the test surface via wavefront forming means including a diffractive optical element which is a spherical wave to be the measurement light, and the measurement light and the reference Interfering light to form an interference fringe, and measuring a difference between the surface shape of the test surface and the wavefront shape of the aspherical wave that is the diffracted light of the predetermined order based on the interference fringe, Replacing the test surface with a reference spherical surface, measuring the difference between the surface shape of the reference spherical surface and the wavefront shape of the spherical wave that is the zero-order light of the diffractive optical element; and The wavefront shape of the aspherical wave is calibrated based on the difference from the wavefront shape of the spherical wave. It is characterized by comprising the step of calculating the surface shape of the test surface by.

According to this process, interference measurement (null test) of the aspheric surface to be measured can be performed using the aspherical wave of a predetermined order of the diffractive optical element. Further, by calibrating the interference measurement result of the aspheric surface to be measured based on the result of interference measurement of the reference spherical surface using the spherical wave which is the 0th-order light of the diffractive optical element, the manufacturing error of the diffractive optical element can be reduced. Since the influence can be removed, the aspherical surface can be measured with high accuracy.

[0009]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing a configuration of an aspherical shape measuring apparatus according to an embodiment of the present invention, which is configured by a Fizeau interferometer. The linearly polarized beam L emitted from the light source unit 1 is converted into parallel light by the collimator lens 2 and enters the polarization beam splitter 3. The polarization plane of the light beam L is selected so as to be reflected by the polarization beam splitter 3.

The light beam L reflected by the polarizing beam splitter 3 is incident on a fuso member 5 via a quarter-wave plate 4. The light beam L incident on the Fizeau member 5 is split into a measurement light LM passing through a reference plane (Fizeau surface) 5a of the Fizeau member 5 and a reference light LR reflected by the reference plane 5a.

The measurement light LM is incident on the aspherical wave forming means 6 composed of a combination of a lens and a diffractive optical element, is converted into an aspherical wave, and is incident on the aspherical surface 7 to be measured. The measurement light LM reflected by the test aspherical surface 7 passes through the aspherical wave forming means 6 and the quarter-wave plate 4 again, and enters the polarization beam splitter 3.

On the other hand, the reference light L reflected by the reference plane 5a
R also transmits through the quarter-wave plate 4 again like the measurement light LM and enters the polarization beam splitter 3.

The measurement light LM and the reference light LR transmit through the quarter-wave plate twice in a reciprocating manner, so that the polarization plane is rotated by 90 degrees, so that they pass through the polarization beam splitter 3.

The measurement light LM and the reference light LR transmitted through the polarization beam splitter 3 have their beam diameters converted by a beam expander 8 and are incident on a two-dimensional image detector 9, where interference fringes are observed.

FIGS. 2A and 2B show the state of the wavefront formed with the aspherical wave forming means 6. FIG. FIG. 2A shows a state of diffracted light of a predetermined order diffracted by the diffractive optical element 6b, and FIG. 2B shows a state of 0th-order diffracted light (directly transmitted light) of the diffractive optical element 6b. FIG.

As shown in FIG. 2A, the diffractive optical element 6
The aspherical wave, which is the diffracted light of the predetermined order b, is designed to be incident perpendicularly on the aspherical surface 7 to be measured.

On the other hand, as shown in FIG. 2B, the zero-order light of the diffractive optical element 6b is designed to be focused at a predetermined focal position F.

Next, the measurement procedure will be described. First, the test aspheric surface 7 is inserted into the optical path. Since the diffracted light of a predetermined order in the diffractive optical element 6b is designed to be incident perpendicularly to the design shape of the aspherical surface 7 to be measured, the measuring light is reflected by the aspherical surface 7 to be measured. Reverse traveling on the outward path while almost maintaining the wavefront shape of the outward path. Therefore, the reference light LR becomes substantially a plane wave.
Interfere with. By analyzing this interference fringe, it is possible to measure the difference between the wavefront shape WA of the aspherical wave at the position of the test aspherical surface 7 and the surface shape WM of the test aspherical surface, φTA = WA−WM. .

However, a wavefront shape error occurs in the aspherical wave due to a shape error and a non-uniform refractive index of the substrate of the lens and the diffractive optical element constituting the aspherical wave forming means, and an assembling error. That is, the shape WA of the aspherical wave at the position of the test aspherical surface 7 is represented by the following equation, where W'A is the designed aspherical wave shape and WE1 is the wavefront shape error: WA = W'A + WE1 (1) Can be expressed as Therefore, the aspherical wave shape W
A needs to be calibrated.

Therefore, in order to calibrate the aspherical wave shape WA, the measurement is performed by replacing the aspherical surface 7 to be measured with the reference prototype 10 as a reference spherical surface. The reference prototype 10 includes the diffractive optical element 6
Assuming that the distance between the condensing position F of the 0th-order diffracted light b and the diffractive optical element 6b is d and the radius of curvature of the reference prototype 10 is r, the reference spherical surface 10a and the 0th order are located at a distance dr from the diffractive optical element 6b. Set up so that the folding light matches.

Since the 0th-order light of the diffractive optical element 6b forms a spherical wave and is designed to be incident perpendicularly to the reference spherical surface 10a, the 0th-order diffracted light is reflected by the reference spherical surface 10a, It reverses on the outward path while substantially maintaining the wavefront shape, and becomes almost a plane wave and interferes with the reference light LR.

By analyzing the interference fringes, it is possible to measure the difference between the wavefront shape WB of the spherical wave at the position of the reference spherical surface 10a and the reference spherical shape WS of the reference prototype 10, φTB = WB−WS. it can. Since the reference spherical wave shape WS is measured with high accuracy, the wavefront shape WB of the spherical wave generated by the wavefront forming means can be known with high accuracy. Assuming that the spherical wave shape at the position of the reference spherical surface 10a is WB and the designed spherical wave shape is W'B, the wavefront shape error WE2 of the spherical wave is given by the following equation (2), WE2 = WB-W'B (2) You can know. Since the wavefront shape error WE1 included in the aspherical wave is equal to the wavefront shape error WE2 included in the spherical wave, the aspherical wave shape WA can be calibrated from the equations (1) and (2). Therefore, the surface shape of the test surface 7 can be measured with high accuracy by the measured value φTA of the difference between the aspherical wave shape WA and the test surface shape WM.

It is desirable that the designed aspherical wave shape WA 'and the spherical wave shape WB' are equal to the designed shape of the test aspherical surface and the reference spherical shape, respectively.

Further, in the aspherical surface shape measuring apparatus and measuring method according to the present embodiment, a Fizeau interferometer is used, but a Twyman-Green interferometer can also be used.

[0025]

As described above, according to the present invention, a spherical wave, which is the zero-order light of the diffractive optical element, is measured with high precision, and the aspherical measurement result is calibrated, whereby the manufacturing error of the diffractive optical element and the like can be improved. And the aspherical shape can be measured with high accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram showing an optical arrangement of an aspherical shape measuring apparatus according to one embodiment of the present invention.

2A is a diagram illustrating a state of diffracted light of a predetermined order of a diffraction element 6b, and FIG. 2B is a diagram illustrating a state of zero-order light.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source unit 2 Collimator lens 3 Polarization beam splitter 4 1/4 wavelength plate 5 Fizeau member 5a Reference plane 6 Aspherical wave forming means 6a Lens part 6b Diffractive optical element part 7 Surface to be inspected 8 Beam expander 9 Two-dimensional image sparing detection Vessel 10 Spherical reference prototype 10a Reference sphere

Claims (2)

[Claims] 1. A light from a light source is split, one light is reflected by a reference surface to be a reference light, and the other light is reflected by an aspheric surface to be measured via a wavefront forming means to be a measurement light. An aspherical shape measuring apparatus that forms interference fringes by causing measurement light to interfere with the reference light and measures the surface shape of the surface to be measured based on the interference fringes, wherein the wavefront forming means includes at least a diffractive optical element. The diffracted light of a predetermined order of the diffractive optical element is an aspherical wave corresponding to the surface shape of the surface to be inspected.
An aspherical shape measuring device, wherein the next light is a spherical wave. 2. A light from a light source is split, one light is reflected by a reference surface to be a reference light, and the other light is an aspherical surface corresponding to a surface shape of a surface to be measured, and a diffracted light of a predetermined order is used. Wave, 0
The next light is reflected on the surface to be measured through a wavefront forming means including a diffractive optical element that is a spherical wave to form measurement light, and the measurement light and the reference light interfere with each other to form an interference fringe. Measuring the difference between the surface shape of the test surface and the wavefront shape of the aspherical wave that is the diffracted light of the predetermined order based on the test surface; replacing the test surface with a reference spherical surface; Measuring the difference between the shape of the spherical wave that is the zero-order light of the diffractive optical element and the wavefront shape of the spherical wave. Calculating the surface shape of the test surface by calibrating the wavefront shape of the spherical wave.