JPH0571913A - Interferometer - Google Patents

Abstract

(57) [Abstract] [Purpose] The difference between the measured light wavefront shape and the reference light wavefront shape is large, and the number of interference fringes generated by the interference between them is too large. Even if it is difficult to observe, it should be possible to measure the DUT with an interferometer. An interference provided with an interference fringe generating means for interfering a measured light wave and a reference light wave to generate an interference fringe, and a light intensity image display means for observably displaying a light intensity image corresponding to the interference fringe. In the total, the light intensity image display means generates a fringe image that generates a fringe image having a smaller number of fringes than the interference fringes by a reference fringe forming means that forms a reference fringe and a calculation operation of the reference fringes and the interference fringes. An interferometer comprising: means and means for observably displaying the fringe image.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention interferes with a light wave to be measured reflected from an object to be measured and a reference light wave to generate interference fringes, so that an optical intensity image corresponding to the interference fringes can be observed. Regarding the total. Such an interferometer is used to measure the shape, inclination angle, position, etc. of the object to be measured with high accuracy.

[0002]

2. Description of the Related Art As the interferometer, a Twyman-Green interferometer, a Fizeau interferometer, etc. are conventionally known. The interferometer includes an interference fringe generating unit that causes interference fringes between a measured light wave and a reference light wave to generate an interference fringe, and a light intensity image display unit that observably displays a light intensity image corresponding to the interference fringe. ing. The interference fringe generating means for generating interference fringes by causing the measured light wave and the reference light wave to interfere with each other, for example, the measured light wave and the reference light wave are a white plate, a surface of a ground glass plate,
Alternatively, it is configured by means for projecting the image on the image pickup surface of the CCD camera or the like. The light intensity image display means for observably displaying a light intensity image corresponding to the interference fringes is a white plate, a ground glass plate, or an image pickup unit of a CCD camera on which the measured light wave and the reference light wave are projected in an overlapping manner. It is composed of a display that displays the image.

[0003]

In the above interferometer,
Since the number of interference fringes increases as the difference between the shapes of the wavefronts of the measured light wave and the reference light wave increases, measurement becomes difficult,
This narrows the measurement range. For example, when the DUT is a surface shape and the surface shape is formed by targeting a predetermined non-ideal spherical surface having a large difference from the spherical surface, using a reference light wave of a spherical wave that is easily obtained, The number of interference fringes of the measured light wave and the reference light wave increases, which makes measurement difficult. In such a case, the wavefront of the reference light wave itself is
There is also an attempt to configure the prototype of the interferometer with a computer generated hologram or the like so as to have an ideal aspherical shape of the object to be measured, but such a prototype is difficult to make,
There is a problem that the prototypes, which are difficult to make, must be prepared for each shape of the object to be measured. Furthermore, it is difficult to verify the accuracy of such prototypes.

In view of the above problems, the present invention has a large difference between the measured light wavefront shape and the reference light wavefront shape, and the number of interference fringes generated by the interference between them is too large, and the light intensity corresponding to the interference fringes is large. Even if it is difficult to directly observe the image,
An object is to enable measurement of an object to be measured with an interferometer.

[0005]

In order to solve the above-mentioned problems, an interferometer of the present invention comprises an interference fringe generating means for causing an interference fringe by causing an optical wave to be measured and a reference optical wave to interfere with each other, and In an interferometer including a light intensity image display means for observably displaying a corresponding light intensity image, the light intensity image display means comprises a reference fringe forming means for forming a reference fringe, the reference fringe and the interference fringe. A stripe image generating means for generating a stripe image having a smaller number of stripes than the interference fringes by an arithmetic operation of, and a means for displaying the stripe image so that the stripe image can be observed.

In the present invention, the "image" means a two-dimensional intensity distribution of light, and the "arithmetic operation" means an interference fringe having an actual two-dimensional light intensity distribution of light. And an operation of superimposing reference stripes, or an operation of calculating image data of digitized light intensities of images of interference fringes and reference stripes distributed on a two-dimensional plane. As a specific type of calculation of the “calculation operation”, the light intensity distribution of the interference fringe image on the xy plane is g ₁ (x, y), and the light intensity distribution of the reference fringe image is g ₂ ( In the case of x, y), for example, the following can be adopted. (A) Operation for calculating moire of the sum or product of g ₁ and g ₂ .

The reference stripe forming means for forming the reference stripe is
For example, it can be composed of a white plate having a reference stripe printed on the surface, frosted glass, or the like. In this case, the measured light wave and the reference light wave may be configured to be projected onto the white plate or frosted glass having the reference stripes printed on the surface. The reference stripe forming means reads the reference stripe image data storage means that stores the image data for generating the reference stripe image and the image data stored in the reference stripe image data storage means and displays it on a liquid crystal display or the like. It can also be configured by means for doing so.

Further, the fringe image generating means for generating a fringe image having a smaller number of fringes than the interference fringes by a calculation operation of the reference fringes and the interference fringes is, for example, a light in which the measured light wave and the reference light wave are superposed. The projection light of the reference fringe image is configured by a means for projecting the projection light on the white plate, the surface of the ground glass plate, the image pickup surface of the CCD camera, or the like, or, for example, the light in which the measured light wave and the reference light wave are overlapped. It is configured by means for projecting the projected light of the reference fringe image on the image pickup surfaces of different CCD cameras and a computer for calculating the sum moiré or the product moiré from the image data picked up by each CCD camera and converted into an electric signal. You can

The means for displaying the fringe image so that it can be observed is a white plate, a ground glass plate or the like onto which the light in which the measured light wave and the standard light wave are superimposed and the projection light of the reference fringe image are simultaneously projected. The image data output from the CCD camera for imaging the light in which the measured light wave and the reference light wave are superimposed and the reference fringe image data read from the reference fringe image data storage means are processed by a computer. The display device may be a CRT display or the like that displays the sum moiré or the product moiré obtained by the above.

[0010]

In the interferometer having the above-mentioned structure, the light intensity image display means forms the reference stripes for forming the reference stripes, and the number of stripes from the interference stripes is calculated by the calculation operation of the reference stripes and the interference stripes. Since it is provided with a stripe image generating means for generating a small stripe image and a means for displaying the stripe image in an observable manner, even when the number of interference fringes between the measured light wave and the reference light wave is too large, It is possible to observe a fringe image with a small number of fringes generated by the calculation operation of the interference fringe and an appropriate reference fringe.

A fringe image having a smaller number of fringes than the interference fringes generated by the calculation operation of the reference fringes and the interference fringes is optically summed between the interference fringes of the measured light wave and the reference light wave and the reference fringes. Alternatively, it can be obtained by generating a product moiré, or performing a calculation by a computer between the image data of the interference fringes and the image data of the reference fringes to generate a moiré of the sum or the product. Next, a sum moiré and a product moiré, which are typical fringe images generated by the calculation operation of the reference fringe and the interference fringe, will be described. Moire has a sum moiré and a product moiré depending on the way of stacking. Here, only a one-dimensional light intensity image will be considered as an image. When the two stripes to be superposed are f ₁ and f ₂ , fwm = f ₁ (x) + f ₂ (x) ………………………… (1) Moire, fsm = f ₁ (x) ・ f ₂ (x) …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… ； The properties and differences between the two are described below.

Japanese moire: Japanese moire as an image is
Usually, it is not often seen, but for example f ₁ as shown in FIG.
And f ₂ are projected by different slide projectors and superimposed on each other. (Note that, as shown in FIG. 5, the image obtained by projecting the slides of f ₁ and f _{2 in} an overlapping manner is a product moire.) Even if it is rare as an image, it may be sufficient for other physical quantities. Can be seen. For example, heterodyne detection, sound wave beating, etc. are sum moire. Consider a sinusoidal stripe for simplicity. If the wavelength of the first fringe is L times that of the second, then each fringe image is

[Equation 1] Can be expressed as (F ₁ and f ₂ are intensity images, so they are always 0 or more.) However, φ is a phase difference, the contrast of both fringe images is 1, and the average intensity is assumed to be equal.

When the moire of the sum of the striped images represented by the equations (3) and (4) is created,

[Equation 2] Becomes Accordingly, the carrier _{wave: λ wc = 2Lλ 1 / (} L + 1) ........................ (6a) envelope _{Wavelength: λ wh = 2Lλ 1 / |} L-1 | ........................ (6b ) Carrier phase: φ _wc = φ / 2 …………………………………… (7a) Envelope phase: φ _wh = −φ / 2 ……………………………………… (7b) As shown in FIG. 6, since the wavelength of the moire corresponds to 1/2 of the wavelength of the envelope, λwm = Lλ ₁ / | L−1 | φwm = −φ.

From the above, the expansion rate R of moire (= wavelength of moire / wavelength of first fringe) is R = (λwm / λ ₁ ) = L / │L-1│ …………………… ……………… (8) On the contrary, if L is expressed by R, when L> 1: L = R / (R-1) ……………………………… (9a) When L <1: L = R / (R + 1) …………………………………… (9b). As can be seen from the equations (6a), (6b), (7a), and (7b), the phase difference φ between the first fringe and the second fringe affects the phase difference between the carrier and moire, but Does not affect.

Product Moire: Most of the moires seen as images are product moiré. For example, when two slides of the images of f ₁ and f ₂ are overlapped and seen through, the moire of the product can be seen. When the fringes of the equations (3) and (4) are superposed by the product,

[Equation 3] Therefore, due to the contribution of the underlined portion of Expression (10), it is inferred that the approximate shape matches the moire of the sum. An example of a product moire is shown in FIG. 7 along with the corresponding sum moire. As shown in FIG. 7, it is presumed that the cycle of the product moire matches that of the corresponding sum moire. The envelope of the product Moire is

[Equation 4] And the Moiré node appears as the intersection of both envelopes.

[0016]

[Equation 5] In other words, the position x of the moire node is calculated by the following equation ().

[Equation 6] By the way, if the positions of the Japanese moire nodes are calculated in the same manner, as shown in FIG.
From equation (5),

[Equation 7] Becomes Since equations (11) and (12) have the same value, it can be seen that the phase and the cycle of the product moire exactly match those of the corresponding sum moire. Therefore, in the future, the equations (7) to (9) can be used for both product and sum moiré.

The difference between the sum moiré and the product moiré is that the sum moiré cannot be seen without resolving the carrier fringes, but the product moiré can be seen without resolving the carrier fringes.
There is a point. Therefore, it is convenient to use product moiré in the present invention. This is because the product moiré makes it possible to use a low-resolution observation device (for example, a television camera).

As described above, the above equation (8), that is,

[Equation 8] The fringe spacing (= wavelength) is converted accordingly. For example, if the wavelength of the reference fringe / the wavelength of the interference fringe = 1.2 is selected, the above (1
The fringe spacing is converted to 5 times by the equation (3). In particular, if the product moiré is used, it is possible to observe the fringe image generated by the calculation operation of the reference fringes and the interference fringes even if the original interference fringes are so small that they cannot be seen. Then, as the arithmetic operation, an operation of directly forming a moire of the sum or a moire of the product from the light forming the reference fringes and the light forming the interference fringes, or the reference fringe image data and the interference fringe image data. It is possible to employ an operation of performing arithmetic processing by a computer to form a sum moiré or a product moiré. Although the above explanation has been made on the one-dimensional stripe, the same argument is essentially established for the two-dimensional image, and the thickness which is easy to observe by the conversion, in other words, the spatial frequency which is easy to observe is obtained. It can be converted to a striped image.

[0019]

Embodiments of the interferometer of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory view of a first embodiment of an interferometer of the present invention, that is, an embodiment in which the present invention is applied to a conventional Wyman Green interferometer. In FIG. 1, a light beam emitted from a light source 1 is expanded by an optical system 2 and divided into two by a beam splitter 3. One of the lights is reflected by the prototype 4 and becomes the reference light wave 5 and reaches the beam splitter 3 again, and the other light is reflected by the surface 5 to be measured and becomes the light wave 7 to be measured and reaches the beam splitter 3 again.

In the beam splitter 3, the reference light wave 5 and the measured light wave 7 are superposed to generate interference fringes 8. In the optical paths of the reference light wave 5 and the measured light wave 7 that generate the interference fringes 8, a ground glass (that is, light intensity image display means) 10 on which reference fringes 9 are printed at appropriate observation positions.
Are arranged. From the back of the ground glass 10,
It is possible to observe the moire of the product generated by the interference fringe 8 and the reference fringe 9.

FIG. 2 is an explanatory view of a second embodiment of the interferometer of the present invention, that is, an embodiment in which the present invention is applied to a conventional Fizeau interferometer. In FIG. 2, the light beam emitted from the light source 21 is expanded by the optical system 22, and the beam splitter 23
To reach the flat prototype 24a. Some of the light passes through the flat prototype 24a as it is, but the rest of the light passes through the flat prototype 2.
At 4a, it is reflected as a plane wave. The light transmitted through the flat plate 24a reaches the object to be measured 25 by converting the light wavefront into a spherical surface by the lens 24b. The flat prototype 24a and the lens 24b can be regarded as a pair of spherical prototypes 24. Light wave reflected by DUT 25 and flat prototype 2
The plane waves reflected by 4a are superposed by a plane prototype 24a, bent by the beam splitter 23, passed through a lens 26, and projected onto a ground glass screen 27. A so-called Fizeau interferometer is configured by the elements indicated by the reference numerals 21 to 27.

Interference fringes 28 are formed on the screen 27 in accordance with the difference between the wavefront shapes of the reflected light wave from the object 25 to be measured and the reflected light wave from the flat prototype 24a. Since the difference in the wavefront shape of the reflected light wave corresponds to the difference between the shape of the measured object 25 and the spherical surface, it is possible to obtain the shape information of the measured object 25 from the interference fringe 28. The above is the principle of shape measurement by the Fizeau interferometer.

On the back surface of the screen 27, a transparent sheet 30 on which reference stripes 29 are printed is arranged so as to overlap the screen 27. From the TV camera 31 disposed behind the transparent sheet 30, the interference fringe 28 is superimposed on the reference fringe 29.
Can be seen. That is, the TV camera 31 captures a moire image of the product of the interference fringe 28 and the reference fringe 29. If the reference fringe 29 is appropriately selected, the interference fringe 2
Even if the stripe interval of 8 is small, an image with a large stripe interval can be displayed on the display 32 for observation.

FIG. 3 is an explanatory diagram of Embodiment 3 of the interferometer of the present invention. In the description of the third embodiment, the second embodiment will be described.
The same reference numerals are given to the constituent elements corresponding to the constituent elements of FIG. In FIG. 3, in the third embodiment, a transmissive liquid crystal panel 40 is used in place of the transparent sheet (that is, the light intensity image display means) 30 on which the reference stripes 29 are printed in the second embodiment. Different from the second embodiment. According to the third embodiment, various reference stripes can be displayed on the liquid crystal panel 40. Therefore, by displaying an appropriate reference fringe according to the interference fringe 28, an image with an appropriate fringe interval can be displayed on the display 32 for observation.

Although the embodiments of the interferometer of the present invention have been described in detail above, the present invention is not limited to the above embodiments,
Various minor design changes can be made without departing from the invention as set forth in the claims. For example, when measuring the shape of an object to be measured that is manufactured with a predetermined aspherical shape, the difference from the ideal aspherical surface, that is, the fringe corresponding to the amount of aspherical surface is used as the reference fringe. By doing so, it is possible to measure an aspherical surface having a large amount of aspherical surface. Further, the present invention can be applied not only to the measurement of the surface shape but also to various measurements using the observation of interference fringes, and can be applied to the measurement of the refractive index of an object, for example. Further, as a specific type of calculation of the calculation operation, the light intensity distribution of the interference fringe image on the xy plane is g ₁ (x, y), and the light intensity distribution of the reference fringe image is g 1.
₂ (x, y), for example, when g ₁ and g ₂ are binary images, the operation of calculating the exclusive OR (XOR) of g ₁ and g ₂ or the operation of the product of differentiated ones, It is also possible to adopt the operation of the following equation. {Dg ₁ (x, y) / dx} × {dg ₂ (x, y) / dx}

[0026]

According to the interferometer of the present invention described above, even if the number of interference fringes between the measured light wave and the reference light wave is too large, the number of fringes generated by the arithmetic operation of the interference fringes and an appropriate reference fringe can be reduced. A small number of striped images can be observed.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an interferometer according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of Embodiment 2 of the interferometer of the present invention.

FIG. 3 is a schematic configuration diagram of Embodiment 3 of the interferometer of the present invention.

FIG. 4 is an explanatory diagram of a moire of a sum which is a typical fringe image generated by a calculation operation of a reference fringe and an interference fringe.

FIG. 5 is an explanatory diagram of a moire of a product which is a typical fringe image generated by a calculation operation of a reference fringe and an interference fringe.

FIG. 6 is an explanatory diagram of carriers, envelopes, and moiré wavelengths of sum moiré.

FIG. 7 is a diagram showing that the sum moiré and the product moiré have the same wavelength.

[Explanation of symbols]

Reference numeral 10 ... Light intensity image display means (frosted glass), 30 ... Light intensity image display means (transparent sheet), 32 ... Means (display) for observing a striped image, 40 ... Reference stripe forming means (liquid crystal panel)

Claims (1)

[Claims] 1. An interference fringe generating means for causing interference between a measured light wave and a reference light wave to generate an interference fringe, and a light intensity image display means for observably displaying a light intensity image corresponding to the interference fringe. In the interferometer, the light intensity image display means is a reference stripe forming means for forming a reference stripe, and stripes for generating a stripe image having a smaller number of stripes than the interference stripes by a calculation operation of the reference stripes and the interference stripes. Image generating means, means for displaying the striped image in an observable manner,
An interferometer characterized by having.