JPS58127109A - Interferometer for measuring surface shape of mirror face - Google Patents

Abstract

PURPOSE:To attain an interferometer permitting to simply measure and inspect the surface shape of a mirror having a relatively large area without a need of enlarging the size of its construction by allowing the incident light beam to enter the surface to be inspected obliquely. CONSTITUTION:An irradiation light from a light source Q becomes a parallel light beam through a lens L1 and then is divided into two beams by a half mirror HM1. One beam hits upon a measured mirror M1 obliquely to cover the overall area of the mirror M1 having a relatively large size, while the other beam hits upon a reference surface M2. The light beams reflected by the mirror M1 and the reference surface M2 are combined through another half mirror HM2 to become an interference light in accordance with the surface shape of the mirror M1. This interference light is projected through a lens L2 on a mirror face M'1. Such oblique irradiation eliminates a need of enlarging the size of both lenses L1 and L2. It is thus possible to obtain an interferometer for measuring the surface shape which can be measure and inspect simply the surface shape of a mirror, etc. having a relatively large area without a need of enlarging the size of its construction.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an interferometer suitable for measuring the flatness of a plane mirror having a relatively large area.

Mirrors used in copying machines have a relatively large area and require fairly high flatness. In particular, since scanning mirrors are moved, their weight becomes a problem, and if a mirror is too thick, it will not be possible to sharpen the mirror. It is desirable that mirrors used in such copying machines be inspected in a non-contact manner. This is to avoid contaminating the mirror surface, and because the mirror is thin, its flatness changes when a heavy load from a prototype is applied.

Normally, a Twyman-Green interferometer as shown in FIG. 1 is used to non-contactly measure the shape of a mirror surface. That is, one of M'l and M2 is a mirror to be inspected and the other is a reference mirror, and the light emitted from the point light source Q is made into a parallel beam by lens Ll, and divided into two beams by half mirror HM. The resulting light flux is incident perpendicularly on the mirrors Ml, M2, and the respective mirrors Ml,
The reflected light beam from M2 is combined again into a single light beam by a half mirror HM, and interference fringes are observed. In this device, the area that can be observed at one time on the surface to be inspected is within the cross-sectional area of the incident light beam. Therefore, when inspecting a mirror with a large area, such as a mirror used in a copying machine, it is necessary to slide the mirror for observation. However, if the mirror is made to slide, it will be more susceptible to vibrations due to the mechanical play of the device, making it difficult to observe, and the time required to inspect the mirror for analysis will be longer. Become.

For inspection equipment for mass-produced items such as mirrors used in copying machines, it is necessary to be able to inspect the entire mirror surface as quickly and easily as possible due to the number of inspection steps, and it must be resistant to the effects of vibration. Must be. Therefore, the apparatus shown in FIG. 1 is not suitable for inspecting large-area mirrors that require mass production. In order to be able to inspect the entire wide mirror surface at once with the apparatus shown in Figure 1, the cross-sectional area of the light flux incident on the surface to be inspected must be made thicker to match the surface to be inspected. L2 etc. must be made with a large diameter, which is difficult from both a processing and cost perspective, and when using a gas laser as the light source,
In order to increase the magnification of the light source and the expander, which is the lens L1, the focal length of the lens L1 must be increased, which increases the space for installing the device, which is also a practical difficulty.

Under the above-mentioned circumstances, there is currently no suitable device that can easily measure and inspect the surface shape of a mirror with a relatively large area. In view of this situation, the present invention enables observation of the entire mirror surface over a relatively wide area with a simple operation without the need to slide the mirror to be inspected, and also requires exceptionally large optical elements such as lenses that constitute the device. The purpose of this invention is to provide a mirror surface shape measuring device which is extremely large in size and the overall size of the device is extremely large.

The basic principle of the present invention is that the surface to be inspected is placed obliquely with respect to the incident light flux. This means that a larger surface can be observed at once compared to the thickness, that is, the vertical cross section. The present invention will be explained in detail below with reference to Examples.

FIG. 2 shows a basic embodiment of the invention. In the figure, Q is a light source, and the light emitted from this light source becomes a parallel beam of light by the lens L1, and is divided into two beams of light by the first half mirror HM1. One of these divided light beams is made to obliquely enter the surface to be inspected M1.

Let this angle of incidence be θ. The other beam is a high-precision reference plane M
2 is made to be incident diagonally. In this example, the angle of incidence of the light beam on the test surface θ and the angle of incidence of the light beam on the reference surface θ! are equal. The light beams reflected by the test surface M1 and the reference surface M2 are combined into one light beam again by the second 7-fumir mirror HM2. To observe the interference fringes, the interference fringes can be seen by inserting the lens L2 into the light beam that has been recombined into one light beam by the second half mirror HM2 and placing the eye at its focal position. Alternatively, if a screen is placed at the position of the image plane M1' of the surface to be inspected M1 formed by the lens L2, interference fringes are projected. Furthermore, if the surface to be inspected is as accurate as a mirror used in a copying machine, then the screen can be inserted at a position not too far from the surface to be inspected... - Insert the screen directly into the light beam that has been combined into a single beam by the Fumirror HM2. In this case, the projected interference fringes can be regarded as interference fringes on the surface to be inspected and there is no problem. The method for observing interference fringes will be described in more detail later.

FIG. 3 shows another embodiment of the invention. Components corresponding to those in the embodiment shown in FIG. 2 are denoted by the same reference numerals, and description thereof will be omitted. The embodiment shown in FIG. 2 is configured so that the optical path lengths of the two beams from the first half mirror HMI to the second half mirror HM2 are the same. On the other hand, in the embodiment shown in FIG. 3, the distance from the HMI to the reference surface M2 is longer than the distance from the mirror HMI to the surface to be inspected M1i, and the incident angle of the light beam to the reference surface M2 is θ1. The angle of incidence of the light beam on the surface M1 to be tested is smaller than θ, and the angle from the first half mirror HMI to the second half mirror HM2-i is 2.
The optical path lengths of the light beams are made unequal. When using a laser as a light source, laser light has good coherence and clear interference fringes can be observed even if the optical path lengths of the two beams do not match exactly. By making it nearly perpendicular to the light beam, it can be made smaller than the surface to be inspected M1. In the embodiment of FIG. 2, the reference plane M
2 needs to have the same size as the surface to be inspected M1. Therefore, the third
In the example shown in the figure, the incident angle θ' of the light beam on the reference surface M2 is
This embodiment is advantageous over the embodiment of FIG. 2 in that the reference surface, which is smaller than the incident angle θ of the light beam, can be smaller than the surface to be measured.

4 to 6 show various modified embodiments regarding the arrangement of observation planes for observing interference fringes, and the one shown is applied to the embodiment shown in FIG. It goes without saying that this can also be applied to the embodiments.

In FIG. 4, a screen S is inserted perpendicularly to the light beam combined into one light beam by the second half mirror HM2 to serve as an observation surface. In such a configuration or in a configuration in which the lens L2 is made perpendicular to the light beam as shown in FIG. This makes it difficult to quantitatively measure the shape of the surface being tested. Fifth. The embodiment shown in FIG. 6 is an improvement on this point, and in FIG. Screen S was installed. In this way, the image of the test surface Ml by the lens L2 becomes parallel to the test surface Ml,
Therefore, the interference fringes on the screen S serving as the observation surface have the same magnification at both the a end and the b end. The embodiment shown in FIG. 6 is the lens L2.
Although the interference fringes are directly projected onto the screen S without using the screen S, the interference fringes can be observed at the same magnification over the entire surface by arranging the screen S parallel to the surface to be inspected M1.

FIG. 7 shows a further improvement of the embodiment shown in FIG. 6 of the coffin.

In order to be able to observe the interference fringes on the screen S brightly in the embodiment shown in FIG.
It is best to look at the light that has passed through the interference pattern, but in that case the distances at both ends of the interference fringes will be different for the viewer, making observation difficult. For this purpose, a Fresnel plate F was placed parallel to the surface to be inspected M1, and a screen S was placed parallel to and close to the back of this Fresnel plate. As shown in Fig. 8, the Fresnel plate F refracts an incident parallel light beam at a certain angle to form a parallel light beam that forms a certain angle with the incident light beam, and the Fresnel plate F refracts the incident light beam almost perpendicularly to the screen S. As shown by the arrow in FIG. 7, the interference filter is designed so that it can be observed brightly when viewed from a direction perpendicular to the screen storage S.

In practice, the reference surface M2 is a part of a spherical surface having a very large radius of curvature, and the interference fringes observed by the above-mentioned apparatus are usually concentric circles. Quantitative inspection of the shape of the surface to be inspected is performed by counting how many of these concentric circles are visible, but the interference fringes become more difficult to count as they become more spaced apart as they go outside the concentric circles. Therefore, in practice, a frame with a circular aperture of about 50 mm in diameter is placed on the observation surface, and when the center of the interference fringes matches the center of the frame, the number of concentric circles of interference fringes that can be seen within the frame is counted. The entire surface of the surface to be inspected is inspected by performing the operation at several locations while moving the center of the interference fringes on the observation surface. In order to move the center of the interference fringes on the observation plane, it is sufficient to slightly change the inclination of the reference plane M2 with respect to the incident light beam. FIG. 9 shows how interference fringes appear on the observation plane. mf is the circular frame mentioned above,
Figure 9A shows a state in which the center of the concentric interference fringes is shifted to the left, Figure 9B shows a state in which the center is shifted to the right, and Figure 9C shows a state in which the center is shifted to the right.In each case, the center of the circular frame mf is Count the number of concentric fringes within the frame based on the center of the interference fringes.

In Figure 10A, three concentric interference fringes are visible within the frame.
Figure B shows a state in which four hyperbolic interference fringes are visible.
This indicates that the surface to be tested is a hyperbolic paraboloid. In reality, it is easier to judge the fringe when two interference fringes appear within a frame with a diameter of about 50 mm, and when the interference fringes are viewed as contour lines, the difference in height between one fringe is 0°3 to 0.4 μm. In this case, it is appropriate that the reference surface has a radius of curvature of about 400 m.

FIG. 11 shows a mechanism for adjusting the inclination of the reference surface.

The mechanism consists of three parts, a, b, and c, where a is a fixed part and is fixed to the device body B, and a reference surface mirror (not shown) is attached to the bottom surface of C with the mirror surface facing downward. . Part b is attached to part a by a hinge l so as to be rotatable about the X-axis, and is pulled together by a spring 2 stretched between parts abb and b. Part 0 is rotatably attached to part b by a hinge 3 around the y-axis, and is pulled together by a spring stretched between b and c on the opposite side, which is not visible in the figure. Reference numeral 4 denotes a screw that passes through part a while screwing together, and its tip touches the upper surface of part b, and by moving this screw forward and backward, part b can be slightly rotated around the X-axis.

In addition, the tip of the screw that passes through 5 while screwing the part 5 is 0.
By moving this screw forward and backward, part 0 can be slightly rotated around the y-axis relative to part b.

FIG. 12 shows the guiding mechanism of the frame mf on the observation plane.

B is the main body of the interferometer, F is the Fresnel plate, S is the screen mounted on top of it, and the chain line is the screen S.
This shows the effectiveness of There are guide rails 6.6 on both sides of the screen S in the longitudinal direction, and the frame holding plate 7 is attached to the guide rails 6.
．． 6 so as to be slidable in the longitudinal direction on the screen S. The frame holding plate 7 has guide grooves 8, 8 in a direction perpendicular to the guide rail 6 on both edges, and the main part is cut out in a rectangular shape. Both edges of the frame mf are slidably fitted into guide grooves 8°8, and the circular opening faces the rectangular window of the frame holding plate 7, allowing it to move two-dimensionally on the screen S. . The interference fringes are observed from above in this figure.

With the above-mentioned configuration, the device of the present invention does not require a particularly large-diameter lens, etc., and can cover the entire surface of a relatively large rectangular mirror without having to slide the examination mirror. The reference surface (the surface to be inspected may also be used)
By making the inclination of the frame finely adjustable, by moving the center of the interference fringes on the observation surface, the shape of each part of the surface to be inspected can be observed continuously while making it easy to count the interference fringes.The frame is movable. Coupled with this, comprehensive quantitative evaluation of the surface to be inspected is very difficult and difficult.Also, if the observation surface is parallel to the surface to be inspected and a Fresnel plate is used, there will be no distortion (the magnification at both ends is equal). )
Effects such as being able to observe bright interference fringes from the vertical direction can be obtained.

[Brief explanation of drawings]

Fig. 1 is a plan view of a conventional example, Fig. 2 is a plan view of an embodiment of the present invention, Fig. 3 is a plan view of another embodiment of the invention, and Figs. 4 to 7 are Fig. 3. 8 is a partially enlarged sectional view of the Fresnel plate used in the embodiment of FIG. 7, and FIGS. 9 and 10 are plan views of various embodiments of the configuration of the observation surface in the embodiment of Front view illustrating how the stripes look, 1st
Figure 1 is a perspective view of the mechanism that finely adjusts the inclination of the reference plane;
The figure is a perspective view of the frame moving mechanism. Q...Point light source, Ll...Lens, HMI...1st
No. half mirror, HM2...second half mirror 1
M1...Test surface, M2...Reference surface, L2...Lens, M1'...Image of Ml by lens L2, S...
・Screen, F...Fresnel board, mf...frame. Agent: Patent Attorney Kosuke Figure 9 Figure 10 Figure 11

Claims (5)

[Claims] (1) After dividing the parallel light beam by the first nof mirror,
One of the divided light beams is made obliquely incident on the test surface, the other light beam is made incident on the reference surface, and the reflected light beams from each surface are combined into one light beam by a second nof mirror,
Surface shape measurement of a mirror surface, characterized in that the combined light beam is observed on an observation surface tilted by the same angle as the angle between the test surface and the incident light beam on the same surface. interferometer. (2) For measuring the surface shape of a mirror surface according to claim 1, characterized in that the angle of incidence and reflection of the luminous flux with respect to the reference surface is smaller than the angle of incidence and reflection of the luminous flux with respect to the test surface. Interferometer. (3) The interferometer for measuring the surface shape of a mirror surface according to claim 1, wherein the inclination of the test surface or the reference surface with respect to the light beam incident thereon is adjustable. (4) The interferometer for measuring the surface shape of a mirror surface according to claim 1, wherein a Fresnel plate is arranged on the observation surface. (5) An interferometer for measuring the surface shape of a mirror surface according to claim 1, which is provided with a circular frame so as to be movable along the observation surface.