JPS63210605A - Apparatus for measuring configuration of optical surface - Google Patents

Abstract

PURPOSE:To perform measurement with good accuracy, by changing over an interference system by opening and closing a plurality of the shutters in a beam path when the non-spherical degree of the surface of an object to be measured is large and when the non-spherical degree is small or said surface is flat and spherical. CONSTITUTION:When a lens 58 has a non-spherical surface, only a shutter 58 is closed and a shearing interferometer is constituted. The reflected beam emitted from a laser beam source 1 to irradiate the lens 58 is split into two beams by a splitter 59 through a splitter 53, and the measuring beam and the reference beam respectively pass through mirrors 62, 61 to reach a TV camera 67 through a lens 66. The mirror 62 is minutely vibrated by a means 64 to perform fringe scanning. When the lens 58 has a spherical surface only a shutter 57 is closed and a fringe scanning Twyman-Green interferometer is constituted. The reflected beam split by the splitter 53 is reflected by a mirror 69 to become reference beam and transmitted beam is reflected by the lens 58 to become data beam; both beams reach the TV camera 67. The mirror 69 is minutely displaced by an electrostriction element 70 to perform fringe scanning. By this method, measuring accuracy is enhanced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention enables non-contact, high-speed, high-precision measurement of soft optical surface shapes, such as those of aspherical plastic lenses, by interferometric measurement. This invention relates to an optical surface shape measuring device.

[Conventional technology]

As a conventional method, a fringe scanning shearing interference method has been proposed that can measure the surface shape of an aspherical surface of an object with high precision (for example, Nikkei Mechanical, February 28, 1983 issue, pp. 34 et seq.). This method creates a measurement wavefront whose shape is to be measured, which is the reflected light from the object to be measured, and a reference wavefront, which is shifted horizontally with respect to the traveling direction of the light, and the optical path length of the light that makes one wavefront. , measure the phase difference at each point in the interference region of both wavefronts, and add this phase difference,
This is a measurement method that specifies the shape of the measurement surface.

FIG. 7 shows an example of a proposed surface shape measuring device using the fringe scanning shearing interference method. Hereinafter, the outline of the surface shape measuring method will be briefly explained using this example of the apparatus, and the problems to be solved by the present invention will also be explained.

In FIG. 7, reference numeral 1 is a laser oscillator, 2 is a beam expander, 3 and 4 are beam splitters, and 6 is a beam splitter.
1 is a plane mirror, 5 is an objective lens, 7 is a piezoelectric element, 8 is a parallel plate, 9 is a beam splitter, 10 is an imaging lens, 11 is an area sensor, and 12 is an object to be measured.

The laser beam emitted from the laser oscillator 1 becomes a parallel beam of light by the beam expander 2, passes through the beam splitters 3 and 4, and then passes through the objective lens 5, where it is once condensed and then becomes a divergent beam of light. , enters the shape measurement surface of the object to be measured 12 and is reflected. The reflected light from the surface to be measured is called information light because it contains information about the surface whose shape is to be measured.

If the wavefront of the spherical wave when it is incident on the surface to be measured of the object to be measured 12 is WO(X)*the surface to be measured is Wt(x), then W(x) for the wavefront W(x) of the information light mentioned above. Wo(X)=2(Wz(x)We(x))−(1). Since Wo(x) can be known according to the setting conditions of the optical system, the wavefront shape W(x) of the information light on the measured surface
By knowing, the shape Wz(x) of the surface to be measured can be known.

Wl (X)= (W(X)+WQ (X))
-(2) Now, the information light is separated by the beam splitter 4 into two light beams, that is, a measurement light and a reference light. Which one is called the measurement light and which one is called the reference light is arbitrary, but here, for convenience, it is transmitted through the beam splitter 4, and the beam splitters 3, 9, . The light that reaches the area sensor 11 via the imaging lens 10 is called measurement light, is reflected by the beam splitter 4, and is reflected by the plane mirror 6. Parallel plate8. Beam splitter9. The light that reaches the area sensor 11 via the imaging lens 10 will be referred to as reference light. When the reference light passes through the parallel plate 8, its traveling direction is shifted by a small distance in the lateral direction. Therefore, the measurement light and the reference light are shifted from each other and overlap each other on the light receiving area of the area sensor 11, and interference fringes are formed due to interference in the overlapped portion. The amount S by which the measuring light and the reference light deviate by a small distance on the area sensor 11 is called the share amount.
If the wavefront reproduced by the measurement light in the light receiving area of 1 is W(x), the wavefront reproduced by the reference light is W(x+S)
The phase difference ΔW(x) between the two wavefronts is therefore,
When ΔW (x) is known, W (x) becomes fΔW(x)d
x=s-W (x) ... (4) W(x
)=-fΔW(x)dx...(5)
It can be obtained by the following calculation.

Now, since the measurement light and the reference light are coherent, interference fringes are generated by interference with each other on the area sensor 11 as described above. Using this interference fringe, the above phase difference ΔW
(x) can be known. That is, by changing the voltage applied to the piezoelectric element 7, the optical path length of the reference light can be changed to λ/N(
λ is the wavelength of the laser beam) and is changed in N steps.

When the optical path length of the reference light is changed in this way, the optical path length of the reference light is changed accordingly.

The pattern of interference fringes on the area sensor 11 changes.

Therefore, each time the optical path length changes by one step, the light intensity IJ(x) of the interference fringe at each point on the area sensor 11 (J=1~
The phase difference ΔW(x) is determined by measuring the phase difference ΔW(x) using a linear equation.

J=IN...(6) In this way, if the phase difference ΔW (x) is known, by integrating this, the wavefront W(x) can be found, and from this wavefront shape, the The surface shape of an object can be determined using equation (2). In practice, measurements are performed by shearing in the X direction and by shearing in the Y direction perpendicular to the X direction, and the surface shape of the object to be measured is comprehensively specified from the results of the single measurement. The above is an overview of surface shape measurement using the fringe scanning shearing interferometry method.

[Problem that the invention seeks to solve]

The above conventional technology has a parallel plate 8 for sharing in one optical path.
Therefore, there was a problem in that the uniformity of the refractive index of the parallel plate 8 and disturbance of the wavefront due to birefringence adversely affected the measurement accuracy. Furthermore, although measurement is possible even if the surface shape of the object to be measured is a plane, two spheres, or a non-test surface, this measurement method obtains differential information of the object as the primary measurement value. Information on the surface shape to be measured can only be obtained by integrating this. For this reason, when the surface shape to be measured is flat or spherical, or when the degree of asphericity is small, interferometric measurement methods that can directly measure these are compared to, for example, Fizeau type or Twyman type interferometric measurement methods. However, the fringe scanning shearing interferometry method has a problem of slightly inferior accuracy.

In view of the above, an object of the present invention is to prevent a decrease in accuracy due to wavefront disturbances, and to use a fringe scanning shearing interference method to measure highly aspherical surfaces, and to measure flat, spherical, and aspherical surfaces. It is an object of the present invention to provide a novel optical surface shape measuring device that can perform measurement using a fringe scanning Twyman interference method when measuring a small degree.

[Means for solving problems]

The above purpose is to generate shearing by moving the mirror 6 in the optical axis direction, excluding the parallel plate 8 for shearing, and to eliminate the tilt error with respect to the optical axis caused by moving the mirror 6 as a new standard. By interfering with the plane waves caused by the reflected light from the surface, it is possible to prevent a decrease in measurement accuracy due to disturbance of the wavefront. In addition, there are two optical path switching shutters that switch between the fringe scanning shearing interference method and the fringe scanning Twyman interference method with a single touch.[Function] The tilt error caused by the movement of the shearing mirror interferes with the reflected light from the reference surface. and operate the shearing mirror to correct the tilt error.

As a result, the operation of the shearing mirror does not cause disturbance of the wavefront, resulting in high precision.

The optical path switching shutter operates to switch between the fringe scanning shearing interference method and the fringe measurement Twyman interference method with a single touch. As a result, aspherical shapes can be measured using the fringe scanning shearing interferometry method, and flat shapes,
Since the spherical shape can be measured using the fringe scanning Twyman interference method, it is possible to perform measurements with high accuracy depending on the surface shape to be measured.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. First, the overall configuration will be explained. 51 is a laser oscillator that emits a laser beam; 52 is a beam expander that expands the laser beam from the laser oscillator 51; 53° and 59.60 are beam splitters; 155 and 513° are shutters that block the laser beam as necessary; 54 is an objective lens, 58 is an aspherical lens to be measured, 61.62 is a mirror, 63 is a finely adjustable mirror holder for tilting the mirror 62, 64
65 is an electrostrictive element that finely moves the mirror 62 to scan the fringe; 66 is a lens that forms an image of interference fringes on the imaging surface of the TV camera 67; 68 is an image processing processor.

Next, the operation of the above-mentioned overall configuration will be explained starting with the operation of the shearing interferometer. When the object to be measured 58 is an aspherical lens, a shearing interferometer configuration as shown in FIG. 1 is used, for example. That is, only 55 of the shutters 55, 56, and 57 are closed, and shutters 56 and 57 are retracted from the optical path.

The object to be measured 58 is set as shown in the figure, and the laser oscillator 51 emits a laser beam. The diameter of this laser beam is expanded by the beam expander 52, and the beam becomes a parallel beam having a predetermined beam diameter and enters the beam splitter 53. Here, the zero reflected light component, which is divided into a reflected light component and a transmitted light component, is blocked by the shutter 55 and is not related to subsequent operations. The transmitted light component is once condensed by the objective lens 54, becomes a divergent light beam, enters the shape measurement surface of the object to be measured 58, and is reflected by the shape measurement surface. This reflected light passes through the lens 54, is reflected by the beam splitter 53, and is separated into two beams by the beam splitter 59. One of the separated light beams (referred to as measurement light) is reflected by a mirror 62, transmitted through a beam splitter 60, and reaches the imaging tube surface of the TV left camera 7 through an imaging lens 66. The other of the separated light beams (this will be referred to as the reference light) is sent to the mirror 61. The beam is reflected by the beam splitter 60 and reaches the imaging surface of the TV left camera 7 through the imaging lens 66.

The wavefront shape of each light beam in the light receiving area is similar to the shape measurement surface of the object to be measured 58 . The magnification of the imaging system consisting of the objective lens 54 and the imaging lens 66 gives a ratio of the size of the shape measurement surface of the object to be measured 58 and the wavefront shape, so by measuring the wavefront shape, the object to be measured 58 The shape of can be specified. The actual shape measurement is performed as follows. The position of the mirror 62 is shifted by a minute distance in the direction orthogonal to the optical axis by the fine movement mechanism 64, and the wavefront of the reference target and the wavefront of the measurement light are shifted by a small distance to the TV.
Interference fringes are generated on the imaging surface of the left camera 7 where the two light beams overlap. The electrostrictive element 65 performs fringe scanning by changing the optical path length of the measurement light by very small distances, and the image processing processor 68 measures the light intensity of each point in the interference region using the interference fringe image from the TV left camera 7. Perform predetermined calculations on the measured values (
(6)) to obtain the phase difference ΔW(x), which is converted into (5
) The surface shape of the object to be measured 58 is specified by integrating according to the equation.

Next, a method for correcting a tilt error with respect to the optical axis caused by moving the mirror 62 by the fine movement mechanism 64 will be described with reference to FIGS. 2, 3, and 4.

After moving the mirror 62 by the fine movement mechanism 64 from a position where the optical path length of the measurement light and the optical path length of the reference light are equal to give an arbitrary share, the shutter 56 is closed from the configuration shown in FIG.
The shutter 55 is opened to create the configuration shown in FIG. 2.

That is, the light reflected from the object to be measured 58 is blocked, the light reflected by the beam splitter 53 is made incident on the mirror 69, and the reflected light is used to correct the optical system. It is assumed that the axis and the optical axis of the reflected light of the mirror 69 are aligned. The light incident on the mirror 69 is reflected, passes through the beam splitter 53, and is split into a reflected light component and a transmitted light component by the beam splitter 59.

The reflected light component is reflected by the mirror 61. The beam passes through a beam splitter 60 and an imaging lens 66 to reach the imaging surface of the TV left camera 7.

The transmitted light component is transmitted through the mirror 62. beam splinter 60. It passes through the imaging lens 66 and reaches the imaging surface of the TV left camera 7.

Both light fluxes that reach the imaging surface cause interference. After the mirror 62 is moved by the fine movement mechanism 64 in the sharing interference configuration shown in FIG.

Shutter 5: Move the z 56 and switch the optical path to the configuration for performing the correction shown in FIG. At this time, the interference fringe pattern shown in FIG. 3 is 5i! If this is detected, the movement of the mirror 62 by the fine movement mechanism 64 means that the mirror 62 is tilted with respect to the optical axis. This causes a difference in the optical path length between when the mirror 62 is aligned with the optical axis and when it is not aligned, which causes an error in the shape measurement value. So mirror 62
Fine movement with the fine adjustment mirror holder 63 that is attached: I
Then, the inclination is corrected so that the interference fringe pattern shown in FIG. 4 is obtained.

Next, measurement when the surface shape of the object to be measured 58 is spherical will be explained. In this case, the shutter 57 is closed and the configuration shown in FIG. 5 is arranged.

In this way, the optical system as a whole becomes a fringe-scanning Twyman-Green type interference device. The beam diameter of the light from the laser oscillator 51 is expanded by the beam expander 52 and enters the beam splitter 53. Here, the light is separated into a reflected light component and a transmitted light component. The reflected light component is mirror 69
It is reflected by the beam splitter 53, transmitted through the beam splitter 59, the mirror 61 . Beam splitter-60
is reflected and enters the imaging surface of the TV left camera 7 via the imaging lens 66. The light reflected by the mirror 69 becomes the reference light in the Twyman-Green type interference measurement method.
The wavefront is a plane. On the other hand, the transmitted light component is once condensed by the objective lens 54 and then becomes a diverging light beam and is incident on the shape measurement surface of the object to be measured 58 . At this time, the focal point of the objective lens 54 and the center of curvature of the surface to be measured are made to coincide.The reflected light from the object to be measured 58 becomes information light and passes through the objective lens 54 in the opposite direction. The light is reflected by the splitter 53 and travels along the same optical path as the reference light described above, reaching the imaging surface of the TV camera 67. Since the information light and the reference light interfere on the imaging surface, the electrostrictive element 70 moves the mirror 69 minutely in the optical axis direction with high precision to perform fringe scanning, thereby determining the surface shape of the object to be measured 58. Measure spherical dust.

FIG. 6 shows another embodiment, and articles having the same reference numerals as those in FIGS. 1, 2, and 5 are the same articles. Reference numeral 71 denotes an optical flat with a reflective film 72, and reference numeral 73 denotes a hollow cylindrical electrostrictive element which is integrated with the optical flat 71. By changing the voltage applied to the electrostrictive element 73, the optical flat 71 can be displaced with high precision in the optical axis direction, that is, in the direction perpendicular to the reflective film. optical flat 71
An optical system 74 consisting of an electrostrictive element 73 and an electrostrictive element 73 can be moved between a state shown by a solid line and a drying state shown by a broken line by an optical system moving means (not shown). The moving means may be used as appropriate, and the movement may be manual or automatic.For measurement using the fringe scanning shearing interference method, the optical system 74 is moved to the state shown by the broken line and retreated from the optical path. Also, the shutter 57 is opened. With such a configuration, it becomes the same configuration as the fringe scanning sharing interference method described in FIG. 1. Therefore, a description of the operation of the sharing interference method according to this configuration will be omitted. When the optical system 74 is set to the state shown by the solid line and the shutter 57 is closed, the optical system as a whole becomes a fringe scanning Fizeau type interference measuring device, and has a configuration for measuring spherical dust when the surface shape of the object to be measured 58 is spherical. When light from the six-laser oscillator 51 enters the optical system 74, part of the incident light is reflected by the reflective film 72 of the optical flat 71, and the rest is transmitted. The light reflected by the reflective film 72 of the optical flat 71 becomes reference light in the Fizeau interference measurement method, and its wavefront is flat.

This reference light passes through the beam splitters 53 and 59, the mirror 61, and the beam splitter 60, and then passes through the objective lens 66.
The light enters the imaging surface of the TV camera 67 through the. On the other hand, the light transmitted through the optical flat 71 is once condensed by the objective lens 54 and then becomes a divergent light beam and enters the shape measurement surface of the object 58 to be measured. At this time, the light condensing point by the objective lens 54 and the center of curvature of the surface to be measured are made to coincide, and the reflected light from the object to be measured 58 becomes information light and is reflected by the objective lens 54.
4 is transmitted in the opposite direction, and is further reflected by the beam splitter 53, following the same optical path as the reference light mentioned above, and reaches the imaging surface of the TV camera 67, where the information light and the reference light are transmitted. Therefore, the optical flat 71 is minutely displaced in the optical axis direction by the electrostrictive element 73 to perform fringe scanning and measure the spherical dust on the surface shape of the object to be measured 58.

Note that drive control of the electrostrictive elements 65, 70, and 73.

Fine movement mechanism 64. Drive control of the finely adjustable mirror holder 63,
Control of the position of the object to be measured 58, control of the attachment attitude, calculation processing for the output of the TV camera 67, etc. can all be performed by a computer.

As described above, according to this embodiment, by switching the optical path switching shutter, it is possible to instantly switch between the fringe scanning sharing interference method and the fringe scanning Twyman interference method, or between the fringe scanning sharing interference method and the fringe scanning Fizeau interference method. I can do things. As a result, when measuring aspherical surfaces, it is possible to measure using the fringe scanning shearing interferometry method, and when measuring planar and spherical surfaces, it can be measured using the fringe scanning Twyman interferometry method or the fringe scanning Fizeau interferometry method. , it is possible to perform measurements with high precision. or,
In measurements using the fringe scanning shearing method, accurate measurements can be made by correcting the inclination of the shearing mirror during shearing.

〔Effect of the invention〕

According to the present invention, a novel surface shape measuring device can be provided. This measurement device instantly switches between the fringe scanning shearing interference method and the fringe scanning Twyman-Green interference method, or the fringe scanning shearing interference method and the fringe scanning Fizeau interferometry method, and when the surface shape to be measured is aspheric, the fringe scanning method is used. If the surface is a flat or spherical surface, it can be measured using the shearing interference method, the fringe scanning Twyman-Green interference method, or the fringe scanning Fizeau interference method, so it is possible to perform measurements with high accuracy depending on the surface shape to be measured. or.

In the measurement using the fringe scanning shearing method, since the tilt of the shearing mirror is corrected during shearing, the measurement can be performed with high accuracy.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the configuration of an embodiment of the present invention when measuring an aspherical surface shape, Fig. 2 is a configuration diagram showing tilt correction of the shearing mirror, and Fig. 3 is an interference pattern when the shearing mirror is tilted. 4 is a diagram showing the interference fringe pattern after correction of the shear mirror, FIG. 5 is a diagram showing the configuration when measuring the spherical shape, FIG. 6 is a diagram showing the configuration of another embodiment, and FIG. 7 is a diagram showing the configuration of another embodiment. The figure is a diagram for explaining surface shape measurement using the conventional fringe scanning shearing interference method. 1.51... Laser oscillator, 2,52... Beam expander, 3, 4, 9, 53, 59, 60... Beam splitter, 5, 54... Objective lens, 6°6
1.62,69...Mirror, 7,6.5,70°73
...Electrostrictive element, 10.66...Imaging lens, 11°67.TV camera, 12.58-.Object to be measured, 55°56.57. ...Shutter, 63...Fine adjustment type mirror holder, 64...Fine movement mechanism, 68...Image processing processor, 71...Optical flat, 72...
・Reflection Figure 1 Figure 2 (z Mirror Figure B Figure 3 Kaya 6 Figure 7F t2 3tL Shigairiashi

Claims (1)

[Claims] 1. A first optical system for measuring the optical surface shape of the object to be measured using the fringe scanning shearing interference method, and a second optical system for measuring the optical surface shape of the object using the fringe scanning Twyman-Green interference method or the fringe scanning Fizeau interference method. It is characterized by having an optical system, a shutter and optical system moving means for switching between the first optical system and the second optical system, and means for correcting the inclination of the shearing mirror in the fringe scanning shearing interference method. Optical surface shape measuring device.