JPH03156305A - Aspherical-shape measuring apparatus - Google Patents

Abstract

PURPOSE:To decrease errors by providing a means for scanning a surface to be mea sured in the direction of an optical axis, providing means for obtaining a position where a differential coefficient becomes zero with respect to the interference fringe obtained at each scanning position, and detecting a position where the curvature of an aspherical shape agrees with curvature of an incident spherical wave. CONSTITUTION:Coherent light outputted from a laser light source 1 is converted into parallel light through an optical system comprising a lens 2, a pinhole 3, a collimator lens 4 and the like. The light is split into two light beams through a beam splitter 5. One light beam is reflected from a reference mirror 8 and returned to the splitters as the reference light. The other light beam becomes a spherical wave through a 5 spherical-wave generating lens 6. The wave is projected on a material to be measured 7. The light reflected from the material to be measured 7 is returned to the splitter 5 as the measuring light. The light is overlapped with the reference light from the reference light 8, and interference fringes are observed on an observing plane 10. Then, the interference fringes are analyzed by a phase shifting method. A position where an aspherical shape agrees with the curvature of the incident spherical wave is obtained. The aspherical shape is obtained by using this position.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention is a method for determining the surface shape of optical components used in optical equipment, video equipment, etc., such as aspherical plastic lenses, molding molds, etc., by interferometric measurement. The present invention relates to an aspherical surface shape measuring device used for highly accurate contact measurement.

[Prior Art] FIG. 5 is a diagram showing the principle of a conventional aspheric shape measuring device using a Twyman-Green interferometer. Laser light source LS
The coherent light outputted from is converted into a parallel light beam by a collimator lens CL, and is split into two light beams by a beam splitter BS.

One of the light rays is converted into a spherical wave S by the spherical wave generating lens L, reflected by the aspherical surface to be measured T, and returns through the lens L to the beam splitter BS as measurement light.

The other beam is reflected by the plane reflecting mirror M and returns to the beam splitter BS as a reference beam. The measurement light and reference light cause interference, and the interference fringes are observed on the observation surface ■. Since interference fringes are caused by the optical path difference between the measurement light and the reference light, they are a figure in which the error (aspherical amount) of the surface T to be measured from a spherical surface is expressed by contour lines. When the laser light source LS is He-Ne, the pitch of the contour lines is one every λ/2=0.3L64μ plow. By analyzing these interference fringes, the aspherical shape can be measured.

By the way, when the bulb rffll (error from the spherical surface) of the surface to be measured T is large, the deviation between the spherical wave S and the surface to be measured T becomes large, and the interval between interference fringes becomes too small, making measurement impossible. It may happen. Therefore, Shunsuke Yokoseki et al.'s ``Aspheric shape measurement method using multiple interference patterns'' (Optics 1
2 (1983), pp. 296-300), the radius of curvature of the incident spherical wave is appropriately selected, and the surface to be measured is shifted in the optical axis direction to partially create areas with large interference fringes. A method has been proposed in which the overall shape of an aspheric surface is determined by linking the fringe analysis results.

[Problems to be Solved by the Invention] In the above-mentioned conventional technology, even when the curvature of the aspherical surface shape and the curvature of the incident spherical wave do not match, the incident light on the surface to be measured and its reflected light pass through the same optical path. It is assumed that However, there was a problem in that the larger the difference between the curvature of the aspherical shape and the curvature of the incident spherical wave, the more the incident light and reflected light no longer travel through the same optical path, resulting in a larger error.

The present invention was made in view of these points, and its purpose is to detect a position where the curvature of an aspherical surface shape matches the curvature of an incident spherical wave, and to detect only the position where this curvature matches. It is an object of the present invention to provide an aspherical surface shape measuring device that reduces errors by determining the aspherical surface shape using the above-described method.

[Means for Solving the Problems] In the present invention, in order to solve the above problems, the first
As shown in the figure, the coherent light is split into a reference light and a measurement light,
The measurement light is incident on the aspherical surface to be measured as a spherical wave,
An aspherical surface shape measuring device that measures the deviation of a surface to be measured from a spherical shape based on interference fringes between the reflected light and a reference light includes a means for scanning the surface to be measured in the optical axis direction, and a means for scanning the surface to be measured in the optical axis direction. and means for determining the aspherical shape of the surface to be measured based on the position where the differential coefficient determined at each scanning position is zero with respect to the interference fringes obtained. It is something to do.

[Operation] In the present invention, as described above, the surface to be measured is scanned in the optical axis direction, and the position where the differential coefficient becomes zero for the interference fringes obtained at each scanning position, that is, the curvature of the aspherical surface shape is determined. The position where the curvature of the incident spherical wave and the curvature of the incident spherical wave match is detected. At this position, the incident wave and the reflected wave pass through the same optical path, so errors can be reduced by determining the aspherical shape using only the position where the curvatures match.

[Embodiment] FIG. 1 shows the overall configuration of an embodiment of the present invention. This device is an application of the Twyman-Green interferometer to aspheric shape measurement. The coherent light output from the laser light source 1 passes through an optical system including a lens 2, a pinhole 3, a collimator lens 4, etc., is converted into a parallel beam, and is separated into two beams by a beam splitter 5. One of the light beams is reflected by a reference mirror 8 made of a flat reflecting mirror and returns to the beam splitter 5 as a reference light. The other light beam is turned into a spherical wave by the spherical wave generating lens 6 and is irradiated onto the object 7 to be measured. The light reflected from the object to be measured 7 returns to the beam splitter 5 as measurement light, overlaps with the reference light from the reference mirror 8, and is observed as interference fringes on the observation surface 10.

In the present invention, this interference fringe is analyzed by the phase shift method (also called the fringe scanning method) to find the position where the aspherical shape and the curvature of the incident spherical wave match, and this is used to find the aspherical shape. . Furthermore, when the amount of aspherical surface is large, the overall shape of the aspherical surface is obtained by connecting them.

First, the measurement principle for detecting the position where the curvature of the aspherical surface and the incident spherical wave match will be explained. Figure 2 is a conceptual diagram of the optical system used in the present invention (excerpted from page 95 of Obtronix's "High Precision Specular Shape Measurement Method"). In Figure 1, L is the lens for generating spherical waves, F is the focal position, f is the focal length, T is the aspherical surface to be measured, S is the incident spherical wave, ■ is the observation surface, and O is the center of the surface to be measured and the observation surface. In this figure, an incident spherical wave S indicated by a broken line is in contact with a surface to be measured T at a point Pi. Although the reference mirror and beam splitter are not shown in FIG. 2 for the sake of simplicity, they are a general Twyman Green interferometer. The phase distribution of the interference fringes appearing on the observation surface (2) is a phase difference distribution between the surface to be measured T and the incident spherical wave S. The 0 phase shift method, which uses the phase shift method (also called fringe scanning method) as a means of determining the phase distribution of interference fringes, is a method for measuring the phase of interference fringes with high precision. ” (Optics 13
(1984), pp. 55-65), it is a means of determining the phase through mathematical calculations using multiple interference fringe patterns obtained by changing the phase of the reference light by a known amount. For example, see ill 8 PZT
9, the four light intensities at one point on the observation surface are expressed as I3. I
2. I3. Assuming I4, the phase at that point can be calculated using the following equation.

W=(λ/4)jan-'((I2I4)
/ (I + I i)1 If this calculation is performed for all observation points on the observation plane, the phase distribution on the observation plane can be determined. Then, by analyzing the phase distribution of the interference fringes determined by the phase shift method, the position of the contact point Pi can be determined.

To explain the specific measurement procedure, first, if there is no aberration in the zero-spherical wave generation lens whose focal point is at the center of the aspherical object to be measured, an interference pattern with uniform brightness will be observed on the tm plane. Next, move the object to be measured in the optical axis direction,
Make sure to obtain coarse interference fringes near the center of the object to be measured. Let the amount of movement from the focal point at this time be 11,
Then, the reference mirror 8 is driven by the PZT 9 and the phase distribution of the interference fringes on the vA measurement surface is determined using the phase shift method, and the position where the differential coefficient of the phase distribution with respect to the observation surface direction (U-axis direction) is zero is determined. The interference fringes on the observation plane are determined as a phase distribution using the phase shift method, and the results are illustrated in FIG.

In Figure 2, one point P at which the concave object is measured is the contact point between the surface to be measured and the incident spherical wave, so the differential of the phase distribution of point Ui on the interference fringe corresponding to point Pi with respect to the observation surface direction is The coefficient should be zero. By utilizing this fact to find the position of point Ui, the coordinates of contact point Pi can be found using the following equation.

In FIG. 2, if the phase difference between the origin O and the point Ui is set as Ai, this A; represents the phase difference between the origin O and the point Pi, and
Angle θi between the incident wave and the reflected wave with respect to the point Pi and the optical axis
is as follows.

θ1-jan-'(U i/f)
-■ From this, the coordinates (Xi, Zi) of point Pi are as follows.

X1=(Ni-Ai) sinθ1 Zi=i'i (1i-Ai) eosθi
- ■ Next, the position Pi = (Xi, Zi
) to find the aspherical shape using only the surface to be measured T
is moved in the Z-axis direction N times, and the shape of the aspherical object is determined from the position of the contact point Pi determined from the phase distribution of the interference fringes each time.

Now, assume that the object to be measured is moved by a minute amount Iδi so that the contact point Pi moves to the outside of the object to be measured. Let the amount of movement from the focal point at this time be 11, (= 1: + δi), and the position where the differential coefficient of the phase distribution of the interference fringe is zero is the point U.
in, the corresponding position on the aspherical surface is point P i41,
Let the angle between it and the optical axis be θi++. From these, the coordinates of point Pi++ are determined using the 0 formula in the same way.

By moving the object to be measured N times by a minute amount δi and arranging the Pi (i=1 to N) obtained for each, the aspherical shape can be obtained. Also, the U axis passes through the origin,
Since it can be taken in any direction as long as it is perpendicular to the optical axis, an aspherical shape can be obtained three-dimensionally.

Furthermore, if the measured area of the aspherical surface is large and the amount of aspherical surface (error from the spherical surface) is large, the method of determining the overall shape of the aspherical surface is to connect adjacent measurement data. When the amount is large, the interference fringes become too fine and the phase difference Ai may not be measurable. At this time, δi is adjusted so that Ui is included in the coarse interference fringes before and after the point U i++, as shown in FIG. 4(b). The phase difference between this point Ui and point Di is B i+1
Anyway, point U! Point P i on the aspheric surface corresponding to 10+
i is determined centering on point Pi. Here, it is assumed that the position of point P; has been determined by the method described above.

In this case, the coordinates of point Pi+ and
++), then these coordinates can be obtained by the following equation, as is clear from examining FIG. 4(,).

Xi+, = Xi・cos(θi+1−θi)+ (&i
++Z i) ・5in(/i+, -θi)→
-B! +I' Sinθi.

Z i++-pi+l-B i+1・sinθ1++(
1i++ Z i) ・cos(1i++−θi)+
X1-sin (θi, -θ1) The interferometer used in the present invention may be any type of device as long as it can obtain a contour interference fringe diagram.・In addition to the Green type interferometer, measurements can be made using exactly the same principle as the Fizeau type interferometer, which is resistant to vibrations and is the most contradictory measurement interferometer.

[Effects of the Invention] According to the present invention, in an aspherical surface shape measuring device that uses optical interference to determine the deviation between an incident spherical wave and a surface to be measured, the surface to be measured is scanned in the optical axis direction, and each scan Since the positions where the differential coefficient becomes zero for the interference fringes obtained at each position are determined, and the aspherical shape of the surface to be measured is determined based on these positions, the curvature of the aspherical shape and the incident spherical wave can be calculated. This has the effect that errors due to differences in curvature do not occur.

[Brief explanation of the drawing]

Fig. 1 is an overall configuration diagram of an embodiment of the present invention, Fig. 2 is a diagram for explaining the measurement principle of the above, Fig. 3 is a diagram showing an example of the phase distribution on the observation plane of the above, and Fig. 4 Figure (,) is a diagram for explaining the principle of connecting the measurement results of the above.
Figure (b) is a diagram showing an example of interference fringes on the observation plane.
The figure is a schematic configuration diagram of a conventional example. 1 is a laser light source, 2 is a lens, 3 is a pinhole, 4 is a collimator lens, 5 is a beam splitter, 6 is a spherical wave generation lens, 7 is an object to be measured, 8 is a reference mirror, 9 is PZT
, 10 is the observation plane.

Claims (1)

[Claims] (1) Split the coherent light into a reference light and a measurement light, make the measurement light a spherical wave, and make it incident on the aspherical surface to be measured. Based on the interference fringes between the reflected light and the reference light, An aspherical surface shape measuring device for determining deviation from a spherical shape includes means for scanning the surface to be measured in the optical axis direction, means for determining the position where the differential coefficient becomes zero for interference fringes obtained at each scanning position, and each An aspherical surface shape measuring device comprising means for determining the aspherical shape of a surface to be measured based on a position where the differential coefficient determined at the scanning position becomes zero.