JPH03243804A - Shape measuring method for aspherical surface - Google Patents

Abstract

PURPOSE:To measure the shape of the aspherical surface which has a >=20mum asphericity quantity with high accuracy by finding the diameter of ring-shaped interference fringes from the amplitude of variation in the intensity of the fringes when a fringe scanning method is used. CONSTITUTION:Assuming that the surface S of a body 7 to be measured is aspherical and its quantity of asphericity is >=20mum as to a Fizeau's interferometer, a light beam which is made incident on the surface S to be measured travels back through its going optical path to reach the surface of an image pickup element, but other light beams do not travel back through their going optical paths, so none of them reach the image pickup element surface. Consequently, the interference fringes are formed in a ring shape. Then when a spherical prime standard 5 is moved in the direction of the optical axis to scan the stripes, the intensity of the interference fringes varies with the phase of reference light in a sine wave shape and the initial phase of this sine wave curve determines the phase of the interference fringes at a point on the surface to be measured. Further, the amplitude of the sine wave curve is found to measure the intensity of reflection from each point on the surface S to be measured, and the intensity is plotted to know the center part of the ring clearly.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method of measuring the shape of an optical aspheric surface using an interferometer.

[Prior Art] The use of aspherical lenses in optical instruments is very significant in reducing aberrations. In addition, recent advances in molding technology have made it possible to mass-produce aspheric lenses of the same shape, which has led to their use in many consumer products such as cameras.

Highly accurate measurement technology is essential to manufacturing highly accurate aspheric lenses. The shape measurement precision required for general aspherical lenses is very high, 0.1 μm or less, and commonly used three-dimensional measuring instruments cannot handle this, so a measuring instrument with higher precision is required.

On the other hand, laser interferometers, such as the Fizeau interferometer, which has been developed to evaluate spherical lenses, measure in units of wavelength of light, making it possible to measure with very high precision. It has the disadvantage that it can only measure objects with a narrow amount of asphericity (maximum deviation of the aspherical shape from the approximate spherical surface) of 20 μm or less, and cannot measure anything larger than that. This is because, as shown in Figure 5, when measuring the shape of an aspherical surface with very large aberrations from the spherical surface, the normal interferometry method in which a spherical wave is incident on the surface to be measured S, Since the light is no longer incident perpendicularly to all surfaces, the rays of the parts that are not perpendicularly do not return along the original optical path and do not reach the image sensor surface -E in the interferometer. It is. At this time, the interference fringes observed by the interferometer have a ring shape as shown in FIG. On the contrary, this ring-shaped interference fringe is where the light beam hits the object to be measured perpendicularly. Therefore, by measuring the position of the object, the diameter W of the ring, and the magnification of the interferometer, it is possible to determine the inclination of the surface S to be measured at the position of the light beam returning in the ring shape.

Therefore, by focusing on this point and sequentially changing the position of the surface to be measured S, measuring the diameter W of the ring at each point, calculating the slope, and connecting and integrating them, the shape of the surface can be determined. The method of measurement is “optics” (Vol. 12, No. 4, 198
It was announced in August 2018.

[Problems to be Solved by the Invention] In order to improve the individualization accuracy in the above measurement method, two steps are required: ■ Determine the diameter of the ring with more precision; ■ Determine the geometrical optical magnification of the interferometer with more precision. Two conditions are considered essential.

In contrast, the present invention calculates the diameter of the ring with high precision from the amplitude of the intensity change of the fringe instead of detecting the phase using the fringe scanning method.
It is an object of the present invention to provide a method for measuring the shape of an aspherical surface that can measure the shape of a large aspherical surface with an aspherical amount exceeding 20 μm with high precision.

1. Means for Solving the Problems] In order to achieve the above object, the present invention has an aspherical surface amount of 20μ.
Measure objects larger than m using a Fizeau interferometer! l! In the aspherical shape measurement method, which calculates the ring diameter of the resulting ring-shaped interference fringes, calculates the inclination of the object to be measured at each point, and measures the shape, the ring diameter is calculated using the fringe scanning method. This is determined from the amplitude of the change in intensity of the stripes when the temperature changes.

[Operation] In a Fizeau interferometer, when the spherical prototype is moved in the optical axis direction to scan the fringe, the intensity of the interference fringe changes sinusoidally with respect to the phase change of the reference light, and the initial phase of this sine curve gives the phase of the interference fringe at a certain point on the selected surface. Furthermore, by determining the amplitude of this sine curve, it is possible to measure the reflection intensity from each point on the surface to be measured, and by plotting this, the central part of the ring becomes clear.

[fruit! IN] Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 is a diagram of the optical system of a Fizeau interferometer used to explain the method for measuring the shape of an aspherical surface according to the present invention. In the figure, 1 is a light source, 2 is a diverging lens, 3 is a half mirror, 54 is a collimator lens, 5 is a spherical prototype, and 6
is a driving device such as a piezo element that moves the spherical prototype 5 in the optical axis direction; 7 is the object to be measured; 8 is a variable spatial filter; 9 is an imaging lens; This is the surface to be measured which is an aspherical surface of the object 7 to be measured. As the light source 1, a He--Ne laser, for example, is used as one that emits visible, infrared, or ultraviolet light beams with good coherence.

The object to be measured 7 is arranged so that the center of curvature of the surface S to be measured coincides with the position of the focal point F of the spherical prototype 5.

In the Fizeau interferometer with such a configuration, the monochromatic laser beam with a wavelength λ emitted from the light source (2) becomes diverging light by the diverging lens 2, and the half mirror 3
After being reflected by the collimator lens 4 and made into parallel light parallel to the optical axis again, it passes through the spherical prototype 5 and illuminates the surface S to be measured of the object 7 to be measured. At this time, part of the light beam is reflected by the reference surface R of the spherical prototype 5, and the rest is reflected by the surface S to be measured. The wavefronts of these reflected lights have shapes corresponding to the shapes of the reference surface R and the surface to be measured S, respectively.

The two reflected lights reflected by the reference surface R and the measured surface S are superimposed on each other by returning along the same optical path, pass through a collimator lens 4, a half mirror 3, and a variable spatial filter 8, and are sent to an imaging device by an imaging lens 9. By being imaged on the No. 10 imaging surface, an interference image consisting of interference fringes based on mutual interference of re-reflected light is formed. Then, by analyzing the interference fringes, the surface accuracy of the surface to be measured S is measured.

During measurement, the driving device 6 vibrates the spherical prototype 5 in the optical axis direction to change the distance between the spherical prototype 5 and the object to be measured 7, and the reflected light from the reference surface R and the m measurement surface S are The reading accuracy is improved by changing the optical path difference between the reflected light and the reflected light, and this method is called a fringe-scan interferometry.

Note that such a Fizeau type interferometer itself is exactly the same as a conventional device.

Now, in the Fizeau type interferometer having the above configuration, if the surface to be measured S is an aspherical surface and the amount of asphericity is 20 μm or more, the incident light is perpendicular to the surface to be measured S as shown in FIG. Light ray a immediately returns along the optical path it came from and reaches the surface of the image sensor, but other light rays do not immediately return along the optical path they came from,
It does not return to the surface of the image sensor. As a result, the interference fringes become ring-shaped as shown in FIG. The width of this ring diameter is varied by a variable spatial filter 8 within the interferometer.

On the other hand, the diameter W of the ring changes depending on the position of the object to be measured (Lp in FIG. 2).

On the other hand, assuming that the magnification of the interferometer is constant, the diameter of the ring (2Xp"
The ray angle ep with respect to > is expressed in a fixed relationship. Furthermore, the beam angle ep and the inclination of the surface to be measured at point P match. Therefore, the relationship between the beam angle ep and the diameter W of the ring is measured in advance, and the diameter W of the ring is measured while measuring the position Lp of the prototype focal point F on the surface S to be measured, and the inclination at each point is determined. , by integrating the results, it is possible to obtain the actual shape.

Here, the ring diameter (2Xp) is converted by the pitch on the image sensor or frame memory. Generally, these prime numbers are 512×512 or less and are very coarse. Also, Figure 3 (
As shown in a) and (b), there is a lot of large noise contained within the ring itself and outside the ring, making it very difficult to detect it.

Therefore, in the present invention, by using the fringe scanning method described above, this ring is extracted from the noise and the diameter of the ring is determined with high accuracy.

Generally, the fringe scanning method attempts to detect the phase state more accurately by scanning the interference fringes with an interferometer.

FIGS. 4(a), (b), and (C) are diagrams for explaining the principle of the fringe scanning method. In order to change the optical path length of the reference light,
- A piezo element 6 is attached to the prototype 5, and the prototype 5 is moved along the optical axis with an accuracy of 1/100 to 1/1000 wavelength. Now, when a voltage is applied to the piezo element 6 and the phase of the reference light is changed, the intensity of the interference fringes at one point A on the surface to be measured S is the same as the phase of the reference light (
b) Changes as shown below. At one point of the interference fringe, the intensity of the interference fringe changes sinusoidally with respect to a change in the phase of the reference light, and the initial phase of this sinusoidal curve gives the phase of the interference fringe at point A. If there is a difference in height between points A and B, their initial phases will be different. The difference from the heterodyne interferometry is that the heterodyne interferometer takes the beat of the reference signal and measurement signal and detects the phase at each point, whereas the fringe scanning method changes the reference phase in stages and detects the phase at each point. The point is that the fringe intensity at a point can be measured simultaneously using a two-dimensional photodetector such as a television camera.

In short, the fringe scanning method is a scan in which either the reference surface of the prototype 5 or the surface to be measured S is moved in the optical axis direction, and the change in intensity at each point of the interference fringe at that time is applied to a sine wave. This is intended to more accurately determine the initial phase P.

However, the present invention is characterized in that the diameter of the ring is measured by finding the amplitude I instead of finding this initial phase.

For example, assume that the reference plane is scanned by λ/2 (λ: wavelength), and four interference fringes are captured at equal intervals (λ/4 in optical path length) during the scan. Generally, if the intensity at one point of the four interference fringes changes as ABCD (FIG. 4(b)), the initial phase P is calculated as -D.

The square of the amplitude ■ is calculated as I2=(A-C)2+(B-D)2.

When this I2 is calculated at each point and plotted, the center of the ring can be clearly seen as shown in FIG. 4(b).

This is because the square of the amplitude is at its maximum when the intensities of the two interfering light rays are equal. If the reflectance of the reference surface and the surface to be measured are equal, the light beam reflected from the surface to be measured S will not be cut by the variable spatial filter 8 (see Figure 1). The intensities of the two light rays are closest to each other at the center of the target ring), and the square of the amplitude is maximum.Of course, there is also a part in the center of the individualization surface S where the light rays hit perpendicularly. The ring diameter can be accurately determined by removing only the portions where the amplitude f is large and determining the center of the maximum value using the method of least squares according to the strength of the square of the amplitude.

When the reflectance of the prototype 5 is greater than the reflectance of the object to be measured 7,
This prototype 5 has no problems at all. On the other hand, if the reflectance of the object to be measured 7 increases, the amplitude will decrease slightly at the point where the light beam is reflected perpendicularly (center of the ring), so a pellicle-shaped ND filter or the like is placed between the reference surface and the individual surface. It is preferable to include it.

[Effects of the Invention] As described above, the method for measuring the shape of an aspherical surface according to the present invention uses the fringe scanning method when measuring an aspherical surface of an object to be measured with an asphericity of more than 20 μm using a Fizeau interferometer. Since the diameter of the ring-shaped interference fringes is determined from the amplitude of the change in the intensity of the fringes when the ring is located, the inclination of the surface to be measured at that position can be determined by determining the ring diameter. Therefore, it is possible to accurately measure the shape of an aspherical surface by moving the object to be measured along the optical axis, determining the diameter of each ring, calculating the inclination, connecting these, integrating them, and converting the coordinates. can.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the optical system of a Fizeau interferometer used to explain the method for measuring the shape of an aspherical surface according to the present invention;
Figure 2 shows the measurement principle of the ring detection method, Figure 3 (a
), (b) are diagrams showing noise-containing interference fringes and their intensity, Figures 4 (a), <b), and (c) are diagrams for explaining the principle of the fringe scanning method, and Figure 5 is a diagram showing the interference fringes containing noise and their intensity. FIG. 6 is a diagram for explaining the conventional measurement of an aspherical shape when the amount of aspherical surface is large, and is a diagram showing ring-shaped interference fringes. l...Light source, 3...Half mirror 15/spherical prototype,
7... Object to be measured, 8... Variable spatial filter, 10...
...Image sensor, S...Measurement surface, W...Ring diameter. Figure 1

Claims (1)

[Claims] An object to be measured whose asphericity exceeds 20 μm is observed with a Fizeau interferometer, the ring diameter of the resulting ring-shaped interference fringes is determined, the inclination of the object at each point is calculated, and the shape of the object is measured. 1. A method for measuring the shape of an aspherical surface, characterized in that the ring diameter is determined from the amplitude of a change in the intensity of a fringe when a fringe scanning method is used.