JPH0132921B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the radius of curvature of the surface of an object and a radius of curvature measuring device used to carry out this method, and particularly to a method for measuring the radius of curvature of a surface of an object. The present invention relates to a method and apparatus capable of measuring the radius of curvature of each minute region almost in a non-contact manner so as to prevent the occurrence of curvature.

A very common measurement method for measuring the radius of curvature of an object's surface is to measure the distance from a common reference plane for each of three points on the object's surface, find an arc or spherical surface that passes through these three points, and then This is a method to obtain the radius of curvature of an object surface area surrounded by three points. In this method, if the interval between the above three points is too small, it becomes difficult to calculate the radius of curvature, and if the interval between the above three points is too large, it is fine when the object to be measured is a sphere with a right circular cross section, but it is difficult to calculate the radius of curvature if the interval between the above three points is too large. An accurate radius of curvature cannot be obtained for objects consisting of . Depending on the shape of the object to be measured, it may not be possible to directly measure three points on its surface. Furthermore, depending on the properties of the object to be measured, when measuring the height and horizontal distance from the reference plane at three points on the surface of the object, bringing the probe into contact with these three points may cause scratches on the surface of the object. I don't feel well because I have to wear it.

In recent years, lightweight and inexpensive plastic lenses have come to be used in optical instruments instead of glass lenses, and their shape accuracy depends on the surface shape precision of the plastic lens mold cavity. Unlike other plastic products, the shape accuracy of the lens can be said to be equivalent to the accuracy of the radius of curvature. In particular, in the case of an aspherical lens, the radius of curvature at each position and point on the lens surface is different, and it is necessary to have a predetermined and accurate radius of curvature at each position. In addition, the lens mold surface is generally finished to a mirror surface with a surface roughness of approximately 0.02 μm Rmax, and scratches and dirt must be avoided at all costs.When inspecting and measuring the curvature of the mold surface, contact This measurement method is inappropriate.

In view of these problems, it is an object of the present invention to provide a method for easily measuring the radius of curvature of an arbitrarily shaped curved surface in a minute area without contact.

Another object of the present invention is to provide a method for measuring the surface accuracy of a surface with an infinite radius of curvature, that is, the flatness of a flat object, since there is no limit to the measurable range of the radius of curvature.

A further object of the present invention is to provide a non-contact optical radius of curvature measurement device that can be used directly to carry out the measurement method described above.

The above object of the present invention is to vertically irradiate a point to be measured with a laser beam that has passed through a circular aperture, to detect the radius of the 0th order diffraction image in the diffraction pattern of the reflected light, and to This is achieved by analyzing the relationship between the point and the radius of curvature.

Therefore, according to the present invention, a laser beam is projected onto the measured point from the normal direction of the measured point through a circular opening, and the 0th order diffraction image of the reflected light from the measured point by this projected light is calculated. Detect the radius value, and use _{this detected} radius value to calculate the radius of curvature _R of _{the point} to be measured _. Distance to the point to be measured, l ₂ ; Distance between the point to be measured and the imaging plane of the 0th-order diffraction image, a; Aperture diameter of the circular aperture, λ : Wavelength of the laser beam, r ₂ ; A radius of curvature measurement method is provided, which is characterized in that it is determined from a detected radius value.

Further, the measuring device used to carry out the method of the present invention includes a laser beam generation source, a circular aperture plate disposed between the object to be measured and the laser beam generation source, and a circular aperture plate disposed between the object to be measured and the circular aperture plate. a first semi-transparent mirror disposed at an angle between the circular aperture plate and the laser beam generation source, and a second semi-transparent mirror disposed at an angle between the circular aperture plate and the laser beam generation source; a device for detecting a reflection diffraction image of a measured point;
a reference optical system that projects a reference beam reflected by a semi-transparent mirror to the measured point and receives the reflected light, and a projection angle and a reflection angle of the projected light and the reflected light of the reference optical system; The object to be measured is positioned so that the values are equal.

First, the measurement principle of the present invention will be explained with reference to FIGS. 1 and 2.

FIG. 1 is a diagram showing the principle of measuring the radius of curvature according to the present invention. 1 is a surface to be measured; 2 is a circular aperture plate in which a circular aperture 4 having a diameter a is bored at the center of an optical axis 3; 5 is a laser light source with wavelength λ; 6 is projection light projected from the laser light source 5 onto the surface to be measured 1 through the circular aperture 4; and 7 is light reflected from the surface to be measured 1 by the projection light 6.

A circular aperture of diameter a is expressed as follows.

Here, √ ² + ² a/2→1 √ ² + ² > a/2→0 Now, xy at the circular aperture position perpendicular to the optical axis
Take polar coordinates on the x ₀ y ₀ plane at the plane and measured surface position, x = ρcosθ, y = ρsinθ, x ₀ = rcos,
If we set y ₀ = rsin and the distance between both planes is l ₁ , then
Fraunhofer with circular opening 4
The amplitude distribution u(x ₀ , y ₀ ) of the diffraction image is Here, J ₁ is a first-order Bessel function of the first kind.
Therefore, the intensity distribution is I(r)=|u(r)| ² = (πa ² /4λl ₁ ) ² [2J ₁ (π
ar/λl ₁ )/πar/λl ₁ ] ² −(2), and a concentric ring image called an Airydisk as shown in FIG. 3 is obtained. In addition, FIG. 3 schematically shows the toroidal image of the above-mentioned Aaridisk using the density of solid lines.
The part where the density of the solid line is high is dark, that is, the light intensity is weak, and the light intensity is the highest in the center circle. The intensity distribution of this early disk is as shown in FIG. In Figure 3, the optical image corresponding to the center circle is 0
This is called the next diffraction image. Note that r is the radius from the optical axis of the reflected light 7 at the x position on the optical axis. x=ar/
If λl ₁ , the light intensity I(x) at the x position is I(x)∽[2J ₁ (πx)/πx] ² The radius r ₁ of the first dark ring, which is the 0th order diffraction image, is r ₁ = 1.22λl ₁ /a − (3).

Next, when the early disk is coupled onto the surface to be measured 1 having the radius of curvature R, the reflected light 7 will cause a similar diffraction pattern of concentric rings to appear on the light-receiving surface 8 of FIG. The radius r ₂ of the 0th order diffraction image formed by this reflected light 7 on the light receiving surface 8 is determined. As shown in the measurement principle diagram in Figure 1, the radius r ₂ of the 0th-order diffraction image of the diffraction pattern on the light receiving surface 8, which is located l ₂ away from the surface to be measured, is ΔP ₁ QO ₁ and ΔP ₂ QO ₂ in the diagram. From the similarity of , it can be expressed as r ₂ = r ₁ (l ₂ + z + ε)/z − (4).

Here, ε=r ₁ ² /2R, z=r ₁ /tan (θ ₁ +2θ ₂ ), θ ₁ is the incident angle of the projected light, and θ ₂ is the reflection angle caused by the curvature. By setting tan ² θ ₂ = (r ₁ /R) ² 0, equation (4) can be expressed as follows.

r ₂ = r ₁ + tanθ ₁ +2tanθ ₂ /1−2tanθ ₁・tanθ ₂ (r ₁ ²
/2R+l ₂ ) =r ₁ (1+l ₂ /l ₁ +2l ₂ /R+r ₁ ² /2Rl ₁ ) −(5) In the end, the radius of curvature R can be found from equation (5) as follows.

R=2l ₁ l ₂ +r ₁ ² /2/r ₂ l ₁ /r ₁ −(l ₁ +l ₂ )2l ₁ l ₂
/ar ₂ /1.22λ−(l ₁ +l ₂ )
-(6) FIG. 4 illustrates the relationship between r ₂ and R in equation (6). It becomes a hyperbola with an asymptote of R=0 and r ₂ =1.22/aλ(l ₁ +l ₂ ). If the surface to be measured is a convex curved surface, the curve in the first quadrant of the coordinates in FIG. 4 is taken, and if the surface to be measured is a concave curved surface, the curve in the third quadrant is taken.

Next, an apparatus configuration for carrying out the method of the present invention will be explained with reference to FIG. FIG. 5 shows an optical system of a curvature measuring device according to an embodiment of the present invention. This device is essentially the same as the measurement principle diagram shown in Fig. 1, but includes means for projecting a laser beam of wavelength λ onto the surface to be measured (point) 1 from the normal direction thereof, and the surface to be measured. A means for guiding the reflected light from the light to a detection device is added. Measured surface 1
A laser beam generator 10 is installed on the top, and a circular aperture plate 2 having a circular aperture 4 with a diameter a between the laser beam generator 10 and the surface to be measured 1 and a circular aperture plate 2 tilted at 45 degrees with respect to the optical axis of the laser beam. A first semi-transparent mirror 11 is placed. A screen 12 is placed on the side of the first semi-transparent mirror 11 and at a distance l ₂ ' from the optical axis of the laser beam.
Place. Note that the circular aperture 4 has its center on the optical axis of the laser beam. The screen 12 is parallel to the optical axis of the laser beam. Immediately after placing a pinhole at the position of the screen 12, a light receiving element (not shown) such as a positioning sensor is mounted on an xy fine movement table that can be moved with an accuracy of, for example, 5 μm. Further, the object to be measured is fixed to a suitable mount (not shown) so as to be rotatable in the three axes of x, y and z and in a direction perpendicular to the z axis.

A second semi-transparent mirror 13 inclined at 45 degrees with respect to the optical axis of the laser projection light is disposed between the laser beam generator 10 and the circular aperture plate 2.
A reference light reflecting plate 14 is installed on the side of the. A reference light receiving element 15 is arranged at an optically symmetrical position opposite to the reference light reflecting plate 14 with respect to the optical axis of the laser beam. Note that the meaning of the optically symmetrical position will be described further later.

In such a configuration, the laser beam generator 1
The laser beam emitted from the laser beam 0 is split by the second semi-transparent mirror 13 into a laser projection beam 6 which is projected onto the surface to be measured and a reference beam 16 which is directed toward a reference beam reflection plate 14 . The reference light 16 is directed to the surface to be measured 1 by the reference light reflector 14.
reflected towards. At this time, the position of the stand on which the object to be measured is placed, for example, the position in the optical axis direction of the laser beam, is adjusted, and the reference light 1 from the reflector plate 14 is adjusted.
6' is made to strike the point to be measured (indicated by reference numeral 17) on the surface to be measured on the optical axis of the laser beam. Furthermore, an angle α (with respect to the optical axis of the laser beam) that is equal to the angle α that this reference beam 16' forms with respect to the optical axis of the laser beam.
The position of the mount, for example, its angular position, is adjusted so that the reflected reference light 16'' is reflected from the measurement target point 17 and received by the reference light receiving element 15. Reference light 16'
is the measured point (measured point 1) on the optical axis of the laser beam.
7) and is reflected by the reference beam light receiving element 15, the angles formed by the reference beam 16' and the reflected reference beam 16'' with respect to the optical axis of the laser beam are equal, and therefore the laser beam in this case The optical axis of coincides with the normal line at the measured point.Reference light reflection plate 1
4 and the reference light receiving element 15 are arranged at optically symmetrical positions with respect to the optical axis of the laser beam, which means such a configuration. In this state, the distance l ₁ from the point to be measured to the circular aperture plate 2 and the distance l ₁ ' from the point to be measured to the first semi-transparent mirror 11 are necessarily determined. In this way, the laser beam projected from the normal direction to the point to be measured through the circular aperture 4 and the first semi-transparent mirror 11 is reflected by the point to be measured and the first semi-transparent mirror 11, and is reflected on the screen 12. A diffraction pattern is formed on top (see Figure 3). If we measure the radius r ₂ of the dark ring in the 0th order diffraction image of this diffraction pattern, we get
The radius of curvature R of the point to be measured can be obtained from equation (6). In addition
In the embodiment shown in FIG. 4, l ₂ in equation (6) becomes l ₁ '+l ₂ '. Note that it is impossible to determine whether the surface to be measured is a concave or convex surface using only the dark ring radius r ₂ of the 0th order diffraction image, and only the radius of curvature of that surface (point) is detected. This can be easily determined by other methods, such as visual inspection, except in special cases.

In addition, when measuring flatness, if R is set to infinity in equation (6), r ₂ = 1.22λ (l ₁ + l ₂ )/a, and the radius r ₂
By measuring , the accuracy of the plane can be immediately detected.

As described above, according to the present invention, by projecting light onto a point to be measured and measuring the radius of the 0th order diffraction image of the reflected light, it is possible to easily and accurately measure almost any surface to be measured having an arbitrary curvature. It is possible to obtain the radius of curvature in a minute area close to a point, preventing scratches and scratches on the surface to be measured.
Since it does not stain, it is useful for measuring the radius of curvature of plastic lens mold surfaces.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the optical path of a laser beam for explaining the measurement principle of the measurement method according to the present invention, Fig. 2 is a diagram showing the intensity distribution of the Early disc, and Fig. 3 is a diagram showing the wheel of the Early disc. FIG. 4 is a diagram schematically showing the image using the density of solid lines; FIG. The figure is a schematic diagram of a curvature radius measuring device according to an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Surface to be measured, 2... Circular aperture plate, 3... Optical axis, 4... Circular aperture, 10... Laser beam generator, 11... First semi-transparent mirror, 12... Screen, 13... ...Second semi-transparent mirror, 14...Reference light reflecting plate, 15...Reference light receiving element, 16, 16'...
Reference light, 16″...Reflected reference light.

Claims (1)

[Scope of Claims] 1. A laser beam is projected onto the measured point from the normal direction of the measured point through a circular opening, and the radius value of the 0th-order diffraction image is reflected from the projected light from the measured point. is detected, and using this detected radius value, the radius of curvature R of the measured point is calculated as follows: R=2l ₁ l ₂ /ar ₂ /1.22λ−(l ₁ +l ₂ ) where l ₁ ; circular opening and measured point Distance to the point, l ₂ ; Distance between the point to be measured and the imaging plane of the 0th-order diffraction image, a; Aperture diameter of the circular aperture, λ: Wavelength of the laser beam, r ₂ : Radius detection of the 0th-order diffraction image A method for measuring the radius of curvature, characterized in that it is determined from the value, . 2. A laser beam generation source, an aperture plate having a circular aperture on the optical axis of the laser beam generation source, and first and second aperture plates arranged on both sides of the aperture plate on the optical axis of the laser beam generation source. an inclined semi-transparent mirror, a device arranged on the side of the first inclined semi-transparent mirror and detecting a reflected diffraction image of a point to be measured obtained through the first inclined semi-transparent mirror, and the laser beam generator. a reference light reflecting plate and a reference light receiving element arranged at positions opposite to each other with respect to the optical axis of the source; The object to be measured is arranged such that light is irradiated onto the point to be measured on the optical axis of the laser beam via the reference light reflector, and the reference light reflected from the point to be measured is received by the reference light receiving element. A curvature radius measuring device characterized by positioning.