JP2003121124A - Method and apparatus for shape measurement using monochromatic rectangular-wave grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a shape measuring method which eliminates a defect in a conventional method and in which the height distribution of an object is obtained in a noncontact manner. SOLUTION: The shape measuring apparatus comprises a step wherein a monochromatic rectangular-wave grating obtained by compositing two different rectangular-wave components whose black-to-white ratios are different, and whose mutual pitch ratio is m:n by setting m and n as three or more mutually prime integers is projected on the object; a step in which the whole cycle of the grating is shifted by 1/(m×n) each so as to image m×n pieces of images; a step in which a phase distribution regarding the rectangular-wave components having a cycle of m/(m×n) is found on the basis of n pieces of images extracted in every m pieces from among the m×n pieces of images, and in which a phase distribution regarding the rectangular-wave components at the cycle of m/(m×n) is found on the basis of m pieces of images extracted in every n pieces; and a step in which a continued phase distribution corresponding to the height distribution of the object is obtained on the basis of both phase distributions regarding both rectangular-wave components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the fields of measuring devices, shape measuring devices, and monitoring devices, and more particularly, to a shape measuring method for obtaining a non-contact phase distribution of contour lines of an object. The invention also relates to a shape measuring device for carrying out such a method.

[0002]

2. Description of the Related Art As a method of measuring the shape of an object in a non-contact manner, a method of analyzing the distortion of a grating projected on the object is often used. Project a grid of equal pitch on the object,
When the image is taken from a direction different from the projection direction, a distorted lattice image is obtained according to the shape of the object. Using the known moire topography technique, a contour image of the object can be obtained from the distorted lattice image by simple image processing. Since the phase value of the contour line is a value representing the height of the object, real-time shape measurement can be realized by obtaining the phase distribution of the contour line in real time.

[0003] In the specification of Japanese Patent Application No. 11-179950, the applicant of the present invention has an integral type phase in which a phase distribution can be obtained from an image taken by continuously phase-shifting a rectangular wave grating having a contrast ratio of 1: 1. The shift method is disclosed. By using the rectangular wave for the projection grating, it is less likely to be affected by the reflectance of the object and the non-linearity of the brightness conversion in the projection / imaging device. Further, since the phase is continuously shifted, the phase shift mechanism and its control are simple and the speed can be easily increased as compared with the conventional method in which the movement of the grating is stopped every time the image is taken. According to this method, it is possible to display the phase difference distribution representing the height distribution of the object for each frame time from the images of the past four frames by using the rectangular wave grating having the contrast ratio of 1: 1.

Further, the applicant of the present invention is in Japanese Patent Application No. 2000-2794.
No. 57 discloses a method of performing phase analysis in real time using a color rectangular wave grating. According to this method, two types of gratings having different pitches and colors are used, and the phase shift is continuously performed to obtain a phase difference distribution representing the height distribution of the object for each frame time of the CCD camera for photographing the grating. Can be displayed.

Further, the applicant of the present invention has applied for a patent application 2001-1950.
No. 32 discloses a two-component separation type phase shift method for interference fringes. According to this method, it is possible to obtain the phase distribution of each grating by combining two types of sinusoidal gratings having different pitches and performing phase shift.

[0006]

The above-mentioned Japanese Patent Application No. 2000-2
The method described in 79457 has a drawback that a special filter and a CCD camera for performing color separation of two types of rectangular waves are required. Further, there is a drawback that it is difficult to perform color separation completely.

The method described in the above-mentioned Japanese Patent Application No. 2001-195032 has a drawback that it is difficult to form a sine wave grating with high precision when the grating is formed from a film.

In view of the above, it is an object of the present invention to provide a shape measuring method which eliminates the drawbacks of the conventional methods. The invention further aims at providing a device for carrying out such a method.

[0009]

In the shape measuring method according to the first aspect of the present invention, the black-and-white ratios are different from each other, and the pitch ratios are such that m and n are 3 or more prime to each other. Projecting a monochromatic square wave grating, which is a composite of two different square wave components of m: n, as an integer;
A step of shooting m × n images by shifting by 1 / (m × n) of the entire period of the monochromatic rectangular wave grating;
A phase distribution regarding a rectangular wave component having a cycle of m / (m × n) is calculated from n images extracted every m images out of n images, and a cycle is calculated from m images extracted every n images. /
Determining the phase distribution for the (m × n) rectangular wave component, and obtaining the continuous phase distribution corresponding to the height distribution of the object from the phase distributions for the two rectangular wave components. Characterize.

A shape measuring method according to a second aspect of the present invention is the shape measuring method according to the first aspect, wherein the image is photographed with an integral value of light intensity during photographing as luminance. It is characterized in that an integral type phase shift method is used in the step of obtaining an image and obtaining a phase distribution for each rectangular wave component.

In the shape measuring apparatus according to the third aspect of the present invention, the black-and-white ratios are different from each other, and the pitch ratios are such that m and n are integers of 3 or more, which are coprime.
A monochromatic rectangular wave grating in which two different rectangular wave components of m: n are combined, a projection means for projecting the monochromatic rectangular wave grating on an object, a grating moving means for moving the monochromatic rectangular wave grating, and the object. The grating moving means includes a photographing means for photographing, and the grating moving means sets the monochromatic rectangular wave grating to 1 / (m of a cycle of the whole monochromatic rectangular wave grating in a photographing cycle of the photographing means.
Xn), and the photographing means continuously photographs m × n images by the photographing means while moving the monochromatic rectangular wave grating by the grating moving means, and the apparatus, A phase distribution relating to a rectangular wave component having a cycle of m / (m × n) is obtained from n images extracted every m images of the m × n images, and m extracted every n images.
Means for obtaining a phase distribution regarding a rectangular wave component having a period of m / (m × n) from a single image, and a continuous phase distribution corresponding to the height distribution of the object based on the phase distribution regarding the both rectangular wave components Is further provided.

A shape measuring apparatus according to a fourth aspect of the present invention is the shape measuring apparatus according to the third aspect, wherein the photographing means photographs the integrated value of the light intensity during photographing as luminance. Then, the means for obtaining the phase distribution of the rectangular wave component obtains the phase distribution by using the integral type phase shift method.

[0013]

According to the method of the first aspect of the invention and the apparatus of the third aspect of the invention, it is possible to perform shape measurement with a higher resolution than in the conventional method and apparatus or in a wide range in the height direction. In the conventional method (Japanese Patent Application No. 2000-279457 cited above), a color grid was used, but since the method and apparatus according to the present invention uses a monochromatic grid, it can be analyzed by a normal CCD camera. it can. Also, since there is no need to perform color separation, no special means or device is required,
Since phase connection can be performed, there is an advantage that shape measurement can be performed with higher resolution than the conventional method and apparatus. Since the phase value of each rectangular wave component can be obtained from the m × n images captured in the past, there is also an advantage that the result can be obtained for each frame capture.

The method of the second invention and the apparatus of the fourth invention are C
It is assumed that the camera is a camera such as a CD camera that shoots the integrated value of the light intensity during the shooting as brightness, and since the integral type phase shift method is used, it can be applied to a high speed camera.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing an example of the configuration of an apparatus for carrying out a shape measuring method according to the present invention. A grating projection projector 2 projects a grating onto an object 1 to be measured, and a CCD camera 3 captures an image of the grating projected onto the object 1. The grating projection projector 2 includes a monochromatic rectangular wave grating 4, a grating moving mechanism 5, a light source 6, and a lens 7. The light emitted from the light source 6 passes through the monochromatic rectangular wave grating 4 moved by the grating moving mechanism 5, and is focused on the surface of the object 1 by the lens 7. An image photographed by such a configuration is analyzed by an analyzing means (not shown) such as a computer connected to the CCD camera 3 by an integral type phase shift method or a phase connecting method described later, Obtain the height distribution of the object 1.

FIG. 2 is a diagram showing an example of the monochromatic rectangular wave grating 4 shown in FIG. 2a and 2b show rectangular wave grating components with a black and white ratio of 1: 1 and 1: 2, respectively. The ratio of the pitch between the grating components having the black and white ratios of 1: 1 and 1: 2 is 4: 3.
Has become. FIG. 2c shows a monochromatic rectangular wave grating in which these two types of rectangular wave grating components are combined, and this monochromatic rectangular wave grating is used in the shape measuring method according to the present invention.

In one embodiment of the shape measuring method according to the present invention, a monochromatic rectangular wave grating 4 as shown in FIG. 2c is moved by a grating moving mechanism 5 at every frame rate (usually 1/30 seconds) of the CCD camera 3. It is continuously shifted at a constant speed so as to change by 1/12 cycle. A phase distribution image regarding each rectangular wave component is obtained by using the integral type phase shift method from the 12 consecutive images thus photographed. The method of obtaining the phase distribution image is shown below.

FIG. 3 is a diagram showing a change in light intensity of the projection grating in one pixel of the CCD camera. FIGS. 3a and 3b show the intensity variation for grating components with a black and white ratio of 1: 1 and 1: 2 respectively, ie for the grating as shown in FIGS. 2a and 2b. Where I _a is the intensity of the 1: 1 lattice component, I _b
Represents the intensity of the 1: 2 lattice component. FIG. 3 shows the intensity variation I _c for a combined square wave, ie a monochromatic square wave grating used in the shape measuring method according to the invention as shown in FIG. 2c. At this time, if I _c is expressed as a function of time t in consideration of the intensity I _{bac k} of the background light, the following formula (1) is obtained.
become that way. I _c (t) = I _a (t) + I _b (t) + I _back (t) (1) The CCD camera shoots the integrated value of the light intensity during the shooting time of one frame as the brightness. Therefore, the luminance I _cn obtained by the CCD camera when the synthetic rectangular wave grating is continuously phase-shifted so as to change by 1/12 cycle in one frame is expressed by the following equation (2).

[Equation 1] Here, n represents the number of phase shifts, and T represents the shooting time of one frame of the camera.

FIG. 4 shows the monochromatic rectangular wave grating of FIG.
It is 12 images obtained by shooting while continuously phase-shifting by 2 pitches at a constant speed. The horizontal direction in this figure represents the position. The luminance distribution of the photographed grid has a shape as shown above for the image of the first frame as an example.

FIG. 5 shows each grid of FIG.
It shows a change in the integrated value of the light intensity when the image is captured with a constant phase shift for each cycle. The dotted line in the figure shows the same value as the luminance change with respect to the position shown in FIG. This is not a value that can be actually photographed, but is shown for convenience of explanation. The brightness captured by the CCD camera is obtained only at the time when t is an integer multiple of T, and has a value on the dotted line at this time. The dotted lines in (a) and (b) respectively represent the 1: 1 lattice component and the 1: 2 lattice component of the luminance change for convenience shown by the dotted line in (c). Therefore, in (a) and (b) as well, on these dotted lines, t
Each component will appear at a position where is an integer multiple of T.

By the way, the phase of a rectangular wave having a black and white ratio of 1: 1 can be calculated from the luminance values of four consecutive frames by using the integral type phase shift method. Similarly black and white ratio 1:
The phase of 2 rectangular waves can be calculated from the luminance values of 3 consecutive frames. How to obtain this phase value is 1: 1
Japanese Patent Application No. 11-179 cited above regarding the rectangular wave
Japanese Patent Application No. 2000-2 for a 1: 2 rectangular wave at 950.
Since it is described in 79457, detailed description is omitted.

With respect to the grating as shown in FIG. 2, 12 images were photographed, and the phase of the 1: 1 rectangular wave was obtained by extracting every 4th image from every other 3 phase image. Can be obtained from each of the three images extracted every four images. This is a 1: 1 square wave,
Black triangle marks in (a) of FIG. 5 are the luminance values of four points extracted every three sheets. If you take out every three sheets, you can see in Fig. 5 (b).
As shown by the circle mark, the integral value of the rectangular wave of 1: 2 becomes constant. Therefore, the luminance change at the four points of the black square mark shown in FIG. 5C is the same as the luminance change of the black triangle mark. Therefore,
By using the integral type phase shift method, the phase value of the rectangular wave component of 1: 1 can be obtained. Similarly, by extracting every four sheets, it is possible to obtain the phase value of the rectangular wave component of 1: 2 assuming that it is not affected by the rectangular wave of 1: 1. Since this analysis method uses the past 12 images, the phase analysis can be similarly performed by discarding the oldest one every time a new image is captured and adding a new one. In this way, the phase analysis result can be obtained for each frame.

A phase connection method using two different pitch gratings will be described. The phase value of the grating increases by 2π for each cycle of luminance change. However, the phase value (and the phase difference) obtained by the phase analysis as described above is too much of the original phase value divided by 2π. Therefore, the change of the phase value with respect to the place is 0 to 2
It becomes a repetition of π and becomes discontinuous. The phase connection (continuous phase) is to obtain a continuous phase value by adding 2nπ (n is an integer) to the repeated phase value of 0 to 2π obtained by the phase analysis. A method using a plurality of pitch gratings is often used as one of the methods for performing phase connection. In the shape measuring method according to the present invention, a monochromatic rectangular wave grating in which two types of gratings having different pitches are combined is projected onto an object, and the phase value of each pitch is obtained by using the phase analysis method as described above. A continuous phase distribution image is obtained from the values in real time.

FIG. 6 shows a phase distribution φ of a 1: 1 rectangular wave.
₁ shows a phase distribution φ ₂ of a rectangular wave of 1: ₂ and a phase distribution φ _c of a rectangular wave of 1: 1 after being made continuous. 0 ≦ φ _c <8π
In the range of, φ _c can be obtained by the following equation (3). φ _c = 4 (φ ₁ −φ ₂ ) (when φ ₁ ≧ φ ₂ ) φ _c = 4 (φ ₁ −φ ₂ + 2π) (when φ ₁ <φ ₂ ) (3)

Using the image of FIG. 4, it is confirmed that the phase connection can be executed by the phase analysis algorithm using the integral type phase shift method using the monochromatic rectangular wave grating in the shape measuring method according to the present invention. From the image of FIG. 4, as described above,
4 for each of a 1: 1 square wave and a 1: 2 square wave
FIGS. 7a and 7b show the results obtained by extracting each of the three and three sheets and performing the phase analysis by the integral type phase shift method. This 2
FIG. 7c shows the result of performing the phase connection as described above from the type of phase distribution images.

FIG. 8 is a diagram showing a result of projecting a monochromatic rectangular wave grating on an actual object and performing phase connection by the shape measuring method according to the present invention. FIG. 8a is a photograph of a monochromatic rectangular wave grating projected on an object, taken by a CCD camera.
An image in which the phase distribution of the rectangular wave component of 1: 1 is calculated from 12 images that are continuously phase-shifted is shown in FIG. 8b, and the phase distribution image of the rectangular wave component of 1: 2 is shown in FIG. 8c. Further, the phase distributions of the reference plate (flat plate) obtained in advance by the same method are shown in FIGS. 8d and 8e, respectively. Phase difference distribution images obtained from the images of FIGS. 8d and 8b and FIGS. 8e and 8c are shown in FIGS. 8f and 8g, respectively. From these figures, it can be seen that a height distribution image of the object can be obtained. However, in these figures, since phase connection is not performed, the phase difference distribution image showing the height distribution is repeated from 0 to 2π. Therefore, FIG. 8h shows the result of performing the phase connection by the above-mentioned method from these two phase difference distribution images.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a shape measuring apparatus using a monochromatic rectangular wave grating according to the present invention.

2A is a diagram showing a rectangular wave grating component having a black-and-white ratio of 1: 1 and a graph showing the relationship between luminance and position, and b is a diagram showing a rectangular wave grating component having a black-and-white ratio of 1: 2 and luminance. It is a graph showing the relationship with the position, and c is a diagram showing a monochromatic rectangular wave grating used in the shape measuring method according to the present invention and a graph showing the relationship between the brightness and the position.

FIG. 3 is a graph showing a change in luminance of a monochrome rectangular wave grating and each rectangular wave component in one pixel of a CCD camera.

FIG. 4 is a diagram showing a phase shift of a monochromatic rectangular wave grating by simulation.

FIG. 5 is a graph showing a change in integral value with respect to time of a monochromatic rectangular wave grating and an influence of each rectangular wave component.

FIG. 6 is a graph showing a phase distribution before and after phase connection.

FIG. 7 is a diagram showing a result of phase connection by simulation, where a is a phase distribution of a 1: 1 rectangular wave component, and b
Is a phase distribution of a 1: 2 rectangular wave component, and c is a phase connected image.

FIG. 8 is a diagram showing an example of shape measurement of an object by a shape measuring apparatus using a monochromatic rectangular wave grating of the present invention, in which a is an image obtained by photographing a monochromatic rectangular wave grating projected on the object with a CCD camera, and b Is a phase distribution image of a 1: 1 rectangular wave component, c is a phase distribution image of a 1: 2 rectangular wave component, and d
Is a phase distribution on the reference plate of a 1: 1 rectangular wave distribution, e is a phase distribution on the reference plate of a 1: 2 rectangular wave component, and f is a phase difference distribution image of a 1: 1 rectangular wave component. Yes, g is a phase difference distribution image of a rectangular wave component of 1: 2,
h is an image resulting from the phase connection.

[Explanation of symbols]

1 object 2 grid projection projector 3 CCD camera 4 monochromatic rectangular wave grating 5 Lattice movement mechanism 6 light source 7 lenses

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazumi Iwai             495-4 Sakaiya, Wakayama City, Wakayama Prefecture F term (reference) 2F065 AA51 DD03 FF08 FF42 HH12                       JJ03 JJ26 LL41 QQ00 QQ14

Claims (4)

[Claims] 1. A shape measuring method for obtaining a height distribution of an object, wherein respective black and white ratios are different, and pitch ratios of m and n are 3 or less.
The step of projecting a monochromatic rectangular wave grating in which two different rectangular wave components of m: n are combined on the object as the mutually prime integers, and 1 / (m × n) of the entire period of the monochromatic rectangular wave grating. ) A step of shooting m × n images at different intervals, and a rectangular wave component having a cycle of m / (m × n) from n images extracted every m of the m × n images. Obtaining a phase distribution and obtaining a phase distribution regarding a rectangular wave component having a period of m / (m × n) from m images extracted every n sheets; and calculating the phase distribution of the object from the phase distribution regarding both of the rectangular wave components. And a step of obtaining a continuous phase distribution corresponding to the height distribution. 2. The shape measuring method according to claim 1, wherein the image is an image photographed with luminance as an integrated value of light intensity during a photographing time, and a phase distribution regarding each rectangular wave component is obtained. , A shape measuring method characterized by using an integral type phase shift method. 3. A shape measuring apparatus for obtaining a height distribution of an object, wherein each black-and-white ratio is different, and the pitch ratios of m and n are 3
A monochromatic rectangular wave grating in which two different rectangular wave components of m: n are combined as the above-mentioned relatively prime integers, projection means for projecting the monochromatic rectangular wave grating onto an object, and the monochromatic rectangular wave grating is moved. And a photographing means for photographing the object, the grating movement means is configured to measure the monochromatic rectangular wave grating in the photographing cycle of the photographing means by 1 / (m of the whole cycle of the monochromatic rectangular wave grating. Xn), and the photographing means continuously moves by the photographing means while moving the monochromatic rectangular wave grating by the grating moving means.
Xn images are taken, and the apparatus calculates a phase distribution regarding a rectangular wave component with a cycle of m / (mxn) from n images extracted every m out of the mxn images. A means for obtaining a phase distribution regarding a rectangular wave component having a period of m / (m × n) from m images extracted every n sheets; and a height distribution of the object from the phase distribution regarding both the rectangular wave components. And a means for obtaining a continuous phase distribution corresponding to the shape measurement device. 4. The shape measuring device according to claim 3, wherein the photographing means photographs the integrated value of the light intensity during the photographing time as luminance, and the means for obtaining the phase distribution of the rectangular wave component integrates. Shape measuring apparatus characterized in that a phase distribution is obtained by using a phase shift method.