JP6527725B2 - Three-dimensional shape measuring device - Google Patents

Description

  The present invention relates to three-dimensional measurement using a light source switching phase shift method by lattice projection.

  In order to measure the shape of the human body, it is necessary to perform measurement at high speed so that the influence of body blur does not occur, and a wide measurement area. Therefore, a measuring device based on a light source switching phase shift method suitable for wide-angle projection with excellent responsiveness has been developed. Non-Patent Document 1 discloses a method of expanding a shape measurement area suitable for the light source switching phase shift method.

Sakaguchi Toshimasa, Fujigaki Motoharu, Murata Yorinobu, Proposal of an expansion method of shape measurement area suitable for light source switching phase shift method, Proceedings of the 19th Mechatronics Workshop Lecture, pp. 185-190 (2014)

  In the method described in Non-Patent Document 1, the luminance is high near the center of the measurement region, and the luminance is low at the periphery. Therefore, although it becomes possible to measure the peripheral part with high accuracy by prolonging the exposure time of the camera, the central part becomes overexposure instead and can not be measured.

  There are commercially available filters called "bull's eye filters" whose transmittance gradually decreases from the periphery to the center. Uneven luminance can be corrected by using this. However, since the luminance of the measurement area is entirely lowered, it is necessary to increase the exposure time of the camera, which causes a problem that the time required for measurement becomes long.

  Therefore, in view of the above-mentioned problems of the prior art, it is an object of the present invention to provide a method of correcting luminance unevenness generated at the time of photographing a grating pattern and an optical system thereof.

The invention according to claim 1 of the present application is a three-dimensional shape measuring apparatus according to a light source switching phase shift method, including a grating projection optical system that projects light from a light source to a target object without using lens imaging. The three-dimensional shape measuring apparatus reduces the brightness by widening the grating pitch projected near the center, and increases the brightness by narrowing the grating pitch at the periphery, the center becomes a concave lens and becomes the periphery According to the above, a single-sided concavo-convex cylindrical lens having the shape of a concavo-convex surface that becomes a convex lens is inserted into the light path of the grating projection optical system to correct luminance unevenness of the projected grating pattern. It is a shape measuring device.

  According to the present invention, it is possible to provide a method and an optical system for correcting unevenness in luminance which occurs at the time of photographing a lattice pattern.

It is a figure explaining the light source switching phase shift method. It is a figure explaining the whole space tabulation method. It is a figure which shows the conversion table which interpolated the relationship of a phase and z coordinate. It is a figure explaining the principle of the multiple table determination method. It is a figure showing a shape measuring device. It is an LED projector layout. It is a figure which shows the optical path which passes a single-sided uneven | corrugated cylindrical lens. It is a figure which shows the change of the light ray by a lens. It is an example of a screen of lens design simulation software. It is a figure which shows the locus | trajectory of the light ray when passing through a lens. It is a figure which shows luminance intensity distribution in a 200-mm position in case there is no lens. It is a figure which shows luminance intensity distribution in a 200-mm position at the time of passing through a lens. It is a figure which shows the effect by a lens. It is an enlarged view which shows the effect by a lens. It is a figure which shows a single-sided uneven | corrugated cylindrical lens. It is a figure which shows the dimension of a single-sided uneven | corrugated cylindrical lens. It is a figure which shows the change of the luminance distribution by a lens. It is a figure which shows the luminance distribution by a single-sided uneven | corrugated cylindrical lens. It is a figure explaining an experimental method. It is a figure which shows the measurement image in the position of height 4 mm. It is a figure which shows height distribution in each measurement position. It is a figure which shows the measurement image of a trapezoid sample (the 1). It is a figure which shows the measurement image of a trapezoid sample (the 2). It is a figure which shows the measurement image of a trapezoid sample (the 3). It is a figure which shows height distribution of a trapezoid sample. It is a figure which shows the height distribution image after compositing. It is a figure which shows height distribution of a synthesized image.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, a method of shape measurement using a line LED will be described.
<Shape measurement by line LED>
FIG. 1 is a diagram for explaining a light source switching phase shift method. First, the phase of the captured image is determined by the light source switching phase shift method. In this method, first, a grid is projected from a light source to an object to be photographed. Next, the phase of the projection grating is shifted by switching the line to be lit. Photographing is performed each time the light source is switched, and a phase shift method is used to obtain a phase value at each pixel from the obtained image.

  The determined phase values are analyzed using a full space tabulation technique. FIG. 2 is a diagram for explaining the whole space table conversion method. A reference plane on which a grating is projected is prepared in advance, the reference plane is moved by Δz in the z direction, and the phase is determined using the light source switching phase shift method at each position. The relationship between the coordinates of each pixel and the phase is obtained for each pixel by repeating this operation n + 1 times until the reference plane reaches the position of zn. From the relationship obtained above, the conversion table interpolated from the relationship between the phase and the z coordinate is created. FIG. 3 is a diagram showing a table to be created.

  When the phase of the measurement object is similarly determined, the value of the z coordinate of the table closest to the phase value is the height of the measurement object. It is possible to obtain three-dimensional coordinates instantly and easily simply by referring to the obtained table without calculating coordinates using a conversion equation. When the light source switching phase shift method is used, there arises a problem that a distorted phase is calculated, but by referring to the conversion table, highly accurate analysis is possible.

<Range expansion by multiple table reference>
Usually, the range in the z direction that can be measured by the grid projection method is determined by the pitch width of the projection grid and the angle of the camera. On the other hand, since the accuracy of measurement depends on the resolution of the analysis phase, there is a problem that the accuracy decreases when the measurement range is expanded. Therefore, it has been difficult to achieve both the measurement accuracy and the range of the lattice projection method.

  Therefore, there is known a method for searching a corresponding table by comparing the phase values of two types of projection grids. FIG. 4 is a diagram showing the principle. In this method, first, a reference surface analysis is performed with one projection grating A, a range in which the phase makes a round in the z direction is created as a single table, and a plurality of such ranges are prepared as Table1, Table2,. For the other grid B, create Table θb that converts coordinates to phase values.

At the time of analysis, first, phase values θa of the projection grid A are substituted into a plurality of tables, and coordinate candidates are calculated by the number of tables. Substituting those candidate values into Table θb to convert them into the candidate values of the phase of the projection grating B, and comparing with the actually measured phase value θb, the true coordinate value zAB is derived.
<Measurement device>
FIG. 5 is a layout diagram of the shape measuring apparatus. This device is composed of a projector and a CMOS camera combining LED devices and grid glass. An external control circuit is used to control the apparatus such as switching the lighting position of the LED.

<Grating projection device>
In order to expand the measurement range, it is necessary to project two types of grids with different pitches. Therefore, the lattice projection apparatus has a structure for projecting two types of lattices by arranging two types of LED devices in the vertical direction and adjusting the distance between each light source and the lattice. FIG. 6 is a view showing the arrangement of the LED projector with a step.

<1. Reduction of uneven brightness of linear LED by lens>
Here, a method for reducing the influence on the measurement result by the change of the luminance distribution of the line LED described above will be described. First of all, the cause of the decrease in luminance is the vignetting or cosine fourth law. The effect of using a wide-angle lens is considered to be large due to the cosine fourth law.

  As a method of solving this problem, it is conceivable to measure the degree of peripheral light reduction of the lens to be used in advance and to perform correction by image processing. In addition to the fact that the present invention can project at a sufficiently wide angle than the lens of the camera used by the line LED, the grid projection method using the light source switching phase shift method is not a grid pattern projection by imaging (see FIG. 1). It utilizes the characteristic that even if a lens that partially bends the light path is inserted into the light path for projecting the grating, the grid can be projected. By using these characteristics, it is possible to reduce uneven brightness with a lens of a special shape.

<2. Single-Surface Irregular Cylindrical Lens>
The single-sided concave and convex cylindrical lens is a lens having a concave and a convex surface on one side, although the entire shape is a cylindrical lens. The central part of the lens has a concave shape, gradually changes to the curved surface of the convex lens, and finally becomes a plane. An image of an optical path when parallel light is passed when this single-sided uneven cylindrical lens is used is shown in FIG.

  As shown in FIG. 7, the light passing through the central concave lens diverges, and the light passing through the convex lens condenses. By providing such a change, it is possible to make the light quantity as seen from the camera close to a constant. If the amount of light from the camera can be made constant, the accuracy over the entire surface can be increased by increasing the sensitivity of the camera.

<3. Simulation>
<3.1 Simulation derivation formula>
The change of the light beam by the lens was simulated using Excel with Microsoft software. In this simulation, the light ray traveling from the point light source is refracted by the lens and is obtained by geometrical calculation until it reaches the screen.

FIG. 8 shows the change of the light beam by the lens. Point is the light source P (p, q) and the slope a ₁ of the straight line PQ connecting the incident point Q (m, n) on the lens becomes equation (1).

The point Q is a point on the function f (θ ₀ ) = A cos (θ ₀ / B). The incident angle θ ₁ of the straight line PQ is an angle that intersects with the normal at the point Q of the number f (θ ₀ ). Assuming that the inclination of the normal line l is a ₂ , the incident angle θ ₁ can be obtained by Equation 2.

The refracting angle θ ₂ can be calculated by Formula 4 from Formula 3 which is Snell's formula.

Be to theta ₂ rotated by the number 5 type rotation matrix lines l (affine transformation), obtains the gradient a ₃ linear QR in equation (6).

Straight QR can be calculated from the coordinates of the gradient _{a 3} and the point Q.

Then, the point Q (m, n) and the point R (r, s) since coordinates is known, determining the incident angle theta ₃ to the rear surface of the lens in equation (7).

The refraction angle θ ₄ can be obtained from Snell's formula by equation 8.

The inclination _{a 4} straight lines RS is tan .theta _4, it is possible to obtain a straight line RS from the coordinates of the point R.

By the above calculation, it is possible to simulate the change of the light ray by the lens.
<3.2 Development of simulation tools>
A simulation tool was developed using Excel from the derivation formula described in 3.1. FIG. 9 shows an example screen of the simulation tool. This tool inputs the lens shape as a trigonometric function of cosine waves. By inputting the position of the point light source, the thickness of the lens, the position of the lens, the refractive index of the lens, and the position of the screen as variables, it is possible to draw a ray trajectory. The luminance distribution can be determined from the imaging position of each ray on the screen.

<3.3 Simulation>
In the conditions shown in Table 1, the result by the simulation calculated | required by 3.2 is shown. The conditions shown in Table 1 were determined in consideration of the configuration of the experimental apparatus. Since it is assumed that the material used for the lens is acrylic, the refractive index is 1.49. The lens shape was set so that the maximum diameter of the depression was 3.0 mm and the width was 60 mm. The graph obtained by simulation is shown in FIG. Further, the light density is obtained from this result, and the light is attenuated by the inverse square of the distance. Therefore, the light density is calculated by multiplying the light density by the attenuation factor.

  First, FIG. 11 shows a graph in which the light intensity without passing through the lens is normalized. Next, FIG. 12 shows a graph in which the light intensity when passing through the lens is normalized by the maximum value when not passing through the lens. The graph of FIG. 13 shows the effect of the lens by dividing by the intensity distribution of light without the lens when passing through the lens. FIG. 14 is a graph in which the vicinity of the center of FIG. 13 is enlarged. Since it can be confirmed that the ratio of the light intensity is lower at the center than at the outer side and higher at the outer side, sufficient effects can be expected in the case of simulation under this condition.

<4. Measurement using single-sided uneven cylindrical lens>
3. In order to confirm the effect with the single-sided concavo-convex cylindrical lens performed by the simulation performed in, the lens was manufactured using acrylic. After cutting out the shape using an NC milling cutter (FIG. 15 (a)), polishing was performed while increasing the number of each of the No. 400 to No. 1000 sandpapers by 200. Finally, the lens shown in FIG. 15 (b) can be obtained by sufficiently polishing with an abrasive (acrylic sand). The shape of the lens is shown in FIG.

  It cuts off with the same curved surface as the case where simulation was done, and 10 mm from each end is a plane. This lens was attached to the device, and the LED was projected onto the reference surface in the absence of lattice glass, and the change in luminance distribution was confirmed with a camera used for measurement. FIGS. 17 (a) and 17 (b) are images taken, and FIG. 18 is a graph of the luminance distribution on a line drawn at the center of the image.

  In the case of no lens, it can be seen that the higher the center, the lower it is. In the case where a lens is used, the difference in the change in brightness can be reduced as compared with the case where no lens is provided, because the brightness on the outside increases and the center slightly decreases.

<4.1 Measurement using a reference plane and measurement using a sample>
Similar to the case where no lens is used, an accuracy confirmation experiment was conducted targeting the reference surface used for calibration. In the experiment, as shown in FIG. 19, the camera and the projector were moved by 4 mm with respect to the reference plane and measured. The measurement conditions shown in Table 2 were set to be the same as in the case of no lens except for the exposure time. FIGS. 20 (a), (b) and (c) respectively show a lattice projection image, a phase distribution image and az coordinate distribution image at a position of 4 mm. At this time, the average and standard deviation of the height distribution in an area of 100 pixels each of A, B, and C3 on a line drawn at the center of the z-coordinate distribution image were obtained (see FIG. 21). Table 3 shows the results of measurement at all positions measured in the same manner.

  By using the single-sided concave and convex cylindrical lens, the value of the standard deviation could be made substantially constant over the entire measurement surface. It is considered that the luminance distribution of the entire measurement surface could be made to be uniformly close by passing the single-sided uneven cylindrical lens. The height average is not significantly different from before using the lens. In addition, there is a possibility that the amount of light may be attenuated and the value of standard deviation may be increased by passing the lens as a whole, but the sensitivity of the camera can be increased by the fact that the amount of light becomes uniform. Compared with the case of the lowest standard deviation without using, the results were not significantly different.

<Evaluation of wide-angle shape measurement accuracy with trapezoidal sample>
An experiment in which the trapezoidal sample was moved within the field of view of the camera and measurement was made at three points was carried out with a lens attached. The image obtained by measurement is shown to FIGS. 22-1, 22-2, and 22-3. Table 4 summarizes the mean and standard deviation of the heights in the region of ○ 1 to 33 of the trapezoidal sample. The regions 1 to 3 differ in measurement positions a, b and c, respectively. a in ○ 1 is i = 144-173, j = 213-270, 22 is i = 111-123, j = 197-292, and 33 is i = 219-242, j = 180-291 pixels is there. ○ 1 of b is i = 326-366, j = 216-268, 22 is i = 253-279, j = 188-291, 33 is i = 402-428, j = 188-291 pixel area is there. ○ 1 of c is i = 499-534, j = 216-269, = 2 is i = 428-458, j = 188-300,, 3 is i = 548-560, j = 198-287 pixels is there.

  Also in the measurement results of the trapezoidal sample, the value of the standard deviation on the outer side can be reduced without increasing the value of the standard deviation at the center value as compared with the case where there is no one-sided concavo-convex cylindrical lens.

  The graphs in FIGS. 23A, 23B, and 23C are obtained by extracting one line on a trapezoid in the horizontal direction from the height distribution images in FIGS. 22A, 22B, and 22C.

<5. Image composition by measurement image from different angles>
The image synthesis by the measurement image from different angles was tried even in the case of using the single-sided concave and convex cylindrical lens. FIG. 24 shows an image of the height distribution obtained by the synthesis. Table 5 shows the height distribution and the standard deviation in the regions 1 to 3 in FIG. 3-12 (b).

  Region 1 is i = 322-369, j = 207-271, region 2 is i = 251-278, j = 181-301, region 3 is i = 397-429, j = 179-301. The height distribution which extracted one line on a sample is shown.

<6. Summary>
The present invention relates to a method of reducing luminance unevenness of a linear LED by wide-angle shooting by inserting a single-sided concave and convex cylindrical lens into an optical path for projecting a grating. According to the present invention, the amount of light as viewed from the camera can be made constant as compared with the case where no lens is used. When shape measurement is performed using a single-sided concave and convex cylindrical lens, the light amount can be made constant, so that although the amount of light as a whole is reduced by inserting the lens, the sensitivity of the camera can be increased. The present invention is a method of correcting uneven brightness in three-dimensional measurement using a light source switching phase shift method using lattice projection when a lens is not used. In the light source switching phase shift method, the grating pattern can be projected even if a cylindrical lens is inserted in the light path in order to project the grating pattern without using lens imaging.

  In the light source switching phase shift method, in order to project the grating pattern without using lens imaging, the grating pattern can be projected even if a cylindrical lens is inserted in the optical path. Therefore, a special shape is used to lower the brightness by widening the grating pitch projected near the center, and to increase the brightness by narrowing the grating pitch in the peripheral part (the center is concave and the convex lens follows the periphery) Make a lens and insert it into the light path. Thereby, the luminance in the central portion is suppressed and the peripheral portion is increased, so that the luminance unevenness can be made uniform.

  According to the present invention, it is possible to correct three-dimensional measurement with high accuracy by correcting uneven brightness and obtaining an optimum exposure time over the entire photographing range, thereby correcting uneven brightness during grid pattern photographing. Is possible.

1 projector 2 grating 3 lens 4 reference plate

Claims (1)

A three-dimensional shape measuring apparatus according to a light source switching phase shift method, comprising a grating projection optical system that projects light from a light source onto a target object without using lens imaging,
The three-dimensional shape measuring apparatus
The center is a concave lens, the center is a concave lens, and the convex lens becomes a convex lens as it gets closer to the periphery, so that the shape of the uneven surface shape so that the brightness is lowered by widening the grating pitch projected near the center and one surface irregularity of a cylindrical lens is inserted in an optical path of the grid projection optical system, the three-dimensional shape measuring apparatus you characterized in that correcting the luminance unevenness of the grating pattern projected with the.