JP2012068021A - Slit light luminance distribution design method and optical cutting irregularity flaw detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slit light luminance distribution design method and an optical cutting irregularity flaw detection device for performing stable optical cutting irregularity flaw detection corresponding to the range widening and speeding-up of surface flaw inspection.SOLUTION: The slit light luminance distribution design method in an optical cutting irregularity flaw detection method for irradiating a measurement object with a slit light, and for picking up the image of a reflected light from an angle which is different from that of a slit light source, and for detecting an irregularity flaw based on the pickup image includes: an average reflection rate measurement process of measuring the average reflection rate of the measurement object to the direction of an image pickup device using a slit light source A whose luminance distribution is already known; and a slit light luminance distribution calculation process of calculating the optimal luminance distribution of the slight light source for picking up the image of the reflected light as uniform luminance based on the measured average reflection rate.

Description

  The present invention relates to a slit light luminance distribution design method for observing slit light as uniform luminance on an image pickup apparatus, and a light cutting unevenness wrinkle detecting device using the designed slit light source.

  As a simple three-dimensional measurement technique for optical wrinkle detection, a light cutting method is known. FIG. 1 is a diagram illustrating a slit light luminance difference that is normally observed on an imaging apparatus. In the figure, 1 is an imaging device, 2 is a slit light source, 3 is slit light on the surface to be measured, and 4 is a captured image. When a slit light source 2 having a uniform luminance distribution is irradiated onto a plane having a uniform surface roughness and picked up by the image pickup apparatus 1 (upper view in FIG. 1), an imaging field of view as shown in the middle or lower view in FIG. There is a phenomenon in which the luminance at the center of the range is high and the luminance at the end is low.

  FIG. 2 is a diagram illustrating a positional relationship between the slit light source, the measurement target, and the imaging device. In the figure, the reference numerals are the same as those in FIG. It is a figure explaining one reason of the phenomenon which the brightness | luminance of the center part of the imaging visual field range mentioned above becomes high, and the brightness | luminance of an edge becomes low.

  That is, as shown in FIG. 2, the central portion is reflected light at an angle (a1) close to regular reflection, whereas the end portion is back diffused light at a shallow angle (a2). This is because the reflection in the direction of the area sensor of the imaging device becomes relatively smaller than that of the central portion. Other reasons include the effects of vignetting.

  The luminance distribution thus generated causes a reduction in accuracy due to halation at the center portion of the slit light, and causes insufficient measurement at the end portion of the slit light, causing a measurement failure.

  In order to cope with this, for example, Patent Document 1 discloses a method of performing imaging twice by adjusting the diaphragm for the purpose of preventing halation due to the luminance distribution of slit light. Patent Document 2 discloses a method of correcting the luminance distribution by calculating a captured image with normal illumination and a captured image with slit light projection.

JP-A-7-120238 JP-A-9-152316

  In recent years, surface flaw inspection has been required to have a wide range and high speed. In the case of the light cutting method, it is necessary to widen the field of view of the imaging device and widen the irradiation range of the slit light source in order to widen the range. In this way, when the slit light source is irradiated over a wide area and the reflected light is imaged by the imaging device, the luminance difference on the captured image becomes larger.

  In order to solve this luminance difference problem, in the methods disclosed in Patent Document 1 and Patent Document 2 described above, it is necessary to adjust the aperture of the same surface and perform imaging twice in Patent Document 1 with respect to the same surface. However, in Patent Document 2, in addition to imaging using a slit light source, imaging with normal illumination is required. That is, there is a problem in that it is impossible to realize a high-speed inspection because a large number of images are required together with a plurality of times of imaging and a wide range is required.

  The present invention has been made in view of such circumstances, and a slit light luminance distribution design method and a light cutting unevenness capable of stably detecting a light cutting unevenness corresponding to the widening and speeding up of the surface defect inspection. It is an object to provide a wrinkle detection device.

  The above problems are solved by the following invention.

[1] A slit light luminance distribution design method in a light cutting uneven wrinkle detection method that irradiates a measurement object with slit light, images the reflected light from an angle different from that of the slit light source, and detects uneven wrinkles based on the captured image. Because
An average reflectance measurement step of measuring an average reflectance in the direction of the imaging device to be measured using the slit light source A having a known luminance distribution;
A slit light luminance distribution calculating step of calculating an optimum luminance distribution of the slit light source for imaging the reflected light as uniform luminance based on the measured average reflectance. Luminance distribution design method.

[2] In the slit light luminance distribution design method according to [1] above,
In the average reflectance measurement step,
The x-axis is set in parallel with the slit light of the slit light source A, the luminance distribution Li (x) of the slit light of the slit light source A on the x-axis, and the longitudinal direction of the slit light (x ′) observed on the captured image The average reflectance R (x) on the x axis is Lo (x) from the average luminance distribution Lo (x ') of the x axis and the relational expression x' = g (x) between the x axis and the x 'axis using the function g. Measure from the equation (g (x)) / Li (x)
In the slit light luminance distribution calculation step,
From the measured average reflectance R (x) and the constant power C of the slit light source, the optimal luminance distribution Ld (x) is calculated from the arithmetic expression of C / R (x).
A slit light luminance distribution design method characterized by the above.

[3] In a light-cutting unevenness wrinkle detecting device configured by a slit light source that irradiates a measurement target and an imaging device that captures reflected light of the slit light source from an angle different from that of the slit light source,
The slit light source is
A light-cutting unevenness wrinkle detecting device comprising a slit light luminance distribution having a higher luminance at an end portion than a central portion of the slit light.

[4] In the light cutting uneven surface wrinkle detecting device according to [3] above,
The slit light luminance distribution Ld (x),
The x-axis is set in parallel with the slit light of the slit light source A whose luminance distribution is known, the luminance distribution Li (x) of the slit light of the slit light source A on the x-axis, and the slit light observed on the captured image From the average luminance distribution Lo (x ′) in the longitudinal direction (x ′ axis) and the relational expression x ′ = g (x) between the x axis and the x ′ axis using the function g, the average reflectance R ( x) is measured from the equation of Lo ((g (x)) / Li (x)
A light-cut irregularity wrinkle detection device, which is calculated from an arithmetic expression of C / R (x) from a measured average reflectance R (x) and a constant power C of a slit light source.

  According to the present invention, the average reflectance in the direction of the imaging device to be measured using the slit light source 1 having a known luminance distribution is measured, and the reflected light has a uniform luminance based on the measured average reflectance. Since the optimal luminance distribution of the slit light source for image capturing is calculated, the slit light source can be observed uniformly on the captured image even in a wide range of inspections, enabling fast and stable eyelid detection became.

It is a figure which shows the slit light luminance difference normally observed on an imaging device. It is a figure which shows the positional relationship of a slit light source-a measuring object-an imaging device. It is a figure which shows the optical system, the coordinate system on a captured image, and the example of a captured image. It is a figure which shows the apparatus structure used for the present Example. It is the figure which plotted the slit light luminance distribution on an imaging device, when using the laser whose brightness | luminance of an edge part is higher than a center part. It is the figure which plotted the slit light luminance distribution on an imaging device at the time of using a luminance uniform laser.

  As described above, there is a phenomenon in which the luminance at the center is high and the luminance at the edge is low because the reflection angle from the measurement target to the imaging device is different at each point of the slit light. Therefore, from the idea that by using a slit light source with higher brightness from the center, the brightness difference on the imaging device can be made smaller or uniform, and accuracy can be prevented and measurement errors can be prevented. The present invention has been conceived. Even when the luminance difference is not necessarily uniform, if the luminance difference can be reduced, it is possible to sufficiently prevent the deterioration of accuracy and prevent measurement failure.

  A method for designing the luminance distribution of the slit light source for making the luminance difference uniform on the image pickup apparatus will be described below. First of all, the light-cut uneven wrinkle detecting device is mainly used for detecting an abnormal portion in a product having a predetermined shape. For this reason, it is considered that the surface roughness of the measurement target is constant because the surface roughness is considered to hardly change during the detection except for the abnormal portion.

  FIG. 3 is a diagram illustrating an optical system, a coordinate system on a captured image, and a captured image example. As shown in FIG. 3A, the x-axis is set in parallel with the slit laser beam. FIG. 3B shows a captured image on the area sensor, and the x ′ axis is provided in a direction parallel to the slit longitudinal direction in the captured image. The relationship between x and x ′ is set to x ′ = g (x) using the function g.

  Note that the relationship between x and x ′ is easily correlated as follows, for example, when approximated as a pinhole camera by the focal length f and imaging distance n of the lens.

  x ′ / f = x / n

  In order to measure the average reflectance of the measurement object, a slit light source A with a known luminance distribution is used, and the luminance distribution on the x-axis is set as Li (x). When the average luminance distribution of the slit light observed on the captured image is set to Lo (x ′) using the slit light source A, the average reflectance R (x) on the x axis is From the equation Lo (x ′) = Li (x) R (x), R (x) = Lo ((g (x)) / Li (x).

  If the luminance distribution of the slit laser to be designed is Ld (x), Ld (x) R (x) = C (constant) may be used in order to design the luminance distribution to be constant on the area sensor. . Thus, a slit laser having a luminance distribution Ld (x) = C / R (x) may be designed. However, since C represents the power of the entire slit light source, it must be set so that sufficient luminance can be secured on the captured image.

  FIG. 4 is a diagram showing a device configuration used in this embodiment. In order to confirm the effect of the present invention, wrinkle detection was performed on a steel pipe sample 6 of a black skin material, and a red slit laser was used as the slit light source 2. As shown in FIG. 4, the slit laser is set so that the angle formed with the sample surface is 30 degrees, and the imaging device 1 is installed on the side opposite to the laser so that the angle formed with the sample surface is 60 degrees. Further, the field of view of the imaging device 1 was configured to be about 300 mm.

  FIG. 5 is a diagram in which the slit light luminance distribution on the imaging device is plotted when a laser having a higher luminance at the end than at the center is used. FIG. 6 is a diagram in which the slit light luminance distribution on the imaging device is plotted when a uniform luminance laser is used.

  FIG. 5 shows the results obtained when a laser whose edge brightness is about four times the center is used. However, since the luminance distribution is almost symmetrical, it is data for a half field of view and is normalized by the maximum luminance after moving average. Since the black skin material has a strong specular reflectivity on the surface, it can be seen that the luminance distribution at the center (left end in FIG. 5) is higher than that at the end (right end in FIG. 5). The luminance ratio between the central part and the edge part is about 9 times. Considering 256 gradations per pixel, when the maximum value of 255 is taken at the center, it becomes about 29 at the end.

  On the other hand, FIG. 6 shows the luminance distribution observed on the imaging device when a slit laser having a uniform luminance is used as the slit laser. Similarly, after moving average, normalization is performed with the maximum luminance. This ratio is about 32 times. When the maximum value of 255 is taken at the central portion, it becomes about 8 at the end portion, and the SN ratio becomes smaller than the result example of the present invention (FIG. 5), and only the end portion is weak against disturbance light and noise. You can see that

  In order to further improve the S / N ratio, the design method used in the present invention may be used and a laser with higher brightness at the center and at the end may be used. However, it is difficult to freely design the brightness distribution of the slit laser. For this reason, it must be realized by using an optical filter.

DESCRIPTION OF SYMBOLS 1 Image pick-up device 2 Slit light source 3 Slit light on measuring surface 4 Captured image 5 Slit light luminance distribution 6 on captured image

Claims (4)

A slit light luminance distribution design method in a light cutting uneven wrinkle detection method that irradiates a measurement object with slit light, images the reflected light from an angle different from that of the slit light source, and detects uneven wrinkles based on the captured image. ,
An average reflectance measurement step of measuring an average reflectance in the direction of the imaging device to be measured using the slit light source A having a known luminance distribution;
A slit light luminance distribution calculating step of calculating an optimum luminance distribution of the slit light source for imaging the reflected light as uniform luminance based on the measured average reflectance. Luminance distribution design method. In the slit light luminance distribution design method according to claim 1,
In the average reflectance measurement step,
The x-axis is set in parallel with the slit light of the slit light source A, the luminance distribution Li (x) of the slit light of the slit light source A on the x-axis, and the longitudinal direction of the slit light (x ′) observed on the captured image The average reflectance R (x) on the x axis is Lo (x) from the average luminance distribution Lo (x ') of the x axis and the relational expression x' = g (x) between the x axis and the x 'axis using the function g. Measure from the equation (g (x)) / Li (x)
In the slit light luminance distribution calculation step,
From the measured average reflectance R (x) and the constant power C of the slit light source, the optimal luminance distribution Ld (x) is calculated from the arithmetic expression of C / R (x).
A slit light luminance distribution design method characterized by the above. In the light cutting uneven surface wrinkle detecting device configured by the slit light source that irradiates the measurement object and the imaging device that captures the reflected light of the slit light source from an angle different from the slit light source,
The slit light source is
A light-cutting unevenness wrinkle detecting device comprising a slit light luminance distribution having a higher luminance at an end portion than a central portion of the slit light. In the light cutting uneven surface wrinkle detecting device according to claim 3,
The slit light luminance distribution Ld (x),
The x-axis is set in parallel with the slit light of the slit light source A whose luminance distribution is known, the luminance distribution Li (x) of the slit light of the slit light source A on the x-axis, and the slit light observed on the captured image From the average luminance distribution Lo (x ′) in the longitudinal direction (x ′ axis) and the relational expression x ′ = g (x) between the x axis and the x ′ axis using the function g, the average reflectance R ( x) is measured from the equation of Lo ((g (x)) / Li (x)
A light-cut irregularity wrinkle detection device, which is calculated from an arithmetic expression of C / R (x) from a measured average reflectance R (x) and a constant power C of a slit light source.