JP2024032350A - Visual inspection device and visual inspection method - Google Patents

Abstract

To provide a visual inspection device and a visual inspection method that can stably detect flaws and other defects without detecting them as contamination.SOLUTION: A visual inspection device includes a bright-field illuminator, a dark-field illuminator, dimming means for adjusting the light intensity of the illumination of the bright-field illuminator and the light intensity of the illumination of the dark-field illuminator, imaging means for imaging the surface of a workpiece, and image processing means for creating a condition that eliminates contamination from the image by controlling the light intensity of the contamination to be the same as the light intensity of a normal area by adjusting the light intensity ratio between the illumination of the bright-field illuminator and the illumination of the dark-field illuminator by the dimming means.SELECTED DRAWING: Figure 1

Description

The present invention relates to a visual inspection device and a visual inspection method.

When producing bearing parts, there is a visual inspection method to check whether the product (work) has any flaws or foreign matter attached to it. As this inspection method, there is a method using image processing. In visual inspection of a workpiece by image processing, the inspection may be performed after removing dirt from the workpiece using a cleaning liquid and then drying the cleaning liquid.

In such a case, if the residual components in the cleaning liquid do not evaporate and remain as dirt, there is a possibility that they will be erroneously detected as a defect. As a means for avoiding this erroneous detection, there is a conventional method that utilizes the difference between the density of defects and the density of dirt in an image, as described in Patent Document 1. In this case, the illuminance of the surface to be inspected is varied to obtain first image information corresponding to dark illuminance and second image information corresponding to bright illuminance, and the density difference between these image information is set to a predetermined value. When it is smaller than the value, it is determined that there is a defect.

Japanese Patent Application Publication No. 2002-116153

The technique described in Patent Document 1 is based on the premise that density changes due to illuminance are not constant for defects such as scratches and stains. However, due to defects and dirt, there are cases where the density change due to illuminance is constant. Thus, if it is constant, it is not possible to distinguish between defects and dirt.

Furthermore, it is necessary to capture two images of low illumination and high illumination under the same positional conditions, making it impossible to inspect the appearance of a moving object. Moreover, when capturing two images of low illuminance and high illuminance under the same positional conditions, it is impossible to do so at the same time, which has the drawback of requiring a large amount of work time.

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention provides an appearance inspection device and an appearance inspection device that can stably detect defects such as scratches without erroneously detecting stains from residual components in a cleaning liquid as defects such as scratches. The present invention provides a method.

The appearance inspection device of the present invention is an appearance inspection device for inspecting the appearance of the surface of a workpiece, and includes a brightfield illuminator that irradiates the workpiece surface with brightfield light, and a brightfield illuminator that irradiates the workpiece surface with darkfield light. a dark-field illuminator for illumination, a dimming means for adjusting the amount of illumination of the bright-field illuminator and the amount of illumination of the dark-field illuminator, and imaging a workpiece surface illuminated by each of the illuminators. The dirt is removed from the image by adjusting the light amount ratio between the illumination of the bright field illuminator and the illumination of the dark field illuminator using the light control means to make the light amount of the dirt the same as the light amount of the normal area. and an image processing means for creating a state to be erased, and the surface of the workpiece is inspected with the same light intensity as that of a normal area determined by the image processing means.

According to the appearance inspection apparatus of the present invention, the workpiece surface is inspected at a light intensity that erases the dirt from the image by making the light intensity of the dirt and the normal part the same, so that the surface of the workpiece is inspected with the light intensity that erases the dirt from the image. Defects will be observed.

A bright field illuminator can be configured with a dome illuminator. Dome lighting is, for example, lighting that includes a dome top plate whose inner surface is a concave curved surface, and that reflects illumination onto the dome top plate to irradiate the light evenly. Further, the dark field illuminator can be configured with a low angle ring illumination. Low-angle ring lighting is lighting that irradiates a workpiece with uniformly diffused light from a low angle to the center.

The image processing means converts the captured image into black and white binarization according to a binarization threshold, and performs blob processing to extract a part where the area of a black part is larger than a predetermined area. can be configured to extract With this configuration, it is possible to stably detect scratches other than dirt.

The appearance inspection method of the present invention is an appearance inspection method for inspecting the appearance of a workpiece surface, and includes a brightfield illumination step of irradiating the workpiece surface with brightfield light, and a brightfield illumination step of irradiating the workpiece surface with darkfield light. a dark-field illumination step for irradiating, a dimming step for adjusting the amount of illumination in the bright-field step and the amount of illumination in the dark-field illumination step, and imaging the work surface illuminated by each of the illuminators. In the imaging process and the dimming process, the light intensity ratio between the illumination of the bright field illuminator and the illumination of the dark field illuminator is adjusted to make the light intensity of the stain the same as that of the normal area, and remove this stain from the image. This process includes an image processing step for erasing the stain, and an inspection step for inspecting the surface of the workpiece using the same light intensity as that for a normal area determined in the image processing process.

According to the appearance inspection method of the present invention, the workpiece surface is inspected at a light intensity that erases the dirt from the image by making the light intensity of the dirt the same as the light intensity of the normal area. Defects will be observed.

The bright field illumination step and the dark field illumination step may be performed simultaneously, and an image taken with both the bright field illumination and the dark field illumination turned on may be used in the inspection step. When performing these operations simultaneously in this manner, the external appearance can be observed in a short time, and furthermore, it can be used even if the workpiece side is moving.

An image captured with the bright field illumination in the bright field illumination step turned on with the dark field illumination in the dark field illumination step turned off, and an image taken with the bright field illumination in the bright field illumination step turned off. The bright-field illumination in the bright-field illumination step and the dark-field illumination in the dark-field illumination step are obtained by adding the brightness values of each pixel with the image captured with the dark-field illumination turned on in the dark-field illumination step. An image equivalent to that obtained when both are turned on at the same time may be generated, and this image may be used in the inspection step.

The workpiece whose appearance is inspected by the appearance inspection device and the appearance inspection method is, for example, one that has been cleaned using a cleaning liquid and then dried with the cleaning liquid. As described above, when a cleaning liquid (for example, cleaning water) is used, residual components in the cleaning liquid often do not evaporate and remain as stains. However, in the appearance inspection using this appearance inspection device or the appearance inspection using the appearance inspection method, such dirt is not recognized as a defect such as a scratch, and stable appearance inspection can be performed. This makes it suitable for visual inspection using equipment and visual inspection methods.

When detecting defects on the surface of a workpiece using image processing, it is possible to reduce the number of products to be discarded or re-inspected due to erroneous detection of dirt, contributing to improved productivity.

1 is an overall simplified diagram of a visual inspection device according to the present invention. FIG. 2 is a simplified perspective view showing a workpiece to be inspected. FIG. 3 is an explanatory diagram of reflection due to bright field illumination. FIG. 3 is an explanatory diagram of reflection due to dark field illumination. FIG. 2 is a flowchart of a visual inspection method according to the present invention. FIG. 3 is an image diagram showing defects and stains under various types of illumination. FIG. 3 is an image diagram showing stains caused by changes in light amount. The images obtained when observing the workpiece are shown; (a) is an image obtained by bright-field illumination, and (b) is an image obtained by bright-field illumination and dark-field illumination.

Embodiments of the present invention will be described below based on FIGS. 1 to 8. FIG. 1 shows an appearance observation apparatus according to the present invention, and this appearance observation apparatus, for example, cleans a workpiece 2 (see FIG. 2) using a cleaning liquid (e.g., water), dries the cleaning liquid, and then, This is to perform an external appearance inspection of the workpiece surface 2a. In this case, the workpiece 2 is a bearing ring of a thrust bearing, which is a flat ring body, and the appearance observation device performs an appearance inspection (defect inspection) of such a bearing ring of a thrust bearing. The defect inspection in this case is an inspection for detecting defects such as scratches and dents.

The external appearance observation device includes a measurement table (mounting table) 1 on which a workpiece 2 is placed and held, an illumination system 3 that illuminates a workpiece surface 2a, which is a surface to be inspected, of the workpiece 2 on this mounting table 1, and an illumination system 3. It is equipped with an imaging means 4 that takes an image of the illuminated work surface 2a. The illumination system 3 includes a bright field illuminator 3a that irradiates bright field light onto the work surface 2a, and a dark field illuminator 3b that irradiates dark field light onto the work surface 2a.

In this case, the bright field illuminator 3a uses so-called dome illumination. The dome lighting shown in the figure is an internal reflection type dome lighting, and includes a dome 10 whose inner surface is a concave curved surface (reflection surface) 10a, and is arranged along the circumferential direction inside the lower opening of this dome 10. It is equipped with a plurality of LEDs 11. In this case, the illumination light from the LED 11 is applied to the dome inner surface 10a, and the reflected light reflected by the dome inner surface 10a is applied to the workpiece surface 2a.

Further, the dark field illuminator 3b uses low angle illumination. The low-angle illumination can illuminate the edge of the workpiece 2 from a low angle, and is placed below the dome illumination. This low-angle illumination includes a ring-shaped main body frame 12 and a plurality of LEDs (not shown) disposed along the circumferential direction on the inner peripheral side inclined surface of the main body frame 12. Note that the inner peripheral side inclined surface is an inclined surface whose diameter increases from above to below.

In this embodiment, the imaging means 4 uses an area camera (area sensor camera, area scan camera) 13. Here, the area sensor is a camera in which image pickup elements are arranged vertically and horizontally and can capture two-dimensional images (planar images).

The illumination light of the bright field illuminator 3 a and the illumination light of the dark field illuminator 3 b are dimmed by a dimming means (dimmer controller: dimmer) 15 . Here, dimming refers to adjusting the brightness of light, and dimmers include a PWM control method, a phase control method, a digital control method, and the like. The PWM (Pulse Width Modulation) control system is a dimming control system that uses pulse modulation, and controls output power by repeatedly switching on and off. In other words, the brightness of the light is adjusted each time the lighting time and extinguishing time of the LED are controlled. The phase control method is a method that converts the heat generated by applying voltage to the filament into light, and adjusts the brightness by cutting off part of the wavelength of the AC power source with a triac. The digital control method is a method that sends digital signals from the dimmer to the lighting equipment to control it, and requires two power lines and a signal line for digital control, but because it is digitally controlled, advanced dimming is possible. It becomes possible.

Therefore, the dimming means 15 may be of any one of a PWM control method, a phase control method, and a digital control method.

Furthermore, the camera 13 that constitutes the imaging means 4 is connected to the arithmetic unit 16 that constitutes the image processing means 18 . By the way, the illumination system 3, the imaging means 4, the light control means 15, the image processing means 18, etc. are controlled by a computer (computer control). Here, a computer basically includes an input means with an input function, an output means with an output function, a storage means with a memory function, a calculation means with a calculation function, and a control function. It is composed of a control means provided. The input function is for reading information from the outside into the computer, and the read data or program is converted into a signal in a format suitable for the computer system. The output function is to display calculation results, stored data, etc. externally. The storage means stores and saves programs, data, processing results, and the like. Arithmetic functions process data by calculating and comparing data in accordance with program instructions. The control function decodes the instructions of the program and issues instructions to each means, and this control function oversees all means of the computer. Input means include a keyboard, mouse, tablet, microphone, joystick, scanner, capture board, etc. Furthermore, output means include a monitor, speaker, printer, etc. Storage means include memory, hard disk, CD/CD-R, PD/MO, etc. The calculation means includes a CPU, and the control means includes a CPU, a motherboard, and the like. In this case, a display device 17 is shown as a monitor.

FIG. 3 shows reflected light L2 when the workpiece 2 is irradiated with the illumination light L1 from the bright field illuminator 3a. When the workpiece surface 2a is a normal surface (a flat surface with no defects or dirt), when the illumination light L1 is irradiated from above the workpiece 2, the workpiece surface will change as shown in FIG. 3(a). The reflected light L2 is reflected upward from 2a, and much of the reflected light L2 enters the camera 13. Therefore, the image of the normal surface will be in a bright state.

Further, as shown in FIG. 3(b), when the workpiece surface 2a has a defect d1 (in the example shown, a flaw in a recess with a V-shaped cross section), the illumination light is directed toward the workpiece 2 from above the workpiece 2. When the light L1 is irradiated, it is reflected diagonally upward at the defect d1, and much of the reflected light L2 does not enter the camera. That is, the amount of reflected light toward the camera is reduced, and the image of the defect d1 appears dark.

Further, as shown in FIG. 3(c), when the workpiece surface 2a has dirt d2 (stains remaining without evaporation of residual components in the cleaning liquid), the illumination light is directed toward the workpiece 2 from above the workpiece 2. When the illumination light L1 is irradiated, the illumination light L1 incident on the dirt d2 is reflected in all directions, the reflected light L2 toward the camera is reduced, and the image of the dirt d2 becomes dark.

FIG. 4 shows reflected light L2 when the workpiece 2 is irradiated with illumination light from the dark field illuminator 3b. When the workpiece surface 2a is a normal surface (a flat surface with no defects or dirt), as shown in FIG. Since the light L2 is reflected in a direction largely shifted from the direction of the camera, the image of the normal surface becomes dark.

As shown in FIG. 4(b), when a defect d1 such as a scratch exists on the workpiece surface 2a, most of the illumination light L1 incident on the defect d1 from a small angle with respect to the workpiece surface 2a is reflected toward the camera. Therefore, the amount of reflected light L2 that enters the camera is small, and the amount of light on the defect d1 remains dark, the same as on the normal surface.

As shown in FIG. 4(c), when the workpiece surface 2a has dirt d2 (stain remaining without evaporation of residual components in the cleaning liquid), the illumination light L1 from a small angle with respect to the workpiece surface 2a will cause the dirt to become dirty. When the illumination light L1 enters the dirt d2, it is diffusely reflected in all directions, and a part of the diffusely reflected light (reflected light L2) heads toward the camera, and the dirt d2 is reflected in the surrounding area. It appears brighter than the normal part.

In other words, as shown in FIG. 6, when observing the defect d1 and dirt d2 using regular reflection (bright field) illumination, the entire screen is bright, the defect d1 and dirt d2 appear black, and the diffuse reflection (dark field) illumination shows that the entire screen is bright and the defect d1 and dirt d2 appear black. If the defect d1 is observed under illumination (for visual field), the entire screen is dark and the defect d1 appears dark, making it impossible to detect the defect d1. However, when observing the dirt with diffuse reflection (dark field) illumination, although the entire screen is dark, the part of the dirt d2 appears brighter than the surrounding normal area. Note that if the illumination for specular reflection (bright field) and the illumination for diffuse reflection (dark field) are performed at the same time, they can be erased from the image and only defect d1 can be detected, as described later. becomes.

Next, a visual inspection method using the visual inspection apparatus shown in FIG. 1 will be described with reference to FIG. 5. First, a bright field illumination step S1 and a dark field illumination step S2 are performed simultaneously. The bright field illumination step S1 is a step of irradiating the work surface with bright field light, and the dark field illumination step is a step of irradiating the work surface with dark field light.

When the bright-field illumination and the dark-field illumination are turned on at the same time, the reflected light L2 from the bright-field illumination decreases and the reflected light L2 from the dark-field illumination increases in the case of dirt d2. Become. Therefore, while imaging the workpiece surface 2a with the imaging means 4 (while performing the imaging step 3), so that the amount of decrease in the reflected light L2 for bright field and the amount of increase in the reflected light for dark field are equal, The light intensity ratio between the visual field illumination and the dark field illumination is adjusted (light adjustment step S4 is performed). As a result, as shown in (specular reflection + diffuse reflection) (bright field + dark field) in FIG. 6, the stain d2 has the same amount of light as the normal area, and can be erased from the image. On the other hand, since the defect d1 remains dark even when the bright field illumination and the dark field illumination are turned on at the same time, it is possible to detect only the defect d1 as a result.

In other words, as shown in FIG. 7, by appropriately setting the light intensity ratio, the dirt d2 disappears from the image, and from this state, the light intensity of the dark field illumination is changed to the light intensity of the bright field illumination. If the light intensity of the bright field illumination is made higher than the light intensity of the dark field illumination, the dirt d2 becomes white, and vice versa. go.

Therefore, the image processing step S5 for the workpiece 2 is performed by appropriately setting the light amount ratio so that the dirt d2 disappears from the image. That is, the workpiece surface 2a is irradiated with bright-field light and the workpiece surface 2a is irradiated with dark-field light, and the next light adjustment step S4 and image processing step S5 are entered. In this case, the captured image is displayed on a monitor screen (display device 17). While visually checking this monitor screen, the operator changes the light intensity of each illumination light using the dimming controller 15 so as to achieve an appropriate light intensity ratio that makes the dirt d2 invisible. Then, the external appearance of the workpiece surface 2a is observed (inspection step S6) using an appropriate amount of light that makes the dirt d2 invisible. That is, in the inspection step S6, the operator inspects the monitor screen of the display device 17.

In an image captured using conventional lighting, both the defect d1 and the dirt d2 appear as dark areas and cannot be distinguished, as shown in FIG. 8(a), but with the lighting of the present invention, an appropriate light intensity ratio In the captured image, only the defect d2 remains as a dark part, as shown in FIG. 8(b). Thereby, the inspection step S6 for detecting the defect d1 can be performed.

In other words, the image obtained in this way is converted into a black and white binary image according to an appropriate binarization threshold, and then blob processing is performed to extract areas where the area of the black part is larger than a certain value. By detecting the dark portion using the dark area, it is possible to detect only the defect d1 without erroneously detecting the stain d2.

When the appearance is inspected using the present appearance inspection device and the present appearance inspection method, the workpiece surface 2a is inspected at a light intensity that erases the dirt d2 from the image by making the light intensity of the dirt d2 and the light intensity of the normal area the same. Therefore, the defect d1 is observed with the stain d2 erased. Therefore, when detecting defects on the surface of a workpiece by image processing, it is possible to reduce the number of discarded products and re-inspected products due to erroneous detection of the dirt d2, contributing to improved productivity.

The image processing means 18 converts the captured image into black and white binarization according to a binarization threshold, and performs blob processing to extract areas where the area of the black part is larger than a predetermined value. can be configured to extract With this configuration, it is possible to stably detect defects d1 such as scratches other than dirt d2.

The workpiece 2 whose appearance is inspected by the appearance inspection device and the appearance inspection method is, for example, one that has been cleaned using a cleaning liquid and dried with the cleaning liquid. As described above, when a cleaning liquid (for example, cleaning water) is used, residual components in the cleaning liquid often do not evaporate and remain as stains. However, in the appearance inspection using the present appearance inspection apparatus or the appearance inspection using the appearance inspection method, such dirt d2 is not recognized as a defect d1 such as a scratch. Such dirt d2 is not recognized as a defect d1 such as a scratch, and a stable visual inspection can be performed, making the present visual inspection apparatus and method suitable for visual inspection.

If the bright field illumination step S1 and the dark field illumination step S2 are performed simultaneously, and the image taken with both the bright field illumination and the dark field illumination turned on is used in the image processing step S5, the external appearance observation can be performed for a short time. Furthermore, it can be used even if the workpiece side is moving.

An image taken with the dark-field illumination in the dark-field illumination step S2 turned off and the bright-field illumination turned on in the bright-field illumination step S1, and an image taken with the bright-field illumination turned off in the bright-field illumination step S1. The bright-field illumination in the bright-field illumination step S1 and the dark-field illumination in the dark-field illumination step S2 are calculated by adding the brightness values of each pixel with the image captured with the dark-field illumination turned on in the dark-field illumination step S2. An image equivalent to the case where the visual field illumination is turned on at the same time may be generated, and this image may be used in the inspection step S6.

In the case where the dark field illumination in the dark field illumination step S2 and the bright field illumination in the bright field illumination step S1 are performed with a temporal shift, the dark field illumination in the dark field illumination step S2 and the bright field illumination in the bright field illumination step S1 are Bright field illumination may be used first.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments and can be modified in various ways. For example, in this embodiment, the area camera 4 is a monochrome type with 640 x 480 pixels. I used the one from However, the type of camera is not limited to the conditions of this embodiment, and for example, a color type area camera with 1600 x 1200 pixels may be used. Note that when performing a visual inspection, there are generally line cameras (line sensor cameras, line scan cameras) in addition to such area cameras. Here, the line sensor has a single line of image pickup elements, and captures an image like a developed view by moving a camera or a workpiece and composing the images one by one line by line. For this reason, it is also possible to use a line sensor as the imaging means 4 instead of the area camera. Note that when using a line sensor, as described above, it is necessary to have a configuration in which the work side or the camera side is moved. That is, one of them may be fixed and the other may be movable, and furthermore, the work side and the camera side may be moved.

In this example, dome illumination was used as bright field illumination. However, bright-field illumination is not limited to dome illumination as long as it can create a bright-field condition, and for example, high-angle ring illumination, coaxial epi-illumination, or the like may be used. Furthermore, in this example, low-angle ring illumination was used as dark field illumination. Dark-field illumination is not limited to low-angle ring illumination as long as it can create a dark-field condition; for example, bar illumination may be used to emit illumination light from a low angle.

In this embodiment, defects are detected using blob processing (a method of analyzing images that have been binarized), but blob processing is not limited to this as long as dark areas can be detected. A method (edge detection processing) of extracting the boundary line between the bright area and the bright area may also be used.

By the way, as the workpiece 2 to be visually inspected, although the bearing ring of a thrust bearing is described in the embodiment, the bearing ring is not limited to such a bearing ring, but threaded parts such as bolts and nuts, gears, shafts, etc. The inspection surface may be a variety of mechanical parts such as a shaft or a spring, but it is preferable that the inspection surface be a flat surface. Further, the cleaning liquid is not limited to water, and various cleaning liquids other than water may be used, and the work may not be cleaned using a cleaning liquid.

Further, in the above embodiment, the defect d1 is a flaw in a recess with a V-shaped cross section, but it is not limited to such a recess with a V-shaped cross section, but may be a recess with a semi-polygonal cross section, The bottom surface may be a concave curved surface. Furthermore, in the inspection step S6, the worker inspects the monitor screen of the display device 17, but the display device 17 is capable of informing the worker that the workpiece 2 has the defect d1. There may be. That is, the operator may be shown that the defect d1 exists, or may be notified by voice that the defect d1 is present, without having to observe it on the monitor screen.

2 Work 2a Work surface 3a Bright field illuminator 3b Dark field illuminator 4 Imaging means 15 Light control means 18 Image processing means d1 Defect d2 Dirt S1 Bright field illumination process S2 Dark field illumination process S3 Imaging process S4 Light control process S5 Image processing Process S6 Inspection process

Claims (9)

An appearance inspection device for inspecting the appearance of a workpiece surface,
a bright field illuminator that irradiates bright field light onto the work surface;
a dark field illuminator that irradiates the work surface with dark field light;
a dimming means for adjusting the amount of illumination of the bright-field illuminator and the amount of illumination of the dark-field illuminator;
imaging means for imaging the work surface illuminated by each of the illuminators;
The light intensity ratio between the illumination of the bright field illuminator and the illumination of the dark field illuminator is adjusted by the dimmer means to create a condition in which the light intensity of the dirt and the light intensity of the normal area are made the same and the dirt is erased from the image. 1. An appearance inspection apparatus comprising: an image processing means, and inspecting a workpiece surface with the same light intensity as that of a normal area determined by the image processing means. The visual inspection apparatus according to claim 1, wherein the bright field illuminator is a dome illuminator. The visual inspection apparatus according to claim 1, wherein the dark field illuminator is a low angle ring illuminator. The image processing means converts the captured image into black and white binarization according to a binarization threshold, and performs blob processing to extract a part where the area of a black part is larger than a predetermined area. The external appearance inspection apparatus according to claim 1, wherein the external appearance inspection apparatus extracts the following. 5. The appearance inspection apparatus according to claim 1, wherein the work whose appearance is to be inspected is one that has been cleaned using a cleaning liquid and dried with the cleaning liquid. An appearance inspection method for inspecting the appearance of a workpiece surface, the method comprising:
a bright field illumination step of irradiating the work surface with bright field light;
a dark field illumination step of irradiating the work surface with dark field light;
a dimming step of adjusting the light amount of the illumination in the bright field illumination step and the light amount of the illumination in the dark field illumination step;
an imaging step of imaging the work surface illuminated by each of the illuminators;
In the dimming process, image processing is performed to erase the dirt from the image by adjusting the light intensity ratio between the illumination of the bright field illuminator and the illumination of the dark field illuminator to make the light intensity of the dirt the same as that of the normal area. process and
1. An appearance inspection method comprising: an inspection step of inspecting the surface of a workpiece with the same light intensity as that of a normal section with dirt determined in an image processing step. 7. The method according to claim 6, wherein the bright-field illumination step and the dark-field illumination step are performed simultaneously, and an image taken with both the bright-field illumination and the dark-field illumination turned on is used in the inspection step. Appearance inspection method. An image captured with the bright field illumination in the bright field illumination step turned on with the dark field illumination in the dark field illumination step turned off, and an image taken with the bright field illumination in the bright field illumination step turned off. The bright-field illumination in the bright-field illumination step and the dark-field illumination in the dark-field illumination step are obtained by adding the brightness values of each pixel with the image captured with the dark-field illumination turned on in the dark-field illumination step. 7. The external appearance inspection method according to claim 6, wherein an image equivalent to that obtained when both are turned on at the same time is generated and this image is used in the inspection step. 9. The method of visual inspection according to any one of claims 6 to 8, wherein the work whose appearance is to be inspected is one that has been cleaned using a cleaning liquid and dried with the cleaning liquid.