Abstract:
A visual inspecting method for an electronic device, comprising steps of: photographing an image of a surface of the electronic device; dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; and comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and determining that a defect is present at a coordination position where the gradation level thereat is lower than the binarization level thereat.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a visual inspecting method for electronic device, a visual inspecting apparatus for electronic device, and a record medium for recording a program which causes a computer to perform the visual inspecting method, and in particular, to those which inspects defects on a surface of a package of an electronic device. 
     2. Description of the Prior Art 
     A visual inspecting method/apparatus for automatically inspecting a defect such as a small hole (hereinafter, referred to as “void”) that takes place on the top surface of the package of an electronic device that has been fabricated is known. 
     In a first prior art of such a visual inspecting method disclosed as JPA 5-280958, a photographed image of the top surface of an inspection object is divided into a plurality of unit regions. With the average value of gradation levels of each unit region, the image is binarized. Whether a defect exists is detected on the basis of the shape of a region with low reflectance in the photographed image. 
     FIG. 1 is a block diagram showing the structure of a defect inspecting apparatus according to the first prior art. 
     In FIG. 1, a scanning beam is radiated from laser light source  20  to the top surface of inspection object  10 . The reflected light is guided to light receiving units  28 . Light receiving units  28  output image signals a 1  and a 2 , respectively. The image signals a 1  and a 2  are supplied to adding unit  32 , A/D converting unit  34 , brightness converting unit  36 , and differentiating filter  38  in the order. Differentiating filter  38  outputs signal a 5  in which the contour of a defect is emphasized. Signal a 5  is supplied to defect address detecting unit  40 . Defect address detecting unit  40  obtains a defect address signal Ad from signal a 5 . Signal a 5  is also supplied to binarizing unit  42 . Binarizing unit  42  binarizes signal a 5 . The binarized signal is supplied to defect region extracting unit  48 . Defect region extracting unit  48  extracts a defect region on the basis of the binarized signal and defect address Ad and outputs the extracted defect region as defect image signal A 2 . 
     In the defect determining method of the first prior art, the address of a defect is obtained on the basis of the differentiated image of the inspection object  10 . Binarizing unit  42  performs the following calculation. 
     
       
           TH   3 = TH   1   −k ( TH   1 − TH   2 ) 
       
     
     where TH 1  is a first threshold value that is obtained from the distribution of gradation levels of the entire differentiated image; TH 2  is a second threshold value obtained from the average value of the gradation levels of adjacent regions of a considered pixel; and TH 3  is a third threshold value. In addition, binarizing unit  42  binarizes the gradation level of each pixel by using threshold TH 3 . 
     The defect region extracting unit  48  determines that a defect on the top surface of the inspection object  10  is present at a pixel position whose gradation level is lower than threshold value TH 3 . 
     FIG. 2 is a schematic diagram showing each region of the inspecting apparatus shown in FIG.  1 . 
     FIG. 2 shows an example of inspection object  10  shown in FIG. 1 which is a concrete electronic device  2 . Referring to FIG. 2, a photographed image that is output from light receiving unit  28  contains a package  2   a  and a part of terminals  2   b.  A region that contains only the package  2   a  is designated as an inspection objective region Rt. The photographed images of package  2   a  contain an image of void B that is a defect on package  2   a,  images of marking characters T marked on package  2   a,  and an image of fluctuating portion P formed on the top surface of package  2   a  or formed due to reflected light. 
     FIG. 3 is a graph showing characteristic curves of gradation levels Lc, binarization levels Ls, versus pixel coordinate positions of the defect inspecting apparatus shown in FIG.  1 . FIG. 3 shows levels taken along line X—X of FIG. 2. A sharp concave portion on the left of the curve of the gradation levels Lc of the photographed image represents a void B. On the other hand, a broad concave portion on the right of the curve of the gradation levels Lc represents a fluctuating portion P. Three protrusion portions in the middle of the curve of the gradation levels Lc represent marking characters T. 
     At the void B, the peak width is narrow and the curve of the gradation levels Lc sharply varies. At the fluctuating portion P, the peak width is wide than that. of the void B and the curve of the gradation levels Lc gradually varies. At each of the marking characters T, although the peak width is narrow, the curve of the gradation levels Lc is higher than that of the package region. 
     In FIG. 3, the “1” level region where gradation level Lc is higher than binarization levels Ls is determined as a normal region (no-void region), whereas the “0” level where gradation levels Lc is lower than binarization levels Ls is determined as a void region. 
     As is apparent from FIG. 3, at fluctuating portion P, although the curve of gradation levels Lc slightly lowers in a wide range, the curve of binarization levels Ls obtained as the average value of curve of the gradation levels also gradually lowers. 
     Next, a second prior art of such a defect inspecting method will be described. The second prior art is simpler than the first prior art. In the second prior art, all the photographed image of the top surface of a package is binarized with a single predetermined binarization level. When the area of the “0” level region of the digitized image (namely, the area of a region whose reflectance is small) is higher than a predetermined value, the region is determined as a void region. 
     FIG. 4 is a graph showing characteristic curves of gradation levels Lc, binarization levels Ls versus pixel coordinate positions of a defect inspecting apparatus according to the second prior art. Similarly to FIG. 3, FIG. 4 shows curves of the photographed image of package  2   a  as shown in FIG.  2 . to As is apparent from FIG. 4, the curve of binarization levels Ls is constant in all the range. 
     In the conventional defect inspecting method/apparatus, it was determined whether or not a defect such as a void is detected on the top surface of a package of an electronic device in a manner as explained above. 
     However, the conventional defect inspecting methods/apparatuses have the following disadvantages. 
     When electronic device  2  is used as an inspection object, the intensity of light radiated from light radiating source  1  to the top surface of the package  2   a  may vary in dependence on the direction and location of light radiating source  1 . In addition, the reflectance of the top surface of package  2   a  may vary position by position because of fluctuation of the ingredients and surface condition of the resin of electronic device  2  and a stain adhered from a die or the like. In these cases, the gradation levels Lc of image signals al and a 2  in a part of of the package  2   a  may differ from those of other parts as shown in FIGS. 3 and 4. Thus, fluctuating portion P tends to take place. 
     When the gradation level of fluctuating portion P is almost the same as that of void B, after the fluctuating portion P is binarized, it may be determined as the “0” level region. Thus, even if package  2   a  does not have a void, it is often mistakenly detected. 
     In the defect inspecting apparatus according to the first prior art, as shown in FIG. 3, the average value of gradation levels Lc of each unit region is used as a binarization level Ls. However, in this method, in the vicinity of a marking character T shown in FIG. 3, since gradation level Lc of the marking character T is high, the binarization level Ls becomes high. Thus, in the vicinity of a marking character T, a void may be mistakenly detected. 
     On the other hand, in the defect inspecting apparatus according to the second prior art, as shown in FIG. 4, the binarization process is performed for all the inspection region Rt with a constant value of the binarization levels Ls. In this method, in the vicinity of fluctuating portion P shown in FIG. 4, since the curve of gradation levels Lc lowers, a void may be mistakenly detected. 
     SUMMARY OF THE INVENTION 
     In order to overcome the aforementioned disadvantages, the present invention has been made and accordingly, has an object to provide a visual inspecting method for electronic device, a visual inspecting apparatus for electronic device, and a record medium for recording a program which causes a computer to perform the visual inspecting method which allow a real defect to be securely detected without an influence of a fluctuating portion, marking characters, and so forth contained in a photographed image of a package of an electronic device. 
     According to a first aspect of the present invention, there is provided a visual inspecting method for an electronic device, comprising steps of: photographing an image of a surface of the electronic device; dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; and comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and determining that a defect is present at a coordination position where the gradation level thereat is lower than the binarization level thereat. 
     According to a second aspect of the present invention, there is provided a visual inspecting method for an electronic device, comprising steps of: photographing an image of a surface of the electronic device; dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and obtaining a coordinate position where the gradation level thereat is lower than the binarization level thereat; labeling a region composed of a succession of the coordinate positions where the gradation levels thereat are lower than the binarization level thereat; and determining that a defect is present in the labeled region when the area of the labeled region is larger than a predetermined area. 
     According to a third aspect of the present invention, there is provided a visual inspecting apparatus for an electronic device, comprising: a unit for photographing an image of a surface of the electronic device; a unit for dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; a unit for subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; a unit for interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; and a unit for comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and determining that a defect is present at a coordination position where the gradation level thereat is lower than the binarization level thereat. 
     According to a fourth aspect of the present invention, there is provided a visual inspecting apparatus for an electronic device, comprising: a unit for photographing an image of a surface of the electronic device; a unit for dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; a unit for subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; a unit for interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; a unit for comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and obtaining a coordinate position where the gradation level thereat is lower than the binarization level thereat; a unit for labeling a region composed of a succession of the coordinate positions where the gradation levels thereat are lower than the binarization level thereat; and a unit for determining that a defect is present in the labeled region when the area of the labeled region is larger than a predetermined area. 
     According to a fifth aspect of the present invention, there is provided a record medium for recording a program that causes a computer to perform a visual inspecting method for an electronic device, the method comprising steps of: photographing an image of a surface of the electronic device; dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; and comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and determining that a defect is present at a coordination position where the gradation level thereat is lower than the binarization level thereat. 
     According to a sixth aspect of the present invention, there is provided a record medium for recording a program that causes a computer to perform a visual inspecting method for an electronic device, the method comprising steps of: photographing an image of a surface of the electronic device; dividing the photographed image into a plurality of unit regions and obtaining a distribution of gradation levels for each unit region; subtracting a predetermined offset value from the gradation level of the highest frequency selected from the gradation levels for each unit region so as to obtain a binarization level for each unit region; interpolating the binarization levels for unit regions so as to obtain a binarization level at each coordination position of the photographed image; comparing the gradation level at each coordination position of the photographed image with the binarization level at each coordination position and obtaining a coordinate position where the gradation level thereat is lower than the binarization level thereat; labeling a region composed of a succession of the coordinate positions where the gradation levels thereat are lower than the binarization level thereat; and determining that a defect is present in the labeled region when the area of the labeled region is larger than a predetermined area. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of the best mode embodiments thereof, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram showing the structure of a defect inspecting apparatus according to a first prior art; 
     FIG. 2 is a schematic diagram showing each region of an inspection object of the defect inspecting apparatus shown in FIG. 1; 
     FIG. 3 is a graph showing characteristic curves of gradation levels Lc, binarization levels Ls versus pixel coordinate positons of the defect inspecting apparatus shown in FIG. 1; 
     FIG. 4 is a graph showing characteristic curves of gradation levels Lc, binarization levels Ls versus pixel coordinate positions of a defect inspecting apparatus according to a second prior art; 
     FIG. 5 is a block diagram showing the structure of a visual inspecting apparatus for electronic devices according to a first embodiment of the present invention; 
     FIG. 6 is a schematic diagram showing each region of an inspection object of the visual inspecting apparatus shown in FIG. 5; 
     FIG. 7 is a flow chart showing an inspecting process of the visual inspecting apparatus shown in FIG. 5; 
     FIG. 8 is a graph showing characteristic curves of gradation levels Lc, binarization levels Ls versus pixel coordinate positions Cd realized in the visual inspecting apparatus shown in FIG. 5; 
     FIG. 9 is a schematic diagram showing each coordinate position of the visual inspecting apparatus shown in FIG. 5; 
     FIG. 10 is a graph showing a distribution of gradation levels Lc in each unit region Ru of the visual inspecting apparatus shown in FIG. 5; 
     FIG. 11 is a schematic diagram showing labeled results of the visual inspecting apparatus shown in FIG. 5; and 
     FIG. 12 is a block diagram showing the structure of a visual inspecting apparatus for electronic devices according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 5 is a block diagram showing the structure of a visual inspecting apparatus for electronic devices according to a first embodiment of the present invention. 
     Referring to FIG. 5, a radiating light source  101  radiates light to an electronic device  102 . 
     The electronic device  102  reflects the light radiated from the radiating light source  101  corresponding to the reflectance of the top surface thereof. 
     A CCD camera  103  photographs a predetermined region of the reflected light of the electronic device  102  and outputs an analog image signal Sa to an A/D converting unit  104 . 
     The A/D converting unit  104  converts the analog image signal Sa into digital gradation image data Db and outputs the digital gradation image data Db to a gradation image data storing unit  105 . 
     The gradation image data storing unit  105  temporarily stores the digital gradation image data Db. 
     A unit region gradation level distribution calculating unit  106  reads digital gradation image data Db from the gradation image data storing unit  105 , calculates a distribution of digital image gradation levels (see FIG. 8) in each unit region Ru (see FIG. 9) with a predetermined center sampling coordinate position Cs in an inspection objective region Rt, and outputs unit region gradation level distribution data Dc as a histogram. 
     A sampling coordinate position binarization level calculating unit  107  searches the highest frequency gradation level Lc of each unit region Ru (see FIG. 9) from the unit region gradation level distribution data Dc, subtracts a predetermined offset value from the searched gradation level Lc (see FIG.  8 ), and outputs the resultant value as sampling coordinate position binarization level data Dd that is a binarization level Ls (see FIG. 8) at each sampling coordinate position Cs (see FIG. 9) to a pixel coordinate position binarization level calculating unit  108 . 
     The pixel coordinate position binarization level calculating unit  108  interpolates the sampling coordinate position binarization level data Dd corresponding to the binarization levels Ls (see FIG. 8) at the sampling coordinate positions Cs (see FIG.  9 ), calculates binarization levels Ls (see FIG. 8) at the individual pixel coordinate positions in the inspection objective region Rt (see FIG.  9 ), and outputs the binarization levels Ls as pixel coordinate position binarization level data De. 
     The pixel coordinate position binarization level data De and the digital gradation image data Db are supplied to a binarizing unit  109 . When the gradation level (see FIG. 8) at each pixel coordinate position is higher than the binarization level Ls thereof (see FIG.  8 ), the binarizing unit  109  sets “1” to the digitizing data at the pixel coordinate position. When the gradation level at each pixel coordinate position is lower than the binarization level Ls thereof, the binarizing unit  109  sets “0” to the digitizing data at the pixel coordinate position. The binarizing unit  109  outputs the resultant binarized image as binarized image data Df. 
     The binaraized image data Df is supplied to a labeling unit  110 . The labeling unit  110  designates the same label to successive pixels with level “0” (as labeled regions Rl 1  and Rl 2  shown in FIG.  11 ), calculates the area of each labeled region (Rl 1  and Rl 2  shown in FIG.  11 ), and outputs the resultant data as label data Dg. 
     The label data Dg is supplied to a determining unit  111 . When at least one of the areas of the labeled regions (Rl 1  and Rl 2  shown in FIG. 11) is larger than a predetermined value, the determining unit  111  determines that there is a void and outputs a determination signal Sh. 
     FIG. 6 is a schematic diagram showing each region of an electronic device inspected by the visual inspecting apparatus shown in FIG.  5 . 
     In FIG. 6, a photographed image of the CCD camera  103  contains a package  102   a  and a part of terminals  102   b  of the electronic device  102 . A region that contains only the package  102   a  is an inspection objective region Rt. The photographed image of the package  102  contains a void B that is a defect on the package  102   a,  a marking character T marked on the package  102   a,  and a fluctuating portion P formed on the top surface of the package  102   a  or due to reflected light. 
     FIG. 7 is a flow chart showing an inspecting process of the visual inspecting apparatus shown in FIG.  5 . 
     Light radiated from the radiating light source  101  shown in FIG. 5 is reflected on the top surface of the package  102   a.  The reflected light enters the CCD camera  103 . The CCD camera  103  converts the light into an analog signal. The analog signal is supplied to the A/D converting unit  104 . The A/D converting unit  104  converts the analog signal into a digital signal. The digital signal is supplied to the gradation image data storing unit  105 . The gradation image data storing unit  105  temporarily stores the digital data as digital gradation image data Db that represents a gradation level Lc at each pixel coordinate position (at step S 301 ). 
     The unit region gradation level distribution calculating unit  106  shown in FIG. 5 reads digital gradation image data Db from the gradation image data storing unit  105  (at step S 302 ). 
     The unit region gradation level distribution calculating unit  106  shown in FIG. 5 obtains the distribution of gradation levels Lc (shown in FIG.  10 ) in each unit region Ru with a predetermined center sampling coordinate position Cs in the inspection objective region Rt shown in FIG.  9  and outputs the resultant data as unit region gradation level distribution data Dc in each unit region Ru (at step S 303 ). 
     When the area of a unit region Ru containing a marking character T shown in FIG. 9 is too small, the gradation level Lc of the marking character T becomes the highest frequency distribution data of the unit region Ru. Thus, the binarization level Ls becomes higher than the normally designated level. Consequently, a detection error will take place. Thus, the area of each unit region Ru is designated so that the gradation level Lc of other than the marking character T becomes the highest frequency gradation level Lc. 
     The sampling coordinate position binarization level calculating unit  107  shown in FIG. 5 obtains the highest frequency gradation level Lc of each unit region Ru from the unit region gradation level distribution data Dc, subtracts a predetermined offset value from the highest frequency gradation level Lc, and outputs the resultant value as sampling coordinate position binarization level data Dd that is a binarization level Ls at each sampling coordinate position Cs (at step S 304 ). 
     The sampling coordinate position binarization level calculating unit  107  pre-designates the offset value so that the gradation level of a void B is “0” and that the gradation level Lc of non-void regions on the top surface of the package  102   a  is “1”. 
     The pixel coordinate position binarization level calculating unit  108  shown in FIG. 5 interpolates the binarization levels Ls at individual sampling coordinate positions Cs in the sampling coordinate position binarization level data Dd to obtain binarization levels Ls at individual pixel coordinate positions in all the inspection objective region Rt, and outputs the binarization levels Ls as pixel coordinate position binarization level data De (at step S 305 ). 
     In this example, a binarization level Ls of a pixel coordinate position Cd=(X, Y) is calculated by the following formula with binarization levels Ls 1 , Ls 2 , Ls 3 , and Ls 4  of four sampling coordinates Cs=(X 1 , Y 1 ), (X 2 , Y 2 ), (X 3 , Y 3 ), and (X 4 , Y 4 ) that surround the pixel coordinate position Cd. 
     
       
         Ls=(1−α)(1−β)Ls 1 +α(1−β) Ls 2 +(1−α)βLs 3 +αβLs 4   
       
     
     where α=(X−X 1 )/(X 2 −X 1 ); and β=(Y−Y 1 )/(Y 3 −Y 1 ). 
     Thus, binarization levels Ls at four sampling coordinate positions Cs that surround a particular pixel coordinate position Cd are interpolated and thereby a binarization level Ls of the pixel coordinate position Cd is obtained. Even if the number of sampling coordinate positions Cs that surround a particular pixel coordinate Cd is three or less, a binarization level Ls at the pixel coordinate position Cd can be obtained. 
     If a binarization level Ls at a particular pixel coordinate position Cd is obtained without using an interpolating calculation with binarization levels Ls at sampling coordinate positions Cs, binarization levels Ls at the boundary of adjacent unit regions Ru become discontinuous and sharply vary. Thus, the measurement accuracy of the area of a void in the vicinity of the boundary deteriorates. Consequently, a void may not be detected. In addition, a fluctuating portion P may be detected as a void. 
     According to the present invention, in order to solve such a problem, a binarization level Ls at a pixel coordinate position Cd is obtained by interpolating binarization levels Ls at sampling coordinate positions Cs. Since binarization levels Ls of the inspection objective region Rt successively and gradually vary, a void can be prevented from being mistakenly detected in the vicinity of the boundary of adjacent unit regions Ru. 
     The binarizing unit  107  shown in FIG. 5 performs a binaraization process in dependence on whether or not the gradation level Lc of the digital gradation image data Db at each pixel coordinate position Cd is higher than the binarization level Ls of the pixel coordinate position binarization level data De and outputs the resultant data as digitized image data Df. When the gradation level Lc of each pixel is higher than the binarization level Ls, it is determined that the gradation level Lc is “1”. When the gradation level Lc of each pixel is not higher than the binarization level Ls, it is determined that the gradation level Lc is “0” (at step S 306 ). 
     The labeling unit  110  shown in FIG. 5 labels successive “0” level regions of digitized image data Df as shown in FIG.  11  and outputs label data Dg that represents the positions and areas of labeled regions Rl 1  and Rl 2  (at step S 307 ). 
     The determining unit  111  shown in FIG. 5 compares the area of each of the labeled regions Rl 1  and Rl 2  shown in FIG. 11 with a predetermined value. When the area of any one of the labeled areas is larger than the predetermined value, the determining unit  11  determines that a defect is present and outputs a determination signal Sh (at step S 308 ). 
     FIG. 8 is a graph showing characteristic curves of gradation levels Lc, binarization levels Ls versus pixel coordinate positions Cd of the visual inspecting apparatus shown in FIG.  5 . 
     FIG. 8 shows levels taken along line II−II of FIG. 6. A sharp concave portion on the left side represents a void B. A broad concave portion on the right side represents a fluctuating portion P. Three protrusion portions in the middle represent marking characters T. 
     In FIG. 8, a “1” level region whose gradation level Lc is higher than the binarization level Ls is a normal region (non-void region). In contrast, a “0” level region whose the gradation level Lc is lower than the binarization level Ls is a defect region (void region). 
     According to the embodiment of the present invention, as long as the area of each unit region Ru is designated so that the highest frequency gradation level Lc of each unit region Ru is a normal gradation level of the package  102 , the inspected result is not adversely affected by a fluctuating portion P, a marking character T, and so forth. 
     When the area of each unit region Ru is too small, the highest frequency gradation level Lc of a unit region Ru containing a marking character T becomes the gradation level Lc of the marking character T. Thus, the actual binarization level Ls becomes higher than the desired binarization level Ls. Consequently, a void is mistakenly detected in all regions that do not contain a marking character T. To prevent that, the area of each unit region Ru is pre-designated so that the sub-area in each unit region Ru that does not contain a marking character is larger than the sub-area in the unit region that contains a marking character. 
     Corresponding to such a theory, as shown in FIG. 8, according to the embodiment of the present invention, the binarization level Ls varies corresponding to the gradation level Lc of the fluctuating portion P. After the fluctuating portion P has been digitized, it is not determined as a “0” level region. Thus, the fluctuating portion P can be prevented from being mistakenly determined as a void. 
     FIG. 9 is a schematic diagram showing each coordinate position of the visual inspecting apparatus shown in FIG.  5 . In FIG. 9, a sampling coordinate position Cs is designated at the center of each unit region Ru. Alternatively, the sampling coordinate position Cs may be designated at any position of each unit region Ru. 
     FIG. 11 is a graph showing labeled results of the visual inspecting apparatus shown in FIG.  5 . After digitized image data has been digitized, successive “0” level pixels are labeled (for example, labeled regions Rl 1  and Rl 2 ). In the example shown in FIG. 11, when the predetermined area value is “3”, the determining unit  111  outputs a determination signal that represents that a void is present in the labeled region Rl 1 . 
     As described above, according to this embodiment of the present invention, (1) the binarization level Ls of each unit region Ru is obtained with the maximum frequency gradation level of each unit region Ru; and (2) the binarization level Ls of each unit region Ru is interpolated and thereby the binarization level Ls at each pixel coordinate position is obtained. Thus, a defect can be securely detected without influences from a fluctuating portion P and a marking character T. Consequently, an incorrect determination can be remarkably reduced. 
     Next, modifications of the embodiment will be described. 
     As a first modification of the embodiment, the labeling unit  110  shown in FIG. 5 designates a non-labeled mask region to a part of the inspection objective region Rt. Labeling unit  110  does not output a label data Dg for a non-labeled mask region. 
     As a second modification of the embodiment, the labeling unit  110  shown in FIG. 5 calculates the distance between two points that are the farthest on the outer periphery of each of the labeled regions Rl 1  and Rl 2 . When the distance is larger than a predetermined threshold value, the labeling unit  110  determines that a void is present in each of the labeled regions Rl 1  and Rl 2 . 
     As a third modification of the embodiment, the labeling unit  110  shown in FIG. 5 calculates the area of each of the labeled regions Rl 1  and Rl 2  and the distance between two points that are the farthest on the outer periphery of each of the labeled regions Rl 1  and Rl 2 . When the area or the distance is larger than a predetermined area threshold value or a predetermined distance threshold value, the determining unit  111  determines that a void is present in each of the labeled region Rl 1  and Rl 2 . 
     FIG. 12 is a block diagram showing the structure of a visual inspecting apparatus for electronic devices according to a second embodiment of the present invention. In the second embodiment, the visual inspecting apparatus according to the first embodiment shown in FIG. 5 is suitably structured with a computer. 
     The visual inspecting apparatus according to the second embodiment comprises an input unit  201 , a CPU  202 , a memory  203 , a radiating light source  204   a,  a CCD camera  204   b,  an A/D converter  204   c,  a display processing unit  205 , a display unit  206 , an external storing unit  207 , an interface unit  208 , and a bus  209 . 
     The input unit  201  is an operation means such as a keyboard and a remote controller that have various operation keys or the like. The input unit  201  supplies various commands to the CPU  202 . In the computer, the input unit  201  is accomplished by for example an alphanumeric keyboard, a dedicated input unit, a computer mouse, and/or a remote controller. 
     The CPU  202  is a micro-computer, a micro-processor or the like. The CPU  202  operates with programs and so forth stored in the memory  203  or received from an external unit and controls various portions. 
     The memory  203  is composed of for example a RAM. The memory  203  stores various types of data under the control of the CPU  202 . In the computer, the memory  203  is accomplished by various record mediums such as a RAM, a flash memory, and a hard drive. 
     The radiating light source  204   a  radiates light to an electronic device  102 . 
     The CCD camera  204   b  photographs reflected light of a predetermined region of the electronic device  102  and outputs an analog image signal Sa. 
     The A/D converter  204   a  converts the analog image signal Sa received from the CCD camera  204   b  into a digital signal and supplies the digital signal as digital gradation image data Db to the bus  209 . 
     The display processing unit  205  is connected to the bus  209 . The display processing unit  206  converts the image data received from the bus  209  into an image signal Sdp. 
     The display unit  206  is an image display means such as a display unit or a monitor. The display unit  206  displays the image signal Sdp as an image. In the computer, the display unit  206  is accomplished by for example one of various types of display units. 
     The external storing unit  207  is a storage medium that stores various process programs for the CPU  202  and data stored in the memory  203 . Various types of data are written to and read from the external storing unit  207  under the control of the CPU  202 . In the computer, the external storing unit  207  is accomplished by various types of storage mediums such as a RAM, a flash memory, and a hard drive. 
     The interface unit  208  interfaces the CPU  202  with an external unit. 
     The bus  209  mutually connects the input unit  201 , the CPU  202 , the memory  203 , the A/D converter  204   c,  the display processing unit  205 , the external storage unit  207 , and the interface unit  208 . 
     The visual inspecting method of the second embodiment is the same as that of the first embodiment. 
     Next, a record medium for recording a program that causes a computer to execute the visual inspecting method for electronic devices according to the second embodiment of the present invention will be described. 
     In the visual inspecting method/apparatus according to the first embodiment of the present invention, a program that causes a computer to execute the visual inspecting method for electronic devices is stored as a control program of a dedicated visual inspecting apparatus. However, according to the second embodiment, the same method of the first embodiment is accomplished by a general purpose computer with a software program or the like. 
     The software program may be stored to the external storing unit  207  such as a memory card, a floppy disk, a hard drive, a CD-ROM, or a DVD-RAM. Alternatively, the software program may be read from the external storing unit  207  and stored to a designated region of a data storing portion  103 . 
     The other structure, method, and flow chart of the record medium according to second embodiment are the same as those of the visual inspecting method, the visual inspecting apparatus, and the visual inspecting process according to the first embodiment. 
     According to the above-described embodiments, each pixel of a photographed image of the CCD cameras  103 / 204   a  is digitized. Alternatively, to perform the process at high speed, a plurality of pixels can be digitized at a time. 
     In addition, another interpolating method may be used. 
     Moreover, as shown in FIG. 9, each unit region Ru does not overlap each other. Alternatively, each unit region Ru may overlap each other in such a manner that the size of each unit region Ru is larger than a predetermined value. In this case, the area of each unit region Ru can be prevented from being too small. Thus, the probability that a marking character T is mistakenly determined as a void can be further reduced. 
     With the above-described method and means, the visual inspecting method for electronic devices, visual inspecting apparatus, and record medium for recording a program that causes a computer to perform the visual inspecting method have the following effects. 
     As a first effect, after a binarization level at each sampling coordinate position is obtained with the distribution of gradation levels in each unit region, a binarization level at each sampling coordinate position is obtained by interpolation and thereby a binarization level at each pixel coordinate position is obtained. Thus, successive binarization levels can be designated without influences from a marking character, a void, and so forth. Consequently, the probability that a fluctuating portion is mistakenly determined as a void can be remarkably reduced. As a result, the inspection accuracy can be remarkably improved. 
     As a second effect, since successive “0” level regions obtained in the digitizing process are labeled and a void is determined corresponding to the areas of the labeled regions, the probability that a fluctuating portion is mistakenly determined as a void can be remarkably reduced. Thus, the inspection accuracy can be remarkably improved. 
     Although the present invention has been shown and described with respect to the best mode embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.