Method of reading optical image of inspected surface and image reading system employable therein

Abstract: A set of red light sources (111-113) are provided above a printed board (20). One (111) of the light sources is located in the angular aperture of an imaging lens system (140). The red light is applied to the printed board and reflected on a wiring pattern (22) provided thereon. Another light source (120) is provided under the printed board, and emits infrared light to the back surface of the printed board. The infrared light passes through a through hole (25) formed in the printed board and then enters the imaging lens system together with the red light reflected. Compound light consisting of the red light and the infrared light passes through the imaging lens system and is then split into red light and infrared light at a cold mirror (150). The respective lights are detected by image sensors (161, 162), to thereby obtain respective images of the wiring pattern and the through hole.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of reading the optical image of an inspected surface and an image reading system employable in an optical appearance inspecting apparatus for a printed circuit board etc. More particularly, the present invention relates to an improvement for increasing incident light intensity in an image sensor equipped in the image reading system. Description of the Prior Art As is well known in the art, a printed circuit board is provided with a metal wiring pattern on its one or each surface, while a through hole for accepting the lead wires of an electronic component therethrough is formed in a direction penetrating the circuit board. In order to inspect whether or not the wiring pattern and the through hole are formed with accuracy within tolerance, various types of optical appearance inspecting apparatuses are employed. Fig. 27 is a conceptual diagram showing a conventional image reading system employed in a wiring pattern inspecting apparatus. Light 2 emitted from a light source 1 is reflected by a half mirror 3, and applied to the surface of a printed circuit board 5. The printed circuit board 5 is provided with a wiring pattern 6 and a through hole 7, and reflected light 8 obtained by reflection of the light 2 on the surface thereof is imaged on a linear image sensor 9 through the half mirror 3 and an imaging lens 4. Fig. 28 shows exemplary incident photo-levels in the linear image sensor 9, which photo-levels correspond to a linear image along the line K - K in Fig. 27. The wiring pattern 6 has a large light reflectance since the same is made of a metal, and an incident photo-level corresponding to the wiring pattern 6 is also large. On the other hand, an incident photo-level from an insulating base 5a of the circuit board 5 is relatively small, while the light 2 is transmitted through the through hole 7 downwardly from the circuit board 5 and the photo-level therefrom is substantially zero. Thus, it is possible to obtain the image of the wiring pattern 6 by discriminating the respective incident photo-levels using a threshold value TH. The conventional apparatus shown in Fig. 27 is on the premise that differences between the photo-levels from the respective regions 5a, 6 and 7 are large. However, the light reflectance on the surface of the wiring pattern 6 is not necessarily uniform, and the photo-level therefrom may fluctuate. Further, the waveforms of the photo-levels are complicated by reflected light from an inner wall portion of the through hole 7, so that the boundary between respective images of the wiring pattern 6 and the through hole 7 is confused. In order to cope with such circumstances, there has been proposed such a system that another light source is provided on a back surface side of a printed circuit board and the image sensor detects not only the reflected light but also the light which is transmitted through a through hole from the light source newly provided. For example, Japanese Patent Publication Gazette No. 62-29737 (1987) discloses such a technique. Further, Japanese Patent Laying-Open Gazette No. 62-276443 (1987) discloses an apparatus for catching only a hole image. EP-A-0126492 describes an apparatus (Fig. 7) for image analysing a printed circuit board using reflected light, with additional underside elimination to detect correct drilling of holes. However, these prior arts have the following disadvantages (1) and (2). (1) In the apparatus shown in Fig. 27, only half the intensity of the light 2 from the light source 1 is reflected by the half mirror 3 and directed to the printed circuit board 5. Further, only half the light 8 reflected by the printed circuit board 5 transmits through the half mirror 3 to reach the image sensor 9. Even if the light reflectance at the wiring pattern 6 is 100 %, therefore, the intensity of light reaching the image sensor 9 is 1/4 of that of the light outgoing from the light source 1. Thus, the intensity of the light received in the image sensor 9 is relatively small, and image detection accuracy is not necessarily high. This is a common problem not only in an apparatus employed for appearance inspection of a printed circuit board, but also in appearance inspecting apparatuses as to various inspected objects. (2) In optical appearance inspection, it is necessary to independently grasp respective states of formation of the wiring pattern and the through hole, while interpositional relation between the wiring pattern and the through hole must also be inspected. Thus, there is required an inspecting apparatus, which can simultaneously catch respective images of the wiring pattern and the through hole. However, the aforementioned prior arts are merely adapted to catch the image of only one of the wiring pattern image and the through hole image, and is not structured to simultaneously grasp both images. When such a prior art is employed, therefore, it is necessary to separately detect the respective images in order to recognize interrelation between the position of the wiring pattern and that of the through hole and then calculate the positional relation between the both images. Therefore, the inspection time is increased and the system structure is complicated. Such circumstances are particularly aggravated in the stream that the diameter of through holes provided in printed circuit boards is so gradually reduced that through holes (mini via holes) of 0.5 to 0.1 mm in diameter, for example, are employed. SUMMARY OF THE INVENTION The present invention as defined in claims 1 and 15 is directed to a method and an image reading system for reading an image of an object. An object of the present invention is to provide an image reading system which can simultaneously and correctly detect respective images of a wiring pattern and a through hole in a printed circuit board. Another object of the present invention is to perform appearance inspection at high speed with high accuracy. These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a partially fragmented plan view showing an optical inspecting apparatus for a printed circuit board in which an image reading system according to a preferred embodiment of the present invention is provided; Fig. 1B is a partially fragmented side elevational view of the apparatus shown in Fig. 1A; Fig. 2 illustrates an exemplary printed circuit board; Fig. 3A is a conceptional side elevational view of an optical head according to a first preferred embodiment of the present invention; Fig. 3B is a conceptional front elevational view of the optical head shown in Fig. 3A; Fig. 4A illustrates relation between the position of arrangement of a light source for regular reflection and incident luminous flux in an imaging lens system; Fig. 4B illustrates incident luminous flux in an imaging lens system in a modification of the first preferred embodiment; Figs. 5A and 5B are waveform diagrams showing image signals obtained by the system of the first preferred embodiment and binarization processing therefor; Figs. 6A to 6C are conceptional diagrams showing ranges of incidence upon imaging lens systems and incident light intensity levels as to the prior art, the first preferred embodiment and an improvement to be compared with the first preferred embodiment, respectively; Fig. 7 is an explanatory diagram showing arrangement in the improvement; Fig. 8 is an explanatory diagram of a mini via hole; Figs. 9A and 9B are explanatory diagrams of a non-telecentric lens system and a telecentric lens system, respectively; Figs. 10A and 10B illustrate hole images obtained through a non-telecentric lens system and a telecentric lens system, respectively; Figs. 11A, 11B and 11C illustrate an electrical structure of the apparatus shown in Figs. 1A and 1B; Fig. 12 is a conceptional front elevational view of a modification of the first preferred embodiment; Fig. 13 is a conceptional side elevational view of an optical head according to a second preferred embodiment of the present invention; Fig. 14A is a conceptional side elevational view of an optical head according to a third preferred embodiment of the present invention; Fig. 14B is a conceptional front elevational view of the optical head shown in Fig. 14A; Fig. 15 is a conceptional view of the major portion of the optical head shown in Fig. 14A; Figs. 16 and 17 are explanatory diagrams showing relationships between angle of receiving imaging light and waveform of a hole image signal; Fig. 18A is a conceptional side elevational view of an optical head according to a fourth preferred embodiment of the present invention; Fig. 18B is a conceptional front elevational view of the optical head shown in Fig. 18A; Fig. 19 is a conceptional view of the major portion of the optical head shown in Fig. 18A; Figs. 20 and 21 are diagrams showing the shape of the edge of selective reflection mirror; Fig. 22 is an explanatory diagram showing influence of the thickness of the mirror on imaging position; Fig. 23A is a conceptional side elevational view of an optical head employed according to a fifth preferred embodiment of the present invention; Fig. 23B is a conceptional front elevational view of the optical head shown in Fig. 23A; Fig. 24 is a conceptional view of the major portion of the optical head shown in Fig. 23A; Fig. 25A is a conceptional side elevational view of an optical head according to a sixth preferred embodiment of the present invention; Fig. 25B is a conceptional front elevational view of the optical head shown in Fig. 25A; Fig. 26 is a conceptional view of the major portion of the optical head shown in Fig. 25A; Fig. 27 is a principle diagram of a conventional image reading system; and Fig. 28 is a waveform diagram showing exemplary image signals obtained by the system of Fig. 27. DESCRIPTION OF THE PREFERRED EMBODIMENTS A. First Preferred Embodiment <A-1. Overall Structure> Fig. 1A is a fragmented plan view showing a printed circuit board inspecting apparatus 10 in which an image reading system according to a first preferred embodiment of the present invention is incorporated, and Fig. 1B is a side elevational view thereof. The apparatus 10 comprises a lower housing 11 and an upper housing 12, and a movable table 13 is horizontally provided in the vicinity of an upper opening of the lower housing 11. The movable table 13 is formed by a rectangular frame 14 and a glass plate 15 mounted therein, and a bottom surface 15a of the glass plate 15 is coarsely-ground. A printed circuit board 20 is placed on a top surface 15b of the glass plate 15, and supported by the glass plate 15. As shown in Fig. 2, the printed circuit board 20 has an insulating base plate 21 which is formed of glass epoxy, and a printed wiring pattern 22 of copper which is formed on one or each surface thereof. The printed wiring patter 22 has a wiring portion 23 and a land 24, and a through hole 25 passing through or penetrating the printed circuit board 20 is formed in the land 24. Referring again to Figs. 1A and 1B, the frame 14 is slidable on a pair of guide rails 16, and a ball screw 17 extends in parallel with the guide rails 16. A ball nut 19 fixed to the frame 14 is fitted with the ball screw 17, so that the movable table 13 is moved in horizontal (±Y) directions when the ball screw 17 is rotated by a motor 18. On the other hand, an image reading system 50 is provided in the interior of the upper housing 12. An optical head array 100, extending in horizontal (±X) directions, is arranged in a central upper portion of the image reading system 50. This optical head array 100 comprises eight optical heads H0 to H7, which are supported by a support member 101 at regular intervals. The support member 101 is slidable on a guide member 102 in the directions (±X), and the guide member 102 is fixed to a pair of side frame members 51a and 51b. The side frame members 51a and 51b are at fixed positions with respect to the housings 11 and 12. The support member 101 is coupled to a motor 103 through a ball nut (not shown) and a ball screw 104. When the motor 103 is rotated, therefore, the optical heads H0 to H7 are movable with the support member 101 in the directions (±X). A light source 120 for transmitting illumination is arranged under the optical heads H0 to H7. This light source 120 is formed by a large number of infrared ray LEDs which are arrayed in the directions (±X), and substantially functions as a linear light source. This light source 120 is supported by support rods 121 and 122 from the side frames 51. Further, another light source 110 for reflective illumination is mounted on lower portions of the optical heads H0 to H7. As hereinafter described in detail, the light source 110 comprises three one-dimensional arrays of red LEDs extending in the directions (±X). Presser roller mechanisms 200A and 200B are provided in front and at the back of the optical head array 100. The front roller mechanism 200a, comprising eight roller units 210A, is mounted on the side frames 51a and 51b through a shaft 201A. The rear roller mechanism 200B, also comprising eight roller units 210B, is mounted on the side frames 51a and 51b through another shaft 201B. The roller units 200A and 200B have rubber rollers which are supported by swingable arms, and the rubber rollers and the arms are urged by springs. The roller mechanisms 200A and 200B are adapted to press the printed circuit board 20, which are fed under the same, to prevent the circuit board 20 from positional displacement and deflection. A pair of control switch panels 26 are mounted on top surfaces of both sides of the lower housing 11. These switch panels 26 are provided with identical switch groups, so that the switches can be easily controlled from either side of the housing 11. The upper housing 12 is provided therein with a data processing unit 300 for performing various data processing and operation control. <A-2. Schematic Operation> Before explaining the structure of the inspecting apparatus 10 in detail, a schematic operation of the apparatus 10 is described. First, the printed circuit board 20 is placed on the glass plate 15 in the state shown in Figs. 1A and 1B. Then either switch panel 26 is operated so that the motor 18 is positively rotated and the printed circuit board 20 is moved in the direction (+Y) with the movable table 13. The light sources 110 and 120 are turned on. When the printed circuit board 20 reaches the position of the image reading system 50 following movement of the table 13, the rollers of the presser roller mechanisms 200A and 200B are rotated following movement of the printed circuit board 20, while pressing the circuit board 20 against the glass plate 15. The optical heads H0 to H7 read images of the wiring pattern 22 (Fig. 2) and the through hole 25 for each scanning line by reflective illumination from the light source 110 and transmitting illumination from the light source 120 respectively. Internal structures of the optical heads H0 to H7 for such reading are described later. Although the optical heads H0 to H7 are linearly arrayed, it is impossible to read the overall image of the surface of the printed circuit board 20 upon movement in the direction (+Y), since gaps are defined between visual fields of the optical heads H0 to H7. Therefore, the motor 103 is driven after complete movement of the printed circuit board 20 in the direction (+Y), thereby moving the overall optical heads H0 to H7 in the direction (+X). The amount of such movement is rendered half the interarray pitch of the optical heads H0 to H7. After such movement, the motor 18 is reversely rotated to move the printed circuit board 20 in the direction (-Y), to read the images of the wiring pattern 22 and the through hole 25 by the optical heads H0 to H7. Consequently, scanning is performed along solid and broken arrows A1 and A2 in Fig. 1A, thereby implementing image reading over the entire surface of the printed circuit board 20. The read images are supplied to the data processing unit 300, which in turn decides whether the wiring pattern 22 and the through hole 25 are defective or non-defective. <A-3. Detail of Optical Heads> Fig. 3A is a schematical side elevational view showing the internal structure of the optical head H0. While Fig. 3A illustrates only one optical head H0, the other optical heads H1 to H7 have the same structures. The optical head H0 has a casing 130, and the light source 110 for reflective illumination is suspended by support members 116 and 117 which are mounted on a lower portion of the casing 130. The light source 110 is formed by combination of a light source 111 for image-reading through regular reflection and light sources 112 and 113 for image-reading through irregular reflection, and each of the light sources 111, 112 and 113 is a substantially linear light source which is formed by a one-dimensional array of red LEDs 115 (Fig. 3B) emitting red light of a wavelength λ 1 (= 600 to 700 nm). In the following description, the light source 111 is called as "a light source for regular reflection", while the light sources 112 and 113 are called as "light sources for irregular reflection". Among these, the light sources 112 and 113 for irregular reflection are arranged at positions considerably separated from an optical axis LA of an imaging lens system 140 which is provided in the optical head H0, while the light source 111 for regular reflection is provided in such a position that its end surface is in contact with the optical axis LA. As hereinafter described, the imaging lens system 140 is adapted to form the images of the wiring pattern 22 and the through hole 25 of the printed circuit board 20 on CCD linear image sensors 161 and 162 respectively. The light sources 112 and 113 for irregular reflection are arranged in the exterior of the visible field or the angular aperture for the imaging, while the light source 111 for regular reflection is arranged in a part of the angular aperture. Light emitted from the light sources 111, 112 and 113 is applied toward an inspected area AR of the top surface of the printed circuit board 20, which is currently located immediately under the optical head H0. The light source 111 for regular reflection and the light sources 112 and 113 for irregular reflection are provided for reflective illumination for the reason that it is preferable to utilize both of regular reflection and irregular reflection from the wiring pattern 22 in order to correctly catch the image thereof, since the surface of the wiring pattern 22 does not necessarily define a mirror finished surface. The light sources 111, 112 and 113 are shaded on the imaging lens system 140 sides, i.e., in the upper sides in Figs. 1A and 1B. On the other hand, the light source 120 for transmitting illumination is formed by a one-dimensional array of red LEDs 125 (Fig. 3B) generating infrared light of a wavelength λ 2 (= 700 to 1000 nm). This light source 120 is provided on a line perpendicularly intersecting with the optical axis LA of the imaging lens system 140. This light source 120 emits the infrared light toward an area of the back surface of the printed circuit board 20 corresponding to the back side of the inspected area AR in a direction (+Z). The red light applied to the inspected area AR from the light sources 111, 112 and 113 for reflective illumination is reflected by the inspected area AR. In the infrared light emitted from the light source 120 for transmitting illumination, part directed to the through hole 25 is penetrated or transmitted through the through hole 25. The reflected light and the transmitted light are spatially overlapped to become compound light, which in turn is directed to the optical head H0. Referring to Fig. 4A, there is shown luminous flux L 0 outputted from the inspected area AR of the printed circuit board 20 and traveling toward a range of an entrance pupil of the imaging lens system 140. Only a part L a of the luminous flux L 0 , which corresponds to half luminous flux L 0 , reaches the imaging lens system 140 and another part L b corresponding to the remaining half is eclipsed or blocked by the light source 111 and the support member 116. Assuming that the parts L a and L b are called as "effective luminous flux" and "ineffective luminous flux", therefore, each of the projection solid angle (the incoming angle of the compound light in the imaging lens system 140) ω a of the effective luminous flux L a and the projection solid angle ω b of the ineffective luminous flux L b is half the projection solid angle (the angular aperture) ω 0 of the luminous flux L 0 . In other words, the following equations (1) to (4) hold: ω a + ω b = ω 0 ω a = α · ω 0 ω b = (1 - α) · ω 0 α = 1/2 As shown in Fig. 3A, the effective luminous flux L a enters a cold mirror 150 through the imaging lens system 140. The cold mirror 150 is adapted to transmit only infrared light. Therefore, the red light contained in the effective luminous flux L a (i.e., the reflected light LR from the surface of the printed circuit board 20) is reflected by this mirror 150 to progress in the direction (+Y), and imaged on a photos-detective plane of the first CCD linear image sensor 161. Further, the infrared light contained in the effective luminous flux L a (i.e., the transmitted light LT through the through hole 25) is transmitted through the mirror 150 and imaged on a photo-detective plane of the second CCD linear image sensor 162. These CCD linear image sensors 161 and 162 have CCD photo-electric converter cells which are one-dimensionally arrayed in the directions (±X). Therefore, the first linear image sensor 161 detects a one-dimensional image of the surface of the printed circuit board 20 through the reflective illumination, while the second linear image sensor 162 detects a one-dimensional image of the through hole 25 through the transmitting illumination. The printed circuit board 20 and the optical head array 100 are relatively moved by the moving mechanism shown in Figs. 1A and 1B, whereby respective areas of the printed circuit board 20 are scanned and two-dimensional images of the wiring pattern 22 and the through hole 25 are obtained for respective areas on the printed board 20. Image signals obtained in the linear image sensors 161 and 162 are digitalized by circuits described below, and thereafter binarized using threshold values TH1 and TH2, as shown in Figs. 5A and 5B. Fig. 5A shows an exemplary image signal PS 0 which is obtained in the first linear image sensor 161, and Fig. 5B shows an exemplary image signal HS 0 which is obtained in the second linear image sensor 162. Fig. 3B is a typical front elevational view of the optical head H0 shown in Fig. 3A, and the first linear image sensor 161 is omitted in Fig. 3B for convenience of illustration. Optical paths in the optical head H0 are shown only as to the light transmitted through the through hole 25. In this embodiment, a telecentric lens system, which is telecentric on both sides being close to the printed circuit board 20 and the linear image sensors 161 and 162, is employed as the imaging lens system 140. Therefore, an imaging optical axis of the transmitted light from each through hole 25, which is in the visual field of this lens system 140, is parallel to the optical axis LA of the lens system 140 itself in both of the object side and the image-detection side of the lens system 140. The image reading system 50 having the aforementioned structure has the following advantages: (1) Half the light outputted from the surface of the printed circuit board 20 toward the entrance pupil of the lens system 140 is imaged on the linear image sensors 161 and 162. In the prior art shown in Fig. 27, on the other hand, the light 8 reaching the image sensor 9 is 1/4 of the light 2 from the light source 1, as described above. Figs. 6A and 6B are schematical diagrams for the prior art and the embodiment, respectively, showing the range in which the light from the printed circuit board is actually incident within the entrance pupil of the imaging lens system and the transmitted optical density thereof. Referring to Fig. 6A corresponding to the prior art shown in Fig. 27, the light is incident upon the overall area of the entrance pupil while the transmitted optical density thereof is 1/4 of or less than the optical density of the light generated in the light source 1. Referring to Fig. 6B for the embodiment, on the other hand, only half the entrance pupil is utilized, while the transmitted optical density thereof is four times that of the prior art. In this comparison, only the light source 111 for regular reflection is considered as to the embodiment. Therefore, the intensity of light incident upon the linear image sensor 161 in the embodiment is twice that of the light incident upon the image sensor 9 in the prior art, and it is possible to obtain images at a high contrast which are hardly influenced by noises. As shown in Fig. 7, also conceivable is a technique of arranging an optical axis LA of an imaging lens system 140a with inclination from a normal direction NA of a printed circuit board 20 by an angle (+) while inclining a light source 111a for regular reflection from the normal direction NA by an angle (-). When the value of the angle  is increased to some extent in this case, reflected light from the printed circuit board 20 is incident upon the imaging lens system 140a without being eclipsed by the light source 111a, and the intensity of incident light upon a linear image sensor is doubled as compared with the embodiment. In this case, however, image detection is performed in a direction inclined from the normal direction NA, and no image detection is performed just on an inspected area. Fig. 6C schematically shows such circumstances, and displacement of a photo-detective range from a cross point in Fig. 6B expresses such inclination. Due to such inclination, it is impossible to attain focusing entirely over the inspected area, whereby blurring or distortion is caused in a detected image of the wiring pattern 22 and detection accuracy therefor is reduced. On the other hand, no such problem is caused in the arrangement of the embodiment. The mirror 150 arranged above the imaging lens system 140 in the embodiment is not a half mirror, but luminous flux, which is incident upon this mirror 150 within regularly reflected light, is substantially entirely reflected, whereby the light intensity is not reduced by half at the mirror 150. (2) The appearance inspection of the circuit board 20 can be performed at a high speed since the images of the wiring pattern 22 and the through hole 25 are simultaneously obtained. Further, since the imaging lens system 140 is commonly employed in image detection of the wiring pattern 22 and that of the through hole 25, there is no need to provide a plurality of imaging lens systems in paralell in each optical head. (3) Since the images of the wiring pattern 22 and the through hole 25 are separately detected by the linear image sensors 161 and 162 as shown in Figs. 5A and 5B, the threshold value levels TH1 and TH2 can independently be set at optimum values in no consideration of interrelation therebetween. Consequently, it is possible to correctly detect the respective images even if photoreceptive levels of the respective images fluctuate. (4) The imaging lens system 140 is formed by a telecentric lens system, so that it is possible to correctly detect an image of a through hole, such as a mini via hole, having a small diameter. The reason for this is as follows: Fig. 6 illustrates an example of a mini via hole in which the ratio (aspect ratio) of a length (depth) H to a diameter D is given as: H / D = 1.6 mm / 0.3 mm = 5.3 When a non-telecentric lens system 141 is employed as shown in Fig. 9A, respective hole spaces in holes 25a and 25b separated from the optical axis of the lens system are partially shaded from the light source for transmitting illumination, and, as shown in Fig. 10A, hole images 25A and 25B corresponding to the holes 25a and 25b are partially lost in image signals obtained therefrom. When the telecentric lens system 140 is employed as in the embodiment, on the other hand, it is possible to correctly catch images of holes 25a and 25b which are separated from its optical axis (Figs. 9B and 10B). (5) Since the bottom surface 15a (Fig. 2) of the glass plate 15 is coarsely ground, the light from the light source 120 for transmitting illumination is uniformly applied to the back surface of the circuit board 20. Therefore, the photo-level on the image of the through hole 25 is uniformalized. <A-4. Electrical Structure> Fig. 11A as combined with Fig. 11B is a block diagram showing an electrical structure of this embodiment. Wiring pattern image signals PS 0 to PS 7 and through hole image signals HS 0 to HS 7 obtained from the optical heads HO to H7 are converted to digital signals by A-D converters 301, and thereafter supplied to binarizing circuits 302 and 303. Fig. 11C shows combination 304 of the binarizing circuits 302 and 303 corresponding to the optical head H0 in detail. The binarizing circuits 302 and 303 are formed by comparators 305 and 306, and the threshold values TH1 and TH2, which are held by registers 307 and 308, are supplied to the comparators 305 and 306, respectively. The comparators 305 and 306 compare the threshold values TH1 and TH2 with the digitalized image signals PS 0 and HS 0 (see Figs. 5A and 5B) respectively, to output binarized signals, which go high when the levels of the signals PS 0 and HS 0 are higher than the threshold values TH1 and TH2 while going low when the former are lower than the latter. The binarizing circuits 302 and 303 corresponding to the other optical heads H1 to H7 have similar structures, and the threshold values TH1 and TH2 from the registers 307 and 308 are commonly used for the respective pairs of the binarizing circuits 302 and 303. Referring again to Figs. 11A and 11B, the binarized image signals thus obtained are supplied to a pattern inspection circuit 400. The pattern inspection circuit 400 constructs two-dimensional images of the wiring pattern 22 and the through hole 25 on the basis of these image signals, and determines whether the wiring pattern 22 and the through hole 25 are defective or non-defective in accordance with prescribed determination rule. The data processing unit 300 is also provided with a control circuit 310. The control circuit 310 supplies turn-on/off instructions to the light sources 110 and 120 through lighting circuits 311 and 312, and outputs driving control signals to the motors 18 and 103. The motor 18 is provided with a rotary encoder 18E, so that a motor rotation angle signal generated therein is delivered to the control circuit 310. This rotation angle signal defines data processing timing. According to the aforementioned structure, the optical inspecting apparatus 10 shown in Figs. 1A and 1B accurately executes appearance inspection of the printed circuit board 20. <A-5. Modification of First Preferred Embodiment> (1) The light source 111 for regular reflection may be arranged on a part of each angular aperture of the optical heads H0 to H7, and may not necessarily be in contact with the optical axis LA of the imaging lens system 140. Namely, the value of the parameter α in the equations (1) to (3) can be selected in various ways in the following range: 0 < α < 1 When the parameter α is taken at a relatively large value, the effective angular apertures of the optical heads H0 to H7 are widened (Fig. 4B). However, since the light from the light source 111 is incident upon the surface of the printed circuit board 20 at a shallow angle in this case, the reflection angle of regular reflection thereof also becomes shallow and the intensity of reflected light (effective luminous flux) incident upon the imaging lens system 140 is inevitably reduced. When the parameter α is taken at a relatively small value, the reflection angle is increased but the effective angular apertures are narrowed, whereby the intensity of the effective luminous flux is likewise reduced. The intensity of the effective luminous flux can be considered as being proportionate to the product of the solid angles ω a and ω b in the equations (1) to (3). The solid angle ω a expresses the width of the effective angular aperture of the imaging lens system 140, and the solid angle ω b influences a reflection angle in reflection of the light, emitted from the light source 111, by the surface of the printed circuit board 20. Therefore, intensity Q e of the effective luminous flux can be written as follows, using a proportional constant C: Q e = C · ω a ω b The following equation (8) is obtained using the theorem that an arithmetic mean is equal to or more than a geometric mean: Q e = C · ω a ω b ≦ C · (ω a + ω b ) 2 /4 Using the condition of the equation (1), the equation (9) is obtained: Q e ≦ C · ω 0 2 /4 where Q e takes the maximum value C·ω 0 2 /4 when: ω a = ω b For this reason, the embodiment shown in Fig. 4A is constructed so that the condition α= 1/2 and the equation (10) hold. (2) Fig. 12 is a typical front elevational view showing a modification of the optical head H0, which is illustrated in a manner similar to Fig. 3B. In the optical head H0 as modified, a lens system which is telecentric only on the object side (i.e.,a side of the printed circuit board 20) is employed as an imaging lens system 142. It is possible to correctly catch the image of a through hole 25 also by such a structure. (3) The light emitted from the light source for reflective illumination and that emitted from the light source for transmitting illumination may be different in wavelength from each other, and it is not requisite to combine the red light with the infrared light as in the aforementioned embodiment. For example, visible light having a first wavelength may be employed for the light source 110 for reflective illumination in Figs. 3A and 3B, and visible light having a second wavelength may be employed for the light source 120 for transmitting illumination. The first and second wavelengths are different from each other. In this case, a dichroic mirror can be employed in place of the cold mirror 150. Such a dichroic mirror is implemented by a mirror which reflects the light of the first wavelength and transmits the light of the second wavelength. As to each of the light sources 110 and 120, combination of a white light source and a color filter may be employed. While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention. B. Second Preferred Embodiment Fig. 13 illustrates an optical head H0 according to a second preferred embodiment of the present invention, which optical head H0 is employable in the image reading system 50 shown in Figs. 1A and 1B. In this optical head H0, combination of white light sources 111A, 112A and 113A and an S-wave polarizing plate 119 is employed as a light source system for reflective illumination. Therefore, the reflective illumination light is an S-wave light, i.e., a light wave whose electric field vector is in the directions (±X). The polarizing plate 119 is supported from the optical head H0 side by a support member (not shown). On the other hand, a light source system for transmitting illumination is formed by combination of a white light source 120A and a P-wave polarizing plate 129, and the P-wave polarizing plate 129 is supported from the white light source 120A side by a support member (not shown). Therefore, transmitting illumination light is a P-wave light, i.e., light wave whose electric field vector is in an XZ plane. Each illumination light is applied toward a printed circuit board 20 similarly to the case of Fig. 3A, and compound light from the circuit board 20 is incident upon a polarized beam splitter 151 through a telecentric lens system 140. The polarized beam splitter 151 reflects the S-wave and transmits the P-wave. Therefore, S-polarized reflected light LR containing image information of a wiring pattern 22 is imaged on a photoelectric converter plane of a linear image sensor 161, while P-polarized transmitted light LT containing image information of a through hole 25 is imaged on a photoelectric converter plane of another linear image sensor 162. The remaining structure is identical to that of Fig. 3A and the light source 112A is in contact with the optical axis of the imaging lens system 140. Also in this embodiment, respective images of the wiring pattern 22 and the through hole 25 are simultaneously detected by the separate linear image sensors 161 and 162, whereby an effect similar to that of the system shown in Fig. 3A is attained. Alternatively, the light for reflective illumination may be P-polarized, and the light for transmitting illumination may be S-polarized. When a light source, such as a laser source, emitting light which is polarized in a specific direction is employed, no polarizing plate may be employed. C. Third Preferred Embodiment <C-1. Construction> Fig. 14A is a conceptional side elevational view of an optical head H0 according to a third preferred embodiment of the present invention and Fig. 14B is a conceptional front elevational view of the optical head H0 shown in Fig. 14A. Although the optical head H0 in Figs. 14A and 14B is comparable to the optical head shown in Fig. 3A and 3B in many points, the former has a characteristic structure as follows: The optical head H0 in Figs. 14A and 14B, the light source 111 for regular reflection is attached to the supporting member 116 in lateral direction (+Y) and is out of the angular aperture of the imaging lens system 140. The arrangement of the other light sources 112, 113 and 120 is the same as the first preferred embodiment. The light sources 111, 112 and 113 emit red light, while the light source 120 emits infrared light. The red light emitted from the light source 111 is reflected at a selective reflection mirror 30a and is directed to the inspected area AR. On the other hand, the red lights emitted from the light sources 112 and 113 are directly applied to the inspected area AR. The selective reflection mirror 30a is attached to the supporting members 116 and 117, and entirely covers the angular aperture ω 0 (Fig. 15) of the imaging lens system 140. The selective reflection mirror 30a is inclined from the optical axis LA of the imaging lens system 140 by 45°. As shown in Fig. 15, the mirror 30a comprises a plane substrate 31a made of transparent glass. Preferably, the thickness of the plane substrate 31a is equal to or less than 10 mm. A major surface of the plane substrate 31a is conceptionally divided into two regions 33a and 34a, whose boundary line BL extends in the direction perpendicular to the drawing sheet. The mirror 30a is positioned so that the boundary line BL substantially crosses the optical axis LA at a right angle, i.e., 90°. The boundary line BL, therefore, substantially divides the angular aperture ω 0 into two equal parts. A thin film 35a formed of TiO 2 , CeO 2 , ZnO 2 or SiO 2 , for example, is provided on the first region 33a. The thickness of the thin film 35a is about 650 nm (=0.65 µm). Therefore, the first region 33a is a cold mirror region which substantially transmits light having wavelength longer than a characteristic critical wavelength but substantially reflects light having wavelength shorter than the critical wavelength. For example, when the thickness of the thin film 35a is about 650 nm, the critical wavelength thereof is about 700 nm. Thus, when the light for reflective illumination having wavelength λ 1 (=600-700 nm) is incident on the first region 33a, the light is reflected at the first region 33a. On the other hand, when the light for transmitting illumination having wavelength λ 2 (=700-1000 nm) is incident on the first region 33a, the light is transmitted through the first region 33a. The light source 111 for regular reflection faces the fist region 33a, and the red light L 1 (Fig. 7) emitted from the light source 111 is reflected on the first regoin 33a to be directed to the inspected area AR of a printed circuit board 20. As already described with reference to Figs. 14A and 14B, the red lights from the light sources 112 and 113 for irregular reflection are directly applied to the inspected area AR. These red lights are reflected on the inspected area AR to become reflected light LR containing image information of the wiring pattern. The infrared light emitted from the light source 120 for transmitting illumination is incident on the back surface of the printed circuit board 20, and a part thereof entering the through hole 25 passes through the through hole 25 to become transmitted light containing image information of the through hole 25. These reflected light and the transmitted light are spatially overlapped to become compound light L RT , which is incident on the mirror 30a. The reflected light incident on the angular aperture or the entrance pupil of the imaging lens system 140 includes regular reflection light LR 1 and irregular reflection light LR 2 . The regular reflection light LR 1 is obtained through regular reflection of the light from the light source 111 on the inspected region AR, while irregular reflection light LR 2 is obtained through irregular reflection of the lights from the light sources 112 and 113 on the inspected region AR. In positional arrangement of the compound light L RT travelling from the inspected area AR to the angular aperture of the imaging lens system 140, the compound light L RT consists of a first portion L RT1 incident on the first region 33a of the mirror 30a and a second portion L RT2 incident on the second region 34a. The irregular reflection light LR 2 is included in both of the first portion L RT1 and the second portion L RT2 , while the regular reflection light LR 1 is included only in the second region L RT2 . This is because the light from the light source 111 for regular reflection is reflected only on the first regoin 33a of the mirror 30a and is directed to the inspected area AR, and therefore, the light from the light source 111 is incident on the inspected area AR from the left side of the optical axis LA in Fig. 15, so that the regular reflection light is obtained only in the right side of the optical axis LA. The transmitted light LT from the light source 120 is included both of the first portion L RT1 and the second portion L RT2 . The transmitted light LT included in the compound light L RT is transmitted through the mirror 30a to enter the upper space of the mirror 30a regardless of whether the same is incident on the first region 33a or the second region 34a. The regular reflected light LR 1 is transmitted through the second regoin 34a and enters the upper space of the mirror 30a. With respect to the irregular reflection light LR 2 , a part incident on the first region 33a is reflected on the regoin 33a and propagation thereof to the upper space of the mirror 30a is prevented, while another part incident on the second region 24a is transmitted through the region 34a and enters the upper space of the mirror 30a. Therefore, within the compound light L c incident on the imaging lens system 140, a portion L f passing through the right side of the optical axis LA in Fig. 14A includes the transmitted light LT and the reflected light LR, while another portion L g passing through the left side of the optical axis LA includes only the reflected light LR. The compound light L c having these components are incident on a cold mirror 150 through the imaging lens system 140. The cold mirror 150 is such a mirror that only infrared light is transmitted therethrough. Therefore, as is in the first preferred embodiment, the reflected red light LR included in the compound light L c is further reflected on the cold mirror 150 and is incident on a CCD linear sensor array 161. On the other hand, the transmitted infrared light LT is transmitted through the mirror 30a and is incident on another CCD linear sensor array 162. Photoelectric conversion of the lights incident on the linear sensor array 161 and 162 and electronic processing in image processing circuits provided in the rear stage of the sensors 161 and 162 are similar to the first preferred embodiment and description thereof is omitted here. Preferably, a telecentric lens system is employed for the imaging lens system 140, as in the first and second preferred embodiments. <C-2. Advantages of Third Preferred Embodiment> The optical head H0 shown in Figs. 14a, 14B and 15 and the inspection device employing the same have the following characteristic advantages in addition to the generic advantages of the present invention. (1) In this optical head H0, all the optical amount of the light transmitted through the through hole 25 is usable in image detection of the through hole 25. With respect to the regular reflection light, a half the light emitted from the light source 111 to the mirror 30a is reflected on the mirror 30a and directed to the inspected area AR. After reflection on the inspected region AR, the regular reflection light propagates and enters the image sensor 161 without subatantial loss of optical amount. Consequently, half the optical amount of the light emitted from the light source 111 is usable in image detection of the wiring pattern 22. The irregular reflection light is incident on the mirror 30a and a half the optical amount thereof is reflected on the first region 33a. However, the remaining half enters the imaging lens system 140. Thus, in the image detection through regular reflection, half the total optical amount incident into the angular aperture ω 0 of the imaging lens 140 is usable in the image detection of the wiring pattern 22. As a result, utilization efficiency of the lights from the light sources 111-113 and 120 is at a high level, and incident optical amounts on the linear image sensors 161 and 162 are large in the third preferred embodiment. The images obtained are at a high contrast and are hardly influenced by noises. Acculate image detection can be attained with the optical head H0. (2) In the first preferred embodiment shown in Figs. 3A and 3B, the effective light flux ω a is only half the angular aperture ω 0 . Thus, as shown in Fig. 16, the angle ω 1 for obtaining the transmitted light through the through hole 25 is also half the angular aperture ω 0 of the imaging lens system 140. Consequently, the hole image signal HS 0 is asymmetric around the center axis of the through hole 25, so that the accuracy in inspecting the through hole 25 is not extremely high. On the other hand, the third preferred embodiment is advantageous in that the angle ω 1 (Fig. 17) for obtaining the transmitted light through the through hole 25 is equal to the angular aperture ω 0 of the imaging lens system 140 and the hole image signal HS 0 is symmetric around the center axis of the through hole 25. For this reason, the accuracy in inspecting the through hole 25 in the third preferred embodiment is improved as compared with the first preferred embodiment. D. Fourth Preferred Embodiment Figs. 18A, 18B and 19 are a schematic side view, a schematic front view and a schematic view showing major portion of an optical head H0 according to a fourth preferred embodiment of the present invention, respectively. In the folowing description, only the difference between the third and fourth preferred embodiment is described. The optical head H0 according to the forth preferred embodiment comprises a selective reflection mirror 30b under an imaging lens system 140. As shown in Fig. 19, the mirror 30b has a plane substrate 31b made of transparent glass, on which a thin film 35b is deposited. The thin film 35b serves as a selective light-reflection layer. Preferably, the thickness of the plane subatrate 31b is 50 µm or less. The material and the thickness of the thin film 35b are the same as the thin film 35a in the third preferred embodiment. Accordingly, respective regions on the mirror 30b reflect the light for reflective illumination and transmit the light for transmitting illumination. The mirror 30b is inclined from the optical axis LA of the imaging lens system 140 by 45° and a light source 111 for regular reflection is arranged to face the mirror 30b. An edge EP of the mirror 30b is located on the optical axis LA and the mirror 30b covers about half of the angular aperture of the imaging lens system 140. About half the light emitted from the light source 111 for regular reflection is reflected on the mirror 30b to be applied to the inspected area AR. Respective light path of the light from the light sources 112 and 113 for irregular reflection is the same as the third preferred embodiment. Within the compound light L RT from the inspected area AR, a part passing through the left side of the optical axis LA enters the mirror 30b, while another part passing through the right side of the optical axis LA progresses in side space of the mirror 30b and reaches the space above the mirror 30b without entering the mirror 30b. Accordingly, the right half portion Lf (Fig. 18A) and the left half portion Lg of the compound light have the same components as the third preferred embodiment. The compound light going out of the imaging lens system 140 is divided into reflected light LR and transmitted light LT with a dichroic mirror 150 and these lights LR and LT are detected by CCD linear image sensors 161 and 162, respectively. Respective images of the wiring pattern 22 and the through hole 25 are detected through this process and are delivered to inspection circuits. Therefore, this fourth preferred embodiment has advantages similar to the the third preferred embodiment. When the thickness D 31 of the plane substrate or glass plate 31b is relatively small, the edge surface 30E (Fig. 20) may be perpendicular to the photo-reflective major surface of the mirror 30b. On the other hand, when the thickness D 31 is relatively large, it is preferred that the edge surface 30E is processed to a wedge of 45 ° so that the edge surface 30E is in paralell to the optical axis LA. Such a modification allows all the light flux passing through the left side of the optical axis LA to another the mirror 30b without causing the condition that part of the light passing through the right side of the optical axis LA is refracted by the substrate 31b. Preferably, the thickness D 31 of the substrate 30b is made thin regardless of whether the edge surface 30E is shaped into a wedge or not. If the thickness D 31 is extremely large as shown in Fig. 22, the light path of the transmitted light passing through a point Q 0 is changed to a high degree due to refraction in the substrate 30b and, as a result, a convergent point P1 of the light passing through the right side of the optical axis LA is largely deviated from a point P2 at which the light passing through the left side of the optical axis LA. Preferably, the thickness D 31 is so determined that the distance between the points P 1 and P 2 is equal to or less than a value Δy (not shown) , where the value Δy is a predetermined allowable limit of the error in conversion of light on the photo-electric conversion plane of the linear image sensor 162. E. Fifth Preferred Embodiment Figs. 23A, 23B and 24 are a schematic side view, a schematic front view and a schematic view showing major portion of an optical head H0 according to a fifth preferred embodiment of the present invention, respectively. Although the third preferred embodiment is similar to the third preferred embodiment, difference therebetween is in that light for reflective illumination and that for transmitting illumination are separated from each other through distinction between polarized lights rather than distinction of wavelength. That is, according to the fifth preferred embodiment, a light source system for reflective illumination comprises combination of white light sources 111A, 112B and 113B and S-wave polarizing plates 119. Accordingly, the light for reflective illumination is S-wave, i.e., light whose elecrtic field vector is in (±X) directions. The polarizing plates 119 are supported from the optical head H0 side through supporting members (not shown). On the other hand, a light source system 120 for transmitting illumination consists of white light sources 125A and P-wave polarizing plate 129. The P-wave polarizing plate 129 is supported in the system 120 through supporting members (not shown). Accordingly, the light for transmitting illumination is P-wave, i.e, light whose electric vector is in XZ-plane. A mirror 30c provided under an imaging lens system 140 covers the whole area of the angular aperture ω 0 of the imaging lens system 140. The morror 30c comprises a plane substrate 31c made of transparent glass as shown in Fig. 24, and a major surface of the plane substrate 31c is conceptionally divided into a first region 33c and a second region 34c similarly to the third preferred embodiment shown in Figs. 14A, 14B and 15. The boundary line BL between the first regon 33c and the second region 34c crosses the optical axis LA of the imaging lens system 140 at a right angle. Accordingly , the boundary line BL of the regions 33c and 34c substantially divides the angular aperture ω 0 into equal two parts. Within these regions 33c and 34c, the first region 33c is provided thereon with a thin film 35c for splitting light into respective polalized components. The thin film 35c is made of combination of MgF 2 and PbF 2 , for example, and the thickness thereof is about 750 nm (=0.75 µm). Similarly to the light-slitting phenomenon in dichroic mirrors, the first region 33c transmits P-polarized light and reflects S-polarized light. Consequently, the light from the light source 111A for regular reflection is reflected on the first region 33c to be applied to the surface of a printed board 20. The S-polarized lights from the light sources 111A, 112A and 113A are reflected on the wiring pattern 22, while the P-polarized light from the light source 120 transmitted through a through hole 25. A compound light L RT , which is a combination the reflected light and the transmitted light, enters the mirror 30c. Optical character and function of the mirror 30c is understood by rewriting "the lights of respective wavelength" in the description of the third preferred embodiment to "the lights of respective polarizations". The imaging lens system 140 receives the compound light consisting of the S-polarized light and the P-polarized light. The compound light enters a polarized beam splitter 151. The polarized beam splitter 151 transmits S-wave and reflects P-wave. Consequently, the S-polarized reflected light LR (Fig. 23A) having image information of the wring pattern 22 is imaged on the photo-electric conversion plane of a linear image sensor 161, while the P-polarized reflected light LT having image information of the through hole 25 is imaged on the photo-electric conversion plane of another linear image sensor 162. The other construction is the same with the third preferred embodiment. Since respective images of the wiring pattern 22 and the through hole 25 are independently and simultaneously detected by the different image sensors 161 and 162, the fifth preferred embodiment has advantages similar to the third and fourth preferred embodiments, which advantages includs improvement in utilization efficiency of photo-amount of lights. In one modification, the light for reflective illumination may be P- polarized light, while the light for transmitting illumination may be S-polarized light. When light sources emitting polarized lights, such as laser, are employed, polarizing plates 119 and 129 may be omitted. F. Sixth Preferred Embodiment A sixth preferred embodiment is shown in Figs. 25A, 25B and 26. An optical head H0 according to the sixth preferred embodiment is a combination of the fourth and fifth preferred embodiments. That is, two kinds of lights are splitted through the technique of polarized light splitting, while the optical head H0 is provided with a mirror 30d having the same shape and located at the same position as the mirror 30b in Fig. 19. The mirror 30d comprises a transparent glass substrate on which a thin film 35d for polarization splitting is formed. An edge EP of the mirror 30d crosses the optical axis LA of an imaging lens system at a right angle. The operation of this sixth preferred embodiment will be understood by those skilled in the art from the description of the fourth and fifth preferred embodiments, and therefore, the description of the operation of the sixth preferred embodiment is omitted here. G. Modifications and Conclusion (1) In the third and fourth preferred embodiments, lights having different wavelength may be employed as light for reflective illumination and that for transmitting illumination, and they are not limited to red light and infrared light. When visible lights having different wavelength are employed, the cold mirror 150 is replaced with a dichroic mirror. Each of the light sources 110 and 120 may be composed of a white light source and a color filter. (2) In the third and fifth preferred embodiments, utilization efficiency of lights takes a maximum value, when the boundary line BL of the first and second regions crosses the optical axis. However, the boundary line BL may be deviated from the optical axis LA. Further, in the fourth and sixth preferred embodiments, the edge EP of the edge of the mirror 30b or 30d may be deviated from the optical axis LA. Although it is preferred that the mirror 30b or 30d is located so as to cover half the angular aperture ω 0 of the imaging lens system, the present invention is effective as long as the mirror 30b or 30d may cover a part of the angular aperture ω 0 . While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.

Claims: A method of reading respective images of a wiring pattern and a through hole of a printed circuit board having first and second surfaces opposite to each other, in which said wiring pattern is formed on said first surface and said through hole is provided between said first and second surfaces, said method comprising the steps of: (a) applying a first light beam of a first optical character, i.e. of a first wavelength or polarized in a first direction, to said first surface of said printed circuit board, wherein said first light beam is reflected on said wiring pattern to become a reflected light beam; (b) applying a second light beam of a second optical character, i.e. of a second wavelength or polarized in a second direction, to said second surface of said printed circuit board, wherein said second light beam is transmitted through said through hole to become a transmitted light beam; (c) obtaining a compound light beam consisting of said reflected light beam and said transmitted light beam; (d) leading said compound light beam to an imaging optical system; (e) receiving with a light splitter said compound light beam having passed through said imaging optical system to split said compound light beam into said reflected light beam and said transmitted light beam; (f) receiving said reflected light beam with a first image sensor to obtain an image of said wiring pattern; and (g) receiving said transmitted light beam with a second image sensor to obtain an image of said through hole. The method of claim 1, wherein the optical character is a wavelength and said step (a) comprises the steps of: (a-1) applying a first component of said first light beam from a position defined in a part of an angular aperture of said imaging optical system to said first surface of said printed circuit board; and (a-2) applying a second component of said first light beam from a position out of said angular aperture of said imaging optical system to said first surface of said printed circuit board. The method of claim 2, wherein said step (a-1) comprises the steps of; providing a light source having a body whose edge is on an optical axis of said imaging optical axis; and enabling said light source to emit said first component of said first light beam. The method of claim 3, wherein said first light beam is red light and said second light beam is infrared light; and said light splitter is a cold mirror reflecting said red light beam and transmitting said infrared light beam. The method of claim 1, wherein the optical character is a direction of polarization and said step (a) comprises the steps of: (a-1) applying a first component of said first light beam from a position defined in a part of an angular aperture of said imaging optical system to said first surface of said printed circuit board; and (a-2) applying a second component of said first light beam from a position out of said angular aperture of said imaging optical system to said first surface of said printed circuit board. The method of claim 5, wherein said step (a-1) comprises the steps of: providing a light source having a body whose edge is on an optical axis of said imaging optical axis; and enabling said light source to emit said first component of said first light beam. The method of claim 1, wherein said step (a) comprises the step of: (a-1) applying the first light beam at a position defined in a part of an angular aperture of said imaging optical system. The method of claim 7, wherein said step (a) further comprises the step of: (a-2) positioning an edge of said light source on an optical axis of said imaging optical system. The method of claim 8, wherein said step (a) further comprises the step of: (a-3) covering half said angular aperture of said imaging optical system with said light source. The method of one of the proceeding claims, wherein said step (a) comprises the step of: (a-1) applying said first light beam from out of the angular aperture of said imaging optical system to a selective reflection mirror covering the whole of an angular aperture of said imaging optical system, wherein: said selective reflecting mirror has a surface which consists of: a first region capable of reflecting the first light beam of a first optical character and capable of transmitting a second light beam of the second optical character; and a second region capable of reflecting said first and second light beams; and said angular aperture of said imaging optical system is divided into two parts with a boundary of said first and second region; to direct said first light beam to said first surface of said printed circuit board, thereby obtaining a reflected light beam through reflection of said first light beam and said wiring pattern; and said step (d) comprises the step of: (d-1) leading said compound light beam to said imaging optical system through said selective reflection mirror. The method of claim 10, wherein the optical character is a wavelength and said light splitter is a mirror reflecting of the one of said first light and said second light beams in said compound light beam. The method of claim 10, wherein the optical character is a direction of polarization and said light splitter is a polarization beam splitter mirror reflecting only one of said first light and said second light beams in said compound light beam. The method of one of the claims 10 to 12, wherein said boundary substantially divides said angular aperture into two equal parts. The method of claim 13, wherein said selective reflection mirror comprises a transparent substrate and a selective photo-reflection film selectively formed on said transparent substrate; said first region is an exposed surface of said selective photo-reflection film; and said second region is an exposed surface of said transparent substrate. An image reading system for reading respective images of a wiring pattern and a through hole of a printed circuit board having first and second surfaces opposite to each other, in which said wiring pattern is formed on said first surface and said through hole is provided between said first and second surfaces, said image reading system comprising: (a) an imaging optical system facing said first surface of said printing circuit board; (b) first light beam source means facing said first surface of said printed circuit board to emit a first light beam of a first optical character, i.e. of a first wavelength or polarized in a first direction, to said first surface of said printed circuit board, wherein said first light beam is reflected on said wiring pattern to become a reflected light beam; (c) second light beam source means facing said second surface of said printed circuit board to emit a second light beam of a second optical character, i.e. of a second wavelength or polarized in a second direction, to said second surface of said printed circuit board, wherein said second light beam is transmitted through said through hole to become a transmitted light beam which is overlapped with said reflected light beam to obtain a compound light beam entering said imaging optical system; (d) light splitter means for receiving said compound light beam having passed through said imaging optical system to split said compound light beam into said reflected light beam and said transmitted light beam; (e) first image sensor means for receiving said reflected light beam to obtain an image of said wiring pattern; and (f) second image sensor means for receiving said transmitted light beam to obtain an image of said through hole. The image reading system of claim 15, wherein the optical character is wavelength and said first light beam source means comprises: (b-1) first illuminant means provided in a position defined in a part of an angular aperture of said imaging optical system to emit a first component of said first light beam; and (b-2) second illuminant means provided out of said angular aperture of said imaging optical system to emit a second component of said first light beam. The image reading system of claim 16, wherein said first illumination means has a body whose edge is located on an optical axis of said imaging optical system. The image reading system of claim 17, wherein said body of said first illumination means covers half said angular aperture of said imaging optical system. The image reading system of one of the claims 15 to 18, wherein said light splitter means comprises: (e-1) a mirror reflecting only one of said first light and said second light beams in said compound light beam. The image reading system of claim 15, wherein the optical character is a direction of polarization and said first light beam source means comprises: (b-1) first illuminant means provided in a position defined in a part of an angular aperture of said imaging optical system to emit a first component of said first light beam; and (b-2) second illuminant means provided out of said angular aperture of said imaging optical system to emit a second component of said first light beam. The image reading system of claim 20, wherein said first illumination means has a body whose edge is located on an optical axis of said imaging optical system. The image reading system of claim 21, wherein said body of said first illumination means covers half said angular aperture of said imaging optical system. The image reading system of one of the claims 20 to 22, wherein said light splitter means comprises: (e-2) a polarization beam splitter reflecting only one of said first light and said second light beams in said compound light beam. The image reading system of one of the claims 15 to 23, wherein said first light beam source means is provided out of the angular aperture of said imaging optical system, for emitting said first light beam toward a selective reflection mirror covering the whole of the angular aperture of said imaging optical system and comprising a surface which consists of: a first region capable of reflecting a first light beam of the first optical character and capable of transmitting a second light beam of the second optical character; and a second region capable of reflecting said first and second light beams; wherein said angular aperture of said imaging optical system is divided into two parts with a boundary of said first and second regions; to direct said first light beam to said first surface of said printed circuit board to obtain a reflected light beam through reflection of said first light beam and said wiring pattern; and said compound light beam enters said selective reflection mirror and then passes through said imaging optical system. The image reading system of claim 22, wherein said boundary substantially divides said angular aperture into two equal parts. The image reading system of claim 25, wherein said selective reflection mirror comprises: (b-1) a transparent substrate; and (b-2) a selective photo-reflection film selectively formed on said transparent substrate; and wherein said first region is an exposed surface of said selective photo-reflection film; and said second region is an exposed surface of said transparent substrate. The image reading system of claim 26, wherein said first light beam source means emits said first light beam in a direction perpendicular to said optical axis; and said selective reflection mirror is inclined from said optical axis by 45°. The image reading system of one of the claims 15 to 23, wherein said first light beam source means is provided out of the angular aperture of said imaging optical system, for emitting said first light beam towards a selective reflection mirror covering a part of an angular aperture of said imaging optical system and capable of reflecting first light beam of a first optical character and capable of transmitting a second light beam of second optical character, to direct said first light beam to said first surface of said printed circuit board and to obtain the reflected light beam through reflection of said light beam on said wiring pattern; and said compound light beam passes through a space in which said selective reflection mirror is provided and then enters said imaging optical system. The image reading system of claim 28, wherein said selective reflection mirror has an edge located on an optical axis of said imaging optical system. The image reading system of claim 29, wherein said edge substantially divides said angular aperture into two equal parts. The image reading system of claim 30, wherein said selective reflection mirror comprises a transparent substrate and a selective photo-reflection film formed on said transparent substrate. The image reading system of one of the claims 24 to 31, wherein said edge of said selective reflection mirror is processed into a wedge.

Inventors: HOKI TETSUO, UEMURA HARUO, HOKI, TETSUO, UEMURA, HARUO, Hoki, Tetsuo, c/o Dainippon Screen Mfg. Co., Ltd, Uemura, Haruo, c/o Dainippon Screen Mfg. Co., Ltd
Applicants: DAINIPPON SCREEN MFG, DAINIPPON SCREEN MFG. CO., LTD.
Application year: 1991
Main classification: G01N21
IPC codes: G01N21, G01R31