referring now to fig1 and 2 , a through - hole void detection apparatus according to an embodiment of the invention will be described . as shown in fig1 a printed wiring board 1 has a multi - layer structure in which each layer has a wiring pattern 1b . a through hole 2 is formed passing through the layers . the inner wall of the through hole 2 and the top and bottom surfaces of the printed wiring board 1 are covered with copper foils 3a and 3b , respectively . the copper foil 3a covering the inner wall of the through hole 2 has a scattering surface which can reflect an exciting beam 13 from a photo - detection optical system 5 to be described later . a portion of the copper foil 3a on the inner wall of the through hole 2 defectively peels off to form a defect , through - hole void 4 . the photo - detection optical system 5 comprises a lamp 6 , a collector lens 7 , an excitation filter 8 , a dichroic mirror 9 , a microscope objective lens 10 , an absorption filter 11 and a detector 12 . the lamp 6 is , for example , an extra high - pressure mercury arc lamp which can emit an exciting beam 13 of high intensity . the collector lens 7 collects the exciting beam emitted from the lamp 6 . the excitation filter 8 can pass a wavelength of exciting beam 13 which is optimum for generating a luminescent beam 14 when it impinges upon a layer material 1a . thus , only the exciting beam 13 of a wavelength limited to a value within a specified wavelength band as shown in fig2 a is permitted to pass through the filter 8 . the dichroic mirror 9 can pass the luminescent beam 14 ( having a longer wavelength than that of the excited beam 13 ) while reflecting the exciting beam 13 , as shown in fig2 b . the microscope objective lens 10 has a large value of numerical aperture na = sin θ . the lens 10 has a focusing angle θ of 10 ° or more for the excited beam 13 and luminescent beam 14 shown in fig1 to set up na ≧ sin 10 ° so that the luminescent beam 14 coming from the through hole 2 can be focused sufficiently . the absorption filter 11 can absorb the wavelength band of the exciting beam 13 and pass only the luminescent beam 14 , as shown in fig2 c . the detector 12 is , for example , a silicon intensifier target tv camera or an image intensifier tv camera of ultra - high sensitivity which can detect a very weak luminescent beam of 10 - 3 to 10 - 2 luxes or less . the detector 12 may also be of a linear image sensor type , particularly , a commercially available ultra - high linear image sensor wherein an image intensifier tube and an ordinary linear image sensor are used in combination . the luminescent beam 14 generated when the exciting beam 13 impinges upon the layer material 1a has no directivity and scatters uniformly from the through - hole void 4 to the through hole 2 . a method of detecting a through - hole void , is carried out using the through - hole void detection apparatus of the above construction , as will be described below . an exciting beam 13 emitted from the lamp 6 is collected by the collector lens 7 . a wavelength of exciting beam 13 limited to a wavelength band shown in fig2 a can be passed through the excitation filter 8 . the output beam from the filter 8 is reflected by the dichroic mirror 9 and focused on a point in the through hole 2 by means of the microscope objective lens 10 objective lens 10 represents a focusing optical system having the large value of numerical aperture na to establish na ≧ sin 10 °, so that the exciting beam 13 may sufficiently diffuse in the through hole 2 to irradiate the entire area of the surface of the copper foil 3a . in the presence of a through - hole void 4 in the through hole , the exciting beam 13 impinges upon the layer material 1a of the printed wiring board 1 through the through - hole void 4 . the luminescent beam 14 generated from the layer material 1a scatters to the through hole 2 by way of the through - hole void 4 . of scattered luminescent beams 14 , beams heading upwards are repetitively reflected at the surface of the copper foil 3a to reach an entrance 2a of through hole 2 and focused on the detector 12 by means of the microscope objective lens 10 &# 39 ; representing the focusing optical system . the dichroic mirror 9 reflecting the exciting beam 13 passes only the luminsecent beam 14 , as shown in fig2 b . since the absorption filter 11 absorbs the wavelength band of exciting beam 13 while passing only the luminescent beam 14 , as shown in fig2 c , the detector 12 detects a bright image at the entrance 2a of through hole 2 in the presence of a through - hole void 4 in through hole 2 but detects a dark image at the entrance 2a of through hole 2 in the absence of any through - hole void 4 , indicating that no luminescent beam is detected . the detector 12 , formed of a linear image sensor of ultra - high sensitivity , detects a luminescent beam of weak intensity which is 10 - 3 to 10 - 2 luxes or less . referring to fig3 another embodiment of the invention shown therein comprises a lamp 6 provided on one side of the printed wiring board 1 to oppose the detector 12 on the other side of the board 1 by way of through - hole 2 . obviously , this embodiment dispenses with the dichroic mirror 9 of the fig1 embodiment but instead needs a condenser lens 15 serving as a condensing optical system capable of condensing the exciting beam 13 . the condenser lens 15 has a large numerical aperture comparable to that of a microscope objective lens 10 &# 39 ;. referring to fig4 another embodiment of the invention shown uses , as the exciting beam 13 , a laser beam emitted from a laser source 16 and collected and broadened by means of the collector lens 7 . the laser source 16 can emit a single wavelength or a plurality of wavelengths limited to a specified wavelength band . therefore , by using a laser source capable of emitting only a laser beam of the wavelength falling within the necessary exciting beam band , the excitation filter 8 of the fig1 embodiment can be omitted . referring to fig5 a further embodiment of the invention wherein the manner of collecting the exciting beam is particularly considered is illustrated . the exciting beam incident on the printed wiring board has an incident angle equivalent to a numerical aperture of about 1 ( one ). in this embodiment , an exciting beam 13 , emitted from a lamp 6 , is reflected downwards at a rotary - parabolic - surface reflection mirror 17 having rotary symmetry with respect to a line connecting the light source and a through hole . reflected beams are collected by means of a similar rotary - parabolic - surface reflection mirror 18 disposed to oppose the reflection mirror 17 and a microscope objective lens 10 disposed inside the reflection mirror 18 , so that a collected beam obliquely enters a through hole 2 in the printed wiring board 1 and is focused on portions in the through hole 2 . more particularly , the rotary - parabolic - surface reflection mirror 18 forms an illuminating / focusing optical system which permits the exciting beam to impinge upon the entire inner circumference of the through hole , thus ensuring that exciting beams 13 of high intensity are scattered obliquely downwardly in the through hole . when a luminescent beam 14 is generated in the presence of a through - hole void ( not shown ) in the through hole 2 , the luminescent beam 14 coming from the through hole 2 is focused on the detector 12 by means of two reflection mirrors 19a and 19b while being passed through the excitation filter 11 , capable of passing only the luminescent beam 14 . in this embodiment , the dichroic mirror of the fig1 embodiment can be omitted . accordingly , in this embodiment , the exciting beam 13 of high intensity obliquely enters the through hole 2 and is reflected obliquely with high efficiency in the through hole 2 to impinge upon the layer material through a fine through - hole void . when a relatively wide area on the printed wiring board 1 is irradiated with the exciting beam 13 , and on rare occasions , a foreign matter 20 is present within the area as shown in fig6 . the foreign matter 20 generates a luminescent beam 14 which leads to erroneous recognition that a spurious through - hole void is present in through hole 2 . to prevent this problem , the illumination area on the printed wiring board 1 is reduced by restricting the exciting beam 13 . for example , as shown in fig7 a and 7b , an iris 21 is interposed between the excitation filter 8 and dichroic mirror 9 included in the photo - detection optical system 5 of the fig1 embodiment to form an image of the exciting beam on the top surface of the printed wiring board 1 . in particular , fig8 a and 8b illustrate an instance where the iris is , a relatively large circular iris 21a . circular iris 21a is slightly smaller than a field of view 10a of the microscope objective lens 10 . in this case , the entire field viewed by the microscope objective lens 10 is irradiated with the exciting beam 13 . fig9 a and 9b illustrate an instance where a small circular iris 21b is used to permit the exciting beam 13 to irradiate only one through hole 2 . fig1 a and 10b illustrate an instance where an iris 21c having a plurality of small apertures , is used to irradiate a plurality of through holes 2 simultaneously . therefore , by using an iris 21 of suitable configuration , mistaking a foreign matter 20 for a through - hole void can be avoided in the event that the foreign matter 20 is present on the surface of printed wiring board 1 . the configuration of the iris has been described with reference to fig8 a , 8b , 9a , 9b , 10a and 10b on the assumption that a tv camera is used as the detector 12 . when a linear image sensor is used as the detector 12 , an elongated iris 21d , as shown in fig1 a and 11b , is used to irradiate only the portion to be detected by the linear image sensor . an iris 21e , having a plurality of apertures at positions corresponding to through holes 2 as shown in fig1 a and 12b , is used to irradiate only the through holes 2 . in fig8 a , 8b to fig1 a , 12b , the iris 21 is described as being circular and oblong but the configuration of the iris 21 is not limited thereto and may be , for example , square and oval . in giving the description with reference to fig1 to fig1 b , the printed wiring board 1 , without a pattern on the outermost layer and available before etching is handled as the object to be checked . since the entire printed wiring board 1 , available before etching , is covered with a copper foil , the detector 12 will detect a luminescent beam 14 only when a through - hole void 4 is present in a through hole in the printed wiring board 1 , except in the rare case when a luminescent beam 14 is generated from foreign matter 20 . therefore , it is easy to detect any through - hole voids 4 in the through - holes of the printed wiring board 1 available before etching . practically , however , a printed wiring board 1 with a pattern on the outermost layer and available after etching is sometimes handled as an object to be checked . in checking this type of printed wiring board 1 , a luminescent beam 14 is generated when an exciting beam 13 impinges upon a layer material 1a &# 39 ; on printed wiring board 1 &# 39 ; excepting a through hole 2 , as shown in fig1 a and 13b . the layer material 1a &# 39 ; is mistaken for a through - hole void 4 in a through hole 2 . to solve this problem , in a further embodiment of the invention as shown in fig1 a and 13b , an iris 21 is provided which restricts the illumination area to an area which is smaller than that of pad 3a . pad 3a is copper foil formed on the outermost layer which is continuous to copper foil serving as an electrical conductor in a through hole 2 . between the iris 21 and the excitation filter 8 , shutter 22 is interposed which is responsive to a commands from controller 24 to open and close . the controller 24 also controls driving of a table 23 which carries the printed wiring board 1 &# 39 ; and which is driven by drive power fed from a drive source to move in x - axis and y - axis directions on the horizontal plane parallel to the sheet of drawing of fig3 a . thus , the shutter 22 , responsive to the command from the controller 24 , is intelocked with the movement of table 23 and is so controlled as to ( 1 .) open , only when the focal position of excited beam 13 coincides with a point in the through hole 2 in the printed wiring board 1 &# 39 ; and ( 2 .) close , whenever the above condition is not satisfied . incidentally , the position of any through holes 2 relative to a reference position on the printed wiring board 1 &# 39 ; is represented by known variables which are obtained from design data . the design data is recorded on a recording device 25 and input to the controller 24 . accordingly , the controller 24 ( 1 .) controls the movement of the table 23 carrying the printed wiring board 1 &# 39 ; in accordance with the input design data , ( 2 .) detects the time when the position of a through hole 2 coincides with the focal position of the objective lens 10 , and ( 3 .) commands the shutter 22 to open at the time of coincidence . in an alternative , the shutter 22 may be provided in association with the optical system for detecting the luminescent beam , as indicated at the phantom line in fig1 a . by using a strobe light source capable of emitting the excited beam , the shutter 22 can be omitted . however , because of a need for an exciting beam of high intensity , the extra high - pressure mercury arc lamp 6 and the shutter 22 may preferably be used in combination . the light source in the form of a laser source can be turned on and off easily by using an acustic optic reflector or the like . in detecting through - hole voids after the formation of the outermost layer pattern ( after etching ), a luminescent beam of high intensity is generated when the exciting beam impinges upon a layer of material from which copper has been removed . detector 12 of ultra - high sensitivity is damaged by receiving the high intensity luminescent beam . the detector 12 must be protected from the reception of the intensive luminescent beam . with the detection apparatus shown in fig1 a and 13b , the detection of the through - hole void can be effected in substantially the same manner as described previously in connection with the preceding embodiments and will not be described herein . as described above , according to the invention , the through - hole void in the presence of a through hole can readily be detected with high reliability to prevent the production of defective printed wiring boards and failure of apparatus such as computers incorporating printed wiring boards .