Electro-luminescent automatic testing apparatus and method for ceramic substrates, printed circuit boards and like items with background illumination suppression

An electro-optical testing apparatus employing gaseous discharge within a sealable substantially gastight chamber for the testing of items such as printed circuit boards, ceramic substrates and other like items for electrical continuity, said chamber having a readily opened and closed access opening for placement of items under test therein, means for introducing a gaseous atmosphere capable of electro-luminescence at low pressures into the sealable chamber with an item to be tested in place, movable probe means disposed within the sealable chamber for coupling one terminal of a source of electric potential to desired test points located on the surface of an item under test, a unipotential array of parallel conductive wires lying in the same plane within the sealable chamber over the item under test and spaced apart therefrom, and means for coupling a remaining opposite polarity potential terminal of said source of electric potential to the unipotential array of conductive wires wherein each individual conductive wire is suitably connected with a plurality of different value voltage limiting resistors each of which are selectively placed in operating series electrical circuit relationship with the respective conductive wire by a respective selector switch connected between each respective voltage limiting resistor and the remaining opposite polarity terminal of said source of electric potential, whereby any particular desired value voltage limiting resistor may be selectively connected in series circuit relationship with the respective electrical conductive wire by selective operation of the respective selector switch, whereby electro-luminescence is produced in the gaseous atmosphere within the sealable chamber around test points contacted by asid probe means and around all conductive points on the item under test having electrical continuity therewith which are exposed to the gaseous atmosphere with a minimum background noise light illumination.