The present invention is a method and apparatus for optical detection and size evaluation of through-penetrating defects such as pinholes in moving foil or film. The invention comprises the installation of at least one image capture device at a first given distance over the moving foil surface, placement of at least one elongated light source comprising an infinite number of point-sources that are not in phase, and are emitting light independently from one another under the foil, periodic automatic computer-controlled image capture of the foil surface with image capture devices, automatic transmission of the image captured by each device to a control computer, and processing of the transmitted image data to detect of defect light spot, followed by determination of generalized index of its initial image. This generalize index value is equal to the brightness averaged within the spot multiplied by the area of the spot. The invention further includes reporting the presence of a through-penetrating defect when the generalized index value exceeds a preset threshold. characteristics of the defect spots.