A pinhole inspection unit that quickly finds tiny pinhole defects in sheet work pieces down to a diameter of only 1 micron.

By making use of a photomultiplier tube with a large photosensitive area and a noise-suppression signal processing technique, Hamamatsu Photonics has developed a pinhole inspection unit, the model C15477 that quickly finds pinhole defects as small as 1 micron (millionth of a meter or μm hereafter) in diameter in sheet work pieces.

This product can detect tiny pinhole defects only a quarter of the size of pinholes detectable up to now, and so will improve inspection accuracy for tasks such as inspecting pinhole defects in fuel cell separators for fuel cell vehicles and aluminum laminate films for pouch rechargeable batteries. It is available as a set with a light source unit and a light collector unit optimized for inspecting pinhole defects in sheet work pieces such as large thin metal sheets measuring 480 mm wide by 180 mm deep.

In optical inspections, the light source irradiates light onto a work piece and any light passing though pinholes is efficiently collected and detected by this inspection unit as electrical signals by the built-in photomultiplier tube that is a highly sensitive photodetector and allows detecting any tiny pinhole defects in the work piece. Other inspection methods rely on fluids or gases for detecting pinholes but the optical method applies no actual physical pressure to the work piece during inspection.

The tinier the pinhole defect, the lower the intensity of the light passing through the pinhole. This reduces the detectable light signal and so makes it hard to find tiny pinholes. Using an amplifier circuit can boost the signal but also raises the noise level in the signal.

Along with using an amplifier circuit to boost the signal level, Hamamatsu revamped the circuit design and employed a lock-in amplifier for signal processing to reduce the noise in the signal by operating the light source unit in pulse mode and using the detection signals of the periodically pulsed light. This way they succeeded in both amplifying the signal and reducing the noise to create an inspection unit capable of detecting pinhole defects as small as 1 μm in diameter in sheet work pieces. They also redesigned the inspection unit and employed a photomultiplier tube with a large photosensitive area. This vastly improves usability when inspecting sheet work pieces with large surface areas. For example, large sheets that required 4 inspection units can now be checked with just a single inspection unit.