Optical InspectionThe basic inspection principle is an optical transmission measurement, whereby a strobe light source delivers an intense illumination to one side of the can, while a high sensitivity light detector "looks" at the other side to detect any light transmitted through a possible hole.The occasional hole where a clear lacquer film stays intact will also be detected as opposed to systems based on vacuum or pressure. The sensitivity of such an optical system is normally limited by the amount of interfering light from the ambient - sun, fluorescent lights etc. - reaching the detector or the surface being inspected. Each OptoScan sensor head is in fact a complete compact vision system operating autonomously. 
This makes the OptoScan principle unique compared to other optical leak testers, since compensation measurements are included, which minimize the influence from the ambient. Because imaging is employed in this process the compensation also takes into account the morphology of the surface being inspected as well as non-uniformities and temperature drift in the camera sensor. Employing this technique achieves a very high and consistent sensitivity that can be maintained without relying on wear-prone mechanical seals as competing optical and pressure systems do. Neither the sensor input window nor the exit surface of the solid state light source needs frequent cleaning. | SensitivityThe basic OptoScan detection system is specified and factory tested to detect min. 95% of all occurrences of a calibrated 1 µm / 2um hole on easy open ends (65mm) or a 5 µm horizontal hole on cans.The definition of sensitivity is closely related with the False Reject Rate FRR- meaning: how many good ends will be falsely rejected when operating at the specified sensitivity. Specifying sensitivity is only meaningful when at the same time listing the accompanying False Reject Rate - since detecting an even smaller hole is quite possible, if a higher FRR is acceptable. The factory FRR at a sensitivity of a 1 µm hole is typically 5 in 1000000 (0,0005%). When applying OptoScan to can inspection the sensitivity is different for holes in the bottom of the can and on the sidewalls. Bottom holes may be detected down to 5 µm (0,0002"), sidewall holes to 50 µm (0,002"). Very large can sizes may result in reduced sensitivity. The sensitivity is monitored during production by inserting specially prepared ends/cans with calibrated holes laser punched in a stainless steel foil attached to the end/can. The operator can then verify on the OptoScan PC the amount of light being detected. |