However, the tubes suffered greatly from fragility during testing and, by 2002, the NVESD revoked the fourth generation designation for filmless tubes, at which time they simply became known as Gen III Filmless. These tubes are still produced for specialist uses, such as aviation and special operations; however, they are not used for weapon-mounted purposes. To overcome the ion-poisoning problems, they improved scrubbing techniques during manufacture of the MCP ( the primary source of positive ions in a wafer tube ) and implemented autogating, discovering that a sufficient period of autogating would cause positive ions to be ejected from the photocathode before they could cause photocathode poisoning.