A commonly used means of grading security features created by the European Central Bank and adopted for use across the banknote industry. Level 1 refers to features that can be verified by the public, untrained examiners or specialist examiners with the naked eye. Level 2 refers to semicovert features that require some form of reading device or tool to be authenticated. They are typically deployed for use by retailers, cashiers and inspectors but are also detected by sensors in machines (the ECB defines Level 2a, 2b and 2c). Level 3 refers to covert features, generally rare chemicals or compounds that can only be identified by sensors in high speed processing machine verification used in cash centres. Level 4 authentication uses forensic examination to verify banknotes and is used solely by central banks/police.