Patent US 6,951,687 (asymmetric pattern - grooves or voids) US 7,163,744 (asymmetric pattern - any optically detectable difference) International patent applications: Mexico*, Canada, EU, Eurasia*, India, South Korea, Republic of China, Peoples Republic of China*, Japan, Australia, New Zealand* (*=issued) Patents cover the article of manufacture, the method of manufacture, and business uses of the article