In one embodiment, a printed security mark comprises a random arrangement of printed LEDs and a wavelength conversion layer. During fabrication of the mark, the LEDs are energized, and the resulting dot pattern is converted into a unique digital first code and stored in a database. The emitted spectrum vs. intensity and persistence of the wavelength conversion layer is also encoded in the first code. The mark may be on a credit card, casino chip, banknote, passport, etc. to be authenticated. For authenticating the mark, the LEDs are energized and the dot pattern, spectrum vs. intensity, and persistence are converted into a code and compared to the first code stored in the database. If there is a match, the mark is authenticated.