A
thermal transfer printing medium that contains a
thermal transfer layer which contains a first
taggant and colorant, wherein: the first
taggant comprises a fluorescent compound with an
excitation wavelength selected from the group consisting of wavelengths of less than 400 nanometers, wavelengths of greater than 700 nanometers. When the
thermal transfer layer is printed onto a white
polyester substrate with a gloss of at least about 84, a
surface smoothness Rz value of 1.2, and a reflective color represented by a
chromaticity (a) of 1.91 and (b) of −6.79 and a
lightness (L) of 95.63, when expressed by the CIE Lab color coordinate
system, and when such printing utilizes a printing speed of 2.5 centimeters per second and a printing energy of 3.2 joules per
square centimeter, a printed substrate with certain properties is produced. The printed substrate has a reflective color represented by a
chromaticity (a) of from −15 to 15 and (b) from −18 to 18, and the printed substrate has a
lightness (L) of less than about 35, when expressed by the CIE Lab color coordinate
system. When the printed substrate is illuminated with
light source that excites the first
taggant with an
excitation wavelength selected from the group consisting of wavelengths of less than 400 nanometers, wavelengths greater than 700 nanometers, the printed substrate produces a light
fluorescence with a
wavelength of from about 300 to about 700 nanometers.