Abstract: 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.