Invisible Ink Recognition Devices
Coded messages written with invisible ink sound like something out of a espionage novel, but they were actually widely used in times of war for sabotage and smuggling purposes. Exceptionally simple chemicals would reveal the secret messages, such as those printed on lemon juice, when exposed to heat, light or a chemical solution1.
More recently, researchers have been developing invisible ink that can be read with a regular printer and a computer. The new ink uses carbon nanoparticles that emit blue when illuminated with UV light. In their latest paper, published in ACS Applied Materials & Interfaces, they report testing an AI model’s ability to recognize symbols illuminated with the ink and decode them using a special codebook.
The current research is based on the use of long wavelength UV illumination for revealing invisible prints. However, such UV light is not easily available and requires specialized close box detection scanners to prevent dangerous exposure to short wavelength UV light. In contrast, the present invention enables use of common black light sources to detect invisible prints.
The invention enables reading in one continuous operation of both invisible barcodes and other invisible insignia printed with invisible printing inks containing phosphorescent luminophores with a medium range of afterglow decay time, such as inorganic phosphors, organic dyes or quantum dots. The inks are printed with low concentrations of the luminophores that make them invisible to the human eye at ambient light levels typical for office and home environments. The luminophores are activated by the white illuminating LED built into the conventional smartphone and the essential photonic properties of the luminophores are measured by the sensor of the phone without any hardware attachment or upgrade of the device.