IR-metameric Inks

Inks, which possess similar spectral properties under usual white illumination (look like of the same colour) and have differences in infrared spectrum (wavelength of 870 nm). Some inks are IR-absorbing and others are IR-reflecting. As a result, when examining the banknotes using special devices, only IR-visible images can be seen in the device monitor (fig. 1).

Infrared protection is based on the property of the material (pigments) to absorb or to reflect IR light.