An inexpensive security method for uniquely marking and identifying physical objects such as documents or labels has been developed. This technology uses the mixing of inks in numerous different ratios to print arbitrary marks on a given object. The ink mixture used in a printed mark produces a unique, nonlinear spectral response or pattern upon its optical excitation. This spectral pattern can then be easily measured for authentication of the object, but cannot be replicated by a counterfeiter without a specific information key, which is knowledge of exactly what inks are used and their original mixing ratios.
The basic method can be implemented with low-cost hardware — a computer, specialized inkjet printer, and optical reader — to create unique security labels for individual items, and is scalable for both low- and high-security applications. This technology implements trap-door printing on a substrate with combinatorial microdrop arrays to form arbitrary patterns on the substrate as a means to authenticate products and documents.
One method of implementing difficult reversibility in the printing and readout relationship utilizes the combination of pigments and phosphors having non-additive color mixing characteristics to make colored microdots. These microdots are produced by microdroppers or inkjet ejectors, each having a certain proportion of pigments and, therefore, would be producing a unique spectral response. Creating and characterizing a microdot with a unique spectral response holds the “cryptographic key.” A re-measurement of the spectral response of a microdot that matches the key authenticates the document on which the microdot is placed. Without knowledge of the key, and because of the non-additive color mixing characteristics of pigments behind a microdot's spectral response, a counterfeiter, taking the spectral measurement of a microdot from an original document and then attempting to determine from this spectral information what pigments and what proportions of each were used to make this microdot, will find it very difficult or impossible to replicate an illegitimate copy.