US Patent 7077329 - Quantum Dots for Document Security

http://www.freepatentsonline.com/7077329.pdf

Quantum dots (i.e. nanosized crystals) are frequently being applied as biological tags to track molecules in analytical devices via a fluorescent effect. In addition, quantum dots may be used as security tags similar to bar codes to mark print media (such as by a printing device) but with the advantage of being undetectable by human or other traditional inspection. This would be highly useful for anti-counterfeiting, document tracking, and other security issues. This patent advances this use of quantum dots via processing algorithms used to decode the data. Claim 1 reads:

1. A method of decoding information encoded by the positions and intensities of spectral lines in the emission spectrum of quantum dots in a carrier medium, wherein a set of codes in a code book is characterized by different combinations of said positions and intensities, comprising: exciting the quantum dots in said carrier medium to initiate fluorescence; sensing the resulting emission spectrum of the quantum dots; performing a deconvolution operation to separate the spectral lines in said emission spectrum; processing the resulting data to find the positions and intensities of the spectral lines in said emission spectrum; and extracting the decoded information by reference to said code book; and wherein the de-convolution operation is represented by the equation .SIGMA..sub.i k(.lamda.i).delta.(.lamda.-.lamda.i)=IFT {FT[f(.lamda.)]/FT[p(.lamda.)]}, where .delta.(.lamda.) represent an impulse function, k(.lamda.i) is the intensity of a .delta.(.lamda.) at .lamda.i, p(.lamda.) denotes the profile function of the spectrum of quantum dots.