Unless you know where to look for the code, what other applications will this be useful for besides authentication.
IBM Japan and Connect Technology have developed an invisible, two dimensional bar-code which can be be read by camera phones.
From JapanCorp IBM Japan and Connect Technology Use Invisible 2D Barcodes
IBM Japan in collaboration with Connect Technology has developed an electronic clipping system, which uses invisible 2D barcodes printed on paper to integrate information from paper and digital data such as information provided on Internet sites.
The new system adds an invisible digital layer to printed materials, enabling the printed materials to be used like a portable site.
Invisible 2D barcodes which store digital data are printed on paper using invisible ink. The barcodes are then extracted from the paper by an image processing application. Since invisible ink is used, 2D barcodes can be printed over paper prints, not restricted by original print designs and layouts.
Invisible barcodes can be printed on each magazine or paper article, allowing users to clip and buy articles instead of magazines and papers