Patent US 4,359,633

A label or identification card has a base with a region which absorbs invisible light and a region which reflects invisible light. Both regions absorb visible light over a predetermined spectral range. A filter is disposed on the base. The filter transmits both invisible light and visible light over the said spectral range, while absorbing visible light over the spectral range outside the said range absorbed by the two regions of the base. Accordingly, the unaided human eye cannot perceive the information represented by the array of the regions on the base, while this array can be detected by scanning with invisible light. In one embodiment, the bar code is a return-to-zero double frequency code.