US Patent 6,100,967
This is a patent that I co-authored during my time working for a small Laser firm based in South Wales.
The patent was developed during some tests using a Carbon Dioxide Laser to mark certain transparently coated materials. When the Laser power was turned down, no obviously visible mark was made, but if viewed under polarised light, a legible mark became clearly visible.
The patent expands on this principle of monitoring marks covertly via polarised light, which could have numerous security based applications.
Monitoring of Covert Marks
Richard Farrar and Barry Hood
A method and apparatus for monitoring a body having a reflective surface, a coating layer of material transparent to visible light present on the surface and producing a covert optical discontinuity in an imagewise distribution on the layer, the imagewise distribution is such that it is visible via polarised light and viewing through a polarised filter.
[PDF : covert_marks_us.pdf 261 kB]