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.
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]