This is a link to the patent that I was granted
Here is the front page of the patent:- The design is for a transparency sensor. It makes use of the fact that light reflected from a transparency is strongly polarised.
It works ! I tested it at home with some polaroid and an LED. I don't think that it has been commercialised though.
"Polaroid" glasses make use of this phenomenon in order to reduce reflected glare from shiny surfaces.
This patent is cited in patents by Lexmark, Rohm and HP :-)