DOCUMENT SENSOR FOR A PAPER AND TRANSPARENCIES WITHOUT WHITE STRIPE
Original Publication Date: 1985-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Xerox Disclosure Journal
It is known to be able to sense transparencies by printing an opaque white stripe on the edge of the transparency. It has been proposed to sense transparencies without a white stripe by placing the transparency between crossed polarizers, using a light source and detector. This works quite well with bi-refringent substrates like transparencies and highly transmissive objects like thin paper but is not well suited to detecting opaque isotropic objects like most papers, due to the crossed polarizers. Diffusive objects like paper almost completely de-polarize the light between the polarizers and allow light to exit the second polarizer but normal papers are too opaque to give a useful signal. Parallel polarizers would allow the sensor to work with paper as well as with existing sensors but then would not give good contrast with transparencies.