Browse Prior Art Database

DOCUMENT SENSOR FOR A PAPER AND TRANSPARENCIES WITHOUT WHITE STRIPE

IP.com Disclosure Number: IPCOM000025426D
Original Publication Date: 1985-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Document File: 2 page(s) / 105K

Publishing Venue

Xerox Disclosure Journal

Abstract

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.

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DOCUMENT SENSOR FOR A PAPER AND TRANSPARENCIES WITHOUT WHITE STRIPE

Robert E. Crumrine

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.

The second disclosure is a sensor using a light source, detector and a twisted- nematic liquid crystal between polarizers to sense transparencies without white stripes and to sense most papers. Transparencies and lightweight papers are sensed with the LCD set to give a wave polarized orthogonal to the detector polarizer and uses the bi-refringence of the transparency and scattering of the papers to provide a light component parallel to the detector polarizer increasing the light intensity. Opaque isotropic objects are sensed with the LCD set to give a wave parallel to the detector polarizer and operation is similar to a normal beam-break sensor.

A small, twisted-nematic liquid crystal display rotates the linear polarized light from the source by 90 degrees on command of an electrical signal, to allow the sensor to work in either a "transparency mode" or a "paper mode", depending on the state of the LCD. The mechanical set-up is shown in the figure, in particular, an LED 10, a first horizontal polarizer 12, a liquid crystal 14, the

Volume 10 Number 3 May/June 1985 115

Proposed Classification
U.S. CI. 271/57 Int. C...