Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

Correction for Non-Uniform Illumination in DOCUMENT Scanner

IP.com Disclosure Number: IPCOM000038619D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 2 page(s) / 40K

Publishing Venue

IBM

Related People

Clarke, DA: AUTHOR [+3]

Abstract

In a document scanner with a single control light source, illumination of the document is not uniform. Thus a scan signal consists of a high frequency data signal superimposed upon a low frequency component due to non-uniform illumination. The scan signal is applied to an amplifier having a low-pass filter in its negative feedback path to remove the low frequency component from its output. In a low-cost document scanner, the intensity of illumination falling upon target documents is not uniform. If the illumination source is central normal to the center line of the document, illumination (and thus output voltage for a blank document) will take the form (Image Omitted) shown in Fig. 1.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 62% of the total text.

Page 1 of 2

Correction for Non-Uniform Illumination in DOCUMENT Scanner

In a document scanner with a single control light source, illumination of the document is not uniform. Thus a scan signal consists of a high frequency data signal superimposed upon a low frequency component due to non-uniform illumination. The scan signal is applied to an amplifier having a low-pass filter in its negative feedback path to remove the low frequency component from its output. In a low-cost document scanner, the intensity of illumination falling upon target documents is not uniform. If the illumination source is central normal to the center line of the document, illumination (and thus output voltage for a blank document) will take the form

(Image Omitted)

shown in Fig. 1. Obviously, any shape of curve is possible; the lowest of the family shown is likely to be achieved by the use of a miniature fluorescent tube placed close to the document. Such tubes characteristically provide reasonably good consistancy of illumination but tend to fall off towards the edges, since the tube is not infinitely long. The effect of this on the perceived image is darkening edges. The method involves the use of an operational amplifier and a feedback circuit incorporating a low-pass filter. The scan signal will consist of a high frequency component actually containing the data superimposed on a low frequency component determined by the illumination level (Fig. 2). The low-pass filter isolates this latter componen...