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Shading Compensation

IP.com Disclosure Number: IPCOM000038722D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Masuda, K: AUTHOR

Abstract

This article describes a shading compensation in a document scanner wherein a curved dichroic filter is positioned between a fluorescent lamp and a document being scanned. Referring to Fig. 1, a document 1 is illuminated by a fluorescent lamp 2 through a dichroic filter 3. Reflected light representing an image pattern of the document 1 is sensed by an optical sensor unit (not shown). Relative movement of the document 1 to the optical sensor unit scans the image of the document 1. Fig. 2(a) shows an uneven distribution of light intensity along the fluorescent lamp 2. The light intensity gradually decreases from the center portion towards both the end portions. The uneven distribution is compensated by the dichroic filter 3 which has a glass layer 4 and a thin film 5, as shown in Fig. 3.

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Shading Compensation

This article describes a shading compensation in a document scanner wherein a curved dichroic filter is positioned between a fluorescent lamp and a document being scanned. Referring to Fig. 1, a document 1 is illuminated by a fluorescent lamp 2 through a dichroic filter 3. Reflected light representing an image pattern of the document 1 is sensed by an optical sensor unit (not shown). Relative movement of the document 1 to the optical sensor unit scans the image of the document 1. Fig. 2(a) shows an uneven distribution of light intensity along the fluorescent lamp 2. The light intensity gradually decreases from the center portion towards both the end portions. The uneven distribution is compensated by the dichroic filter 3 which has a glass layer 4 and a thin film 5, as shown in Fig. 3. Assuming that I1 is an intensity of incident light to the dichroic filter 3 and I2 is an intensity of transmitted light by the dichroic filter 3, a transmission ratio I2/I1 of the dichroic filter 3 is mainly determined by an angle A. In other words, the transmission ratio is controlled by varying an angle of the surface of the thin film 5 to the incident light. The angle of the surface, i.e., curvature of the dichroic filter 3, is controlled to have a curve of the transmission rate, as shown in Fig. 2(b), by which the characteristic curve of the Fig. 2(a) is compensated to realize a flat characteristic at the surface of the document 1.

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