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Intelligent focus in document scanner

IP.com Disclosure Number: IPCOM000208098D
Publication Date: 2011-Jun-23
Document File: 3 page(s) / 209K

Publishing Venue

The IP.com Prior Art Database

Abstract

Some document image scanners use a SELFOC lens to focus the document image on a Full Width Array (FWA) image sensor. In high speed scanning systems of this type, the lens is constructed at the expense of focal depth. This means that the document is constrained to a narrow position range from the SELFOC lens, compared to other types of focusing systems. This results in focus problems for book folds and for highly crumpled or folded sheets of paper. This idea proposes a feedback loop which measures document distance from the platen and adjusts the focus accordingly to provide a greater focus depth above the platen as needed. This adjustment is performed dynamically during the image scan process by moving the SELFOC lens system. If the entire scan bar, including the illumination is moved, consistent illumination and focus could be maintained for variation in document position from the platen.

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Intelligent focus in document scanner

Some document image scanners use a SELFOC lens to focus the document image on a Full Width Array (FWA) image sensor.  In high speed scanning systems of this type, the lens is constructed at the expense of focal depth. This means that the document is constrained to a narrow position range from the SELFOC lens, compared to other types of focusing systems. This results in focus problems for book folds and for highly crumpled or folded sheets of paper. This idea proposes a feedback loop which measures document distance from the platen and adjusts the focus accordingly to provide a greater focus depth above the platen as needed. This adjustment is performed dynamically during the image scan process by moving the SELFOC lens system. If the entire scan bar, including the illumination is moved, consistent illumination and focus could be maintained for variation in document position from the platen.

To enable a high scanning speed, the SELFOC lens used in image scanners with FWA image sensor modules must have a small f-number to allow sufficient light to pass through the lens. The effect of this is to reduce the focal depth of the lens. This causes no problems under normal circumstances, but can cause text in folds of books to be blurred.  Beyond the focal point of the lens, images are found to become blurred very quickly, meaning the scanner is unable to provide high quality scanning of images more than a couple of millimeters from the platen glass.

Equally important, the light from the scanner is directed so that it is brightest at the focal point of the lens. Moving the document away from this point also results in less illumination, therefore the image would be of a poorer quality even if it was in focus.

To address these issues, it is proposed that the scanning module’s position could be adjustable so that the focal point of the lens can be adjusted. By moving the entire scanning module up and down, the light remains brightest at the focal point of the lens and coincides with the document image position.

As an example implementation, the FWA module has three plastic "buttons" which press against the lower side of the platen glass (see figure below). These are used to ensure that the FWA remains a fixed distance from the glass and that the focal point of the lens is at the upper side of the glass. These buttons could be changed so that they are adjustable, and by moving the buttons up and down the focal point of the lens can be moved upwards, away from the upper surface of the platen glass and into the fold or binding of a document.

Piezoelectric positioning devices are known for their accuracy and could be used in this application. The distance the buttons are raised from the FWA must be accurately controlled.  Otherwise there is a risk that the system will be out of focus.

The height above the platen glass could be measured using carefully angled lasers projected at the document to be scanned. These las...