|Sunday, 27 August 2006|
Optical scanners generally are of two types which correspond to the two types of graphic images available. The first type is reflective scanners for use with original images such as prints which depend upon light being reflected from them to form the desired image. In a reflective-type scanner, the document is defined as a reflective document and the light source is positioned in front of this document. Light is reflected by the document and then passed to the optical sensing device. The other type are transmissive scanners which sense the density of graphic images formed by light passing through them such as slides and movie film. In a transmissive scanner, the document is defined as a transmitted document and the light source is positioned behind this document. Light travels from the light source to the document, then travels to the optical sensing device to be converted into image signals. Typically, the reflective document image scanner is usually made in the form of flat bed scanner, and the penetrative document image scanner is made in the form of film scanner.
A scanner generally comprises a transparent window for allowing light reflected from a document to be scanned back to the scanner, a charge coupled device (CCD) having a plurality of optic sensors for converting a line image reflected from the document into an array of analog signals, a set of lenses for conveying light reflected from the document to the CCD, an analog-to-digital (A/D) converter for converting the analog signal array into an image data array, a control unit for controlling operations of the scanner, and a memory for storing the image data array. The scanners can be classified into flatbed scanners that put all scanned documents on the top of their transparent windows, and feed-in scanners feed the scanned documents into the scanners for generating the scanned images. Various types of optical scanners are currently commercially available, including drum scanners, hand-held scanners, and flat bed scanners. The most commonly used scanners are flatbed scanners. A flat bed image scanner is widely used today is a periphery equipment for scanning documents, books and pictures and for furnishing the scanned information to a computer to be processed. Flatbed scanners have been the mainstream of scanners as peripheral devices of computers.
A flatbed scanner generally comprises a board (platen) on which a user can lay books, magazines, and other documents to be scanned, and a scanhead apparatus or carriage which generally moves along the document scanning the document. The carriage moves beneath the glass scanning the document on the glass board. The imaging system, typically an input scanner, is typically comprised of an illumination unit, focusing optics, and a linear imaging array. Flatbed scanners typically include a linear sensor array that is moved relative to the original image, along an axis that is perpendicular to the axis of the sensor array. A step motor is used to move the scanning module forward and backward along the shaft for scanning a complete document placed above the transparent window of the scanner. A flatbed scanner usually further comprises a document holder for fixing transmitted documents. To scan a transmitted document, the document holder is mounted on the transparent window of the flatbed scanner. The transmitted document is fixed onto the document holder. A flat bed scanner includes a light-receiving device mounted in a light-receiving seat. The light-receiving device includes mirrors and a lens for focusing the image upon an electro-optic transducer, also known as a charge coupled device (CCD) which then converts the light images into electrical signals. A typical flatbed scanner is based on charge coupled device (CCD) technology. A typical CCD may comprise an array of individual cells (sensing elements) which collects or builds-up an electrical charge in response to exposure to light. The size of the electrical charge built up is dependent on the intensity and the duration of the light exposure. In optical scanners, CCD cells are aligned in linear arrays. The resolution of the flatbed scanner depends on the density of the CCD and is limited by the mechanical accuracy of the scanning of the light source and optical detector.
A flatbed scanner casing generally includes an upper housing and a lower housing in which the upper housing is a rectangular frame and a transparent platform. The lower housing is usually used to accommodate optical, electronic and mechanical devices of the flatbed scanner. The top housing comprises a rectangular opening in the middle portion of its surface for installing a transparent planar board. A typical flatbed scanner mounts the illumination system and a portion optical system to a moveable carriage assembly. A typical flatbed scanner may include illumination and optical systems to illuminate a portion of the object, whereas the optical system collects light reflected by the illuminated scan region and focuses a small area of the illuminated scan onto the surface of a photosensitive detector positioned within the scanner. A flat bed scanner may also be provided with an automatic document feeder (ADF). The ADF automatically feeds pages, one by one, over a second predetermined area on the upper surface of the platen. Flat bed type image scanners are classified into two different kinds in accordance with their different operation ways: reflection-type scanners and transmission-type scanners. The reflection-type scanners are used for scanning non-transparent media such as paper, objects and books. On the other hand, the transmission-type scanners are used for scanning transparent media such as films and slides.
In use of a flat bed scanner for reflective scanning, the document that is to be scanned is fixedly placed on a transparent glass plate on the flatbed scanner. Reference marks are provided on the transparent glass plate that allows the user to place the document at correct positions. The light reflected from the scan line is directed through an optical system to form an image of the scan line on a sensor such as a CCD array, which converts the optical signal to an electronic representation of the scan line. After the page is placed over the fixed page scan area, the user then typically closes the cover over the page and initiates the scanning operation. During the scanning operation, the document is illuminated and the image sensor is optically exposed to each section of the page through the fixed page scan area on the platen. A line light source moves inside the machine body from one end to the other end to allow the paper to be fully scanned by the light. Reflection of the scanning light provides information of the image printed on the paper to the scanner. The optical sensor array produces electronic data which is representative of each scan line portion of the document. Image data representative of the entire object is obtained by sweeping the scan line across the entire object by moving the illumination and optical systems with respect to the object.