What You Always Wanted to Know About Scanning

One of the most useful attachments of a computer is a scanner which has now become indispensable for both home and office usage. Scanners analyze an image and text and process them in such a way that the optical characters can be saved in a file on your computer. These images and text can be altered as per the need and then printed or used on your web page.

Among all the various types of scanners available, Flatbed scanners (also called desktop scanners) are the most versatile and common. The other types are sheet-fed scanners and hand-held scanners. Whereas the sheet-fed scanners are like the flatbed scanners except that the document is moved while keeping the scan head stationary, hand-held scanners need to be moved by the user in place of a motorized bed. Hand-held scanners do not give good image quality but they can capture the text very quickly. Finally, there is another type of scanner called a Drum scanner that is used to capture extremely detailed images and is normally used by the publishing industry.

The following steps describe how a scanner works:

- The glass plate is used to place the document and the cover is closed. The cover gives a uniform background, which is used by the software of the scanner to fix a reference point in relation to the size of the document that is being scanned. The inside of the cover is mostly flat white although some of them are black too.

- In order to illuminate the document, new scanners use a cold cathode fluorescent lamp (CCFL) or a xenon lamp whereas older scanners use a standard fluorescent lamp.

- The scan head is made up of mirrors, lens, filters and CCD array. A stepper motor runs a belt that moves the scan head slowly over the document. A stabilizer bar is attached to the scan head to eliminate wobble or deviation in the pass, which is the term to indicate that a complete scan of the document has been completed, by the scan head.

- An angled mirror is used to reflect the image of the document to another mirror. Whereas some scanners use two mirrors there are others that use three mirrors. In order to focus the image onto a smaller surface, the mirrors are slightly curved.

- The last mirror reflects the image onto a lens, which focuses the image through a filter on the CCD array.

Each type of scanner has its own filter and lens arrangement. In some scanners there is a three-pass scanning process with each pass using a different color filter (red, green or blue) between the lens and CCD array. After the completion of the three passes, the three filtered images are assembled by the scanner software into a single full-color image. However, the single pass process is used by most scanners nowadays whereby the image is split into three smaller versions of the original by the lens. These smaller versions are then passed through a color filter (red, green or blue) onto a discrete area of the CCD array. The data from the three areas of the CCD array is combined by the scanner into a single full-color image.

A newer imaging array technology is contact image sensor (CIS), which has become very popular nowadays. It replaces the CCD array, mirrors, filters, lamp and lens with rows of red, green and blue light emitting diodes (LEDs). However, the CIS scanners are not capable of providing the same degree of quality and resolution as is given by the CCD scanners although the CIS scanners are cheaper, lighter and thinner.

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