How Does a Document Scanner Work?

A document scanner is a device that electronically copies content from printed pages and turns it into a computer text file. Small businesses use scanners to digitize legal documents, correspondence, business cards and other types of printed material, saving many hours of laborious manual data entry. The technology has two main parts: hardware that turns a page into an image, and software to extract text from the image.

Document scanners come in different types. A flatbed scanner is a shallow rectangular box with a glass top and a light source inside. You place a document on the glass and a mechanism moves the light progressively across the page while sensors capture the image. These scanners are useful for processing a relatively small number of pages at a time. An enterprise document scanner has an input bin into which you can stack hundreds of pages; it images them automatically, one at a time. A portable scanner is a small device you can pack into a briefcase; you pull it slowly across a page and it processes the printed material with its own lighting system and sensor.

Flatbed and enterprise document scanners use charge-coupled device image sensors; these are components that move across the page, picking up variations in light intensity and converting them into electronic signals. Portable scanners employ a different device called a contact image sensor; these are more compact than CCDs and work close to the surface of the document. A digitizer processes the sensor signal, turning it into a stream of numbers that indicate the brightness or darkness of points on the document. The scanner's interface transmits these numbers through a USB or other cable to your computer, which receives the data.

When you connect a document scanner to your computer for the first time, the Windows operating system detects the new device and installs the appropriate drivers for it. Drivers are specialized programs that are developed specifically for each model by the scanner manufacturer. Windows copies the drivers into its files. The document-processing application program you use communicates with Windows, which accesses the drivers and runs the scanner.

To use the scanner, you run an application program, typically one provided by the manufacturer. The program receives image data from the scanner and performs a process called optical character recognition. OCR recognizes individual letters in the image and builds text from them automatically. Once the computer has this information, you can edit, search and save it as if it were a word processing document. This is what distinguishes document scanning from simple image scanning -- the ability to extract text from document images.