How Does a Document Scanner Work?

The Scanner

A document scanner is a vital tool for many workplaces that often work with hard copy documents. There are three major types of document scanners: flatbed, sheet-fed and hand-held. The flatbed scanner requires the user to place the document to be scanned on the top of a glass inside the machine. The image sensor within the flatbed scanner moves across the page, copying its contents. A sheet-fed scanner, which usually resembles a printer, moves the paper over the sensor to achieve the same effect. Hand-held scanners are operated by the user, requiring the user to manually move the sensor over the document in order to scan it.

The CCD Sensor

The image sensor of almost all document scanners is an array of charge-coupled devices. The CCD arrays work through photo-sensitive diodes. These diodes convert the light (or lack of light) they catch into an electrical signal with the same information. When scanning a document, they don't scan for words; they scan for the lack of light black ink has relative to a white page. The CCD array's job is made easier in flatbed scanners; there's usually a light inside the scanner to make the page brighter and easier for the array to read. Most sensors operate in a RGB color mode (if scanning), meaning the image of the document will be separated into red, green, and blue images by a lens before they are put back together when they show up on a computer.

The CIS Sensor

The contact image sensor differs. It consists of hundreds of red, green, and blue LED lights. When these lights combine, it becomes a white light. The sensor is placed very close to the document (as the name suggests), with only a piece of glass separating the two. When it scans, the white light is produced, at which point the sensors pick up the image. This method is much more power efficient than using CCD, but its image quality is poor.

Getting to the Computer

To finish the scanning process, the new electronic image must transfer to a computer. Scanners can be connected to computers in a variety of ways, such as USB, FireWire and Small Computer System Interface. It's also common to use a network that allows for a group of computers to connect to the same scanner. Special drivers that speak the scanner language of TWAIN are needed in order to read the output. These drivers sometimes come with programs such as PhotoShop but usually come in the form of an installation CD with the scanner upon purchase. Once the image has been transferred to the computer, its information can be put into a number of different file types. PDF is the standard for document viewing because of its clarity, ease of use and ubiquity. A PNG is an uncompressed image that allows higher quality but uses more memory. A JPG is a compressed image that makes the file smaller, with lower image quality.