An optical scanner is any scanning technology that converts a physical image, text, or object into a digitized form that can be stored as a 2D image file on a computer. 3D scanners also exist, but they are not widespread and are sometimes based on different technologies. Optical scanners have been around for decades and their price has dropped accordingly. You can buy low-level used optical scanners on eBay for as little as 20 US dollars (USD) and a new scanner on newegg.com for around 50 USD. Better optical scanners sell for hundreds of dollars.
The basic technology underlying optical scanners is that of the charge-coupled device (CCD). A CCD consists of a capacitor array that is sensitive to light. The technology was invented in 1969 by Willard Boyle and George Smith at AT&T Bell Labs, and was commercialized shortly thereafter.
The capacitors in a CCD respond to up to 70% of incident light, in contrast to photographic film, which only responds to 2%. Incoming light causes the capacitors to become charged to a degree proportional to the light intensity. The capacitor array passes the charge from node to node until it reaches a final capacitor. Then the charge is amplified into voltage, which flows into a wire. The charge pattern is read by another device and recompiled into a pixel map that represents the scanned object.
The most popular optical scanner in use today is the flatbed scanner, which consists of a pane of glass above a rod-shaped scanning device mounted on a track. The scanning device moves back and forth on the track to scan whatever is placed above it. Optical scanners are also used to record fingerprints and votes from ballots, scan entire books or libraries, and serve as "eyes" for the blind that convert text into automated speech.
Because different exposure conditions can cause radically different images to appear, optical scanners are equipped with proprietary software that corrects for these conditions to produce a consistent image. 24-bit color images are a common standard. An external scanner is attached to a computer through an input-output channel, such as SCSI, USB, or firewire. Wireless adapters for optical scanners also exist.