The Scanning Process
A scanner initially operates much like a photocopier. An image such as a photograph or a drawing is placed on a transparent plate. The lid of the scanner is closed to keep stray outside light from entering. When the scan is begun, an incandescent or fluorescent light illuminates the image on the transparent plate. The light that reflects off of the image enters a lens. At this point the scanner operates differently from a photocopier. In a photocopier the lens focuses the light onto a plate that creates an electric charge that attracts the toner particles that produce the mirror image of whatever document is being copied. In a scanner, however, the lens focuses the reflected light onto a row containing many electronic light sensors.
The sensors convert the light to electric current. The strength of the current produced by each sensor (in volts) is proportional to the intensity of light striking the particular sensor. The electrical output is in digital form. The signal, which now represents the tonal values of the original image in digital form, is ready to be read by a computer.
After being scanned by a digital scanner, a photograph consists of a grid of points called pixels. A pixel is the smallest unit of information in a digital image. Each pixel has data attached that tells the computer what color to assign the pixel. For black-and-white-images, that data is usually a level of gray from 0, which is black, to 255, which is white. It sometimes may have more or fewer levels, depending on the sensitivity of the scanner.
More expensive scanners often have more pixels per square inch of the scanned image than do less expensive scanners. More pixels translate to more information, which produces a digital image that more closely mirrors the actual image. In other words, the resolution of the denser pixel system is greater.
Science EncyclopediaScience & Philosophy: Jean-Paul Sartre Biography to Seminiferous tubulesDigital Scanners - The Scanning Process, Color Scanning, Digital Scanners For Publishing, Scanners That Can Read