Dots per inch refers to the image resolution that scanners produce, and dpi is the number of dots that fit in a 1-inch line. Scanners reflect images to charge-coupled device arrays via mirrors, filters and lenses and then transfer the images to computers using compatible software. The dpi of a scanner is determined by the number of sensors in a single row of the CCD array, the x-direction sampling rate and the precision of the stepper motor.
A CCD array is a collection of small, light-sensitive diodes called photosites, which covert light into an electric charge. Scanners move a scan head, which consists of mirrors, filters, a lens and a CCD array, across a lighted image with a belt that is attached to a stepper motor and a stabilizer bar. The image is reflected through angled, slightly curved mirrors to focus it on a smaller surface, and the last mirror reflects it to a lens, which focuses the image through color filters onto the CCD array. A 300 by 300-dpi scanner that can scan letter-sized documents has 2,550 sensors in each 8.5-inch horizontal row, and single-pass scanners have three rows.
Image quality is determined by sharpness and resolution, and the quality of lighting and optics used in a scanner determine the sharpness of an image. Scanners communicate sensed images to computers via drivers, and most scanners speak a common language called TWAIN. Scanners usually come with additional software, such as a scanning utility and editing software, and many scanners feature Optical Character Recognition software.