• shines bright light onto image being scanner while scan head moves from top of document to bottom at constant rate
• scan head measures intensity of light reflected back from document -> converted to digital information
• resolution measured in dots per inch
• for colour scanning, three passes of scan head are required (using red, green and blue filters)

• user touches area of screen in order to enter data
• used in industrial environments such as manufacturing, warehousing and security systems and in avionics and medicine
• suitable in situations where the operator is moving about and can quickly and easily enter commands by touching the screen
• hand-held portable model used by airlines to record receipts for bar sales, by hospitals to collect information on maternity patients and by local authorities for door-to-door collection of taxes

• consists of a glass substrate covered with a tight fitting plastic sheet
• conductive coatings applied to inner surface of both elements
• separator dots separate cover sheet from glass substrate
• light finger pressure -> internal electrical contact at point of touch -> controller supplied with analogue voltage needed for digitisation

• screen is single glass panel with electrical transducers in the corners
• interfacing controller sends electrical signal to transducers which convert the signal into surface acoustic waves
• waves are reflected across the active area of the glass by an array or reflector stripes located on the outer edges of the glass
• when a finger or pen touches the screen, a portion of the wave is absorbed and the change in the received signal is analysed by the microprocessor in the interfacing controller
• digitised co-ordinate pairs are transmitted to the computer

• input signal range divided up into a number of discrete levels (number of levels determines the resolution with which analogue signal can be reproduced)
• samples of analogue signals taken at frequent intervals and converted to digital values

Interrupt-driven system e.g. traffic lights

• computer receives a signal as soon as data is collected and can then decide what action to take if any

Polled system - computer periodically checks the input level from a device

Turnaround documents - produced as output for one computer process and can be used as input for another