A computer-controlled microdensitometer has been designed and built in this laboratory for measuring Guinier powder diffraction photographs. It consists of a microcomputer-controlled single-beam microdensitometer using as a measuring unit an IR emitting diode ( lambda =940 nm), a collimator system and a photodiode. The scan process is controlled by a microcomputer, Zilog MCZ 1/05, with two floppy disc units. The microcomputer is also used for data reduction. A direct connection with a large scale computer, Amdahl 470/V7, makes it possible to run indexing, search-match or profile refinement programs within 15 minutes from the moment the film is mounted in the scanner.