The term plate measuring machine has been used for equipment that records information about the content of photographic plates, specifically providing information necessary to determine the celestial coordinates of objects imaged on astronomical photographic plates. The term automatic plate measuring machine (or APM) is used, often specifically for a specific example of such machines located at Cambridge University, UK (the Cambridge Automatic Plate Measuring Machine).
The machine must sense the images, by passing light through the plate to/from a known location in relation to the plate (a device called a densitometer), and record that location. Then, locations of the imaged objects can be determined from information about the coordinates of the plate's target region of the celestial sphere and through use of measured distances across the plate to recognizable objects of known location.
Such machines have been used to gather data from plates, often decades old, which has been useful regarding astrometry as well as locating galaxies imaged on the plates. The large-scale photographic surveys, such as POSS and the UK Schmidt Survey have been processed in such fashion. Examples of plate measuring machines: