Abstract: Apparatus for determining the location of graduated marks in measuring comprising source means for emitting electromagnetic radiation towards electromagnetic radiation sensors, and a measuring mark moveable between the source means and the sensors from one measuring position to another in a direction of measurement transverse to the direction of radiation, the mark being adapted to cause a restriction in the cross-sectional measurement of radiation between the source means and the sensors, the sensors being adapted to detect the location of the radiation restriction by sensing radiation intensity proportional to the area of their receiving faces upon which such radiation falls in use.

Abstract: The invention provides a method of measuring comprising: directing sources of electro-magnetic radiation, having a predetermined displacement relative to each other in a direction of measurement, to radiate over the receiving areas of electromagnetic radiation sensors also having a predetermined displacement relative to each other in a direction of measurement; sequentially switching the sources; moving a mark between the sources and the sensors, the mark causing a restriction in a cross-section measurement of the radiation passing beyond it, and the mark moving in a direction of measurement from a reference position to a position determined by the measurement being taken; detecting any changes in the sensors as a result of movement of the radiation restriction caused by movement of the mark and the switching of the sources; noting between the switching of which sources each such change occurred; and ascertaining what distance the radiation restriction moved to cause the change to occur between the noted sour