Abstract: Optical apparatus for monitoring the relative positioning of first and second members which are movable relative to each other includes radiation means for generating an optical wavefield which provides, over a given area which has a predetermined location relative to the first member at least when the first member is in a given position relative to the second member, a distinctive light intensity distribution. The distinctive light intensity distribution results from at least two wavefronts that differ from each other in curvature in each of two mutually orthogonal planes. The radiation means preferably includes a source of radiation, a diffracting element and a focussing element. The diffracting element coacts with the source of radiation to provide said at least two wavefronts and the focussing element produces an image of the diffracting element at said predetermined location at least when the first member is in said given position relative to the second member.