Optical scanning apparatus

In optical scanning apparatus comprising a source of light, a rotatable reflecting element having a plurality of reflective facets which are inclined to the axis of rotation, means for directing a beam of light derived from the source at said reflecting element in a direction parallel to its axis of rotation to impinge upon successive facets thereof as it is rotated, means for defining a position for a recording element having a surface that requires scanning, means for focussing the beam as reflected by said reflecting element onto said surface, a motor coupled to rotate the reflecting element, so that each facet in turn causes the focussed beam to scan across said surface in one direction, and a drive for moving the recording element substantially perpendicular to said on direction, past said focussed beam, the improvement comprising a generator responsive to the rotational movement of the reflecting element to generate a signal indicative of the angular position of said reflecting element, said generator including a readout device comprising a further light source and a photocell and a shutter comprising a pair of elements each provided with a grating formed by an array of alternate opaque and transparent portions, said photocell being arranged to view the light source through the gratings, said elements being mounted for relative movement in accord with the rotation of the reflecting element, whereby illumination of the photocell is modulated cyclically at a frequency which varies in accord with the speed of rotation of the reflecting element.