Abstract: A laser engraving head (6) for use in preparing a medium for a printing process includes a deflector (AOM). The deflector, which may be an acousto-optic modulator, deflects the laser beam which is focused by a lens (L) onto the surface of the medium. The lens (L) may be arranged telecentrically with respect to the deflected laser beams so that the exit pupil (P) defined by the range of deflected beams is located at the back focal plane of the means for focusing. A number of interchangeable beam expanders (20) may be provided mounted within the head and moveable so that one of the expanders is brought into the path of the beam to provide a desired cell size. Control circuits for the head may modify the control signal applied to the deflector compensate for variations as a function of deflection angle in the cell size produced on the medium.

Abstract: An apparatus for preparing a medium for use in a printing process supports the medium (2) for rotation about an axis. A head (6) is mounted for movement parallel to the axis and includes means to direct a laser beam onto the surface of the medium. The laser beam engraves or exposes the series of cell sites (c) around the medium and an acousto-optic modulator (12) positioned in the path of the laser beam deflects the beam substantially in the direction of rotation of the medium (2). In a preferred example the beam tracks the surface of the medium at a speed matched to that of the medium so as to keep the beam stationary on the cell site while that site is being engraved or exposed. The beam may also be deflected to skip backwards or forwards over immediately adjacent cell sites.