| 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 to compensate for variations as a function of deflection angle in the cell size produced on the medium. Where the deflector is formed of a material, such as germanium, which is subject to thermal run-away, then the head may include sampling optics which deflect a portion of the beam upstream of the deflector and a power sensor which receives the deflected beam and is monitored to detect the incidence on the deflector of optical power exceeding a predetermined threshold. |