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: A method of patterning a perforated screen printing screen (9) having a lacquer coating filling its perforations (10), using a laser engraving device, includes the steps of locating the position of the locus of aligned perforations (10), aligning an engraving head (7) of the laser engraving device with the locus of the perforations (10) and then selectively actuating the laser to remove the lacquer individually from those perforations (10) which it is required to unblock.

Abstract: A laser engraving machine for engraving rotary printing screens includes a shaft (1) having one end connected to a rotary drive (3)for rotating the shaft (1), two end ring assemblies (7, 8) at least one (8) of which is movable along the shaft (1) and which are arranged to locate concentrically and fix opposite ends of a rotary printing screen (2) to the shaft (1). The machine also includes a laser engraving device including a head (4) movable backwards and forwards parallel to the axis of rotation of the shaft (1) to engrave a printing screen (2) and a support (9, 13). The support (9, 13) is movable along the screen (2) with the laser engraving head (4) to provide local support for the screen (2) so that the part being engraved is supported at a constant predetermined position with respect to the head (4) of the laser engraving device.

Abstract: A method of, and apparatus for, determining the position of an unknown fixed location, e.g. the end of an underground passage inaccessible to human beings, involves establishing a chain of measuring station of known station spacing between a base and a position at the unknown fixed location. Each station (except, optionally, the foremost one) of the chain is provided with light sources and photodetectors as well as a goniometer for measuring in at least one plane the angle of inclination of the station immediately ahead of it. The stations of the chain are spaced apart by a known distance and arranged so that each station is within the operative range of the immediately preceding station. By transmitting the measurement results from each station to the base, and processing the data, the position of the fixed location from the measurement results.