Incremental printing of symbolic information – Light or beam marking apparatus or processes – Scan of light
Patent
1994-11-21
1998-05-05
Le, N.
Incremental printing of symbolic information
Light or beam marking apparatus or processes
Scan of light
347239, 347244, 347243, B41T 247, G01D 1514, G02B 2600, G02B 2700
Patent
active
057482225
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
The present invention relates to a laser engraving head suitable for use, for example, for preparing anilox rollers. It is particularly concerned with a head incorporating an acousto-optic beam deflector, such as that described in our co-pending international application no. PCT/GB 91/01829 filed Oct. 18, 1991.
Conventionally, when a laser engraving head is used to prepare an anilox cylinder, a ceramic-coated roller is rotated and the laser engraving head is moved parallel to the longitudinal axis of the roller. Typically the engraving head includes a plane mirror which receives light from the laser which is fixed at one end of the engraving apparatus. The head focuses light from the laser onto the surface of the roller. The combined effect of the rotation of the roller and lateral movement of the engraving head is to produce a helical pattern of cells on the roller. Typically the laser is pulsed, with one pulse corresponding to each cell.
Alternatively, to produce the helical cell pattern the roller may be moved laterally past the head whilst rotating, the head being fixed. This has the advantage of a fixed optical path length from the laser.
As described in our above-cited application, hitherto the speed with which cells can be engraved has been limited by such effects as channelling between adjacent cells, and smearing or deformation of cells. A significant increase in the frequency of engraving can be achieved by using a beam-deflecting device such as an acousto-optic deflector to deflect the engraving beam between cell sites. This may be used to track a given cell site as the roller moves at high speed, and also to skip between cell sites so that the cells are engraved in an order which serves to minimize problems such as channelling.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, a laser engraving head for use in an apparatus for preparing a medium for use in a printing process includes means for deflecting the laser beam, and means for focusing the laser beam onto the surface of the medium, the means for focusing being arranged telecentrically with respect to the deflected laser beams, the exit pupil defined by the range of deflected beams being located at the back focal plane of the means for focusing.
The present inventors have found that significant advantages are obtained by the adoption of a telecentric arrangement for the final focusing lens. The principle ray of each pencil of rays is then normal to the roller target. This has the effect that the distance of deflection of the laser beam on the surface of the roller is independent of the focus of the lens. This makes it possible to adjust the focal position of the lens in order, for example, to optimise the cell size and minimize the unengraved land between the cells.
Although the use of a telecentric configuration is much preferred, alternatively the effects of a non-telecentric configuration may be corrected for in software. The degree of divergence of the principle rays leaving the focusing lens then need to be calibrated with respect to the focal position of the roller. In use, the operator would set up the separation of the engraving head from the roller for the optimum cell shape (aspect ratio of cell opening to cell depth) . Next the precise separation of the head from the roller would need to be sensed and fed to the controlling hardware. This could be achieved by a dial indicator with electrical feed out or a low voltage differential transformer (LVDT) displacement transducer. A look-up table or algorithm in software or hardware is used to apply the appropriate correction to the control signal for the acousto-optic deflector so that the deflection distance on the surface of the roller is correct.
The use of a telecentric configuration places constraints on the design of the other elements of the optical system of the engraving head. As described in further detail below, the present inventors have developed a head offering significantly improved performance, within those c
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Gordon Raquel Yvette
Le N.
ZED Instruments Ltd.
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