Patent ID: 8835798
Filing Date: 2014-09-16
Classification: B23K,B41C

Abstract:
1. A method for exposing a surface of a cylindrical drum in a continuous helical path to illumination from a pulsed laser source to cause ablation of the surface so as to form an array of repeating 3D microstructures thereon, the method comprising: generating pulses of radiation using a laser; providing a mask of rectangular shape with a first axis and a second axis, the mask having a plurality of features arranged on a fixed pitch in a line parallel to the first axis thereof corresponding or proportional to the pitch of the 3D microstructures to be formed, the features in the line corresponding to different depth contours of the 3D microstructure which together define a complete microstructure; locating the mask relative to a projection lens with the first axis thereof substantially parallel to the axis of rotation of the drum and the second axis thereof substantially parallel to the direction of rotation of the drum; exposing the mask to the rectangular shaped laser beam such that all the features thereof are exposed by the laser beam; projecting an image comprising the plurality of features on the mask through the projection lens onto a target area of the drum surface; rotating the drum about its axis; moving the projection lens and mask with respect to the drum in a direction substantially parallel to the axis of rotation thereof such that after each complete rotation of the drum the projection lens and mask have moved a distance equal to said fixed pitch of the microstructure array in the direction parallel to the axis of rotation of the drum whereby said plurality of features are successively exposed onto the same target area to form the different depth profiles of the microstructure; and controlling the timing of the laser pulses in relation to an angular position of the rotating drum such that laser pulses occur whenever the target area on the drum surface has moved a distance corresponding to a repeat distance of the array of microstructures in the direction of rotation such that a whole number of the microstructure repeat distances fit into a helical path around the drum in each revolution thereof, wherein a width, in the direction of rotation of the drum, of the image projected onto the target area of the drum surface by each exposure of the mask to the rectangular shaped laser beam is less than or equal to a minimum repeat pitch of the array of microstructures in the direction of rotation, and a length, in a direction perpendicular to the direction of rotation, of the image is greater than or equal to twice the minimum repeat pitch of the array of microstructures in the direction perpendicular to the direction of rotation.