High Speed Laser Scanning Unit (LSU) Using 12-Beam Laser Diode Array and Image Tracking System (ITT) for High Quality Color Printing

Abstract

The exposure system of the digital printing engine transforms the digital image data stream into a charge image on the Photo Imaging Plate (PIP). The system developed by HP-Indigo is based on a conventional scanning unit design principal. It consists of laser light source, collimator, polygon scanner and anamorphic F- scanning optics. The uniqueness of this system is in the combination of very high process speed (1.2 m/sec) and high resolution (800 dpi) at A3 size printing format resulting in printing at extreme data rate of 440 Mpixel/sec.To achieve high quality for such challenging requirements, a conventional optical design was uniquely optimized for the use of a 12-beam laser diode array together with a 6-facet high speed (33,000 rpm) polygon scanner. In order to maintain the imaging spot uniformity within sufficient focal depth, we had to achieve high dynamic optical performance (especially flatness) of the polygon system as well as to keep under stringent control the surface shape quality and adjustment accuracy of the scanning optics.The printed image quality depends very much on color plane registration between separations, as well as on local placement accuracy of image lines in the process direction within each color separation. The free running polygon that is not synchronized with the process direction movement, is able to introduce color registration errors up to half of a 12-line scan (187.5 μm). Such a big error is not acceptable and should be corrected. Writing an image at a constant scanning speed while the machine process speed is inevitably instable may result in severe banding artifacts on the printed image. Solution for both problems is given by using a controlled high bandwidth Dynamic optical system that places the image lines at correct equidistant positions detected by a rotary encoder attached to the PIP drum (ITT).