Fundamental mechanisms of lithographic printing plate imaging by near-infrared lasers

The fundamental mechanisms of exposure by near-infrared pulses of multilayer films, which can be used as lithographic printing plates, are investigated using time-resolved optical microscopy. The films were developed by Presstek, Inc. (Hudson, NH) for use in the PEARL™ imaging system. Exposure by 10-μs duration Gaussian profile pulses is shown to occur with an extremely sharp fluence threshold of J_th = 0.34 J/cm². Exposure greatly alters the surface affinity for inks. Time-resolved microscopy shows the mechanism of surface alteration to involve thermochemical decomposition of the surface coating material, which results in its debonding from the film. Using threshold measurements and a theoretical thermal conduction model, debonding is shown to occur when the temperature in the film is about 500°C.