NEW METHOD FOR EXPOSURE THRESHOLD MEASUREMENT OF LASER THERMAL IMAGING MATERIALS

A new method is described for measuring the exposure threshold of lase r thermal imaging materials exposed by pulses from a laser with a Gaus sian radial spatial profile. This method, termed the maximum spot meth od, involves placing the material in the focused beam of a repetitivel y pulsed laser and exposing a series of spots of different sizes by mo ving the material through and beyond the beam focus. The exposure thre shold can be deduced by knowing the radius of the largest exposed spot and the pulse energy. The method is demonstrated on a laser ablation transfer film (Lasermask(TM)) and a direct imaging film developed by P resstek, Inc., for computer-to-plate imaging applications. It is shown that the new method, which is convenient and quick, gives the same re sults with fewer sources of experimental errors as conventional thresh old measurement methods. The convenience of the new method permits sys tematic studies of the dependence of exposure properties on material p roperties or laser imaging conditions. As an example, the imaging thre shold of Lasermask is measured as a function of laser pulse duration f rom 10(-12) to 10(-4) s.