Abstract: A composite structure for making printing plates and other surfaces in relief is improved by reducing the density of a polymer substrate thereby increasing the amount of material removed per unit of time by a given total amount of energy. The density of the polymer substrate is reduced by making it cellular or poriferous using expandable micro capsules made of thermoplastic shell and a nucleus of a liquified blowing agent.

Abstract: Inherent defects in the angular relationship between facets of a rotating polygonal mirror used to sequentially scan a beam of radiation are corrected by an optical reflecting or refracting element pivotally mounted in the path of the radiation. With the use of electromechanical devices energized by timed electrical signals of appropriate value, the mirror or refracting element is pivoted to correct the scanning errors caused by angular defects in the rotating mirror. The effect of variations in the angles between facets of the polygonal mirror are also corrected by an electronic circuit which includes a delay device capable of delaying the scanning by a predetermined amount. Control means for the electromechanical devices and for the delay device are pre-programmed so as to make the proper adjustments for each facet of the rotating polygonal mirror.