Application of a modulated light beam for the determination of the charge pattern

Investigations of the charging and discharging processes of electrets and other objects require in many cases determination of surface distribution of charge density. For such investigations induction capacitive probes are frequently used. Such probes, usually equipped with a small area measuring electrode (or aperture) are placed above and very close to the surface of investigated objects and moved along a straight line. By analyzing the surface line by line one can obtain a map of the distribution of the surface charge density. As well as simple induction probes one can use a commercial one with a DC-AC converting and electric field compensation in the air gap between the investigated sample and the probe. Other method was suggested by Alquie et al. They suggest to apply local perturbation of the geometry of a measuring system, where pressure pulse (generated on a small surface by laser ablation of a target deposited on the back electrode of the investigated electret) was used for creation of the transcient perturbation. A similar method is proposed below. Periodical and local heating of the absorption layer deposited on the back electrode of an electret is suggested for local and periodical perturbation of the geometry of a sandwich containing the investigated electret. A laser generated light beam (modulated and focused on the absorption layer) was used for local heating