Desensitization Effects on Solarizing Layers Containing Iridium

Abstract

Around 1950, H. A. Wark observed a very peculiar desensitization effect on solarizing emulsions containing iridium. This photographie materiel was exposed quite normally in a camera but could afterwards be exposed safely to full daylight and was even developed in bright light. During development a direct positive picture of the original scene appeared, regardless of any second exposure. Following essentially the formulae given in a patent by Wark, a material with similar properties could be reproduced. This emulsion shows the following sensitometric behaviour: After a first exposure (with an exposure time shorter than one second) it loses its sensitivity gradually during a subsequent dark period (of about 1000 seconds) in a well defined manner. The material is now much less sensitive towards a second exposure than it was towards the first exposure. The rate of this sensitivity loss depends on the amount of the first exposure approximately iike a second-or third-order reaction does on concentration. The temperature dependence of the reaction rate is in accordance with an activation energy of 0.8 ±0.2 eV.