Abstract: A light-activated transducer includes a transparent electrically-insulating substrate (2) having on one surface an electrode structure. The electrode structure includes an electrode portion (5a) containing an aperture for passage therethrough of light which has passed through the substrate, a contact pad (5c) spaced from the electrode portion, and an electrical feedthrough (5b) connecting the electrode portion to the contact pad. An insulator layer (3) is adhered to the surface of the substrate and on the feedthrough, and surrounds the electrode portion while leaving uncovered the contact pad and the electrode portion and a corresponding region of the substrate. A conductive or semiconductive cover sheet (1) is adhered to the insulator layer and supported thereby in spaced overlying relationship with the electrode portion and the corresponding region of the substrate. The, cover sheet, insulator layer, and the substrate form a cavity that, when sealed, contains an ionisable gaseous filling.