Security devices and methods of manufacture thereof

A method of manufacturing at least part of a security device, the method comprising:

• conveying a substrate web comprising a photosensitive film along a transport path, the photosensitive film being adapted to exhibit an increase in optical density upon exposure to radiation of a predetermined wavelength and concurrent or subsequent heating, the increase in optical density being due to the formation of bubbles within the photosensitive film;
• exposing the photosensitive film to radiation of the predetermined wavelength through a mask, wherein the mask comprises a predetermined pattern of regions which are substantially opaque to radiation of the predetermined wavelength and at least semi-transparent to radiation of the predetermined wavelength, respectively;
• during the exposure, moving the mask alongside the substrate web along at least a portion of the transport path at substantially the same speed as the substrate web, such that there is substantially no relative movement between the mask and the substrate web;
• and
• heating the substrate web comprising the exposed photosensitive film;
• whereby regions of the photosensitive film exposed to the radiation of the predetermined wavelength undergo an increase in optical density such that the photosensitive film displays a reproduction of the predetermined pattern.