(57) A recording material contains on a 20 - 200 µm thick paper or plastics support a 2 - 30 µm layer of a dispersion of particles of a light-sensitive organic substance which forms a decomposition product when light-­exposed and an organic substance which reacts imagewise (e.g. a diazo compound and its coupler or a photo-oxidiz­ing agent and a leuco dye); at least one of these may be microencapsulated. The material is imagewise exposed, e.g. from a fluorescent lamp, and overall heated in two steps, firstly at a temperature (e.g. 60 - 90°C) at which no formation or fixing of the image occurs and then at a higher temperature (e.g. 80 - 150°C) to cause the re­action to form a visible image.
The first heating can be at the time of the imagewise exposure, e.g. by the heat from the lamp. The material is transferrd between the heating steps as fast as possible. The use of the first heating allows less energy to be used for the development step.
Diazo material used can contain other com­ponents such as a color forming aid or substance fusible at 50 - 150°C.
Visible images are formed of good colour density, low background fog and good shelf life before use.