This is the first time that small fluorescent sensors performing complicated computational functions, edge detection, which can be regarded as a new area of research in form of image processing. With a combination of florescent sensor, photo acid generator and sodium carbonate spreading on a filter paper, clear and visualized edges of 1-2 mm width were delivered, based on the light dose-driven ‘off-on-off fluorescent behaviour. The fluorescent intensity of sensor molecules in the irradiated region gradually enhanced upon irradiation of 254 nm UV light, due to generation of protons by photo-decomposition of the photo acid generator in that area. Then, it was slowly quenched by a strong quencher produced along with protons; whereas the fluorescence at the edge of irradiated and unirradiated regions was switched ‘on’ by the diffusion of protons neutralizing the pH there. A number of parameters, such as the pKa value of the fluorescent sensors, the light intensity from the UV lamp at two wavelengths, the number of molecules involved in producing the edge of the object and the values of Ksv between sensors and quenchers were determined. The proof of light dose-driven ‘off-on-off fluorescent behaviour of the molecules in solution and the development of a minimal model were built.