Autoradiography uses a photographic emulsion to visualize molecules labeled with radioactive elements. The emulsion contains silver halide crystals that become reduced to metallic silver when exposed to radiation from the radioactive material. Photographic developers then show these silver grains as blackening of the film. Techniques of autoradiography have become important in molecular biology for cell and tissue localization experiments using weak beta-emitting isotopes like tritium, carbon-14, and sulfur-35 that result in discrete images. Electron microscopy can then be used to locate the image in the developed film.