The process of forming stable or permanent visible images directly or indirectly by the action of light or other forms of radiation on sensitive surfaces. Traditionally, photography utilizes the action of light to bring about changes in silver halide crystals. These changes may be invisible, necessitating a development step to reveal the image, or they may cause directly visible darkening (the print-out effect). Most photography is of the first kind, in which development converts exposed silver halide to (nonsensitive) metallic silver.