PHOTOCONDUCTIVITY

that portion of the ability of a material to allow an electric current to flow in it that is affected by exposure to light, frequently used to detect and measure the amount of light. Many crystals, including those made from the chemical elements silicon and germanium, are poor current conductors because their electrons are unable to move freely within the material when an electrical voltage is applied. Light directed on such materials is absorbed by some electrons, however, freeing them to pass more easily from one atom to the next when a voltage is applied. In contrast, good conductors such as metals have an abundance of free, current-carrying electrons. When photoconductive materials are removed from the light, the freed electrons return to their more tightly bound state. The time required for the return to normal conductivity varies widely from one type of crystal to another. See photoelectric effect.