Luminescence .is .the .spontaneous .emission .of .light .from .the .excited .electronic .states .of .physical .systems. The .emission .is .preceded .by .the .process .of .excitation, .which .may .be .produced .by .a .variety .of .agents. .If .it .is .achieved .by .the .absorption .of .light .it .is .called .photoluminescence, .if .by .the .action .of .an .electric .field .electroluminescence, .if .by .a .chemical .reaction .chemiluminescence, .and .so .on. Following .the .excitation, .if .the .system .is .left .alone .without .any .additional .influence .from .the .exciting .agent, .it .will .emit .spontaneously
