Devices that create light pulsations at intervals varying from one flash every few seconds to several flashes per second. They have become a familiar part of the atmosphere of excitement at some dance halls and discotheques, but in different situations they have proved of some assistance in inducing hallucinatory experiences, especially when seen with half-open or even closed eyes.
Pierre Janet did pioneering work with stroboscopes. He flashed lights at mental patients and saw some immediate breakthroughs in their health. The strobes work by a process called entrainment, in which the brain tends to align itself with the frequency of the flashing light. A beam flashed at the right frequency can induce a more relaxed state and cause the brain to change its frequency.