REM dreamer
The REM Dreamer is a mask-like device that claims to aid the user in attaining lucidity in dreams. The technique it uses is a flashing light and/or sound that the device generates when the sleeper is in the REM phase of sleep (where most dreams occur). The user must then recognize these cues in his dream so that he realizes that he is asleep and dreaming. The timing of the cues are programmed by the user, who may vary their intensity so that they will reach his dream but will not wake him.
The REM Dreamer is one of many induction devices that takes advantage of the NATURAL phenomenon of incorporating external stimuli into one's dreams. This way when the cues are initiated, the user's dream will transform them into something else, like a reflection of the sun on a window. The flashing cues in the dream may come in any form reminiscent of light, which adds to the difficulty of identifying them and realizing one is dreaming.
Once the user has become lucid, he can stop the cues from flashing by moving his eyes up and down for a few seconds. The REM dreamer recognizes this signal and turns off. This system is possible because during a dream the physical eyes move in a similar way as dream eyes do. Also, on a final note, the use of these stimuli have not been proven one hundred percent safe, so naturally caution is advised.