Long persistent light emitting diode indicators

An undergraduate laboratory was designed for undergraduate students to make long persistent light emitting diode (LED) indicators using phosphors. Blue LEDs, which emit at 465 nm, were characterized and used as an excitation source. Long persistent phosphors, SrAl 2 O 4 :Eu 2+ ,Dy 3+ (green) and CaS:Eu 2+ ,Tm 3+ (red), were used as light converters. These phosphors were coated onto the LEDs to produce an afterglow emission once the LEDs were switched off. The indication time of the indicators was extended due to the afterglow emission from phosphors. The emission spectra, afterglow emission spectra and afterglow decay curves of the LED indicators were measured by using a photodetector. These long persistent LED indicators have important applications in emergent situations when electrical power is unavailable.