Alarms {burglar alarm} can use ultraviolet light, which reflects from windows and doors. If reflectors move, light-path interruption sounds alarm.
Door sensors {electric eye}| can use a collimated light source on one side and a vacuum tube {phototube} on other side. Tube has a half-cylinder plate {cathode, electric eye} that emits electrons when light hits. Electrons travel to other electrode {anode, electric eye}. Breaking light path signals a relay to open door.
Lasers {maser}| can use microwaves.
7-Machine-Kinds-Electronic-Light
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Date Modified: 2022.0225