Essentially, an infrared transmitter is an electronic device that generates IR light--light with a wavelength of between 1 mm and 750 nanometers--which lies between visible light microwaves in the electromagnetic spectrum.
Infrared transmitters are widely used as a means of wireless communication by remote controls for televisions and other electronic devices. Police, firefighters and ambulance drivers, for example, carry a special IR transmitter that they can use to change traffic lights from red to green within two seconds, as they approach an intersection.
How Infrared Works
An infrared transmitter is simply a Light Emitting Diode, which emits invisible IR light, and some associated circuitry. In an infrared remote control, for example, pushing a button sends an electric signal to the LED, which converts the signal into a beam of infrared light. The receiving device detects the light with a photo diode and converts it to an electrical signal which, via an integrated circuit, controls its actions.
The main limitation of IR technology is that it requires line-of-sight between the transmitter and receiver. It cannot be used around corners or between rooms with intervening walls or ceilings.