An infrared transmitter is a device that emits a beam of light in the infrared range, which is just slightly out of range of normal human vision, and encompasses a wavelength of light longer than visible red light. Devices that incorporate infrared technology stretch from the mundane, such as television remote controls, to the exotic, such as night vision goggles used by the military. Many natural objects emit their own spectrum of infrared radiation, including the human body, the Sun, and the Earth. This makes sensors and optical detectors that operate in the infrared range useful devices in astronomy, wireless telecommunications and medicine.
Other terms for infrared light are heat radiation and black body radiation. This is because infrared light is emitted by objects cooling off in the dark, and the wavelength of light is so long that it disperses quickly and tends to be absorbed by anything nearby, which generates heat. For this reason, an infrared transmitter in wireless technology, such as a computer mouse or keyboard, is only functional at a short range, and the object must have an unimpeded path to the infrared receiver, as the light cannot pass through thick or dense structures such as walls or metal.
Infrared light is easily absorbed by anything in its path, and disperses quickly. While this limits its use for long-range transmissions, it makes it ideal for short range sensors such as in faucet controls in public restrooms and hand air dryers, where the devices turn on when hands are placed in close proximity to the path of the light beam and otherwise remain off to conserve power and water. Television remote controls have one of the longest range capabilities for a consumer infrared transmitter, and can usually function at a distance of 15 feet (4.6 meters) or more. The effective distance of infrared devices is dependent on the wavelength of light used, with shorter wavelengths transmitting farther. Infrared light ranges from the nearly visible or near-infrared to the microwave or far-infrared range of the spectrum at 750 nanometers to 1 millimeter wavelengths.
One important application of infrared light is the design of a mobile infrared transmitter (MIRT), which is installed in many emergency vehicles such as police cars, ambulances, and fire trucks. The vehicle has a near-infrared transmitter with an effective range of 1,500 feet (457 meters). The device allows what's known as traffic signal preemption, allowing emergency vehicles that are equipped with infrared receiver controls to alter the automatic switching of traffic lights in their favor as they move down city streets. This speeds their progress toward emergency locations, and, in the US as of 2001, 55 of its 75 largest metropolitan areas were using the technology.