The light path in an optoelectronic link can be in air or space between an emitter and detector in discrete packages, a link through a transparent medium such as an optical fibre, or between closely-spaced devices in a single package, in which the beam can be interrupted or reflected, or else in which the beam does not appear externally. The most-used optoelectronic devices are the LED and the optocoupler. In most applications, the light is simply another information path, and is not otherwise significant. Optoelectronics is also used to measure, monitor or study radiation, usually as discrete sources, and there are other forms of radiation detector that do not involve semiconductors. Among these detectors are the phototube and photomultiplier, the thermocouple and others, important in special applications. Here, we are mainly concerned with optoelectronics as circuit elements.
Emitters based on a pn-junction, like LED's, are all pretty much alike electrically. They require a series resistor to limit the current through them, and protection against reverse voltages. Distinguish between an LED (or IRED) and a photodiode, which is a pn-junction intended as a radiation detector, not a source.
via Electronics 4.