emitters

An emitter is a source that releases energy, particles, or signals into its surroundings. The term is used across science and engineering to describe objects or devices that radiate or emit something rather than absorb or reflect it. Emitters may be characterized by what they emit, how much they emit, and the spectrum or type of emission.

Common categories include electromagnetic emitters, which release photons as light or other radiation. Examples include LEDs, lasers, lamps, and thermal emitters such as hot surfaces. Radioactive materials act as particle emitters, releasing ionizing radiation in the form of alpha, beta, or gamma radiation. Acoustic emitters generate sound waves, while thermal emitters release heat.

In electronics, the word emitter also designates a terminal of a transistor that injects charge carriers into the device, as in the emitter in a BJT or the emitter follower circuit.

In software, an emitter (often called an event emitter) is an object that emits events to which listeners can subscribe, a pattern widely used in event-driven programming.

Other contexts include environmental policy, where emitters refer to sources of pollutants or greenhouse gases whose emissions are tracked and regulated, and astronomy, where stars and other bodies are natural emitters of electromagnetic radiation.

Emission properties—intensity, spectrum, directionality, and coherence—are central to the design and analysis of emitters, and they are governed by underlying physical processes and material properties.