The beam diameter of an electromagnetic beam is the diameter along any specified line that (a) intersects the beam axis and (b) lies in any specified plane normal to the beam axis, the distance between the two diametrically opposite points at which the irradiance is a specified fraction, e.g., ½ or 1/e, of the beam's peak irradiance. The items in the Metroid series are the driving force behind its gameplay. ... Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. ...

Beam diameter is usually used to characterize electromagnetic beams in the optical regime, and occasionally in the microwave regime, i.e. cases in which the aperture from which the beam emerges is very large with respect to the wavelength. Microwaves are electromagnetic waves with wavelengths longer than those of Terahertz (THz) wavelengths, but relatively short for radio waves. ... As an emitter, antenna aperture is the diameter of the cross-section of an antennas radiation pattern in the direction of highest gain. ... The wavelength is the distance between repeating units of a wave pattern. ...

Beam diameter usually refers to a beam of circular cross section, but not necessarily so. A beam may, for example, have an elliptical cross section, in which case the orientation of the beam diameter must be specified, e.g. with respect to the major or minor axis of the elliptical cross section.

For a more detailed description of one specific optical beam diameter, see Gaussian beam. In optics, a Gaussian beam is a beam of electromagnetic radiation whose transverse electric field and intensity (irradiance) distributions are described by Gaussian functions. ...