Beam diameter
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The beam diameter of an electromagnetic beam is the diameter along any specified line that is perpendicular to the beam axis and intersects it. For this purpose, the diameter is often defined as the distance between the two diametrically opposite points at which the irradiance is a specified fraction of the beam's peak irradiance. The most common choices for the fraction are ½ and 1/e2.
Beam diameter is usually used to characterize electromagnetic beams in the optical regime, and occasionally in the microwave regime, that is, cases in which the aperture from which the beam emerges is very large with respect to the wavelength.
It is recommended that laser beam diameter measurements be made by reference to the international standard ISO 11146-1:2005, which specifies methods for measuring beam widths (diameter), divergence angles and beam propagation ratios of laser beams (if the beam is stigmatic) and for general astigmatic beams ISO 11146-2 is applicable.
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, for example 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.
