| distance between two diametrically opposed points where the irradiance is a specified fraction of the beam's peak irradiance
Note 1 to entry: The concept is most commonly applied to beams that are circular or nearly circular in cross-section.
Note 2 to entry: The term "beamwidth" has different meanings in relation to antennae (see IEV 712-02-33) and in the field of ultrasonics (see IEV 802-01-20).
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