| 1 – the increase in the cross-section of a beam with increasing distance from the source
2 – in the far-field region, angle subtended by two diametrically opposed points in a plane perpendicular to the optical axis, at which points the irradiance is a specified fraction of the beam's peak irradiance
NOTE – Generally, only the maximum and minimum divergences (corresponding to the major and minor diameters of the far-field irradiance) need be specified.
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