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Introduction
The beam divergence of an electromagnetic beam is an angular measure of the increase in beam
diameter with distance from the optical aperture or antenna aperture from which the
electromagnetic beam emerges. The term is relevant only in the "far field", away from any focus
of the beam. Practically speaking, however, the far field can commence physically close to the
radiating aperture, depending on aperture diameter and the operating wavelength.
Beam divergence is often used to characterize electromagnetic beams in the optical regime, for
cases in which the aperture from which the beam emerges is very large with respect to the
wavelength.
Beam divergence 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 divergence must be specified, for example with respect to the major or minor axis of the
elliptical cross section.
The divergence of a beam can be calculated if we know the beam diameter at two separate points
(D1, D2), and the distance (L) between these points. The beam divergence is given by
θ~tan-1(r2-r1/L)
If the beam has been collimated using a lens or other focusing element, the divergence expected
can be calculated from two parameters: the diameter, Dm, of the narrowest point on the beam
before the lens, and the focal length of the lens, f. The divergence is then given by
Divegence= rm/f
Like all electromagnetic beams, lasers are subject to divergence, which is measured in
milliradians (mrad) or degrees. For many applications, a lower-divergence beam is preferable.
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