The light emitted by a laser is confined to a rather narrow cone. But, when the beam propagates outward, it slowly diverges or fans out. For an electromagnetic beam, beam divergence is the angular measure of the increase in the radius or diameter with distance from the optical aperture as the beam emerges.
The divergence of a laser beam can be calculated if the beam diameter d1 and d2 at two separate distances are known. Let z1and z2 are the distances along the laser axis, from the end of the laser to points “1” and “2”.
Usually, divergence angle is taken as the full angle of opening of the beam. Then,
Half of the divergence angle can be calculated as
where w1 and w2 are the radii of the beam at z1 and z2.
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.