Beam Divergence of a laser beam is a measure for how fast the beam expands far from the beam waist. Beams with very small divergence, with approximately constant beam radius over significant propagation distances, are called collimated beams. It 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. It is the derivative of the beam radius with respect to the axial position in the far field, at a distance from the beam waist which is much larger than the Rayleigh length.