View source for Beam Divergence

The path of light emitted from an aperture can be approximated by a cone. The divergence is usually specified as the half-angle of the two-dimensional triangle formed by the emitted light at the aperture (when a source refers to “full angle,” that is simply twice the “half angle”). This means that power/area density falls off with the square of distance. For long distance use of lasers, beam divergence becomes an important factor, because the energy emitted by the laser will be spread over a fairly large area, so power considerations are key.