Fluctuations of intensity caused by laser beam wandering

A derivation is given of an approximate expression for the density distribution of the radiation power passing through a specified aperture which is illuminated by a laser beam subject to random displacements. The expression provides a good approximation to the exact distribution plotted graphically by means of a table of Q functions if the dimensions of the receiving aperture are such that up to ~50% of the incident beam power passes through it.