The angular intensity profile of a single beamlet is expressed as a sum of a main Gaussian and a bump-on-the-tail representing the optical aberration. Transmission of a total beam comprised of a large number of beamlets is then analytically derived. By an iterative matching of the calculated transmissions to the measured transmissions through at least two downstream openings, the parameters that describe the single beamlet angular profile are determined. Some of the neutral beam injectors employed for heating fusion plasmas are characterized by this modeling. Also presented is an improved algorithm for calculating the intensity distribution at a downstream plane.