Do not confuse the pilot beam size with the effective beam size. Although the two are related if the beam is a Gaussian beam, the effective width is more general. As the beam propagates and is truncated, aberrated, develops secondary maxima or demonstrates any non-Gaussian behavior, the pilot beam and effective width will not give the same values. The primary purpose of the pilot beam is to control the Physical Optics propagation algorithms.

The pilot beam will only be a good indicator of the real beam size if the real beam is a good Gaussian.