Abstract We present a semi-analytic theory for calculating light–particle interactions in shaped laser beams even when the paraxial beam description is invalid. It requires weighting the expressions for the cross sections with a beam normalization parameter, ϰ , associated with the incident power. An analytical formula for ϰ in terms of the beam shape coefficients [1] , [2] , [3] is derived. We show that approximate expressions for this beam normalization parameter based on either a Parseval or paraxial type approximation are inadequate for optics involving high numerical apertures.