  A controllable energy method, which considers the undersampling issue of the
transfer function and valid spectral energy of a source signal, is proposed to
implement angular spectrum diffraction calculation in near and far fields. The
proposed method provides an optimized frequency boundary $f_{CE}$ within which
it always keeps controllable energy to be diffracted. The controllable energy
angular spectrum method significantly reduces the number of samples while
having the same accuracy as previous angular spectrum methods, implying a
higher calculation efficiency. The new perspective of analyzing spectral energy
is shown to improve the performance of relevant diffraction calculations.
