Pointwise Fresnel gain versus integrated transmitted power

A Fresnel diffraction aperture can increase the detected intensity at one selected observation point by removing contributions that would otherwise interfere destructively there. The increased local signal is a redistribution of transmitted radiant power, not an increase in total energy supplied by the incident beam.

For a passive absorbing screen under fixed illumination, closing part of an aperture cannot increase the total transmitted radiant power. Nevertheless the remaining complex amplitudes can have a larger sum at a particular point. Comparing the single-point response with a measurement integrated over the whole transmitted field distinguishes the largest contiguous Fresnel aperture optimization objective from an efficiency claim.