Electric field induced second harmonic (EFISH) generation typically increases with laser intensity. Therefore, EFISH signals generated at the rear Si=SiO2 interface by a laser beam transmitted through a thin silicon membrane should be much weaker than the signals generated in reflection at the front surface. The counter-intuitive finding that the signal generated at the rear surface is stronger than the front surface signal [Appl. Phys. B 104, 735 (2011)] is nowconfirmed by detailed experiments applying 10 to 30 _mthin membranes.Using, in addition, the local laser beam intensity [Appl. Phys. B 123, 253 (2017)] leads to the hypothesis that the quasi-static electric field at the rear interface consists of two counteracting components.