The invention describes a method for hardening a security mechanism against physical intrusion and substitution attacks. The cap member seals a discharge surface in the head. To accomplish this, a light pipe is constituted of two adjacent portions, and an infrared transceiver module is coupled into both portions. Additionally, the individual SST dies can have a cross-connection contact coupled to the interconnection. Temporally-interleaved image-subtraction is particularly well suited for use in video eyetracking applications where ambient light and scene motion can cause significant problems.