placed at a variable distance from the back surface of the si wafer across from an intact region and pcr signals were monitored fig9 it is observed that the pcr amplitude remains constant for all three frequencies up to a distance of u02dc 1 mm away from contacting the back surface where it starts to increase the curves are normalized to their value on the surface to show that the rate of increase is independent of frequency the pcr phase remains essentially flat throughout to determine the origin of pcr signal variations with backing whether due to ir photon internal reflections or backing emissivity changes 31 the laser was turned off and a mechanical chopper was placed at some distance away from the ir detector the lockin amplifier signals from the ingaas detector nearly vanished at u02dc 5 u03bcv a baseline value that remained constant for all combinations of wafer chopper and the three substrate materials these dc emissivity experiments with the ingaas detector in place are clear evidence that its spectral bandwidth lies entirely outside the thermal ir planck emission range of the si wafer with or without substrate therefore the pcr amplitude enhancement for mirrored and rubber backings fig8 a is consistent with simple reflection of exiting transmitted cw generated ir photons at the surface of the backing with no possibility for thermal infrared emissivity contributions from the backing itself the order of.