1. An apparatus comprising:a chamber;
a heat source disposed within the chamber, wherein the heat source comprises a laser positioned to direct focused laser radiation at a reduced area of a first surface of a substrate to be processed;
a radiant interface detector comprising a radiation source, a partially reflective mirror, a control surface, and a radiation sensor,
wherein the partially reflective mirror is positioned to direct control radiation towards the control surface and interference radiation towards a reduced area of a second surface of the substrate to be processed, the reduced area of the first surface opposite the reduced area of the second surface,
and wherein the control surface is perpendicular to the direction from which the control radiation is received; and
a substrate support disposed within the chamber, the substrate support having a substrate contact surface and a back side opposite the substrate contact surface, wherein the substrate contact surface is disposed between the laser and the radiation source, between the laser and partially reflective mirror, and between the laser and the radiation sensor.