Seebeck effect

Seebeck effect imaging (SEI) uses a laser to generate thermal gradients in conductors. The thermal gradients induced generate corresponding electric potential gradients. This correlation of thermal and electric gradients is known as the Seebeck effect. The SEI technique is used to locate electrically floating conductors.

When the laser changes the thermal gradient of a floating conductor, its electrical potential changes. This change in potential will change the bias of any transistors connected to the floating conductor, which affects the heat dissipation of the device. These changes are mapped to a visual image of the device in order to physically locate the floating conductors.

Original text: http://en.wikipedia.org/wiki/Thermal_laser_stimulation#Optical_beam_induced_resistance_change