Differential thermal measurements have been extended beyond simple fault isolation to quasi and fully dynamic test conditions. A new technique of Dynamic Digital Modulation has been developed that allows highly sensitive differential thermal measurements during active device operations. A quadrature version of the modulation also produces a thermal time constant map that allows for direct visualization of heat flow within a device structure. A wide range of potential applications in failure analysis, reliability and reverse engineering are opened up. Examples include in situ identification of resistive bonds, internal heat flow in packaged devices and die, dynamic heat loading, and localization of structural elements for reverse engineering.