To investigate differential PopTea (dPopTea) measurements, the three configurations shown in Fig. 2 were studied. The TBC/substrate/subsystem is axisymmetric with respect to the centerline of the laser, as shown in the illustration. Dimensions and materials associated with these P50515 configurations are provided in Table 1. Configurations (i) and (ii) show measurement conditions where heat transfer beyond the length scale of the substrate Lsub is characterized by the ill-defined subsystem. The subsystem is composed of a composite layer of finite thickness L 1 overlying a semi-infinite material (material 2). The composite layer contains a void space with surrounding material (material 1). Configuration (i) of Fig. 2 has a 323 μm APS coating whose thermal properties are to be determined. Configuration (ii) illustrates conditions where heat transfer into the substrate/subsystem is characterized in the absence of the TBC. Configurations (i) and (ii) are created by using thermal paste to attached identical substrates to the subsystem and differ only by the existence of the TBC for configuration (i). The model used to fit experimental data for configuration (ii) is the same as configuration (i) in the limit of a vanishingly thin TBC with coating properties assigned to be the same as the substrate. Finally, configuration (iii) defines the standard measurement conditions for PopTea, where heat transfer into the substrate/subsystem is “well defined.” This means out of Africa hypothesis the substrate and subsystem are a continuous semi-infinite body with negligible thermal contact resistance. In the current investigation, the thin substrate material allows thermal penetration depths into material 2. This is not ideal for standard PopTea because of the additional interface. However, in the present investigation, care is taken to provide good thermal contact (with silicon thermal paste) between the substrate and material 2, such that heat transfer into the substrate/subsystem satisfies the “well-defined” semi-infinite requirement.