One thermocouple is coated with a thin insulator and the other with a thick insulator.
Heat passing through the insulators produces a different temperature at each thermocouple.
The incident heat flux is directly proportional to this temperature difference. The thin film heat flux sensors being developed at NASA are fabricated as a plug-type sensor on the surface of a ceramic material. They can also be fabricated directly on the surface of a part such as a turbine blade. The sensitivity is increased by adding many thermocouple pairs. The top and bottom contact pads in figure 2 are the connections to the heat flux sensor.