With this thermal resistance in mind, the designer can then take a look at the relationship between applied power and the tab copper temperature, also known as the derating curve. As applied power to the circuit is gradually increased, the heat generated in the film is not completely conducted through to the ambient, thus the resistor film temperature rises above the tab temperature. Even though the thermal resistance between the resistor and the tab are quite good in thin film designs, the tab temperature must be adequately controlled at a lower temperature than the resistor in order to keep the resistor below 155°C.