Gradients caused by paint chips or other emissivity changes on the surface of the structure are not sensitive to the direction of the heat conduction. In other words, the sign of the differential is independent of the direction of heat flow. As a material (e.g. steel) is heated from the left, the region under the paint chip is uniformly heated because there is no flaw in the steel substrate. On the left edge of the paint chip, the differential is apparently hot on the left and apparently cold on the right. Similarly, if the heat counducts from the right, the area under the chip is again heated uniformly and the left edge has the same differential as before with the left edge of paint appearing hot and the chip appearing cool. Therefore, the gradient is independent of the direction of the heat flow. This is the cornerstone of Coating Tolerant Thermography.