Also referred to as HSR. The thermal path from the hot side of the TE cooler to the ambient, including mounting interfaces, fans, etc., is measure of the effectiveness of a heat sink. In other words, how well does the heat sink remove the heat from the TE cooler? Its units are °C/W and is used to determine the number of degrees the hot side will rise in temperature for a given amount of heat that is dumped into it. For example, a heat sink resistance of 0.1°C/W will result in a hot side temperature rise of 1°C when 10 Watts is applied. The effectiveness of the heat sink greatly affects the performance of the TE cooler. Therefore, the better the heat sink (better is a lower °C/W) results in less input power to the TE cooler or a colder cold side temperature.