Off-state leakage currents account for roughly half of the total current is today’s ICs, and with each new generation of technology, the problem is getting worse. Failure analysts, however, see things differently. Light emission associated with leakage current is a rich source of information about the operation of ICs. In this article, the authors explain how they use this light to monitor logic states, measure temperatures, analyze cross-talk and power distribution noise, and diagnose broken scan chains. Light emission from off-state leakage current (LEOSLC) is shown to be especially useful for diagnosing faults that reside in scan clock trees, which are otherwise very difficult to detect.