A mechanism for diagnosing broken scan chains based on leakage light emission is provided. An image capture mechanism detects light emission from leakage current in complementary metal oxide semiconductor (CMOS) devices. The diagnosis mechanism identifies devices with unexpected light emission. An unexpected amount of light emission may indicate that a transistor is turned off when it should be turned on or vice versa. All possible inputs may be tested to determine whether a problem exists with transistors in latches or with transistors in clock buffers. Broken points in the scan chain may then be determined based on the locations of unexpected light emission.