The proliferation of counterfeit Integrated Circuits (ICs) poses a big challenge for the electronics industry as today's supply chain of ICs becomes more complex and expensive. For this reason, it is imperative to mitigate these risks by developing counterfeit IC detection techniques, especially at the chip level. In this paper, a novel counterfeit IC detection method is proposed. It is based on the measurement of intrinsic light emission from CMOS ICs that is a strong function of the device threshold voltage (Vt) for a given technology. In contrast to currently available counterfeit IC detection techniques, the proposed method measures IC degradation after it has been used for a long time and does not require a reference IC to provide a baseline signature. A differential technique is used to detect different levels of degradation inside the same chip in order to determine if it is counterfeit. Experimental results presented in this paper show that emission can be effectively used to detect degradation, thus making this technique a very effective way to detect counterfeit ICs.