Spontaneous near infrared (NIR) emission is used for measuring transistor channel temperature in ICs. Emission modeling and data analysis are leveraged to estimate the peak temperature and thermal time constant inside the channel of CMOS transistors. The non-invasive nature of the technique allows one to reliably monitor the temperature of any device on-chip with high spatial and temporal resolution, without the need for circuit modifications or dedicated on-chip sensors. This method can be used to model heat dissipation during early process development, to localize hot spots, to calibrate on-chip sensors, etc. In this paper, temperature is estimated by fitting empirical emission data to an emission model that can be solved for device channel temperature.