Infrared (IR) lock-in thermography (LIT) has been successfully used for defect detection in solar cells. Depending on the experimental setup, defects such as shunts, series resistances, pre-breakdown regions, etc., can be qualitatively visualized or quantitatively measured. IR-LIT results improve the spatial resolution (SR) in defect visualization and the signal-tonoise ratio (SNR) between defects and sound regions compared to classic DC thermography. The same results have been accomplished on solar modules, considered as an ensemble of solar cells electrically connected. The main problem that arises in IR-LIT technique is encapsulated modules/cells measurement, because most glasses used for encapsulation are opaque in the IR region. In this research, IR opacity of encapsulating glass is treated from a thermal point of view. Solar cells have been considered as a heat source with a heating frequency actively generated by a modulated forward polarization. The encapsulation behaves as a thermal low pass filter, whose cutoff frequency has been calculated. In the experiments, a modulated forward electrical polarization has been applied to a solar module in the dark. The tested sample has been a standard solar module of 36 cells connected in series. Thermal images have been acquired from the side of the external glass surface by an IR camera. A large improvement of SR and SNR has been demonstrated for shunt detection when the modulation frequency is below the cutoff.