Lock-in thermography (LIT) can be used both for single solar cells and for modules consisting of a series connection of single cells. Until now LIT has been used nearly exclusively for single cells and only rarely for modules, which have been investigated only by standard (steady-state) thermography. There are some differences between LIT applied to single solar cells and to modules. The purpose of this contribution is to point out these differences. One issue is the fact that, in a module, the

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... cell biases are floating. Another issue is that, in readily processed modules, the cells are covered by glass, EVA, or some laminate. Even before lamination the active layer is often covered by some metal layer which shows a low infrared emissivity. It is shown that in this case a good emissivity may be obtained by sucking-on a thin black plastic foil. Glass-laminate modules may be imaged best "from behind" through the laminate layer. Even glass-glass modules may be imaged "through the glass" if the lock-in frequency exceeds 0.5 Hz. However, in this case the temperature drift during the measurement is especially disturbing and must be compensated properly. It is found that the advantages of LIT compared to steady-state thermography (improved sensitivity and spatial resolution) are valid also for investigating solar modules.