Many photovoltaic cells and OLEDs are covered by cover glass or foil to protect the device against oxygen and moisture. The cover glassor foil is semitransparent for infrared radiation and conducts the heat and blurs lock-in thermograms. In lock-in thermograms of OLEDs with cover glass short circuits raise typically 10 mK in temperature,where as literature and observations suggest that polymers decompose into gas at way higher temperature behind the glass. A good interpretation of lock-in thermograms may be done by the understanding of the heat conduction and radiation through cover glass. Modelling of heat conduction and radiation around a short circuit in cover glass will be presented as well as first validation experiments where a short circuit has been imitated by a laser.