The cover image of Felix Dreckschmidt and Hans-Joachim Möller, represented also in their article on pp. 1356ff., shows etched grain boundaries in multicrystalline silicon. This colourless microscopic background is overlaid with luminescence intensity from defects in colour. Red means high and blue low luminescence intensities. The defect luminescence with energies smaller than the silicon band gap is located at grain boundaries. In the white rectangle we found GB4 luminescence located at twin boundaries (see also abstract image). GB4 has a peak position at 1350 nm (0.92 eV). Outside the white rectangle GB1 luminescence is found with a peak position around 1600 nm (0.78 eV). The implemented figure shows the measured temperature dependence of the GB4 luminescence intensity. The modelled dependence with the expected activation energy of around 0.2 eV is included in red in the figure.