Different dust temperatures for NGC 4151 at a distance of 33 ld from the black hole assuming mean temperatures between 1000 and 1500 K during the third epoch of the NGC 4151 dust reverberation campaign from Koshida et al. (2009). The temperatures were calculated using Eq. (8) with the assumption of a constant dust grain size. The color coding for the assumed mean temperatures during the third reverberation period is shown in the upper left corner of the plot. This shows that it is very hard to reach temperatures below 1000 K at a distance of 33 ld from the black hole, which are needed for dust production above the accretion disk.