Closed-loop feedback control systems have shown to be able to stabilize optimum process conditions for reactively sputtered aluminum doped zinc oxide (ZnO:Al) films on glass, but the problem of long term drift has not been addressed so far. In the present work we describe an online control method which is able to detect process drifts and offers the possibility to adjust operation point settings in long-term operation. The control system is based on the evaluation of spectroscopic photometry measurements in the visible and near-infrared wavelength regime. The measured spectra are evaluated with respect to their band-gap and free carrier absorption. We found that the band gap and the plasma frequency are directly correlated with the oxygen partial pressure during deposition. Comparing the plasma frequency with Hall measurements, it was shown that the carrier concentration in the films can be monitored. This enables a control of free carrier absorption during a production process. (C) 2009 Elsevier B.V. All rights reserved.