In combustion power plants, the flame stability is affected by varying fuel properties especially in low load conditions. The destabilization of the flame can be circumvented by adapting fuel, air and swirl settings of the burner system. For this adaptation a quantitative real-time measurement of the flame stability, e.g. through a camera system, is necessary. Existing approaches for the flame stability measurement already exist for continuous combustion systems and are based on flame flicker analysis, which is rather empirical and give no defined value range and interpretation. Moreover no flame stability measures exist for forced oscillating combustion systems with pulsed fuel flow rate, aiming at reducing the NOx-formation. In this paper, we present an image processing based measure for the quantitative online flame stability monitoring applicable in non-oscillating as well as for oscillating combustion systems. The new flame stability measure provides a geometric interpretation and can thus be directly applied to different burner and camera configurations. We demonstrate the flame stability measure at a 1 MW pilot-scale power plant with co-combustion of torrified coal with hard coal using cameras sensitive in the visible and near-infrared spectral range.