In contrast to other welding methods, laser beam welding still suffers from a lack of established quality control or even closedloop control systems in industrial production. One promising solution is the observation of the image feature called "full penetration hole" indicating the penetration depth of keyhole welding processes. This article studies its stochastic behavior in aluminum. As a consequence of the nonlinear temporal response to laser power, a robust feedback strategy was developed to control penetration depth by adapting laser power to preset detection rates of the full penetration hole.