V/K-band cross-correlation functions of NGC 4151. Based on the observed V- and K-band light curves, 50 interpolated light curves for both V- and K-band have been simulated and CCFs calculated accordingly, shown as gray dotted lines. These reflect the observations and the uncertainty in the determination of the time lag owing to the gaps and non-uniform sampling of the data. The CCF based on the light-curve model is overplotted as a red dashed line. All CCFs have been normalized to their peak value.