1. A method for determining signal quality in optical transmission systems by measuring an amplitude histogram of a signal, the method comprising the steps of: calculating characteristic histogram moments for determining an effective signal-to-noise ratio, the characteristic histogram moments including average values of amplitude distributions and standard deviations of the amplitude distributions; and assigning the characteristic histogram moments, for further determining the effective signal-to-noise ratio, to an optical signal-to-noise ratio of a transmission system that is not impaired by further interference effects.