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In order to extract maximum information from electroretingraphic waveforms obtained under clinical conditions, it is necessary to have reliable quantitative methods to characterize both the amplitude and shape of these waveforms. When sinusoidally modulated light is used to stimulate the retina, the resulting ERG potentials are, in general, not sinusoidal due to the nonlinearities in the system. However, the responses are very reproducible and can easily be characterized by a few parameters based on a Fourier analysis. The more conventional flash ERG, although usually of higher amplitude, is much less reproducible in shape and needs many more parameters to characterize completely. Amplitude and phase characteristics can be understood on the basis of a simple model for the scotopic B-wave system and additive interaction by the photopic system. Changes in amplitude and phase characteristics with various experimental conditions could be predicted and were confirmed by subsequent experiments. The ultimate goal of this work is to improve the quantitative basis for clinical electroretinography and to provide the clinician with additional data which can be useful in the diagnosis of retinal diseases.