Transfer characteristics of the human visual system at well above contrast threshold conditions when viewing luminance distributions varying sinusoidally in one direction in space have been examined by psychophysical means. Information over a wide spatial frequency range was achieved by restricting the measurements to extreme values of the response as well as of the sine-wave stimulus variation. The ratio between the peak-to-peak value of the apprehended brightness variation and the average luminance of the stimulus distribution has been found to be approximately linearly related to the stimulus modulation when viewing a sinusoidally varying luminance distribution of fixed spatial frequency in the mesopic region. Image quality parameters and the non-existence of a modulation transfer function of the visual system are discussed.