Objects which vary strongly as a function of x but only slowly in y occupy a cigar-shaped area in the spatial-frequency domain. Such an object spectrum is badly matched to the frequency-transfer domain of a lens, which is usually circular. By means of spatial modulation, the object spectrum can be adapted to the transfer domain of the lens. In this way, the one-dimensional resolution limit or bandwidth of the lens can be overcome, as shown by experiment and theory.
M. A. GRIMM and A. W. LOHMANN, "Superresolution Image for One-Dimensional Objects," J. Opt. Soc. Am. 56, 1151-1156 (1966)