Spatial frequency filter

A spatial frequency filter including: an optical system having detector means and means for projecting onto the detector means an image of an object area; and means for operating the optical system alternately in a first mode to develop a first system transfer function and a second mode to develop a second system transfer function which is matched to the first system transfer function in low spatial frequencies and divergent therefrom in high spatial frequencies, for imposing on the detector means a first representation of the image in the first mode and a second representation of the image in the second mode, thereby providing a fluctuating elemental image output from each element of the detector means which derives from a high spatial frequency object; and also including means for limiting the wavelength range of input radiation to the detector means.