(d) input signal producing means positioned close to said first lens and between said first lens and said second lens for producing said first input signal behind said first Fourier transform lens to introduce a wavefront distortion quadratic phase term and (c) a second Fourier transform single lens in optical alignment with said filter for taking the inverse Fourier transform of the product of the Fourier transform of said first signal and said information of said second signal, and for forming said inverse Fourier transform at a second position along said optical axis, said inverse Fourier transform being substantially equivalent to the mathematical correlation function between said first signal and said second signal (b) a filter located at said first position providing information obtained from a second signal which is to be correlated with said first signal (a) a first Fourier transform single lens for taking the Fourier transform of a first signal representing an input image and forming said Fourier transform at a first position along an optical axis

2f system lens fourier