1. Method of determining the nature of an image under examination having numerous picture elements comprising the steps of: (a) producing, over a wavelength scanning time interval, a first group of sequentially generated time varying spectral analysis input signals representing intensities of at least several wavelength components of said numerous picture elements of said image under examination during frequency spectrum scanning thereof; (b) producing a second group of time varying spectral analysis reference signals representing the intensities of wavelength components of said numerous picture elements of a reference image being compared to said input image; and (c) correlating said first and second groups of time varying spectral analysis signals.