From the point of view of the optical information processing, we consider the spectrogram in which the spectral lines are immersed in the bright continuous background as a low contrast optical picture. By means of the coherent optical processing system and a special high-pass filter, the Fourier spectrum of the continuous background can be reduced and then the final result may appear as a spactrogram without continuous background. This may be considered as a new method for enhancement of the ratio S/N of a spectrogram.