20 the image memory contain a discrete addressable memory cell for each point in the image the contents of the image memory 20 are scanned by addressing the memory on an image point by image point basis and the successive values at image points 22 in the image matrix are fed to a first input of a digital dividing circuit 30 at the same time the values of all image points in a local area 24 of the matrix which surrounds the addressed image point 22 are fed to an average value computing circuit 40 the average value of the image points in the area 24 is a measure of the low spatial frequency background components f x y at the addressed image point 22 the high order bits from the output of the average value circuit 40 are fed to a second input of divider circuit 30 the average value circuit 40 and divider circuit 30 function as a two dimensional homomorphic low pass filter which extracts a slowly varying background component from the image signal alternately other known types of low pass filters may be utilized in some applications it is also possible to extract the background component of the image signal using a one dimensional filter for example the background component can be extracted from echo ultrasound or radar signals with a one dimensional low pass filter which operates on the logarithm of the a line signals in the time domain alternately if the image signal is subjected to logarithmic compression before it is stored in the image memory a subtraction circuit can be utilized in place of the divider circuit.