Abstract: An image is scanned by a TV camera 9 and the resultant signal digitized in an analogue to digital converter 12 the output of which represents the measured intensity of each picture element of the image. The values relating to selected lines may be captured in a line store 13. Alternatively the number of picture elements of a selected intensity may be counted during the count of a scan in a level counter 14, using a look-up table 15 as a mask to select the desired level. After all levels have been similarly counted the resultant histogram is modified by mapping each intensity into a new value with a smaller number of bits in such a way as to tend to equalize the number of picture elements in each new level. The new values are loaded into the look-up table 15 and in subsequent scans the measured intensities are converted into the new values, which may be displayed on a monitor 16 or captured in the line store 13.