An apparatus for expanding illuminated picture elements in a CRT display (91) utilises an image memory (93) wherein video bit signals are written into only addresses whose binary x coordinate has a predetermined first digit, and whose binary y coordinate has a predetermined first digit.Signals in addresses (92) corresponding to picutre elements P I,J ; P I-1,J ; P I-1, J+1 ; and P I,J+1 are read from the image memory, and a Boolean OR sum signal (94) is generated therefrom which is converted to an analogue signal (95). The picture element P I,J is illuminated in response to an analogue signal representing the Boolean OR sum signal 1.