Abstract: Apparatus for testing a printed wiring board comprising means for optically scanning a board to be tested in two orthogonal, X-Y axes such that for each scan in the Y direction it scans a narrow strip of the board of predetermined width, w in the X direction. The optical scanning means is arranged to provide n.m discrete signals for each scan in the Y direction, wherein n is a predetermined number of signals representative of characteristics of the board across the width w of the strip (typically 1000) and m is a number dependent upon the dimension of the board to be scanned in the Y direction. Processing means is provided for processing the signals in groups of n'.times.m' where n' is less than or equal to n and m' is less than or equal to m (typically n' and m' are 40) to provide signals representative of the characteristics of the board along said strip and for storing said signals.