1. A method of detecting anomalies in a reproduction of a pattern on a workpiece comprising the operation of irradiating that surface of the workpiece carrying the pattern, scanning the said surface with detection means sensitive to such radiation to derive a video signal representing an array of points on the said surface, the signal having at least two levels relating to respective features of the pattern, deriving a digital value of the signal for each point, comparing each such value with a predetermined threshold value to determine those points which constitute a first feature of the pattern, determining those points which define a boundary of the first feature, determining changes of direction between successive points on the boundary, and using the changes in direction so determined to identify any anomaly in the boundary which has at least one dimension which is comparable to the smallest dimension of the pattern.