Abstract: An optical test head comprises one or more detectors for providing output signals indicative of the condition of a workpiece surface. Data from these detectors are stored in one or more memories only when the data from the detectors satisfy one or more conditions (e.g. the data exceed than a particular threshold). The data are then passed from the one or more memories to an electrical circuit for processing. In addition, location information is stored in one or more memories and passed on to the electrical circuit when the data from the detectors satisfy the one or more conditions.