Inspection of surface flaws by comparator microscopy

The increasing attention paid in recent years to control of surface quality has exposed the lack of objectivity of existing standards relating to flaws such as digs and scratches. The present requirements of the customer and supplier of optical components for improved standards are discussed, and recent attempts to satisfy these requirements are reviewed. It is concluded that a technique, described as comparator microscopy, in which a width of line or slit is identified which removes from a transmitted or reflected beam the same amount of light as the flaw under examination, has much in its favor in terms of user needs. The method, which is under consideration as the basis of an ISO standard, is described, and results are presented indicating how this concept of line-equivalent width can be used to compare different national standard flaws. Extending this technique to measure flaws and polish on-machine is suggested as a future possibility.