The rapid and automated detection of manufacturing flaws is becoming increasingly important in order to maintain competitive advantage in many production environments.
In the case of natural and ornamental materials, the presence of both surface colouration and surface topography is often such that manual inspection, along with many conventional imaging techniques, fails to isolate physical or structural defects in the presence of complex and random patterns.
In this project the concepts of photometric stereo are adapted and extended for application in manufacturing environments. A case study on the high speed inspection of ceramic tiles is used for the analysis of surfaces at production line rates of up to 30 m/min. This new technique, for the first time, demonstrates a genuine and commercially attractive potential for the practical automated quality control of complex surfaces. A commercial system, based on this research, is currently being developed.
Photometric Stereo (PS) emerged in 1978 and has developed into a robust technique for object shape recovery using a simple methodology. However, it has largely remained dormant in its range of application. The classical application of photometric stereo (CPS) has been based on the interaction of collimated, uniform lighting with stationary continuous Lambertian surfaces. Although the technique is widespread, in the main it has remained essentially a laboratory tool, which is capable of recovering discrete surface orientation information for the regeneration of surface form.
In this project we have adapted the classical static approach into one which is able to handle dynamic scenes. The key to the adaptation of PS for moving scene analysis lies in the multiplexing of directional light, acquisition, and object location. In CPS the object location and the vision system components can be and are fixed. In a dynamic environment only the vision system components can be fixed, if any. Consequentially, the classical approach can no longer be applied due to the breakdown of that particular type of multiplexing which is based primarily on being able to observe the same point at the same location from the same reference position at any time within the process. In a non-static scene the observation of the same point at the same location may not be possible if there is a temporal difference between observations. The separation of image capture by time is now likely to be subject to some degree of miss-registration and this in turn is likely to de-couple the consistency of the brightness-gradient based relationship between the corresponding pixels of the photometrically disparate images. To overcome these issues the system is able to capture multiple images instantaneously, i.e. without a temporal difference.
The results are amazing. It is as if you can separate the surface colour by peeling it off the surface geometry!
A patent application has been made in connection with this work.