Ideally, inspections would be designed on the basis of the initial information outlined above. Inspection parameters would be chosen so that the desired defects would be detected and sized with the desired level of performance. Inspection equipment would then be chosen as the next step to ensure the procedure which has been designed can be applied without problems. Information about the environment in which the inspection is carried out and any time or access restrictions is also needed for effective equipment design.

In practice, the ideal approach outlined above is not always followed. Often, inspections are designed on the basis of general code requirements or in the light of previous practice rather than from an analysis of the defects which might occur. In either event, qualification requires the parameters of the inspection procedure and equipment which can influence the outcome of the inspection to be identified. The purpose of qualification is then to show that the values chosen by those responsible for designing and specifying the inspection are appropriate to the defect, component and performance parameters.

Where parameters are specified within a tolerance, qualification must show that variation over the entire range of values of each parameter of this type will not adversely affect inspection performance and reduce it to an unacceptable level. This is also the basis on which substitution of equipment for that used in the actual qualification is allowed. It is also the basis on which qualification on one component can be extended to other, similar components without the need for full scale re-qualification.

This paper discusses the concept of essential parameters, their classification and their importance in the qualification process.