Visual inspection is one of the most common and most powerful means of non-destructive testing.
To be most effective visual inspection merits special attention because it requires training (knowledge of product and process, anticipated service conditions, acceptance criteria, record keeping ) and it has its own range of equipment and instrumentation.
It is also a fact that all defects found by other NDT methods ultimately must be substantiated by visual inspection. Visual Inspection can be classified as Close Visual Inspection(CVI) and General Visual Inspection(GVI). The most common NDT methods MPI and DPI are indeed simply scientific ways of enhancing the indication to make it more visible.