Visual inspection, with or without optical aids, is the original method of NDT. Many defects are surface-breaking and can be detected by careful direct visual inspection.
Visual testing is a popular NDT method because it is easy to perform, it is a low-cost method, and it requires minimal equipment.
Visual testing can be done by looking at the test piece directly, or by using optical instruments such as magnifying glasses, mirrors, borescopes and computer-assisted viewing systems. Visual testing can be performed on components that show visible corrosion or degradation such as welds, storage tanks, piping, boilers, and pressure vessels.