Visual inspection (VT) relies upon the detection of surface imperfections using the eye. Normally applied without the use of any additional equipment, VT can be improved by using aids such as a magnifying glass to improve its effectiveness and scope.
VT is considered to be the primary NDT method. Since it relies on an evaluation made using the eye, VT is generally considered to be the primary and oldest method of NDT. With its relative simplicity and because it does not require sophisticated apparatus, it is a very inexpensive method thus provides an advantage over other NDT methods. A further advantage of VT is that it is an ongoing inspection that can be applied at various stages of construction.
The primary limitation of VT is it is only capable of evaluating discontinuities, which can be seen on the surface of the material or part. Sometimes there may be some visual indication of a subsurface imperfection that may need an additional NDT method to provide verification of the subsurface discontinuity.
VT is most effective when it is performed at all stages of any new fabrication, and is the main method used during the inspection of pressure equipment. If applied say after welding has been completed, it is possible that subsurface flaws may not be detected.