To look at an object with one’s own eyes is the most natural form of examination. Surface examination of cracks and other irregularities under visible light is the cheapest, simplest and quickest NDT method. However, all defects cannot be seen as such by unaided eyes and therefore, techniques to enhance the contrast of the discontinuity to render it visible are applied.

The main point of visual testing is that the inspector must be able to see the surface being tested. That may seem simple, but it is not always so. Sometimes, the area to be tested is not adequately lluminated. A minimum of 160 lx (15 ftc) is necessary for general visual testing with 500 lx (46 ftc) being recommended for critical testing.

The Illuminating Engineering Society suggests 1100 to 3200 lx (102 to 297 ftc) for critical work (ASNT, 1996). If the specimen is not well illuminated or is shadowed, a light meter should be used to determine whether additional light is needed. The specimen must also be clean.

The instrument being used for testing is the human eye. It needs to be up to the job being done meaning it needs to be tested and corrected if necessary. Too often, we believe we can see better than we really can.