Direct visual examination is the only NDT method whose entire capability rests on three numbers a technician can measure in seconds, and which are the first thing a good auditor asks for. The eye must be within 600 mm of the surface, the line of sight must be at least 30° to the surface, and the surface must be lit to the level the governing standard demands. Miss any one and the examination is not a compliant VT, however experienced the examiner.

The reason for each is optical. At more than 600 mm the angular size of a fine crack or undercut falls below what the eye reliably resolves; at less than 30° the surface is seen so obliquely that foreshortening hides depth and surface-breaking features merge into the texture. Illuminance sets contrast sensitivity: the eye’s ability to separate a dark line from a grey background falls off sharply as light levels drop, which is why the required levels are far above ordinary workshop lighting.

The numbers differ by standard, and using the wrong one is a common finding. ASME BPVC Section V, Article 9 requires a minimum of 100 fc (1076 lx) at the examination surface. ISO 17637 sets a minimum of 350 lx and recommends 500 lx. AWS D1.1 requires 50 fc (500 lx). Illuminance is measured at the surface with a calibrated light meter, and light meters need calibration records like any other measuring instrument.

Meeting the conditions is necessary but not sufficient — resolution has to be demonstrated as well. The benchmark most procedures adopt is the one ASME Section XI sets for a VT-1 examination, IWA-2211: the technique must resolve a 0.8 mm (1/32 in) black line on an 18% neutral grey card, or equivalent lower-case lettering, at the maximum distance and minimum angle used. Run that demonstration at the actual conditions, not at a comfortable bench distance, and record it.

Worked example

Governing standard	asme_v
Viewing distance	400 mm
Viewing angle to the surface	45 deg
Measured illuminance at the surface	800 lx
Maximum permitted distance	600 mm
Minimum permitted angle	30 deg
Required illuminance	1076 lx
Illuminance shortfall	276 lx
Distance acceptable	1
Angle acceptable	1
Illuminance acceptable	0
Conditions compliant	0

ASME V Article 9 requires 100 fc = 1076 lx. Distance 400 mm is inside the 600 mm limit and 45° exceeds the 30° minimum, but 800 lx is 276 lx short of the requirement, so the examination conditions are not compliant until the lighting is improved.