In brief
- The first method in every inspection — and mandatory before others under most codes
- ISO 17637 sets the conditions: ≥350 lx illumination, eye within 600 mm, viewing angle ≥30°
- Detects surface and geometric non-conformities — profile, undercut, cracks, misalignment
- Remote visual testing (RVT) extends it into vessels, pipework and confined spaces
- Cheapest method by far, and the one that decides what else is needed
Visual testing (VT) assesses the surface condition and geometry of a component by direct observation, or with optical aids. It is the oldest non-destructive method, the least expensive, and — under most welding codes and inspection plans — mandatory before any other method is applied.
The reason is economic and practical. There is no point radiographing a joint with a visible burn-through, and a weld cap that has not been dressed will produce indications on a radiograph that have nothing to do with the weld’s internal quality. VT removes the obvious, establishes that the joint is fit to be examined further, and determines which further methods are actually needed.
It is also the method most often done badly, because it looks like something anyone can do.
Direct and remote visual testing
Direct VT — an uninterrupted optical path between the inspector’s eye and the surface. Mirrors, magnifiers and endoscopes may be used, provided that path is maintained.
Remote visual testing (RVT) — used where direct observation is impossible. Borescopes, videoscopes, crawler cameras and drones reach the inside of vessels, heat exchanger tubes, pipework, boilers and confined spaces.
RVT is a distinct service, not a variation. It removes the need to enter a confined space — with everything that implies for permits, standby personnel, rescue provision and time — and it produces a photographic or video record that direct observation does not. Where a tank interior, a header or a long pipe run has to be assessed, RVT is often the only economically sensible option.
The resolution requirement is the point to check: an RVT system must be demonstrated capable of resolving the smallest feature that the acceptance criteria call for, usually against a reference such as a line pair chart placed at working distance.
Conditions — the part usually skipped
VT is only valid if performed under defined conditions. ISO 17637 specifies them for welds:
| Parameter | Requirement |
|---|
| Illumination at the surface | Minimum 350 lx; 500 lx recommended |
| Viewing distance | Eye within 600 mm of the surface |
| Viewing angle | Not less than 30° to the surface |
These are not guidelines. An inspection carried out at 200 lx, or at a grazing angle from two metres away, does not meet the standard regardless of how experienced the inspector is — and a report issued on that basis is not defensible.
Surface preparation is equally part of the test. Slag, spatter, scale, oil and loose coating must be removed from the weld and the adjacent parent material. A joint that has not been cleaned cannot be assessed, and cleaning it is not a preliminary to the inspection — it is the first step of it.
Personnel also require an eyesight examination, verified periodically as part of ISO 9712 certification: near vision to a defined standard, and a colour or contrast differentiation check.
What visual testing detects
Weld profile and geometry
- Undercut at the weld toe
- Excess weld metal, and excess penetration at the root
- Insufficient throat thickness, incorrect leg length on fillet welds
- Irregular or asymmetric cap profile
- Incorrect weld width, poor blending at the toes
Surface discontinuities
- Surface-breaking cracks, including crater cracks
- Surface porosity and exposed pores
- Lack of fusion where it breaks the surface
- Incomplete penetration and root concavity, where the root is accessible
- Burn-through
- Arc strikes on parent material — often overlooked and a genuine crack initiation site
- Spatter
Assembly and fit-up
- Linear and angular misalignment
- Incorrect gap, incorrect bevel preparation
- Missing or incorrectly positioned components
In-service condition
- Corrosion, pitting and erosion
- Mechanical damage, dents, gouges
- Coating breakdown
- Leakage traces and deposits
Measurement, not just observation
Experienced inspectors do not simply look. Undercut depth, cap height, leg length, throat thickness and misalignment are measured, because acceptance criteria are numerical.
The standard toolkit: weld gauges (multi-purpose and fillet), callipers, a steel rule, feeler gauges, a mirror for root access, low-power magnifiers — ISO 3058 governs their selection — and an illumination meter to demonstrate that the lighting requirement was met.
Recording the measured value rather than a pass/fail opinion is what makes a VT report useful later, particularly when a weld is re-assessed or a dispute arises.
Acceptance criteria
VT does not decide anything on its own. Findings are assessed against a quality level:
ISO 5817 defines quality levels B (stringent), C (intermediate) and D (moderate) for fusion-welded joints in steel, nickel and titanium, with numerical limits for each imperfection type. ISO 6520-1 provides the classification and reference numbers for the imperfections themselves.
Which level applies comes from the design specification or construction code — not from the inspector, and not from the laboratory. Establishing it before inspection avoids the most common dispute in weld acceptance.
Limitations
Surface only. VT sees what is visible. A weld can be visually perfect and contain lack of fusion, slag inclusions or internal cracks throughout. Internal assessment requires ultrasonic or radiographic testing.
Fine cracks are easily missed. A tight crack at the weld toe may be invisible to the eye and obvious under magnetic particle testing or penetrant testing. Where surface cracking is the concern, VT alone is not sufficient.
Access governs coverage. Root conditions cannot be assessed on a single-sided joint. Obstructions, insulation and adjacent structure create areas that cannot be seen and should be identified in the scope rather than quietly omitted.
Conditions govern validity. Poor lighting, an unfavourable angle or an unprepared surface invalidate the inspection.
It depends on the inspector. More than any other method. This is precisely why the standard defines conditions, why eyesight is verified, and why certification levels exist.
Standards
| Scope | Standard |
|---|
| Visual testing of fusion-welded joints | ISO 17637 |
| Aids to visual inspection — magnifier selection | ISO 3058 |
| Quality levels for weld imperfections | ISO 5817 |
| Classification of weld imperfections | ISO 6520-1 |
| Personnel certification | ISO 9712 |
| US practice | ASME BPVC Section V, Article 9 |
| Structural steel welding | AWS D1.1 |
Evaluation against acceptance criteria requires ISO 9712 Level 2 as a minimum; procedure approval requires Level 3.
Frequently asked questions
What illumination is required for visual testing of welds? ISO 17637 requires a minimum of 350 lx at the surface under examination, with 500 lx recommended. The eye must be within 600 mm of the surface and the viewing angle not less than 30°.
What is the difference between direct and remote visual testing? Direct VT maintains an uninterrupted optical path between the inspector’s eye and the surface. Remote visual testing uses borescopes, videoscopes or cameras to reach areas that cannot be viewed directly — tank interiors, pipework, confined spaces.
Is visual testing always required before other NDT methods? Under most welding codes and inspection plans, yes. It removes gross defects, confirms the joint is fit for further examination, and determines which methods are actually needed.
Can visual testing detect cracks? Surface-breaking cracks of sufficient size, yes. Fine, tight cracks are frequently missed and require magnetic particle or penetrant testing.
What qualification does a VT inspector need? ISO 9712 certification in the VT method, with a verified eyesight examination. Level 2 as a minimum to evaluate against acceptance criteria; Level 3 to write and approve procedures.
Which standard sets the acceptance criteria? ISO 17637 governs how the inspection is carried out; ISO 5817 provides the quality levels and numerical limits. Which quality level applies comes from the design specification or construction code.