Visual Inspection

Visual Inspection (manufacturing-quality/ndt/visual-inspection)

Use when the task is executing visual inspection (VT) on a part: computing the borescope aperture ratio that sets image brightness, sizing the magnification needed to resolve a target surface indication size at a given working distance, converting lamp intensity to illuminance under the inverse-square law and checking the lighting requirements in lux or foot-candles, sizing the field of view and the number of scan positions for full surface coverage, and comparing measured surface indications with the acceptance criteria for a pass-fail disposition.

Domain quick reference

• Aperture ratio: A = D / d, with D the objective aperture diameter and d the working distance in meters. The ratio sets light-gathering power: a 6 mm objective at a 100 mm working distance gives A = 0.06, and holding the same optic at 50 mm doubles the ratio, which is why a borescope is advanced toward the surface before judging fine indications.
• Eye resolution: s = d * tan(theta) with theta the acuity limit of 1 arcminute (2.909e-4 rad). At a 300 mm viewing distance the unaided eye resolves about 8.727e-5 m (87.3 micrometers); at the 250 mm near point about 7.272e-5 m (72.7 micrometers).
• Magnified resolution: s = d * tan(theta) / M. A 10x magnifier at the 250 mm near point resolves about 7.272e-6 m (7.3 micrometers); a 5x system at 300 mm about 1.745e-5 m (17.5 micrometers).
• Required magnification: M = d * tan(theta) / s. Resolving a 25 micrometer indication at 250 mm needs about 2.91x; a 50 micrometer indication at 300 mm about 1.75x. If the tool magnification falls short, the indication class is not reliably seen.
• Inverse-square law: E = I / d^2, with I the lamp intensity in candela and d the distance in meters. A 250 cd lamp at 0.5 m gives 1000 lux, the level commonly required for close inspection of fine detail; the same lamp at 1 m gives 250 lux, too dim.
• Lamp intensity: I = E * d^2. Reaching 1000 lux at 0.5 m needs a 250 cd lamp; reaching it at 1 m needs 1000 cd, four times the intensity because the light spreads over four times the area.
• Lamp distance: d = sqrt(I / E). A 250 cd lamp must sit within 0.5 m to hold 1000 lux on the surface.
• Units: 1 foot-candle = 10.76391 lux, so 100 fc is about 1076.4 lux and 1000 lux is about 92.9 fc; a 100 fc requirement is slightly stricter than a 1000 lux requirement on the same surface.
• Field of view: FOV = 2 * d * tan(full_angle / 2). A 40 degree borescope at a 50 mm working distance covers about 3.64e-2 m (36.4 mm); at 100 mm with a 60 degree field about 1.155e-1 m (115.5 mm).
• Scan positions: n = ceil(part_area / (field_area * (1 - overlap))). A 0.01 m2 surface covered with a 1.6e-3 m2 field at 20 percent overlap needs ceil(0.01 / 0.00128) = 8 positions.
• Acceptance: an indication is acceptable when its measured length is at or below the acceptance limit in the engineering specification. A 1.2 mm indication against a 1.0 mm limit is rejected; a 0.8 mm indication is accepted. The limit comes from the specification, not from the math.

Workflow

1. Establish the viewing geometry: working distance d, objective aperture D for direct or borescope viewing, and the smallest indication class the procedure must resolve.
2. Compute the aperture ratio with aperture_ratio (ratio = D / d) and confirm the optic gathers enough light for the working distance.
3. Size the magnification with magnification_for_resolution for the target indication size at the viewing distance, then confirm with resolvable_size that the available magnification actually resolves the class; if not, the optic or the method must change.
4. Verify the lighting: convert the procedure requirement to lux with foot_candles_to_lux or lux_to_foot_candles, back out the needed lamp intensity with intensity_for_illuminance, and confirm the lamp is within distance_for_illuminance so the surface holds at least 1000 lux (or the procedure minimum) at the inspection distance.
5. Plan the coverage with field_of_view and scan_positions, applying the procedure overlap fraction so no seam between fields hides an indication.
6. Measure each surface indication, compare it with the acceptance criteria with acceptance_verdict, record the results, and disposition the part under special-process control by NAS 410 qualified personnel.

Pitfalls

• Confusing VT with liquid penetrant inspection: VT sees only what is open and visible on the surface; tight surface-breaking cracks that are invisible to the eye are the domain of liquid penetrant inspection, whose capillary action pulls penetrant into the crack. VT is the first pass, PT is the follow-on for tight cracks.
• Confusing VT with method selection: ndt-method-selection decides which method fits the defect class and material; VT is one execution method and cannot see internal or subsurface conditions at all.
• Forgetting the acuity limit: 10x magnification does not resolve a 1 micrometer crack, it resolves about 7.3 micrometers at the near point; indications finer than the magnified resolution are rated as not resolvable, not as acceptable.
• Ignoring the inverse-square falloff: doubling the lamp distance quarters the illuminance, so a lamp that meets 1000 lux at 0.5 m gives only 250 lux at 1 m and the inspection is out of procedure.
• Mixing lux and foot-candles: 100 fc is about 1076 lux, not 100 lux; converting the procedure requirement before comparing lamps avoids accepting a dim setup.
• Treating the indication as the flaw: VT indication length is a surface projection; an indication at or near the acceptance limit must be dispositioned against the criteria and, where the procedure requires, confirmed with a higher-sensitivity method before a conditional acceptance.
• Skipping coverage overlap: scanning field to field without overlap leaves seams where indications hide; the overlap fraction closes the gaps at the cost of extra scan positions.
• Routing the disposition to the QMS leaves: VT is an inspection method, not a quality-management process; acceptance decisions still flow through the nonconformance-control and MRB process of the as9100 quality leaves when product does not conform.

Behavior contract (gate 3)

The inspection math is exercised by the gate 3 contract test: scripts/test_visual_inspection.py against scripts/visual_inspection_logic.py (stdlib unittest, offline). Run: python3 scripts/test_visual_inspection.py

Compliance

• Standards referenced, not reproduced: AS9100 clause 8.5.1.3 frames NDT as a special process requiring controlled procedures and qualified personnel, and NAS 410 sets the qualification and certification requirements for the NDT personnel who execute visual inspection; the aperture ratio, resolution, illuminance, coverage, and acceptance calculations above are common VT methodology, summary-only per standards-map.yaml.
• compliance: STANDARDS-REF, gated: false.