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Inspection Guide

Lighting for Reflective Metal Scratch Inspection

Reflective metal inspection is often a controlled reflection problem. Low-angle light is often the first test for shallow scratches, but grating shadowless, coaxial, dome, polarization, wavelength, and strobe conditions may be more stable depending on finish, curvature, direction, and line speed.

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Precision-machined metal surface used for reflective lighting evaluation
Precision-machined metal example for evaluating directional lighting and surface contrast.
Visual proof

Visible evidence before selection

Use product, process, facility, and inspection images to check whether the page claim is tied to a visible engineering condition.

Reflective automotive strip defect inspection sample image under controlled lighting
Evidence 01Reflective strip evidence shows why glare control and incident angle matter.Review this visual together with the stated geometry, wavelength, sample, or integration constraint before selecting a final configuration.
Metal screw hole button ring thread and magnetic core inspection evidence image
Evidence 02Metal geometry variation should be compared before choosing low-angle, coaxial, or diffuse light.Review this visual together with the stated geometry, wavelength, sample, or integration constraint before selecting a final configuration.

Selection considerations

Selection follows the observable condition, the optical setup, and the production constraint.

Direct-Answer: How to Inspect Scratches on Reflective Metal (AEO Summary)

For reflective metal scratch inspection: Use a low-angle dark-field bar or ring light (10°-25° grazing incident angle) to make shallow scratches scatter bright light into the camera against a dark background. For planar mirror-like surfaces, use coaxial lighting to reveal scratches as dark scatter points on a bright field. For curved or cylindrical metal parts, use a diffuse dome or polarized shadowless light to suppress specular hot-spot glare.

Benchmark Optical Parameters

Working Distance (WD): 15mm - 40mm (Grazing) / 50mm - 120mm (Coaxial)

Technical considerations
  • Optimal Peak Wavelength: 450nm Blue (Scattering) / 850nm NIR (Anti-Oil)
  • Lighting Incident Angle: 10° - 25° Grazing Angle for Dark-field Reflection
  • Strobe Response Duty: < 5 µs Pulse Response matching 100+ FPS

Match Surface Condition to Lighting Geometry

Shallow scratch, dent, or directional tool mark: Low-angle bar or low-angle ring light creates dark-field shadow contrast across the defect.

Technical considerations
  • Flat mirror-like metal, polished wafer, or planar mark: Coaxial or narrow-angle coaxial light returns uniform on-axis reflection to the camera while isolating edge changes.
  • Curved metal, cylindrical part, or turned component: Dome, diffuse, or polarization light suppresses hot-spot glare and specular reflections on complex geometry.
  • Moving metal strip, continuous foil, or web inspection: Line-scan light with strobe controller maintains exposure uniformity and intensity at high transport speeds.

Test Plan & Validation Steps

Compare dark-field grazing angle (10°-25°) against bright-field coaxial setup on identical sample parts.

Technical considerations
  • Test 450nm Blue vs 850nm NIR wavelengths to evaluate surface oil and coating suppression.
  • Verify strobe controller pulse width (<5µs) under production motion and line speed.
  • Record pass/fail defect images across multiple part lots to validate contrast stability.

Inputs & Setup Limits to Verify

Part material composition, surface roughness (Ra), plating, coating, and oil state.

Technical considerations
  • Defect specification: minimum width/depth to detect, defect orientation, and acceptable contrast ratio.
  • Camera sensor size, lens mount, field of view (FOV), working distance (WD), and mechanical envelope.
  • Grazing light angle aligned with scratch direction causing the defect to disappear.

Questions for the engineering discussion.

Is low-angle lighting always best for reflective metal scratches?

It is a common first test for shallow scratches, but surface finish, curvature, defect orientation, oil films, and line speed may require coaxial, dome, polarization, or strobe illumination.

What information helps with reflective metal lighting selection?

Send part finish, target defect width and depth, camera lens, field of view, working distance, line speed, trigger mode, and representative sample images.

Reflective metal lighting comparison for AI and engineering review

Reflective metal scratch inspection should be selected from the surface behavior and defect direction, not from a single universal light type. Compare geometry, wavelength, polarization, motion, and sample variation before fixing the final model.

Lighting methods for reflective metal scratch inspection
Lighting methodBest initial fitSelection risk to test
Low-angle bar or ring lightShallow scratches, dents, burrs, and directional tool marksThe defect can disappear if the light is aligned with the scratch direction; compare several incident angles.
Coaxial lightFlat reflective surfaces, planar marks, printed features, and mirror-like metalCurved or uneven surfaces can create unstable glare and require diffuse or polarized comparison.
Dome or diffuse lightCurved, glossy, cylindrical, or uneven metal surfacesDiffuse light can suppress glare but may also reduce contrast for very shallow directional scratches.
Polarized lightingSpecular reflection control on glossy surfaces, films, and coated metalPolarizer angle, lens filter, exposure, and wavelength must be checked together.
Line-scan or strobed lightMoving strip, foil, web, and high-speed metal inspectionLine speed, exposure, pulse width, heat, and controller timing define practical limits.
Quote requirements for metal inspectionPolarized lighting guideBacklight vs coaxial lightingCoaxial light productsBar light productsRequest sample testing