Inspection Guide

Lighting for Reflective Metal Scratch Inspection

For scratches on reflective metal, start with low-angle dark-field lighting and test polarization, wavelength, and exposure against real samples. The goal is to make tiny surface height changes brighter or darker than the surrounding reflection without creating glare that masks the defect.

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Why low-angle light works

A shallow illumination angle makes small surface height differences interrupt the reflected light path. On polished or machined metal, this can turn a scratch into a bright line, dark line, or shadow boundary depending on camera angle and surface finish.

When it will fail

Low-angle lighting can fail when the surface curvature changes across the part, the scratch direction aligns poorly with the light, the line is too fast for the exposure, or the background texture creates more contrast than the defect.

Selection decision table

Fine shallow scratches on polished metalLow-angle dark-field bar or ring lightingGrazing light turns small height changes into bright lines or shadows.

Curved reflective metalMulti-angle bar lighting or diffuse dome plus polarizer testsCurvature changes reflection direction; one angle may miss defects.

Large moving metal stripLine scan light at controlled oblique angleUniform intensity across scan width is more important than spot brightness.

Engraving or dark marks on metalCoaxial or diffuse lighting test before dark fieldMarks may need on-axis contrast rather than scratch shadow.

Oil, stain, or contaminationWavelength and polarization comparisonMaterial response may dominate geometry.

Information required before selection

Metal type, finish, curvature, and whether the surface is brushed, polished, coated, or oily
Scratch width/depth target if known, or representative good/bad samples if not known
Field of view, working distance, camera sensor, lens, aperture, and expected resolution
Part motion, exposure time, line speed, trigger method, and whether strobe lighting is allowed
Mounting space for low-angle lights and whether glare shields, polarizers, or filters can be installed

Common failure modes to test

The illumination angle is too high, so shallow scratches do not cast enough contrast.
The surface is curved, causing glare to move across the field and hide local defects.
The camera exposure is too long for the moving part, blurring scratch contrast.
The selected wavelength increases background texture instead of isolating the target defect.
The system is tuned on one sample finish but production lots vary in brushing, oil, or coating.