Scratch Defect Visual Inspection
Scratch defect visual inspection: for linear scratches on surfaces such as metal, plastic, glass and coatings, low-angle light or stripe light highlights raised and recessed shadows, distinguishing machining marks from real scratches.
defect Overview
A linear relief feature that low-angle light renders most clearly; the difficulty is distinguishing machining marks from real scratches
Scratches are linear raised or recessed damage on the surface; they appear through shadows and bright edges under low-angle light or striped light, and are often invisible under flat diffuse light. Machine vision uses directional illumination to highlight their linear shadow and then judges by morphology and contrast; the biggest risk is misjudging the material's own brushed pattern or machining marks as scratches, and oblique scratches may be masked by single-direction illumination, usually requiring multi-angle or striped light coverage.
On an image, a scratch is a linear feature with a direction and light-and-dark edges: when low-angle light comes from one side, the "lit edge" of the scratch brightens and the "shadowed edge" darkens, while under flat light it may be completely invisible. Scratch inspection therefore depends largely on illumination — with the right light, even a faint scratch forms a clear image; with the wrong light, even a deep scratch may be missed.
Scratches are most easily confused with a material's normal texture. The brushed lines on metal parts, the ejector pin marks on plastic parts, and the flow-leveling marks on coated parts are linear appearances inherent to the design or process and are not defects. Judgement requires first knowing the material's "normal grain direction", then flagging as suspect any linear feature that deviates from that direction or appears in an area where no grain should exist.
Directional illumination is a double-edged sword: when light comes from one direction, scratches perpendicular to that direction are most visible, but scratches parallel to the light path are instead darkened. To cover scratches in any orientation, engineering practice commonly uses multi-directional low-angle light in turn, or striped light / photometric stereo to obtain multi-directional information in a single pass and then synthesize a scratch saliency map.
Occurrence Causes
Only when the cause is known can you decide which station should check for it
- Incoming material and transfer: stacking friction, scratching by hard objects on fixtures / conveyor belts
- Machining tools: linear damage left by cutting, polishing, and deburring process steps
- Assembly and packaging: screws, clips and packaging film edges scratching the surface
- Normal material texture: brushed grain, ejector pin marks and flow marks are misread as scratches (normal)
- Cleaning and handling: stiff brushes and dust particles dragging fine scratches across the surface
imaging Key Points
Whether it can be detected depends first on whether it can be captured
Low-Angle Light
Light grazing along the surface produces clear bright and dark edges where a scratch rises and falls, making scratches easiest to reveal
Fringe light / structured light
Provides multi-directional illumination information in a single pass, covering scratches in any orientation and suppressing directional blind spots
Photometric Stereo
Combine multi-directional illumination into a normal or height map to separate scratches from texture
Coaxial Light
Suppress specular reflection on highly reflective surfaces so that glare does not hide scratch shadows
judgement Method
Scratch judgement is based on "the contrast and continuity of the linear feature and whether its direction is consistent with the normal grain". First exclude the material's inherent grain direction, then measure the length, width and maximum contrast of the linear connected regions in the saliency map.
Thresholds need to be biased: escaping one deep scratch usually costs far more than passing one extremely shallow hairline mark, so the reject line for shallow scratches should be defined separately for appearance surfaces and functional surfaces by the quality department, and saved with the recipe. Inspection blind spots caused by the direction of parallel light paths must be eliminated in the illumination plan.
| Judgement Dimension | Description |
|---|---|
| Contrast | The maximum difference between the light and dark edges of a scratch, to separate real scratches from faint grain |
| Length / Width | Short, shallow hairline marks and long scratches are handled by grade |
| Direction | Whether it follows the same direction as the normal material grain; only anomalous directions are suspicious |
| Position | Visible surfaces and hidden assembly surfaces use different rejection criteria |
| Illumination Coverage | Multi-directional or stripe light eliminates blind spots along the direction of parallel light paths |
Applicable algorithm
Directional Saliency Map Synthesis
Fuses multi-directional / stripe-light images to obtain a scratch map independent of direction
- Eliminates blind zones from single-direction lighting
- Photometric stereo captures them in one pass
Linear feature extraction
Use edge / Hessian-type operators to extract linear structures and suppress texture
- Distinguish scratches from brushed lines
- Filter by length and width
Deep learning segmentation
Apply pixel-level segmentation to shallow scratches of varying morphology
- Requires OK/NG sample training
- Outputs position and dimensions for review
Common Materials
Common Industry
Common Question
Why Are Some Scratches Invisible Under Ordinary Lighting?
How are brushed texture and true scratches distinguished?
Will single-direction illumination miss certain scratches?
How Do You Image Scratches on Highly Reflective Surfaces?
How Shallow Can a Scratch Be and Still Fail?
How small a scratch can scratch inspection detect?
Does Switching Models of Reflective Material Require Retuning?
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The key to scratch inspection is "using directional illumination to form a clear image and distinguishing machining marks from real scratches".
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Whether a defect can be detected depends on whether imaging captures the defect features. Provide OK and NG samples and we will run actual imaging and judgement tests on the equipment.