Lighting for scratch detection on plated plastic parts

An electroplated ABS part behaves like a chrome mirror: the flat plating returns your light exactly where it came from, and a scratch is the one feature that breaks the mirror. Pick the wrong light and the whole field saturates toward white while the defect you are paid to find vanishes into it.

This guide gives the lighting decision, the angles and distances to start from, the camera arithmetic that decides how small a scratch you can see, and a bench procedure you can finish in an afternoon. It is written for anyone setting up scratch detection on plated plastic parts — bezels, knobs, trim, switch caps — on a static station or a moving line.

What lighting finds scratches on plated plastic fastest?

Dark-field side lighting is the fastest working answer: aim the light so its rays graze the surface 15-20° above the plane, which puts the illumination 70-75° away from a vertical camera axis, and the plating mirrors that light past the lens while every scratch scatters light into it.

The law of reflection does the work. Light arriving at 15° leaves a mirror surface at 15°, travelling sideways across the station instead of up into the lens. A scratch is a tiny V-groove whose walls sit at every angle, so some wall always directs light up to the camera. The plated field reads near black and each scratch reads as a bright line.

A low-angle shadowless light delivers this from four sides at once, so scratch orientation stops mattering. The same recipe finds dents and pits: see Detecting dents and pits in metal surfaces with low-angle lighting. If parts travel past the camera, a bar-shaped low-angle light covers one direction of travel, as in Low angle bar lights for scratch detection on moving boards.

Why do coaxial and dome lights wash scratches out?

Coaxial and dome lights return the plating's specular reflection straight into the lens, so the flat field saturates and a scratch that scatters only a few percent of the incoming light disappears into it; for pure height defects on flat shiny plating, both are the wrong tool.

A coaxial light couples its LEDs through a beam splitter onto the camera axis, so flat plating acts as a mirror and sends the light straight back: the field climbs toward 255 grey. A dome surrounds the part with diffuse light from nearly every direction, which is exactly why curved parts look even — but on a flat chrome surface most of that light still arrives near the viewing axis and comes back the same way. The scratch has to fight a saturated background, and it loses. The physics, and when bright field wins instead, is laid out in Bright field vs dark field lighting: the law of reflection decides.

One station can still cover both defect families: strobe a dark-field shot and a coaxial shot in alternation, or use a combined dark-and-bright-field source, and judge each image with its own thresholds.

Where should the light sit, and at what angle?

Put the light exit 30-50 mm above the part plane, offset it so the rays leave 15-20° above the surface, and the specular reflection travels 70-75° away from the camera axis — far outside the acceptance cone of any normal lens.

How far off-axis is enough? A lens accepts light inside a cone whose half-angle follows its f-number:

Even wide open at f/2.8, a reflection 70° off-axis misses the lens by 60°. The old shop habit of setting light and camera about 90° apart is simply the flat end of a 70-90° working band. Flatter gives a blacker field but fewer photons out of each scratch, so start at 15-20° above the surface and steepen only if the background grey climbs past about 20% of full scale.

Which camera and lens numbers pair with dark-field lighting?

Work backwards from the narrowest scratch you must catch: give it at least 2 pixels, keep the pixel scale at or under half the scratch width, and everything else follows. The example below lands on a 0.045 mm pixel, a 110 mm field of view and a 12 mm lens.

Take a plated bezel 100 mm wide and a 5 MP camera with 2448 × 2048 pixels at a 3.45 µm pitch, so the sensor is 2448 × 0.00345 mm = 8.45 mm wide. These are example inputs, not a required spec.

1. Field of view: 100 mm × 1.10 margin = 110 mm.
2. Pixel scale: 110 mm ÷ 2448 px = 0.045 mm/px, so a 0.1 mm scratch spans 2.2 px.
3. Magnification: m = 8.45 ÷ 110 = 0.0768.
4. Lens at a 150 mm working distance: f = 150 × 0.0768 ÷ (1 + 0.0768) = 10.7 mm, so order 12 mm.
5. Working distance with the 12 mm lens: WD = 12 × (1 + 0.0768) ÷ 0.0768 = 168 mm.

The lens focal length calculator runs the same numbers in one step, and the calculator below links straight to matching lenses.

How do I set it up on the bench?

Eight bench steps take you from an empty plate to a locked recipe: mount the camera square, fit the low-angle light at 15-20° above the surface, strobe through a controller, then tune exposure until the field reads near black and a known scratch reads bright.

1. Mount the camera square to the part plane at the working distance from the example above — 168 mm.
2. Offset the low-angle light so its exit sits about 40 mm above the part and its rays graze at 15-20° above the surface.
3. Feed the light from a 24 V supply through a light controller and start at half output.
4. Set exposure so the flat field reads 10-20% of full scale and a known scratch peaks at 60-80%.
5. Fire the light pulse inside the exposure window in strobe mode, so room light never adds to the image.
6. Shroud the station — enclosure curtains or a simple matte-black tunnel around the field.
7. Run three good parts and three scratched parts; confirm the background sits within ±5 grey levels and scratch contrast stays above 50 grey levels.
8. Rotate a scratched part in 90° steps; a low-angle ring should hold contrast through all four orientations.

A low-angle shadowless light does this from four sides at once, so part orientation stops mattering. The LS-SDL-0067 is a 113 mm low-angle shadowless light that runs on 24 V.

Flat, square and ring shadowless bodies cover small bezels through panel-size trim:

How do I keep fingerprints and room light from faking defects?

Fingerprints and room light fool dark-field checks too, so shroud the station, handle parts with gloves, and separate thin bright lines from soft oil blobs in software before you count a defect.

Skin oil scatters grazing light exactly like a scratch wall does, so a greasy bezel produces soft bright smears that a naive blob counter calls defects. Gloves from the plating line onward remove most of it. The shroud matters because a fluorescent tube at a grazing angle is a second dark-field source, and strobing the LED keeps mains flicker from beating against the exposure.

In software, run a line-shaped filter before counting: a scratch is long and thin with high gradient across it, an oil smear is wide and soft. Distinguishing scratches from dust with lighting covers the lighting side, and Scratch defect detection with lighting and dynamic thresholding the threshold logic.

An infrared band at 850 nm or 940 nm also sees through thin oil films better than visible light does.

When is a dome or coaxial light still right for plated plastic?

A dome or a coaxial light is still the right tool when the defect you chase changes reflectance instead of height: missing plating, engraved or recessed marks, or a curved part whose overall appearance you must judge evenly.

Missing plating is a reflectance change, so bright field is exactly right: the bare plastic where chrome peeled off reads dark against the mirror. Engraved or recessed marks behave the same way, and a coaxial light delivers illumination straight down the viewing axis to make them pop — the set-up is described in Coaxial lighting for engraved characters in recessed metal parts. For that job, the LS-CXL-0227 is a 50 x 50 mm blue coaxial light on 24 V.

Curved plated parts are the dome's home ground: it wraps even light around a knob or handle so the whole surface is judged at once, which Dome lights for shiny, curved and wrinkled parts covers in depth. Watch for the dark spot the camera hole casts at the centre of a dome image; the fix is in Dark spot from the camera hole in dome lights: how to fix it.

The LS-DOM-0085 provides a 116 mm emitting area inside a 166 mm outer dome.

Which checks do I run before ordering?

Before you order anything, check four things on paper: the light exit covers the part diagonal, the rays clear the fixture at your chosen angle, the pixel scale halves the narrowest scratch, and the station can be shrouded from room light.

Light

• Exit size at least the part diagonal, so grazing light reaches the far corner and not just the centre.
• 24 V feed from a controller output, so you can strobe and set output level.
• Colour matched to the camera's strongest response band.

Geometry

• Rays 70-90° away from the camera axis, starting at 15-20° above the surface.
• Fixture lips below the light path and matte black.

Camera and lens

• Pixel scale at most half the narrowest scratch width — 0.045 mm/px in the example.
• Field of view at least 1.1 × the part width.

Environment

• Shroud or tunnel so room light cannot graze in.
• Vertical room for the light: 30-50 mm above the part in the example geometry.

When the list is filled in, the shadowless lights category holds the low-angle bodies, the lights finder filters them by size and colour, and the light configurator walks the rest of the chain. For a mixed defect set, budget a dark-field and a coaxial source from day one — or bring the part callouts to the contact page and we will spec the pair with you. More lighting recipes are collected on the lighting topic page.