- Lighting geometry decides what a defect looks like. Camera resolution comes second.
- Shiny metal sends light in one direction, so glare moves when the part tilts.
- No single light is best. Test each one on your own finish with real defects.
- A closed, matte-black light box with a fixed setting removes room light as a variable.
- Color temperature and CRI matter far more for color checks than for mono surface checks.
Why shiny metal is hard to light
A rough, matte surface scatters light in many directions. A camera sees about the same brightness from almost any angle. Polished or machined metal behaves differently. It acts partly like a mirror, so most light leaves in one direction. If that direction points at the lens, you get a bright glare band. If it points away, the surface looks dark.
This matters because the glare band moves. Tilt the part a few degrees and the band slides across the face. A glare band can hide a scratch, or it can look like a defect on a good part. Both outcomes cause wrong decisions, whether a person or software is judging the image.
Machined faces make this harder. Tool marks run in one direction and reflect light unevenly. A mill finish can look bright from one side and dull from the other. So the goal of lighting is not to make the part bright. It is to make the part look the same every time, while letting a real defect stand out.
Diffuse light
Diffuse light comes from a large, soft source such as a cube, a dome or a lit tent. The light arrives from many angles at once. This softens harsh reflections and gives an even look across the part. It is a common first choice, and it is forgiving when parts sit in slightly different positions.
The weakness is contrast. Because light arrives from everywhere, a shallow scratch may fill with light from other directions and almost disappear. In one published simulation of a scratch on satin aluminum, a diffuse dome gave far lower contrast than bright-field or dark-field light. The authors note the result depends on the groove shape, so treat it as a warning, not a rule.
Diffuse light also shows its own reflection. A flat face can mirror the bright cube back at the lens, and a tilted face can mirror it into a glare band. If you use a diffuse source, check what each face reflects, and move or mask the source until no face mirrors it into the lens.
Coaxial and low-angle light
Coaxial light travels along the same path as the camera, usually through a half-mirror. A flat, shiny face sends it straight back to the lens and looks bright. A dent, nick or scratch tilts the surface, sends light elsewhere, and shows up dark. This works well on flat reflective faces. It does poorly on curved or tilted ones.
Low-angle light, also called dark-field, comes from a ring or bar at a shallow angle to the surface. A flat face reflects it away from the lens and looks dark. A scratch, burr or raised edge catches the light and glows. This is a strong way to show directional scratches, because the defect is bright on a dark background.
Low-angle light has its own limits. A scratch that runs parallel to the light direction can vanish, so a bar light may need the part rotated, or several directions of light. Tool marks may also glow and look like defects. As with every type, you rotate the part under the light and confirm that real defects stay visible.
Switching light directions and polarization
Because glare depends on geometry, one way to beat it is to change the geometry. Take several frames, each lit from a different side, and combine them. Research on highly reflective stainless steel, and on machined surfaces, has shown that combining images under switched lighting helps find defects that a single light misses. The price is more hardware and more software.
Polarizing filters are often suggested as a quick fix, but the physics is limited. For bare metal, a single polarizer on the lens does not remove glare. A crossed pair can help: one polarizer over the lights and a second one over the lens. Metal tends to keep the polarization of the light it reflects, while scratches scatter it and change it.
That crossed pair costs light. Expect to lose about two stops of brightness, so you need stronger lights or longer exposure. The film also needs mounting on both the lights and the lens. It is a useful option, but it is an addition to good geometry, not a replacement for it.
Color temperature, CRI and a closed box
Color temperature describes how warm or cool white light looks, measured in kelvin. About 5500 K is a neutral daylight white. CRI, the color rendering index, describes how faithfully a light shows colors, on a scale up to 100. These two numbers matter when you judge color, such as anodize shade or staining. For a mono camera checking scratches, they matter less than geometry.
What matters more is that the light does not change. Room lights flicker, windows change through the day, and someone opens a door. An enclosure with a matte-black interior and a closed front blocks that. Many vision projects fail because ambient light changed after setup, not because the camera was weak.
Fix the brightness once and leave it. A dimmer knob that people nudge will shift every image, and that shift can look like a defect to software. Lock the exposure and gain on the camera too, so that brightness is set by the light and not by an automatic control.
How to choose for your part
Start with the defect, not the light. List what you need to catch: scratches, dents, burrs, stains. Collect a few real examples and a few good parts. Then photograph all of them under each candidate light, rotating each part to several orientations. Keep the camera and exposure fixed so only the lighting changes.
Judge each setup by two questions. Do the defects stand out from the good parts? Do the good parts look consistent when you re-seat and tilt them? A light that shows defects but also makes good parts vary wildly will cause false rejects. Change one thing at a time, and write down what you changed. Expect compromise. The best light for scratches may not be the best for dents. Shops often settle on one good-enough geometry and a closed box, because a simple setup that stays the same every day beats a clever one that drifts.
VZSOR puts the part in an enclosed LED box with a closed front, CRI 97+ light at 5500 K and a fixed dimmer setting, so the light is the same for every part. The station is pre-pilot and validated on one part, a 2 inch by 2 inch mill-finish aluminum L-bracket. Your own parts may need a different geometry, and the pilot tests that on your finish.