Dark-field lights for industrial machine vision

Describe the feature more precisely

Dark-field lights are incident-light lights with a very shallow angle of incidence: they sit on the camera side so that only edges, engravings and defects scatter light into the camera. Dark field is a lighting principle, not a ready-made defect detector. At a shallow angle of incidence, a smooth surface reflects most of the light away from the camera, while edges, elevations, recesses or particles scatter part of it back. Whether your particular feature becomes visible enough is not something the lighting class alone can tell you.

A rough base material scatters light of its own and brightens the background. The direction of a scratch governs how strongly it responds to directed lighting. Transparent, curved, multi-layer or soiled parts open up further reflection paths. Base the selection on the size, depth or height, orientation and permissible variation of the feature.

Match form factor to the installation

Ring-shaped dark-field lighting brings shallow light onto the part from every side, which suits structures with varying orientation. A directed low-angle geometry favors one direction and separates features aligned with it more sharply. Field of view, part geometry and the space you have available all feed into the decision.

Many listed versions are specified for short working distances, though the ranges do differ, so a blanket distance figure would be misleading. Check the model-specific working distance together with the illuminated area, housing dimensions, camera field of view and any shading.

These points matter most when comparing products:

• Part and limit feature: material, surface, orientation and the smallest deviation that still matters.
• Imaging geometry: field of view, working distance, camera angle and available installation height.
• Spectrum: wavelength in combination with material contrast, sensor, lens and filters.
• Process: motion, cycle time, exposure time, triggering, and continuous or strobe operation.
• Integration: power supply or controller, connector, cable length, heat dissipation, IP rating and cleaning conditions.

Power supply, connectors and IP rating differ from version to version, so never carry missing information over from a similar model. For IR and UV, confirm the spectral suitability of the setup and the product-specific safety notes as well.

Alternatives and evidence

For pure print or color features, steeper front lighting usually works better. On flat reflective surfaces, coaxial lighting is a genuine alternative. Where you are mainly inspecting the outer contour and can reach the part from behind, look at a transmitted-light geometry instead. Dark field is not the answer simply because a surface is glossy; what counts is whether the shallow angle separates your specific feature.

A meaningful test covers several good parts and parts showing the smallest relevant deviation, including variation in position, surface and process. Keep camera, lens, exposure and setup as constant as possible while comparing versions. The result applies to the conditions you tested, not as blanket approval for other materials or distances.

Before testing, define which image characteristic leads to acceptance or rejection. Beyond the visual impression, look at the contrast in the relevant image area, how stable it stays across permissible variation in position and surface, and how well it separates from acceptable contamination. Lighting makes a feature visible; it does not set the evaluation or acceptance threshold on its own. You establish that threshold together with camera resolution, lens, exposure and the evaluation method.