Brightfield vs Darkfield Lighting: Which to Choose?

The Physical Problem

A scratch, engraving or embossed mark is a small, local discontinuity in a surface's angle relative to the surrounding flat area. A single, arbitrarily chosen lighting angle often fails specifically on this class of defect: the same discontinuity that reflects light one way toward the camera at one lighting angle reflects it a completely different way — or not at all — the moment the angle changes, and choosing the wrong one can hide the very defect the inspection is meant to find.

The Optical Solution

This is exactly the distinction between brightfield and darkfield illumination, a well-established principle from optics and microscopy that applies directly to machine vision. In brightfield lighting, the source is positioned so its light reflects straight back into the camera off the flat, undamaged surface — the image is bright by default, and a scratch or defect that scatters that light away from the lens appears as a dark mark against it. In darkfield lighting, the source instead sits at a grazing, oblique angle chosen so that specular reflection off the flat surface never reaches the lens at all — the image is dark by default, and only a raised edge, scratch or engraving that happens to scatter light back toward the lens at that specific angle appears as a bright mark against the dark field. Brightfield suits flat, mirror-like surfaces best viewed on-axis (a job for coaxial lighting); darkfield suits surface-relief defects like scratches and embossing, and is typically achieved with a bar or dome light mounted at a low, grazing angle instead of straight on.

Integration Tools

• M12 5-Pin Wiring Diagram