Tech note: backlight, coaxial or ring light — which light for which defect?
Missing OLED traces show best in backlight, crushed traces in coaxial light; probe counting and ceramic thickness rely on backlight; bright and dark defects on UV tape use coaxial plus bottom light. Four cases that explain the basics of lighting.
An old saying in machine vision: lighting decides half of success. The same defect can go from invisible to obvious just by changing the light. Four real cases show where the three most common lights fit.
Backlight: for outlines and presence
Backlight shines from behind the object, turning it into a silhouette with the sharpest outline.
- Missing OLED traces: gaps transmit light, giving the best detection
- Probe counting: transparent boxes imaged with backlight count most reliably
- Ceramic substrate thickness: the edge is imaged against backlight to compute thickness; the mechanism must keep part, camera and backlight mutually perpendicular for accurate measurement
Coaxial light: for changes on flat surfaces
Coaxial light shares the camera's optical axis; flat areas reflect evenly while dents and pits turn dark.
- Crushed OLED traces: best detected with coaxial light, using self-comparison within the image
- UV tape appearance: coaxial plus bottom light; dark defects marked red, bright ones blue, with the inspection region defined by a circumscribed circle
Ring and side light: for scratches and relief
Low-angle or side lighting makes the edges of scratches and pits cast shadows or glints, so they stand out on flat surfaces. High-angle ring light gives even imaging on metal, suited to scratches on metal areas.
Summary
First ask under which light the defect differs most from normal areas, then choose — often the answer is a combination. That is why Shin Shou always compares several lights during evaluation.
