Why Does a Vision Inspection System Reject Good Parts?

• time：2026-10-10 15:07:07
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A vision inspection system rejects good parts when the image, the parts, or the decision logic drift apart: lighting and glare changed since validation, part presentation or position moved outside the trained envelope, focus or exposure degraded, tolerance limits were set from unrepresentative samples, or trigger timing captures the wrong moment. The first safe check is to open the rejected-part images — the system's own evidence — and compare them against the images used at validation. Fixing false rejects starts with the image, not the thresholds.

Key Takeaways

• The rejected image is the data. Comparing reject-time images against validation images localizes the cause in minutes: darker? glare spot? part rotated? edge of frame? blurred?

• False rejects usually mean the world changed, not that the tolerances were wrong to start. New part batch surface finish, a replaced lamp, ambient light through a new skylight, a worn fixture shifting presentation.

• Tolerances are only as good as the sample set behind them. Limits trained on one production batch fail on the next batch's normal variation — validation sets must span the intended production population.

• Trigger timing is a silent killer. The right part captured in the wrong instant (moving, half-in-frame, still decelerating) looks defective to any algorithm.

• Do not loosen tolerances to chase the reject rate. Loosening without re-validation trades false rejects for false accepts — the worse failure. Fix the image conditions or revalidate with proper samples.

Symptom Definition

This article covers an elevated rate of rejects whose parts are confirmed good on manual review:

1. Sudden step in false rejects — from a specific date or event;

2. Gradual creep — slowly rising over weeks (lighting aging, fixture wear, lens contamination);

3. Batch-correlated false rejects — one part batch rejects, the next passes;

4. Position-correlated false rejects — parts present slightly differently reject more.

True defect detection that operators dispute ("the scratch is fine") is a tolerance-policy question, not a false-reject fault — it needs a documented quality decision, not a technical fix.