Optical lens inspection: when 2D cannot see a shallow scratch, use a 3D laser
Scratches, bright spots, bubbles, coating colour, black spots, spray marks and encapsulation defects on lens elements were almost all detected in 2D. For the shallowest scratches, a 3D line laser works clearly better. Decentration is checked by fitting the circle centre under coaxial light.
Lenses are transparent and reflective, so defects are often visible to the eye but hard to capture on camera. Shin Shou has run several evaluation rounds for camera-lens and glass-lens makers, collecting more than a hundred defect images. Here are the highlights.
2D camera: most defects detected
With a 5 MP colour camera at about 4–6 μm, flat backlight and coaxial light, we tested scratches, bright spots, bubbles, coating colour, black spots, spray marks and encapsulation defects:
- Bubbles, coating colour, black spots, spray marks, encapsulation: nearly all detected, imaging both faces of the lens
- Bright spots and some shallow scratches: low contrast, but detected after filtering noise by area and length/width
- The shallowest scratch sample: not visible in 2D
3D line laser: the answer for shallow scratches
Scanning the same samples with a 3D line laser (X 0.5 μm, Z 0.4 μm), scratches and bright spots stood out far more in the height map than in 2D — even the shallow scratch 2D missed could be detected. Where specs demand very shallow scratches, we recommend combining 2D and 3D.
Glass lenses: scratches, pits and decentration
- Scratches and pits: highlighted with side light; judged at ≥ 25 pixels
- Decentration: coaxial light highlights the convex boundary; the ideal circle and centre are fitted and compared. Good parts showed about 5.8 px offset, rejects about 37.9 px — a clear separation
Recommendations
The number of samples decides whether side-light angles and criteria can converge. Please provide several samples of each defect type and mark which face is up for a more accurate assessment.
