Micro-Defect Detection on TWS Earphones using Photometric Stereo

Project Background

In the manufacturing of high-end consumer electronics, such as TWS (True Wireless Stereo) earphones, cosmetic quality is a critical factor influencing brand perception. However, automated inspection is notoriously difficult due to the product's curved geometry, highly reflective surfaces, and monochromatic (often pure white) designs.

Challenges

Our client faced significant challenges with traditional 2D machine vision solutions:

• Extremely Low Contrast: Under standard single-source lighting, shallow scratches or micro craters on white surfaces blend into the background, making them nearly impossible to detect.

• Feature Confusion: Conventional imaging struggles to distinguish between harmless surface stains (2D color changes) and actual physical damage (3D depth changes), leading to high false rejection rates.

The Solution

To overcome these limitations, we deployed a high-precision inspection solution based on Photometric Stereo technology. Unlike standard imaging, this system utilizes advanced computational photography to separate shape from texture.

Technological Highlights:

• Multi-Spectrum & Multi-Segment Illumination:The system utilizes a specialized lighting controller to rapidly capture multiple images under varying illumination angles and wavelengths.

• Surface Normal Analysis:By processing the sequence of images, the algorithm reconstructs the Surface Normal Map (X/Y direction). This effectively generates a slope map of the surface, completely isolating physical defects from surface color or pattern interference.

Inspection Results

The implementation of this solution achieved distinct visibility of micron-level defects on complex curved surfaces.

• Standard Imaging (Left): Under normal lighting conditions, the defects are virtually invisible to the camera, resulting in potential quality escapes.

• Photometric Stereo / Normal Map (Right): After algorithmic processing, the background noise is filtered out, revealing the defects clearly:

  • Scratch: Revealed as a distinct linear depth feature.

  • Crater: Identified clearly as a localized surface depression.

Client Value

The successful deployment of this system delivered immediate benefits to the production line:

1. Enhanced Detection Rate: Successfully identified low-contrast defects (shallow scratches/dents) that were previously undetectable.

2. Reduced False Positives: accurately distinguished between cosmetic stains and physical damage, saving costs on unnecessary scrap.

3. Versatility: The solution is highly adaptable and has since been applied to other components, including charging cases, smartphone back covers, and precision injection-molded parts.