Evaluating Optical CMM Accuracy for Micro Molded Features
Evaluating optical CMM accuracy for micro molded features requires quantifying optical lens resolution, polymer light dispersion, and sensor thermal drift.
Refractive Edge Distortion
Meaning
Optical imaging errors occur when light bends as it passes through different media or along the curved boundaries of a transparent specimen. The occurrence of refractive edge distortion alters the perceived boundary of a molded plastic part, leading to inaccurate dimensional measurements during automated optical inspection. This phenomenon is a major challenge when scanning transparent lenses or clear plastic housings.
It arises because the transition between the dense polymer and the surrounding air acts as a tiny lens, skewing the coordinate data collected by non-contact metrology systems.
Bending Mechanism
Light rays change direction when they encounter materials with different refractive indices. In the context of refractive edge distortion, the light bends away from the straight path as it transitions from the plastic edge into the surrounding air. This redirection of light creates a false border on the sensor.
Measurement Uncertainty
Blurred or shifted boundaries can cause optical coordinate measuring machines to miscalculate critical dimensions. When refractive edge distortion is present, the measurement software might report that a conforming part is outside its tolerance limits. This error can result in the high-volume rejection of perfectly good components.
Software Correction
Manufacturers employ advanced software algorithms to compensate for the bending of light at part boundaries. Ray-tracing models can calculate the expected deflection and adjust the edge-detection coordinates accordingly. This correction step improves measurement accuracy.