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.
Sub Surface Scattering Bias
Meaning
Error in optical height measurement occurs when light penetrates the surface of a translucent material. Precision metrology of plastics and resins must account for sub surface scattering bias to avoid false readings. The light returns from a point deeper than the physical top layer.
Material Interaction
Translucent substances allow photons to travel through the upper layers before returning to the sensor. High levels of sub surface scattering bias result in measurements that suggest a part is thinner than its physical reality. Calibration targets must match the material properties of the production run.
Surface finish also influences how much light enters the material.
Correction Algorithm
Software filters apply mathematical offsets to the raw scan data to estimate the true surface position. Developing a sub surface scattering bias model involves testing samples with known dimensions under varying light conditions. An inaccurate correction leads to the rejection of good parts.
This step requires a controlled laboratory environment.
Sensor Selection
Different wavelengths of light penetrate materials to different depths. Reducing sub surface scattering bias often involves switching to shorter wavelengths or using structured light patterns. Choosing the right hardware for the material avoids the need for complex software adjustments.
Blue lasers provide higher accuracy on semi-transparent surfaces than red alternatives. This selection happens during the initial design of the inspection station to ensure long-term reliability.