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.
Telecentric Lens Distortion
Meaning
Projection of parallel light paths eliminates magnification changes while introducing small geometric inaccuracies. Machine vision tasks use telecentric lenses to maintain constant scale, but telecentric lens distortion can still affect the shape of the captured image. These errors appear as slight warping at the edges of the field of view.
Magnification Consistency
Constant image size ensures that the dimensions of a part do not change as it moves along the optical axis. Minimal telecentric lens distortion allows for highly accurate gauging of part diameters and hole placements. If the distortion is not mapped, the measurements lose their precision.
Small errors in the lens assembly cause these deviations.
Calibration Routine
Grid patterns of known dimensions allow software to calculate the correction factors for each pixel. Measuring telecentric lens distortion involves a high-precision audit of the entire imaging area. This step is mandatory before a tool is released to the production floor.
Each lens has a unique distortion profile that must be stored in the control system.
Measurement Tolerance
Quality control limits must be wider than the residual error left after software correction. Even with a high-end system, some telecentric lens distortion remains and must be accounted for in the uncertainty budget. This budget determines if the tool is capable of inspecting the tightest part specifications.
Production yields drop if the measurement system is not significantly more accurate than the parts it tests. The final verification step ensures that the system meets the requirements of the manufacturing standard.