Optical Deviation
Physical aberration in a lens where the magnification changes with the distance from the optical axis causes straight lines to appear curved. Small amounts of radial distortion are present in almost all lenses used for board inspection and must be corrected through software. Distortion is most visible at the corners of an image where the light rays pass through the outer edges of the glass.
Geometric Alteration
Barrel effects occur when the magnification decreases toward the edges, making the image look like it is wrapped around a sphere. When radial distortion takes the form of pincushioning, the magnification increases at the edges and pulls the corners outward. Both types interfere with the ability of an automated system to measure the exact center of a component.
The magnitude of this effect is typically proportional to the distance from the center of the sensor, requiring a non-linear correction function.
Image Normalization
Mathematical models help the inspection software straighten the image before any measurements are taken. By applying a correction algorithm, the vision system can treat the entire field of view as a flat plane. This process is essential for verifying the alignment of large connectors or high-pin-count chips that span a wide area of the sensor.
