Resolution Standard
Mathematical requirement for digitizing a physical layout specifies that the sampling frequency must be at least twice the highest spatial frequency present in the image. The spatial nyquist criteria dictate the minimum resolution for optical inspection systems to accurately detect small features like trace widths or spacing. If the imaging sensor fails this requirement, aliasing occurs and the resulting data misrepresents the actual board geometry.
This rule ensures that the digital representation of the circuit contains enough information to reconstruct the original physical form without distortion.
Pixel Density
Calculation of the pixel size is the primary application of this principle in automated optical inspection. To resolve a 50 micron feature, the spatial nyquist criteria suggest a sampling interval of no more than 25 microns to ensure the edges are clearly defined. High resolution cameras are selected based on their ability to meet this limit across the entire field of view.
Detection Reliability
Detection reliability depends on maintaining a margin above this theoretical floor. When the spatial nyquist criteria are ignored, the system may miss intermittent shorts or open circuits that fall between the sampling points. Advanced systems often oversample by a factor of four to provide the contrast needed for reliable defect classification.
