To develop and model an optical system for machine vision, the first step is to define the requirements. Often the starting point includes requirements for a specific focal plane to be used in the imaging system, with a certain sensitivity. This has implications on the illumination system, including the light source, power, and collection efficiency. The reflectivity of the inspected surface (how much light is reflected into the imaging path) affects the development of the illumination system.
On the imaging system side, key factors include:
- The feature size to be resolved on the inspected part
- The numerical aperture needed for resolution and signal level
- The stand-off distance between the inspected part and the lens
- The area (field of view) of the inspected part to be imaged
Also, there may be system-level metrics dealing with more general aspects of the system performance. For example:
- Requirements for high signal to noise ratio, including requirements for high speed camera function and exposure
- Hyper-centric imaging of parts with acceptable distortion and large viewing angles for sidewalls of units under test can create unique illumination requirements
- Requirements to have very large (or narrow) part scales can drive unique requirements as well
For the overall system, other questions to be determined are the overall envelope and the cost.
As it is challenging for post-processing to improve the inferences that can be made from the raw data, it is crucial to design an optical system that captures as much information that is relevant to the task as possible. Design parameters may include structured illumination, coherence of the light sources, reflectivity, absorption, scattering, spectral response, phase contrast, polarization property, radiometry, etc.
To aid the development of a machine vision system, Synopsys offers LightTools for illumination design and CODE V for optical system design.