Vision Basics - Camera Calibration and Lighting Directional Incandescent floods, ring lights, and fiber lights mounted above or to the side of an object provide directional lighting. This lighting is the simplest to install, but effective vision oper- ations depend on this type of light source remaining constant. If the light source dims, the object appears different to the camera. If the angle changes, shadows may be added that are interpreted as features of the object. This type of lighting is most effective with simple objects or objects where specific, highly-identifiable features are being inspected.
Shiny surfaces or objects with variable surface brightness are difficult to inspect with directional lighting. Structured In structured lighting, a highly collimated light source is applied to the object. Lasers are sources of collimated light. This type of lighting allows the vision system to detect three- dimensional features, such as depth changes in the surface plane or holes in the object. Shiny surfaces are not amenable to structured lighting. Strobe Strobe lighting is required when the speed of moving objects exceeds one pixel every 17 mil- liseconds. Strobes cast harsh shadows, but can be diffused (see above).
Filtering and Special Effects In many cases, specific lighting problems can be solved by placing an optical filter on the cam- era lens. The three most common filters used for black and white cameras are polarizing fil- ters, color filters, and infrared filters. Polarizing Filters Reflected light is highly polarized (the light waves have a predominant orientation about the wave axis). A polarizing filter can be adjusted so that light waves with a predominant ori- entation are filtered. If reflected glare from an object is a problem, a polarizing filter may be able to minimize the problem.
A polarizing filter reduces the overall scene brightness so more intense lighting sources are needed with this type of filter. By adjusting the orientation of polarizing filters on both the light source and lens, you can significantly reduce ambient light and reduce shiny reflections. Color Filters Color filters allow you to reduce or eliminate different colors of light that reach the camera. Color filters may enable the system to ignore irrelevant object features that are a given color, or ignore non-significant differences in an object that develop due to differences in the colors of the feature.
Color filtration is difficult and should be attempted only when other avenues have been exhausted. ACEUser's Guide, 3.6.x, 18316-000 Rev A Page 313