Could collimated light cause interference effects?
Collimated light is the best choice if you need to inspect objects with curved edges; for this reason this illumination technique is widely used by our customers, especially when working with measurement systems of shafts, tubes, screws, springs, o-rings and similar samples. However, collimated light generates interference effects, both destructive and constructive, due to the partial coherence of the radiation source and physically inherent to this type of illumination.
These effects appear as a image luminosity distribution different than the one expected from a standard illuminator such as a common backlight, which provides a perfectly homogeneous border but at the same time cannot operate effectively with other types of objects (for example cylinder shapes).
Image processing libraries are routinely able to compensate for higher or lower luminosity near the object edge (lighter border or expanded shadow areas), adjusting dimensional analysis to standard lighting conditions.
Changing image processing parameters is required if you have to measure objects with significally varying shapes and surfaces: by diffusing, reflecting or blocking the incoming light, the object becomes an integral part of the optical system.
In that sense, a contactless optical measure examines the disturbance
caused by the inspected object: this can noticed when measuring (under
any illumination) two dimensionally identical objects but with different
surface textures.