A fluorescent lamp is an example of a diffuse light source. Spheres and domes can also provide diffuse light. Directional illumination comes from a localized source. Collimated light and light from lasers are highly directional because the light rays propagate parallel to one another. You can also obtain directional light from LEDs and fiber-optic sources.
Directional lighting produces shadows that can help locate surface defects such as burrs or pits.
Polarized light helps reduce glints from shiny or highly reflective parts. Structured illumination finds use in inspection systems that need to obtain 3-D information about a part. A line of light, perhaps supplied by a laser, is an example of a structured light source. Unpolarized illumination is typical of lighting found in offices or factories, and it is useful for lighting diffuse parts. The types of lighting described above are not mutually exclusive.