Coherent light can have some disadvantages such as causing speckling and diffraction, and lasers have stringent safety requirements. Diffuse illumination comes from multiple directions and provides uniform illumination across the field of view. You use it to illuminate reflective or shiny parts because it produces little, if any, glinting, and it reduces shadows. 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.