The term light propagation describes the manner in which light strikes an object. We will be concerned with the effects of collimation, structure, focus and diffusion. In the diagram below, these have been arranged together on a scale that has structured laser light at one extreme, and diffuse light at the other. In this context it should be noted that the terms diffuse and even when related to illumination, are not explicitly the same.
Coherent laser light where the waves/photons share the same phase
Collimated on-axis illumination, where the rays of the light are all parallel
Focused laser light, or a line light that is focused onto the surface of a web
LED light, where the light from the array has a direct path to the target
LED light, where the light from the array passes through a diffusing material