https://physics.stackexchange.com/questions/252393/why-cant-incoherent-light-be-collimated-as-well-as-laser-light-e-g-in-a-laser

You could put an <LED> in a cavity with a thin long hole but then, most rays, which are not aligned with the hole, will just bounce inside forever producing heat.

So you would have a very hot device, and very little efficiency on the light output. This heat might also behave like a <black-body radiation> source, so you would not have a single frequency.

The beauty of lasers is the laser cavity (two parallel mirrors around the medium) selects parallel motion preferentially, see e.g.: https://youtu.be/_JOchLyNO_w?t=832 from <video How Lasers Work by Scientized (2017)>


Why can t you collimate incoherent light as well as a laser

ID: why-can-t-you-collimate-incoherent-light-as-well-as-a-laser