Next Level Spirit Level (for billiard / pool table)

From a channel Lumen includes in full: Jeroen Vleggaar, a professional optics engineer, explains coherence, interferometry, lens and mirror making and photolithography with bench experiments; rare expert-level optics teaching.

A 13-minute video from Huygens Optics's catalogue; the title names its topic.

Video by Huygens Optics, played through YouTube's embedded player.