An artificial sky canopy for use in arcologies, ships, and stations. It not only simulates the appearance of a terrestrial sky, but emits bio-supportive radiation sufficient to provide a comparable Vitamin D input to a life on Earth (back when it was more habitable), and even to grow plant life. They are generally considered a crucial element of artificial environments, only a couple of steps less so than air and water.
Higher-end models might display images of a sun, clouds, moon & stars. More basic versions are a simple monotone or gradient. They can be set to fluctuate according to a customizable clock. Many ships and stations stagger its rotation to allow rotating shifts of workers to have equal access to daylight.
Generally referred to as an ArSol, the genericized name of their most prominent manufacturer.