When I was working in Berlin a colleague mentioned that each employee has to be within a certain distance of a window.

Apologies - but I don't have a written source - perhaps someone else can shed some light..

French law mandates eye-level transparent windows to outdoors in workplaces, except for those where it is incompatible with the activities. It also mandates sufficient natural light, and for rooms with no windows intended for long work, at least 200 lux of light on the desk (but the lighting also has to be "adapted" to the activities).

Rooms with no doors in large buildings are sometimes used as meeting rooms (since they aren't intended for permanent work). I have seen once a large room with just a few windows that were far away from most desks, I guess it was technically legal but would probably not pass should one of the employees sue (comfort is an important part of the law).

I'm in Denmark and I don't think the law applies to factory workers. however due to the prices on electricity you would be crazy if you didn't try to illuminate with day light, but I'm not sure a skylight counts as a window.