The above says it all. That and one little UV bulb just wouldn't do it. It's interesting that there are reports and pictures of daylight enlargers from the 19th century. These show enlargers that open to the sun. They were sometimes built into a room, sometimes they were free standing and could be adjusted to keep the condensers facing the sun. Those required that the sensitized paper be in something akin to a film holder. I haven't seen any references to what material they were exposing (albumen?), or how long the exposures were.
Originally Posted by Gerald C Koch
If one were to build a UV enlarger I suppose focusing could be done with UV protective glasses and/or a yellow filter.