Why does Romeo call the tomb a lantern?

He says that the tomb will be a "lantern," which means that it is a dome with windows that can make the room more bright. He says it is bright in there because Juliet is there and lights the room up with her beauty.

Romeo refers to Juiliet's beauty as the lantern (light) filling the tomb.

A grave? Oh, no. A lantern, slaughtered youth,

For here lies Juliet, and her beauty makes

This vault a feasting presence full of light.

Death, lie thou there, by a dead man interred.