I have a copper john pro II and a widowmaker. The pro II has exposed fiber optics and can't be replaced in the field. they broke while hunting 70 miles from home. copper john sent me some new fibers and they broke. I ordered replacement fibers from cabelas and cut them to size and replaced them. the widowmaker sights' fibers can be replaced but the yellow fibers are not very bright even with the light. I am now looking for a new sight and won't try copper john sights in the future.