The TFO models are both. {Tritium Fiber Optic}. The Tritium being the glowing at night part of it. TruGlo has other models that are only fiber optic that only work in day light. Read here at product descriptions.... http://www.truglo.com/IW_Products.m4p.pvx?;PRODUCTS_NO_TREE?cat=30500?company=TGI
The TruGlo TFOs show in both day and night, a buddy of mine showed me the ones he has installed on his 17 i believe it was. I tested it out too. They look pretty cool!