When I was a soldier I would write letters after the sun went down. The problem with that was other soldiers would be bothered by the light pollution from my flashlight. This pen focuses light right onto your page. No more angry soldiers. I left the cap clear for use as a safety light. It also works much better then holding a flashlight under your chin. The idea is that the pen body works like a fiber optic. The top is illuminated and transmits the light down to the writing tip.