The latest development in sub-miniature LED technology makes the Mark-Lite Fire Fly possible. Its five eye-catching red LEDs operate for an incredible 100 hours on a single set of 2 N-size alkaline batteriesÂ—with an operating cost of about 2 cents/hr. The Mark-Lite Fire Fly is the perfect marker light for people active in the dark. Use one for night scuba diving, or while biking, jogging or walking. Comes with a Velcro strap.