Many models feature bright daytime reticles using fiber optics which collect ambient light.
BAC models feature reticles illuminated both day and night. A fiber optic collects ambient light during the day, and a tritium lamp lights the pattern at night. The Tritium lamp lasts 20 years. Amber color is used in compact ACOG models because its brighter. Because only the prism housing moves, the ACOG is much more rugged than conventional designs.