A weapon-mountable designator/illuminator unit having a light source adapted to generate a light beam in a direction away from the unit. The light beam is variable between a collimated light beam and a divergent light beam for, respectively, designating a target and illuminating a target area of the weapon. At least a first modular power supply is adapted for installation in the designator/illuminator unit. The power supply includes a cage adapted to receive a power source, a first end adapted to be received within the designator/illuminator unit, and an end cap adapted to be exposed when the power supply is assembled with the unit. The power supply is adapted to deliver power from its power source to the light source of the designator/illuminator unit. The end cap may contain a light source for generating a light beam in addition to the light beam generated by the designator/illuminator unit.