Methods, apparatus, and systems are provided concerning modular lamp controllers. In one embodiment, a modular lamp controller dims a high intensity discharge lamp based upon occupancy. The lamp has a port within its fixture, and the port is coupled to a lamp capacitor. The modular lamp controller includes a housing, a plug, a capacitor, a switch, an occupancy sensor, a switch controller, a power supply, an interchangeable lens assembly, and a mounting adapter assembly. The plug is adapted to connect to the port. The capacitor couples in series with the lamp capacitor when the plug is attached to the port. The switch defines a full and reduced power state of the lamp. The occupancy sensor generates a signal indicating the presence of an occupant within a monitored space. The switch controller operates the switch to activate the full or reduced power state of the lamp based upon the occupancy signal. The power supply operates using current from the lamp when the plug is attached to the port. The interchangeable lens assembly acts as optics for the occupancy sensor, and the mounting adapter assembly mounts and locks the modular lamp controller into position.