A modular germicidal light grid system for use inside an air treatment apparatus that has a plenum in which a stream of air is enclosed. The system comprises at least one elongate member and at least one lamp assembly. Each lamp assembly comprises a housing defining at least one socket and is mounted to one elongate member at a predetermined position. The system further comprises at least one linear germicidal light source. Each light source has a longitudinal axis and a distal end constructed and arranged to mount within one socket of the housing. The elongate member is mounted within the plenum and the lamp assembly is mounted to the elongate member such that the longitudinal axis of the light source extends therein the stream of air and is positioned at an acute light angle relative to the direction of flow of the stream of air.